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The ERS-1 radar altimeter mission
Acta Astronautica Vol. 14, pp. 287-295, 1986 Printed in Great Britain.
0094-5765/86$3.1)0+.00 (~) 1986PergamonJournalsLtd.
THE ERS-1 RADAR ALTIMDETER MISSION
C R Francis
European
Space
R e s e a r c h a n d T e o h n o l o g l ; C e n t r e (E3TEC) Noordwijlc, T h e N e t h e r l a n d s
e n a b l i n g w i n d s p e e d a n d d i r e c t i o n t o be d e t e r m / n e d o v e r a s w a t h 400 km wide.
ABSTRACT
A v i s i b l e a n d i n f r a - r e d i m a g e r , t h e Along Traclr S o a r m i n g R a d i o m e t e r (ATSR), o p t i m / s e d For m e a s u r e m e n t s o f s e a - s u r f a c e temperature. A nadir viewing t w o - f r e q u e n c y m i c r o w a v e r a d i o m e t e r is included, the full instrument being r e f e r r e d t o a s ATSR/M.
T h e R a d a r A l t i m e t e r t o be c a r r i e d o n E R S - I will p r o v i d e m s u r a s e n t s or altitude, significant waveheight and windspeed over t h e o c e a n s t o a l a t i t u d e o f 82". In a d d i t i o n , t h e i n s t r u m e n t h a s a d e d i c a t e d Ice Mode w h i c h is d e s i g n e d t o e n s u r e o p e r a t i o n o v e r t h e e x t r e m e t e r r a i n Found a t t h e margins or ice sheets, though with reduced performance. A description of the instrument, its operating environment, the mission objectives, and the particular problems or calibration and validation, are presented. A t t e n t i o n is a l s o F o c u s s e d o n t h e synergystic nature or Radar Altimeter data with respect to data from other sources. Thus independent nmasuremmtts are required to provide corrections to altimeter data, while altlsmtar data in turn may be combined with data from other sources to enable studies not possible with either sensor alone.
INTRODUCTION
T h e ERS-I s a t e l l i t e is s c h e d u l e d f o r l a u n c h in 1989. a n d a s p a r t o f i t s p a y l o a d i t will carry Europe's first Radar Altimeter. The mission am for this instrmmnt are stated as the measurement of seastate, and measurements over ice and major ocean currents, though by making suitable auxiliary moasurmmmts, and aesociatd data analyses, these am can be extended to other areas.
Precise Range and Range-rate Experiment (PRARE) w h i c h u s e s d u a l f r e q u e n c y . pseudo-random modulated, two-way links to measure the range to special ground transponders. T h e SAR a n d t h e Wind S c a t t e r o m e t e r a r e combined in t h e Active Microwave I n s t r u m e n t (AMI). T h e ATSR/M a n d t h e PRARE a r e nationally Funded (by Britain, Prance and Germany), following an Announcement of O p p o r t u n i t y i n 1981. T h e ERS-1 s a t e l l i t e will f l y i n a sun-synchronous orbit at an altitude of 777 kin, w i t h a n i n c l i n a t i o n o f 98.5 °. T h i s reference orbit repeats after 3 days. but other repeat periods are also possible with m i n o r c h a n g e s in o r b i t a l t i t u d e . T h e r e f e r e n c e o r b i t will b e u s e d d u r i n g t h e Commissioning Phase, of 3-6 months d u r a t i o n . when the major calibration and validation a c t i v i t i e s will be u n d e r t a k e n . D u r i n g t h e Operational Phase, following this. different r e p e a t p e r i o d s will be u s e d , t h o u g h t h e strategy has yet to be established. T h i s p a p e r will d e s c r i b e t h e A l t i m e t e r i n s t r u m e n t , which is similar in many r e p e c t s t o t h e S e a s a t i n s t r u m e n t ( M a c A r t h u r . 1976). a n d will c o n c e n t r a t e o n n o v e l o r u n i q u e aspects. Then the overall systes~ including t h e g r o u n d p r o c e s s i n g e l e m e n t s , will be outlined, and finally the approach to c a l i b r a t i o n a n d v a l i d a t i o n will be described.
An i l l u s t r a t i o n o f E R S - I is s h o w n a s Fig. 1. T h e p a y l o a d i n c l u d e s t h e Following instruments :
s
R a d a r A l t i m e t e r (RA), t h e s u b j e c t paper.
of this
S ~ I n t h e t l c A p e r t u r e R a d a r (SAR), w h i c h c a n produce high resolution imagery of oceans a n d ice. I t c a n o p e r a t e i n Image-Mode (while in c o n t a c t w i t h a g r o u n d s t a t i o n ) . o r i n Wave-Mode t o g e n e r a t e i m a g e s p e c t r a of
the
THE INSTRUMENT T h e g e o p h y s i c a l m e a s u r e m e n t o b j e c t i v e s of t h e A l t i m e t e r a r e s h o w n i n T a b l e 1. In addition to these quantitative performance o b j e c t i v e s t h e r e is t h e r e q u i r e m e n t t o return the power envelope shape of averaged echoed pulses, and to be capable or o p e r a t i o n o v e r ice c o v e r e d s u r f a c e s .
ocean wave-field.
Wind S c a t t e r o m e t e r , w h i c h m e a s u r e s t h e radar backscatterlng coefficient of the ocean in three different orientations. 9R~
288
PeaccfulSpaceand GlobalProblemsofMankind
TABLE I
Geophysical
Requirements
Precision ±0.5 m or whichever
Time Delay
p r e c i s i o n : 0 . 6 7 ns ( l o for ls a v e r a g e s )
o°
10~ is greater
stability:
0.67
0 . 5 dB f o r 1 dB for
o ° = 8 dB o ° = 2 0 dB
ns
In g e n e r a l t h e r e q u i r e m e n t s a r e v e r y s i m i l a r to those for the Seasat altimeter, except t h a t t h e ERS-1 A l t i m e t e r is b r o a d e r in scope, since it has specific requtrmments o v e r ice. T h i s p a p e r a s s u m e s f a m i l i a r i t y with the Sea~at altimeter, I~lrti©ularly the
Fig.1
t h e ERS-I R a d a r
Altimeter
Dynamic Range
Significant Wave Height (SWH)
Surface
for
1
to
20
m
5.13
to
5.50
(height 769 to
Rates of Change
ms
range: 8 2 5 km)
- 8 dB t o + 4 0 dB (assuming no atmospheric losses)
Ocean Mode: 50 m / s 0.6 m/s/s Ice Mode: 200 m/s 2.4 m/s/s
Ocean Mode: 6 dB/s Ice Mode: 24 d B / s
principle of its operation (puise-eamprmmion / full-dermmp) and the characteristics of radar echoes using a pulse-llatited alt4amter. A brief description o f t h e s e t o p l ~ will b e f o u n d i n t h e A p p e n d i x if n e c e s s a r y .
An I m p r e s s i o n o f ER S -I o v e r
Europe
36th IAF Congress, Stockholm, Sweden
A b l o c k d i a g r a m o f t h e RA is s h o w n i n Fig. 2. E a c h o f t h e e l e m e n t s ( e x c e p t t h e a n t e n n a ) is d u p l i c a t e d f o r r e d u n d a n c y . Most o f t h e d e t a i l i n t h i s d i a g r a m is w i t h i n t h e M i c r o w a v e S u b - s y s t e m (MWSS), f o r i t is h e r e that there are clearly identifiable units. T h e c r i t i c a l elements o f t h i s s u b s y s t e m a r e :
Ultra Stable OsciUator (use), whose quality determ/nes the quality of the time d e l a y m e a s u r m m n t . The use within t h e RA w o u l d b e c a p a b l e o f k e e p i n g time t o a b o u t a s e c o n d in t h r e e y e a r s . Chirp Generator, which generates the chirp for both the transmitted pulse and t h e l o c a l o s c i l l a t o r p u l s e . Two d i f f e r e n t bandwidth chirps are available, for the O c e a n a n d Ice Mode r e s p e c t i v e l y . The c h i r p b a n d w i d t h in t h e Ice Mode is f o u r t i m e s l e s s t h a n i n t h e O c e a n Mode, w h i l e a l l o t h e r b a n d w i d t h s r e m a i n t h e same this has the effect of reducing precision b y a f a c t o r o f 4, a n d i n c r e a s i n g t h e t r a c k i n g window w i d t h b y the same f a c t o r . T h e T r a v e l l i n g Wave T u b e ('r~r) a m p l i f i e r a n d its power s u p p l y t o g e t h e r form t h e High P o w e r Amp lifier. T h i s is t h e o u t p u t s t a g e o f t h e r a d a r t r a n m d t t e r . T h e T w r is a v a c u u m tube device, and has to be operated carefully. Thus the instrument operation has t o be c a r e f u l l y s c h e d u l e d t o allow a d e q u a t e w a r m i n g - u p time. a n d s e v e r a l p a r t i a l l y p o w e r e d mo des a r e a v a i l a b l e s o t h a t t h e l i f e t i m e may be m a x i m i s e d b y o p t i m a l o n / o f f switching. T h e S i g n a l P r o c e s s o r S u b - A s s e m b l y (SPSA) h a s two f u n c t i o n a l s u b - d i v i s i o n s . F i r s t is t h e spectrum analyser, which performs Analog to Digital conversion of the incoming signals, a t a h i g h r a t e (3.2 m e g a s a m p l e s p e r s e c o n d for each of the I and Q inputs) and then a F a s t F o u r i e r T r a n s f o r m (FFT) u s i n g a h a r d w a r e Multiplier-Accumulator device. This FFT p r o c e s s e f f e c t i v e l y f o r m s a f i l t e r b a n k . An o p e r a t i o n e q u i v a l e n t t o s h i f t i n g t h i s
Microwave Sub-System
Freer
/I
End ~ Electronic|
=
,
f i l t e r b a n k c a n be p e r f o r m e d b y p h a s e r o t a t i o n o f t h e i n c o m i n g s i g n a l . T h i s is u s e d f o r f i n e tim/rig a d j u s t m e n t s i n t h e r e c e i v e r . T h i s b l o c k w i t h i n t h e SPSA a l s o performs averaging of the sampled pulses. o v e r g r o u p s o f 50. T h e s e c o n d f u n c t i o n a l d i v i s i o n is t h e m / c r o c o m p u t e r i t s e l f , w h i c h processes the averaged echoes with the p u r p o s e o f e s t i m a t i n g t h e time o f a r r i v a l o f r e t u r n i n g p u l s e s . T h i s is v i t a l f o r t h e c o n t i n u e d o p e r a t i o n o f t h e r a d a r (s e e Appendix). The mcrocomputer also controls t h e AGC c i r c u i t s i n t h e r e c e i v e r a s a r e s u l t o f t h i s r e a l time d a t a p r o c e s s i n g . T h e science data from the instrument are output f r o m t h e SPSA. T h e s e d a t a a r e : *
t h e 50 p u l s e a v e r a g e d r e t u r n w a v e f o r I L s a m p l e d In 64 c o n t i g u o u s windows. T h e s e are generated by the spectrum analyser a n d a r e t h e same a s t h e i n p u t t o t h e parameter estimation process in the microcomputer.
