PEER.REVIEWED ARIICTE TheUse of ReferenceSurfaces to Determine Repeat-OrbitVariability in SatelliteAltimetry W. Cudlip,H.A. Phillips,and A.H.W.Kearsley Abstract topography and radar backscatterare constant.Previous re- searchinto altimeter measurementsover land has shown that Two alternative techniquesfor estimating the variability of the there are several regions in radial orbit error for collinear tracks arc investigated using large, uniform desert the world that give "good-quality" radar echoes(Rapley et ol., L987i Geosataltimeter data. The first usessinusoidal fitting to ocean height differences around an orbit, and the second usesrela- Guzkowska et a1.,1990). For example, areasof the Libya tively flat areas of land (in the Simpson Desert, Australia, and Sand Sea,Taklimakan Shamo,and the Kalahari and Simpson the Antarctic Plateau).using a non-oceansurface rcquires Desertsare uniform over hundreds of kilometres and are- knowledge of the local surface slope, and we obtain ihis very flat (i.e., have large-scalesurface slopes less than 0.1'). principle, through the fitting of a plane to the set of repeat height meas- In these deserts,together with the flatter regions in urements. The difference in the relative-orbit-enor estimates Greenlandand on the Antarctic Plateau.could be used to de- from the two techniques is 12 cm root-mean-square(nus), termine the relative orbit error at a number of points around from which we conclude that relative orbit etor can be re- an orbit arc. The relative orbit error at other locations around the orbit could then be determined through sinusoidal duced to less than 9 cm using ocean fitting, and to between 9 inter- polation, as in the and 1.2cm using land fitting. The Antarctic p)ateau could not oceantechnique. Many orbits will not be used as a reference as the orbit error appeared correlated cross suitable referencesurfaces, and this may be a drawback with the cross-trackdisplacement of repeat backs, preventing compared to using the ocean for certain applications.The ef- the determination of the local surface slope. The land analysis fect of the distribution of suitable referencesurfaces is not was also limited by lack of waveform data and Geosatoff- addressedhere as it is application dependent. pointing; current altimeter mr'ssions(e.g., ERS-1and Topex/Po- The two techniqueshave relatively independent sources seidon) should be able to achieve higher accuracies. of error, and so their merits and accuracyare assessedby comparing the relative orbit error derived using the ocean with that derived at two land referencesites for the same set Introduction of orbits. The Simpson Desertin central Australia was cho- Radial orbit error is the single largestsource of error in de- sen as one of the r-eferencesites because it was already termining surface heights from satellite radar altimetry. How- known to provide "well-behaved" altimeter returns (Chua ef ever, for many applications of altimeter data requiring repeat o1.,fssf); and Australia has extensiveocean on three sides, observations, e.g., determining changesin large-icaleocean enabling good interpolation of the oceanresults. A region on topography or measuringthe variation in water level in lakes the Antarctic Plateauaround 56o8,72oS was chosen as the or wetlands, it is sufficient to know the variation in the satel- other referencearea because it was crossedby two of the or- lite orbit rather than its absolutevalue. This can be achieved bits that passedover the Simpson. In principle, much of the through the use of height referencesurfaces that are assumed Antarctic Plateaucould be used as a referencesurface due to to be fixed or varying in a known way. In this paper, we in- the uniformity of its terrain and its low surfaceslopes. Geo- vestigatetwo alternative techniquesfor estimating the varia- sat GeophysicalData Records(Cnns) for four orbits with re- bility of the radial orbit error for collinear tracks using peat data in 1987 and 19BBwere used in the analysis. Geosatdata. The first technique uses the mean ocean surface,cor- TheGeosat Data rected for tides, etc., as a reference.The effect of the dy- The Geosataltimeter satellite, Iaunched bv the U.S. Naw in namic topography of the ocean surfaceis reduced by taking 1985, had the primary objective of mapping the marine ge- advantage of the fact that difference in height measurements oid. Initially, it performed a classifiedGeodetic Mission be- repeat (the for orbits relative orbit error) has a once per cycle tween 1 April 1985 and 30 September1986. During this sinusoidal (Cartwright form and Ray, 1990).A number of re- period, the satellite was in a very long repeat orbit, with a searchershave used collinear repeat tracks to estimateorbit -5 km cross-trackspacing at the equator,in order to perform in (e.g., error this way Cheney ei al., 1991:Van Geysenef fine-resolutiongeoid mapping. The second phase of the mis- o1.,1992). sion beganon B November 1986 and was known as the exact The secondtechnique uses