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https://ntrs.nasa.gov/search.jsp?R=19840003479 2020-03-21T01:46:55+00:00Z

o^

THE UNIVERSITY OF KANSAS SPACE TECHNOLOGY CENTER

Raymond Nichols Hall

22?1 Irving Hill Drive—Campus West Lawrence, Kansas 66045

Telephone: 913-864-4832

1- -009[84 - -Made -i%, 'ible under NASA sponsorship

in the t of early and vii;$ a di,-

ine'. :1sern ,;:j w Earth Resources survey

MICROWAVE REMOTE SENSING OF SNOWp j og_a, j and without IWA.,

-for any use .nadWere0t." EXPERIMENT DESCRIPTION AND PRELIMINARY RESULTS

Remote Sensing LaboratoryRSL Technical Report 340-1

W. H. Stiles

B. C. Hanson

F. T. Ulaby

,q C

'V,,&, s f I E I ZJune, 1977

Fawwaz T. Ulaby, Principal Investigator

Supported by

NATIONAL AERONAUTICS AND SPACE ADMINISTRATIONGoddard Space Flight CenterGreenbelt, Maryland 20771

NAS 5-23777

(984-100091 MICROWAVE REMOTE SENSING OF $84-11547SMOV ZXkV*Z 1AZVT DESCRIPTION AND PRELIBINAi.yRESULTS (Kansas Univ.) 123 P iiC A06/1111 A01

CSCI 08L UnclazG3/43 00009

CRES

^If^^^l 'REMOTE SENSING LABORATORY 4

ACKNOWLEDGEMENTS

The work reported herein was principally supported by NASA/GSFC

under contract NAS5-23777 and in part by NASA/JSC under contract

NAS9-15003. The principal investigator wishes to acknowledge the

following individuals and their respective organizations for the tech-

nical assistance they provided in support of t tie snow experiment con-

ducted at Steamboat Springs, Colorado:

Dr. Albert Rango, NASA/GSFC

Dr. Jim Shiue, NASA/GSFC

Dr. Vincent Salomonson, NASA/GSFC

Dr. William Linlor, NASA/ARC

Dr. Lawrence Klein, NASA/LRC

Capt. Ted Lane, Eckland AFB

Mr. George Ewell, Georgia Institute of Technology, EngineeringExperiment Station

ff^4 Mr. Bruce Jones, M. W. Bittinger and Associates, Inc. {

Also acknowledged is the hard work and dedication of the Remotesi

Sensing Laboratory,-experiment team under the leadership of Mr, Herschel

Stiles and Mr. Bradford Hanson and the guidance provided by Professors

r" R. K. Moore, L. F. Dellwig and K. C. Carver. In addition to Mr. Stiles

and Mr. Hanson, the experiment team consisted of the following RSL

,personnel: Mr. D. Brunfeldt, Mr. J. Lyall, Mr. M. Lubben, Mr. K. Scott

and Mr.^ L. Gulley.

+5

Fawwaz T. Ulaby

Principal Investigator °!

Page

a.

ACKNOWLEDGEMENTS .. .................................... ............ iABSTRACT ............................................................. xi1.0 INTRODUCTION .............................................. 1

2.0 EXPERIMENT DESCRIPTION ......................................... 5

2 .1 Test Site Description ..................................... 5

2.2 Microwave Sensors ......................................... 5

2.2.1 MAS 1-8 and MAs 8-18/35 ............................ 5

2.2.2 Radiometers ........................................ 15

2.3 Ground Truth Instrumentation .............................. 21

2.3.1 Snowpack Conditions ................................ 21

2.3.1.1 Snow Depth ................................ 24

2.3.1.2 Snow Density ............................ 24

2.3.1.3 Snow Wetness Measurement............ .... 24

2.3.1.3.1 Capacitance Measurement of

Snow Wetness ................... 24

2.3.1.3.2 Calorimeter Measurement of

Snow Wetness ................... 28

2.3.1.4 Snow Temperature .......................... 31

2.3.1.5 Stratification ............................ 312.3.1.6 Grain Size, Shape and Texture ............. 31

2.3.1.7 Surface Roughness ......................... 312.3.2 Soil Conditions........... ................. .... 31

2.3.3 Atmospheric Conditions ............................. 342.4 Data Acquisition.. .... ................. .................. 34

2.4.1 Daily Backcatter and Emission Measurements........ 342.4.2 Diurnal Backscatter and Emission Measurements...... 34

2.4.3 Attenuation ........................................ 36

2.4.4 Single Cell Diurnal Fluctuation Measurement........ 36

2.4.5 Snowpile Experiment ................................

ii

40

4

Y

Y

V ^

TABLE OF CONTENTS (CONTINUED)

Page

3.0 FADING .. ....................... ..._........................ 41

3.1 Rayleigh Fading Statistics ................................. 42

3.2 Fading Reduction.......,.. ................................. 45

3 .3 Measurement Precision .... ............... ............ 49

3.4 Measurement Variability with a Pulsed Radar ................ 53

4.0 PRELIMINARY RESULTS............ ............................ 56

4.1 Measurement Variability., ............................... 57

4.1.1 Test Site Spatial Variability ....................... 57

4.1.2 Precision of Microwave Measurements ................. 59

4 .2 Diurnal Experiment ......................................... 64

4.3 Single Cell Diurnal Fluctuation Experiment.......... ..... 89

4.4 Snowpile Experiments............ ......................... 95

4.5 35 GHz Attenuation Experiment .............................. 101

REFERENCES........ ........... .............................. .... 109

xiik

LIST OF TABLES

Page

Table 1. Data Base of 1977 Snow Experiment atSteamboat Springs, Colorado. 4

Table 2. MAS 1-8 and XMAS 8-18/35 NominalSystem Specifications. 9

Table 3. Radiometer Specifications. 15

Table 4. MAS 8-18/35 Antenna Beamwidths. 53

Table 5. Calculated values of 1) range variation d(Fig. 39), 2) Decorrelation Afd(eq. 37),3) Number of independent samples providedby frequency averaging N ff (eq. 38), 4) 95%/5%confidence interval for N f samples (Fig. 38for N=N ), 5) Number of spatially independent

Fsamples acquired N , 6) Total number ofindependent samples N=N xN , 7) 95%/5% confidence

f (Fsg.interval for N samples 38). 55

Table 6. Mean snowpack height and standard deviationbased on N samples acquired along the perimeterof the test plot as indicated in Figure 41. 59

Table 7. Mean snowpack water equivalent and standarddeviation. 59

Table 8. Scatterometer measurement variation withspatial position, 62

Table 9. Ground truth for snowpile experiments. 99

Table 10. 35 GHz attenuation experiment data. 106

iv

_.

t

1

LIST OF FIGURES

Page

Figure 1. Experiment timetable showing data acquisitionperiods of the various systems.

Figure 2. View of the test site south of Steamboat Springs,Colorado.

Figure 3. Steamboat Springs test site layout.

Figure 4. Test plot layout.

Figure 5. Closeup of MAS 1-8 RF section.

Figure 6. Closeup of the MAS 8-18/35 RF section.

Figure 7. MAS 8-18 block diagram.

Figure 8. Overall schematic of the 35 GHz radar module.

Figure 9. Functional block diagram of 10.69 GHzradiometer [71.

Figure 10. Functional block diagram of 37 GHz radiometer.

Figure 11. Functional block diagram, 94 GHz radiometer 181.

Figure 12. Calibration curve of 37 GHz H-polarizationradiometer-.

Figure 13. Calibration curve of 37 GHz V-polarizationradiometer.

Figure 14. Calibration curve of the 94 GHz radiometer.

Figure 15. Snow depth measurement.

Figure 16. A given volume of snow was removed from eachsnow interval with an aluminum cylinder of knownvolume (500 cc) and placed in a pan. The pan,snow and cylinder were transported to thebalance and weighed. The data were thenrecorded for the appropriate date and time.

v

m

I

3

6

7

6

8

8

10

12

16

17

18

22

22

23

25

25

t

4

LIST OF FIGURES (CONTINUED)

Page

Figure 17. Dependence of dielectric constant on wetness. 26

Figure 18. Quality factor of snow capacitor versuswetness. 26

Figure 19. Dependence of dielectric constant onfrequency. 26

Figure 20. Capacitor Sampling Procedure. 27

Figure 21. The cold calorimeter, used for measuring theamount of free water present in a sample ofsnow, consists of a thermos bottle with a

' thermocouple probe inserted through the lidand extending down into the central cavityof the thermos. 29

Figure 22. The temperatures of the solution were recorded` using a digital thermometer, and the weights

of snow and toluene were measured. 29f^.

Figure 23. a) Temperature was measured at 2 cm intervalswith thermocouple probes and a Doric digitalthermometer. b) Temperature was alsomeasured at 10 cm intervals using thermisters

4tencased in Pier tubing. 32

.tFigure 24. Snow stratification profiles were measured.

aThis photograph shows three distinct layers. 32

t"" Figure 25. Photomicrographs of the snow were made using a

" fiberoptic light source. This technique wassuccessful because of the cold light sourcecharacteristics of the fiberoptic system. 33

Figure 26. Surface roughness was measured by insertinga ruled panel vertically into the snow andphotographing the surface irregularitiessuperimposed against it. 33

i• Figure 27. The weather station was equipped with aMeteorgraph (model 701, Weather MeasureCorp., Sacramento, Ca.) which recordedtemperature, relative humidity and pressure. 35

vi

I


Page

Figure 28. Two pyronameters (model SR71, Spectrolab, Inc.)mounted back to back enabled measurements ofincident and reflected solar radiation. 35

Figure 29. Diagram illustrating the attenuation measure-ment procedure. 37

Figure 30. Attenuation measurement. 38

Figure 31. Diagram illustrating the procedure used tomeasure the attenuation of the snow at 35.6 GHzas a function of layer thickness (t). 39

Figure 32. MAS 8-18/35 and radiometers during one ofthe snowpile experiments. 35

Figure 33. Diagram showing scattered E field components. 43

Figure 34. Chi-square probability distribution for threedifferent values of k, the number of degreesof freedom [11]. 46

Figure 35. Spectral response curves for two differenteffective bandwidths '[17). 47

Figure 36. Spectral response curves for two differenteffective bandwidths [17]. 48

Figure 37. Histograms of v° for bare ground normalizedby Q°, the mean value of a o . N is thenumber of independent samples based on themeasured data and N is the nt=. , *qber calculated onthe basis of Rayleigh fading. [151 50

Figure 38. 90% confidence interval for Rayleigh distribution. 51

Figure 39. Illustration defining D; the range resolutionafforded by the antenna beam. 52

