The scenario orbital data, defined in the constellation file, specifies satellite orbital positions for a particular moment in time. In the scenario that time is referred to as the Orbital Reference Time. But for satellite orbital positions to be correct for the start of the scenario, PosApp must extrapolate the satellites forwards or backwards from their orbital position at the orbital reference time to the orbital position at the scenario start time.
By default Spirent provides perfectly circular orbits with no intentional errors such as clock or perturbation terms. With such orbital definitions there are no constraints on the difference between orbital reference time and scenario start time as motion of the satellite around a circular orbit will not lead to any unintentional orbital position error.
But if a real almanac is loaded into the scenario or orbital parameters are manually changed such that orbital terms such as Inclination Rate or Right Ascension Rate are non-zero a large time difference between orbital reference time and scenario start time can cause the extrapolated orbit to diverge significantly, giving unexpected results. In this case the orbital reference time should be 'close' to the scenario start time. Worst case, we recommend that the scenario start time is not more than ±2 weeks from the orbital reference time.