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Session D4: PNT Situational Awareness: Operations

Trusted GNSS Ephemeris for Military Applications from a Long Term Orbit and Clock Correction Source
Eric Vinande, Jason Pontious, Mark Carroll, AFRL/RYWN; Jason Drotar, Michael Merrigan, NSWCDD
Location: Room 6-8
Date/Time: Monday, Jun. 3, 5:05 p.m.

To make full use of multi-GNSS signals in a trusted manner, a user must have confidence in the source of a ranging signal along with the location of the signal source (orbiting satellite). This presentation will focus on a trusted GNSS replacement ephemeris framework for receivers able to accept externally loaded ephemeris parameters (satellite orbit and clock correction). The generation of replacement ephemeris by a US government entity is a key enabler for the trusted military use of GNSS signals.
Previous work assessed the accuracy over propagation time of a long term orbit and clock correction (LTOC) ephemeris representation and propagator developed by Naval Surface Warfare Center, Dahlgren Division (NSWCDD) with planned demonstration on the upcoming NTS-3 mission [1]. The present work will describe the conversion of LTOC to ephemeris parameters and utilization by a commercial satellite navigation receiver and a government reference satellite navigation software defined receiver platform.
Since the LTOC representation provides useful accuracy levels for many missions for up to two weeks, the present use case being explored is one where a user accesses updated LTOC information when convenient and produces ephemeris for specific mission time windows on demand. The workflow will utilize an LTOC propagator based on the Estimation and Prediction of Orbits and Clocks to a High Accuracy (EPOCHA) software maintained by NSWCDD with the addition of conversion to GNSS ephemeris parameters with a 2-4 hour fit interval. This interval will be chosen to align with what is utilized for existing broadcast ephemeris messages (e.g. a 4 hour fit interval for GPS). The ephemeris parameters will be provided in a Navigation Message Replacement “NMR-Lite” file. This file is in XML format and mimics the GPS NMR standard [2], by providing the same orbit and clock parameters, but lacks most of the auxiliary information normally contained in a navigation message.
For use by an existing commercial satellite navigation receiver, the NMR-Lite XML file will be converted to match the ephemeris log format that receiver accepts. Positioning accuracy results with varying LTOC propagation lengths will be presented for a hardware receiver and for the GNSS Test Architecture (GNSSTA) receiver platform developed by MITRE and AFRL.
References:
[1] Brannon, Chris, Han, Wen, Vinande, Eric, Drotar, Jason, “Multi-GNSS Long Term Orbit and Clock (LTOC) and Integrity Experimentation with NTS-3 User Equipment,” Proceedings of the 2023 Joint Navigation Conference, San Diego, CA, June 2023, pp. 988-1009.
[2] GPS Operations Center Interface Description Document (IDD), Version 12 (16 Dec 2009); Section 3.6: Navigation Message Replacement, pp. 26-31.

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