Satellite Radar Altimeter

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SYSTEM DESI GN AND ADVANCED TECHNOLOGY DEVELOPMENT FOR A LI GHTWEI GHT, LOWCOST, SATELLI TE RADAR ALTI METER

The si mul ati on i ncl uded the i nteracti ons showni n Fi gure 2. Radar al t i meter subsystemoutput si gnalparameters are shown i n Tabl e 3 . The i ntegrated

Autonet i cs St r ategi c Syst ems D vi si onDefense El ect r oni cs

Rockwel l I nt ernat i onal Corporati on

P .O . Box 4192, Anahei m CA 92803-4192

Abstract-A light weight (16.1 kg), low power

(35-watt) small satellite radar altimeter suitablefor launch on a Pegasus launch vehicle is described.

The altimeter is capable of measuring ocean surface

height with a precision of 3.5 cm from an altitude

of 800 km. Risk reduction is achieved by the use

of non-developmental items and devices demonstrated

under IUD. This radar altimeter will permit

8CientifiC and military ocean topography data to be

collected well into the 2lst century.

JNTRODUCTION

A sat el l i t e radar al t i met er measures t hedi st ance between i t sel f and t he surf ace of theocean di rectl y bel ow w t h a preci si on of a fewcent i met ers. Si nce t he sat el l i t e' s orbi t can bei ndependentl y ascer t ai ned, the shape of t he ocean

surf ace can be preci sel y determned al ong a l i neunder t he sat el l i t e. The resul t i ng shape can becompared t o the ocean' s l ong-t erm geopotent i alsur f ace ( geoi d) and ocean mesoscal e f eat uresi dent i f i ed. These f eat ures have sci ent i f i c andml i t ary i mport ance. ' ** Radar al t i meters were carr i edaboard SKYLAB, SEASAT, GEOSAT- I , ERS- 1 and TOPEXsat el l i t es. The abi l i t y to provi de preci si onal t i meter data i n the f uture depends on t hedevel opment of a l i ght wei ght radar al t i metercompat i bl e w th a smal l satel l i te bus and a rel at i vel yi nexpensi ve l aunch vehi cl e such as the ai r - l aunchedPegasus. 3 Thi s paper descr i bes systemdesi gn of aradar al t i meter sat i sf yi ng t hese requi rement s andIR&D per f ormed by Rockwel l t o reduce r i sk associ atedw t h the concept .

RADAR ALTIMETRY HISTORY

Space- based radar al t i metr y began w t h Skyl abi n 1973 f ol l owed by ot hers shown i n Tabl e l . 4 ,5

Earl y mssi ons were concerned w t h mappi ng theeart h' s geopot ent i al f i el d whi l e more recent mssi onsf ocused on measurement of ocean t opographi calf eat ures. GEOSAT- 1 i s a representat i ve exampl e ofa radar al t i met er sat el l i t e. GEOSAT-1 was a 1450-

pound, compl et el y redundant sat el l i t e w t h a 207-

pound, 165-wat t r adar al t i meter devel oped by t heJohns Ho~!:;ns Uni versi t y Appl i ed Physi cs Laboratoryand l aunched on an At l as- E.

Tabl e 1. Radar Al t i met er Sat el l i t es

1s 1TOPEX

SATELLITE RADAR ALTIMETER REQUIREMENTS

Per f ormance speci f i cati ons based on the provenGEOSAT- 1 radar al t i meter are shown i n Tabl e 2.

Tabl e 2. Radar Al t i meter Per f ormanceSpeci f i cat i ons

iwxlePreci s on 3. 5 cm (10) @ 2m

si gni f i cant wavehei ght ( SWH)

Measurement r ate 10 Hz

Modul ati on l i near FMBandw dth 32 0 MHz

PRF 1020 Hz

Frequency 13. 5 GHz

Waveform

Durati on 102. 4 ps

Ti me- t o- t r ack i sec

I 15 cmreci si onAccur acy 10. 1 SWH or 0. 5 m

whi chever i s l arger

wave hei aht CSWKL

Sur f ace radar cross s ec t unL BXLAccuracy 1 dBCal i brat i on mode requi red for RCS

and hei ght bi as

Detai l ed sat el l t e bus desi gn perf ormed byRockwel l ' s Space Syst ems D vi si on i ndi cated a maxi mumal l owabl e wei ght and average power f or t he al t i meterof 17- kg and 35- wat t s, r espect i vel y, i ncl udi ngredundancy. Thi s al l ocati on was based on anoveral l sat el l i t e wei ght of 272 kg and a 240-wat t

sol ar panel . The resul t i ng sat el l i t e can be pl acedi n an 800-kmci rcul ar orbi t by a Pegasus l aunchvehi cl e.

