Satellite geolocation

Identifying the geographical location of an interfering signal informs the mitigation activity.

There are many biases within the measurement system that, if not accounted for properly, will manifest themselves as time delays or frequency offsets.

In order to determine the position of signal source, a second set of measurements is required.

The measurement of the reference signal is purely passive and simply serves to remove the biases in the system.

A single reference geographically close to the target will give a high degree of cancellation of the location effects of ephemeris error.

TDOA – TDOA geolocation is ideal for moving targets, since the movement of the target will introduce varying and random frequency changes, causing an FDOA result to be useless, unless obtained from a highly inclined satellite.

Due to the repetitive nature of the signal, no unique TDOA solution will exist.

One problem with using only TDOA lines of position is that they tend to be north-south orientated and close to parallel, so that the “crossing point” of a TDOA-TDOA measurement can be error prone and uncertain, as it is “hidden” in a long intersection of the lines.

Moving targets are not likely to be located using FDOA methods, unless using a highly inclined satellite.

TDOA-FDOA geolocation also has an interesting limitation in that there are generally two times per day, separated by around 12 hours, where the FDOA becomes very small and hard to relate to an accurate LOP.

These times can be calculated based on known satellite ephemeris information and approximate transmitter location, and can therefore be avoided when taking FDOA measurements.

Nevertheless, a nominally CW transmission can contain imperfections, especially if a station transmits near its maximum EIRP.

Hence, it often has a phase noise component which might be recognized as a modulated signal and therefore used to make TDOA measurements.

However, it is generally more accurate to locate a CW carrier using FDOA-FDOA geolocation, even for non inclined satellites.

US 5570099, DesJardins, Gerard A., "TDOA/FDOA technique for locating a transmitter", issued 1996 US 6018312, Haworth, David Patrick, "Locating the source of an unknown signal", issued 2000 Haworth DP; Smith NG; Bardelli R; Clement T (1997).