*
the parameter estimates theaselves, from t h e o n - b o a r d r e a l - t i m e p r o c e s s o r . The q u a n t i t i e s in t h e d a t a s t r e a m a r e : time d e l a y rms w a v e h e i g h t target renectivity a u x i l i a r y i n f o r m a t i o n , s u c h a s tinm o f m e a s u r e m e n t ( d a t a t i o n ) , o p e r a t i n g mode, etc.
These Science Data are output to the Instrument Data Handling and Tranmstesion (IDHT) S y s t e ~ w h i c h is a c o m p l e x p a r t o f the payload dedicated to Science Data from the various instruments. Cmmmnlcatlon with t h e IDHT is v i a a d a t a b u s . C o n t r o l o f t h e RA is p e r f o r l m d b y t h e I n s t r u m e n t C o n t r o l U n i t (ICU). w h i c h is t h e s t a n d a r d a p p r o a c h f o r t h e ER S -I i n s t r u m e n t s . This device communicates with the on-board c o m p u t e r w i t h i n t h e main p a r t o f t h e satellite by means of the On-Board Data
(MWSS) AGC Re( iver
f
EPC
Signal
Processor
Sub Assembley (SPSAI
Instrument Control
Unit (ICU)
Fig.2
289
Block d i a g r a m o f t h e r a d a r
altimeter
Instrument DataS Handling and , ) Transmission /
v'
On - Board X Data Handling
/
290
Peaceful Space and Global Problems of Mankind
H a n d l i n g (OBDH) bus. N o t e t h a t t h i s is d i s t i n c t from t h e IDHT b u s r e f e r r e d t o a b o v e , t h o u g h d a t a o n t h e OBDH b u s will a l s o be t e l e m e t e r e d t o t h e g r o u n d v i a t h e s a t e l l i t e ' s t e l e m e t r y link. T h i s is u s e d f o r housekeeping data, telecommands, etc. The ICU is a n i n t e l l i g e n t d e v i c e a n d e x p a n d s c o m m n d s i t r e c e i v e s f r o m t h e OBDH b u s i n t o c o m p l e t e c o n f i g u r a t i o n s o f t h e RA. T h i s of course includes appropriate selection from the redundant subsystems. Each of the major functions described above is p h y s i c a l l y a c c o m m o d a t e d in a s e p a r a t e e l e c t r o n i c s box, a n d e v e r y t h i n g is d u p l i c a t e d , e x c e p t f o r t h e a n t e n n a . The c o m p l e t e i n s t r u m e n t mess is a b o u t 100 kg. T h e e l e c t r i c a l p o w e r r e q u i r e m e n t o f t h e RA is 155 W. T h e k e y f e a t u r e s o f t h e i n s t r u m e n t a r e s h o w n in T a b l e 2.
PARAMETER ESTIMATION One o f t h e m o s t s i g n i f i c a n t i n n o v a t i o n s i n t h e ERS-I RA is t h e m e t h o d u s e d f o r real-time parameter estimation in the m i c r o - c o m p u t e r . T h i s p a r a m e t e r e s t i m a t i o n is required to close the tracking loops which k e e p t h e RA o p e r a t i n g . T h e r e a r e in f a c t two d i s t i n c t m e t h o d s u s e d , f o r t h e Ice Mode a n d O c e a n Mode r e s p e c t i v e l y . T h e Ice Mode u s e s an established method, called Centre of G r a v i t y t r a c k i n g . This is simple, u n a m b i g u o u s a n d e a s i l y u n d e r s t o o d . The centre of gravity of the waveform within the t r a c k i n g window (which is 4 thnes w i d e r t h a n i n t h e O c e a n Mode) is f o u n d , a n d t h e H e i g h t Tracking Loop updated to keep this centre of gravity at a reference point, say at the mid dle o f t h e window. T h i s t e c h n i q u e will not be disturbed by returns with multiple leading edges or other effects observed over i c e - c o v e r e d s u r f a c e s . By d e f i n i n g t h e r e f e r e n c e p o i n t t o be l a t e r t h a n t h e c e n t r e o f t h e window, t h e r e is a n i n c r e a s e d probability that the leading portion of the w a v e f o r m will b e o b s e r v e d s o m e w h e r e in t h e window. The m e t h o d u s e d f o r t h e O c e a n Mode is m o r e comp lex, a n d is c a l l e d t h e S u b - O p t l m a l Maximum L i k e l i h o o d E s t i m a t o r (SMLE). T h e p r i n c i p l e s b e h i n d t h i s m e t h o d a r e s h o w n in Fig. 3. The k e y p o i n t s a r e : * t h e p r o c e s s is i t e r a t i v e , f r o m o n e 50 pulse average to the next. * t h e r e a r e 3 i n t e r a c t i n g 2nd o r d e r tracking loops. These are : Height Tracking Slope Tracking
L o o p (HTL)
TABLE 2
Key C h a r a c t e r i s t i c s
Mass Power TX P o w e r Pulse length
TX f r e q u e n c y
Antenna
diameter
c a l l e d AA, BB, CC, These combiations the leading edge controlled by the
o f t h e RA
100 k g 155 W 50 W p e a k 20 t t s u n c o m p r e s s e d 3 . 0 3 n s ( o c e a n Mode) 1 2 . 1 n s (Ice Mode) 1 3 . 8 GHz 1 . 2 ms
DD a n d EE a s shown. are intended to contain w i t h i n BB a n d DD, a n d a r e STL.
each loop operates by generating an error s i g n a l w h i c h is i n p u t t o a d o u b l e i n t e g r a t o r c i r c u i t w h o s e o u t p u t be c ome s a c o n t r o l s i g n a l w i t h i n t h e RA. T h e e r r o r s i g n a l s a r e g e n e r a t e d as follows (asstm~ng in e a c h c a s e t h a t o n l y t h e r e l e v a n t p a r a m e t e r is i n c o r r e c t l y e s t i m a t e d ) . AGC Loop. T h e sum o f a l l t h e g a t e s AA t o EE is f o u n d ( a n d d i v i d e d b y 64 t o o b t a i n the average power per filter) and compared t o a r e f e r e n c e l e ve l . The d i f f e r e n c e is the error signal. HTL. T h e a v e r a g e v a l u e o f g a t e BB a n d DD is f o u n d ( s e e p o i n t s B a n d D in Fig. 3(a)). T h e a v e r a g e o f t h e s e two p o i n t s , H, s h o u l d i d e a l l y e q u a l t h e r e f e r e n c e level if t h e r e is no h e i g h t e r r o r . T h e r e f e r e n c e l e v e l is s u b t r a c t e d f r o m H, a n d the result divided by the expected slope to give the height error s i g n a l . T h i s is s h o w n i n Fig. 3(a). STL. A g a i n t h e a v e r a g e v a l u e s o f BB a n d DD a r e F ound b u t now B is s u b t r a c t e d f r o m D a n d d i v i d e d b y t h e w i d t h o f BB (which is the half-width of the expected leading e d g e s l o p e ) . With n o e r r o r this should e q u a l t h e e x p e c t e d s l o p e . The d i f f e r e n c e is t h e STL e r r o r s i g n a l . T h i s is i l l u s t r a t e d i n Fig. 3(b). Each loop uses a selection of the gates AA-EE, t o g e n e r a t e a n i n p u t t o a 2nd o r d e r f i l t e r . T h e STL g e n e r a t e s a c o n t r o l s i g n a l w h i c h is u s e d t o e v a l u a t e AA-EE o n s u b s e q u e n t s a m p l e s . T h e AGC l o o p c o n t r o l s t h e AGC c i r c u i t w i t h i n t h e r e c e i v e r . T h e HTL generates a high precision estimate of h e i g h t ( d e l a y time). T h e m o s t s i g n i f i c a n t bits (coarse) control the timing of the d e r a m p p u l s e (s e e A p p e n d i x ) w h i l e t h e l e a s t s i g n i f i c a n t bits (fine) c o n t r o l t h e spectrum analyser fine tuning.
L o o p (STL)
AGC L o o p * t h e r e t u r n w a v e f o r m is a s s u m e d t o h a v e a n i d e a l s h a p e , d e f i n e d b y t h e h e a v y l i n e in Fig. 3. T h i s s h a p e is a s i m p l i f i e d f o r m o f t h e i d e a l r e t u r n , a s d e f i n e d b y Brown (1977). T h e t r a c k e r will o p e r a t e c o r r e c t l y with real signals, whose shape often varies considerably from this simplified form. S e t s o f c o n t i g u o u s f i l t e r a r e combined fully adaptively, to form gates
AUTO CALIBRATION A novel method of auto-calibration
of the t i m i n g p a t h i n t h e RA h a s b e e n d e v i s e d . T h i s is i n t e r e s t i n g b e c a u s e it i l l u s t r a t e s one of the subtleties of the instrument: the r e q u i r e d time p r e c i s i o n is muc h f i n e r t h a n t h e e q u i v a l e n t time r e s o l u t i o n o f t h e f i l t e r b a n k . T h e e c h o w a v e f o r m is s a m p l e d a t a r e s o l u t i o n o f a b o u t 3 ns , w h i l e t h e r e q u i r e d 10c a p r e c i s i o n implies a time p r e c i s i o n o f
36th IAF Congress, Stockholm, Sweden
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HTL e r r o r signal ° ................
