Electronic Intelligence
Electronic Intelligence (Elint) is the process of observing the signals transmitted by radar systems to obtain information about the capabilities of these radars. This allows the measurement of radar signal parameters from a remote location. The Elint analyst then derives various operating parameters of the radar from the measured parameters. This information allows the determination of radar capabilities for the purpose of threat classification.
The parameters most commonly measured are the Pulse Repetition Interval (PRI), Pulse Width (PW), Pulse Amplitude (PA), signal frequency and the Direction/Angle of Arrival (DOA/AOA). PRI measurements enable determination of maximum unamibiguous range of the radar. Pulse to pulse signal amplitude variation provides information regarding the antenna scan pattern, antenna rotation period (ARP), side lobe level etc. The direction of arrival measurements are used to locate the emitter by triangulation. However, the measurement of emitter parameters becomes increasingly difficult with the increase in the complexity of radar signals and emitter density.
The measured and derived parameters are finally processed to identify and classify the emitter. Emitter identification is possible by the comparison of measured and derived parameters with the ones stored in the threat library. The threat library contains the known parameters of the various operating modes of different radars. Each operating mode is assigned a threat level which is an indication of the lethality of the emitter. All unknown emitters (or emitters operating in unknown modes) can be regarded as potentially dangerous.
An Elint System
Threat classification on the basis of emitter identification has a severe limitation. In hostilities, enemy emitters generally operate with unknown parameters or in unknown modes, also known as the wartime modes. Thus the number of unknown, and therefore potentially dangerous emitters increases significantly. This can tremendously increase the workload of the operators as they would attempt to analyse or counter emitters which appear to be threatening but are actually not threatening.
Threat classification should, therefore, be independent of emitter identification. The threat level of an emitter can be inferred directly from the measured and derived signal parameters. For instance, consider the following threat classification heuristics generally used by Elint operators,
Similar heuristics may be used by an artificially intelligent threat classification system to classify the unknown emissions. Such a system would isolate the tracking radars from the search radars on the basis of the detected emissions so that the operator may give necessary attention to more threatening emissions.
Elint is generally employed to have a strategic view of the Electronic Order of Battle (EOB), e.g. to determine the radar deployment patterns over a long period of time. Elint data generally forms the basis of threat libraries in radar warning receivers. Due to the nature of Elint, Elint receivers need to be extremely sensitive and should be able to provide accurate values of the measured parameters. Moreover, Elint analysts have, at their disposal, sophisticated processing engines. These machines not only assist them in analysing the collected information but also provide them with user friendly interfaces. Elint operators can use a wide variety of display formats and visualisation techniques. Some of the most common display formats are given below
Panoramic and Geographical Displays
Radar Warning Receivers (RWRs), on the contrary, are designed to give nearly immediate warning if threatening emitters illuminate the platform. These receivers have sensitivites lower than those of the Elint receivers, but are still sensitive enough to intercept emissions and process the measured parameters even when the platform is outside the emitter’s range. Thus RWR platforms have the advantage that they can determine a potential threat even when they are not visible to the threat. Moreover, they measure only enough parameters with sufficient accuracy to identify the emitter and classify the threat posing the platform. The processors used in RWRs employ near real time processing techniques. The output to the operator is generally in the form of well defined symbols (one for each emitter in the threat library and class of threat) on a circular display. The azimuth location of a symbol with respect to the centre of the display indicates the approximate angle of arrival of the emission. Its distance from the centre of the display indicates the lethality of the threat. The closer the emission is to the centre of the display, higher is its lethality or the threat level.
RWR Display
ESM (Electronic Support Measures) receivers search, intercept, locate and identify electromagnetic emitters. The information collected and processed from ESM receivers about various radars, their operating modes, their threat levels and their deployment is used for the tactical employment of military assets such as ECM (Electronic Counter Measures) or SEAD (Suppression of Enemy Air Defense) equipment.
ESM systems are generally more sensitive than the RWRs and perform more accurate measurements and analysis of the radar parameters. The collected, processed and analysed information is used to update the local Electronic Order of Battle (EOB), ECM and SEAD deployment, early warning of enemy approach and fusion with information from other sensors.
Bibliography
© Omar Bashir, November 1998
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