Marine Gravity Recovery for the Malaysia Region from TOPEX,
ERS-1 and Geosat Satellites Radar Altimeter Data
Shahrum Ses, Majid Kadir and Hassan Hashim
Faculty of Geoinformation Science and Engineering
University Technology Malaysia
Locked Bag 791
80990 Johor Bahru, Malaysia
E-mail:sharum@fksg.utm.my
Abstract
Over the last 20 years, considerable developments in the measurement of sea surface heights using a satellite radar altimeter have been observed. The measurement precision has improved from 300cm to 2cm the and the resolution has been reduced from 70km to less than 20km on the ground. The dense satellite altimeter data gives an insight into the detail recovery of marine gravity field which can be used for geological exploration of the sea floor.
The combined Topex, ERS-1 and Geosat altimeter data have been used for the determination of the sea surface (MSS) and marine gravity field in the Malaysian region
(
j=0
0 to 10
0,
l=96
0E to 120
0E). A substantial amount of the combined altimeter records have been used in the elevation. The gravity field was derived from the height anomalies using Fast Fourier Transfer (FFT) technique. The free air gravity anomalies were obtained on a regular grid of 3'45" * 3'45" corresponding to 7km spacing at the equator. The precision of the satellite altimeter derived gravity anomaly values over the open marine area of the Malaysian region is expected to the better than 2mGal. The quality of anomaly recovery in the semi-enclosed seas around the region was also verified by comparison with the available shipboard measurements.
Overview on the Altimetry Missions
Radar altimeter measurements of the sea surface height gave marine geodesists and geophysicists an insight into the detail recovery of gravity field over all the ocean and basins. Experience with GEOS-3 and Seasat in the 1970s had demonstrated the enormous potential of altimetry, but neither mission provided such complete long-term global coverage. From March 1985 until January 1990, the U.S. Navy satellite Geosat generated a new data set with unprecedented spatial and temporal coverage of the global oceans (Dougals and Cheney, 1990). Geosat paved the way for a series of highly-successful altimeter missions that followed it, i.e. ERS-1 (!((!-96), TOPEX/POSIEDON (1992-), and ERS-2(1995-).
The inclination and the repeat period determine how the altimeter data will be distributed on the earth. Geosat has an inclination of 1080 which means that the satellite's ground tracks are located between -720 and 720 latitude. Since the ERS-1 has an inclination of 980 , the covered area is extended by 100 south and north. The period, at which the satellite completes one revolution also determines the pattern of satellite's ground tracks on the surface of the earth. The repeat period of the satellite orbit governs the spacing of the altimeter tracks on the ocean surface. Longer repeat cycles such as 168-day ERS-1 geodetic phase or the non-repeat (drifting) orbit of the Geosat GM provide the high-density coverage of the altimeter tracks.
The acronym "Geosat" was derived from geodetic satellite, because its primary mission was to obtain a high-resolution description of the marine geoid up to latitudes of 72 degrees. This goal was achieved during the first 18 months, known as the geodetic mission (GM). During this time the ground track had a near-repeat period of about 23 days (330 revolutions in 23.07 days). The drifting orbit resulted in a dense, global network of sea level profiles separated by about 4km at the equator. Because of the military significance of this unique set of observations, the GM data were initially classified but in 1995 were released to NOAA in their entirely for public distribution. At the conclusion of the GM on September 30, 1986, the satellite orbit was changed, and the exact repeat mission (ERM) began on November 8, 1986. This produced sea level profiles along sea track that repeated themselves within 1-2 km at intervals of about 17 days (244 revolution in 17.05 days). The ERM covered 62 complete 17-day cycles before tape recorder failure in October 1989 terminated the global data set.
The global coverage of the European Earth Resource Satellite (ERS-1) altimeter data (up to latitudes of 80 degrees) have been used for the determination of the marine gravity field. The use of the one year 35-day repeat ERS-1 data (from November 10, et alkalinity., 1992; Yi, 1995; Hwang, 1997). The shorter repeat periods of 35 days for ERS-1 do not provide dense track coverage. On April 10, 1994, the ERS-1 satellite went into its first 168-day repeat cycle, the so-called geodetic phase. Very long repeat cycles of 168-day for ERS-1 geodetic mission provide the high-density coverage needed for complete resolution of the gravity field.