PERFORMANCE ANALYSIS

r-



Tabl e 3 . Al t i meter out put si gnal parameters

Number of I&Q sampl esI&Q sampl e rate 1. 25 MHzAnti al i asi ng f i l ter 1. 28 MHz

Range span extentSampl e si ze 8-bi t sSampl e rate 1020 Hz

I EU bus r ate 2. 5 Mbps

The al t i meter processes the recei ved si gnalusi ng a st retch techni que where r ange resol ut i on i sgi ven by t he expressi on,

60=MaximumVelociW= I26Wsec

Maximum Acceleration =

0.84m/sec2

Track Used

Derived From

GEOSAT Data ITonga Trench

0 I1 I

600 800 1,000 1,200 1,400

Time (sec) MTD 920116-2821

Fi gure 1. Perf ormance w a s Obtai ned by Si mul ati onl rsing Real i st i c Sea Sur f ace Data

f uncti ons, i ncl udi ng hei ght t racki ng. A di gi t alsi gnal processor i n t he I EU per f orms a Fast Four i erTransf orm ( FFT) of t he t i me-domai n al t i met er out putt o t ransf orm t he recei ved ret urn i nt o ampl i t udeversus r ange bi ns.

Common Antenna

PretriggerT (Blanking) PulseI

Radar Altimeter Subsystem II I

128 ComplexSamples Calibration '-

a

IEU in TT& C Subsysterr

Coarse Heightand Rate, AGC,SelectiveAttenuation,PRI, Mode

t

71FFT and Fine Heighta p Tracker for HeightAGC Tracker LoopAcquisition Processing and ControlCalibration Processing and ControlCombine Two 50-Pulse Averages10.2 Hz Telemetry

b Telemettv Data

Ground Segment

SWH and RC S Estimation

User Seg me nt MTD 920204-3034

Fi gur e 2. The I nt egrated El ectroni c Uni t ( I E U J

Cont rol s the Al t i meter

I -

where,c = 3 x 10 8 m s

Bc = chi r p bandw dt h.

For B = 320 MHz, t he resul t i ng range resol ut i oni s 47 cm The accuracy (oh)w t h whi ch t he ocean' ssurf ace can be measured can be rel ated to thesi gnal - t o- noi se (SNR) i n a range cel l by,

1+ 2 I SNRoh=-

2 K , K , a

where,

KW = sl ope of ocean returnKG = t rack gate parameter

PRF = pul se repet i t i on f requencyTi nt = t rack f i l t er i nt egrati on t i me.

N = PRF x Ti nt

The deri vati on of Equat i on 2 assumes oceanscatteri ng i s Rayl ei gh, N measurements are averaged,and the t rack gate i s opt i mal l y f ormed based on theocean surf ace' s si gni f i cant wave hei ght (SWH).6 Forexampl e, i f SNR = 18 dB, Kw = 0. 713 (corresponding toa SWH of 2 m, = 1, PRF = 1020, and Ti nt = 1 sec,then oh = 2. 13 cm Note that Equat i cn 2 i ndi cates thatas l ong as SNR remai ns above 12 dB, accuracy i s l i mt edpri mari l y by ocean surf ace scat ter i ng stat i st i cs.

A more detai l ed si mul ati on of the hei ght t racki ng

process was created and i ncorporated actual GEOSAT-1al gori thm. The ocean return wavef orm si mul ati oni ncl uded the anal yti c convol ut i on of ( 1) the averagef l at sea-surf ace i mpul se response, ( 2) a Gaussi anhei ght probabi l i t y densi ty of ocean- surf ace specul arpoints, and ( 3) a Gaussi an systempoi nt - target response.Other model ed ef f ects were a Gaussi an antenna gai npat tern and spher i cal earth correcti ons.

