Sameet Deshpande, Arun Raghupathy and Payam Dehghani Rahimzadeh, NextNav Inc.

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Timing systems have been the backbone of various critical infrastructure networks across the world and are required to provide an accurate, precise information all the time. The timing systems mainly rely on satellite based GNSS signals. Recent studies have shown the GNSS signals are easily prone to interference and spoofing which can lead to denial of service and result in catastrophic consequences. Hence, there is strong need to build resiliency into the timing systems which can continue to provide accurate and precise timing under all conditions. Multiple independent timing sources can be utilized to achieve resiliency and robustness. NextNav’s TerraPoiNT system, a terrestrial navigation system, can provide accurate timing independent of GNSS and is a viable alternative for a timing source. TerraPoint system is Time-Of-Arrival (TOA) multilateration system offering 3D positioning and timing through a dedicated terrestrial network of synchronized transmitters operating in the 920-928 MHz frequency band, using Code Division Multiple Access (CDMA) spread spectrum signals. The transmitters, beacons, are typically synchronized to absolute time through GPS but can also use other absolute time sources such as a disciplined Cesium clock, LEO satellite systems, eLoran or Time over Fiber. In the absence of an absolute time source or during extended outages the transmitters can maintain relative timing synchronization sufficient to provide position and navigation capability as well as relative timing without any significant loss in performance. TerraPoiNT beacon network is a geographically distributed terrestrial network of beacons, with each beacon containing a GPS receiver and an atomic clock. TerraPoiNT network can be deployed across the city or over a small campus depending upon the application. TerraPoiNT beacons have a timing sub-system which is used to maintain time and synchronize with other beacons in the network. The beacons are classified as “Leader”, which have access to absolute timing source like Time-Over-Fiber (ToF), LEO-satellite based as well as atomic clock like Cesium which allows the beacon to maintain synchronization with the UTC time and “Follower”, which derive timing from leader beacon or other follower beacons in the network. The beacons can also “listen” to each other to transfer time and self-synchronize. Since beacons 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 non-GPS time sources 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 GPS through measurement data consistency check using historical data as well as stability comparisons with respect to the atomic clock. A GNSS-free TerraPoiNT network of four beacons was deployed in San Jose Downtown to demonstrate the timing capability of the system. One of the beacons was classified as “Leader” which had access to absolute time source and other beacons derived timing from remaining beacons in the network. Although any follower can derive timing from a leader or any other follower which it can observe and perform measurements, the beacons in the network were configured to derive time from a specific beacon in these tests. A 3-hop network was setup and distance between leader and last follower was 16 Km with two followers in between. This allowed the time transfer capability over multiple hops to be demonstrated. The beacons were deployed in urban environment with significant building clutter in a heavy-multipath environment. The ability of beacons to maintain time synchronization in such an urban environment demonstrates the capability and reach of TerraPoiNT signals. In these tests, the absolute time source at the leader beacon were sufficiently filtered to remove micro jump and obtain a stable reference to maintain the synchronization to the UTC. All the follower beacons were able to maintain time within 10ns of the leader beacon irrespective of the number of hops used to derive timing. NextNav Timing receiver (NTR) was used to measure the timing performance of the network. NTR is used to derive timing and provide PPS + time of day (TOD) message using the TerraPoiNT signals. It can automatically select the best beacon signal for extracting timing or choose a user-configured beacon. In these tests, NTR is configured to select a particular beacon to extract timing to meet the objectives of showing performance with different hops. The NTR antenna is co-located with the receiver eliminating any need for an open sky placement as needed in GNSS-based timing receiver. A standard off the shelf timing truth equipment (Calnex) was used to measure the time interval error for the NTR. The tests were performed at two different locations, one inside an office on the 7th floor and other inside the sheltered room on the top of a building over different durations ranging from 1 hour to 72-hours. The tests were also performed by configuring the NTR to derive timing for each of the 3 followers, which helped to analyze any impact of time transfer over multiple hops. The timing accuracy over a 72-hour test to be within 45 ns of the UTC time. This indicate the TerraPoiNT system can provide accurate timing independent of any GNSS system and at any locations where TerraPoiNT signals are available including inside the buildings. Various timing tests done at different locations and using different Followers to obtain the timing show that the timing accuracy is within 50 ns under all conditions. These results indicate that TerraPoiNT system can provide accuracy timing within its coverage area using any beacon in the network by maintaining accurate and precise relative synchronization among beacons without the need for any absolute source at each beacon. The TerraPoiNT system can be easily scaled to transfer time across cities.