H ................ ~ ' - - J ~
Reference Level
/
AGC loop error signal
I--
I-
AA
BB
I
CC
DD
EE
Y
STL error signal
expected slope
I-
z
-I-
AA
BB Fig. 3 (a) (b)
C'C
EE
DD
Conceptual o p e r a t i o n or t h e SMLE tracker : HTL and AOC STL
a b o u t 0.7 n s , a f a c t o r o f a b o u t 5 b e t t e r . In t h e m e a s u r e m e n t modes t h i s is a c h i e v e d because of the " curve-fittlng" nature of the p a r a m e t e r e s t i m a t o r . In o r d e r t o d e t e r m / n e a n y c h a n g e s in time d e l a y w i t h i n t h e RA it is a r e l a t i v e l y simple m a t t e r t o couple some o f t h e t r a n s m i t t e r o u t p u t book i n t o t h e r e c e i v e r (Fig. 4). This p r o d u c e s a single c h i r p echo. e q u i v a l e n t t o a p o i n t t a r g e t . However, c u r v e - r i f t i n g would n o t w o r k with s u c h a s i g n a l a n d t h e b e s t p r e c i s i o n would be a b o u t 3ns, which would n o t be a d e q u a t e f o r calibration. Therefore the a v e r a g i n g o v e r 50 p u l s e s p e r f o r m s d w i t h i n t h e SPSA h a s b e e n e x p l o i t e d , b y s h i t t i n g e a c h e c h o in t h e 50 pulse a v e r a g e using a comb/nation o f t h e coarse a u d fine t u n i n g described above. The normal c u r v e - f i t t i n g SMLE estimator can t h e n be u s e d . By t h i s m e a n s a l e a d i n g edge c a n be c o n t r u c t e d , Fig. 5. However, b e c a u s e t h e l o o p r e c e i v e s a n i n p u t s i g n a l o n c e e v e r y 50 pulses (instead o f once p e r pulse) t h e
I
1 Fig. $
I
16
I
32
I
1.8
k leading edge generated discrete tones
!
64 f r o m 50
s y s t e m is slowed down b y a f a c t o r 50. This t e c h n i q u e is called c l o s e d - l o o p c a l i b r a t i o n . A much f a s t e r (-150 ms) o p e n - l o o p c a l i b r a t i o n will also be p e r f o r m e d a t i n t e r v a l s of a b o u t $ minutes, in which the l o o p s a r e n o t closed a n d t h e e r r o r s i g n a l s themselves are used to determine the offset, using • characteristic curve dcterm/ned using closed-loop calibration data. Using t h i s m e t h o d c a l i b r a t i o n o f most o f t h e s i g n a l p a t h in t h e i n s t r u m e n t is achieved to the normal o p e r a t i n g precision. Only t h e a n t e n n a a n d t h e F r o n t End E l e c t r o n i c s (which a r e p a s s i v e a n d t h e r e f o r e r e l a t i v e l y s t a b l e ) a r e excluded. SYSTEM ASPECTS
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T h e o v e r a l l RA d a t a flow is s h o w n in Fig. 6. This d i a g r a m makes c l e a r t h e d i s t i n c t i o n b e t w e e n t h e IDHT a n d OBDH d a t a b u s s e s . The t w o - w a y flow o f c m m m n d s a n d h o u s e k e e p i n g d a t e o n t h e OBDH b u s is e x t e n d e d b y t h e
292
Peaceful Space and Global Problems of Mankind
I_
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Fest Delivery Products
Housekeeping
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Fig. 6
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RA d a t a flow t h r o u g h system
S - b a n d t e l e m e t r y link. via t h e p r i m a r y TTC s t a t i o n a t K i r u n e t o t h e Mission Management a n d C o n t r o l C e n t r e (k~ICC). Here t h e s t a t u s o f t h e i n s t r u m e n t is m o n i t o r e d , a n d t h e complex p a y l o a d o p e r a t i o n s p l a n implemented a s a s e r i e s o f telecemmands. T h e S c i e n c e Data o n t h e IDHT b u s is t r a n s m i t t e d a t X - b e n d , b o t h in r e a l - t i m e a n d a s a dump f r o m t h e o n - b o a r d t a p e r e c o r d e r which r e c o r d s m N s u r e m s n t s made while o u t o f c o n t a c t with a g r o u n d s t a t t l o ~ This d a t a t r a n s m i s s i o n is r e c e i v e d a t K i r u n a a n d o t h e r s t a t i o n s . At t h e t o p e - d u m p s t a t i o n s a quasi-real-t~ d a t a p r o m m s o r is u s e d t o generate the Fast Delivery products. These will h a v e a d e l a y o f l e s s t h a n 3 h o u r s w/thin the ground station, and cannot t h e r e f o r e b e full p r e o i s l o n p r o d u c t s b e c a u s e m a n y r e q u i r e d a u x i l i a r y d a t a will n o t be a v a i l a b l e (e.g. p r e c i s i o n a l t i t u d e restitution). The Fast Delivery p r o d u c t s
foreseen f o r t h e RA a r e stlptiflmmt wavehldght, windspeed and altitude (accuracy ~I m). Raw d a t a f r o m t h e X - b a n d r e e e l v l n g s t a t i o n s will be a r c h l v e d , a n d u l t i m a t e l y p r o c e s s e d t o full p r e c i s i o n , a t t h e Proeis/cm Proooeslng a n d Are]hiving Facilities (PAP's). These facilities a r e
provided by various m r states, and do n o t t o m p a r t o f the main proeuremmtt of ERS-I. T h e PAF's a r e n o t e x p l i c i t l y s h o w n in Fig. 6. CALIBRATION T h e o v e r a l l c a l i b r a t i o n o f t h e a l t i m e t e r is a a m l t i s t e p p r o e e d u r ~ T h e i n s t r t m m n t will be c a l i b r a t e d a n d v a l i d a t e d b e f o r e l a u n c h b y a c o m b i n a t i o n o f t e s t mmmuremmtts, a n a l y s i s a n d e c h o simulation. A R e t u r n S i g n a l Simulator (RSS) is being developed as p e r t o f t h e i n s t r u m e n t t e g t - e q u i p m m t t . This d e v i c e wUl be i n s t a l l e d in p i e c e o f t h e a n t e n n a , and can simulate echoes with an a p p r o p r i a t e d e l a y tiara a n d w i t h m o d u l a t i o n
1 I I
J
t h e ERS-I
which r e a l i s t i c a l l y simulates o c e a n o r o t h e r s u r f a c e s . This device d e p e n d s o n a n i n t i m a t e c o n n e c t i o n t o t h e a l t i m e t e r , s i n c e it inhibits and generates various trigger s i g n a l s w i t h i n t h e i n s t r u m e n t . T h i s is e s s e n t i a l in o r d e r t o a c h e i v e t h e r e q u i r e d p r e c i s i o n a n d a c c u r a c y . The RSS will be u s e d , with a n a l y s i s o f i n d i v i d u a l t e s t m e a s u r e m e n t s , t o f u l l y c a l i b r a t e all t h e altimeter measurements before launch. Following l a u n c h t h e i n t e r n a l c a l i b r a t i o n t e c h n i q u e d e s c r i b e d e a r l i e r will be used. b u t a n e x t e r n a l c a l i b r a t i o n is still n e c c e s s a r y t o p r o v i d e a complete c a l i b r a t i o n o f t h e I n - o r b i t s y s t e ~ This e x t e r n a l c a l i b r a t i o n will be r e s t r i c t e d t o time delay. A l t h o u g h m e a s u r e m e n t s o f t h e p o w e r return from special transponders (either a c t i v e o r p a s s i v e ) will b e made. t h e e x p e c t e d a c c u r a c y o f s u c h m e a s u r e m e n t s will be i n s u f f i c i e n t t o allow c a l i b r a t i o n , a n d t h i s is r e g a r d e d m o r e a s v a l i d a t i o n . External calibration of the significant
wavehe/ght ( e f f e c t i v e l y t h e echo leading e d g e s l o p e ) will n o t be p e r f o r m e d . T h e e x t e r n a l c a l i b r a t i o n o f t i m e - d e l a y will be p e r f o r m e d b y c o m p a r i s o n w i t h m e a s u r e m e n t s from a ground-based laser to a laser retro-refloctor fitted near to the altimeter a n t e n n a . A s i / l a r t e c h n i q u e was u s e d with S e a s a t (Kol~nkiowicz a n d Martin. 1982). u s i n g a l a s e r o n t h e i s l a n d o f Bermuda. A number of problesm were encountered, including : * t h e a l t i l m t e r c o u l d n o t make m e a s u r e m e n t s o v e r a 25 km s e g a l e n t a r o u n d t h e i s l a n d , so e x t r a p o l a t i o n f r o m m e a s u r e m e n t s in t h e o p e n o c e a n was r e q u i r e d . This r e q u i r e d c o r r e c t i o n s f o r tides, w a v e h e i g h t etc. *
atmo6phertc corrections for altimeter tiara-delay.
