conceptual performance of a satellite borne wide swath sar

IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. GE-19, NO. 2, APRIL 1981

tic atmospheric models with aerosols and common absorbinggases," Solar Energy, vol. 21, pp. 361-369, 1978.

[24] P. C. Shields, Elementary Linear Algebra. New York: Worth,393 pp., 1973.

[25] K. L. Coulson, "Effects of reflection properties of natural sur-faces in aerial reconnaissance," Appl. Opt. vol. 5, pp. 905-917,1966.

[26] D. D. Egbert and F. T. Ulaby, "Effect of angles on reflectivity,"

Photogrammetric Eng., vol. 29, pp. 556-564, 1972.[27] J. A. Smith, "MRS literature survey of bi-directional reflectance,"

prepared for NASA/Goddard Space Flight Center, Greenbelt, MDby ORI Inc., 1979.

[281 K. T. Kriebel, "Reflection properties of vegetated surfaces: Tablesof measured spectral biconical reflectance factors," UniversitaetMuenchen-Meteorologisches Instit., Wissenschaftliche Mitteilung.no. 29, 1977.

Conceptual Performance of a Satellite Borne,Wide Swath Synthetic Aperture Radar

KJYO TOMIYASU, FELLOW, IEEE

Abstract-A satellite borne synthetic aperture radar can image a wideswath in the order of 700 km with one-look 100-m resolution. If thedesign meets the ambiguity constraints at the far edge of the swath, themaximum swath width is independent of both radar wavelength andshape of the physical antenna aperture. The antenna pattern can be apencil beam scanned in the elevation plane, or a fan beam formed bya long antenna. The scanning pencil beam antenna may be a phasedarray or multiple-feed reflector which may be more practical than along antenna to image a wide swath. Design performance trade com-putations are presented involving resolution, swath width, antenna area,average transmitter power and digital data rate.

I. INTRODUCTIONA SYNTHETIC aperture radar (SAR) which comprises a

14&1 pulsed transmitter, an antenna, and a phase coherentreceiver produces a two-dimensional image of a collapsedthree-dimensional scene [1]-[3]. The SAR is borne by anaircraft or satellite, and the antenna is oriented typically atright angle (broadside) to the velocity vector. The image isin the plane defined by the radar platform velocity and radarslant range vectors.A synthetic aperture radar was successfully flown on the

SEASAT satellite [4]. The antenna was 2.2 m high in theelevation plane and 10.7 m long in the azimuth plane. Theradar frequency was 1275 MHz and the beamwidth wasabout 60 in the elevation plane and 10 in the azimuth plane.

Manuscript received April 2, 1980; revised December 12, 1980. Thiswork was supported by the National Aeronautics and Space Adminis-tration under Contract NAS-1-15657.The author is with General Electric, Valley Forge Space Center,

Philadelphia, PA 19101.

The swath width was 100 km. For some applications such asgeologic mapping, which is relatively static, a 100-km swathwidth is adequate. Other scenes which are dynamic, such assea ice detection, ocean oil spill detection, hydrology and soilmoisture determination, a swath much wider than 100 km isdesired to shorten the revisit period with complete globalcoverage from a satellite. A limited area, coarse resolution,short revisit period SAR has been configured for a nutatinggeosynchronous satellite platform [5].To obtain a swath wider than 100 km, for example 300 km,

the SAR antenna used on SEASAT would have to be narrowerin height, such as 0.7 m, and longer in length, such as 32 m,with resulting tradeoffs in azimuth resolution and/or the num-ber of independent azimuth looks. Another approach toobtain a wide swath was proposed by Moore and first reportedby Claassen [6] in 1975, and utilized an antenna beam that isscanned crosswise to the satellite ground track. (See Fig. 1.)Subsequent work has been reported by workers at the Univer-sity of Kansas [7]-[10]. Multiple antenna beams to achievewide swath coverage have also been considered by Cutrona[11], [12].The scanned beam SAR configuration described in this paper

is similar to that analyzed by Claassen [61 except that a spher-ical earth geometry is used here as was used by Lin [8]. Thediscussion assumes a satellite platform for the SAR, a broad-side antenna beam and coarse azimuth resolution imagery. Acoarse azimuth resolution requires a short length of syntheticaperture, and hence, a short integration time. The permissiblenumber of scan beam positions is given by the ratio of thetime period available for scanning across the swath to the shortintegration time period necessary to form the synthetic array

