Using GPS and GPS/INS Systems to Assess Relative Antenna Motion Onboard an Aircraft Carrier for Shipboard Relative GPS M.G. Petovello, G. Lachapelle and M.E. Cannon Position, Location And Navigation (PLAN) Group Department of Geomatics Engineering University of Calgary BIOGRAPHIES Dr. Mark Petovello holds a PhD from the Department of Geomatics Engineering at the University of Calgary where he is a senior research associate in the Position, Location and Navigation (PLAN) group. He has been involved in various navigation research areas since 1998, including satellite-based navigation, inertial navigation, reliability analysis, and dead-reckoning sensor integration. Dr. Gérard Lachapelle is a Professor of Geomatics Engineering at the University of Calgary where he is responsible for teaching and research related to location, positioning, and navigation. He has been involved with GPS developments and applications since 1980. He has held a Canada Research Chair/iCORE Chair in wireless location since 2001. Dr. M. Elizabeth Cannon is a Professor and Head of Geomatics Engineering at the University of Calgary. She has been involved in GPS research and development since 1984, and has worked extensively on the integration of GPS and inertial navigation systems for precise aircraft positioning. Dr. Cannon is a Past President of the ION. ABSTRACT The shipboard component of the Joint Precision Approach and Landing System (JPALS) is intended to ultimately provide auto-landing capability for an inbound aircraft. An inherent requirement therefore is to know the relative position of the aircraft relative to the touchdown point (TDP) during approach. Although GPS will be used for this purpose, it is obvious that a GPS antenna cannot be placed at the TDP for operational reasons. As such, a geometrical translation of the GPS data collected elsewhere on the ship to the TDP must effectively be performed. In order to do this properly, the vector components between the GPS receiver and the TDP must be known. Under operational conditions however, these vector components may become dynamic quantities due to the flexure of the ship. In order to meet the accuracy and integrity requirements, the effect of ship flexure must be accounted for, either explicitly via compensation, or statistically in accuracy allocations. Since information on the magnitude and frequency of ship flexure is not currently available, appropriate measurement methods need to be developed. This paper investigates two methods of estimating the baseline length between two GPS receivers located on an aircraft carrier, as a means to determine ship flexure. The first method uses standard differential GPS techniques, while the second employs a dual-GPS/INS approach. To evaluate the latter approach, a test was conducted to simulate ship flexure. Results indicate that using tactical-grade IMUs in the dual-GPS/INS approach is effective at reducing GPS errors which are likely be more prevalent onboard an aircraft carrier under operational conditions. Also, in an attempt to obtain an initial estimate of ship flexure, GPS-only data collected onboard an actual aircraft carrier is processed herein. Deviation in the estimated baseline length between various GPS receivers on the aircraft carrier, which is indicative of ship flexure, is shown to have peak-to-peak variations of 6 to 7 cm, and correlate well with ship dynamics. INTRODUCTION The Joint Precision Approach and Landing System (JPALS) being developed by the United States Department of Defense is intended to provide accurate and reliable guidance information to military aircraft landing on land and aircraft carriers using GPS augmented with other sensors. For land-based operations, Local Differential GPS (LDGPS) will be used, whereas Shipboard Relative GPS (SRGPS) techniques will be employed for aircraft carrier landings. Presented at ION NTM 2005 - San Diego, CA - January 24-26, 2005 1/11