“GPS On A Chip” - An Advanced GPS Receiver for Spacecraft

T. Meehan, C. Duncan, C. Dunn, D. Spitzmesser, J. Srinivasan, T. Munson, J. Ward, J. C. Adams, J. How

Abstract:	Three years ago, facing a growing list o f stringent navigation and science requirements for future low-cost Earth missions, NASA began a program to develop a new high performance spaceborne GPS receiver. The program acquired the (inaccurate) title “GPS on a Chip” (GOAC), reflecting the long-term goal to reduce the receiver to a credit-card size package of unprecedented versatility and power. The GOAC development is a collaborative effort involving the Jet Propulsion Laboratory, the Goddard Space Flight Center, and Stanford University. A principal goal is to produce a low-cost modular design that can be configured efficiently for a wide range of unique space applications. These include (1) real time uses such as few-meter navigation and timing, precise attitude and rate determination and pointing control, formation flying, rendezvous and proximity operations; (2) post-processing uses such as centimeter-level orbit determination for ocean altimetry and precise relative tracking for gravity recovery; (3) direct remote sensing uses such as tracking rising and setting signals as they pass through the earth’s atmosphere and acquiring signals reflected off the surface of the oceans; (4) advanced utility functions from receiving and interpreting uplink commands to operating an entire satellite autonomously throughout its mission by means of a powerful embedded processor. In late 1995, the GOAC development team laid down a set of basic requirements: l Channels (simultaneous signals tracked): configurable from 8 to 96. l Tracking methods: C/A, Pl, P2, or “codeless” tracking assignable to any channel. l Observable: C/A, Pl, P2 pseudorange and Ll, L2 continuous carrier phase. l Code precision: <10 cm pseudo range and <0.2 mm phase ( 1 sec integration). l Codeless precision: <30 c m pseudorange and d mm phase (1 sec integration). l Antennas: configurable from 1 to 12 with parallel inputs. l Data sample rates: up to 100 sampleslsec (dual frequency pseudorange and phase) l Uplink data extraction: up to 1 Mbit/see l Operation: on-orbit cold start in 15 min with no initialization; restart within 60 sec In addition to providing low cost GPS receivers for spacecraft designers, the GOAC program shall be used to demonstrate several new concepts that improve the G]% receiver’s functionality in orbit. Sc\me of these new algorithms include real-time precision orbit determination, carrier phase based attitude determi nat ion on. nonaligned antennas, calibration free attitude determination, GPS-only attitude rate measurements, on-orbit Wide Area Augmentation Satellite (WAAS) capability, single antenna safehold operation, and autonomous receiver and spacecraft operations. These new capabilities are expected to greatly improve the GPS receiver’s usefulness as an engineering sensor for spacecraft and lower overal 1 satellite operating costs. The first full version of the GOAC (the “Blackjack”), with 4 antennas and 48 parallel channels, will begin testing i n the summer of 1998. Configured as a cube 10 cm on a side, it features a 4-layer, modular design. The Blackjack will weigh under 3 kg and consume about 10 watts. Initial units will be rated for about 15 Krad total radiation dose, but can be hardened to higher levels. The first two operational flights will be on the German CHAMP mission and the Argentine SAC-C mission, both scheduled for launch in the summer of 1999.
Published in:	Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998) September 15 - 18, 1998 Nashville, TN
Pages:	1509 - 1517
Cite this article:	Meehan, T., Duncan, C., Dunn, C., Spitzmesser, D., Srinivasan, J., Munson, T., Ward, J., Adams, J. C., How, J., "“GPS On A Chip” - An Advanced GPS Receiver for Spacecraft," Proceedings of the 11th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1998), Nashville, TN, September 1998, pp. 1509-1517.
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