0196-2892/81/0400-0108$00.75 © 1981 IEEE

108

TOMIYASU: SATELLITE BORNE, WIDE SWATH SAR

SARANTENNA

Fig. 1. Antenna beam footprints of elevation-plane scanned beam syn-thetic aperture radar.

length [13]. The amount of scan angle in the elevation planeto image the swath is the product of the number of permissiblebeam positions and the elevation beam width of the antenna.A wide swath favors a small antenna area, however, the areamust be sufficiently large to satisfy the ambiguity constraintsat maximum slant range. Finally, the antenna area exerts adominant influence on the signal-to-noise ratio (SNR) atmaximum range. This paper describes conceptual performancetrades in terms of swath width, resolution, antenna area,average power and data rate. The calculations are approximateand are intended for configurational purposes.In a paper by Claassen and Eckerman [14], the SAR imaged

a wide swath with constant incidence angle. This entailedtransmitting a single, wide, curved fan beam, and receivingsimultaneous, multiple, contiguous pencil beams whose com-posite pattern matched the transmitted fan beam. The radarcould map on either side of satellite ground track. A variationof this method would be to perform the transmit and receivefunctions with one pencil beam that would be scanned acrossthe same arc sector. This type of conically scanned pencilbeam SAR is not discussed in this paper.

II. WIDE SWATH COVERAGEWith a conventional broadside SAR, the ground swath

coverage using a narrow beam antenna is governed by the slantrange, incidence angle, radar wavelength, and the effectiveaperture dimension in the elevation plane. The other antennadimension in the azimuth plane must be equal to or exceedthat value required to satisfy the minimum antenna area Aambcriterion dictated by the range and azimuth ambiguity con-straints derived from the antenna pattern, pulse repetition fre-quency and signal processing characteristic [15] . An equationfor Aamb is given by Harger [2, p. 29, eq. (2.12)]

8vscXR tan biAamb = C(1)

where vc,cis spacecraft velocity, X is radar wavelength, R isslant range, c is light velocity and qi is the incidence angle.

The equation becomes invalid for wide elevation-plane beam-widths and large incidence angles on a spherical earth.With the conventional broadside SAR the synthetic aperture

length LSA is equal to the distance of satellite travel when thescene area is illuminated by the antenna beam of azimuthalangular width X/LA. The distance LSA = TB vSC and the sceneillumination time TB is

XRTB =

LA Vgt(2)

where vgt is ground track velocity. If the full length of thesynthetic aperture LSA is processed to form a single focusedimage, the intrinsic azimuth resolution is LA /2. This is calleda "one-look" image. With a modest antenna length LA, theintrinsic one-look resolution of LA /2 may exceed the require-ment for some coarse resolution applications such as forimaging sea ice [16]-[18] and geological features [19]. Forcoarse resolution the synthetic aperture length required forimaging is less than LSA, and therefore time becomes availableto perform other functions. In one option, the total LSA canbe divided into NL lengths with each shorter length corre-sponding to that value required for the coarse resolution. Inthis manner NL images can be derived from the initial LSAwith each image having an azimuth resolution of 8az = NLLAI2.These NL images can be added incoherently or superimposedto reduce "speckle" effects and improve the image inter-pretability in some cases [20]-[23]. The composite productis called a "multilook" image.

In another option, the available time resulting from coarseresolution imagery can be utilized instead to scan the antennabeam to other directions in the elevation plane in order toincrease the width of the swath to be imaged [6], [7]. Thisoption is called scanned beam SAR and its design performanceis considered in this paper. The objective of the paper is topresent a conceptual performance trade and not a hardwaredesign trade.With a scanned beam SAR the time period Tw available to

scan across the swath is given by the illumination time avail-

109


able from the beam geometry at the nearest slant range RN toprovide contiguous mapping so that

X RNTW =--LA Vgt

where vgt is ground track velocity. The antenna beam dwelltime Td required to form the synthetic aperture is

_NL X12Td -NLIa.zVsc!RF

Td is proportional to the slant range and it is computed foroperation at the farthest range RF to provide a conservativedesign. NL is the number of azimuth looks. The number ofpossible scanned beam positions NB is given by the ratioTWITd and

