This paper will describe an enhancement to the GPS double difference carrier phase measurements on a single airborne platform by smoothing them with inertial measurements while preserving the dynamic bandwidth. This enhancement will reduce the impact of carrier phase multipath and carrier phase noise on baseline determination between multiple antennas on an aircraft when in flight. This type of measurement system has numerous applications where platform pointing and relative body motion must be determined at the mm-level for applications such as sensor stabilization, Synthetic Aperture Radar, long range RADAR (i.e. angle-of-arrival measurements). Lower noise levels (mm-level and below) enable more performance to the stabilized system such as increased aperture for longer range, operation at higher frequencies, and more image resolution. The focus of this paper will be on a technique to provide this enhanced performance for these various applications using the available navigation systems. Additionally, this type of smoothing can effectively remove the additional noise induced by carrier phase measurement differencing. The noise level of a double or triple difference can be reduced below that of the raw measurement. A complimentary synthesized double difference quantity with ultra-low-noise characteristics will be used to smooth the GPS carrier phase double difference measurements without losing dynamic bandwidth since it follows the airborne dynamics. Flight test data will be presented to demonstrate the performance improvement in the midst of aircraft dynamics. Results will show that the noise reduction follows the theoretical prediction.
This paper explores the concept of using a high rate (i.e. 100 Hz), high accuracy integrated velocity (i.e. mm accuracy) estimate from stand-alone GPS measurement for sensor stabilization. The velocity algorithm uses GPS L1 code measurements at a rate of 2 Hz and L1 carrier measurements at 100 Hz. This velocity can be used for heading determination and then for inertial alignment or stabilization of other sensors. The integrated velocity vector accuracy is at the mm level and can be used to provide heading measurements better than 1 deg. This paper addresses several issues such as the velocity propagated position, relation between the velocity error and position error due to sensor lever-arms, timing accuracy of measurement association between various sensors, and a statistical technique to estimate the velocity error on a dynamic platform using two or more GPS antennas. High update rate position estimates, formed using the propagated velocity is shown to improve upon the noise performance of a triple difference technique. A velocity vector alignment technique is compared to a navigation-grade inertial heading alignment over a long lever-arm. A tradeoff discussion illustrates some measurement alignment and integration considerations for a remote sensor. Analysis of these concepts is provided using flight test data collected on April 12, 2006. 1,2
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