Figure 40. Diagram illustrating elevation plane geometryfor a pulsed radar. 54

Figure 41. Spatial variability of snow depth and densityat test site. 58

vii

a

t

3

C

}

f

!c


Wage

Figure 42. Spatial variability of microwave radiometric;temperatures. 60

Figure 43. Spatial variability of received backscatterpower at two frequencies and angles. 63

Figure 44. Diurnal temperature variation. 65

Figure 45. Diurnal variation of free water content asmeasured with the freezing calorimeter. 66

Figure 46. Diurnal variation of AC of the top 5 cmlayer as measured with the Q-meter. 68

Figure 47. Diurnal variation of AC of the 5-10 cmlayer from the top as measured with theQ-meter. 69

Figure 48. Diurnal variation of AC of the 10-15 cmlayer from the top as measured with theQ-meter. 70

Figure 49. Diurnal variation of Q of the top 5 cmlayer as measured with the Q-meter. 71

Figure 50. Diurnal variation of the received powermeasured by the radar at 5° angle ofincidence. 72

Figure 51. Diurnal variation of the received powermeasured by the radar at 25 0 angle ofincidence. 73

Figure 52. Diurnal variation of the received powermeasured by the radar at 55 0 angle ofincidence. 74

Figure 53. Angular response of the radar backscatterpower for a wet and a dry case. 75

Figure 54. Diurnal variation of the radiometrictemperature at 10.69 GHz, horizontalpolarization. 77

viii

^ i

{


Page

Figure 55. Diurnal variation of the radiometrictemperature at 37 GHz, horizontalpolarization. 78

Figure 56. Diurnal variation of the radiometrictemperature at 37 GHz, verticalpolarization. 79

Figure 57. Angular response of radiometric temperaturesat 10.69 GHz and 37 GHz for a dry and a wetcase. 80

Figure 58. Diurnal temperature variation. 81

Figure 59. Diurnal variation of free water content asmeasured with the freezing calorimeter. 82

Figure 60. Diurnal variation of AC of the top 5 cmlayer as measured with the Q-meter. 83

Figure 61. Diurnal variation of the received power measuredby the radar at 0' angle of incidence. 84



Figure 64. Diurnal variation of the radiometric temperatureat 10.69 GHz, horizontal polarization. 87

Figure 65. Diurnal variation of 37 GHz radiometrictemperature at 0' and 50 0 . 88

Figure 66. Temperature variation over the measurementperiod of the single cell experiment. 90

Figure 67. Snow free water content variation of the surfacep ayer as measured by the freezing calorimeter. 91

Figure 68. Variation in AC of the surface layer as measuredby the Q-meter. 92

ix

If, A117Maw a

PM

w


Page

Figure 69. Time variation of 50° backscatter powerat three frequencies. 93

Figure )0. Time variation of 70° backscatter powerat three frequencies. 94

Figure 71.. Time variation of the 50° radiometrictemperature at three frequencies. 96

Figure 72. Time variation of the 70 0 radiometrictemperature at three frequencies. 97

Figure 73. Radiometric temperature variation withsnow depth. 98

Figure 74. Radiometric temperature variation withwater equivalent. 100

Figure 75.. Radiometric temperai:gya variation withsnow depth. 102


Figure 77. Radiometric temperature variation withsnow depth. 104


Figure 79. Measured path loss as a function of snowthickness for three snow conditions. 107

1-

ABSTRACT

The active and passive microwave responses to snow were investigated

at a site near Steamboat Springs, Colorado during the February and March

winter months of 1977• The microwave equipment was mounted atop truck-

mounted booms. Data were acquired at numerous frequencies, polarizations

and angles of incidence for a variety of snow conditions. This report

documents the experiment description, the characteristics of the microwave

and ground truth instruments and presents the results of a preliminary

analysis of a small portion of the total data volume acquired in Colorado.

xi

z,

Snowpack water is a major component of the total water supply for

the Western United States, Alaska and many other parts of the world.

Since the runoff from snowmelt is usually limited to the spring and

early summer, conservation of this water is very important. Accurate

prediction of runoff is therefore needed on a seasonal, monthly, weekly

and daily basis for flood control, hydroelectric power generation, ir-

rigation, domestic and industrial water supplies, and recreation.

Present methods for monitoring snowpack characteristics use statistical

models based on a few widespread sample points. Recently aerial photo-

graphy and ERTS imagery have shown some promise for mapping snow extent

[1,2]. Microwave remote sensing has several potential advantages over

optical remote sensing. Snow covered areas tend to be cloud covered for

large portions of the time. The ability of microwave sensors to make

timely observations unhampered by cloud cover is very important to snow-

pack monitoring. Moreover, the greater penetration of microwaves into

the snowpack can potentially provide information on the depth profile

of the properties of interest.

S:gYiff et al. [3l made measurements on snow covered ground with a

CWPdoppler scatterometer and observed that snow had the effect of lower-

ing the backscatter of the underlying terrain. The University of Kansas

Remote Sensing Laboratory conducted an experiment in early 1975 to investi-

gate the radar response to snow cover [4]. This experiment showed strong

sensitivity of the radar measurements to snow wetness (free water in the

snowpack). Both of the above experiments were hampered by inadequate

snowfall and incomplete ground truth data

Edgerton et al. [5] made radiometric measurements of snow and deter-

mined that a direct relationship existed between brightness temperature

and water equivalent for dry snow. For wet snow, a complex behavior was

reported. More recently, Kurzi et al. [6] examined possibilities of

large scale mapping of snow cover using Nimbus-5 data.

The conclusions derived from the above experiments are limited in x,

scope because of the limited data base and the often narrow range over

I'

which the sensor or snow parameters varied. In an effort to better

I

understand the microwave scattering and emission properties of snow,

the University of Kansas conducted an extensive experimental program dur-

ing the period from 2 February to 25 March 1977 at a site near Steamboat

Springs, Colorado. The microwave instruments used included the MAS 1-8

(1 to 8 GHz radar), the MAs 8-18/35 (8 to 18 and 35 GHz radar), and

three radiometers (10.69, 37 and 94 GHz). Ground data were also acquir-

ed to facilitate interpretation of the remotely sensed data. Figure 1 gives

the timetable of the experiment and operation periods for the various

measurement systems and Tabie 1 is a summary of the data volume acquired.

The measurement systems will be described in more detail in Section 2.

This report covers the experiments, equipment descriptions, a section

on fadng,and some preliminary results. Specifically, the preliminary results

include two of the diurnal experiments, the single spot fading time history

experiment, the snowpile experiments,and the 35 GHz attenuation experiment.

i

2

F

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TABLE 1. Data Base of 1977 Snow Experiment

at Steamboat Springs, Colorado.,a

SYSTEM DATA SETS

MAS 1-8

Diurnal subsets 34Regular sets 46

TOTAL 80

MAS 8-18/35

Diurnal subsets 90

Regular sets 40

TOTAL 130t"

Radiometers

X-band

Diurnal subsets 96

Regular sets 40fi

TOTAL 136

37GHz1Diurnal subsets 77

ii Regular sets 26 iTOTAL 103

j

94 GHz rDiurnal subsets 29

j Attenuation Sets

2-8 GHz 6712-18 GHz 40 {

35 GHz 4

Capacitance Measurements 201 }1

Calorimeter Measurements 217T

Temperature Profiles 270

Ground Truth Sets 1918

Photographs 576i

4

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kk NZ/

2.0 EXPERIMENT'DESCRIPTION

The following section covers the test site selection, equipment

descriptions and data acquisition.

2.1 Test Site Description

After considering several potential test sites for conducting the

snow experiment, the area around Steamboat Springs, Colorado was select-

ed and a 40 acre hayfield was rented from Mr. Ben Hibbert of Steamboat

Springs. Figure 2 shows the location of the test site. The surface of

the field was very flat and was covered with closely cut hay approximate-

ly six centimeters tall. Figure 3 illustrates the experiment layout.

Test plot #1 was the main test area for observation with the microwave

sensors. Test plot #2 contained buried enclosures for the attenuation

experiment. It was also to serve as the back-up area for the main test

plot. The radar trucks were parked between plots #1 and #2 and remained

stationary for the duration of the experiment. Connections were made

to the electric company and the telephone company for the experiment

duration. Figure 4 shows the trucks and ancillary equipment in place

at the test site.

2.2 Microwave Sensors

Both active and passive systems were employed to measure the micro-

wave properties of snow. The MAS 1-8, MAS 8-18/35 and three radiometers

were used in the experiment. The following section describes these

sensors.

2.2.1 MAS 1-8 and MAS 8-18/35

The MAS 1-8 and the MAS 8-18/35 are truck mounted wideband FM-CW

radars. Figure 4 shows both the MAS 1-8 and MAS 8-18/35 in operation

at the Steamboat Springs test site. Figure 5 shows a picture of the

MAS 1-8 radar. Figure 6 is a closeup of the antenna section of the

MAS 8-18/35. The system specifications for both MAS systems are given

'in Table 2. A block diagram of the MAS 1-8 system is shown in Figure 7.

The MAS 8-18 system is identical to -the MAS 1-8 system block diagram

except for frequency coverage. The 35 GHz channel is shown in block

5I

'/weatherstation

-00— Py roname ter

ORiGiFW-.' Parr.OF POOR QUALI TY

- .^► .. j ^ _ ^ "rte

^^^^ - - ..^` ^._°`^..

Colorado 131-

Figure 2. View of the test site south of

Steamboat Springs, Colorado.

to Steamboat

--' Spr i rigs

I nstrumentation Van

MAS 1-8N, /-MAS 8-18/35

Figure 4. Test plot layout.

6

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k

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ORIGINW PAuU FV`OF POOR QUALITY

Scale: 112" = 100 ft.--•-- 0 Fence Lines

o Temperature ThermisterProfile Poles

+ Snow Depth Ganging Stations

1-300 ft: —j

— t LunebergTest PlotLens Locations----, #2

300 ft. rAttenuation

---^ j Pits

Hay Bales 15 ft.Gate •

300 ft, tiTest Plot

#1

50 ft. ---^,r

Colo. -131

Road to U. S. 40

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Figure 3. Steamboat Springs test site layout,,.

7

W 1111111111111011

7Z

^ 1

V-- MAS 8-18 antennas

10.69 GHzradiometer

Of pUcR QUALITY

Figure 5. Closeup of MAS 1-8 RF section.

94 GHz radiometer

35 GHz scatterometer antennas

electronics enclosure

/37 GHzf radiometer

Figure 6. Closeup of the MAS 8-10/35 RF section.

8

II

TABLE 2 .

