introduction • radiometric calibration • geometric
TRANSCRIPT
Dr. Jacques Louet
ESA
The Netherlands
CALIBRATION
PRACTICALEXPERIENCEWITH ERS-1
/_ _ _/
N94-15907
• Introduction
• Radiometric Calibration
• Geometric Calibration
° Phase Calibration
• Polarimetric Calibration
PI_IEOEDtNG PAGE BLANK NOT F)LMED 299
Introduction
Basic SAR measurement parameters are :
• Radar backscattering
• Target position
• Target speed
• Polarisation
SAR calibration facilitates quantativemeasurements needed to derive geophysi-cal parameters of the area under observa-tion from basic SAR measurements (e.g.soil moisture, biomass,ocean wave en-ergy,ocean currents, ice type, ice flow, ..... )
Inversion Model
Sensor
Calibration
Direct Model
Geophysical Parameter
Data
Calibration
Geophysical
Calibration©
GeophysicalParameterEstimate
-9m
Geophysical
Validation
__ 30_.
Radiometric Calibration
Radiometric calibration is relating the SARoutput in terms of digital number to _o.
Design for stability
Sensor calibration
pre-flight characterisation
internal calibration
external calibration
• Data calibration
302-
Characterisation parameters for ERS-1
Transmitter Parameters
Frequency
Pulse power
Pulse duration
Chirp bandwidth
Antenna Parameters
Relative azimuth pattern
Absolute elevation pattern
Receiver Parameters
Sensitivity
Bandwidth
Power transf er function
Calibration Parameters
Internal calibration stability
3o_
Instantaneous Amplitude
Center Fre(_.. nc_.
S300M_z" _
l Amplitude
Corresponding toPeak RF Power
v
Time
Instantaneous FrequencyII linear Up-Chirp
__,, ', lB= 15.5 MHz
I .1I
37.12 ps I
of Spectrum
B=IS.SMHz
I!I
i5300MHz
Center Frequency
v
Frequency
_3o4-
z
z:
.o-i
IIIllllll
\.\
illllllll
0
i
IIlJllllll liliiilil
lllllllll
0 0IIIIII!1
IIIii111
IIIIIIII1_1
0
-10
-20
dB -3O _
-40
-50
-60-3
A
I I I L
-2 -1 0 11 I
2 3degree
50_
dB
4O
30
2O
-10
-10 _
-20-15 -10
\
I I I
-5 0 5
3O6
I I
10 15degree
.EXAMPLE OF 2-BIT CODE
IIm
I0m
-V;n0
+v ;n
EXAMPLE OF 5-BIT CODE
--V ;.
i £TC
" ;l|OI!
r,mJ iO|lO
01111 F
011_ 'w0110|
OllO0
FTC0
+V ;n
307
d -10B
-15DC
S
a
t- -20
-15 -I0dB tel ADC sat
-S
0.6
dB
r
e
l
ADC
0.4
0.2
-0.4
\
\
-30 -25
308
-20 -15 -i0-dB rel ADC sat
-5
Internal Calibration
The objective of internal calibration is moni-toring system parameter changes withtime, such as
Output, power
Receiver grain
Linear distortion (amplitude & phase)
Non-linear distortion ( amplitude &phase)
Noise level
Level of spurious signals
Internal calibration methods include
Calibration using transmit pulse sample(ERS-1 method)
Reference signals : noise, cw
Transmit power measurements
309
aft
antennas
HPA
5177 MHs
5177 MI_
tor
123 MHs
r
310
E
|
External Calibration
The objective of external calibration is to
measure and monitor system componentsoutside the internal calibration loop.
External calibration uses made-made ornatural targets.
External calibration can be used to deter-mine the radar image calibration factoreither in combination or without referenceto internal calibration.
