Satellite geolocation

Satellite geolocation is the process of locating the origin of a signal appearing on a satellite communication channel. Typically, this process is used to mitigate interference on communication satellites. Usually, these interference signals are caused by human error or equipment failure, but can also be caused by deliberate jamming. Identifying the geographical location of an interfering signal informs the mitigation activity.

Many communication satellites share a given frequency band. As a signal is transmitted to a particular satellite there is some amount of side lobe or spillover energy that is transmitted to adjacent satellites. At a receive station that has two antennas, one pointed at the primary satellite (the satellite the signal is intended for) and a secondary satellite (a satellite that is receiving side lobe energy), both paths of the signal are received and measured. From a comparison of those paths, two measurements can be made: Differential Time Offset (DTO) and Differential Frequency Offset (DFO). These measurements are often implemented through correlation processing. DTO represents the difference in time that it takes the signal to travel through the two satellites, while DFO represents the difference in frequency the received signals present through the two satellites. The frequency differences observed are due to different Doppler shift resulting from relative satellite motion and differences in the translation frequencies of the two satellite channels. Channel translation frequencies and downlink Doppler shift and delay can be calibrated out of the measurements by observing transmitters of known location simultaneously on the channels. This leaves the uplink DTO and DFO as the observables. See 'Reference Signals' below.

Once a DTO calculated, it can be combined with the known position of the satellites and the receiving station. This combination provides a locus of positions on the Earth’s surface for the source of the signal; from this result a line of position (LOP) can be derived. A similar line can be derived for the frequency differences. Where the two LOPs intersect is the signal transmission location. In addition to geolocation with a time LOP and a frequency LOP, a location can also be determined by finding the crossing point of two time LOPs. The second time LOP is an identical measurement using a different secondary satellite, or using the same secondary satellite, but later in time. Similarly, two frequency LOPs can be used to determine a location. It can be shown that, in general, it is expected that the two LOPs intersect in two places. In many circumstances it is possible to discount one of the intersections e.g. due to it not being in the coverage area of one or both satellites. In some circumstances, it is not possible to distinguish intersections from a pair of LOPs, in which case, additional LOPs need to be determined.

While measuring the DTO and DFO will give you an idea of the location of the signal source, the location will be inaccurate. There are many biases within the measurement system that, if not accounted for properly, will manifest themselves as time delays or frequency offsets. For example, while a satellite translation frequency is known to within a few kHz, accurate geolocation requires frequency measurement accuracies of single mHz.

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