Terrestrial Timing
SystemSECURE, RESILIENT SOLUTION FOR POSITION,N AVIGATION, A N D TIM IN G

> Need for resilient timing> TerraPoiNT system

Agenda> Network and beacon timing> Timing in urban areas with multiple hops
> Trials and results> Conclusion

Resilient Timing• Timing has become critical for major infrastructure and requires a resilient timing system− Most of the timing systems utilize GNSS receivers to derive and provide timing information• GNSS-based Timing systems are susceptible to the following problems:− Low GNSS signal strength creates constraints in antenna placement & cable calibration− Easily prone to interference: weak signals can be easily jammed− Spoofing: Various incidents have indicated that GPS system is vulnerable to spoofing which cancause more damage than undesired interference• The NextNav TerraPoiNT system provides a better timing solution that overcomes thechallenges of existing systems

System Attributes• Provides full position, navigation and timing capabilities in urban and indoor environmentswith or without the presence of GPS− Timing only services offered with 1-2 beacon overlap− Full PNT services with 4 beacon overlap− Resilient timing and operation in absence of GPS• “Mission critical” reliability: support for an encrypted signal and resistant tospoofing/jamming• Nationwide spectrum owner (920 – 928 MHz) – 95% urbanized POPs

• Supported in global cellular standards - 3GPP (Rel. 13+) and OMA (SUPL 2.0.3) as “MBS”, atype of terrestrial beacon system (TBS), as well as at other SDOs
• PNT IC based capability available from 1H 2022

4System Architecture• Beacons deployed throughout the coverage area
• Each beacon has an onboard timing source• Beacons share time with each other to maintain local synchronization• UTC sync to a small subset of beacons can be provided using:− Any absolute alternate timing source (eg. ToF, LEO satellite, TWSTT) and can also be
maintained across outages using a local Cesium.
− Or… holdover from GNSS maintained using local Cesium during GNSS outages• Signal is encrypted to enable user authentication and to provide a level ofspoofing protection

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Resilient mesharchitecture:
if any beacon iscompromised, theservice will continue

THE CASE FOR GPS ALTERNATIVESTiming Fusion Technology
Leader Beacon

Follower Beacon• Can accept multiple absolute time sources: i.e GPS,Time-Over-Fiber (TOF), Low-Earth Orbit satellites• All beacons have a two-way time transfer receiversynchronizing through measured paths from other beacons• Rb atomic clock provides ST-2 holdover and ability foran extended holdover source such as Cesium• Follower-Leader timing is maintained through two-way timetransfer based corrections

TerraPoiNT
Tx/Rx
Antenna

GPS Receiver
GPSAntenna

CesiumTimeRubidium
Clock

TerraPoiNT
Transmitter

Radio/RFOtherabsoluteTime source
like ToF, LEO
Telemetry
Modem* Note that the beacon can operate completely GPS free

• Follower control loop with mix of centralized and distributedcontrol which applies the corrections to the Rb clockMesh Network Wireless Sync of Beacons• The network is a mesh of beacons (transmitters) consisting ofLeader beacon(s) and Follower beacons− Leader beacon(s) have access to one or more absolute time sources(UTC)− Follower beacons listen to one or more Leader and Follower beacons,and compute ToA measurements for time transfer purposes• TWTT technology is used to take ToA measurements andcompute synchronization information for each beacon
APNT Demo Networkat SJ7

Time transfer over multiple-hops• In a network, the follower beacon can derive timing using another follower resulting inmultiple hops to synchronize the beacon with leader which has the UTC time
− The mean of the time transfer error between a pair of beacons (one hop) can be positive or
negative− When time is transferred across multiple hops, the cumulative mean time error need not growalways. However, the variance of time error grows as the number of hops increase− TerraPoiNT network can have multiple time transfer beacon paths where the best path can be preselected during installation or dynamically to get minimum time error growth• In this APNT network, the follower beacons were able to maintain time within 10ns of theabsolute time8Time Distribution Demonstration• A network of 4 beacons in SanJose (SJ) downtown:− Leader, Follower 1 (F1), Follower 2 (F2) and Follower 3 (F3)

Network and Test Sites
F1 beacon− F1 follows Leader, F2 follows F1 and F3 follows F2 beacon .