MAS 1-8 and MAS 8-18/35 Nominal System Specifications

MAS 1-8

MAS 8-18

35 GHz Channel

Type

Modulating Waveform

Frequency Range

FM Sweep: Af

Transmitter Power

Intermediate Frequency

IF Bandwidth

Antennas

Height of Ground

Type

Feeds

FM-CW

Triangular

1 -8 GHz

400 MHz

10 dBm

50 KHz

10 KHz

20 m

122 cm Reflector

Crossed Log-Periodic

HH, HV, VV

12 0 at 1.25 GHzto

1.8 0 at 7.25 GHz

0° (nadir)-80°

Signal Injection(delay line)

Luneberg LensReflector

FM-CW

Triangular

8-18 GHz

800 MHz

10 dBm

50 KHz

10 KHz

26 m

46 cm Reflector

Quad-Ridged Horn

HH, HV, VV

V at 8.6 GHzto

2° at 17.0 GHz

0° (nadir)-80°



FM-CW

Triangular

35.6 GHz

800 MHz

1 dBm

50 KHz

10 KHz

26 m

Scalar Horn

HH, HV, VV,RR, RL, LL

3°

0° (nadir)-80°



PolarizationCapabilities

Beamwidth

Incidence Angle Range

Calibration:

Internal

External

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diagram form in Figure B. Radar data were taken at eight frequencies

between 1.125 GHz and 7.75 GHz for MAS 1-8. MAS 8-18/35 took data

at 11 frequencies between 8.6 and 17.0 GHz and at 35.6 GHz. The

systems operate in three polarization configurations, HH (horizontal

transmit-horizontal receive), HV (horizontal transmit-vertical receive),

and VV (vertical transmit-vertical receive). In addition, at 35.6 GHz,

RR (right circular transmit-right circular receive), RL (right circular

transmit-left circular receive `•, and LL (left circular transmit-left

circular receive) polarizations are measured. Data can be obtained at

any angle of incidence between 0° and 80° from nadir. Receive power levels

are converted to scattering cross section by a two step calibration

procedure. Short term power variations due to oscillator power, mixer

temperature, etc., are normalized by referencing the power to the power

through a coaxial delay line of known toss. Actual calibration to

radar cross section is accomplished by referencing the return power to

the power returned from an object of known radar cross section. ALuneberg lens is used for that purpose. The Luneberg lens has a radar

cross section which has been calibrated against a metal plate. The

advantage of using the lens for this purpose is its relative insensitivity

to orientation.

The calculation of a' results from evaluating the radar equation

for an area extensive target.

ff P t t

G G a26° dAPr - 3r

(1)A (4,r) R t

where

P r = received power

P t = transmitted power

Gt = transmit antenna gain

G = receive antenna gain

X = wavelength

a° = scattering coefficient

Rt = target range

dA = differential element of illuminated area

11 ,

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ORIGINAL RAGC 15'OF POOR QUALITY

. 1

If the assumption is made that the parameters inside the integral

are constant over the illuminated area the radar equation becomes:

P = Pt G Gra2CT A il l(2)

r(4Tr) 3 R4 t

Note that P r represents the received power at the receive antenna

terminal. If we introduce an unknown constant. Kt to represent the effects

of cable loss, mixer conversion loss etc., we can write

1/2

V = K Pt G GrA2ao A il 1

(3)

t t (kr)3Rt

where V is the voltage at the mixer output.

Shortly before or after recording the return from the target of

interest, switches at the transmit and receive RF lines are actuated to

replace the transmit antenna-receive antenna path with a coaxial delay

line of loss L. Thus, the voltage received is given by:

1/2Vt d = Kt [Pt

L] (4)

a The ratio of the two voltages, M t ; is given by:

1/2V G G X26° A.

" _ t _t r i l l (5)Mt

Vtd (47T)3 R t

Thus any variations in Pt or Kt are removed by this internal calibration

technique.

In addition to internal calibration, external calibration is also

conducted by recording the voltage corresponding to the return from a

standard target of known radar cross section, in this case a Luneberg lens.

The measured voltage is given by:

13

1/2P t G t 0ra2ac

4i c = Kc(4,r) 3 R c

Ii ORll PMS 15

OF pOUR QUALITY

(6)

where K is the receiver transfer constant during calibration against

the lens, R is the range to the lens and ac is the radar cross section

of the lens. Again internal calibration is conducted shortly before

recording the voltage due to the calibration target:

Vcd = Kc [Pt L) 1/2(7)

and the ratio is given by:1/2

M = Vc =Gt Gra2ac

(8)c Vcd (4Tr) 3__ R4

Combining Eqs. 5 and 8 yields the following expression for a° in dB:

a°(dB) = 20 log M t - 20 log Mc + 10 log ac

(9)

10 log A ill + 40 log R - 40 log R

The first two quantities are measured and recorded by the system

and a is known (measured by the manufacturer)with respect to a flatc

plate. A ill is calculated from the geometry on the b;sis of measured>

values of the beamwidths (for each frequency-polarization configuration)

f and the range Rt . Finally, Rt and RC are determined through measurement

of the modulation frequency #m:

^` I

where

c = the velocity of propagation

f 1F = intermediate frequency

of = FM sweep

f = modulation rateM

2.2.2 Radiometers

Passive microwave data were acquired with radiometers operating at

10.69, 37 and 94 GHz. These devices were all either borrowed nr rented

for the experiment. The manufacturer's specifications are given in

Table 3. Block diagrams of the radiometers are shown in Figures 9-11,

TABLE 3. Radiometer Specifications

Manufacturer Aerojet Aerojet merry

Frequency 10.69 GHz 37 GHz 94 GHz

Type Dicke Dicke Total-power

Polarization H H and V H

Bandwidth 200 MHz 300 MHz 730 MHz

Sensitivity (At min) .2K (1-sec.) .5K (1-sec.) 3.5K

Accuracy 1K +1K +[.05(300-Ts)+6]

Temperature Range 50-350K 0-500K 0-500K

Approx. Gain (Volt/K) -.012 .010 .020

AGC No Yes Yes

i

15 f

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16

17

fs

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TC Ta2 Tay ATTEN,

S1 S2 --+LSd3 pISOLATOR---4* MIXER-1.L

4

ERF AMPI

VIDEOPREANIP

TEMPERATUREPRATUREAUDIO

EME:MrFE,v.-^Htj CONTR O L& (1) SYNC

CO.M p

AMP

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ENSATION IDETECTOR

REF OSC DEMOD SYNC

LOGIC DRIVERS DEMODS 1

DCOUTPUTS

Baseline Ta2 Ta t

Calibrate T C T al

Vertical T. - Ta 1

Horizontal Th ' T al-

Figure 10. Functional block diagram of37 GHz radiometer.

OF pOOR QUALITY

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The 10.69 and 37 GHz radiometers were operated with 1-second

integration time. Data were taken at the same angles as the scat-

terometers since the radiometers were located (Figure 6) on the boom

with the scatterometers. Calibration of the radiometers was checked

when possible by looking at the sky or microwave absorber.

X-band (10.69 GHz) radiometer

The following equations were used to convert the measured voltages

and physical temperatures of components to Tap , the radiometric

temperature of the scene viewed by the antenna.

T = thl + thl + tsw + tsw (11^HL 1..108' 136.$-- 12.06

T _ twl twl + tsw + tsw

(12)W L 11 .10^' + 136.5 ^ 12.3$

G = TCA---- L_ TBL

HL A (13)

T THL

_VANT

G

- BBL (14)

T in = 1.042 ITR - t1 239 t7 - t273 6W 3 1T25 J (tS)

_ _ 1(Tap 1 .009 LANT T i n CANT (1 LANT

9951 TRAD

FO 5 (16)ij

20

where

}

t hl = physical temperature of hot load

tsw = physical temperature of Dicke switch

twl = physical temperature of warm load

t = physical temperature of waveguide l

t2 = physical temperature of waveguide 2

CANT= physical temperature of antenna

LANT= loss of the antenna

G = gain factor

THL= radiometric temperature of hot load

TWL = radiometric temperature of warm load

T in = radiometric temperature at receiver input

Tap = radiometric temperature at the input to the antenna

VANT= voltage measured with receiver connected to the antenna

VCAL = voltage measured with receiver connected to the warm load

VBL = voltage measured with receiver connected to the hot load

TRAD= radiometric temperature of energy emitted by the

radiometer RF section

Equations 11-15, provided by the radiometer manufacturer's manual [ 71,represent the system transfer function between the input to the receiver

(T in ) and the final output voltages. Equation 16 relates T ap to T i n by

taking into account absorption and mismatch losses of the antenna and

waveguide section. The mismatch losses were measured and supplied by Dr.

Lawrence Klein of NASA Langley Research Center. Matching Equation 16

against calculated values of T in from measurements of the emission by an

anechoic absorber of known physical temperature yields a value for LANTapproximately equal to 1.0.

37 GHz Radiometer

The following two equations were used to calculate Tap for the

two channels from the measured voltages. These equations are best fits

to the calibration points shown in Figures 12 and 13 .

TGn (horizontal)= 95.06 V - 1..02 (17)

Tap (vertical)= 102.8 VV - 23.52 (18)

where V and VV are measured voltages.

94 GHz Radiometer

Figure 14 shows the best fit curve for the 94 GHz radiometer

calibration:

Tap = 53.4 VANT - 20.86 (19)

where VANT is the measured voltage.

Fluctuation in the absolute calibration of the 94 GHz radiometer

occurred when the instrument's ambient temperature fell outside the 25°C

to 35% range. Note will be made in the data analysis if the operating

range was exceeded.

2.3 Ground Truth Instrumentation

This section covers description of the snow sampling, soil sampling,

and atmospheric measuring equipment.

2.3,1 Snowpack Conditions

The following parameters were measured for the snowpack:

1. Depth

2. Density

3. Wetness

4. Temperature

5. Stratification

6. Grain size, shape and texture

7. Surface roughness

21

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(AA 001)

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23

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2.3.1.1 Snow Depth

Two permanent gauging stations, shown in Figure 3 , were located

on opposite ends of the test plot. These stations were monitored twice

daily. Figure 15 is a photograph of one of the stations.

2.3.1.2 Snow Density

Snow density profiles were obtained using a horizontal sampling

technique. Figure 16 illustrates the sampling and weighing procedures.

Also, the standard vertical (Mount Rose snow tube) sampling technique

was used periodically. It was determined that for detailed snowpack

analysis the horizontal sampling technique was easier to use and had

better accuracy.

2.3.1.3 Snow Wetness Measurement

Since one of the major parameters affecting the microwave properties

of the snow is free water, two methods were employed to measure wetness.

2.3.1.3.1 Capacitance Measurement of Snow Wetness

The use of a capacitor to measure the free water content of snow

was proposed by 1_inlor [ 9 1 who loaned us his equipment for this experi-

ment. The amount of free water in the snow affects the dielectric con-

stant of the snow capacitor and thus its Q and capacitance. Figures 17

and 18 show the experimental relationships between dielectric constant

or Q and the amount of free water. The response of dielectric constant

and therefore capacitance is a linear function of wetness. Determination

of wetness from capacitance is simples than using the Q because of

linearity. Also, low frequencies give the most sensitivity to wetness.

