Precise Orbit Ephemeris Research Articles

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.

Remote sensing precipitable water with GPS

The principle and method of remote sensing precipitable water (PW) by the Global Positioning System (GPS) are introduced. A series of tropospheric delay have been obtained from the dual frequency receiver's data of global tracking stations in East Asia and precise satellite ephemeris of IGS from July 31 to August 20, 1997. The continuous PW at the interval of 30 min have been estimated at Shanghai and Wuhan GPS sites. The results are compared with PW from conventional radiosonde during the same period, and the RMS is about 0.50 cm.

Satellite doppler determination of differential sea ice motion in the vicinity of the north pole∗

Abstract During April and May 1979 multidisciplinary observations were conducted in the central Arctic Ocean from three manned ice stations to explore the nature of the submarine Lomonosov Ridge (Weber, 1979). Positions of the LOREX (Lo‐monosov Ridge Experiment) ice camps were monitored continuously using geodetic satellite doppler receivers and the U.S. Navy Navigation Satellite System. Near real‐time doppler data reduction on the ice using broadcast satellite ephemeris provided operational horizontal positions with accuracies of about ± 250 m. A precise geodetic satellite doppler reduction program has been adapted to accommodate linear station motion in a simultaneous multistation, three‐dimensional adjustment in phases using postfitted precise satellite ephemeris, if available. Complete reprocessing of the LOREX doppler data set has produced average errors of ± 48 m for single pass solution and ± 24 m for mean three‐four horizontal station positions. The station ellipsoidal heights show strongly correlated variations in excess of 5 m, with the average error of ± 0.45 m. Station velocities are also strongly correlated, with the mean of about 225 m/h and the maximum of 1,240 m/h. Total strain and hourly strain rate components of sea ice have been evaluated for a homogeneous two‐dimensional strain model. The configuration of the ice camps provides a unique solution for mesoscale strain, indicating a major change in the pattern of pack ice deformation over the Lomonosov Ridge. The total strain reflects plastic deformation, which takes place mainly in episodic events. The strain rate also indicates nearly diurnal oscillations, with amplitudes an order of magnitude smaller than those corresponding to the major deformation events.