Light Speed Delay in Signal Propagation

STK Communications accounts for light speed propagation delay while computing access times between transmitters and receivers. Link budget analysis is carried out with the light speed adjusted times.

The impact on short-range links (e.g. to and from LEO satellites) is very small. Yet, the delays increase in significance for link distances to GEO and beyond. Transmission start and stop times for uplinks and downlinks will differ, and the reception start and stop times shift because of the signal propagation delay.

A transmitter on a high altitude satellite or a deep space object may start the transmission well before the receiving Earth station appears over the horizon. STK accounts for the signal propagation delay to compute access to the receiving station.

Often, STK may show a valid access, even when the transmitter and receiver may not have a clear, direct line of sight. Yet, the dynamic line of sight will be complete after signal propagation delays have been accounted for. The receiver will be in the line of sight when the signal arrives at the receiver.

Similarly, access range and direction of antenna tracking will be computed taking into consideration the signal propagation delays. The transmitting antennas will point toward the direction where the receiver will be when the signal arrives at the receiver. The receiving antennas will track the transmitter position where the signal originated, not the current position of the transmitter.

Transmission Timing Computation

To compute transmission timing -- i.e. to determine when a transmitter may start and stop transmitting -- compute access from the transmitter to a receiver and generate a Link Budget report. All times in that report will reflect the transmitter start and stop times, taking the light speed delay into consideration.

Reception Timing Computation

To compute reception timing, compute access from the receiver to a transmitter, and then generate a Link Budget report. The report will show the signal arrival times. Interval end times represent the times when the transmission stops arriving at the receiver. The report times will be from the receiver's perspective and most likely will differ from the times as viewed from the transmitter end.