The Direct Method in Strapdown Airborne Gravimetry - a Review

摘要

This paper reviews various aspects of the direct method in airborne gravimetry, using a strapdown inertial measurement unit (IMU) and GNSS. The direct method (also referred to as "accelerometry approach" or "cascaded approach") is a cascaded two-step approach: inertial accelerations are computed from GNSS positions using numerical differentiation in the first step, which are then removed from IMU specific force measurements in the second step. The transformation of strapdown measurements into the navigation frame requires the knowledge of the vehicle's attitude, which is commonly computed using a commercial IMU/GNSS strapdown navigation software. In contrast, the indirect method (also referred to as "inertial navigation approach" or "one-step approach") combines all available measurements (GNSS positions and IMU measurements) in a single Kalman-Filter, which is, however, more difficult to implement and requires a substantial tuning of the filter. A comparison of both approaches based on real campaign data indicates the overall quality of the two approaches being on a par. The accuracy achieved is 0.6 mGal (after line adjustment) and 1.3 mGal (without adjustment) at flight altitude, with a spatial resolution of about 6 km (half-wavelength). These results demonstrate for the first time that both processing approaches can achieve gravity disturbances at the same level of accuracy. This paper discusses the fundamentals of the direct method, addressing readers who are striving for their own implementation. The various corrections and data processing aspects are presented in detail as well as a standard evaluation scheme.