Signals of opportunity for space navigation: An application-oriented review

Abstract

Space navigation using X-Ray pulsars is known as opportunistic navigation, where we rely on signals of opportunity from natural phenomena to determine navigation solutions for spacecraft. However, this definition becomes too small or shallow, as it doesn't include other forms of navigation using natural beacons reported over the last two decades. Since a classification or review of all opportunistic techniques is still missing, this article presents the first survey on the field of signals of opportunity (SOOP), and introduces a framework for this scientific area, where natural beacons could be associated to stars, distant planets or natural phenomena, and signals could come with distinct characteristics and wavelengths. The new classification groups dozens of existing technologies of navigation using natural beacons, making easier for newcomers to interpret quickly a variety of works and mathematical formalisms. In addition, we deepen into an engineering perspective on the ongoing status of SOOP-based navigation, highlighting what is required to achieve the level of readiness on the methods that still on theory. We also give answer to a number of open questions, such as e.g. what criterion defines if a signal from a given natural beacon could be used or not for navigational purposes, as well as what is the minimum intersatellite distance that we can detect using an opportunistic signal, what type of signal characteristics we need from natural phenomena to generate navigation solutions or how far apart natural beacons preferentially need to be. These results enrich the field and become fundamental with an increasing interest of the space industry to bring novel forms of navigation into low-cost spacecraft of limited hardware capabilities (such as Cubesats).Furthermore, using results from past remote sensing missions, we preliminary found that Earth's lightning is a natural resource available in large scale and in sufficient variability that can be potentially used for relative opportunistic navigation. The physics of the problem is introduced, where at least theoretically, multiple lightning events can be used to map the arrival times to cooperating spacecraft, fixing their positions relative to each other independently from ground.