Session A1: Alternatives, Backups, Complements to GNSS

On the Feasibility of a Carrier Phase-Based Velocity Solution from LEO Satellites
Zhen Zhu, East Carolina University; Sanjeev Gunawardena, Air Force Institute of Technology; Eric Vinande, and Jason Pontious, Air Force Research Laboratory
Location: Beacon A

The potential position, navigation and timing (PNT) capability of existing and future Low Earth Orbit (LEO) satellite systems has been considered an alternative or supplement to GNSS. Recent development includes LEO-assisted GNSS precise point positioning (PPP) [1], standalone LEO positioning solution, differential LEO solution [5][6], or LEO integrated with inertial sensors [7]. The analyses in [2][3][4] showed that a Doppler-based positioning solution could potentially reach meter-level accuracy, which could indeed serve as a backup system to GNSS. In practice, however, it would be much more challenging to achieve this level of accuracy with existing commercial LEO systems, mainly due to limitations in ephemerides and the onboard clocks.
For most commercial LEO systems, it is assumed that the satellite position can be computed with the two-line element (TLE) datasets published by the North American Aerospace Defense Command (NORAD). A simplified general perturbation (SGP4) model is used to propagate the satellite position over time. It has been recognized that the satellite position and velocity estimated with this approach bear significant errors [8][9]. Furthermore, these LEO satellites are not equipped with the same time references as GNSS, which means that the satellites may not have high-quality onboard atomic clocks, and they may not be synchronized within the constellation [10].
In a LEO-based position solution, the contribution of the spatial and temporal error sources has been studied. These error sources may be corrected or mediated via differencing, or be estimated in an integrated solution. The impact of LEO error sources on the receiver velocity solution will be analyzed in this work.
It has been well-documented that the velocity of a GNSS receiver can be estimated based on time-differenced carrier phase. This solution also requires a unit vector that points from receiver to the satellite, which is computed based on the approximate positions. The carrier phase measurement should also be corrected using the broadcast model of the GNSS satellite clock.
Time-differenced carrier phase can be extracted from LEO signals as well. However, the receiver and satellite position errors may be significantly greater than those from GNSS, which results in an erroneous unit vector. As afore mentioned, a LEO satellite clock is expected to be of lower quality than GNSS, and there may not be a correctional model for it either. Given these constraints, is it still feasible to compute a meaningful velocity solution from an existing commercial LEO constellation?
This paper will seek to answer this question using practical error models derived from existing LEO systems. The contribution of major error sources will be analytically determined, based on which possible correction or mitigation processes will be discussed. The integration of LEO satellites into a GNSS/inertial solution will also be explored.
This work was supported by Air Force Research Laboratory, FA8650-22-C-1017. This document has been approved for public release, distribution unlimited. Case Number: AFRL-2024-5190.

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