Test Results from a Precise Positioning and Attitude Determination System for Microsatellites using a Software-Defined Radio

Alison Brown, Peter Brown, Benjamin Mathews

Abstract:	Current GPS technologies for satellite navigation are costly, heavy, and utilize high amount of power. This makes such systems difficult for microsatellites to support. In this paper we describe a low-cost, lowweight, low-power GPS navigation system to support micro/nanosatellites. A key component of our solution involves our patented TIDGET-based receiver design, which takes a brief snapshot of GPS data and powers off until the next position fix is desired. The processing of the TIDGET snapshot data is implemented in a software defined radio (SDR) using a GPS software application. This approach shares the resources available within a spacecraft’s SDR to support both communication and navigation functions reducing the size, weight and cost of the hardware on a micro/nanosatellite. An important aspect of the TIDGET is its modular size, which allows multiple TIDGETs to be placed on the satellite shell for full GPS visibility and robustness to satellite spin. The current state of GPS receivers for spacecraft onboard position and velocity measurements to update orbit propagators is to employ multiple stand alone GPS receivers or a multi-antenna GPS receiver connected to different antennas placed around the spacecraft to remain in the field of view of the GPS satellites. These GPS receivers obviously require valuable resources of power, mass, volume and cost to perform their function. If a system could be developed to achieve similar performance while realizing savings in one or more of these areas, there would be a strong demand in the industry especially as smaller spacecraft platforms gain popularity. The NAVSYS TIDGET technology offers several key advantages over currently available spacecraft GPS receivers. TIDGET receivers are low-weight, small, and consume much less (peak and average) power than traditional receiver designs. The TIDGET receiver captures only a small snapshot of GPS data, on the order of 10s of milliseconds, and does not run and draw power continuously. Since the TIDGET only captures data and does not process it, the TIDGET offers “on demand” processing anywhere that the data can be sent. In other words, as long as the navigation solution is not needed in real-time, the GPS data snapshots may be processed on an as-needed basis, when convenient for the processors, and multiple snapshots may be queued/stored for later processing. Finally, the TIDGET is a small-form factor module that may be easily attached to the spacecraft shell. In a current design in development, we are using three TIDGET modules that are interconnected so that snapshots are synchronously collected and jointly processed. This allows the spacecraft to have full 360° visibility in both azimuth and elevation and can account for spacecraft spin, if necessary. The multiple TIDGET signals can also be used to estimate the attitude of the spacecraft if sufficient common satellites are in view. In this paper we describe the design, simulation, and testing of the TIDGET receiver developed for microsatellite operations and describe the benefits of this approach and the processing employed within the SDR to perform both positioning and attitude determination. We will present a discuss software modifications necessary to account for the high velocities associated with space flight, and present design plans for integrating our MATLAB processing software to a representative microsatellite avionics module for onboard processing. Test results obtained with a GPS RF simulator will demonstrate the performance of this system in a space environment.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	769 - 774
Cite this article:	Brown, Alison, Brown, Peter, Mathews, Benjamin, "Test Results from a Precise Positioning and Attitude Determination System for Microsatellites using a Software-Defined Radio," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 769-774.
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