F o r ERS-1 a n u m b e r o f p o s s i b i l i t i e s h a v e
Peaceful Space and Global Problems of Mankind
been suggested, including the Dakar p e n i n s u l a r . On e n o v e l p r o p o s a l is t h e G r e e n l a n d i c e - s h e e t . In t h e c e n t r e o f t h e ice-sheet the altimeter could track the i n v a r i a n t i c e s u r f a c e (which c a n be s u r v e y e d ) u s i n g t h e O c e a n Mode. a n d i t c o u l d continue over the laser site. Tropospheric c o r r e c t i o n s w o u l d a l s o be m u c h e a s i e r w i t h the effective absence of water vapour. Expected problems due to small-scale ionospheric variability and radar p e n e t r a t i o n o f t h e i c e s u r f a c e c o u l d be s o l v e d b y u t i l i s i n 8 t h e d i f f e r e n t i a l PRARE measurements, and transponders fixed to the surface, respectively.
MULTISENSOR D A T A Measurements from o t h e r s e n s o r s on ERS-I will b e u s e d t o p r o v i d e c o r r e c t i o n s t o t h e a l t i m e t e r d a t a . T h e ATSR/M will b e u s e d t o mak e m e a s u r e m e n t s o f a t m o s p h e r i c w a t e r vapour, particularly using the nadir-viewing microwave radiometer. The water vapour c o r r e c t i o n is t h e m u s t v a r i a b l e o f t h e atmospheric corrections and in-situ m e a s u r e m e n t s a l o n g t h e same RF p a t h will b e most valuable. The use of t h e d i f f e r e n t i a l ( S - b a n d . X - b a n d ) PRARE m e a s u r e m e n t s i n o r d e r t o a s s e s s t h e T o t a l E l e c t r o n C o n t e n t (TEC) of the ionosphere has been referred to above. The RA f o o t p r i n t s a r e c o i n c i d e n t w i t h t h o s e o f t h e ATSR/M, a l l o w i n g j o i n t s t u d y of, f o r example, o c e a n c u r r e n t s , combining sea-surface temperature measurements from t h e ATSR/M w i t h c o i n c i d e n t e l e v a t i o n m e a s u r e m e n t s f r o m t h e RA. T h e AMI d o e s n o t h a v e c o i n c i d e n t s w a t h s w i t h t h e RA. s o j o i n t s t u d i e s must make allowances f o r t e m p o r a l o r spatial differences between measurements. However, slowly v a r y i n g p h e n o m e n a (eg level a n d e x t e n t o f c l o s e d - l a k s e ) s h o u l d be o b s e r v a b l e in t h i s w a y . C o m b i n i n g ERS-I measurements with those from other s a t e l l i t e s (eg TOPEX/POSEIDON. NROSS etc.) is a l s o a n a r e a o f a c t i v e i n t e r e s t . CONCLUSION
T h e m a k i n g o f RA m e a s u r e m e n t s a s p e r t o f t h e E R S - t s y s t e m is a c o m p l e x a n d d e m a n d i n g t a s k . Some o f t h e c h a r a c t e r i s t i c s o f t h e system have been described, and these reveal some o f t h e n o v e l d e s i g n f e a t u r e s o f t h e instrument and the interrelationships between many elements of t h e systmn.
APPENDIX OPERATING PRINCIPLES T h e E R S - I R a d a r A l t i m e t e r , in common w i t h t h e S e a s a t i n s t r u m e n t , o p e r a t e s in t h e p u l s e - w i d t h l l a d t e d mode. a n d u s e s t h e f u l l deramp system of pulse compression. The m e a n i n g o f t h e s e t e r m s will be d e s c r i b e d h e r e . Fig. 7 s h o w s a n a r r o w p u l s e f r o m a n altimeter encountering a flat surface, which is a s s u m e d t o be r o u g h on t h e s c a l e o f t h e r a d a r w a v e l e n g t h , so t h a t , in t h e a b s e n c e o f antenna pattern effects, the returned power is p r o p o r t i o n a l t o t h e a r e a i l l u m i n a t e d . T h e n o m i n a l a n t e n n a b e a m w i d t h is i n d i c a t e d b y
293
t h e d a s h e d l i n e s f r o m t h e a n t e n n a . As t h e pulse propagates the illum/nated area. and therefore the returned power, grows rapidly. E v e n t u a l l y a n a n n u l u s is f o r m e d , a n d t h e g e o m e t r y is s u c h t h a t t h e a r e a of t h i s remains constant as the diameter Increases. H ow e ve r, t h e r e t u r n e d p o w e r d o e s n o t r e m a i n constant, since the antenna pattern causes an attenuation as the annulus expands. The distribution function of surface scatterers, for the normal case where the reflecting s u r f a c e is n o t f l a t , a l s o i n f l u e n c e s t h e r a d a r e c h o . T h e e c h o w a v e f o r m is t h e convolution of the probability distribution of the heights of surface scatterers and the flat surface response described above, which is i t s e l f t h e c o n v o l u t i o n of t h e point t a r g e t r e s p o n s e ( w h i c h is r e l a t e d t o t h e pulse shape) and the two-way antenna p a t t e r n . S u c h a mode o f o p e r a t i o n is c a l l e d p u l s e width limited b e c a u s e n o t all of t h e t a r g e t is i l l u m i n a t e d s i m u l t a n e o u s l y , a n d t h e limit t o t h e r e c e i v e d p o w e r is controlled by the illumination.
The full deramp pulse compression technique i s s h o w n i n Fig. 8. In a c o n v e n t i o n a l p u l s e c o m p r e s s i o n r a d a r a c h i r p waveform is obtained by /repulsing a dispersive delay l i ne . T h i s s p r e a d s t h e r e q u i r e d e n e r g y o v e r time t o allow r e d u c e d p e a k p o w e r . When t h e r a d a r e c h o is r e c e i v e d i t is p a s s e d t h r o u g h another dispersive delay line with an inverse characteristic. For a point target a s i n g l e p u l s e e c h o is t h u s g e n e r a t e d f r o m t h e incoming chirp. This approach could cause p r o b l e m s w i t h i n t h e RA f o r t h e f o l l o w i n g reasons : * To m e e t t h e c o m p r e s s e d p u l s e w i d t h a n d peak power requirements a very high t i m e - b a n d w i t h p r o d u c t is r e q u i r e d f o r t h e c h i r p . T h i s c a n be a c h i e v e d o n transmission by frequency multiplying a chirp with a smaller time-bandwidth p r o d u c t , b u t s u c h a n a p p r o a c h c a n n o t be taken for the matched compression filter. * To s a m p l e t h e e c h o w a v e f o r m w i t h a resolution of 3 ns extremely high speed t i s d n g a n d o t h e r c i r c u i t r y w o u l d be required. The a p p r o a c h t a k e n in all known s p a c e a l t i m e t e r s s i n c e S e a s a t is t o t r a n s l a t e f r o m t h e time d o m a i n i n t o t h e f r e q u e n c y d o m a i n v e r y e a r l y in t h e r e c e i v e r , b y t h e full d e r a m p m e t h o d , w h e r e t h e i n c o m / n g s i g n a l is mixed w i t h a r e p l i c a o f t h e t r a n s m i t t e d c h i r p . Timing t h e n be c ome s t r a n s l a t e d t o spectrum analysis and a matched compression f i l t e r is n o t r e q u i r e d , i n Fig. 8 a representation of the frequency-time c h a r a c t e r i s t i c o f t h e c h i r p is s h o w n b y t h e transmitter. A representation of the echoed s i g n a l is s h o w n b y t h e f r o n t e n d o f t h e r e c e i v e r . T h i s is a s e r i e s o f - , a n y overlapping chirps, each echoed from an individual scatterer. The density of these c h i r p s a l o n g a c u t p a r a l l e l t o t h e time a x i s would h a v e t h e s h a p e of t h e r e t u r n e d power envelope. Individual chirps vary from this ideallsed waveform because of interference effects between the echos from different scatterers.
294
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When the echo is expected to r e t u r n the chirp generator is retriggered and a second chirp generated. During the upoonversion and multiplication process a slight frequency offset is introduced, and this beoomss the
has the duration of any individual echoed c h i r p , b u t m m n o t e n c o m p a s s , in a d d i t i o n . t h e s p r e a d o f a r r i v a l times. H o w e v e r , t h e s k e w is o n l y a b o u t o n e p e r t in I000.
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The next Important stage is the Spectrum Analyser! here the spectrum of the tones is found. TA/s speatrum exactly represents the time s t r u c t u r e of the echo waveform Finally this inlroramtton is used by the Parameter h t t l m t o r . This step is essential in order to provide the estimate of when the next echo is exl:xmted to return, for the chirp
series of tones is generated, centred on the f i r s t IF. where a o u t parallel to the f r e q u e n c y a x i s is e q u i v a l e n t t o t h e time axis cut referred to above. The r e p r e s e n t a t i o n o f t h e s e t o n e s in Fig. 8 s h o w s a s l i g h t s k e w ( w h i c h is highly e x a g e r a t e d ) . T h i s is b e c a u s e t h e LO c h i r p
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36th IAF Congress, Stockholm, Sweden
a c c u r a t e t h i s e s t i m a t e m u s t be, t h e full b a n d w i d t h o f t h e $ p e c t r u a A n a l y s e r Is e q u i v a l e n t t o a h e i g h t window o f a b o u t 30 • in t h e O c e a n Mode. The maximum h e i g h t r a t e s p e c i f i e d in Table t in t h i s m e is 50 m/s. If t h e h e i g h t e s t i m a t e w e r e n o t c o n t i n u o u s l y u p d a t e d t h e s i g n a l c o u l d be c o m p l e t e l y l o s t in less t h a n a s e c o n d . REFERENCES Brown, O. S. (1977). The a v e r a g e impulse r e s p o n s e of a r o u g h s u r f a c e and its a p p l i c a t i o n . IEEE T r a n s o n A n t e n n a s a n d P r o p a g a t i o n , AP-2$, 1, 67-74. M a c A r t h u r , J. L. (1976). Design o f t h e S e a s a t - A r a d a r a l t i m e t e r . WFS-IEEE O o e a n s - 7 6 , lOB___. Kolenklewicz, I~ a n d Martin, C. F. (1982). SEASAT a l t i m e t e r h e i g h t c a l i b r a t i o n . J. G e o p h g s . Res., 87. C5, 3189-3197.
