Passive utilization of the TWSTFT technique

As an active technique TWSTFT is limited in the number of concurrent users on the same communication channel. Using the transmissions of the reference network, passive users can observe the signals in a common view sense and by knowing the orbit of the relaying satellite, corrections describing the dynamic path delay can be used to estimate the clock difference between passive stations. Reliable orbit determination can be achieved using all the active measurements performed by the regular active users. For the EU/US network using Telesat Telstar 11N, the orbit estimation uncertainty is in the order of 1m for the area spanned by the contributing stations. The corresponding time uncertainty for a passive user is about 1 ns, slightly larger than achievable by active links. A passive use of TWSTFT may provide increased timing resiliency for a number of applications. The design of a passive ground station based on a SDR receiver is suggested.