Airborne Gravity Data Downward Extension Technology

Airborne gravimetry is a new kind of technique for surveying the gravity acceleration over the earth's surface and is advanced in gravimetry history, which basing on aircraft, including gravimeter, GPS, altimeter and attitude sensor etc.. Airborne gravimetry is applied to rapidly get extensively and precisely well-distributed information of the earth gravity field in order to meet the needs of Geodesy, Geophysics, Oceanography, Exploration, Space-Science and the other fields.Airborne gravimetry includes scalar airborne gravimetry and vector airborne gravimetry. Scalar airborne gravimetry is used to get the value of the gravity acceleration, and vector airborne gravimetry can get not only the value but also the direction of the gravity acceleration. Airborne gravimetry which is discussed in this paper denote scalar airborne gravimetry only. The gravity anomaly got from airborne gravimetry were at flight level, but which we always used are on the earth surface or on the geoid, thus we must reduce them into the earth surface or the geoid, and then can be used to calculate the disturbing gravity fields e.g. geoid undulation according the BVP's theory for physical geodesy. The reduce accuracy of the airborne gravity downward to the earth surface or the geoid have large effects on the finale results.There has been considerable growth in the field of airborne gravimetry over the last decade. Among them an airborne gravimetry system named CHAGS (CHinese Airborne Gravimetric System) has been developed in Xi'an Research Institute of Surveying and Mapping. In Mar and April 2002, we have carried out the first airborne test using the CHAGS and obtained some good results. Using parts of the results, the problems of the downward continuation for the airborne gravity were detailed discussed in this paper.First, various of the downward methods were discussed and compared, based on the analysis results we advanced the real method which suit for the CHAGS, and designed the software for downward continuation of the airborne gravity anomaly.