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0094-5765/86$3.1)0+.00 (~) 1986PergamonJournalsLtd.
THE ERS-1 RADAR ALTIMDETER MISSION
C R Francis
European
Space
R e s e a r c h a n d T e o h n o l o g l ; C e n t r e (E3TEC) Noordwijlc, T h e N e t h e r l a n d s
e n a b l i n g w i n d s p e e d a n d d i r e c t i o n t o be d e t e r m / n e d o v e r a s w a t h 400 km wide.
ABSTRACT
A v i s i b l e a n d i n f r a - r e d i m a g e r , t h e Along Traclr S o a r m i n g R a d i o m e t e r (ATSR), o p t i m / s e d For m e a s u r e m e n t s o f s e a - s u r f a c e temperature. A nadir viewing t w o - f r e q u e n c y m i c r o w a v e r a d i o m e t e r is included, the full instrument being r e f e r r e d t o a s ATSR/M.
T h e R a d a r A l t i m e t e r t o be c a r r i e d o n E R S - I will p r o v i d e m s u r a s e n t s or altitude, significant waveheight and windspeed over t h e o c e a n s t o a l a t i t u d e o f 82". In a d d i t i o n , t h e i n s t r u m e n t h a s a d e d i c a t e d Ice Mode w h i c h is d e s i g n e d t o e n s u r e o p e r a t i o n o v e r t h e e x t r e m e t e r r a i n Found a t t h e margins or ice sheets, though with reduced performance. A description of the instrument, its operating environment, the mission objectives, and the particular problems or calibration and validation, are presented. A t t e n t i o n is a l s o F o c u s s e d o n t h e synergystic nature or Radar Altimeter data with respect to data from other sources. Thus independent nmasuremmtts are required to provide corrections to altimeter data, while altlsmtar data in turn may be combined with data from other sources to enable studies not possible with either sensor alone.
INTRODUCTION
T h e ERS-I s a t e l l i t e is s c h e d u l e d f o r l a u n c h in 1989. a n d a s p a r t o f i t s p a y l o a d i t will carry Europe's first Radar Altimeter. The mission am for this instrmmnt are stated as the measurement of seastate, and measurements over ice and major ocean currents, though by making suitable auxiliary moasurmmmts, and aesociatd data analyses, these am can be extended to other areas.
Precise Range and Range-rate Experiment (PRARE) w h i c h u s e s d u a l f r e q u e n c y . pseudo-random modulated, two-way links to measure the range to special ground transponders. T h e SAR a n d t h e Wind S c a t t e r o m e t e r a r e combined in t h e Active Microwave I n s t r u m e n t (AMI). T h e ATSR/M a n d t h e PRARE a r e nationally Funded (by Britain, Prance and Germany), following an Announcement of O p p o r t u n i t y i n 1981. T h e ERS-1 s a t e l l i t e will f l y i n a sun-synchronous orbit at an altitude of 777 kin, w i t h a n i n c l i n a t i o n o f 98.5 °. T h i s reference orbit repeats after 3 days. but other repeat periods are also possible with m i n o r c h a n g e s in o r b i t a l t i t u d e . T h e r e f e r e n c e o r b i t will b e u s e d d u r i n g t h e Commissioning Phase, of 3-6 months d u r a t i o n . when the major calibration and validation a c t i v i t i e s will be u n d e r t a k e n . D u r i n g t h e Operational Phase, following this. different r e p e a t p e r i o d s will be u s e d , t h o u g h t h e strategy has yet to be established. T h i s p a p e r will d e s c r i b e t h e A l t i m e t e r i n s t r u m e n t , which is similar in many r e p e c t s t o t h e S e a s a t i n s t r u m e n t ( M a c A r t h u r . 1976). a n d will c o n c e n t r a t e o n n o v e l o r u n i q u e aspects. Then the overall systes~ including t h e g r o u n d p r o c e s s i n g e l e m e n t s , will be outlined, and finally the approach to c a l i b r a t i o n a n d v a l i d a t i o n will be described.
An i l l u s t r a t i o n o f E R S - I is s h o w n a s Fig. 1. T h e p a y l o a d i n c l u d e s t h e Following instruments :
s
R a d a r A l t i m e t e r (RA), t h e s u b j e c t paper.
of this
S ~ I n t h e t l c A p e r t u r e R a d a r (SAR), w h i c h c a n produce high resolution imagery of oceans a n d ice. I t c a n o p e r a t e i n Image-Mode (while in c o n t a c t w i t h a g r o u n d s t a t i o n ) . o r i n Wave-Mode t o g e n e r a t e i m a g e s p e c t r a of
the
THE INSTRUMENT T h e g e o p h y s i c a l m e a s u r e m e n t o b j e c t i v e s of t h e A l t i m e t e r a r e s h o w n i n T a b l e 1. In addition to these quantitative performance o b j e c t i v e s t h e r e is t h e r e q u i r e m e n t t o return the power envelope shape of averaged echoed pulses, and to be capable or o p e r a t i o n o v e r ice c o v e r e d s u r f a c e s .
ocean wave-field.
Wind S c a t t e r o m e t e r , w h i c h m e a s u r e s t h e radar backscatterlng coefficient of the ocean in three different orientations. 9R~
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TABLE I
Geophysical
Requirements
Precision ±0.5 m or whichever
Time Delay
p r e c i s i o n : 0 . 6 7 ns ( l o for ls a v e r a g e s )
o°
10~ is greater
stability:
0.67
0 . 5 dB f o r 1 dB for
o ° = 8 dB o ° = 2 0 dB
ns
In g e n e r a l t h e r e q u i r e m e n t s a r e v e r y s i m i l a r to those for the Seasat altimeter, except t h a t t h e ERS-1 A l t i m e t e r is b r o a d e r in scope, since it has specific requtrmments o v e r ice. T h i s p a p e r a s s u m e s f a m i l i a r i t y with the Sea~at altimeter, I~lrti©ularly the
Fig.1
t h e ERS-I R a d a r
Altimeter
Dynamic Range
Significant Wave Height (SWH)
Surface
for
1
to
20
m
5.13
to
5.50
(height 769 to
Rates of Change
ms
range: 8 2 5 km)
- 8 dB t o + 4 0 dB (assuming no atmospheric losses)
Ocean Mode: 50 m / s 0.6 m/s/s Ice Mode: 200 m/s 2.4 m/s/s
Ocean Mode: 6 dB/s Ice Mode: 24 d B / s
principle of its operation (puise-eamprmmion / full-dermmp) and the characteristics of radar echoes using a pulse-llatited alt4amter. A brief description o f t h e s e t o p l ~ will b e f o u n d i n t h e A p p e n d i x if n e c e s s a r y .
An I m p r e s s i o n o f ER S -I o v e r
Europe
36th IAF Congress, Stockholm, Sweden
A b l o c k d i a g r a m o f t h e RA is s h o w n i n Fig. 2. E a c h o f t h e e l e m e n t s ( e x c e p t t h e a n t e n n a ) is d u p l i c a t e d f o r r e d u n d a n c y . Most o f t h e d e t a i l i n t h i s d i a g r a m is w i t h i n t h e M i c r o w a v e S u b - s y s t e m (MWSS), f o r i t is h e r e that there are clearly identifiable units. T h e c r i t i c a l elements o f t h i s s u b s y s t e m a r e :
Ultra Stable OsciUator (use), whose quality determ/nes the quality of the time d e l a y m e a s u r m m n t . The use within t h e RA w o u l d b e c a p a b l e o f k e e p i n g time t o a b o u t a s e c o n d in t h r e e y e a r s . Chirp Generator, which generates the chirp for both the transmitted pulse and t h e l o c a l o s c i l l a t o r p u l s e . Two d i f f e r e n t bandwidth chirps are available, for the O c e a n a n d Ice Mode r e s p e c t i v e l y . The c h i r p b a n d w i d t h in t h e Ice Mode is f o u r t i m e s l e s s t h a n i n t h e O c e a n Mode, w h i l e a l l o t h e r b a n d w i d t h s r e m a i n t h e same this has the effect of reducing precision b y a f a c t o r o f 4, a n d i n c r e a s i n g t h e t r a c k i n g window w i d t h b y the same f a c t o r . T h e T r a v e l l i n g Wave T u b e ('r~r) a m p l i f i e r a n d its power s u p p l y t o g e t h e r form t h e High P o w e r Amp lifier. T h i s is t h e o u t p u t s t a g e o f t h e r a d a r t r a n m d t t e r . T h e T w r is a v a c u u m tube device, and has to be operated carefully. Thus the instrument operation has t o be c a r e f u l l y s c h e d u l e d t o allow a d e q u a t e w a r m i n g - u p time. a n d s e v e r a l p a r t i a l l y p o w e r e d mo des a r e a v a i l a b l e s o t h a t t h e l i f e t i m e may be m a x i m i s e d b y o p t i m a l o n / o f f switching. T h e S i g n a l P r o c e s s o r S u b - A s s e m b l y (SPSA) h a s two f u n c t i o n a l s u b - d i v i s i o n s . F i r s t is t h e spectrum analyser, which performs Analog to Digital conversion of the incoming signals, a t a h i g h r a t e (3.2 m e g a s a m p l e s p e r s e c o n d for each of the I and Q inputs) and then a F a s t F o u r i e r T r a n s f o r m (FFT) u s i n g a h a r d w a r e Multiplier-Accumulator device. This FFT p r o c e s s e f f e c t i v e l y f o r m s a f i l t e r b a n k . An o p e r a t i o n e q u i v a l e n t t o s h i f t i n g t h i s
Microwave Sub-System
Freer
/I
End ~ Electronic|
=
,
f i l t e r b a n k c a n be p e r f o r m e d b y p h a s e r o t a t i o n o f t h e i n c o m i n g s i g n a l . T h i s is u s e d f o r f i n e tim/rig a d j u s t m e n t s i n t h e r e c e i v e r . T h i s b l o c k w i t h i n t h e SPSA a l s o performs averaging of the sampled pulses. o v e r g r o u p s o f 50. T h e s e c o n d f u n c t i o n a l d i v i s i o n is t h e m / c r o c o m p u t e r i t s e l f , w h i c h processes the averaged echoes with the p u r p o s e o f e s t i m a t i n g t h e time o f a r r i v a l o f r e t u r n i n g p u l s e s . T h i s is v i t a l f o r t h e c o n t i n u e d o p e r a t i o n o f t h e r a d a r (s e e Appendix). The mcrocomputer also controls t h e AGC c i r c u i t s i n t h e r e c e i v e r a s a r e s u l t o f t h i s r e a l time d a t a p r o c e s s i n g . T h e science data from the instrument are output f r o m t h e SPSA. T h e s e d a t a a r e : *
t h e 50 p u l s e a v e r a g e d r e t u r n w a v e f o r I L s a m p l e d In 64 c o n t i g u o u s windows. T h e s e are generated by the spectrum analyser a n d a r e t h e same a s t h e i n p u t t o t h e parameter estimation process in the microcomputer.
