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EW101 Communications EW – Part 16 Location of Communications Emitters By Dave Adamy This month and next month, we will discuss precision emitter location techniques. In general, these techniques provide emitter location with sufficient accuracy to support targeting. This means that the location accuracy is expected to be equal to the burst radius of a weapon (tens of meters). However, there are other applications that may profit from extremely accurate location – for example, determining if two emitters are co-located. We will discuss two precision techniques – time difference of arrival (TDOA) and frequency difference of arrival (FDOA) – followed by the combination of these two techniques. Both TDOA and FDOA require the presence of a highly accurate reference oscillator at each receiver site. In the past, this required an atomic clock at each location, but now GPS provides the equivalent at a significantly lower size and weight. (continued) The Journal of Electronic Defense | September 2008 T Time Figure 2: A single analog signal received at two distant stations will have the same modulation, but will be offset in time by the difference in distance. TDOA TDOA depends on the fact that signals travel at the speed of light, thus a single signal will arrive at two receiving sites at a time difference that is proportional to the difference in distance, as shown in Figure 1. The distance from the emitter to each receiver is the propagation time multiplied by the speed of light (d = C x t). If we knew the precise time at which the signal left the transmitter and the time at which it reached each receiver, we could calculate the distance from each receiver site to the transmitter and would thus know the precise emitter location. This is done in cooperative systems such as GPS in which the transmitted signal carries information about the time the signal is transmitted. However, when dealing with hostile signals, we have no way to know the time the signal leaves the transmitter. We thus can only determine the difference between the two times of arrival. Since communication signals are continuous, the only way to determine this time of arrival difference is to delay the received signal in the receiver closest to the emitter until the modulation from the two signals is correlated (see Figure 2). This requires that each receiver have a variable delay capability (either might be closest to the emitter). The whole range of relative delays, in effect, searches through the area of possible emitter locations. 47 DELAY CORRESPONDING TO TDOA CORRELATION VALUE DELAY SETTINGS AMOUNT OF DELAY IN RECEIVER CLOSEST TO EMITTER Figure 1: Because the signal travels at the speed of light, the time difference of arrival is proportional to the difference in distance to the two receiving sites. Figure 3: Delaying one of the two received analog signals will produce a soft correlation peak when the delay is equal to the time difference of arrival.

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