N vsc RN 6azlINLvgt RF LA /26az/INLLA/2

The ratio vsclvgt is slightly greater than unity, and RN/RF isslightly less than unity. In words, the potential number ofbeam positions is given approximately by the ratio of thequantity 6azINL to the intrinsic focused azimuth resolutionLA /2. For a given value of 6azINL the product of NBLA isfixed so that a hardware design trade is indicated betweenLA and NB. The antenna length LA = 26az1(NBNL), so thatLA assumes its longest value of 26az when NB = 1 andNL = 1.For SEASAT, NB= 1 and LA = 10.7 m, so that the radar

design with aiZINL = 25 m/4 is consistent with the foregoingdiscussion. The coherent phase history in SEASAT could havebeen used to process the image for a finer azimuth resolu-tion of 6azINL = 6.25 m (with the same 25-m ground rangeresolution).For the subject scanned beam SAR, it is assumed that the

beam is scanned in the elevation plane and that the narrowelevation beamwidth OE is constant with scan angle which isnot valid for a wide scan angle [8]. The available swath anglerange Osw to map the swath is approximately NBOE, so that

W-_az 2 XNL LA LE

Since the antenna area Aant = LALE,

- 6az 2XVAsw = N ==NBLAA . (3)

NL Aant Aan?t(3The swath angle Osw is inversely dependent on antenna area

Aant but is independent of the shape of the antenna aperture.The selection of particular values of the antenna dimensions isa hardware design trade involving the values of NB and LA.An example of a SAR with NB = 18 beam positions has beenreported [181, [24]. A wide swath favors a small area Aant,however, this is constrained by ambiguity considerations.Further, a small value of Aant will require greater transmitterpower to achieve a given SNR.Since the minimum antenna area criterion Aamb given by (1)

and computed for the far edge of the swath must always be

satisfied, the swath angle in the elevation plane can be re-written as

o az cambN RANsL 4 vscRF tan OiF Aant

=NBLA 8vcRF tan OiF Aant

The incidence angle is kiF at the far edge of the swath. Theangle 0sw is measured from the swath far edge towards nadir.Maximum O. occurs when Aant = Aamb. A coarse azimuthresolution with NL = 1 (one azimuth look) will permit wideswath coverage whereas NL > 1 will reduce the swath angle. Itis noted that when constrained by ambiguity (Aant = Aamb),Osw is independent of wavelength. The minimum usable valueof incidence angle may be dictated either by the scene to beimaged or by the degradation in range resolution. From thegeometry, the projected surface (or ground) range resolution5gr is governed by the radar slant range resolution bsr (com-mensurate with the radar bandwidth) through the equation6gr = 6sr cosecant Oi. An incidence angle near zero is notusable by any SAR [3].As for all sensors on spacecraft, the subject of reliability

requires consideration. A potential low reliability componentin the scanned beam SAR is the electrical network to scan thebeam rapidly. Electrical scanning of pencil beams have beenused in two operational satellites and will be used in anotheroperational satellite in the near future. In the operationalNIMBUS-5 satellite launched in 1972, a 19.35-GHz pencilbeam for a radiometer is electrically scanned cross trackthrough nadir [25]. In the operational NIMBUS-6 satellitelaunched in 1976, a 250-MHz bandwidth, 37-GHz pencil beamfor a radiometer is conically scanned from left to right andforward of the spacecraft to enable measurements on theearth's surface at a constant incidence angle [26]. For theDefense Satellite Communication System-IlI (DSCS-III), soonto be launched for operational use, there are electrically con-trolled beam forming networks for three multiple beam lensantennas to provide selectable and commandable coverage ofthe earth from a geostationary satellite [27]. The downlinkbeams radiate 40 watts of power at X-band. These hardwareexamples are given to illustrate the degree of operationalmaturity of electrically scanned antenna beams.

III. TRANSMITTER POWER

A major concern of a SAR in a satellite is the amount of dcelectrical power since this has a direct relationship on the sizeof the solar panels. The power for the SAR can be dividedsimply into two parts, i.e., that required by the transmitterand that for the remainder of the SAR. With different designs,the average transmitter power may vary widely but the re-mainder is relatively fixed. Thus, the average transmitterpower is of major concern for a satellite borne SAR.The average transmitter power of a SAR depends on many

parameters. The average power is independent of antennashape but decreases rapidly by increasing At and decreasingRF. Large values of Aant and small values of RE are counterto wide swath coverage so that a performance trade is indi-cated. In order to maintain a given value of ground image

110

(4)


resolution 5gr across the swath, 6sr must be varied in accor-dance to 6sr = 5gr sin ji, where ki is the incidence angle. Thereis a hardware design limit on 6sr and an operational limit on4i which is scene dependent.