*
the parameter estimates theaselves, from t h e o n - b o a r d r e a l - t i m e p r o c e s s o r . The q u a n t i t i e s in t h e d a t a s t r e a m a r e : time d e l a y rms w a v e h e i g h t target renectivity a u x i l i a r y i n f o r m a t i o n , s u c h a s tinm o f m e a s u r e m e n t ( d a t a t i o n ) , o p e r a t i n g mode, etc.
These Science Data are output to the Instrument Data Handling and Tranmstesion (IDHT) S y s t e ~ w h i c h is a c o m p l e x p a r t o f the payload dedicated to Science Data from the various instruments. Cmmmnlcatlon with t h e IDHT is v i a a d a t a b u s . C o n t r o l o f t h e RA is p e r f o r l m d b y t h e I n s t r u m e n t C o n t r o l U n i t (ICU). w h i c h is t h e s t a n d a r d a p p r o a c h f o r t h e ER S -I i n s t r u m e n t s . This device communicates with the on-board c o m p u t e r w i t h i n t h e main p a r t o f t h e satellite by means of the On-Board Data
(MWSS) AGC Re( iver
f
EPC
Signal
Processor
Sub Assembley (SPSAI
Instrument Control
Unit (ICU)
Fig.2
289
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Instrument DataS Handling and , ) Transmission /
v'
On - Board X Data Handling
/
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Peaceful Space and Global Problems of Mankind
H a n d l i n g (OBDH) bus. N o t e t h a t t h i s is d i s t i n c t from t h e IDHT b u s r e f e r r e d t o a b o v e , t h o u g h d a t a o n t h e OBDH b u s will a l s o be t e l e m e t e r e d t o t h e g r o u n d v i a t h e s a t e l l i t e ' s t e l e m e t r y link. T h i s is u s e d f o r housekeeping data, telecommands, etc. The ICU is a n i n t e l l i g e n t d e v i c e a n d e x p a n d s c o m m n d s i t r e c e i v e s f r o m t h e OBDH b u s i n t o c o m p l e t e c o n f i g u r a t i o n s o f t h e RA. T h i s of course includes appropriate selection from the redundant subsystems. Each of the major functions described above is p h y s i c a l l y a c c o m m o d a t e d in a s e p a r a t e e l e c t r o n i c s box, a n d e v e r y t h i n g is d u p l i c a t e d , e x c e p t f o r t h e a n t e n n a . The c o m p l e t e i n s t r u m e n t mess is a b o u t 100 kg. T h e e l e c t r i c a l p o w e r r e q u i r e m e n t o f t h e RA is 155 W. T h e k e y f e a t u r e s o f t h e i n s t r u m e n t a r e s h o w n in T a b l e 2.
PARAMETER ESTIMATION One o f t h e m o s t s i g n i f i c a n t i n n o v a t i o n s i n t h e ERS-I RA is t h e m e t h o d u s e d f o r real-time parameter estimation in the m i c r o - c o m p u t e r . T h i s p a r a m e t e r e s t i m a t i o n is required to close the tracking loops which k e e p t h e RA o p e r a t i n g . T h e r e a r e in f a c t two d i s t i n c t m e t h o d s u s e d , f o r t h e Ice Mode a n d O c e a n Mode r e s p e c t i v e l y . T h e Ice Mode u s e s an established method, called Centre of G r a v i t y t r a c k i n g . This is simple, u n a m b i g u o u s a n d e a s i l y u n d e r s t o o d . The centre of gravity of the waveform within the t r a c k i n g window (which is 4 thnes w i d e r t h a n i n t h e O c e a n Mode) is f o u n d , a n d t h e H e i g h t Tracking Loop updated to keep this centre of gravity at a reference point, say at the mid dle o f t h e window. T h i s t e c h n i q u e will not be disturbed by returns with multiple leading edges or other effects observed over i c e - c o v e r e d s u r f a c e s . By d e f i n i n g t h e r e f e r e n c e p o i n t t o be l a t e r t h a n t h e c e n t r e o f t h e window, t h e r e is a n i n c r e a s e d probability that the leading portion of the w a v e f o r m will b e o b s e r v e d s o m e w h e r e in t h e window. The m e t h o d u s e d f o r t h e O c e a n Mode is m o r e comp lex, a n d is c a l l e d t h e S u b - O p t l m a l Maximum L i k e l i h o o d E s t i m a t o r (SMLE). T h e p r i n c i p l e s b e h i n d t h i s m e t h o d a r e s h o w n in Fig. 3. The k e y p o i n t s a r e : * t h e p r o c e s s is i t e r a t i v e , f r o m o n e 50 pulse average to the next. * t h e r e a r e 3 i n t e r a c t i n g 2nd o r d e r tracking loops. These are : Height Tracking Slope Tracking
L o o p (HTL)
TABLE 2
Key C h a r a c t e r i s t i c s
Mass Power TX P o w e r Pulse length
TX f r e q u e n c y
Antenna
diameter
c a l l e d AA, BB, CC, These combiations the leading edge controlled by the
o f t h e RA
100 k g 155 W 50 W p e a k 20 t t s u n c o m p r e s s e d 3 . 0 3 n s ( o c e a n Mode) 1 2 . 1 n s (Ice Mode) 1 3 . 8 GHz 1 . 2 ms
DD a n d EE a s shown. are intended to contain w i t h i n BB a n d DD, a n d a r e STL.
each loop operates by generating an error s i g n a l w h i c h is i n p u t t o a d o u b l e i n t e g r a t o r c i r c u i t w h o s e o u t p u t be c ome s a c o n t r o l s i g n a l w i t h i n t h e RA. T h e e r r o r s i g n a l s a r e g e n e r a t e d as follows (asstm~ng in e a c h c a s e t h a t o n l y t h e r e l e v a n t p a r a m e t e r is i n c o r r e c t l y e s t i m a t e d ) . AGC Loop. T h e sum o f a l l t h e g a t e s AA t o EE is f o u n d ( a n d d i v i d e d b y 64 t o o b t a i n the average power per filter) and compared t o a r e f e r e n c e l e ve l . The d i f f e r e n c e is the error signal. HTL. T h e a v e r a g e v a l u e o f g a t e BB a n d DD is f o u n d ( s e e p o i n t s B a n d D in Fig. 3(a)). T h e a v e r a g e o f t h e s e two p o i n t s , H, s h o u l d i d e a l l y e q u a l t h e r e f e r e n c e level if t h e r e is no h e i g h t e r r o r . T h e r e f e r e n c e l e v e l is s u b t r a c t e d f r o m H, a n d the result divided by the expected slope to give the height error s i g n a l . T h i s is s h o w n i n Fig. 3(a). STL. A g a i n t h e a v e r a g e v a l u e s o f BB a n d DD a r e F ound b u t now B is s u b t r a c t e d f r o m D a n d d i v i d e d b y t h e w i d t h o f BB (which is the half-width of the expected leading e d g e s l o p e ) . With n o e r r o r this should e q u a l t h e e x p e c t e d s l o p e . The d i f f e r e n c e is t h e STL e r r o r s i g n a l . T h i s is i l l u s t r a t e d i n Fig. 3(b). Each loop uses a selection of the gates AA-EE, t o g e n e r a t e a n i n p u t t o a 2nd o r d e r f i l t e r . T h e STL g e n e r a t e s a c o n t r o l s i g n a l w h i c h is u s e d t o e v a l u a t e AA-EE o n s u b s e q u e n t s a m p l e s . T h e AGC l o o p c o n t r o l s t h e AGC c i r c u i t w i t h i n t h e r e c e i v e r . T h e HTL generates a high precision estimate of h e i g h t ( d e l a y time). T h e m o s t s i g n i f i c a n t bits (coarse) control the timing of the d e r a m p p u l s e (s e e A p p e n d i x ) w h i l e t h e l e a s t s i g n i f i c a n t bits (fine) c o n t r o l t h e spectrum analyser fine tuning.
L o o p (STL)
AGC L o o p * t h e r e t u r n w a v e f o r m is a s s u m e d t o h a v e a n i d e a l s h a p e , d e f i n e d b y t h e h e a v y l i n e in Fig. 3. T h i s s h a p e is a s i m p l i f i e d f o r m o f t h e i d e a l r e t u r n , a s d e f i n e d b y Brown (1977). T h e t r a c k e r will o p e r a t e c o r r e c t l y with real signals, whose shape often varies considerably from this simplified form. S e t s o f c o n t i g u o u s f i l t e r a r e combined fully adaptively, to form gates
AUTO CALIBRATION A novel method of auto-calibration
of the t i m i n g p a t h i n t h e RA h a s b e e n d e v i s e d . T h i s is i n t e r e s t i n g b e c a u s e it i l l u s t r a t e s one of the subtleties of the instrument: the r e q u i r e d time p r e c i s i o n is muc h f i n e r t h a n t h e e q u i v a l e n t time r e s o l u t i o n o f t h e f i l t e r b a n k . T h e e c h o w a v e f o r m is s a m p l e d a t a r e s o l u t i o n o f a b o u t 3 ns , w h i l e t h e r e q u i r e d 10c a p r e c i s i o n implies a time p r e c i s i o n o f
36th IAF Congress, Stockholm, Sweden
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HTL e r r o r signal ° ................