The t rack al gor i t hmwas the same U- P t rackerused on GEOSAT- 1. Hei ght est i mates were i nput t ot he f i l t er every 49 ms ( i . e. , each f i l t er i nputconsi st s of 50 r awmeasurement s averaged together ) .Automati c gai n control ( AGC) out put was used tomai nt ai n a const ant pul se l evel t o avoi d hei ghtt racki ng er rors whi ch are a funct i on of si gnall evel . vol t ages proport i onal t o SW H ( Vsm) and

sat el l i t e at t i t ude ( Vat t ) deri ved f romt he recei vedsi gnal were al so computed f romthe si mul ated di gi t i zedwavef orm Af t er r aw hei ght ( and raw hei ght r ate)have been cal cul ated on board, the hei ght measurementi s corr ect ed dur i ng ground processi ng usi ng the SWHand sat el l i t e at t i t ude paramet ers.

7



Si mul ated ground cor recti ons are based onbi cubi c pol ynom al s i n Vswh and Vat t si m l ar t ot hose used f or TOPEX. The coef f i ci ent s are f oundf roml i near l east squares based on dat a af t er a-P

t racker and AGC l oop st abi l i zati on.Fi gure 3 shows predi cted range preci si on

versus SNR w t h PRF as a parameter and rangepreci si on versus sat el l i t e of f - nadi r angl e w t h SWH

as a parameter. The upper curve shows that a SNR =

18 dB and a PRF of 1 0 2 0 Hz resul t i n a rangepreci si on of 2 . 4 cm consi derabl y bet t er t han t he

requi red 3 . 5 cm Li kew se, the l ower cur ve showsthat a sat el l i t e at t i t ude of f - nadi r angl e of 0 . 7 5

degr ees and SWH of 2 m w l l resul t i n a rangepreci si on of 2 . 8 cm

Radar range equat i on parameters used to achi evea SNR 2 1 8 dB ( w t h3 dB margi n) are shown i n Tabl e 4 .

Parameter

tecei ved Power (S)

Peak RF power (P, )Transm t t er

l osses (L, )Antenna gai n (G )Wavel engt h (1)

Ref l ecti vi ty ( 0 0 )

Sur f ace area ( A)4rr

Al t i t ude ( H)At mospheri c

At t i t ude

Recei ver l oss (L,)Processi ng l oss ( Lp)

Subt otal ( dBm

l oss (Latm

l oss (L* t t )

Recei ved power ( dBm

Selected Requirements Yield 2 20 %Margin at SW H =2 m

PRF Requirement: 1,020 HzSNR Requirement: 2 18 dBOff-Nadir Angle Requirement: s .75 deg

Value

7 W0. 9dB

4 0 . 3

2 . 2 2 cm9dB63.72 dBsm1 2 . 5 7

800 km

1 dB

0 db

2 . 1 1 dB1 dB

PRF (Hz):

p 10n

0 5 10 15 ' 20

SNR (dB)

8smcmcabon I margin

11 I I I II

0 0.2 0.4 0.6 0.8 1

Off-Nadir Angle (deg)

MTD 920116-2818

Figure 3. SNR, PRF and Attitude Control Accuracy

were Selected to Provide Robust Range Precision

Measurements

The si mul ati on al so predi ct ed SWH esti mati onaccuracy w t h the resul t s shown i n Fi gure 4 . Thedi sconti ni 7; ' i es i n t he per f ormance curve are t heresul t of i hangi ng t rack gat e si zes as a f unct i on

of SWH.

RADAR ALTIMETER IMPLEMENTATION

The l i ght wei ght r adar al t i met er i ntenti onal l yuses a majori ty of nondevel opmental i tem to mni mzecost , and where necessary, demonst rati on of cri ti calcomponents under I R&D to reduce r i sk. A l ayout of theal t i meter mounted on the f orward bul khead of the

Table 4. Altimeter Parameters were Selected to

Provide a 21 dB SNR based on GEOSAT-1 OceanRef1 c t i ty Da ta

P, G, ~~L~G~A A = I cHc/Bc = 3x108m/s=

(4rr )3H4LatmattL,LpB = 320 MHZ

Noi se power ( N)Bol t zmann' s

Reference

Noi se fi gure (F )

Processi ngbandw dt h (B)Subtotal

const ant ( k)

t emperature (To

Noi se power ( dBm

Si gnal - t o- noi serati o ( dB)

EW.Yn13W-1

1 . 3 8 ~ 1 0 - ~ '

mw/K / Hz290K

2 . 3 3 dB9.77 KHz

38. 5

80.6

9 . 0

63.72

1 9 1 . 8 2

2 4 . 6 2

39.89

6 4 . 5 1

21.07

-dEm

0 . 9

33.06

32.9E

236.11

1 . 0

0

2.13

1 . 0

307. li

115.3E

1 9 8 . 6 (

2.3:

200.9:

136.4 :

lo

=Performance

MarginE

SW H (m )

MTD 020121 2930

Figure 4. SWH Measurement is Accurat e

Over 0 to 20 m SWH

satel l i te i s shown i n Fi gure 5 . Al t i meter el ementsand bui l t - i n redundancy are i l l ustrated i n Fi gure 6.