Figure 19 illustrates the variation of capacitance and Q with frequency.

A trade off is involved between sensitivity (low frequency desirable)

and Q (high frequency desirable). The problem with using Q measurements

alone is that the structure of the snow and of the capacitor will affect the Q.

Therefore an alternative is to measure the change.in capacitance between a

snow sample with free water, and the same sample after freezing with

dry ice. For this case, the change in capacitance will be independent

of structure and only a function of the free water. The measurements

of capacitance and Q were made with an HP-4342A Q--meter at the following

frequencies: 100 KHz, 230 KHz, 500 KHz, 1.0 MHz and 3.2 MHz. Figure 20

24

a.

4^

a

y .,j .^^^fl ,.,...emu. _ .1.i.. • w •.- _. -.

4 ,,^^^^

i r. 7 AIk

I"

% rc,ti,^c ^^r,^^ + r

Figure 15. Snow depth measurement.

Figure 16. A given volume of snow was removed from each snow interval

with an aluminum cylinder of known volume (500 cc) and

placed in a pan. The pan, snow and cylinder were trans-

ported to the balance and weighed. The data were then

recorded for the appropriate date and time.

25

\:. JI

A

OF PC+mt

x 7 fOAM POLYURETHANE 14^: 3FN

FOAM POLYURETHANE KHZ

K' (DRY) • 1.055

0.01E gm/CMS (DRY)6

K' (DRY) •LOSDENSITY: 0.018 !DRY) INTRINSIC 0, W • 4.3 %

12

0DENSITY:

100x

INTRINSIC 0, W • 10.5 VOL %,u zs ^1Ow

a200

F ; K' FOR W • 10.5 VOL % ^

cr2W a{J V0 500 3 - 6 yz

I^!%

t~j

w 2 " K' FOR W • 4.3 VOL %^ ?

ti 4 za vIr

_1 2, 4 MHZz '^`^9.3 GHZ

0 00 2 4 6 B 10 104 106 106 107

WETNESS (W)IN VOLUME PERCENT FREOUENCYIN HZ

FIG. 17. DEPENDENCE OF DIELECTRIC CON- FIG. 19. DEPENDENCE OF DIELECTRIC CON-STANT ON WETNESS 191 STANT ON FREQUENCY [91

SNOW SAMPLE DESCRIPTION

20 f VOLUME: 2.36 x 10 3 CM3WEIGHT: 1.27 x 103 gm (DRY)ELECTRODES ARE 13' x 13'SNOW IS 12'x 12'x I' THICK

a PLEXIGLAS FRAME

3 FREQUENCY: 3.84 MHZ

°z 15 WATER WAS MANUALLY MIXED WITH SNOW INin REFRIGERATED ROOM, 0• CU.

KO

Q10

rf-

aa

5 8 I 2 3 4 5 6WETNESS (W)IN VOLUME PERCENT

FIG.18. QUALITY FACTOR OF SNOW CAPAC-ITOR VERSUS WETNESS 191

(a) Obtaining sample. (b) Preparing snow capacitor.

fn

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OF POOR QUAUTY

I JI} I

•o r

(c) The filled snow capacitor,

• '" ^T

' f

i

(d) fleas,-rement of capacitanceand Q showinq Q-meter,

inductors and snow

capacitor.

Figure 20. Capacitor Sampling Procedure.

27

shows the procedure used in obtaining samples and making the

measurements with the Q-meter. At a given frequency, the change in

capacitance is given by:

DG=Cs - Cf

(20)!; C W

0

where

Cscapacitance with snow sample

C capacitance after freezing snow sample

Co capacitance of empty capacitor

A = calibration constant

W = volume percent wetness

The quantities Cs , C and Co are measured for each sample but the

calibration constant A has not yet been determined. Thus, at this time

the capacitance method provides a measure of free water content on a

relative scale only.

2.3.1.3.2 Calorimeter Measurement of Snow Wetness

The calorimetric method of measuring snowpack wetness was investi-

gated by Leaf [101. Either a freezing or melting calorimeter could have

been used. However, for snowpack wetness the freezing calorimeter is

more accurate. The calorimeter is an insulating container with provisions

for measuring the temperature. A known amount of toluene (cooling

agent) is allowed to reach equillibrium inside the calorimeter, then the

wet snow is added and the solution is again allowed to reach equillibrium.

Figure 21 shows the cold calorimeter and Figure 22 shows the procedure11

used in obtaining the samples and performing the calorimeter measurements.1z

If the calorimeter is assumed lossless, heat will be conserved between

initial and final states:

H i = Hf(21)

wheret

H. = initial heat content of all constituents {z

H f = final heat content of solution

28

t

(FN

04

OF POOR QAjAUrf

Figure 21.

The cold calorimeter, used for measuring theamount of free water present in a sample of

snow, consists of a thermos bottle with athermocouple probe inserted through the lidand extending down into the central cavityof the thermos.

Figure 22. The temperatures of the solution were recordedusing a digital thermometer, and the weights

of snow and toluene were measured.

29

The heat content of the final solution of toluene and ice is

; .t t f (W i +E) C t f + t f S Csf

(22)

whe re

tf = final equiliibrium temperature

W = weight of toluene

E = calorimeter constant

S = total weight of snow

C tf specific heat of toluene at tf

Csf = specific heat of ice at tf

The two terms are the heat content of the toluene and calorimeter, and

the heat content of the ice. The heat content of the initial constituen.ts

(toluene, snow, free water) is

H i = t i (W i+E) Cti + LF + tsDC s + ts FCw(23)

whe re

t i = initial temperature of toluene

t = snow temperaturesF = weight of free water in snow

D = we i gl; of dry snow, S = D + F

C,ti = specific heat of toluene at ti

C S = specific heat of snow at is

Cw = specific heat of water at is

L = heat of fusion of water = 79.7 cal/g

The first term of Equation (23) is the initial heat content of the

toluene and calorimeter and the second term is the heat required to

change the state of the free water. The third term is the heat content

of the ice component of the snow and the last term is the heat content

of the free water in the snow. If the temperature of the snow (ts) is

less than 0°C, then the last term of Equation (23 ) should be changed

to tsFCs.30 t

e i

ti

Solving for the fraction of free water

^W + E ) (t C - t Ci)) (t C - t C__ F_ i f tf i ti ^ F sf s sT S S(t C- t C + L) ht C t C

s w f sf / s w f sf + L(24)

2,3.1.4 Snow Temperature

Temperature profiles of the snowpack were measured at 10 cm height

intervals using thermisters incased in PVC tubing. The thermisters

were measured with a bridge circuit that read out in degrees Celcius.

Profiles were also obtained with a Doric digital thermometer and

thermocouples at 2 cm intervals. Figures 23a and 23b show these two

instruments.

2.3.1.5 Stratification

Snow stratification or layering is illustrated in Figure 24.

Location, thickness and boundaries were measured for eaci; layer. A vertical

face is cut in the snow-pit sampling area to expose the layers. The layer

thicknesses were then measured and photographs taken.i

{ 2.3.1.6 Grain Size, Shape and Textureic Photomicrographs were used to record the grain sizes, shapes and

?

structural relationships for each layer. Figure 25shows the microscope

and light source.

2.3.1.7 Surface Roughness

Surface roughness was recorded by photographing a metal grid inserted

into the snow. Figure 26shows the panel in use. This particular snowc

condition occurred after high winds.

2.3.2 Soil Conditions

The soil temperature was measured at the soil surface and at 5 cm

below the surface with the Doric thermometer.. Temperature was also monitored

at the surface, 2 cm and 5 cm below the soil surface with the thermister

bridge meter. Late in the measurement period when the soil was thawed, soil

moisture samples were taken. These samples were dried in a microwave oven to

determine the soil moisture content.

31

I

a) (b

^nOF POOR QUALITY

Figure 23a) Temperature wi^,s measured at 2 cm intervals withthermocouple probes and a Doric digital Lhernometer.

b) Temperature was also measured at 10 cm intervals usingthermisters encased in PVC tubing.

Figure 24. Snow stratification profiles were measured.

This photograph shows three distinct layers.

32

D 11

JORIGINAL nt-. E i5OF POOR QUALITY

Figure 25. Photomicrograohs of the snow were made usinga fibe roptic light source. This technique

was successful because of the cold light

source characteristics of the fihercptic

system.

Figure 26. Su r face roughness was measured by inserting

a ruled panel vertically into the snow andphotographing the surface irregularities

superimposed against it.

33

r

I.a

2.3.3 Atmospheric Conditions

A weather station was installed at the test site to continuously

record temperature, relative humidity, and barometric pressure.

Figure 27 shows the weather station. Two pyronameters were used to

measure both incident and reflected solar radiation. figure 28 shows

the pyronameters.

2.4 Data Acquisition

This section describes the specific experiments performed to better

understand the microwave characteristics of snowpacks.

2.4.1 Daily Backscatter and Emission Measurements

The daily data sets covered a more complete set of sensor param-

eters than the special experiments. The MAS 1-8 and MAS 8-18/35 operated'

at all frequencies and polarizations. The incidence angles observed

were: 0°, 10% 20% 30 0 , 50 0 and 70 0 . Spatial averaging was employed to

reduce the effects of fading. The radar returns from 20 resolution cells

were averaged at 0°. The number of spatial samples was decreased with

increasing angle to a minimum of five at 70°. The microwave radiometers

measured five cells at each angle of incidence. In addition to the

remotely sensed data, ground truth was taken with each set.

Between one and three daily sets were obtained depending on snow

conditions and equipment status. Approximately three hours were required

per data set. The desired time period for the daily sets were predawn,

noon and late afternoon. These time periods cover the widest range of

snow conditions within one day.

2.4.2 Diurnal Backscatt r and Emission Measurements

Four diurnal data sequences were conducted. This measurement

program was implemented to observe short term variation in snow conditions

such as appearance of free water in the snowpack and structural changes

within the Layers. Each diurnal experiment consisted of continuous data

acquisition over a 28 hour period commencing at 6:00 am.

To improve the temporal resolution of the variations under obser-

vation, the time span of an individual data set was reduced by reducing

the number of system parameters at which measurements were made.

34

_^..

OF POOR QUALITY

ORIGINAL s AC'Z t`iOF POOR QUALI'T'Y

Figure 27. Figure 28.

Figure 27. The weather station was equipped with a Meteorgraph(model 701, Weather pleasure Corp., Sacramento, Ca.)which recorded temperature, relative humidity and pressure.

Figure 28. Two pyronameters (model SR71, Spectrolab, Inc.) mountedback to back enabled measurements of incident and reflected

solar radiation.