H ................ ~ ' - - J ~
Reference Level
/
AGC loop error signal
I--
I-
AA
BB
I
CC
DD
EE
Y
STL error signal
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I-
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-I-
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DD
Conceptual o p e r a t i o n or t h e SMLE tracker : HTL and AOC STL
a b o u t 0.7 n s , a f a c t o r o f a b o u t 5 b e t t e r . In t h e m e a s u r e m e n t modes t h i s is a c h i e v e d because of the " curve-fittlng" nature of the p a r a m e t e r e s t i m a t o r . In o r d e r t o d e t e r m / n e a n y c h a n g e s in time d e l a y w i t h i n t h e RA it is a r e l a t i v e l y simple m a t t e r t o couple some o f t h e t r a n s m i t t e r o u t p u t book i n t o t h e r e c e i v e r (Fig. 4). This p r o d u c e s a single c h i r p echo. e q u i v a l e n t t o a p o i n t t a r g e t . However, c u r v e - r i f t i n g would n o t w o r k with s u c h a s i g n a l a n d t h e b e s t p r e c i s i o n would be a b o u t 3ns, which would n o t be a d e q u a t e f o r calibration. Therefore the a v e r a g i n g o v e r 50 p u l s e s p e r f o r m s d w i t h i n t h e SPSA h a s b e e n e x p l o i t e d , b y s h i t t i n g e a c h e c h o in t h e 50 pulse a v e r a g e using a comb/nation o f t h e coarse a u d fine t u n i n g described above. The normal c u r v e - f i t t i n g SMLE estimator can t h e n be u s e d . By t h i s m e a n s a l e a d i n g edge c a n be c o n t r u c t e d , Fig. 5. However, b e c a u s e t h e l o o p r e c e i v e s a n i n p u t s i g n a l o n c e e v e r y 50 pulses (instead o f once p e r pulse) t h e
I
1 Fig. $
I
16
I
32
I
1.8
k leading edge generated discrete tones
!
64 f r o m 50
s y s t e m is slowed down b y a f a c t o r 50. This t e c h n i q u e is called c l o s e d - l o o p c a l i b r a t i o n . A much f a s t e r (-150 ms) o p e n - l o o p c a l i b r a t i o n will also be p e r f o r m e d a t i n t e r v a l s of a b o u t $ minutes, in which the l o o p s a r e n o t closed a n d t h e e r r o r s i g n a l s themselves are used to determine the offset, using • characteristic curve dcterm/ned using closed-loop calibration data. Using t h i s m e t h o d c a l i b r a t i o n o f most o f t h e s i g n a l p a t h in t h e i n s t r u m e n t is achieved to the normal o p e r a t i n g precision. Only t h e a n t e n n a a n d t h e F r o n t End E l e c t r o n i c s (which a r e p a s s i v e a n d t h e r e f o r e r e l a t i v e l y s t a b l e ) a r e excluded. SYSTEM ASPECTS
J
TWT
Fig. 4
~
Line Delay
I"~'~
IJ Expanderl saw I
Auto-calibration signal path
T h e o v e r a l l RA d a t a flow is s h o w n in Fig. 6. This d i a g r a m makes c l e a r t h e d i s t i n c t i o n b e t w e e n t h e IDHT a n d OBDH d a t a b u s s e s . The t w o - w a y flow o f c m m m n d s a n d h o u s e k e e p i n g d a t e o n t h e OBDH b u s is e x t e n d e d b y t h e
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Peaceful Space and Global Problems of Mankind
I_
Commands
Fest Delivery Products
Housekeeping
MMCC
I
-I
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Fig. 6
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RA d a t a flow t h r o u g h system
S - b a n d t e l e m e t r y link. via t h e p r i m a r y TTC s t a t i o n a t K i r u n e t o t h e Mission Management a n d C o n t r o l C e n t r e (k~ICC). Here t h e s t a t u s o f t h e i n s t r u m e n t is m o n i t o r e d , a n d t h e complex p a y l o a d o p e r a t i o n s p l a n implemented a s a s e r i e s o f telecemmands. T h e S c i e n c e Data o n t h e IDHT b u s is t r a n s m i t t e d a t X - b e n d , b o t h in r e a l - t i m e a n d a s a dump f r o m t h e o n - b o a r d t a p e r e c o r d e r which r e c o r d s m N s u r e m s n t s made while o u t o f c o n t a c t with a g r o u n d s t a t t l o ~ This d a t a t r a n s m i s s i o n is r e c e i v e d a t K i r u n a a n d o t h e r s t a t i o n s . At t h e t o p e - d u m p s t a t i o n s a quasi-real-t~ d a t a p r o m m s o r is u s e d t o generate the Fast Delivery products. These will h a v e a d e l a y o f l e s s t h a n 3 h o u r s w/thin the ground station, and cannot t h e r e f o r e b e full p r e o i s l o n p r o d u c t s b e c a u s e m a n y r e q u i r e d a u x i l i a r y d a t a will n o t be a v a i l a b l e (e.g. p r e c i s i o n a l t i t u d e restitution). The Fast Delivery p r o d u c t s
foreseen f o r t h e RA a r e stlptiflmmt wavehldght, windspeed and altitude (accuracy ~I m). Raw d a t a f r o m t h e X - b a n d r e e e l v l n g s t a t i o n s will be a r c h l v e d , a n d u l t i m a t e l y p r o c e s s e d t o full p r e c i s i o n , a t t h e Proeis/cm Proooeslng a n d Are]hiving Facilities (PAP's). These facilities a r e
provided by various m r states, and do n o t t o m p a r t o f the main proeuremmtt of ERS-I. T h e PAF's a r e n o t e x p l i c i t l y s h o w n in Fig. 6. CALIBRATION T h e o v e r a l l c a l i b r a t i o n o f t h e a l t i m e t e r is a a m l t i s t e p p r o e e d u r ~ T h e i n s t r t m m n t will be c a l i b r a t e d a n d v a l i d a t e d b e f o r e l a u n c h b y a c o m b i n a t i o n o f t e s t mmmuremmtts, a n a l y s i s a n d e c h o simulation. A R e t u r n S i g n a l Simulator (RSS) is being developed as p e r t o f t h e i n s t r u m e n t t e g t - e q u i p m m t t . This d e v i c e wUl be i n s t a l l e d in p i e c e o f t h e a n t e n n a , and can simulate echoes with an a p p r o p r i a t e d e l a y tiara a n d w i t h m o d u l a t i o n
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which r e a l i s t i c a l l y simulates o c e a n o r o t h e r s u r f a c e s . This device d e p e n d s o n a n i n t i m a t e c o n n e c t i o n t o t h e a l t i m e t e r , s i n c e it inhibits and generates various trigger s i g n a l s w i t h i n t h e i n s t r u m e n t . T h i s is e s s e n t i a l in o r d e r t o a c h e i v e t h e r e q u i r e d p r e c i s i o n a n d a c c u r a c y . The RSS will be u s e d , with a n a l y s i s o f i n d i v i d u a l t e s t m e a s u r e m e n t s , t o f u l l y c a l i b r a t e all t h e altimeter measurements before launch. Following l a u n c h t h e i n t e r n a l c a l i b r a t i o n t e c h n i q u e d e s c r i b e d e a r l i e r will be used. b u t a n e x t e r n a l c a l i b r a t i o n is still n e c c e s s a r y t o p r o v i d e a complete c a l i b r a t i o n o f t h e I n - o r b i t s y s t e ~ This e x t e r n a l c a l i b r a t i o n will be r e s t r i c t e d t o time delay. A l t h o u g h m e a s u r e m e n t s o f t h e p o w e r return from special transponders (either a c t i v e o r p a s s i v e ) will b e made. t h e e x p e c t e d a c c u r a c y o f s u c h m e a s u r e m e n t s will be i n s u f f i c i e n t t o allow c a l i b r a t i o n , a n d t h i s is r e g a r d e d m o r e a s v a l i d a t i o n . External calibration of the significant
wavehe/ght ( e f f e c t i v e l y t h e echo leading e d g e s l o p e ) will n o t be p e r f o r m e d . T h e e x t e r n a l c a l i b r a t i o n o f t i m e - d e l a y will be p e r f o r m e d b y c o m p a r i s o n w i t h m e a s u r e m e n t s from a ground-based laser to a laser retro-refloctor fitted near to the altimeter a n t e n n a . A s i / l a r t e c h n i q u e was u s e d with S e a s a t (Kol~nkiowicz a n d Martin. 1982). u s i n g a l a s e r o n t h e i s l a n d o f Bermuda. A number of problesm were encountered, including : * t h e a l t i l m t e r c o u l d n o t make m e a s u r e m e n t s o v e r a 25 km s e g a l e n t a r o u n d t h e i s l a n d , so e x t r a p o l a t i o n f r o m m e a s u r e m e n t s in t h e o p e n o c e a n was r e q u i r e d . This r e q u i r e d c o r r e c t i o n s f o r tides, w a v e h e i g h t etc. *
atmo6phertc corrections for altimeter tiara-delay.
