TerraPoiNT - An Advanced Terrestrial Technology to Enhance Navigation in a GNSS-Denied Environments

Sameet Deshpande

Abstract: The past few years have seen the navigation systems have become an important part of the everyday infrastructure. Although satellite based navigation systems like GPS have been the primary source, they are not sufficient to meet the accuracy and reliability needs of various modern applications like autonomous cars and unmanned aerial vehicles. To meet the requirements of these newer applications, a terrestrial based system is an ideal system to complement the traditional GNSS navigation systems. NextNav has developed a TerraPoiNT system, a terrestrial based positioning, navigation and timing system which can complement existing navigation systems as well as serve as a backup to the GNSS system. TerraPoiNT system (a.k.a Metropolitan Beacon System (MBS)) is a type of Terrestrial Beacon system standardized in global cellular standards - 3GPP (Rel. 13+) and OMA (SUPL 2.0.3) . TerraPoiNT is a Time-Of-Arrival (TOA) multilateration system offering 3D positioning and timing through a dedicated terrestrial network of synchronized transmitters using Code Division Multiple Access (CDMA) spread spectrum signals like GPS. TerraPoiNT also offers vertical positioning through integration with a barometric altimeter and environmental reference information provided over the data payload of the beacon. FCC authorized TerraPoiNT transmitter EIRP (Effective Isotropic Radiated Power) at 30 Watts (45 dBm) resulting in typical received signal power of about -80 dBm at 1 km range, which is about 100,000 times (50 dB) higher than the nominal received signal power for GPS. TerraPoiNT network is a geographically distributed terrestrial network of beacons. The beacons “listen” to each other and can perform relative and self-synchronize among themselves. The beacons in the network are classified as “Leader” and “Followers”. Each beacon has a timing sub-system which is used to synchronize and maintain time. “Leader” Beacons have a timing sub-system which has access to one or more absolute time sources like GNSS, LEO or ToF, along with a precise atomic clock (such as Cesium/Rb) allowing long holdover time to be maintained during outage of absolute time sources. As each beacon can listen to multiple beacons, a high degree of resilience is built into the network. In addition, this self-synchronization capability is scalable to a large network with only a few beacons requiring absolute time source or atomic clocks due to multi-hop beacon time transfer capability. Disruption and/or manipulation can be detected by looking at a drift in phase of the timing. The beacons are designed to detect anomalies in position and timing. This capability enables TerraPoiNT network to detect local anomalies in any absolute time system like GPS through measurement data consistency check using historical data as well as stability comparisons with respect to the atomic clock. TerraPoiNT’s innovative methods to address the near-far reception problem coupled with the capability to resolve very short multipath resolution enables it to provide a robust performance in any urban or indoor environment. TerraPoiNT signals can penetrated inside the building with sufficient signal strength to enable the TerraPoiNT receivers conquer the frontier of indoor positioning. The TerraPoiNT receiver technology has been licensed to several IC (Integrated Circuit) manufacturers (Broadcom, GCT Semiconductor, Intel) and it has been integrated in their standalone GPS chipsets, or LTE (Long Term Evolution) based GPS chipsets. TerraPoiNT System capabilities were assessed by the U.S. Department of Transportation (U.S. DOT) at LaRC as part of the alternate navigation technology assessment for positioning, navigation and timing. The DOT sponsored GNSS Backup trials were performed in the NASA’s LaRC campus and covered different positioning, navigation and timing scenarios. Twelve TerraPoiNT beacons operating without reference to GPS were deployed at the test site and synchronize with each other using two-way time transfer method. In the trials conducted by the DoT, the “Leader” beacon in the network used the Cesium clock and rest all beacons referred as followers performed observations to maintain time synchronization between beacons. This method allowed the time to be maintained within few nanoseconds. This method demonstrated an approach to maintain the timing in absence of any GNSS signals and robustness to any individual site failure making the timing system more resilient to any malicious attacks. DOT sponsored trial assessed different aspects of the technology like navigation performance, system readiness, commercial deployment, etc. There were a total of 12 technologies which participated in the assessment; However TerraPoiNT was the only technology which participated in the all aspects (positioning, navigation and timing) of the evaluation. The purpose of positioning and navigation tests were to evaluated different attributes like accuracy, availability and reliability of the system under test [1]. Another important aspect of navigation system is to provide accurate timing. The requirement to have accurate, precise and robust timing has become the backbone for various critical infrastructures. DoT trials also focused on evaluating timing accuracy of the different systems to determine a viable option for an alternate timing system. Tests were conducted in outdoor and indoor locations for static performance, vehicle navigation tests to evaluate 2D performance and Drone navigation tests for 3D performance. TerraPoiNT was the sole technology to participate in all the tests demonstrating the versatile nature of the system and capability to provide back-up to all aspects of GNSS system. TerraPoiNT receiver used fusion of TerraPoiNT signals and inertial sensor for 2D navigation to provide an accurate and reliable solution while for 3D navigation; only the TerraPoiNT signals were used. TerraPoiNT receiver data were collected for every test scenario. The reference system’s data were post-processed and corrected for system errors to obtain a cm-level accurate truth result. The TerraPoiNT receiver position data were compared with the truth data obtained from the reference by aligning them in time and then converting them to a local tangent plane coordinate frame with local east, north and up (ENU) frame [1]. Various error statistics were calculated for the positioning and timing error to obtain the accuracy of the system. TerraPoiNT technology showed exemplary performance in the various positioning tests under different dynamics. Static results were accurate within a couple of meters. The 2D navigation performance accuracy was within 3m (50%) and 8m (95%) of the entire test. The 3D accuracy was < 3m (50%) and < 5m (95%) over the different tests. In these drone tests, TerraPoiNT receiver utilized only the TerraPoiNT signals without any inertial sensor fusion. The vertical component was provided using the barometric data because VDOP is fundamentally limited for wide-area terrestrial systems. The drone test involved different dynamic condition and flight trajectory up to 100 m of altitude above the ground. The results showcase the ability of TerraPoiNT technology to provide accurate and reliable performance. These tests indicate that the TerraPoiNT beacons can self-synchronize within few nanoseconds enabling seamless navigation to the user in scenarios where GNSS system is compromised. TerraPoiNT system demonstrated accurate, reliable and robust performance in entire trials and it was rated #1 among all the technologies evaluated both in position and timing trials [1]. A combination of factors including performance accuracy, system/UE maturity, spectrum protection, signal robustness; service resilience, system interoperability and service longevity were considered to arrive at this rating. This proves that TerraPoiNT technology can complement as well as serve as back-up for GNSS system References [1]: Complementary PNT and GPS Backup Technologies Demonstration Report published in Jan 2021
Published in: Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021)
September 20 - 24, 2021
Union Station Hotel
St. Louis, Missouri
Pages: 2253 - 2271
Cite this article: Deshpande, Sameet, "TerraPoiNT - An Advanced Terrestrial Technology to Enhance Navigation in a GNSS-Denied Environments," Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri, September 2021, pp. 2253-2271.
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