TECHNOLOGY AREA(S): Space Platforms
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OBJECTIVE: Develop scalable, flexible lower SWaP-C GPS crosslink capability that allows future operational systems to forego significant dependence upon ground clock and ephemeris refresh with concurrent support of real-time command and control and age of data.
DESCRIPTION: Global Positioning System (GPS) space segment is currently comprised of 31 satellites in MEO at an approximate altitude of 20,200km (radius approx. 27,600km). The nominal space segment for GPS space consists of 24 space vehicles (SVs) in six orbital planes. Each GPS orbital plane nominally contains four satellites at a 55-degree inclination each (tilt relative to the equator).
GPS satellites receive updates from dedicated ground antennas located at Cape Canaveral, Ascension Island, Diego Garcia, and Kwajalein, as well as the Air Force Space Control Network (AFSCN). The purpose of these updates is to synchronize the atomic clocks on board the satellites, update ephemeris data onboard the satellite, and provide command and control of satellite functions, to include critical data and capabilities needed by U.S. and allied forces in theaters of operation. Each satellite is normally updated once per day, resulting in an average age of data of 11 hours and 58 minutes. Between these updates, as the age of the ephemeris and clock data increases, positioning and timing errors experienced also increase.
GPS Blocks IIR and IIF utilize UHF crosslinks (260 MHz to 290 MHz). This band is allocated on a primary basis to the mobile and fixed services. The Navy UHF Follow-On program has priority for the use of this band, and several other primary users such as paging services limit the utility of this band. Furthermore, the current UHF antenna is omnidirectional, which increases the interference potential. The GPS Directorate has considered other bands such as Ka Band (22.55-23.55 GHz) and V-Band (59.3-64.0 GHz) with directional antennas, but the large size, weight, and cost of this system hindered further development.
To minimize reliance on ground uploads to individual satellites; it is desirable to have GPS satellites communicate via reliable crosslinks at high rates to all assets that minimizes the age of clock and ephemeris. The implementation of this high data rate crosslink provides ephemeris and clock updates across the constellation from a single ground to SV upload with the remaining SVs obtaining updates via the crosslinks. (Updated data transmitted are currently relayed via the crosslinks.) A realistic approach to understanding the viability of these high rate crosslinks is to design and develop capabilities to synchronize clocks and to generate onboard ephemeris via inter-satellite range determination on the crosslinks and geo-location from a crosslink ring with a specific focus on reduced SWaP-C.
Peak data rates per crosslink initially are expected to be approximately 1.5 - 3 Mbps and average data rates will be approximately 200-700 kbps.
Any relevant proposal should clearly indicate how the intended effort conclusion result will improve the GPS system's capabilities via a validation and verification plan.
Proposers are highly encouraged to work with relevant PNT system prime contractors to help ensure applicability of their efforts and the initiation of technology transition design and development.
Proposers should clearly indicate in their proposals what government furnished property or information are required for the success of the effort. Requests for other-DoD contractor intellectual property will be rejected.
PHASE I: Design and develop an innovative concept, which includes a preliminary design for a low SWaP-C space based crosslink system for GPS that meets or exceeds government-specified application requirements.
PHASE II: The selected proposer will design and build an EDU for the GPS crosslink system ground test and evaluation. Phase II efforts should ensure compatibility with component interface descriptions which support overall payload and space vehicle reference designs as part of their commercialization effort. Interface descriptions will be supplied to Phase I awardees invited to propose for Phase II.
PHASE III DUAL USE APPLICATIONS: Military application: GPS space and MilSatcom constellations. Commercial application: Wide Area Augmentation System (WAAS). Commercialization of the proposed innovation through a Phase III should motivate partnerships with other GPS system contractors.
- "On-Orbit Validation of GPS IIR Autonomous Navigation," John A Rajan, Matthew Orr, and Paul Wang (ITT), Proceedings of the Institute of Navigation 59th Annual Meeting, 23-25 June 2003, pp. 411-419.
- "The Future of the Global Positioning System," Defense Science Board, October 2005, p. 44 - 45.
- "National Positioning, Navigation, and Timing Architecture Study Final Report," National Security Space Office, September 2008.
- "Alternative Architectures for Reduced Age of Data," Ranwa Haddad, John R. Berg, Bernard B. Yoo, Proceedings of the 22nd International Meeting of the Satellite Division of the Institute of Navigation, Sept 22-25 2009, pp 1519 - 1529.
- “GPS Modernization: GPD III On the Road to the Future”, Dr. Keoki Jackson (Lockheed Martin Space Systems Company), Stanford PNT Symposium, November 13-14, 2012.
- “Laser Cross-link Systems and Technology,” D.L. Begley, IEEE Communications Magazine, 06 August 2002, pp. 126-132.
- “Free-Space Optical Communications for Next-generation Military Networks,” J.C. Juarez, A. Dwivedi, A.R. Hammons, and S.D. Jones, IEEE Communications Magazine, 20 November 2006, pp. 46-51.
KEYWORDS: GPS, crosslink, space-based networks, data transfer, free space optical data transfer, FSO