Potential LEO Satellite Augmentation for Rescue-21 in Alaska

Dahnyoung McGarry, Richard J. Hartnett, Peter F. Swaszek, Bryan Chan, Banks Evans, Aidan Kenna

Abstract:	Rescue 21 (R21) is a land-based, very high frequency (VHF) positioning and communications system used by the US Coast Guard to locate mariners in distress and save lives at sea. In the lower 48, R21 has been celebrated as a resounding success as an invaluable aid for Search and Rescue (SAR), and system availability is generally acceptable. Unfortunately, R21 has not been as available in the far north, either directly or indirectly due to the harsh Alaskan climate. For this reason, we are looking at possible space-based solutions as promising alternatives to augment existing CG distress localization and communications capabilities. In recent years low Earth orbiting satellites (LEOS) have become extremely popular. Individual CubeSats are commonly mentioned in research, development, and educational efforts; LEOS constellations are in vogue at ION conferences for their potential in GNSS-denied positioning and navigating. The fundamentals of navigating with such satellites, primarily based on the observation of Doppler shifts of whatever signals they broadcast, are well established as per the TRANSIT system of the 1970’s and subsequent studies. As signals-of-opportunity (either with or without knowledge of their orbits and signal structures), they still provide additional positioning and navigation observables. The reverse problem, Doppler processing of radio signals received at the satellite, has also been considered. From the late 1970’s, ARGOS has received low duty-cycle transmissions of telemetry data from scientific platforms, using the Doppler shifts of the transmissions to locate the platforms. Similarly, COSPAS/SARSAT uses a constellation of satellites in low and medium Earth orbits to measure Doppler shifts of data transmissions for search and rescue efforts. Our concept for the use of LEOS in R21 is quite similar, yet different in that we work with the existing transmission protocol – VHF radio (voice over narrowband FM), a totally unstructured analog broadcast with no timing information – available to the typical mariner. As part of a larger Coast Guard program to leverage modern space-based technology toward its missions, we are formulating the use case for LEOS in R21, including conducting some experiments with existing LEOS transmissions in the VHF band to develop and test system algorithms. Specifically, we are mentoring projects for Coast Guard Academy cadets (senior capstone projects) to both work on the system level details and to develop in-house expertise. Since we currently have no satellite in space to listen for a ground-based transmission, our measurement and testing efforts have focused on the downlink problem: (low power) satellite signals of opportunity to measure Doppler on the ground, solving for our own position to test algorithms and methodology. Other groups within the Coast Guard Academy are planning on a CubeSat launch in the next 12-18 months with a software defined radio payload for proof of concept of our approach. In this paper, we review the project status to date. Topics include: converting the received narrowband FM signal into a Doppler measurement, the convergence of a iterative least squares positioning algorithm with the knowledge obtained from the LEO footprint, accounting for oscillator instabilities such as bias, drift, etc., and experiments with real data to validate our methods.
Published in:	Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023) September 11 - 15, 2023 Hyatt Regency Denver Denver, Colorado
Pages:	608 - 628
Cite this article:	McGarry, Dahnyoung, Hartnett, Richard J., Swaszek, Peter F., Chan, Bryan, Evans, Banks, Kenna, Aidan, "Potential LEO Satellite Augmentation for Rescue-21 in Alaska," Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023), Denver, Colorado, September 2023, pp. 608-628. https://doi.org/10.33012/2023.19381
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