Figure 32. MAS 8-18/35 and radiometers during one ofthe snowpile experiments.

35

Generally, only HH polarization was observed with the exception of the

35.6 GHz scatterometer for which all polarizations were measured. Also,

only 0 0 , 20 and 50° angles of incidence were sampled. The time span

for a data set was reduced to approximately 1.5 hours. For the last two

diurnals, only 50° data were acquired and the time span was approximately

0.75 hours. The ground truth data sets were obtained at hourly intervals

except for the calorimeter and capacitance measurements which required

a slightly longer duration.

2.4.3 Attenuation

One basic question to any study of snow is that of penetration. Is

the microwave response related only to the snow (and to what depth beneath

the snow surface) or is there a contribution by the underlying ground?

To measure this attenuation, two boxes for each radar system were placed

in the field before the first snowfall. Figure 29 illustrates the place-

ment. The MAS 1-8 and MAS 8-18/35 were used as transmit sources. The

one-way path loss was measured with a small 2-8 GHz spiral and a 12.4-

18.0 GHz waveguide horn and a Boonton power meter. Figure 30 shows the

measurement system. The power loss was measured at six frequencies in the

2-8 GHz band and at five frequencies in the 12.4-18.0 GHz band for two

snow thicknesses. The one-way loss is not totally due to attenuation, in

fact for low loss cases, the dominant loss factor may be mismatch at the

snow-air interfaces. Using multiple frequency ,measurements for each of

two layers may allow separation of attenuation and mismatch.

Also, an experiment was carried out independently to measure attenu-

ation at 35 GHz. The path loss in this case was measured horizontally.

Readings were taken for varying snow thicknesses and for three snow

conditions. Figure 31 illustrates the layout. A signal generator and

doubler were used for a transmit source.The receiver consisted of a

crystal detector and a VSWR meter. Due to some equipment problems,

sensitivity was limited. Alignment of the antennas was by manual peak-

ing of the VSWR meter.

2.4.4 Single Cell Diurnal Fluctuation Measurement

This experiment is a variation on the diurnal experiments already

described. In this case it was desirable to observe the microwave

l1

36

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LZZE

8co

a^csm

-vc0L

a:vcv

cn

30,cN

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37

C"I

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OF POOR QU ►:LiTY

a) Attenuation pit showing

b) Surface level power

the power meter, antenna

measurement.

an6 shadow of theMAS 8-18/35 transmit source

c) Closeup of the antenna

d) Closeup of the receive

and power meter and

horn.antenna box at +20 cm

above ground level.

Figure 30. Attenuation measurement.

38

4

Mwo&

r r

f ,

+0

JJL

-^^-'.iII CV1 U

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U p ^

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response for a single observation cell. The look direction and

position were therefore set and the scattering and emission properties

measured for 12 hours during the daytime. Measurements at all polariza-

tions and frequencies were taken at 50 0 and 70° angles of incidence.

2.4.5 Snowpile Experiment

The winter of 1976-1977 was the most severe drought in the last

10 years in Colorado and as a result the sn owpack in the test area

reached a maximum of only 57 cm. To test the microwave response to snow-

depth, an artifical snowpack was created by piling snow up to depths of

144 and 170 cm (two experiments were conducted). Figure 32 shows a

ph^:.,jraph of the MAS 8 -181/35 and radiometers in operation during this

experiment. Since the size of the target allowed only one independent

look (spatially), emphasis was placed on acquiring radiometric measure-

ments. MAS 8-18135 data were acquired, however the error bars are quite

large due to signal fading. Ground truth was obtained in conjunction

with the microwave measurements.

4o

L.i

1

iA

(FY

f

s

4

a

as

f

i

3.0 FADING

Fading is a result of constructive and destructive interference

of the signal components of the scattering elements within the

resolution volume. This fluctuation complicates the study of the scat-

tering properties of a target. A single measurement of the return power

therefore can only give an estimate of the target properties. In general,

unless the target can be described exactly, the fading contribution is

unknown. In most radar measurements, fading has the effect of intro-

ducing an uncertainty in the estimated value. For example, fading can

cause degradation in the probability of detection of a search radar. The

speckle in a radar image is the result of' fading. Similarly, the precision

of radar cross section measurements with a scatterometer is limited by

fading. Averaging of independent measurements provides fading reduction and

hence a better estimate of the target radar cross section. If the target

is area extensive and homogeneous, spatial averaging can be employed.

Also, excess system bandwidth can be used to give more than one independent

sample per measurement. A combination of both spatial and frequency

averaging can also be used.

One of the original investigations into fading was performed by

Marshall and Hitshfeld [ 111. Later, Swerling [121 developed models to

investigate the effect of radar cross section fluctuation on the probability

of detection. Several different types of statistical distributions have

been proposed and used to account for the effects of signal fading. If

the power returned from a single scatterer is large compared to the

power from all of the remaining scatterers of the target, the Rice

distribution 1131 is used to describe the statistics. If, on the other

hand, the target is assumed to consist of a large number of scatterers

of approximately equal backscatter amplitude, the Rayleigh distribution is

applicable. The Rayleigh distribution has been widely used for terrain

surfaces and has been shown to provide good agreement with experimental

results [14,151.

l

41

4

3.1 Rayleigh Fading Statistics

If a target is area extensive and can be represented as a large

collection of approximately equal amplitude scatterers, then the resultant

signal voltage can be represented as a phasor .summation over all the

scatterers in the resolution volume (Figure 33)'

t

Et = V ej8 = Z E ej6k (25)n

where

E t = resultant signal voltage

V = resultant magna; ..Ade

A = resultant phase

E = single element amplitude

td k = single element phase

n = number of scatterers

The signal can then be separated into its components:

X =V sin8=E E cosh kn k ( 26)

Y = V cos 8 = Z E sin 6kn

If E and 6k are uncorrelated, it has been shown that the probability

density function of the signal voltage V is given by [15 l:

2

ON) = a—V e-V /a( 27)

which is recognized as the Rayleigh density function with a being the

variance of V. Since power W varies as the square of the voltage,

W= K V 2(28)

42

e^tc q t.:^

OF POCIR 3

Y i Xk1 ^

i

' ---Yk--_ tk

Y — —e '

X

x

Figure 33. Diagram showing scattered E field components.

74qx

. OF POOR QUALITY

the density function of W can be obtained from the condition:

P (W) dW = P (V) dV

(29)

which yields:

2P (W)= 12 a

-W/26 (30)2cr

where a has been set equal to 26 2 . Equation 30 is the power Rayleighdensity function or chi-square distribution with two degrees of freedom.

The general chi-square density function with i degrees of freedom is

given by:

_ ^ 1

Pi (W) 2 i/2 6i ^ I exp ( 2x21 W 2 (31)1V ` JJ

The distribution for one independent sample is therefore chi-square with

two degrees of freedom.

If N samples from the distribution are averaged,

Nw = ^ E w ( 32)N

k=1 k

The probability density function of W is a gamma density function and is

given by [ 15 1:

_ N —N-1

PN(w) 2NQ2N r(N)exp (

2o2W) (33)

The above distribution has the following `important properties:

WW— = E [W] = 262 (34)

a-2 = [w21 = 464 (35 )W N

44

Hence the variance to the square of the mean ratio is given by:

vW

NW

Figure 34 presents plots of P N (W) for several different values of N.

3.2 Fading Reduction

Marshall and Hitschfeld [l]] evaluated the statistical independence

of fading with respect to time, range, aspect and frequency. Moore,

Waite and Rouse [16] proposed that bandwidth, in excess of that required

for resolution, could be used to reduce variations due to fading. If

the bandwidth of the measurement system i5 large such that the signal

return decorrelates in frequency, more than one independent sample

may be obtained from one measurement [17]. Waite 1171 showed that the

required frequency spacing to decorrelate r.-d thus given ;.vidependence

was

Afd = 1550

MHz (37)

where D is in meters. D is the maximum range variation across the reso-

lution volume. Waite performed an expeti,nent to show the effects of

averaging in frequency. Figures and show the variation in scat-

tering coefficient for two targets and two effective bandwidths. Notice

the decrease in fading with increasing bandwidth.

Improvement in measurement precision through the use of frequency

averaging is determined by the number of independent samples provided

by the frequency bandwidth AF employed in the averaging process:

_ AF

N f tlfd

where N is the number of independent samples per measurement due to

45

(36)

(38)

K.)

ORIGINAL PAGE PS

OF POOR QUALITYF;

t.^

t. o

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i Figure 34. Chi-square probability distribution for threek

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f of freedom [ 1 1 ] .f' n

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frequency averaging. Figure 37 shows the frequency distributions of

the measured radar scattering coefficient of three different cases. In

each case the value of N calculated on the basis of the measured values

and the value calculated on the basis of Equations 37 and 38 are

indicated.

If in addition to frequency averaging, spatial averaging is also

used, the total number of independent samples is given by

N = N s N

(39)

where N s is the number of independent spatial measurements. Once N is

determined, the variance of W can be calculated from Eq. 36 and the

desired confidence interval can be, calculated from gamma density function

tables. As an example, the 5% and 95% probability levels are shown in

Figure 38 as a function of N.

Under certain conditions, averaging returns over time will give

independence. In the case of a stationary radar observing the ocean,

wave motion provides a continuously varying observation area therefore

for a single look direction averaging in time is effective. However,

for the case of a stationary radar and stationary target, the relative

phases of the scatterers 66k in Eq. 25 ) will remain constant and the

returns will be completely correlated.

Variation in aspect angle can also provide independent samples if

the angular change is sufficient. Spatial averaging may be employed

to give independence and reduce fading. The assumption is made that

returns from non-overlapping cells are independent and that the target

is homogeneous over the areas of observation. Averaging may be applied

either in azimuth or range. Averaging in range, however, is valid only

for high incidence angles where the magnitude of the scattering coef-

ficient is known to be slowly varying with angle.

3.3 Measurement Precision

The MAS 8-18/35 operates at 11 frequencies over the 8-18 GHz band

and at 35.6 GHz. Table 2 and Table 4 give the system specifications

and antenna beamwidths at selected frequencies. Figure 39 shows the

geometry of the illuminated cell in the elevation direction.

49F, .

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TABLE 4, MAS 8-18/35 Antenna Beamwidths

Antenna Product Beamwidths

8.6 GHz, HH polarization 3.60



Antenna Height 26 m

Table 5 is a summary of the calculated values of the various param-

eters leading to the 90% confidence intervals with and without spatial

averaging. The 90% confidence interval is defined by two values, both in

dB with respect to the mean. The lower value represents the level at

which the probability that it will be exceeded is 95% and the higher

value represents the level at which the probability that it will be

exceeded is only 5%. The need for spatial averaging is most apparent at ir

nadir where frequency averaging does not appear to provide any increase

in the number of independent samples. In practice, however,; the assumption

that no penetration into the medium takes place and that the surface is

perfectly smooth are too conservative and hence the effective number of

independent samples is usually between 2 and 4 [15]. On the other extr,ime, Ei

the confidence interval with spatial averaging at 70° is smaller than the

isystem stability factor estimated to be about + 0.5 dB.