F o r ERS-1 a n u m b e r o f p o s s i b i l i t i e s h a v e
Peaceful Space and Global Problems of Mankind
been suggested, including the Dakar p e n i n s u l a r . On e n o v e l p r o p o s a l is t h e G r e e n l a n d i c e - s h e e t . In t h e c e n t r e o f t h e ice-sheet the altimeter could track the i n v a r i a n t i c e s u r f a c e (which c a n be s u r v e y e d ) u s i n g t h e O c e a n Mode. a n d i t c o u l d continue over the laser site. Tropospheric c o r r e c t i o n s w o u l d a l s o be m u c h e a s i e r w i t h the effective absence of water vapour. Expected problems due to small-scale ionospheric variability and radar p e n e t r a t i o n o f t h e i c e s u r f a c e c o u l d be s o l v e d b y u t i l i s i n 8 t h e d i f f e r e n t i a l PRARE measurements, and transponders fixed to the surface, respectively.
MULTISENSOR D A T A Measurements from o t h e r s e n s o r s on ERS-I will b e u s e d t o p r o v i d e c o r r e c t i o n s t o t h e a l t i m e t e r d a t a . T h e ATSR/M will b e u s e d t o mak e m e a s u r e m e n t s o f a t m o s p h e r i c w a t e r vapour, particularly using the nadir-viewing microwave radiometer. The water vapour c o r r e c t i o n is t h e m u s t v a r i a b l e o f t h e atmospheric corrections and in-situ m e a s u r e m e n t s a l o n g t h e same RF p a t h will b e most valuable. The use of t h e d i f f e r e n t i a l ( S - b a n d . X - b a n d ) PRARE m e a s u r e m e n t s i n o r d e r t o a s s e s s t h e T o t a l E l e c t r o n C o n t e n t (TEC) of the ionosphere has been referred to above. The RA f o o t p r i n t s a r e c o i n c i d e n t w i t h t h o s e o f t h e ATSR/M, a l l o w i n g j o i n t s t u d y of, f o r example, o c e a n c u r r e n t s , combining sea-surface temperature measurements from t h e ATSR/M w i t h c o i n c i d e n t e l e v a t i o n m e a s u r e m e n t s f r o m t h e RA. T h e AMI d o e s n o t h a v e c o i n c i d e n t s w a t h s w i t h t h e RA. s o j o i n t s t u d i e s must make allowances f o r t e m p o r a l o r spatial differences between measurements. However, slowly v a r y i n g p h e n o m e n a (eg level a n d e x t e n t o f c l o s e d - l a k s e ) s h o u l d be o b s e r v a b l e in t h i s w a y . C o m b i n i n g ERS-I measurements with those from other s a t e l l i t e s (eg TOPEX/POSEIDON. NROSS etc.) is a l s o a n a r e a o f a c t i v e i n t e r e s t . CONCLUSION
T h e m a k i n g o f RA m e a s u r e m e n t s a s p e r t o f t h e E R S - t s y s t e m is a c o m p l e x a n d d e m a n d i n g t a s k . Some o f t h e c h a r a c t e r i s t i c s o f t h e system have been described, and these reveal some o f t h e n o v e l d e s i g n f e a t u r e s o f t h e instrument and the interrelationships between many elements of t h e systmn.
APPENDIX OPERATING PRINCIPLES T h e E R S - I R a d a r A l t i m e t e r , in common w i t h t h e S e a s a t i n s t r u m e n t , o p e r a t e s in t h e p u l s e - w i d t h l l a d t e d mode. a n d u s e s t h e f u l l deramp system of pulse compression. The m e a n i n g o f t h e s e t e r m s will be d e s c r i b e d h e r e . Fig. 7 s h o w s a n a r r o w p u l s e f r o m a n altimeter encountering a flat surface, which is a s s u m e d t o be r o u g h on t h e s c a l e o f t h e r a d a r w a v e l e n g t h , so t h a t , in t h e a b s e n c e o f antenna pattern effects, the returned power is p r o p o r t i o n a l t o t h e a r e a i l l u m i n a t e d . T h e n o m i n a l a n t e n n a b e a m w i d t h is i n d i c a t e d b y
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t h e d a s h e d l i n e s f r o m t h e a n t e n n a . As t h e pulse propagates the illum/nated area. and therefore the returned power, grows rapidly. E v e n t u a l l y a n a n n u l u s is f o r m e d , a n d t h e g e o m e t r y is s u c h t h a t t h e a r e a of t h i s remains constant as the diameter Increases. H ow e ve r, t h e r e t u r n e d p o w e r d o e s n o t r e m a i n constant, since the antenna pattern causes an attenuation as the annulus expands. The distribution function of surface scatterers, for the normal case where the reflecting s u r f a c e is n o t f l a t , a l s o i n f l u e n c e s t h e r a d a r e c h o . T h e e c h o w a v e f o r m is t h e convolution of the probability distribution of the heights of surface scatterers and the flat surface response described above, which is i t s e l f t h e c o n v o l u t i o n of t h e point t a r g e t r e s p o n s e ( w h i c h is r e l a t e d t o t h e pulse shape) and the two-way antenna p a t t e r n . S u c h a mode o f o p e r a t i o n is c a l l e d p u l s e width limited b e c a u s e n o t all of t h e t a r g e t is i l l u m i n a t e d s i m u l t a n e o u s l y , a n d t h e limit t o t h e r e c e i v e d p o w e r is controlled by the illumination.
The full deramp pulse compression technique i s s h o w n i n Fig. 8. In a c o n v e n t i o n a l p u l s e c o m p r e s s i o n r a d a r a c h i r p waveform is obtained by /repulsing a dispersive delay l i ne . T h i s s p r e a d s t h e r e q u i r e d e n e r g y o v e r time t o allow r e d u c e d p e a k p o w e r . When t h e r a d a r e c h o is r e c e i v e d i t is p a s s e d t h r o u g h another dispersive delay line with an inverse characteristic. For a point target a s i n g l e p u l s e e c h o is t h u s g e n e r a t e d f r o m t h e incoming chirp. This approach could cause p r o b l e m s w i t h i n t h e RA f o r t h e f o l l o w i n g reasons : * To m e e t t h e c o m p r e s s e d p u l s e w i d t h a n d peak power requirements a very high t i m e - b a n d w i t h p r o d u c t is r e q u i r e d f o r t h e c h i r p . T h i s c a n be a c h i e v e d o n transmission by frequency multiplying a chirp with a smaller time-bandwidth p r o d u c t , b u t s u c h a n a p p r o a c h c a n n o t be taken for the matched compression filter. * To s a m p l e t h e e c h o w a v e f o r m w i t h a resolution of 3 ns extremely high speed t i s d n g a n d o t h e r c i r c u i t r y w o u l d be required. The a p p r o a c h t a k e n in all known s p a c e a l t i m e t e r s s i n c e S e a s a t is t o t r a n s l a t e f r o m t h e time d o m a i n i n t o t h e f r e q u e n c y d o m a i n v e r y e a r l y in t h e r e c e i v e r , b y t h e full d e r a m p m e t h o d , w h e r e t h e i n c o m / n g s i g n a l is mixed w i t h a r e p l i c a o f t h e t r a n s m i t t e d c h i r p . Timing t h e n be c ome s t r a n s l a t e d t o spectrum analysis and a matched compression f i l t e r is n o t r e q u i r e d , i n Fig. 8 a representation of the frequency-time c h a r a c t e r i s t i c o f t h e c h i r p is s h o w n b y t h e transmitter. A representation of the echoed s i g n a l is s h o w n b y t h e f r o n t e n d o f t h e r e c e i v e r . T h i s is a s e r i e s o f - , a n y overlapping chirps, each echoed from an individual scatterer. The density of these c h i r p s a l o n g a c u t p a r a l l e l t o t h e time a x i s would h a v e t h e s h a p e of t h e r e t u r n e d power envelope. Individual chirps vary from this ideallsed waveform because of interference effects between the echos from different scatterers.
294
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has the duration of any individual echoed c h i r p , b u t m m n o t e n c o m p a s s , in a d d i t i o n . t h e s p r e a d o f a r r i v a l times. H o w e v e r , t h e s k e w is o n l y a b o u t o n e p e r t in I000.
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The next Important stage is the Spectrum Analyser! here the spectrum of the tones is found. TA/s speatrum exactly represents the time s t r u c t u r e of the echo waveform Finally this inlroramtton is used by the Parameter h t t l m t o r . This step is essential in order to provide the estimate of when the next echo is exl:xmted to return, for the chirp
series of tones is generated, centred on the f i r s t IF. where a o u t parallel to the f r e q u e n c y a x i s is e q u i v a l e n t t o t h e time axis cut referred to above. The r e p r e s e n t a t i o n o f t h e s e t o n e s in Fig. 8 s h o w s a s l i g h t s k e w ( w h i c h is highly e x a g e r a t e d ) . T h i s is b e c a u s e t h e LO c h i r p
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36th IAF Congress, Stockholm, Sweden
a c c u r a t e t h i s e s t i m a t e m u s t be, t h e full b a n d w i d t h o f t h e $ p e c t r u a A n a l y s e r Is e q u i v a l e n t t o a h e i g h t window o f a b o u t 30 • in t h e O c e a n Mode. The maximum h e i g h t r a t e s p e c i f i e d in Table t in t h i s m e is 50 m/s. If t h e h e i g h t e s t i m a t e w e r e n o t c o n t i n u o u s l y u p d a t e d t h e s i g n a l c o u l d be c o m p l e t e l y l o s t in less t h a n a s e c o n d . REFERENCES Brown, O. S. (1977). The a v e r a g e impulse r e s p o n s e of a r o u g h s u r f a c e and its a p p l i c a t i o n . IEEE T r a n s o n A n t e n n a s a n d P r o p a g a t i o n , AP-2$, 1, 67-74. M a c A r t h u r , J. L. (1976). Design o f t h e S e a s a t - A r a d a r a l t i m e t e r . WFS-IEEE O o e a n s - 7 6 , lOB___. Kolenklewicz, I~ a n d Martin, C. F. (1982). SEASAT a l t i m e t e r h e i g h t c a l i b r a t i o n . J. G e o p h g s . Res., 87. C5, 3189-3197.
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