-2
AMI Transponder Specifications
Parameter Wind Mode Image/Wave
Radar Cross Section _ 87.5dBm 2 65.0dBm 2
Adjustment Range +0,-5dB +0,-5dB
Calibration Accuracy +.5dB +.5dB
Cross-calibration +.2dB +.2dB
Accuracy
Stabilityt'Over 3 Years) '2_.ldB +.ldB
312
1
z_W_ o_J_ _ o
LI Z
CD
F
I
ID E NJ i
t@(D .._)._Ir_ b.U)
..N_I .._
_ _W._.3h-
>_
d6131 _13814WI_
D _ .._Om Z_.-_ _-_3 __ t_ •
315
• !
i
+ + +
t_W
r_WI--
W
>,&.
o
ml
0
ml
OdX
ERS-I Transponder Antennas
Gain Values for 12 Antennas
27.9
27.8
27.7
_27.6
v27.$¢-
"-- 27._
27.3-
27.2
2T.i_
i , t" ! . i "1
_, ! i ! ! If I i , i I! _ l t .,4,,. ,! I........_. ...._ _ I_i\ _ i_i---+_l _ _',,f- -,_1 :!It.
_T--BI
it!
!i
1
fTtA
i! !aT! iI t
2 3 4
I tI it I
i- !,! t_i |t tl i
t i!
; !i
IItt!!
1 1- t t
t t!
lll
iS i 7
" t8,
l!!!t
!
!
i
I .t
t
. !t !t! t
il 9 I0 11 12
AH_mllmmrs II serial rmlxrs 5(_ to 576
Average G_ = 27.81 dB
Standard Deviation = 0.06 dB
316
Transponder Error Model
antennaerrors
electronicgainerror
incoherent _- /
_coherent _ multipath _k /I/t/'y I / / clutter'
multipath __
317
0 I ! { _ '_ I /_ I I I I i
/-10
a5
.o..
Q. -40 n_ .-_.lO
-so ,..;i_': , , , ,""_ , _,' ' '-t80 -l. I_ /
-120 -60 O 60 120 t8# -[. 0o
ANGLE (Oeg)
Sideiobe structure in H-plane pattern (plane of offset).
-0.20
_04o
_,NGLE (Oeg)
I I t
I.io 1
Pointing evaluation E-plane, based on LMS curve fll
-tO
_B
-20
-30
-40
- I - ;
p:O*
/'/-p/,,,,le
-( o . c l_oss
" I I --
-180 -120 -60 O 60 IZO 180
Degrees
E and H plane patterns, co and cross-polarisation.
! I
l I
120 180
318 •
! ' I !
L
if-
g
1
//!
!
u n-
(1
f, 0. Iuo 2 \
0
i ! r-rr Z
It 0 M
M o.NZ X
_). >. i _ ° <
w /u
I,,I
"_--< -'_ (1. I
=" 0
j _
I
Z _
_U
o
-4,
z__Z
om
o
mn
, Un-,<
i
mo>ri-
m
n_r_Z Io
t-I
00. I
rr_JI-
n_r
! o--- u
o
7
u
E
,Z
®a:
U I<OE
LM
zmI-
320
_rr4c
a
0.03
0.0_
Q01
<
0<
.J
-O01
i -30
i°
!-Oealm_ i :
•(102 ..........................:.....................i....i
i(6_I)(nm i
"0.03 _ i _ , , , , ; ' " •
-200 -100 0 100 200
o
E]_S1;Sk9kl_'2MIJLTIPA'IHESlllkTERFi%T¢571,R1572
-2.4 -.,. .....................
,%
\ _-26 .... '' 0 o ': ...... ' _ •
: o"_. O !! '-
O "%:
................. _'-L.... i.............o
'%i ! "%. :
_::"3.4 ......... i.............. '". ..... o ..
