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INTRODUCTIONGPS has made robust, all-weather, precise global navigation a reality since the 1980s [1], but only recently has this technology been applied as a sensor to formations of vehicles. GPS provides an array of sensing options for a formation, including absolute positioning, relative vehicle positioning, attitude estimation, and precise timing. It can provide sensing across the entire array of formation-flying missions in space, in the air, and on the ground.One application for this work is for formation-flying space science missions in which the objective is to replace the traditional approach of using a single monolithic satellite (such as Landsat-7) [2 -4] with a virtual spacecraft bus consisting of a cluster of autonomously controlled vehicles. This technology could then be applied to a number of distributed observation missions, including earth mapping (synthetic aperture radar [SAR], magnetosphere), astrophysics (stellar interferometry), and surveillance [5 -7]. A second application is the use of GPS sensing for groundbased formations of farm equipment (i.e., tractors, harvesters, combines), which could result in increased efficiency and a significant reduction in waste [8].Formation-flying applications also require a sophisticated interspacecraft communications system [4]. Missions requiring low-to-medium formation knowledge and control will typically use radio frequency (RF) -based telecommunications. However, several researchers are currently developing RFbased formation-flying communication devices that can also be used as local ranging systems. Examples include the Applied Physics Laboratory Cross Link Transceiver (CLT) [9], the Jet Propulsion Laboratory (JPL) Autonomous Formation-Flying Sensor (AFF) [10], the Stanford Pseudolite Transceiver Crosslink (SPTC) [11], the Star Ranger by AeroAstro [12], and the (ITT) Low Power Transceiver (LPT). By providing additional interspacecraft range and Doppler measurements to the navigation solution, these on-board transmitters could facilitate the use of carrier-phase differential GPS (CDGPS) for orbits in which adequate visibility/geometry to the NAVSTAR constellation is not available [11]. Furthermore, these RF ranging devices could be designed to provide far more accurate measures of the vehicle relative motions than those derived from the traditional GPS pseudolites. These highly accurate measurements could then be combined with the CDGPS solution to obtain a better overall estimate of the spacecraft relative positions and attitudes.

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