Global Navigation System Based on Master Clocks and Two-way Links in Space
Svehla, D.
Technische Universitaet Muenchen

In the first part we discuss advantages and disadvantages of the one-way ranging navigation systems such as existing GPS and GLONASS, and future GALILEO and COMPASS. We show how the standard global GPS parameters, like station coordinates, troposphere zenith delays, satellite orbits and clocks and Earth rotation parameters are improved by combining measurements from all four GNSS systems, which amounts to about 110 GNSS satellites. Although GPS system was designed some 20-30 years ago, future navigation systems follow the same concept based on the one-way ranging. Here, we present how such a concept can be improved by making use of the recent developments in the atomic clock technology and advanced two-way frequency transfer systems.

We present a concept of using an atomic clock in the e.g. geostationary orbit (GEO) as a master clock for all GNSS satellites in the medium Earth orbit (MEO). Since there is a clear visibility between a satellite in the GEO orbit and all GNSS satellites in the MEO orbit, two-way microwave or/and collocated optical link may be established to directly transfer frequency from the master clock in the GEO orbit and further be re-transmitted by the GNSS satellites as the one-way navigation signal. Advantage of the two-way links is cancellation of the first order Doppler effect and therefore possibility for real time frequency dissemination between the master clock and the GNSS satellites. In order to introduce redundancy in such a system, at least two or three master clocks would be required in the GEO orbit. Since two-way links provide range as additional observable, continuous ranging between GEO and GNSS satellites and continuous ranging between three GEO satellites can be obtained at the same time. This opens the possibility to form a network of e.g. three GEO satellites in space that can be used as a reference frame for all GNSS satellites in the MEO orbit. We demonstrated before, that orbits of all GPS satellites can be estimated based on at least 6 LEO satellites forming a dual constellation in space. However, in such a system orbit of at least one LEO satellite has to be known and tied to the earth-fixed system, in order to estimate orbits of all LEO and GPS satellites together. Here we show that a similar principle can be applied to three satellites in the GEO orbit forming a dual constellation with the GNSS satellites in the MEO orbit. Orbits and master clocks of the three GEO satellites can continuously be monitored using two-way links from the ground. The system could be further stabilized if the three GEO satellites would be drag-free. The concept of such a navigation system based on the dual constellation between “fast moving” GNSS satellites and “stable” GEO satellites enables very accurate real-time orbit and clock determination of the GNSS satellites and considerably reduces a need for a dense real-time network on the ground. With this concept, the use of H-masers and Cs- or Rb-clocks in the GNSS satellites can be reduced to USO of higher quality. In this way, GNSS satellites are used just as re-transmitters of the time scale formed by the GEO master clock(s) controlled from the ground using two-way links. Such a navigation system based on the GEO/MEO dual constellation and frequency dissemination can be extended to other orbit altitudes between LEO and GEO, e.g satellites dedicated to transmit microwave signals like ASTRA and IRIDIUM constellation. However, the use of a LEO orbit in such navigation system considerably reduces tracking time for a ground station to only few minutes. As alternative to this concept, master clock(s) could be placed on the ground and the two-way link used for frequency dissemination from the ground. Disadvantage of such a concept is reduced tracking visibility of a GNSS satellite from a ground station and reduced performance of the two-way links due to the ionosphere scintillations and weather conditions.

Using simulated data we show all advantages of the navigation system based on master clocks and two-way links in space. The first optical clock reached stability down to one part in 100^-17 over few hours of averaging and the first cesium clock for space have been under development for the ACES mission on board the Space Station with stability of 10^-16. Microwave link developed for the ACES mission follows these orders of stability.