Geographically correlated errors observed from a laser‐based short‐arc technique

The laser‐based short‐arc technique has been developed in order to avoid local errors which affect the dynamical orbit computation, such as those due to mismodeling in the geopotential. It is based on a geometric method and consists in fitting short arcs (about 4000 km), issued from a global orbit, with satellite laser ranging tracking measurements from a ground station network. Ninety‐two TOPEX/Poseidon (T/P) cycles of laser‐based short‐arc orbits have then been compared to JGM‐2 and JGM‐3 T/P orbits computed by the Precise Orbit Determination (POD) teams (Service d'Orbitographie Doris/Centre National d'Etudes Spatiales and Goddard Space Flight Center/NASA) over two areas: (1) the Mediterranean area and (2) a part of the Pacific (including California and Hawaii) called hereafter the U.S. area. Geographically correlated orbit errors in these areas are clearly evidenced: for example, −2.6 cm and +0.7 cm for the Mediterranean and U.S. areas, respectively, relative to JGM‐3 orbits. However, geographically correlated errors (GCE) which are commonly linked to errors in the gravity model, can also be due to systematic errors in the reference frame and/or to biases in the tracking measurements. The short‐arc technique being very sensitive to such error sources, our analysis however demonstrates that the induced geographical systematic effects are at the level of 1–2 cm on the radial orbit component. Results are also compared with those obtained with the GPS‐based reduced dynamic technique. The time‐dependent part of GCE has also been studied. Over 6 years of T/P data, coherent signals in the radial component of T/P Precise Orbit Ephemeris (POE) are clearly evidenced with a time period of about 6 months. In addition, impact of time varying‐error sources coming from the reference frame and the tracking data accuracy has been analyzed, showing a possible linear trend of about 0.5–1 mm/yr in the radial component of T/P POE.