Very precise synchronization of distributed pseudolites

Pseudolites are GNSS transmitters that have a fixed position on the ground and transmit original satellite signals. Very often pseudolites are used around airports and harbors to ease the navigation and increase integrity and availability of satellite navigation systems. Pseudolites are also used in the European "Gate"-Projects. "GATE" is the name of a Galileo test bed where several pseudolites are located around a valley in southeast Bavaria transmitting Galileo signals so that receivers are able to use Galileo signals for testing purpose in “real live” before the satellites are in the sky. Besides this first Gate project a number of similar pseudolite projects have been established. One of these projects is named “aviation gate” and is located at the research airport of Braunschweig. This project shall show the use of Galileo signals in aviation - from approach to landing to taxiing. It is extremely important to synchronize the pseudolites as good as possible to minimize the navigation errors. The system in Braunschweig consists of 9 pseudolites and a reference station. The pseudolites are mounted in an inner circle of 5 systems around the airport and 4 systems in distances between 30 and 60 km. The full pseudolite constellation is working now, the results are remarkeable. The paper describes the basics and goals of the project and includes the latest results of the project - 9 free running GPS-receivers are synchronizing 9 pseudolites. The results look very promising - the receivers are far better synchronized than anticipated, they work in the low nanosecond range. To enhance the accuracy of the timing receivers into the sub-nanosecond range the receivers have a remote adjustment capability: The reference station continuously measures the range of the pseudolites and is able to adjust the phase of the GPS receivers 10 MHz output with a resolution of 25 ps over a WLAN link. This type of application is ideally suited at locations w- ere independent stations require a very high degree of synchronization.