We are heavily reliant on Global Navigation Satellite Systems (GNSS) for navigation, positioning and timing services, both for critical infrastructure and military operations as well as in normal life. However, GNSS signals are weak, making then vulnerable to both deliberate and accidental interference and deception (spoofing). An alternate means of navigation is required to take over when GNSS is completely denied and to validate it when it is not.
Stronger satellite signals at similar frequencies can be used to supplement and take over from GNSS signals, creating signals of opportunity to support positioning and timing. They are designed to communicate with small terminals such as satellite phones or modems, so can be processed to a similar size as a traditional GNSS receiver. Unlike terrestrial signals of opportunity like TV transmitters or mobile basestations, satellite signals have the advantages of providing global coverage and are not vulnerable to local power disruptions or outages. Roke was funded by the Defence Science and Technology Laboratory (Dstl) to investigate this approach, and we constructed a prototype antenna array capable of receiving satellite signals from a range of different sources in L and S bands. This enabled measurement of direction, Doppler and propagation time for each satellite. We then conducted a number of 4x4 trials to confirm the outcome.
This research shows that that is possible to exploit the transmissions from the existing Iridium, Globalstar and Orbcomm constellations to achieve navigation and timing for a moving platform. This is currently realised using an antenna array but could be achieved from a single antenna for more space constrained platforms. Future work would seek to further improve positioning accuracy by exploiting more satellite signals and to mature the concept to higher Technology Readiness Levels (TRL), in collaboration with government, military users and industrial partners.
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