The wei ght and power consumpti on of the al t i meter areal so shown i n t he i nsert contai ned i n Fi gure 6 .

Two approaches for LFM chi rp generati on wereexamned as part of the I R&D ef f ort . The f i rst was anovel approach8 where the 102.4 ps, 32 0 MHz w deband

EPS.50713.PO1-38



Altimeter

-Amplifiers

Ambient Temperature Load

MTD 920120-2918

RadiometerSky HornA

Figure 5. The Altimeter Consists of Two Redundant Sides ( A & B ) Sharing

a Common Transmi /Recei e Swi h

b o IEU-POrt Waveguide

Solid-statePower Amplifier

Peak RF Power

7 t o 9W

I

Electronics I

Low-NoiseElectronics Amplifier

l b I Solid-state JAltimeter

II r I

(Side6)1 Electronics Power Amplifier

Component Weight (kg)-TransmiVReceive Switch (1) 1.6

+Port Waveguide Switch (2) 0.3

Low-Noise Amplifier(2) 0.73

Altiineter E lectronics (A and B) 7.3

Solid-State Power Amplifier (2) 5.5

Waveguide 0.7-otal 16.13

DC P ower (W)

1.2

0.8

19.0

14.0

35

Source Heritage_ _ _ ~

ElectromagneticSciences

Transco


Rockwell


Commercial Supplier

ERS-1, TOPEX

ERS-1

Mars Observer

MTAS, RIMS, IR&D

NROSS

-

MTD 920121-2914

Figure 6. Use of Nondevelopmental Items and Components Demonst rated

Under IR&D were Emphasized



l i near FM pul se was synthesi zed f romei ght l i near FM

subpul ses 12.8 ps l ong each w t h a bandwdth of 40 MHz.

The desi red f requency versus t i me character i st i c i sshown i n Fi gure 7. Two surf ace acoust i c wave ( SAWdevi ces operati ng i n “pi ng- pong”mode were used generatet he cont i gnuous subpul ses. Thi s technique was cal l edt he Hybri d Stepped Frequency ( HSF) techni que. HSFbreadboard hardware i s shown i n Fi gure 8 and aspectrumanal yzer output shown i n Fi gure 9.

A second approach usi ng di rect di gi tal synthesi s(DDS) technol ogy si ml ar t o that used for t he GEOSAT-

32 0

28 0

24 0

2 200E.x

16 0

t 12 0

a)

3 80a

40

0

*

F i g

40-MHz LFM

Subpulse Chirp

Time-Bandwidth Product I 2,768

I l l

12*8 Subpulse DurationPS I I-02.4 ps Pulse Duration-4

re 7 . HSF was Used to S y n t h e s i z e the

MTD 920204-3030

Wideband LFM Pu ls e Pr ove n on GEOSAT-1 wi t h Eig ht

Con t ing uous LFM Subpu lse s wi th D i f f e r e n t C e n t e r

Frequencies

1 chi rp generator was al so examned under IR&D I nthi s case, the w deband chi rp waveformwas synthesi zedf roma si ngl e baseband l i near FM pul se 102.4 ps l ong.The baseband pul se consi st ed of a pul se w th astart i ng f requency of - 20MHz varyi ng l i nearl y to +20

MHz. The baseband pul se I &Q channel s were combi ned i na si ngl e si deband mxer to forma real si gnal at 125MHz. The real si gnal was t hen l i mted and f requencymul t i pl i ed by a factor of ei ght to achi eve the f ul l 320

MHz bandw dth. A bl ock di agramof the I R&Dhardware i sshown i n Fi gure 10 al ong w th the baseband I&Q

out put s. Thi s i mpl ement ati on i s based on a Stanf ordTel ecommuni cati ons ( STEL) Chi r p- Numeri cal l yCont rol l ed Osci l l at or ( C- NCO) speci f i cal l y desi gnedf or chi rp waveform generat i on. The STEL C- NCO i st he smal l ret angul ar devi ce i n t he upper l ef t - handport i on of Fi gure 11.