: 0
_dlmm : i _,.- Um_nmFm_m : o: _-.. C_ _m ml=,',,_ ,,
i ! Ii5_ cm
• i . . . ; • • . , • • • L . • •-$8 , , . i .-300 -2O0 -tO0 0 100 2OO
MB_'_ Ik_Ebll
3OO
3OO
321
Transponder Calibration Accuracy
Antenna Gain Error
SAR SCATT
full
range
fun
range
Pointing Error
Electronic Gain Error
Antenna Coupling Error
t Transponder StabilityA+B+C
F
G
l I[I
Transponder Stability
1 a = (A+B+C)/3Clutter & Noise
against a ° = 0 background
Coherent Multipath
Incoherent Multipath
Single Transponder Observation Error
v/D 2 + E 2 + F 2 + G 2
Atmospheric Loss Uncertainty
Combined Error -
v/D 2 + E 2 + F 2 + G 2 + H 2
I Transponder Calibration Accuracy
0.028
0.53
0.05
0.002
.O8
1 dr
0.027
0.0495
0.032
0.56
0.056
0.088
I dr
0.029
0.0217
0.0207 0,0388
0.0410 0.0767
0.07
0.07
0.093
0.07
0.1 I 0.1170.2 0.2
dB
deg
dB IdR
j °t
iB
dB I
dR I
I
322
Representative SAR Calibration Budgets
SAR Radiometric Accuracy
A Absolute Calibration Error
B Cross Swath Calibration Error
C Radiometric Stability Drift
I ]Nominal Calibration Accuracy
D Radiometric Stability Drift
Nominal- Observation
E Across Swath Characterisation Error
F Across Dynamic Range Characterisation
G Atmospheric Loss UncertaintyTotal Radiometric Error =
VIA 2 +' B2 + C 2 + v/D 2 + E 2 + F 2+ G _
(a) Dominate_ by t_der calibration
(b) Single sa_le exror,re_ced by in-flight monitoring
(c) Random e_or per
Absolute CzJibration Error
l_r
a 0.14 dB
b 0.08 dB
b 0.25 dB
I 0.3 IdBIc 0.25 dB
c 0.1 dB
c 0.1 dB
c 0.07 dB
0.6 dB
H
I
J
K
Transponder Calibration Accuracy
Antenna Gain Characterisation
Single Transponder Observation Error
Number of
Samples
3
3
3
Atmospheric Loss Uncertainty 3
Combined Error =
v/H2/3 +/2/3 + J2/3 + K2/3
Single Sample
Error(1 a)0.2
0.1
0.07
0.07
0.14
dB
dB
dB
dB
dB
.323
I
!
7tI!
!II
ii1
I
• iiJ
i2 ..........
I
I
SAR- SWATH VENICE ORBIT
ESA/ESTEC, ERS-I, 3-day repeat orbit (43), 24.365de_SAR
Figuze 7: Transponder Sites Flevoland, Commissioning Phase Covezase
- 324 -
X
I
\ Iq
I y
"ID
L,9_D
6O
5O
4O
3O
t"
2ol0
theoretical RCS of the ca ibrato_s......... i ......... j,,- .... ,,,,, ......... _ ........ ,7
3 ESA Transponders
f JPL CornerReflector
"- FEL-TNO
Corner
Reflector
I0 20 30
incidence angle 8 [degree]4O
1
5O
325
i
•Z# U._ONOdSNVI41 VS._
(_iusp ut) NOI.I.O_S SSOUO UVOIti# ._./.fi'/OSSI# a._l:inslt--Jl,#io _o (D(D u') iD ,
I I I I I I ! I 1 1 1 I I l 1 ! _ _ ,
cro _o_. p_mI ,_ _,e _,_
li. iri _ Ill -- Ill -- Qd I
E,,., o- -. o-I, I ..i n- I-.. O n,.
I m_ i,, "l Q- i o '-.. zi
_1 _ .-._..._. _-.