^F

3.4 Measurement Variability with a Pulsed Radar

Like the FM-CW system, a pulse system can have the capability toY

provide more than one independent sample per look. If the return from

a chirp pulse is not processed for the best range resolution, multi-

samples may be obtained from one pulse. However, most systems are

monochromatic and will have the bandwidth matched to the pulse width. - ;rt

Under the above circumstance , only one independent sample is obtainedF

per look.

Consider a pulse radar with a pulse width T (Figure 40 ) and a #,^

matched IF bandwidth AF IF = 1/T The maximum range resolution across!t

ia resolution cell is:

;r

53 l,t?3 t

x

j 4m

i.y

x

m

CN

a

M0nQa

40—

L

EO0a?

N'

OQCC5

a

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aL.i.

C^GaLas^

O

N

LL

L

1

RPRP "Ir,

a

ir't

ixi,

54

1

ORKWMAL PAGE,. r-OF POOR QURLrN

TABLE 5. Calculated values of 1) Range variation d (Figure 39), 2) DecorrelationAfd (Eq .37), 3) Number of independent samples provided by frequency {

averaging N (Eq. 38), 4) 95%/5% confidence interval for N f samples

(Figure 38 for N-N p), 5) Number of spatially independent samplesacquired N, 6) Total number of independent samples N-N f x Ns , 7) 95%/5%

confidences interval for N samples (Figure 38).

Angle or Incidence I D(m) I Afd jMHz) I N

1

95%/5% for NF samples I Ns I N 95%/5% for N samples

8.6 GHz

0° 0.013 1.2 x 10 41 -12.9 dB/+4.8 dB 20 20 -1.8 dB/+1.4 dB

50° 3.04 49.3 16 -2.1 dB/+I,6 dB 5 80 -0.76 dB/+0.73 dB

70° 13.2 11.4 70 -1.0 dB/+0.9 dB 5 350 -0.35 dB/+0.35 dB

i17.0 GHz

^`t

0° .003 5.2 x 1041 12.9 dB/+4.8 dB 20 20 -1.8 dB/+1.4 dB

50 1.43 104.9 7 -3.3 dB/+2.3 dB 5 35 -1.3 dB/+l.l de

70° 6.2 24.2 33 -1.4 dB/+1.2 dB 5 165 -0.5 d8/+0.47 dB

435.6 GHz

i

o .009 1.7 x 1041 -12.9 dG/+4.8 dB 20 20 -1.8 dB/+1.4 dB }

50° 2.53 59.3 13 -2.3 dB/+1.8 de 5 65 -0.82 dB/+0.79 dB

70° 11.0 13.6 5B -1.0 dB/+0.9 dB 5 290 -0.38 dB/+0.38 de

i

55

D = CT (40)

where c is the velocity of light 0 x 108 m/s). Hence the decorrelationfrequency bandwidth is:

A' fd = ; 3 MHz = 2cxT150 x 106 Hz (41)

and the number of independent samples provided by the IF bandwidth

AF IFis:

NoFl

F _ 1 c T = 1 (42)f ofd T 2 x 15

If Rayleigh fading is assumed, the 90% confidence limits on a single

measurement are between -12.9 dB and +4.8 dB relative to the mean.

If a chirp pulse radar is used with a frequency variation of ) 0 /T

and an IF bandwidth of 10/T , the number of independent samples per look

will be 10. Therefore, for a given observation the 90% confidence limits

with Rayleigh fading are -2.7 dB to +2.0 dB. A large reduction in fading

is obtained without changing pulse width (or range resolution).

4.0 PRELIMINARY RESULTS

The following section presents some of the preliminary analyses of a

small portion of the data. It should be noted that the radar backscatter

measurements reported herein have not been converted to absolute scattering

coefficient values. The conversion involves the incorporation of the range

to target, the area of the illuminated cell and the Luneberg lens cross

section. This conversion will be completed over the next two months and

all future reports will report values of Q°, the scattering coefficient

instead of relative backscatter power values.

56

iiA

* .4

!J(t

l

4.) Measurement Variability

This section examines the horizontal spatial variability of the

test site and the measurement precision of the radar systems.

4.1.1 Test Site Spatial Variability

Ground truth data collection was conducted in a snowpit along the

eastern edge of the test plot. A single pit was chosen to minimize

the time span required for each complete ground truth set. To test

the applicability of the ground truth data obtained in the snowpit to

the rest of the field, four tests were made to examine the uniformity of

the snow parameters across the field. Table 6 provides a summary of

the snow depth variations of the snowpack. The samples were acquired

along the perimeter of the field (at the locations indicated in Figure

41) for the first three tests. The last test, which was performed after

the last microwave date set was acquired, sampled the field itself at

the numbered locations shown in Figure 41.a:

ff

It is clear from Table 6 that the snowpack was spatially uniformE in depth as indicated b the he i ght standard deviation. Before theP Y 9

March 14, 1977 test, high winds had caused the back edge of the test

i plot to drift and reduce the snow depth in that area. These points

were not really in the radar field of view, since at 70° angle oft

incidence, the maximum ground 7ange of the radar was 90 meters and thea

'• outer perimeter of the field was at a range of 110 meters. If the

measurements along the back edge of the plot are deleted, then the

standard deviation is 1.4 cm.

In addition to measuring the snowpack height, depth profiles were n

obtained of the snowpack density. The combination of height and density ({

r E profile was used to calculate the snow water equivalent m (cm). Figure

41 includes plots of m as a function of position around the field for

. the February i5, 1977 and March 12, 1977 tests and for the interior

A Ij^

^x

57

1 ; ^{

Qmuou,u. c^

o

^co00

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58

of the field for the March 26, 1977 test. Table 7 is a summary of

the measurements statistics.

TABLE 6. Mean snowpack height and standard deviationbased on N samples acquired along the perimeterof the test plot as indicated in Figure 41.

Date N h (cm) oh (cm)

February 15, 1977 11 31.3 1.1

March 12, 1977 9 46.7 1.9

March 14, 1977 21 37.9 1.9

March 14, 1977 12 (excluding 39.2 1.4far range,points E throughN in Fig. 41)

March 26, 1977 15 40.3 1.$

TABLE 7 . Mean snowpack water equivalent and standarddeviation.

Date N m (cm) om (cm)

February 15, 1977 11 7.3 0.22

March 12, 1977 9 12.1 0.71

March 26, 1977 15 12,9 0.61

4.1.2 Precision of Microwave Measurements

Having established in the previous section that the snowpack was

fairly uniform, both in terms of height and water equivalent, we now

proceed to examine the'measurement precision of the microwave sensors.

More specifically, the spatial homogeneity of the snowpack makes it

possible to average dat_: from different spots across the field, thereby

improving the measurement precision of the microwave sensors, partic-

ularly the radar.

Figure 42 shows the 10.69 GHz and 37 GHz radiometric temperatures

for 13 measurements at 13.different spots across the field. The

59

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60

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Pi

positions of the spots were chosen randomly. The measurements werer

acquired over a span of approximately two hours during the late hours

of the night during which the snowpack conditions remained essentially

constant. According to Figure 42, the 10.69 GHz radiometric temperature

variation was only 3 K. At 37 GHz, the total variation is 9 K for the

vertically polarized channel and 12 K for the horizontally polarized

channel. About 5 K of that variation can be explained as a response to

variations in the downward emitted sky radiometric temperature. The

remainder is probably due to small variations in the snowpack properties

over the two-hour measurement period. It may also be argued that the observ-

ed'variation is totally due to drifts in the amplifier gain of the radiom-

' eter receiver. In either case, the variation is small in comparison to

t the changeg (about 80 K) observed during the diurnal cycles (section 4.2)

which appears to be in response to variations in snow wetness.

Variability of the radar data is shown in Figure 43 for 17.0 GHz

and 35.6 GHz at nadir and 50 0 . Table 8 provides a list of the measured

values of the received power and the mean value and the variance of the

{ received power., Also provided in Table 8 are the number of independent

i samples calculated on the basis of the measured data (Eq. 36 and on the

t basis of Rayleigh fading and frequency averaging (Eqs. 37 and 38). Two

values of the predicted number of independent samples are given in Table

8 the smaller value corresponds to the assumption that the backscatter

Ij is only from the snowpack surface while the larger value corresponds to

' the assumption that the entire snowpack depth contributes to the back-

f^. scatter. Comparison of the calculated and predicted values of N clearly

1 demonstrates that Rayleigh fading is a good descriptor of the fading

statistics of snowpacks.N

l The variations shown in Figure 43 are in agreement with the pre-

dictions of section 3.3 and Table 5. At 5° angle of incidence the

variations are much la rger than the variation at 50° for both frequencies.

This is precisely the reason for acquiring more spatially independent

' samples at nadir than at the higher angles. it

.1 awl

OF POOR QUALITY

TABLE 8. Scatterometer measurement variation with spatialposition.

i

Date: 2/18/77 Time: 0300 Angle of Incidence (degrees): 5 Date: 3/3/77 Time: 2045 Angle of Incidence (degrees): 50

Measurement Number Measured Received Power (dB) Measurement Number Measured Received Power (dB)

17.0 GHz 35.6 GHz 17.0 GHz 35.6 GHz

1 -38.9 - 8.3 1 -40,1 - 5.6

2 -4o.3 - 2 .7 2 -43.2 - 6.5

3 -36.1 - 6.9 3 -42.8 - 5.7 ?

4 -41.6 - 7.3 4 -44.1 - 7.7

5 -39.2 - 3.5 5 -44.3 - 7.3

6 -44.0 - 4.6 6 -42.8 - 6.2

7 -44,o - 2.4 7 -43.o - 7.8

8 -42.7 - 2.9 8 -39.4 - 6.5

9 -39.9 - 9.0 9 -43.1 - 6.8

10 -42.4 - 7.4 10 -42.5 - 6.6

11 -37.4 - 6.8 11 -41.8 - 8.o

12 -37.2 - 5.8 12 -42.7 - 6.6

13 -48.9 - 9.2 13 -42.8 - 8.7

14 -47.8 -10.8 14 -45.1 - 6.0

15 -38.2 - 7.1 15 -43.3 - 5.8

16 -42.1 - 9.8 a

17 -43.5 - 3.5

18 -45.3 -10.9

19 -45.2 - 7.9

20 -38.7 - 3.3

V 9.02x10-5 .270 u 5.64x10-5 .214W

a-2 4.02x10-9 .0264

w

a_2 4.569x10-10 .00173w w

Ncalc u?2.0 2.8 4calc _ P? 7.0 26.5

a2 a2

Npred(no penetration) 1 I Npred (no penetration) 7 13

Npred(complete 1.9 1.9 Npred(complete 10.8 16.5

penetration penetration I35 cm) 35 cm)

62 t

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(SP) JGNOd 8AIJBIOS

4.2 Diurnal Experiment

To investigate the effects of diurnal changes in snowpack conditions,

data were acquired over four diurnal periods. During the first diurnal

experiment on February 17-18, 28 MAS 8-18/35 and radiometer data sets were

taken at 55 0 angle of incidence and 14 MAS 8-18/35 and radiometer data

sets were taken at 5° and 25°. Oround truth was obtained approximately

once per hour. The calorimeter measured 13 surface samples over the

28 hour period. Seven surface samples were checked for wetness with the

capacitor method. Also samples were measured for wetness at deeper layers.