If the expression for average transmitter power required forclutter scene imaging is combined with the ambiguity antennaarea expression given in (1), then

rrC2kTSLS RF (Aab2Pave = SNR 8772 jsrU tan2 iF Aantm)

where

SNRk

Ls

RF

vscx

NL6sr0

PiF

S/N at beam centeris Boltzmann's constantis system noise temperatureis system loss factor, greater than unityis slant range to far edge of swathis antenna efficiencyis spacecraft velocityis radar wavelengthis number of looksis slant range resolutionis normalized radar cross section at incidence angle Nbis incidence angle at far edge of swath.

If the design is ambiguity limited (Aant = Aa.mb) in a givengeometry, the average power decreases with increasing radarwavelength.The transmitter peak power Ppk is related to the transmitter

average power Pave and the transmit duty cycle DT by PVae =

PpkDT. The duty cycle can be computed by considering 1)the pulse transit time across the antenna beam footprint, 2) a

design margin to minimize the range ambiguous response

[15], and 3) a time margin TM during the interpulse period(IPP). The transmit duty cycle DT is the ratio of transmitpulse duration rT to the IPP which is the reciprocal of thepulse repetition frequency (PRF)

DT = TT/IPP = TTPRF.

The sum of the transmit pulse duration TT and the receivepulse duration TR is made almost equal to IPP; some timemargin TM is allowed to permit timing variations. Thus

TT + TR + TM = IPP.

The receive pulse duration is longer than TT by the amount oftime TFP required for the pulse to undergo a round triptraverse across the beam footprint; thus

TR = TT + TFP

where

2ROE tanTFP =

If the ambiguity constraint (1) is combined with the expres-

sion for TFP,

AambTFP -4vSC LE

A typical value for the pulse repetition frequency, and hencethe interpulse period, is required to complete the computa-tion. A very critical parameter in a SAR is the pulse repetitionfrequency (PRF). To achieve high quality imagery, the PRFis typically about 30 percent higher than the minimum valuerequired that meets the azimuth ambiguity constraint. Theminimum value is given by vSCI(LA /2) which in words statesthat the satellite platform travel distance cannot exceed one-half the physical antenna length between successive pulses.The typical PRF is nominally about 2.6 vsc/LA. Usually thereare several pulses in transit along the round trip slant range atany instant of time.The transmit duty cycle DT in terms ofAamb is given

2__6_Aamb

DT = 05(1 TM) 2.6(A /1 8 A~~ant

If TM/IPP is assumed to be 0.1, then

DT = 0.45 - 0.325 amb)Aant

The largest value for DT is 0.45 and this occurs when theswath is very narrow, i.e., Aant >> Aamb. For a wide swath,Aant - Aamb and DT = 0.125 so that relatively high peakpowers become necessary.A given signal phase history obtained over a satellite travel

distance of LSA if processed as an NL-look image will resultin an image with a degraded focused azimuth resolution ofNLLA/2, a smaller amount of speckle, and a narrower swathangle by a factor ofNL when compared to those of a one-lookimage. If the radar bandwidth is adjusted so that the groundrange resolution matches the processed azimuth resolution,the NL -look image will have a degraded resolution of NL6 inboth coordinates, and for Pave fixed, a higher SNR by a factorof NL NL . Besides summing multiple images differingslightly in azimuth angle (multi-azimuth look imaging), it ispossible to sum multiple images formed by multiple radarfrequency bandwidths; this is called multi-range look imaging[20] and for this type of radar the transmitter power mustbe increased by the multiple factor.

IV. DATA RATE

A synthetic aperture radar can produce data at a high rate.Data from the SAR has to be either sent real-time over a com-munication link to a ground station, or recorded on-board forlater transmittal to ground. With digital data rates in excessof about 15 Mbit/s, the rate can become a problem. Since anon-board type recorder was not available to SEASAT the rawanalog radar data was sent directly to a ground station whichlimited SAR operation to only those regions accessible fromthe ground station. On ground, the data was digitized and therate was 107 Mbit/s. In general, a wide swath requires a highdata rate so that the subject of data rate becomes of interest.The return signal received by the radar is usually recorded

first and processed later to produce images of scenes. Be-cause the received signal pulse length is a fraction of the inter-pulse period, it is assumed that signal buffering is employedto stretch out in time the data stream until it almost fills theinterpulse period. This technique will reduce the data rate

