Integrating Multiple GPS Receivers With a Low Cost IMU For Aircraft Attitude Determination

Many airborne applications, such as altimetry or photogrammetry without ground control points, strongly rely on the precise evaluation of the aircraft’s attitude. This paper presents an approach to the attitude determination problem, where a system combining Litton’s LN-200 Inertial Measurement Unit (IMU) with differential GPS (DGPS) is described. The methodology is based on the integrated processing of raw inertial measurements and GPS data collected from at least two receivers on board the aircraft and one, or possibly more, reference stations. It is well known that GPS and inertial navigation systems have complementary operational characteristics that can be exploited in an integrated system for continuous tracking of the aircraft’s navigational parameters (attitude, velocity and position). The incorporation of DGPS observations of one of the aircraft’s GPS antennas, specifically absolute position and doppler velocity readings, improves the overall system accuracy, including the estimates of roll and pitch angles that result from filtered raw inertial measurements, since it enables the continuous alignment of the strapdown analytical platform. By processing carrier phase measurements in differential mode between a pair of the aircraft’s GPS antennas (without information from any reference station), good attitude observations, especially true heading, are obtained. This is critical to align the low cost inertial platform in heading, since azimuth errors are weakly coupled with velocity or position errors. In fact, by this method alignment is achieved in motion and very quickly. The inertial system also provides high rate (DGPS filtered) position and velocity estimates that can be valuable during GPS signal outages and in DGPS ambiguity fixing procedures that occur whenever a new satellite becomes visible or cycle slips are detected. The estimated accuracy is 0.01º (1s) for roll and pitch and 0.1º (1s) for heading. Should the reference station data be absent and stand-alone doppler and position estimates be used instead, some degradation of the roll and pitch accuracy are to be expected. Besides a description of the relevant theory, this paper also includes post-processing results from one of a series of flights that took place in the Azorean area during a gravimetric and altimetric survey. The present methodology was successfully employed to process all data from this campaign.