Efficient Communication Concepts for Low-Earth-Orbit Pico-Satellite Formations

Advances in miniaturization during the last decades have enabled the construction of small spacecraft with total masses reaching down to 1kg and below. At the same time standardization of components, interfaces, and platforms for the design of small satellites have further reduced their development costs. This trend has recently made formations of several small satellites in favor of one large unit an economically feasible alternative for university-scale organizations. While the satellite-formation approach provides new opportunities regarding Earth observation applications, it also poses new challenges to their communication system: the increased number of nodes at large distances and therefore propagation delays complicates the problem of medium access; the distributed collection of Earth observation data creates the necessity to gather the data over different links and possibly multiple hops. At the same time, the simultaneous operation of a number of identical satellites performing one task cooperatively can lead to redundancy in the data that needs to be communicated; exploiting this redundancy improves the overall efficiency of the communication system. In this work we discuss design approaches for communication protocols for Earth-observing satellite formations. In doing so we cover several layers from medium access control over network and transport up-to and including compression of payload data. Cooperative position awareness beaconing is nowadays required for vessels at the high seas and serves as a convenient example for the in-orbit reception of data from large terrestrial sensor networks. We demonstrate how a purpose-made medium access control protocol can improve both, terrestrial performance for cooperative awareness and in-orbit overhearing for the purpose of global tracking. Regarding higher network layers we discuss modern coding techniques like network coding and distributed source coding. These are less frequently used in terrestrial general-purpose communication networks like the Internet but can be employed to take advantage of the communication redundancy that is inherent to satellite formation operation. We show how use cases that differ in terms of kind of payload data and network topology each can benefit from their own, best-suited communication technique. We find that network coding is well-suited for over-the-air programming of satellite formations, that is, for ground-station-to-satellite broadcast transmissions. To adapt the concept of random linear network coding to typical topologies of satellite-and-ground-station networks, we introduce a novel decoding algorithm that enables protocols to use simpler feedback mechanism. Satellite-to-ground-station transmissions of multiple satellites' payload measurement data is often correlated across nodes. Here we demonstrate the applicability of distributed source coding techniques to increase the efficiency of communication resource utilization. As a common bottom line that applies to all of these subtopics we conclude in the end that in Earth-observing satellite formations, there is a plethora of different types of information redundancy across the satellites. The exploitation thereof allows tailored communication protocols to significantly outperform their state-of-the-art terrestrial counterparts