Time synchronization of a TDOA-based position finding system

In this report the possibilities and problems on synchronous data acquisition for geographically separated TDOA receivers are considered. In this case, the receiver´s synchronization is specified to max 10 ns deviation for 24 hours. An inventory of reliable methods and existing products has been made. Performance, price, complexity and availability are factors that were taken into consideration in the evaluation. The evaluation shows that undisciplined oscillators are unsuitable for a TDOA system, and because of that a common reference are necessary to make the receivers work synchronous. The proposed solution is to use common-view against GPS signals which means that several receivers observes the same GPS satellites to reduce errors in satellite clocks, delays in the atmosphere etcetera. Another solution, with even better performance, is using the carrier phase of the GPS signal which means that the carrier phase is measured and compared between respective receiver station. For a tactical TDOA system redundancy is necessary. This is issued using a very stable oscillator disciplined by the GPS when present.