Essentials of ESM Systems

An ESM System allows the visualisation in real-time of the RF environment in which it is operating and usually the recording of the RF data so that detailed analysis can be done at a later date.

ESM systems are an essential part of the mission systems of military aircraft and ships in a world where the collection and interpretation of ISTAR data is becoming increasingly important for national and international security.

ESM systems work by detecting pulses from Radars, measuring the parameters of the pulses, sorting the received pulses into groups (de-interleaving), producing "tracks" and "fixes" for display in the ESM Picture and attempting to identify the type of Radar from which the pulses have been transmitted.

Here is a typical picture that may be produced by an ESM system.

There are many complicated processes in the production of an ESM picture and the analysis of the ESM system performance is a pain-staking and time-consuming business.

Pulse Parameters

Parameters of the pulses which are measured by the ESM are:

The pulse parameters which are calculated by the ESM are:

Frequency Agility

The RF signal emitted by a radar can be fixed, or agile from pulse-to-pulse or from batch-to-batch. Some agile signals follow a specific pattern of changing RF from pulse-to-pulse or from batch-to-batch, whereas others change RF on a pseudo-random basis.

The extent of RF agility varies from just a few MHz difference between the RF levels to hundreds of MHz difference between maximum and minimum values of RF in some cases. Some radars have up to 20 different RF levels available, with tens of MHz difference between levels.

In general radars with differences in RF of more than 20MHz between RF levels are batch-to-batch agile, while those with smaller RF differences between levels are pulse-to-pulse agile.

Radars which have batch to batch RF agility often have switch and dwell PRI patterns so that the PRI changes at the same time as the change in RF.

Pulse Repetition Interval

The Pulse-Repetition Interval (PRI) of a signal is an important parameter both in the de-interleaving and identification stages of ESM processing. PRI types are can be classified as fixed, staggered or jittered, with pulse groups (doublets or triplets) used for some applications such as missile guidance. PRI is the inverse of Pulse Repetition Frequency (PRF).

Techniques for Angle of Arrival Measurement

There are three techniques in general use for the measurement of the angle of arrival of radar signals


De-interleaving is the process of sorting pulses from different radars into individual sets of pulses (an intercept), one for each radar. The simplest de-interleaving technique is the construction of a grid with RF on one axis and DOA or DTOA on the other. Here is an example of such a grid.

There are four clusters of pulses shown in the picture, each of them has been received from a different radar. If there is more than one cluster at the same DOA or DTOA there is a possibility that the pulses from the clusters have come from a RF agile radar.

Once the pulses have been assigned to intercepts, the Pulse Repetition Interval (PRI) can be calculated using a technique such as Time of Arrival Difference (TOAD) histogramming.

The following diagram shows how the PRIs are calculated

Here is an example of the TOAD histogram which has peaks from two radars.

The correct determination of PRI often proves to be the main stumbling block in the operation of an ESM, often leading to multiple tracks being in existence simultaneously for a single radar, or that tracks are derived from intercepts received from more than one radar. The consequences of these limitations are that the RWR does not produce a true picture of the RF environment.

The ESM Picture

The ESM picture contains "tracks" or "Lines of Bearing" which indicate the direction of arrival (DOA) of a radar signal to the ESM platform and sometimes "fixes" which show where the ESM thinks that the radar is located.

A typical ESM picture might look like this:-

If the ESM produces fixes, it is usual for the display to include an error ellipse which shows the ESM confidence in the calculation of the radar location.

The ESM will contain a library of radar signatures consisting of parameters such as Radar Frequency (RF), Pulse Width (PW) and Pulse Repetition Interval (PRI) so that the tracks can be identified and alerts can be set up when threat radars are detected.

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