27-02-2013, 03:18 PM
SIGNAL SORTING METHODS and DIRECTION FINDING
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INTRODUCTION
As shown in Figure 1, signal processing is basically a problem of signal detection, emitter parameter measurement and correlation, emitter sorting, identification, and operator notification. The ultimate goal of this processing is to classify radar signals by their unique characteristics and to use this data to identify enemy radars operating in the environment, determine their location or direction, assess their threat to friendly forces, and display this
information to the operator.
PASSIVE DIRECTION FINDING AND EMITTER LOCATION
Direction finding (DF) systems provide several important functions in modern EW systems. We have already
discussed the importance of measuring the emitter's bearing, or angle of arrival (AOA), as an invariant sorting
parameter in the deinterleaving of radar signals and in separating closely spaced communication emitters. In addition,
the conservation of jamming power in power-managed ECM systems depends on the ability of the associated ESM
system to measure the direction to the victim emitter. A function which is becoming increasingly important in defense
suppression and weapon delivery systems involves locating the emitter's position passively. This can be accomplished
from a single moving platform through successive measurements of the emitter's angular direction, or from multiple
platforms which make simultaneous angular measurements.
The emitter identification function requires identifying and associating consecutive pulses produced by the
same emitter in angle of arrival (AOA) and frequency. The AOA is a parameter which a hostile emitter cannot change
on a pulse-to-pulse basis. However, to measure the AOA of pulses which overlap in the time domain first requires
them to be separated in the frequency domain. The advanced ESM receivers which accomplish this function must
operate over several octaves of bandwidth while providing RMS bearing accuracies on the order of at least 2 degrees
with high POI and fast reaction time in dense signal environments.
Scanning Beam
The mechanically scanning beam, or "spinner," requires only a single receiver and also exhibits high sensitivity
due to the use of a directive antenna. The disadvantage is that the "spinner" usually exhibits slow response because it
must rotate through the coverage angle (e.g., 360 degrees) to ensure that it intercepts an emitter. Also, if the emitter
uses a scanning directional antenna, both beams must point at each other for maximum sensitivity, which is a low
probability occurrence. Both of these effects cause the mechanically scanning beam technique to have a low probability
of intercept (POI).
Amplitude Comparison
The two primary techniques used for direction finding are the amplitude-comparison method and the
interferometer or phase-comparison method. The phase-comparison method generally has the advantage of greater
accuracy, but the amplitude-comparison method is used extensively due to its lower complexity and cost. Regardless of
which technique is used.