11 1


to a minimum. The digital data rate is given by the productof three quantities, namely, 1) number of range bins per

pulse, 2) the number of bits per range bin, and 3) the pulserepetition frequency.The number of range bins is given by the ratio of the slant

range extent of the antenna beam footprint to the slant range

resolution, 6sr. Since the phase of the received signal isrequired in addition to its amplitude, both the in-phase (I) andquadrature (Q) phase components are recorded. When theanalog received signal is digitized, a sufficient number of bits(BIQ) is required for each range bin to provide adequate ac-

curacy or fidelity. The nominal PRF is governed by ambiguityconsiderations and may vary across a wide swath. The precisePRF must be such as to prevent eclipsing the receive pulsesby the transmit pulses.The digital data rate DR is given approximately by

2_6 c (Aamb\DR2_BIQ 8- ( am)

6sr A ant

It is noted that the data rate increases with finer slant range

resolution, is independent of the shape of the antenna aper-

ture, but decreases if the area Aant =LALE is increased. Ifthe design is ambiguity limited, the swath is widest and thedata rate assumes its highest value. The value of 6sr is deter-mined by the ground resolution 6g7 and the incidence angleat the near edge of the swath. If a wide swath is mapped bya single fan beam, the SAR would provide too fine a groundrange resolution at the far edge of the swath just to meet theground range resolution at the near edge. The data rate can

be minimized by using a scanned beam SAR with variabletransmit-pulse RF bandwidth and hence variable 8sr across

the swath. For a given antenna area and geometry, it shouldbe stated that AamblAant decreases at a faster rate than does5sr towards nadir. In a 100-m resolution, 290-km swathcoverage synthetic aperture radar design [18] three RF band-widths were utilized to keep the digital data rate to below10 Mbit/s.

V. SIGNAL PROCESSINGThe principles of processing SAR signals to produce two-

dimensional images have been discussed in the literature [1],[2]. Optical techniques have been widely used, and recentlydigital techniques have been reported [28], [29]. Digitalprocessing of SEASAT SAR data using general purpose com-

puters has been accomplished by Jet Propulsion Laboratory[30], MacDonald Dettwiler and Assoc., Ltd. [31], [32], andprobably others. In the proposed scanned beam radar, theantenna beam at any instant of time illuminates a fraction ofthe swath width. The beam dwell duration in this position issuch as to generate the required synthetic aperture length. Inthis time period a sufficient number of radar signals are trans-mitted and received to produce an image of this illuminatedarea called a "patch." All of the patches across the swathare combined after processing to form an image of the fullswath width. After traversing the swath, the antenna beamreturns and an image is formed of the next in-track patchadjacent to the first patch. (See Fig. 1.) All of these cross-

track and in-track patches are combined to produce an image

FAR EDGE IS AMBIGUITY LIMITED100 INCIDENCE ANGLE AT NEAR EDGE

400

SWATH WIDTH, KM

200 _400 500 600 700 800 900

FAR EDGE GROUND RANGE, KM

Fig. 2. Ambiguity limited swath width as a function of range to the faredge of the swath. The one-look image azimuth resolution is 30 m.The swath width is independent of SAR wavelength.

MINIMUM ANTENNA AREA, M2

400 500 600 700 800 900

FAR EDGE GROUND RANGE, KM

Fig. 3. Minimum antenna area for ambiguity limited operation as afunction of range to the far edge of the swath. The SAR frequency is9375 MHz and the azimuth resolution is 30 m.

of the scene [18]. The transmitted radar signal and receivedsignal processing and timing have to be sufficiently accurateto permit joining adjacent patches to within a fraction of theresolution dimension. Resampling of the data may be re-

quired to achieve patch alignment. A digital buffer memoryis required to store all of the image data of the patches for afull swath and to read out the data to produce a single imageof the full swath scene. This is, in effect, a digital mosaicingproblem, and is not considered further here.

