NATO STO

The Global Navigation Satellite System (GNSS) has transformed society. GNSS is the combination of the existing and future navigation satellite systems. Prominently featured in GNSS are the American GPS, the European Galileo, the Russian GLONASS, and the Chinese Beidou. The usage of the different systems increases coverage, accuracy, and reliability of the overall service. Based on the EUSPA Earth Observation and GNSS market report, the market potential of GNSS solutions is estimated to 405 billion US$ in 2031(1). In addition to navigational services, farming, trading, and urban development, are mentioned as use cases for the GNSS services. To operate navigation satellites reliable and precisely, equally resilient and robust ground stations are required. Ground stations are distributed around the world and equipped with redundant operation systems. On board of the satellites, the timing signal, which is at the base of GNSS service, is produced by using frequency references. Different frequency references exist. Most of those are based on the stabilization of a radio frequency source by means of a gas cell. Ground based systems usually achieve better precisions than those in orbit. This is due to factors, such as the control of the environment, required reliability, and physical properties, i.e. size, mass, and power requirements. Current developments promise to improve the quality of space-based frequency references, for instance by deploying degenerate quantum gases. Even though ground stations are very well distributed and the large number of active satellites with precise clocks on board operate well, coverage of the distribution of the GNSS signal is limited. Consequently, the signal is inaccessible at a lot of places, especially under ground or far away from land masses. Another limiting factor in GNSS are jamming and spoofing, which reduces the accuracy of the position signal on ground. It includes actions, such as blocking or interfering with signal transmission, intersecting and altering the signal, and actively reducing the precision of the signal. Consequently, other options to navigate without GNSS have been investigated. There are options that enable navigation without reliance on GNSS altogether and others that are designed to bridge limited gaps in GNSS coverage. Those options include navigation based on inertial sensing, mobile networks, and along (magnetic) maps. (1) https://www.gsc-europa.eu/electronic-library/gnss-market-and-user-reports