F i g u r e 9 . HSF Techn olog y Gene rat es the Wideband

P u l s e f r o m C o n t i g u o u s L i n e a r FM S u b p u l s e s .

Fi gu re 8. HSF Perf orma nce was Demonstr ated Under IR&D

PS50713POl-5

10--r-



Fine Track Inputs- Chirp Rate (Autom atically Updated Every 12.5 ns)- Pulse Duration

Initial A-Phase word

Coarse Acquisition InputA-Phase Word Every 0. 8 s

From ReferenceFrequency Gsnerator

DigitalCommanc

Logic

t80MH z

AD9721

ModulatorI SSF-IE I

Buffer

125 MHzFrom Reference

FrequencyGenerator

-b

ToUpconverter/Multiplier

Fine Track Pulse

(8192 4882.8 Hz Steps)(in 102.4 ps )

125 MHz- 0 MHz

Coarse Acquisition Pul-

(128 4882.8 HzSteps)(in 102.4 ps )

125 MHz-312.5 KH Z

~ ~

Fi gure 10. The STEL- 1180 C- NCO was t he Key El ement of t he DDS Demonst r at i on

As a r esul t of t hi s I R&D work, t he DDSapproach was f el t to be more mature and of l owerr i sk t han the HSF techni que and was sel ected asbasel i ne. However , i t was al so recogni zed t hat HSFt echnol ogy of f ers a pat h t o chi rp waveforms ofarbi t r ar tl y l arge bandw dt hs and enhanced rangeresol ut i on. I n addi t i on, a si gni f i cant degreeof j ammng i mmuni t y can be real i zed w t h HSF si ncesubpul se center f r equenci es can be generat edi n random or der w t hout af f ecti ng the st retchprocess.

Fi gur e 11. LFM Pul se Generat i on Usi ng Modern DDSTechnol ogy was al so Demonst rated

EPS 50713 POL6

Concl usi on

The radar al t i meter descri bed i n thi s paper w l lenabl e space-based al t i metr y data to be col l ected wel li nto the 21st century where hi gh perf ormance must beachi eved at modest cost . The al t i meter (excl udingsi gnal processi ng) wei ghs onl y 16. 1 kg and consumes35- watt s Dc power. I t consi st s of of f - the- shel fcomponents and technology demonst rated under I R&D andw l l resul t i n l i t t l e devel opment r i sk.

References

1.

2.

3 .

4 .

5 .

6.

1

Char l es C. Ki l gus, et. al . , "Moni t ori ng t heOcean w t h Navy Radar al t i meter Li ghtsat s, "AIAA Space Pr ograms & Technol ogy Conf erences,March 24- 27, 1992.J . Fi nkel st ei n, C. Ki l gus, "Ocean Moni tori ng by

Satel l i te Radar Al t i met er , " A I M Space Programs &

Technol ogy Conferences, March 24-27, 1992.CDR Paul F. Qui nn, USN, et . al . , 'US. Navy

Conferences, March 24-21, 1992Al f red R. Zi eger, et. al . , "NASA Radar Al . ti metf or t he TOPEX/ Posei don Proj ect , " Pr oceedi ngsof the IEEE , Vol . 79, No. 6 , J une 1991.J ohn L. MacArt hur, et . at . , "Evol ut i on of t heSatel l i t e Radar Al t i meter , " J ohns Hopki ns APL

Techni cal Di gest , Vol . 10, No. 4 , 1989) .J . L. MacAr t hur, "SEASAT-A Radar Al t i meter Desi gnDescri pti on, " The J ohns Hopki ns-Appl i ed Physi csLaboratory, Document SI K-5232, November 1978.G. S. Hayne, et . al . , "Corr ect i ons f or t heEf f ects of Si gni f i cant Wave Hei ght and At t i t udeon GEOSAT Radar Al t i meter Measurements, " J ournalof Geophysi cal Research, Vol . 95, No. C3,

March 15. 1990.

GEOSAT Fol l ow- on, A IM Space Program & Technol ogy

Satellite Radar Altimeter

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