TECHNOLOGY AREA(S): Space Platforms
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OBJECTIVE: Develop advanced VHF/UHF inter-satellite radio network utilizing dynamic spectrum access and non-directional antennas to achieve 1-2 Mbps average data rates, providing GPS autonomous navigation and near real-time command and control (C2).
DESCRIPTION: Crosslinks have been an integral part of GPS satellites since the early days of the program. With the Block IIR satellite, the role of the GPS crosslinks expanded to include an Autonomous Navigation (Autonav) capability. By using crosslink ranging and onboard computation of ephemeris and clock states, the Block IIR satellites have the ability to maintain accuracy for a specified period of time after loss of contact with the GPS control segment. Original plans for GPS III included directional crosslinks at a much higher frequency to support new capabilities. A fully populated GPS III crosslink network would have provided control of the constellation from a single Continental United States (CONUS) ground antenna, zero age of data availability with a single upload, and near real-time command and control. Due to the high Size, Weight, Power, and Cost (SWAP-C) of the directional crosslinks, this capability was deferred from the first ten GPS III satellites.
The current UHF/VHF crosslinks have a few advantageous features, such as low SWAP-C and non-directional antennas that simplify the network architecture. The primary disadvantage of UHF/VHF is the crowded spectrum, where GPS is a secondary user that must operate on a non-interference basis. New techniques in cognitive radio and Dynamic Spectrum Access (DSA) may permit reconsideration of UHF/VHF for enhanced GPS crosslinks. Fortunately, research into utilizing the television “white space” as permitted by the Federal Communications Commission (FCC) may yield applicable techniques and technologies that can be exploited for GPS crosslinks.
This topic is focused on assessing the feasibility of enhanced UHF/VHF crosslinks for GPS using cognitive radio and DSA. The objective is to provide a 1-2 Mbps average data rate while ensuring that GPS crosslinks do not interfere with other users in the band. Using the existing crosslink antennas on GPS, the crosslink architecture should include spectrum sensing and Dynamic Frequency Selection (DFS) to utilize the band on a non-cooperative basis with other users of the band.
PHASE I: Create a model of existing UHF/VHF spectrum utilization in the Medium Earth Orbit (MEO) environment. Develop an architecture, operational concept, and preliminary design for enhanced GPS UHF/VHF crosslinks, to include DSA technologies, network architecture, spectrum sensing, and information assurance.
PHASE II: Demonstrate enhanced UHF/VHF crosslinks with brassboard hardware and simulated interference environment.
PHASE III DUAL USE APPLICATIONS: Develop prototype UHF/VHF crosslink payload, suitable for flight experiment testing. Commercial: Application to commercial satellites and/or terrestrial use.
- P. Brodie, “Evolution of the GPS Navigation Payload – A Historical Journey,” Stanford Center for Position, Navigation & Time (SCPNT), October 2009.
- Sonntag, Henry E., "Block IIR UHF Crosslink Enhancements Study," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 819-828.
- Shellhammer, S.J.; Sadek, A.K.; Wenyi Zhang, "Technical challenges for cognitive radio in the TV white space spectrum," Information Theory and Applications Workshop, 2009, pp. 323,333, 8-13 Feb. 2009.
KEYWORDS: GPS, crosslinks, cognitive radio, dynamic spectrum access, UHF, VHF