Airborne GPS Positioning With cm-Level Precisions at Hundreds of km Ranges

G.L. Mader

Abstract:	Accurate airborne remote sensing (LIDAR, photogrammetry, gravimetry) is critically dependent on accurate positioning of the aircraft´s trajectory using GPS in conjunction with INS. This paper will examine the techniques and results by which we have achieved precisions and estimated accuracies at the cm-level over ranges of hundreds of km for GPS determined aircraft positions. This is customarily done by using several ground-based reference stations, to each of which a separate differential, kinematic GPS solution is computed. These solutions use precise GPS orbits computed by the IGS and the L1 and L2 carrier phases to form the double differenced, ionosphere-free observable with the phase bias´s fixed to their integer values. Differences between the trajectories computed using several base stations will check the consistency of the integer fixing and also reveal the effects contributed from those base stations, e.g. multipath, position errors, etc. Common mode errors arising from the aircraft however, will not be seen. The KINPOS program has been used to compare trajectories for a large number of LIDAR and other mapping projects. For example, a project, which used base stations distributed over 2200m in elevation and with distances to the aircraft ranging from 0 to 250km, have shown the importance of estimating tropospheric delay as a function of aircraft altitude. The resulting trajectories for this project had RMS vertical agreements of 2cm and horizontal agreements of 0.5cm. These results are independent of the range to the base stations, the base station elevation and the aircraft altitude. This estimation requires that the GPS antenna on the aircraft be calibrated for phase center offsets and phase center variations. The kinematic data itself is then used to model the variation of total zenith delay. Other flight scenarios as well as quality control will be discussed.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	425 - 433
Cite this article:	Mader, G.L., "Airborne GPS Positioning With cm-Level Precisions at Hundreds of km Ranges," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 425-433.
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