On February 17 the sky was generally a light overcast and the winds were

calm.

The temperature of the snow was measured as a function of depth in

the snow, and also at the ground level. Figure 44 shows the variation

in temperature of the air, snow at 26 cm above the ground and the ground

level. During the diurnal period the snow depth was approximately 30 cm;

so the 26 cm measurement was the closest to the snow surface. As expect-

ed, the air temperature exhibited the greatest variation over the 28 hour

period. The peak temperature at 1030 hours corresponded to a light cloud

cover. The clouds were variable during the day resulting in the non

uniformity in the temperature curve. The snow temperature rose to 0°C

by 1030 hours and remained around 0°C until about 2200 by which time the air

temperature had dropped sufficiently to cause cooling of the snow. During

the period at 0°C, varying amounts of free water were present in the snow

and were measured by the calorimeter and capacitance methods. The ground

temperature was observed to vary less than the snow temperature because

it was insulated by 30 cm of snow._ The ground was in a frozen state during

the 28 hour period.

Figure 45 presents the free water content of the surface layer and v

one underlying layer as measured by the calorimeter. The reason

for the dip in free water around 1100 hours is not known since it a

appears to lead the corresponding temperature drop. The peak value of

the free water content, 8.3%, occurred in late afternoon just preceding

the temperature drop to below 0°C. The free water of the 15-20 cm layer

above ground level indicates a similar trend to the snow surface measurements

64'it

r.

3

{

zr

f

OF POOR WALITY

10

5

0

OV

2 -5LDate: 2117 - 2118177

CL ' Air Temperature-10 ' Snow Temperature at 26 cm

n Ground Temperature

-15

-2010800 12001600 2000 2400 0400 0800Noon Midnight

Time of DayJ

'Figure 44. Diurnal temperature variation.

651

^a

ORIGINAL PAGE, IS

OF, POOR QUALITY.

1 01

I

10

8

6

4

sL

2

LU-

0

-2

?0 cm)und

I`

T -40800 1200 1600 2000 2400 0400 0800

Noon Midnight

Time of Day

k

Figure 45. Diurnal variation of free water content as measured with the freezingcalorimeter.

66

r,

with a smaller magnitude. The snow was dry between 2300 hours, 17 February

and 0800 hours, 18 February.

Capacitance measurements were obtained at three depth layers

throughout the diurnal period. Figures 46 to 48 show the change in

capacitance at several frequencies. The lower frequencies are much

more sensitive to wetness; however, when the snow is very wet they tend

to have Q's lower than can be measured by the Q-meter. Some measure-

ments are missing due to operator errors. Examination of the curves,

for the ranges of wetness observed during the diurnal, indicates that

500 KHz provides the best compromise between sensitivity and Q. Data

from the daily experiments indicate that the frequency choice should

be around 1 Mliz for higher values of wetness. Figure 49 shows the

variation in Q over the diurnal period. The minimum measurable Q was 5.

Therefore, 500 KHz or 1 MHz would be the best choice for frequency.

The changes in capacitance are very similar to the changes in free water

as measured with the calorimeter. The deeper layers show progressively

smaller values of wetness.

The diurnal radar data at 8.6, 17.0 and 35.6 GHz are plotted in Y

Figures 50 through 52. The 5° data show no obvious response to a singles

ground truth parameter whereas the 25° and 55 0 data indicate an inverse

response to free water content (Figures 45 and 46 ). Over the period

when the ground data was relatively constant, 0200 to around 0800 hours,

there is 'Less than 2 dB variation in power return. The response to free

water content appears to be stronger the higher the frequency. At 25°,

fine dynamic range at 8.6 GHz is about 6 dB while it is 8.5 dB at 17.0 GHz

and almost 12 dB at 35.6 GHz. Figure 45 indicates that the greatest

rate of change in the amount of free water was from 1100 hours to about

1400 hours which matches well with the decrease in backscatter power. The

55° backscatter power data show a slight increase in dynamic range over the

25° data: 7 dB at 8.6 GHz, 9 dB at 17.0 GHz and 14 dB at 35.6 GHz. Figure 53

gives the angular responses for a wet and a dry data set. The slopes of the

responses are all greater for the wet case than for the dry case. Also

the change in slope between the wet and dry cases increases with

frequency.R

67

WV

ORW-AfNAL PAGr. 1-3

OF POOR QUALITY

60

Date: 2117 - 2118177Layer: Top 5 cm

3.2 MHz

50 1.0 MHz• 500 KHzn 230 KHz

40

30r

20

0

A

........................................

1800 1200 1600Noon 2000 2 0400 0800M idn Ight

9ure. 46. Diurnal variation of AC of the top 5with the 0-meter. .,m foyer as measured

68

ORtOli4AL PAGE 10OF POOR QUALITY

50

Date: 2/17 - 2118177

Layer: 5 to 10 cm from top40 3.2 MHz

• 1.0 MHz• 500 K.Hz

30 n 230 KHz

q N

20

N

10W

TJ

m

X0800 1200 1600 2000 2400 0400 0800Noon Midn ight

Time of Day

Figure 47. Diurnal variation of AC of the 5 to to cm layer from thetop as measured with the Q-meter.

69

Date: 2117 - 2118177Layer: 10 to 15 cm from top

3.2 MHz• 1.0 MHz

500 KHz

230 KHz

E I

i

I

OF POOR QUALITY

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Figure 48. Diurnal varictiore of AC of the 10 to 15 cm layer from thetop as measured °wl the Q-meter.

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30 Date: 2117 - 2118177Angle of Incidence (Degrees): 55Polarization: HH

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Figure 52,, Diurnal variation of the received power :measured by the radarat 550 angle of incidence.

N 74

7

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ORIGIN AL PAGE 1.9OF POOR QUALITY

17.0 GHz, HH

Date: 2/17 - 2/18/77Ti me:

100600 Hours (Cold & Dry

---- 1430 Hours, (Warm & Wet)

0

35.6 GHz, HH

co

-20

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r(av

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8.6 GHz, HHi

-500 10 20 30 40 50 60 70Angle of Incidence (Degrees)

Figure 53. Angular response of the radar backscatter powerfor a wet and a dry case.

Radiometric data were acquired at 10.69 GHz and 37 GHz in addition

to the radar data. Figure 54 shows the diurnal response at 10.69 GHz.

The mid-day notch is surprising since there were no obvious ground

truth changes; this effect may be due to penetration bnd therefore caused

by layering effects. The 37 GHz responses at 5° and 55° from nadir

are shown in Figures 55 and 56 . Note the 80 K rise in temperature

with the appearance of the free water. Edgerton et al. [ 5 1 observed

the same effect at 13 and 37 GHz but not at 1.5 and 5 GHz for a 40 cm

water equivalent snowpack. Figure 57 shows the radiometric angular

response for a wet and a dry snow case. When the snow is wet, the drop

in temperature with angle is less.

During the second diurnal experiment on March 3 and 4, sixteen MAS 1 -8,

twelve MAS 8-18/35 and twelve 10.68 GHz and 37 GHz radiometer data sets

were obtained over a 26 hour period. Ground truth data were also recorded.

These data included eight calorimeter surface samples and seven capacitor

measurements. The sky was very lightly overcast and light snovq was fall-

ing. The temperatures are shown in Figure 58 . The snow temperature

was near 0°C for only a short time during the day. The calorimetric

and capac;taice measurements again agree qualitatively (Figures 59 and

60 ) and show the peak value of the free water to be between 1400 and

about 1700 hours. Figures 61 through 63 present the relative radar

response at nadir, 20° and 50°. Similarly to the first diurnal experi-

ment, no simple response is apparent for the 0° data. At 20° and 50°

angles of incidence, the response to free water is quite strong, especially

at the higher frequencies: 35.6 GHz and 17.0 GHz. Figure 64 shows the

variation of the 10.69 GHz radiometric temperature over the diurnal

period. The radiometric temperature response is approximately a mirror

image of the cross section curve exhibited by the radar. Figure 65

presents the 37 GHz vertical channel data at 50°. The response in this

case also agrees with the 10.69 GHz and radar data although the change

between 1100 hours and 1600 hours is much more pronounced than that of

the 10.69 GHz.

In conclusion, the following preliminary observations are deduced

from the first two diurnal experim.nts:

7F.

:L

ORIGINAL PAGE 'jS

OF POOR QUALITY

Date: 2/17 - 2/18/77

Angle of Incidence (Degrees):.:)Uu

280

260

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220

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horizontal polarization.

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OF POOR QUALITY Data: 2/17 - 2/18/77Ti me:

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Figure 57. Angular response of radiometric temperatures at10.69 GHz and 37 GHz for a dry and a wet case.

80

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Figure 61. Diurnal variation of the received Power measured by the radar at Qoangle of incidence.

84

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Date.. 3/3 - 3/4/77Angle of Incidence (Degrees): 20Polarization: HHo 1.25 GHz* 7.625 GHzin 8.6 GHz• 17.0 GHz• 35.6 GHz

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e.

1. Passive Microwave

a) Wetness of the snow surface layer strongly affects the

microwave emission at all angles of incidence (between 0° and

55°) and both frequencies employed in these two diurnal

experiments (10.69 GHz and 37 GHz).

b) The response to wetness is much more pronounced at

37 GHz than at 10.69 GHz.

2. Active Microwave

a) Weti;°ss of the snow surface layer strongly affects the

microwave backscatter at angles of incidence away from nadir

(20° or higher).

b) The response to wetness increases with frequency.

