Today, more than six percent of Europe’s economic output is dependent on satellite navigation – and this figure is continuously increasing. Economies such as the economy of Germany, with large automotive, energy, and banking sectors, are highly dependent on navigation services. Therefore, the logical approach was to support the European effort in developing an independent navigation system, known as Galileo. At present, the first generation of the Galileo system is being deployed. The satellites are produced in Germany and one of the control centers is located in Oberpfaffenhofen. The institute supports these developments, specifically through the verification of signals. The first Galileo satellites have already spent 50 percent of their operational life in Earth orbit. Due to the long lead times, it is thus necessary to start with the development of the next generation(s) of satellites and systems. A particular focus of the institute is on enabling the use of satellite navigation in safety-critical applications.
In the 1970s, the solar mission Helios required a large antenna, which was built at the DLR site in Weilheim. Today, this antenna, featuring a diameter of 30 meters, is a big opportunity for satellite navigation, since it is capable of raising navigation signals 30 m antenna dish at DLR premises in Weilheim, used for GNSS signal verification above the noise floor. This permits a detailed analysis of these signals, which are otherwise drowned in noise. The institute
has developed processes for measuring many important signal properties and in particular for characterizing signal deformations. The resulting capabilities are regularly used by ESA during the in-orbit testing of Galileo satellites. Support was also given to the US GPS Wing.
A precise understanding of the worst signal deformations is of particular importance for the use of GPS and Galileo in safetycritical applications, such as the landing of aircraft.
The extremely weak satellite signals are typically superposed by interference when reaching the receiver and this interference can be many orders of magnitude stronger than the desired satellite signal. To handle such situations, the institute developed receivers capable of suppressing interference that is 10 orders of magnitude larger than the desired signal! Furthermore, these receivers are also able to recognize spoofed signals or signals transmitted by a repeater (meaconing). Such capabilities are essential for the use of satellite navigation in safety-of-life applications.
Ionospheric monitoring and prediction
The ionospheric plasma significantly affects satellite navigation. In extreme cases, it can cause delays of the satellite signals by several tens of metres. With the new signals from GPS and Galileo, this delay can generally be estimated – at least when the necessary receiver upgrade can be afforded. In all other cases, the receiver must resort to externally provided correction data. A corresponding service has been developed at the institute. It relies on different measurements performed on the ground and on various satellites. To date, the institute has developed models for predicting not only signal delays, but also ionospheric storms. The latter are phases of particularly high fluctuation in the ionisation caused by solar events. When this occurs, some signals can become completely unusable. Correspondingly, the ability to forecast such phases has a direct economic benefit.
Pathfinder for the next generation – Galileo Evolution
GPS is the blueprint for all current satellite navigation systems. Its architecture has proven its benefits and will therefore not be changed fundamentally in the two upcoming generations. In the opinion of the institute, these generations should be focussed on improving the suitability for safety-critical applications, the availability of high precision, and on the reduction of operating costs. Measures that are useful to this end include a robust time system on board the satellites, self-assessment capabilities for signals on the satellites, inter-satellite links, potentially two-way measurements between the satellite and the ground, as well as the transmission of a number of parameters useful for accurate positioning. Some of the additional parameters shall improve the compatibility with other systems. Selected developments of the signals will also be considered under the condition that backward compatibility is maintained.