SATELLITE ALTIMETRY AND ITS APPLICATION
Chinese Academy of Surveying and Mapping, Beijing 100039, China
During the period of this report, Chinese scientists in this field devised the editing criteria of multi-satellite altimeter data, improved each geophysical correction, and analyzed the error sources of satellite altimetry and the corresponding methods to eliminate those errors. They also developed the method of dual satellite crossover adjustment and a new technique of local crossover adjustment between multi-tracks, and used sea surface anomaly model to reduce the influence of sea surface time change on altimeter data of geodesy mission. In this method, the adjusted crossing pairs are composed of ERS-235 (ERS-135), Geosat/ERM (GM), ERS-1/168 and T/P. In the adjustment, the radial orbits of ERS-1, ERS-2, and Geosat SSH data are improved by a crossover adjustment with fixed T/P SSH data. After the data are processed, the root mean square (RMS) values of the difference at the crossover points for ERS1/168, Geosat/ERM and ERS2 are 0.114 m, 0.029 m, and 0.024 m respectively.
The co-line processing method is also studied, which is designed to eliminate the satellite orbit errors and determine mean sea level and its changes according to the satellite altimetry mission. Co-line processing can be used to compress the original observation data significantly, to reduce the influence of all kinds of time change factors and middle-short wave sea surface topography effectively, to reduce random noise and improve the precision of the altimeter data.
The question of establishing a unified datum is also solved. All sea surface height (SSH) data are constructed in a frame. There are systematic differences between SSH data of ERS-2 or SSH data of Geosat/ERM and SSH data of T/P because the reference frames and ellipsoids of them are different. When they are used together, all SSH data should be in a frame. In data processing, ERS-1, ERS-2 and Geosat SSH data are consisted with T/P SSH data in a frame after frame transformation and the crossover adjustment.
At School of Geodesy and Geomatics, Wuhan University, the methods for determining mean sea surface (MSS) are developed and the WHU2000 mean sea surface (MSS) model grided in 2¢´2¢ size is determined for latitudes below ±82°. The precision of this model is better than 0.05 m.
The methods for the recovery of gravity field from satellite altimetry are summarized, and the geoid and gravity anomaly grided in 2.5¢´2.5¢ size over China are recovered from multi-satellite altimeter data. Two methods are used in gravity recovery. The one is the deflection of the vertical along-track profiles with inverse Vening-Meinesz formula and the other is the least square collection (LSC) with shipboard gravity. Compared to the shipboard gravity and SIO gravity, the RMS of difference is 9.0 mGal and 15.03 mGal. After the geoid is determined, the dynamic ocean topography with the same grid over China oceans is separated from MSS height. The RMS of difference between the computed dynamic topography and EGM96 SST model is 0.220 m.
Based on the relationship between the gravity anomaly and mass deficiency of the ocean, the 2.5¢´2.5¢ bathymetry model in the South China Sea is inversed using multi-satellite altimeter data and geophysical data. In the study, the FFT technique has been put in practice, and the effect of proportion has been considered. On the other hand, the calculated result is compared with TBASE model and 49096 values of ship depth by GEODAS.
Scientists at Institute of Geodesy and Geophysics, CAS established two types of MSL models over China's offing and vicinity, using Geosat/ERM and T/P data and grided dual-conicoid model, respectively. The resolution of the models is 30¢´30¢, and RMS comes up to 8.5 cm. The geoid extracted by subtracting long wave long sea surface topography model (Nerem Model) from this model shows there is an obvious anomaly area in Okinawa Trough and Philippine Gouge. Comparing with OSU91A, the RMS of this geoid is 36 cm in East China Sea, 41 cm in South China Sea, 18 cm in the Pacific area. Based on the long wave sea surface topography model Nerem, 20 order sea surface topography model of over China's offing and vicinity is established according to mean sea level model of altimeter data and geoid model JGM3/OSU91A.
2¢´2¢ marine gravity anomaly model over the South China Sea (0o-25oN，105o-122oE) IGG99 S is established by along-track inversion of vertical line deviation. RMS of IGG99 S is 14 mGal at the whole South China Sea, 10 mGal at South China Sea basin (10°-18°N，110°-118°E). The iterative-cut solution of altimetry-gravity is improved and the gravity model over Chinese continent and its offing is established, whose interior precision comes up to 8.7 mGal (gravity anomaly) in continental area, 0.13 m (geoid) in marine area. Benthal topography and its structure are also recovered and a 600 km zone of fracture between 15°N, 120°E and 10°N, 111°E is detected, which is considered as a sea mount bounding resulting from the collision between India Plate and Eurasia Plate. In addition, the obvious vibrational periodical signal from China's offing sea surface and its space distribution are studied with altimeter data.
Scientists at research group of “THE STUDY ON CONTINENTAL AND SEA LEVEL VERTICAL MOVEMENT IN CHINESE COASTAL AREA”, which consists of Chinese Academy of Surveying and Mapping, China National Marine Information Center, School of Geodesy and Geomatics of Wuhan University, and Shanghai Astronomical Observatory, extracted tidal harmonic constants of 11 main partial tide (O1, K1, P1, Q1, M2, S2, K2, N2, M4, M6, MS4) and annual partial tide Sa of Huanghai Sea and East China Sea by T/P altimeter data. The interior precision of partial M1, M2 are calculated according to crossover residue; their amplitudes are 2.4 cm and 0.8 cm respectively; the precision of epoch is 2.3°and 2.5°respectively. Compared with the harmonic constants of 13 island tide gauges, the amplitude precision of M1 and M2 is 3.8 cm and 2.0 cm respectively, epoch precision is 1.09°and 7.4°respectively. Harmonic constants of 8 main daily and half-day partial tides of South China Sea (Sa, Saa, Mm, Mf, Q1, O1, P1, K1, N2, M2, S2, k2) are also acquired. Compared with the tide gauge data, amplitude errors of the partial tide are better than 2 cm; the bigger epoch error is 7°. Amplitudes and epochs of 28 partial tides of northwest Pacific are also calculated with method of difference comparison. Compared with the data of 12 tide gauges in that sea area amplitude precision of O1, K1, M2, S2 is 1.0 cm, 1.2 cm, 2.97 cm, 1.87 cm respectively. With T/P altimeter data and tide gauge data, an assimilation test has been made to optimize nonlinear open-boundary tidal data by adjoint method; it aims to establish a tidal model over China's offing.
This research group's study of the seasonal change of the global and China sea level with 7 years' T/P altimeter data shows there is a falling trend in the sea level of China Sea in the first season and fourth season, a rising trend in the second season and third season. But for the global sea level, it rose in the first season and fourth season, fell in the second and season third season. The monitoring by T/P altimeter data shows that the change rate of global sea level during January 1993 and May 1999 is 2.0±02 mm/year. In addition, 1997-1998 El Nino over tropic Pacific is monitored by T/P altimeter data; the precision of the monitoring is better than 7 cm.REFERENCES
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