Comparison DUT and UT/CSR Orbits for ERS-1

UT/CSR provided ERS-1 orbit generated with the TEG3 gravity field model for Phase C (April 1992 till December 1993). DUT used the ERS-tailored gravity model DGM-E04 to produce orbits for ERS-1 mission Phases C, D, E, F, and G and ERS-2 Phase A (April 1992 till December 1995). Comparison of the orbits for Phase C follows a number of steps:

Convert UT/CSR orbits to the GRS80 reference ellipsoid as for the DUT orbits.
Interpolate orbits every 30 seconds for the entire Phase C period covered by both orbits.
Identify periods on which either orbit is erroneous. Especially during the first half of Phase C the number of orbital maneuvers is extensive. Also periods with reduced SLR and altimeter tracking data are notoriously bad.
When the rms radial orbit difference for a particular revolution exceeds 9 cm it is disregarded in the comparison. Thus a table of blackout periods was devised. The rms radial orbit difference of the remaining period is 3.7 cm.
Make a time series of points separated by 35 days. Those pertain to approximately the same location (within approximately 100 km along-track). We assume that the constant part as well as the time variant part of the orbit error is nearly identical over such a distance. This gives us 100800 time series of orbit differences at locations separated 30 seconds (approx 200 km) along track.
Fit a 4-parameter function in each time series, consisting of a mean, trend, and annual cycle.
Keep statistics and plot the 4 parameters geographically.

Results

Plots of the geographical global distribution of the four parameters show only some systematic behaviour in the annual cycle. This is likely to be related to difference in the gravitational tide model used by the two institutes in the orbit computation. Some tracks are more pronounced. They are likely associated with remaining excessive errors in one or a few orbital repeats. On the whole, also regionally, over Antarctica, we see no evidence of localised secular trends in the orbits.

Statistics of the four parameters are given in the table below, making a division between Global (82S-82N) and Antarctic (South of 65S). Important is that for either area there is no clear average trend or yearly cycle. The rms values also indicate that overall the estimated trends and cycles are moderate. Note that because of the limited period (20 months) some non-cyclic variations of the orbit differences may be aliased into the estimated trend and cyclic components. This, however, doesn't change the conclusion that there is no significant trend or seasonal variation in the orbits.

                          GLOBAL            SOUTH OF 65S
                     --mean--- ---rms--- --mean--- ---rms---
Mean                      .184     2.683     -.039     2.459 (cm)
Trend                    -.043     1.286      .008     1.487 (cm/year)
Annual Amp (In-phase)     .041     1.171     -.423     1.290 (cm)
Annual Amp (Quadrature)   .015     1.057     -.569     1.230 (cm)
           Residuals               2.318               2.556 (cm)

Conclusion

Judging from the radial differences between two highly precise ERS-1 orbits for Phase C there is no reason to believe that there is an erroneous secular trend or a yearly cycle to be detected in either orbit that might cause unrealistic trends or variations in the (global) sea level rise or (regional) ice sheet mass balance studies.

More analyses on ERS-1 orbit stability

Questions or comments:

Remko Scharroo, remko.scharroo@lr.tudelft.nl.

22 January 1997.