I. THE ESTABLISHMENT OF THE MARINE GEODETIC NETWORK

Based on the national Order A and B GPS networks, the Chinese Ministry of Communications, National Ocean Administration, Chinese Navy and so on have undertaken GPS geodetic connection to the present long-term tidal gauges and set up a monitoring network of tidal gauges. In order to get local vertical control with GPS leveling, some departments have set up small local GPS networks.

In Xiaolangdi Reservoir profile survey, GPS network was laid out by using active densification method. With maneuverable and high efficient operation, the accuracy obtained was well above the requirement of Order D.

The layout proposal and operating method of geodetic network under the tough conditions of South China Sea were studied. After data processing, the accurate coordinate of the control point of South China Sea in ITRF was obtained and thus the geodetic networks of South China Sea and the mainland were linked together as a whole.

Chinese scholars have also studied the basic principles of GPS altimetry and its application. Since Jan. 1 of 2002, founded by the Maritime Safety Administration of Chinese Ministry of Communications, RBN-DGPS has been fully accomplished and come into operation. Within 300 km offshore along Chinese coast, the positioning accuracy reached up to 5-10 m.

II. BATHYMETRY, GRAVITY SURVEY AND THE DATA PROCESSING

During the past four years, 2 500 000 km² of sea areas, including East China Sea, South China Sea and the Northwest Pacific Ocean, have been surveyed and the survey line was 1 800 000 km. More than 1 000 charts on different scales were published. More than 2 000 marine research stations were set up. Chinese scholars have made a further and more extensive study on multi-beam echo sounding technology and the data processing. A series of position reduction formulas and their error equations for swath sounding were derived. The dynamic effect of the marine sounding, i.e., the influence of vessel attitude, was taken into full account in derivation of the equations.

Chinese scholars also researched into the mechanism as a result of sound velocity influencing on multi-beam sounding, and the technical method of sound velocity correction by taking SeaBeam 2100 Multi-beam Sounding System for example. The detection of abnormal data in marine survey was studied and the robust interpolation comparison test based on robust M-estimation by an iterative calculation procedure was proposed. Based on discrete fast wavelet transform technique, an optimum wavelet threshold filter algorithm for seabed topographic signal reconstructing from echo sounding signal affected by ambient noise was put forward. This method is of particular advantage to reduce experience error and raise labor efficiency. By using geodetic height from GPS, the reduction of soundings was proposed. It is a real-time depth sounding method without taking into account of dynamic draught and tidal data. In view of the different kinds of error sources from multi-beam sounding, the influence on fair chart and correction method were studied. The strict analysis and correction formulas were proposed in accordance with the time lag and offset effect.

Chinese scholars have recovered the seabed topography from altimeter data and set up the improved model with consideration of the compensation effect of isostasy. The accuracy of the recovery was in the same level of the world.

Chinese scholars have developed a Hydrographic Data Processing System. The system function includes control survey calculation, coast topographic data processing and charting, soundings processing and charting, marine gravity data processing and charting, tidal observation and analysis and hydrographic data management, etc.

In respect of the adjustment of gravity network, mathematical model was adopted to characterize the change of the dynamic systematic errors, the crossover adjustment was carried out simultaneously to determine the model parameters and the compensation of systematic errors was realized. This is the self-calibrating adjustment suggested by Chinese scholars and it has achieved a good result in practical application. By making use of altimeter data, analytical and numerical inversion of the Stokes formula and inverse Vening-Meinesz formula for the recovery of marine gravity anomalies were studied and the estimated gravity anomalies were compared to the real marine gravity data from shipboard measurements at sea. The result shows that by using the recovery of gravity anomalies from altimeter data, the derived magnitude of accuracy in the gravity anomalies gridded in 15′×15′is better than 5 mGal, 30′×30′better than 4 mGal and 1°×1°better than 3 mGal.

Chinese scholars also derived a strict formula for the convolution formula and determined the marine gravity anomalies gridded in 2.5′×2.5′over the South China Sea using inverse Vening-Meinesz formula.

III. COAST TOPOGRAPHIC SURVEY

In resent years, electronic plane-table or GPS RTK has been used for the coast topographic survey in China. By using this technique, either control point for surveying can be set up quickly or even the topographic survey can be conducted directly without control point. More than six kinds of topography survey softwares have been in the use. During the past two years, aerial photogrammetry has been spread out in coast topographic survey.

IV. HYDROGRAPHIC INSTRUMENTS

The technology of GPS carrier phase measurement was put forward to determine vessel attitude, the test of the technology was taken and the measurement accuracy reached up to 0.28°. The two main error sources influencing the depth accuracy of airborne laser sounding, i.e., the accessories error and the depth error, were analyzed and studied. The concept of Shipborne Shallow Water Laser Bathymetry was put forward and a measurement plan of laser double-frequency phase method used for shallow water bathymetry was studied. The simulated test of the system was accomplished in laboratory and it approved the feasibility of the measurement plan. Now the Shipborne Shallow Water Laser Bathymetric System is under study and development. The pressure gauges suitable for different kinds of ocean environments were produced and they were used in marine survey and tidal study. Airborne Laser Sounding System was developed and produced and the test was taken in the South China Sea. The repetition frequency of laser was 200 Hz, the sounding depth was 50 m and the depth accuracy was ±0.3 m.

V. VERTICAL DATUM

Present situation of marine surveying vertical datum and its existing problems were researched into and the proposal for improvement of marine vertical datum was recommended. Based on the frequency spectrum of mean sea level (MSL) and relation model between two tidal stations, the mathematical model for hydrographic datum transfer were studied and more precise models were developed.

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