• Total time transfer distances:

5.13Km
7.57Km− First hop distance (L→F1) is 5.13 km

Leaderbeacon− Two-hop distance (L → F1 → F2) is 12.7 km− Three-hop distance (L → F1 → F2 → F3) is 16.24 km• Two test locations, in challenging indoor environments• NextNav Timing Receiver (NTR) configured to track F1, F2 orF3 depending on the scenario.
9F2 beacon

Receiver attest site 2
F3 beacon

3.54KmReceiver attest site 1

This work was funded by the Department of Homeland Security, Science andTechnology Directorate under contract # 70SAT19CB0000023

Demo Timing Receiver Capabilities• Provides HW based PPS + 10MHz + time of day (TOD) message including residual
time corrections• Performs self-survey or accepts pre-surveyed location as input• Automatically selects the best beacon signal for extracting timing• Based on an OCXO
• Present receiver takes ~10 minutes to provide a valid PPS output after thereceiver is configured/powered-on• Configuration and monitoring is through a serial port

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Demo Performance Assessment Methodology @Timing Receivers

NextNav Timing Receiver (NTR)• Timing receiver is used in this demo to derive timing from aspecific beacon• Performance of the receiver against timing truth is measured
using Calnex or an equivalent system

Ref-PPS
NTR-PPS• The Calnex or equivalent system measures and displays thetime error of timing receiver pps with respect to timing truth

Time Error(ns) (ns)Sample Calnex plot

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Calnex or Equivalentsystem
Time Elapsed (Ks)

NTR connected
to PC toconfigure andderive timingfrom aparticularreceiverTest Setup at Site 1
The test location is in San Josedowntown

Antenna for timing receiver

The timing receiver and its antenna arelocated inside a sheltered equipment
room on the top floor of a 14-storybuilding.
12TimingreceiverTest Setup at Site 2
The test location is in San Josedowntown.

The timing and its antenna
are located inside the officespace, on the 8th floor of a12-story building.
The office is in a deep
indoor location, ~25 ft fromthe window.
13Calnex GPS antenna

CalnexNTR

NTR Antenna

Results @ Test Sites• Short Term tests over 1 hour duration wereperformed

High accuracy @ errors well under 100 ns
Stable performance @ less than 20 ns variability
100 nsLowest errors for F3 cases
due to benign multipath from
F3 to receiver at site 1Test
Site1

F1Test

F3F2

Highest errors for F1 cases due
to challenging multipath from
F1 to receiver at site 2

100 nsSite2

14F1F2

F3

Long Term Test
• A test was performed over 72 hour long todetermine the performance over a long term

• Timing performance approximates astationary process with stable mean andvariance• The results indicate absolute accuracy within50ns• Sub-50ns absolute timing performance
was maintained through the 3-day test

• This indicate TerraPoiNT timing is stable overlong term
Day 1

15Day 2Day 3

Beacon Time Sync Performance
Time error (ns)Time error (ns)Beacon timing compared to the GPS Timing truth

Time of day (m)Time of day (m)Follower 1

Time error (ns)Time error (ns)

LeaderTime of day (m)Time of day (m)Follower 2
Followers track the Leader
Timing errors within [-15 ns to +15 ns]
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Follower 3

Conclusions• Demonstration showed a functional, stable and high precision timing system that is independent of GPS, in areal-world environment• Time transfer system, combined with a Cesium clock or other time source can deliver high precision timing
wirelessly across a wide coverage area
• The demonstration showed the system can accept multiple time references and maintain relative and absolute
sync across the beacon network and in a robust manner• Beacon network timing for end user timing applications was delivered in difficult urban environments using atiming receiver accurate within 50ns, which could be built with the same form factor as a GPS chipset-basedsolution• Demonstration shows the TerraPoiNT system provides the capability to distribute the UTC time wireless withaccuracy required to support applications such as 5G time sync, timing for automotive applications

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