Further analysis of the data in conjunction with the results of the third

and fourth diurnal experiment, and the daily experiments should provide

a more detailed picture of the paszive and active microwave response to

snow parameters:

4,3 Single Cell Diurnal Fluctwat;on Experiment

In this experiment all the data were acquired at 50° and 70° from

nadir. The azimuth position remained un,:hanged for the entire ll hour

duration of the experiment. Sky conditions were clear in the morning

and became partly cloudy to cloudy late in the da y . Figure 66 showsIr

the variations of the air temperature, snow surface layer temperature,

snow temperature at 10 cm above the ground. and the snow-ground inter-

face temperature. Note that the temperature at 10 cm above the ground

i

was'cooler than either the snow surface temperature or the ground

temperature. Figures 67 and 68 show the calorimeter and capacitance

data for the surface layer. The dip in the capacitance readings at

1540 hours is due to the snow being so wet that the capacitor could

not be filled properly. Figures 69 and 70 show th€:. radar backscatter

fluctuations over the measurement period. The trends at 50' and 70a

tare very similar. The 8.6 GHz data were stopped at 1500 hours due to

}equipment malfunction. When the snow was dry, in the early morning,

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the fluctuation in the measurements was on the order of 3 dB. Then

the surface layer underwent a rapid melting phase from about 1030 to

1200 hours. During this period, the cross section dropped by 12 dB,

16 dB and about 18 dB at 8.6 GHz, 17.0 GHz and 35.6 GHz, respectively.

The 8.6 GHz data appeared to be continually decreasing until the

equipment malfunctioned, this trend contrasts with the higher frequencies

which seemed to exhibit a saturation effect after about 1300 hours.

Figures 71 and 72 present the temperatures measured with the radiometers

at 50° and 70°. The 94 GHz radiometer overheated and had to be shut off

at 1500 hours. All three radiometers appear to saturate after about

1200 hou_ ,s. The dynamic range from the wetest to dryest case was 40 K..

for the 10.69 GHz radiometer increasing to about 120 K for the 37 GHz

channels and about 80 K for the 94 GHz radiometer. The 94 GHz data show

a decrease after 1200 hours at 70% this may have been the result of gain

changes since a similar change is observed for the sky temperature at

94 GHz but not at 10.69 GHz or 37 GHz. Between 0745 hours and 1500 hours

the 94 GHz sky temperature decreased from 45 K to -?5 K! This obvious drift

in the radiometer gain will be investigated as a means of correcting the x

snow response.

4.4 Snowpile Experiments

Two snowpile experiments were conducted. In the first experiment,

the snow was piled to a depth of 144 cm in five steps. The air temperature r

was below -2°C for the duration of the experiment. The snow temperature

varied from a maximum of -1.6 °C at 134 cm above ground to -2.4°C at

74 cm. Calorimetric measurements were not obtained, however, these

temperatures indicate dry snow conditions. Figure 73 shows the variation

of radiometric temperature with depth, Bare ground was observed, then

fresh snow was added in increments of about 40 cm. Table 9 is a

summary of the ground truth data. In Figure 74 the radiometric data are

plotted versus water equivalent. The temperature decrease is seen to be

approximately linear above 3 cm of water for both 37 GHz channels and

at 10.69 GHz.

95

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TAB,_ 9. Ground Truth for Snowpile Experiments

Date Snow Layer (cm) Density Temperature

Feb. 24 144-113 .196 - 1 .8°C

113-80 .176 -2.2°C

80-40 .22 -2.4°C

40-12 .226 -3°C

12-0 .10 -4°C

March 21 170-120 .456 -1.7 °C

120-71 .425 - .8°C

71-51 .413 -1.2°C

51-32 .510 - . 7 °C

. 32-13 .512 -1.O'OC

13-0 .340 - . 5 °C

March 22 170-140 .447 < 0°C

140-120 .385 < 0°C

120-105 .381 < 0°C i

i105-85 .382 < 0°C

k 85-70 .411 < 0 °C

`. 70-52 .420 < 0°C

52-37 .420 < 0°C

^ 37-14 .462 < 0°C k

14-0 .462 < 0°C

`^ l

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ORIGINAL PAGE 1WOF POOR QUALITY

Date.. 2/24/7'Angle of Incidence (Degrees): 27n 10. 69 GHz

37 GHz, H-polarization• 37 GHz, V-polarization

280

260 Compacted

240 •

M

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0 200

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Figure 74. Radiometric temperature variation with water equivalent.

100

+ t

The second experiment included the 94 GHz radiometer. In this

experiment snow was piled to a depth of 170 cm in seven layers.

Figures 75 and 76show the radiometric temperatures plotted against

depth and water equivalent. Calorimeter measurements were obtained

for each layer and indicated that no free water was present. Note

that there appears to be a saturation effect at about 30 cm water

equivalent at 37 GHz. The 94 GHz response saturates at about 15 cm

water equivalent.

The following day, the snowpiie was uncovered by layers and the

experiment repeated. Figures 77 and 78 show the results. The fluctu-

ation in the 94 GHz response for depths greater than 70 cm was the

result of temperature fluctuations of its ambient temperature (and

hence its gain). The saturation effect is also apparent for this

experiment. The 94 GHz radiometer response levels off after about

20 cm water equivalent and the 37 GHz response saturates at 23 cm water

equivalent while the 10.69 GHz response continues to drop up to the

maximum value of water equivalent although the rate of change decreases

above 40 cm water equivalent. This observation indicates that the

effective depth of penetration of the snow varies inversely with fre-

quency which points to the potential use of muiti-frequency microwave

sensors to reconstruct the profile of snow parameters of interest.

4.5 35 GHz Attenuation Experiment

This experiment was performed on March 23 and March 25. Section

2.4.3 describes the method used. The total path loss through several

thicknesses of snow was measured for three different layers of the snowpack.

Table 10 gives the results of this experiment. Case i was the third

layer from the top of the pack. No calorimeter measurements were obtained,

but the snow was fairly wet. Case 2 was the second layer from the bottom.

The wetness as measured by the calorimeter was 1.6. Case 3 was the third

layer from the top late in the day on March 25. Calorimeter measure-

ments were not made, but the snowpack was near saturation in wetness. For

this case only one reading was possible at 6 cm thickness before the

layer collapsed. Figure 79 is a plot of the measured loss as a function

of snow layer thickness for each of the three cases.

101

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Date. 3121177Angle of l nciden;:e ( Degrees): 57

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Figure 76. Radiometric temperature variation with water equivalent.

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280

260

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Figure 78. Radiometric teriiperature variation with water equivalent.

105

INC

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TABLE 10. 35 GHz Attenuation Experiment Data

Snow Received Power- Received Power-Case Thickness (cm) Air Path Snow Path Range (cm)

I (wet 20 -31.5 dBm -62 (noise) dBm 30snow)

14 -30.5 -50.6 30

10 -31 -41.5 30

7 -31.2 -42 30

5 -31.5 -43 30

2 (dry 57 -42.5 -54 67snow1.6% 47 -38.5 -50.5 57

free 39 -40 -52.5 57water) 28 -39 -49 57

17 -4o -46.5 57

15 -4o -46 57

7 -40 -41 57

3 (very 8 -34 noise 40wet

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The difference between the power received through the air path

and the power received through the snow path is the loss in dB. This

loss is composed of two parts, mismatch loss and attenuation loss.

Moreover, since the measurement is performed with coherent transmission,

the two types of losses cannot be easily decoupled (even with multiple

thickness measurements) because of the interference effects caused by

multiple reflections, unless the measurements cover a wide range of

thickness at intervals smaller than the effective wavelength in the

snow. These conditions were not realized respectively because of the

sensitivity limitation of the power meter and the difficulty involved

in physically reducing the snow thickness while maintaining a flat

interface.

108

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REFERENCES

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1

[1] Leaf, C. F., "Aerial Photographs for Operational Streamflow Forecastingin The Colorado Rockies," 37th Western Snow Conf. Proc., Salt LakeCity, Utah, 1969.

(21 Barnes, J. C. and C. V. Bowley, "Snow Cover Distribution as Mapped fromSatellite Photography," water Resources Research, v. 4, n. 2,

pp . 257-272, 1968.

131 Cosgriff, R. L., W. H. Peake and R. C. Taylor, "Terrain ScatteringProperties for Sensor System Design (Terrain Handbook II),"Ohio State University Experiment Station, 1960.

[4] Stiles, W. H., F. T. Ulaby, B. C. Hanson and L. F. Dellwig, "Snow Back-scatter in the 1-8 GHz Region," RSL Technical Report 177-61, Universityof Kansas Center for Research, Inc., Lawrence, Kansas, 1976.

[5] Edgerton, A. T., A. Stogryn and G. Poe, "Microwave Radiometric Investi-gation! of Snowpacks," Final Report for USGS, Aeroj,et General Corp.,El Monte, California, 1971.

[61 Kunzi, K. F., A. D. Fisher, D. H. Staelin and J. W. Waters, "Snow andIce Surfaces Measured by the Nimbus-5 Microwave Spectrometer,"Journal of Geophysical Research, v. 81, pp. 4965-4980, 1976.

[71 Aerojet General Corporation, "Operations and Maintenance Manual for X-bandMicrowave Radiometer," prepared for NASA Langley Research Center.

[81 Sperry Microwave Electronics, Clearwater, Fla., "Technical Manual--94 GHz Radiometer," prepared for Air Force Avionics Laboratory,Air Force Systems Command U.S.A.F.,Wright Patterson AFB.

[9] Linlor, W. I., F. D. Clapp, M. F. Meier and J. L. Smith, "Snow WetnessMeasurements for Melt Forecasting," NASA Special Publication SP391,in Operational Applization of Satellite Snowcover Observations,A. Rango, Ed., Proc. Workshop, Waystation, South Lake Tahoe, Ca.,August 18-20, 1975•

[10] Leaf, C. F., "Free Water Content of Snowpack in Subalpine Areas,"Western Snow Conf. Proc., 1966.

[111 Marshall, J. S. and W. Hitchfeld, "Interpretation of the FluctuatingEcho from Randomly Distributed Scatterers, Part l," Canadian Journalof Physics, v. 31, n. 6, pp. 962-994, September, 1953•

[12] Swerling, P., "Probability of Detection of Fluctuating Targets," IRETransactions, v. IT-6, pp. 269-308, April, 1960.

i

[131 Rice, S. 0., "Mathematical Analysis of Random Noise," Bell S stemTechnical Journal, v. 22, pp. 282-332, 1944; v. 23, pp. -156,

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1151 Bush, T. F. and F. T. Ulaby, "Fading Characteristics of Panchromatic RadarBackscatter from Selected Agricultural. Targets," IEEE Transactionson Geoscience Electronics, v. GE- 13, n. 4, pp. 149-157, October, 1975.

[161 Moore, R. K., W. P. Waite and J. W. Rouse, Jr., "Panchromatic andPolypanchromatic Radar," Proc. IEEE, V. 57, pp. 590-593, 1969 •

[171 Waite, W. P., "Broad —Spectrum Electromagnetic Backscatter," CRESTechnical Report 133-17, University of Kansas Center for Research, Inc.,Lawrence, Kansas, 1970.

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