,=,,_ i ....-.", .I _" / / " """ " ' .I,,.,,I / / ._ .-" ,'" - '
/ / .I ,,.-" ," ./ t
/ / !..-" .." - ..",,=, / _ ., • ,./ # ,'"1 ," I
/ / f { , i¢
i I" II ' ' i _ ! I
''"''"''"" ksuoioa'i_au U_NUO00NJ_ t _dr
__ (r,ussP u!) NOIiO._S SSOUO 14VOifl4 ._JJ'l'lOSl_d O._UI'ISV'WJtll,, 326 - ,
/i
'T,
2
(/) t,.
o
(Jepuodsue.ri qoee Io) "S'07d "IVNlYiON _HJ. ONIIOb'V
(ZWBp ul) NOII O_S SSOL/O brvOV'd OgIdI'ISV'-JIN gILL .-I0 NOI.LVlb'VA
(VJ.VO X3"ldWO0) ONV'IOA.-TI:I NI SU._ONOdSNV'di VS._
Illom
>Cg
-I0rel--ZoOI-o
cl0re
I
rrilJ
Z
el"b9UJ
m
327
Calibration of other areas
The application of the calibration factor toother areas requires knowledge of geome-try (range & antenna angle) and stabilityover time.
High resolution information about the an-tenna diagram can be obtained from im-agery of extended uniform targets such asthe tropical forest or ice shelves.
No absolute information is required forthese 'targets of opportunity'. Stabilitv overtime and a smooth dependence of G" onincidence angle is necessary.
328-
_v
i
i
Data calibration
Data calibration include the following steps
Correct raw data for a number of systemparameters
Linear distortion (amplitude & phase)
Non-linear distortion
Detector imperfections (DC-biases,amplitude & phase imbalance)
time variations as measured by inter-nal calibration
noise bias
Application of calibration factor to theentire image, taking into account ge-ometry and antenna angle.
: 33f
0
I I I I l I l I l I l I I I _"
I i t
]11.1 Iill i i tttt 11_
!
!:
!_
i'-
_m
E
E
o
; o0
I I I I_li I I I I al ill lJJ[ 0")
, /,:.. I - ! I
.'lOi
(/)
._(2- 0
0F).E
oE3
{_1 E
(/_
T- "0r_T--
I! I I ! I ! II1: i I I I!I I II_lr,l_" 0 I_ 0
• _ ........... i_IdU.}o_ u_:_I,_, _o_cl
Geometric calibration
Geometric calibration can be achieved fol-
lowing the same principles as for radiomet-ric calibration.
Design for stability
Characterise SAR system in terms ofdelay, phase error and frequency offset
Calibrate using point target with accu-rately known position
Geometric image calibration is carried outin the data calibration stage by correctionfor kn#wn system biases and applying ge-omet_ information (platform position & atti'tude and terrain models).
I
33_
I J , , , I , , , , I
-#tm-_o
°_"
'i',,
I I I IQ 0 0 0
X X X
• . o
I I0 0
x X0 LC)
(',,#
334"
c,qo3
I>
J OZI
0
r
--3
I0
olo3
1r'v-<
I
c'q
I- >
0Z
I00
03o3
Iij
I
c'N
FE
Phase calibration
During the formation of an intensity imagethe phase information in the radar data islost. For applications in interferometry andpolarimetry complex images are formed
which provide two data points per pixel,amplitude & phase or real & imaginaryparts of the complex signal.
Careful and elaborate phase characterisa-tion of both the SAR sensor and the SAR
processor are required. Time stability iscritical for the above applications.
335
Polarimetric calibration
L
Polarimetric calibration can be treated es-sentiai|y as a multi-channei extension ofthe methods discussed above. PolarimetricSAR differ from single channel SAR in thefollowi .....ng ways. _
There are two orthogonal transmit chan-nels to COnsider and four receive chan,nets.
Ra_ometric calibration can be sepa-rated into absolute calibration and rela-tive between channel calibration
Geometric calibration can be Separatedinto absolute calibration and relative be-tween channel calibration
Phase calibration between channels isessential
Crosstalk has to be taken into account
c
L
336