VI. PERFORMANCE CALCULATIONSThe preceding performance design equations were applied

for specific examples to compute various geometrical andradar parameters. The principal trade involves far edge ground(access) range, swath width, antenna area, azimuth resolutionand number of azimuth looks as given in (4). Because of per-formance degradation, a minimum value of incidence angle is

I

112


100

10

,

w

z

0

z

zz

uiI-

z<

1.0

0.1

NUMBER OF ELEVATION BEAMS

Fig. 4. Antenna dimension options for 358-km swath from 1100-kmorbit altitude.

imposed. A value of 10° has been arbitrarily set althoughthere are instances when this value should be larger. Thus, toachieve wide swaths, it is implied that the far edge of theswath should have a large ground range and a large incidenceangle, and it is at this edge where it is most difficult to satisfythe minimum antenna area ambiguity criterion. Again, be-cause of performance degradation, a maximum value of inci-dence angle has been imposed. The degradation may appearas low SNR for the particular scene, or ambiguous response.An upper limit of 450 has been arbitrarily set, although thereare instances when this value should be smaller.For an assumed swath far edge incidence angle, the spherical

earth access ground range and the minimum antenna area arecomputed. All computations were made on a card-program-mable TI-59 calculator. The swath angle is then calculatedusing (4) by assuming values for _az and NL. The swath angleis transformed into ground coordinates, and the swath widthis determined. Examples of computations of these parameterswere made and are presented in graphic form to indicate theperformance of a coarse resolution, wide swath syntheticaperture radar. The computations involved two values ofazimuth resolution, viz., 30 and 100 m, and three satellitealtitudes of 700, 900 and 1100 km.

VII. COMPUTATIONS FOR 30-METER RESOLUTIONThe available swath widths under ambiguity limited opera-

tion were computed for one-look imaging and 30-m resolu-

tion. The ground ranges from the subsatellite point to the faredge of the swath were computed for several incidence anglesand three satellite altitudes. (See Fig. 2.) From an altitude of1100 km, the incidence angle is 450 when the ground rangeis 880 km from the subsatellite point. The minimum antennaarea required to meet both the range and azimuth ambiguityconstraints for this geometry is 9.07 M2. (See Fig. 3.) Anambiguity limited swath width of 358 km is attainable andthis is shown in Fig. 2. For this geometry, the effect of thenumber of scan beams in the elevation plane on the apertureshape is illustrated in Fig. 4. With a single beam configura-tion, the antenna dimensions are 0.166 m high and 54.6 m longto meet the 9.07-m2 minimum aperture area. With an 18scanned beam configuration, the antenna shape is essentiallysquare which is regarded to be far more practical for a satelliteborne synthetic aperture radar.With an incidence angle greater than 450, the ground range

can be increased further, but then the antenna area must beincreased to avoid ambiguity and the swath width decreases.On the other hand, if the incidence angle and ground range areboth decreased, the minimum antenna size decreases and theambiguity limited swath width increases. An arbitrary mini-mum incidence angle of 100 is selected at the near edge of theswath because of excessive degradation in ground rangeresolution for a given slant range resolution. With this mini-mum incidence angle limit, a maximum swath width of 453.5km is obtained from the 1100-km altitude when the far edge

113


incidence angle is 34.250 and the ground range is 619 km.The antenna area for this geometry is 5.46 m2. If a swathwidth narrower than 453.5 km and a ground range closer than619 km are acceptable, then the antenna area can be madelarger than 5.46 m2 and the average power and data rate can

be both decreased. From Fig. 2 it can be seen that the maxi-mum swath width decreases with decreasing satellite altitude.The beam dwell or integration time Td was computed for one

example. A value of 0.106 s was obtained for a one-look 30-mresolution image at the far edge of the swath where the inci-dence angle was 450, the surface ground range was 880 km,and the slant range was 1455 km from a 1100-km altitudeorbit.The average power and data rate were computed for several

incidence angles and the three satellite altitudes. The follow-ing assumptions were made:

S/N = 9 dB at beam center, 3 dB at beam edgef= 9375 MHz

NL = 1 look

6az = bg, =30 mTs = 728 K

Ls =6 dBa0= 0.005 cot2 PiF

6sr = 6gr sin /i71 = 55%

v = \/3960l.2I(H+Re), km/sH = satellite altitude, kmRe = earth radius, 6378 km.

The average powers for ambiguity limited operation, one-

look imaging and 30-m resolution are

ra

faar

in

thsc

da

haW

thin

ANTENNA

AREA, M2

300 400 500 600 700SWATH WIDTH, KM

Fig. 5. Maximum antenna area as a function of swath width with 100near edge incidence angle, 100-m azimuth resolution, one-look image,and 9375-MHz SAR frequency. The average transmitter power anddata rate are minimized.

