THE ACCURACY ASSESSMENT OF PRECISE ORBITS COMPUTED FROM DOPPLER TRACKING DATA

The use of the Guier-plane navigation solution in the accuracy assessm ent of precise orbits computed from doppler tracking data is investiga ted. Observation residuals obtained from the DORIS doppler tracking of TOPEX/POSEIDON (T/P) are used to compute high elevation pass (HEP) sl ant-range errors that approximate the radial orbit error. The HEP slan t-range errors are compared to the estimated radial orbit error for th e T/P orbit, which is determined using external validation tests. The DORIS HEP errors for T/P are also compared to those obtained from the DORIS tracking of SPOT-2. It is shown that the Guier-plane navigation errors for TIP are larger than those for SPOT-2, which contradicts the fact that T/P is known to have a radial orbit error three times small er than that of SPOT-2. To understand this result, orbit simulations a re performed to obtain observation residuals with known sources of for ce and measurement model error. The HEP slant-range errors obtained fr om the simulated residuals demonstrate that the Guier-plane navigation solution can sense orbit error due to mismodeled dynamics, but that m easurement errors lead to pessimistic estimates of the orbit error. It is concluded that given the current noise level of the DORIS tracking system, the HEP slant-range errors provide little quantitative inform ation when the orbit error is below the decimeter level. Such a conclu sion provides a strong impetus to include additional tracking systems on future oceanographic satellites where DORIS is the primary tracking system and where validation of centimeter-level radial orbit accuracy is required.