Funded in 2000 by the Earth Science Technology Office under the Advanced Information Systems Technology (AIST) Program, the GDGPS system set out to develop and demonstrate a GPS-based technology that will enable Earth-orbiting satellites, airplanes, and terrestrial systems to achieve unprecedented levels of real-time positional accuracy globally. Specifically, decimeter-level real-time orbit determination accuracy for satellites in Earth orbit, and 10-20 cm real-time positioning accuracy for airplanes and terrestrial vehicles. These capabilities are needed to enable a broad range of NASA ESE goals: autonomous navigation, intelligent sensor webs, science data compression, and enhanced capability to monitor, respond to, and predict natural hazards. In developing such a breakthrough capability we have leveraged the significant investment NASA has made in its Global GPS Network (GGN), as well as the government investment in the Wide Area Augmentation System (WAAS) technology developed at JPL. Although a number of private and government organizations provide real-time positioning services in localized regions to users on or near the ground, a global system such as we have developed has never been achieved before, nor even attempted, due to the perceived technical and cost challenges. For more information about the GDGPS system, including live performance monitoring, see http://galia.gdgps.net/igdg.

Highlights of achievements and benefits

Now in its third and final year of funding, the AIST GDGPS Project has achieved and surpassed its goals. We have developed a prototype operational system that provides revolutionary end-to- end capabilities for precise global positioning and orbit determination. The underlying software, Internet-based Global Differential GPS (IGDG), was selected as NASA Software of the Year, 2000. A series of ground tests and simulations, airborne tests, and space flight tests have demonstrated the advertised performance of the system.^[1,2,3,4,5] We have also demonstrated some of the tangible, cross-cutting benefits that can be realized from such a remarkable capability, including near-real-time (3 hours latency) science quality orbits (2.5 cm vertical accuracy) for Jason-1, near-real-time science quality (5 cm) sea surface height estimates from Jason-1 science data, sub-centimeter real-time zenith tropospheric delay estimates at ground GPS sites, sub- nanosecond real-time clock monitoring in support of Deep Space Network (DSN) operations, and few-centimeter accuracy for the geodetic coordinates of ground GPS sites.

Partnerships and customer base

The obvious commercial benefits from such as system have led to fruitful collaborations with industry and to significant technology transfers. Particularly noteworthy is the collaboration with Navcom Technology (a Division of John Deere & Co.), which enhances the robustness of the system by supporting redundant real-time data centers (see below). Navcom also provides one of the dissemination mechanisms for the differential corrections through leased global beams on three Inmarsat geosynchronous communications satellites. This saves NASA roughly $1M/year in communications costs. At the moment, JPL has long-term reimbursable support contracts with several major commercial companies for a variety of GDGPS products and services. In addition, several NASA missions and programs increasingly depend on the system's unique capabilities. These include the DSN and the Jason-1 Project. SRTM has used the real-time GPS orbit and clock products of the system for rapid orbit determination and data validation. The Ocean Surface Topography Mission (OSTM) has expressed strong interest in our capability to produce near-real-time sea surface height, as we have demonstrated with Jason-1 data. While the present customer support covers only a fraction of the total cost of operations we see excellent prospects to increase the customer support to the break-even point within the 5-year horizon of this work. Perhaps the most significant application is the potential of the system as a GPS integrity monitoring system, since it is the only available real-time system that tracks all the GPS satellites all the time. The U.S. Air Force, NIMA, and the Aerospace Corporation have expressed strong interest in this unique capability.

System architecture and operational status

While the system was developed as a proof-of-concept prototype (for example, it does not provide 24 hour support, nor any formal service level), it was designed with the capability to support reliable operations, which is key to the acceptance of this technology by the ultimate users. Innovative use of the Internet provides for high reliability through redundancy. This extremely cost effective architecture ensures that there are no single points of failure, and explains the track record of 99.99% reliability since 2000. The real-time data stream of GPS observables (pseudorange and phase) is provided by a subset of the NASA Global GPS Network (GGN) (Figure 1) that was configured with Internet connectivity, a computer, and the IGDG client software. The network is large enough to provide for multiple-redundancy. Each site in this network streams its GPS observable data to two data centers (operated by Navcom) that are widely separated geographically. The centers can echo the data to a number of operation centers that can act in parallel. The GDGPS Operations Center (GOC) receives this data stream through a dedicated frame relay line or through the open Internet. To ensure a reliable environment for operations, the GOC is run at a contractor's facility outside JPL. The concept of multiple redundancy is also employed in the computing architecture of the GOC, with hot dual-string computer chains that operate in parallel. Similarly, user access is enabled through two independent, high bandwidth communications channels to the Internet backbone. All resources are supported by uninterrupted power supply. The real-time corrections to the GPS broadcast orbits and clocks are disseminated through the open Internet, through customer-installed dedicated lines, or through Navcom's Inmarsat global beams. We have demonstrated direct access to the Internet and to the corrections data stream from any point in the world (on the ground or in the air) through Iridium satellite modems.

1. Bar-Sever, Y.E., R. Muellerschoen, and A. Reichert, The Development and Demonstration of NASA's Global Differential System, ESTC 2002 Conference, Pasadena, 2002 [http://esto.nasa.gov/conferences/estc-2002/Papers/A5P2(BarSever).pdf]^{(back to text)}

2. Muellerschoen, R., A. Reichert, D. Kuang, M. Heflin, W. Bertiger, Y. Bar-Sever, "Orbit Determination with NASA's High Accuracy Real-Time Global Differential GPS System," Proceedings of the ION GPS 2002, Salt Lake City, UT, 11-14 September, 2001.^{(back to text)}

3. Haines, B., W. Bertiger, S. Desai, D. Kuang, T. Munson, L. Young, P. Willis, "Initial Orbit Determination Results for Jason-1: Towards a 1-cm Orbit," Proceedings of the ION GPS 2002, Portland, OR, September 24-27, 2002.^{(back to text)}

4. Bertiger, W. I., Y. E. Bar-Sever, B. J. Haines, B. A. Iijima, S. M. Lichten, U. J. Lindqwister, A. J. Mannucci, R. J. Muellerschoen, T. N. Munson, A. W. Moore, L. J. Romans, B. D. Wilson, S. C. Wu, T. P. Yunck, G. Piesinger, and M. Whitehead, "A Real-Time Wide Area Differential GPS System," Navigation: Journal of the Institute of Navigation, Vol. 44, No. 4, , pp. 433-447, 1998.^{(back to text)}

5. JPL Press Release, October 14, 2002: http://www.jpl.nasa.gov/releases/2002/release_2002_191.cfm^{(back to text)}