Satellite Altitude area, edge angle

to 100. The use of a larger antenna area will advantageously700 km 900 km 1100 km decrease both average power and data rate. The largest antenna

areas were calculated which provided specified swath widths400

path 1423 W 18403W 2263 Wsatisfied

edge incidence angle limit

save swath widths of 300, 500 and 700 km were selected for each¢450 swath 326 km 344 km 358 km of the three satellite altitudes of 700, 900 and 1100km. The

Pave 1383 W 1784 W 2188 W radar frequency was 9375 MHz. The largest antenna areas asa function of swath width are plotted in Fig. 5. From the

The ambiguity-limited data rate depends only on the slant 1100-km altitude, the 700-km swath has a far edge incidenceLnge resolution, and it is 46.7 and 42.4 Mbit/s for 40 and 450 angle of 44.50 and 13.03 m2 antenna is required. The farir edge incidence angles respectively. It is assumed that 8 bits edge incidence angles are also given in the figure for othere required to record the amplitude and phase of the signal swath widths and satellite altitudes.Leach range resolution cell. If the satellite is in a 700-km altitude orbit, a 700-km swathIf the number of azimuth looks is increased to NL = 4, then cannot be imaged since the ambiguity constraint is violated[e average power values can be reduced to one-half, the beam at the far edge of the swath. The maximum swath widthan angle range would be reduced to one-quarter, and the which can be imaged from this altitude is 691.5 km, and theita rate would remain unchanged. far edge incidence angle is 54.050. If the far edge incidence

angle is limited to 450 due to operational reasons, a 500-kmVIII. COMPUTATIONS FOR 100-in RESOLUTION swath can be imaged from the 700-km altitude. The required

Computations were performed for one-look imaging and antenna area for this swath is 11.86 m2.)O-m azimuth resolution. Images with 100-m resolution may With the antenna area optimized for each combination ofwve utility in mapping sea ice [18] and lineaments [19]. swath and altitude the average powers were computed, and'ith ambiguity limited operation at the far edge of the swath, a wide variation was found. As before, it is assumed thatie computed swath widths were so wide that the near edge ao = 0.005 cot2 biF. The average powers were calculated forcidence angle became less than 100. By increasing the detection at the far edge of the swath. The computational

'I26 f2.9 \

24

22 29.60

20

1100 KM ALT.18 33.60

16

900 40.10 * iF1414 ~~~~~700KMX|\

12 4<4

10 7 0

AMBIGUITYLIMIT

114


100-M RESOLUTION

ONE LOOK

9375 MHZ

ao0= .005 COT2 0, r

AVERAGE PO'WATT

SWATH WIDTH, KM

Fig. 6. Average transmitter power with optimum antenna area as a

function of swath width with 100 near edge incidence angle.

results are shown in Fig. 6. For a 300-km swath the average

power of about 18.5 watts is almost independent of satellitealtitude between 700 and 1100 km. With wider swaths, theaverage power decreases with increasing altitudes.The digital data rates were computed for particular swaths

and altitudes assuming the use of the optimum antenna area.

It was assumed that 8 bits are required to record the amplitudeand phase of the signal in each range resolution cell. For a

700-km swath the digital data rate is in the vicinity of 10Mbit/s.

IX. CONCLUSIONSThe swath coverage capability of a synthetic aperture radar

is proportional to its azimuth resolution and inversely pro-

portional to the number of azimuthal looks and its antennaarea. The widest swath is obtained when the geometry isambiguity limited. In instances where the maximum obtain-able swath exceeds requirement, increasing the antenna area

will reduce the transmitter power and digital data rate.Performance trade computations suitable for configurational

purposes are presented involving resolution, antenna area,

swath width, average transmitter power and digital data rate.The antenna aperture shape controls the number of beampositions in the elevation plane to cover the swath, and this isa hardware design trade factor.The processing of radar data to produce an image of a wide

swath is probably best achieved by employing digital tech-niques. Images of subswath scenes illuminated by the antennabeam in each scan position are combined to form an image

of the full swath. An alignment accuracy of a fraction of theresolution dimension is required to combine the subswathimages.

REFERENCES

[11 L. J. Cutrona, "Synthetic aperture radar," in Radar Handbook,M. I. Skolnik, Ed. New York: McGraw-Hill, 1970, ch. 23, pp.23.1-23.25.

[2] R. 0. Harger, Synthetic Aperture Radar Systems, Theory andDesign. New York: Academic Press, 1970.

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