Gravity anomalies from airborne measurements - experiments using a low cost IMU device

The gravity field may be determined using different techniques, but airborne gravity surveying is becoming a powerful tool mainly due to its potential in remote areas. One of the main problems in airborne gravity is the separation of the vertical accelerations acting on the airborne platform from the natural gravity anomalies. GPS information can be used for INS in-flight alignment, calibration of inertial sensor readings and ensure long term navigational accuracy for low cost inertial navigation systems. Such a system was developed in support of an airborne gravimetric and altimetric campaign that took place in the Azores region, Portugal in October 1997, in the scope of the AGMASCO (Airborne Geoid MApping System for Coastal Oceanography) project. This system was originally developed with the only purpose of determining the navigational parameters (attitude, velocity and position) of the aircraft. However, it is also capable of producing estimates of the vertical gravity anomaly. Since this anomaly is one of the main perturbations to the performance of the navigation system, it had to be estimated together with the inertial sensor biases in order to improve the final solutions. Therefore it was decided to test the ability of a low cost Inertial Measurement Unit (IMU) as a gravity measuring device. This paper describes the methodology followed to obtain estimates of the local gravity anomaly by integrating GPS and inertial measurements. The results show that the gravity anomaly measured by diferential GPS/IMU integrated system matches quite well the results obtained with Lacoste & Romberg (L&R) sea and air gravimeters. The ability of this kind of inertial system to recover gravity anomalies, focused on medium wavelengths perturbation signals, was evaluated. The lack of long term stability of the IMU accelerometers and gyros renders long wavelength gravity anomaly determination impracticable. This integrated DGPS/IMU system can be seen as a complement to a gravimeter, which exhibits a long term stability.