Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Dr. C.R. Carter
The possibility of establishing accurate timing on board a navigation satellite in inclined orbit using a timing reference from either an earth station or geostationary satellite has been an important task in the last fifteen years. More recently, there has been considerable effort placed in designing and developing the NAVSTAR system which uses atomic clocks on-board satellites in inclined orbits to establish accurate time.
In this thesis we discuss another possible mode of operation which is based on the transponding of timing information from an earth station to a navigation satellite in inclined orbit through a satellite in geostationary orbit. Assuming that the satellite in the geostationary orbit has constant space delay with an earth station, then the only change in the space delay between the earth station and the satellite in inclined orbit occurs between the two satellites.
The main advantages of our solution to this problem are:
1) The atomic clocks on board the navigation satellite are no longer required.
2) A communication link now exists between the earth station and all users of the system through the navigation satellite. This is due to the fact that the satellite in geostationary orbit has the capability of observing both the navigation satellite in inclined orbit and the earth station located inside its coverage area.
We assume the following:
1) The location of geostationary satellite is accurately known. This is usually true since its motion with respect to earth stations is small.
2) The space delay from earth station to geostationary satellite can be determined to within less than 1 ns using conventional timing methods in TDMA. Thus, accurate time at the geostationary satellite is established.
3) The distance between the geostationary satellite and the navigation satellite varies smoothly with time.
4) The location of the navigation satellite is known to lie within a sphere of certain radius centred at a known point.
If we calculate in advance the actual space delay between a geostationary satellite and a navigation satellite; and know the timing on-board the geostationary satellite, we can establish timing on-board the navigation satellite. The technique which we use in computing the uplink and downlink delays between the two satellites depends on the estimation of the navigation satellite location. The error in estimating the location of the navigation satellite does not affect the calculation of the space delay between the two satellites directly, but its effect will be reflected by the change in the space delay.
The computed results show that the estimated uplink and downlink space delay between the two satellites can be calculated to a high degree of accuracy (a fraction of a nanosecond or less). Thus it appears that this system could be practical especially for commercial use which may include communications links as well as navigation information.
Sabry, Ehab Ibrahim, "Precision Synchronization of Navigation Satellites in Inclined Orbit From an Earth Station" (1984). Open Access Dissertations and Theses. Paper 1303.