land surfacesfor which the repeat mission (nntra).During this period the satellite was placed in a 17-day repeat orbit with a -160 km cross-track W. Cudlip is with the Mullard SpaceScience Laboratory, University CollegeLondon, Holmbury St. Mary, Docking, PhotogrammetricEngineering & Remote Sensing, Suney, RH59 6NT, United Kingdom. vol. 6r, No. 7, July 1995,pp. BB1-8S0. H.A. Phillips and A.H.W. Kearsleyare with the School of 0099-1112l95/6107-BB1$3.00/0 Surveying,University of New South Wales, P.O. Box 1, Ken- O 1995 American Society for Photogrammetry sington, New South Wales 2033, Australia. and RemoteSensing PEER.REVIEWED ARIICTE p ( \ ) 4 t '1/ \ 7 \.4 ( I h /t l r/ ) ) r\ 1 \ f \ 16 it a ) (-, \ ) ( r \ \ \ 8t' Figure1. Globalground tracks for the four Geosatorbits that crossthe SimpsonDesert in Australia.The tracks are labeledwith the track numberassigned sequentially to the 243 orbits within each I7-day repeatperiod. spacing at the equator,resulting in the orbit ground track re- the antenna not pointing to the nadir, causes a change in the peating every 243 orbits. Sixty-two repeat cycles (usually re- pattern of radar illumination of the surface, and this distorts ferred to as ERMs)were completed before the satellite finally the shape of the return echo. This has an effect on the on- failed in fanuary 1990, almost five years after launch. The or- board height tracker that is dependent on the surface topog- bit of the satellite was controlled by rocket burns, with up- raphy. Over the ocean, the topography is quantified using the datesbeing required every few weeks, to ensure that the significant wave height (swu) parameter. The SWH itself can ground tracks repeatedto within + 1 km. The data from this also alter the tracking characteristics, and so the combined part of the mission are not classified,and 1 Hz averageddata effect of off-pointing and sWH is theoretically calculated be- (GeophysicalData Records)are available from the NOAANa- fore the mission and entered into look-up tables. Off-pointing tional OceanographicData Center in the U.S.A. (Cheneyef and swu estimates derived from the return-echo waveform o1.,19911. shape are used to extract the appropriate conection from the There are four Geosatground tracks that crossthe Simp- tables, and the value is inserted into the cDR data. son Desert,two ascending(tracks 81 and 1,24)and two de- In this analysis, when calculating the surface heights, scending (tracks 16 and 21,7).The track numbers used here the ionospheric corrections and the Fleet Navy Operational refer to the orbit number within each tz-day repeat period. Center (r'Noc) atmospheric corrections given in the GnR data Figure L shows the location of the ground tracks on a map of were used. Data over the ocean were also corrected for tides the world. The results from the analysis of track 16 are pre- and the swu/off-pointing correction. For data over the land, sented in this paper. The Antarctic Plateauis crossedonly only the Earth tide correction was added. The swtt/off-point- by the two descendingtracks due to the asymmetric distribu- ing correction was not applied because the off-pointingesti- tion of land around the South Pole, and the latitudinal limit mates are known to be incorrect over land. This is a result of of 72" for Geosat.The ERS-1satellite has a larger latitudinal the return-echo waveform shape being non ocean-like, and Iimit (82') which will result in over half of its orbits crossing the long averaging time of -2 minutes required for the off- the Antarctic Plateau. pointing calculation. The land data are therefore uncorrected Geosatcompleted 62 ERMsin all. Improvedorbit ephem- for Geosat off-pointing. This can introduce errors of up to eris data, based on the GEM-T2geopotential model (Hainesef + 17 cm in individual surface height measurements for SWH o1.,1390), were available to us for only the first 43 ERMs values of up to S m (the maximum value allowed in the (covering1987 and most of 19BB).Data error problemsmeant analysis). that ERMs1, 5, and 20 could not be included in the analysis, resulting in 40 ERMsof data for processing.Excluding the re- AltimeterConections maining ERMsllom the analysis is not a serious drawback be- Many corrections have to be applied to convert the time de- causetowards the end of 19BBthe stability of the spacecraft lay measured by a radar altimeter into first a range measure- began to be seriously affectedby the increasedsolar activity ment and then a surface height measurement, The associatedwith the maximum of the sunspot cycle which oc- corrections required and their accuracies have been dis- curred in 1989. The pendulum motion of the satellite in- cussed extensively elsewhere (SeasatSpecial Issue I, 1982; creased,resulting in hore frequent "off-pointing," which Geosat Special Issue I, 1990; Guzkowska ef al.,'1990; Cheney causeda loss of data quality and the instrument to loose lock et al., 19s1), and a brief but comprehensive overview of the- of the surfacemore often.