Positioning in GPS Challenged Locations - The NextNav Terrestrial Positioning Constellation

In this paper, we explore the limits of GPS-based positioning solutions with specific emphasis on the challenges presented to GNSS systems in indoor locations and urban canyons. It is in this context that we introduce NextNav's location technology and system architecture, and its significance as a reliable, ubiquitous, low power, fast (order of 6s cold-start TTFF) positioning service. Conceptually, NextNav is deploying a "terrestrial positioning constellation", which combines accurate horizontal positioning with floor-level height precision. NextNav's technology is based on the deployment of long range terrestrial beacons across a wide area (for example, a large metro like the San Francisco – San Jose metropolitan area) transmitting positioning signals over licensed spectrum. NextNav beacon's signals can be received using the upcoming generation of multi-radio GNSS ASICs. Compared to the current generation of GNSS chipsets, NextNav’s technology requires only firmware modifications to the digital baseband and appropriate modifications to the RF frontend of a receiver to support NextNav’s frequency band. This minimizes the cost impact to the overall receiver solution and the time and effort required for implementation. A receiver that is authorized to receive NextNav's signals employs trilateration methods to compute position at the receiver without the need for other signals or cellular data/control channels. NextNav’s signal processing emphasizes the key challenge of terrestrial positioning, fixed reflected-path bias, as opposed to the differing challenges imposed by remote, satellite-based transmitters. The NextNav beacons are designed to be autonomously synchronized to within a few nanoseconds of each other. Further, the beacon signals are broadcast on an encrypted basis to help provide a secure location service and prevent spoofing or other unauthorized uses of the signal information. The key features of NextNav’s technology are: * Consistent, wide-area indoor and outdoor location accuracy – the network is deployed with metropolitan-area coverage, and is not limited to specific venues. * High horizontal accuracy (currently ~20m) and precise vertical accuracy (~1 – 3m) available today, with yields of 98% where deployed. * Designed to be integrated with GNSS chips, minimizing handset cost. * Designed for seamless core network integration across both control-plane and user-plane-based applications. * Operation as an “overlay” network similar to GPS, independent of GSM/EDGE, UMTS, LTE and future communications air interface technologies. A data channel is not required for positioning. * Fast Time to First Fix from a cold start - of the order of 5-6s; in assisted mode or from a warm start, TTFF of less than 1 second is possible. * Position computation is done on the device. * Nextnav's signals are complementary to GPS signals. The system has been operational in the San Francisco Bay Area for over three years and more recently has been deployed in 40 markets across the US. NextNav's technology has been subjected to numerous third party trials to verify system performance. Extensive internal testing across the San Francisco Bay Area in malls, office buildings, homes and hotels (including deep indoors in downtown San Francisco), yielded cumulative 2D performance numbers of 20m position error 50% of the time, 26m error 68% of the time and 47m error 90% of the time. Most recently, the FCC-sanctioned CSRIC Working Group 3, tasked among other things with exploring indoor location accuracy standards for wireless E911 autolocation, conducted a side-by-side trial of various location technologies at its national test bed. This test program examined horizontal and vertical indoor location performance across rural, suburban, urban and dense urban morphologies. NextNav was a participant in that test bed (1) and the results validated system performance in a test program designed to challenge competing location technologies for safety-of-life applications. Thus far, there has been no positioning technology that provides accurate vertical positioning in a multi-story building. NextNav has addressed this issue co-locating weather reference stations with its beacons. This weather information, including pressure and temperature among other factors, is included in the NextNav transmission and provides localized reference information to handsets equipped with built-in MEMS pressure sensors. By combining the handset pressure data with the reference information, vertical positioning to the floor level has been demonstrated. In extensive testing across dozens of multi-story buildings, NextNav has demonstrated median vertical accuracy of better than 2m. The paper also explores the challenges of deploying such beacons in a wide area and maintaining highly precise time synchronization across all weather conditions. We also discuss the multipath and jamming issues at the receiver with specific real world examples. The paper concludes with the current state of the technology and deployment, receivers that are being built to work with the technology, and comparative performance results of NextNav's technology against other carrier grade technologies. (1) CSRIC III is Communications Security, Reliability and Interoperability Council was established by FCC. The tests were conducted by an independent 3rd party, sponsored by WG3 working group of CSRIC. The official results will be available for general public in mid March.