FLIGHT TEST-RESULTS FROM A STRAPDOWN AIRBORNE GRAVITY SYSTEM

In June 1995, a flight test was carried out over the Rocky Mountains t o assess the accuracy of airborne gravity for geoid determination. The gravity system consisted of a strapdown inertial navigation system (I NS), two GPS receivers with zero baseline on the airplane and multiple GPS master stations on the ground, and a data logging system. To the best of our knowledge, this was the first time that a strapdown INS ha s been used for airborne gravimetry. The test was designed to assess r epeatability as well as accuracy of airborne gravimetry in a highly va riable gravity field. An east-west profile of 250 km across the Rocky Mountains was chosen and four flights over the same ground track were made. The flying altitude was about 5.5 km, i.e., between 2.5 and 5.0 km above ground, and the average flying speed was about 430 km/h. This corresponds to a spatial resolution (half wavelength of cutoff freque ncy) of 5.0-7.0 km when using filter lengths between 90 and 120 s. Thi s resolution is sufficient for geoid determination, but may not satisf y other applications of airborne gravimetry. The evaluation of the int ernal and external accuracy is based on repeated flights and compariso n with upward continued ground gravity using a detailed terrain model. Gravity results from repeated flight lines show that the standard dev iation between flights is about 2 mGal for a single profile and a filt er length of 120 s, and about 3 mGal for a filter length of 90 s. The standard deviation of the difference between airborne gravity upward c ontinued ground gravity is about 3 mGal for both filter lengths. A cri tical discussion of these results and how they relate to the different transfer functions applied, is given in the paper. Two different math ematical approaches to airborne scalar gravimetry are applied and comp ared, namely strapdown inertial scalar gravimetry (SISG) and rotation invariant scalar gravimetry (RISG). Results show a significantly bette r performance of the SISG approach for a strapdown INS of this accurac y class. Because of major differences in the error model of the two ap proaches, the RISG method can be used as an effective reliability chec k of the SISG method. A spectral analysis of the residual errors of th e flight profiles indicates that a relative geoid accuracy of 2-3 cm o ver distances of 200 km (0.1 ppm) can be achieved by this method. Sinc e these results present a first data analysis, it is expected that fur ther improvements are possible as more refined modelling is applied.