De-interleaving in Comm ESM

De-interleaving is one of the main functions of Comm ESM systems; the goal of this function is twofold, as it must provide:

• a correct estimation of the number of transmitting assets surrounding the Comm ESM

• a correct characterization of each of the detected emitters

The above mentioned information is essential to an effective Picture Compilation, enabling a fast and reliable detection of hostile platforms, precise and timely execution of countermeasures and continuous tracking of emitters of tactical interest.

Precision is paramount; underestimating the number of emitters is clearly a major flaw, creating opportunities for enemy assets to go undetected, but also overestimating (proliferation) has to be avoided because it would typically lead to wrong characterization of the emitters, creating again the potential for lack of recognition of threats.

However, the task of the De-interleaving is challenging; in particular

• The electromagnetic scenario may complicate the task.
  Communication emitters are – by policy – allocated to specific bands in tightly packed communication channels; therefore the spectrum in communication bands is typically very crowded, and several emitters (each with exactly the same waveform) occupy neighboring channels.
  Depending on the geometry of the scenario, most of the emitters may be received from the same DOA, complicating the matter even further.

• The emitters may be intentionally designed to evade detection and characterization. That is the case, for instance, of Frequency Hopping (FH) emitters, which change their transmission frequency in a pseudo-random fashion.

• Comm ESM and surrounding transmitters may move relative to each other, over time, so that each emitter will change its DOA.

The De-interleaving has to properly compensate for the time intervals when the emitters cannot be received and measured.
This may happen because the RF receiver cannot provide instantaneous coverage of the entire operating frequency band, but also for example because there may be temporary disruptions caused by the transmitting activity of on-board radar and communication emitters hosted on the same platform.

The goals of the de-interleaving process are actually achieved as the concatenation of the process of “parameters measurement” and “Emitter separation”; the latter in fact exploits the measurements obtained in the former to discriminate correctly between potentially ambiguous and similar emitters.

In the context of Communication emitters, the main parameters which can be measured pertain either to the type of waveform (measurements and data which refer to frequency domain and time behavior of the emitters) or to the geometry of the scenario (DOA and received amplitude).

The “emitter separation” algorithm uses frequency, time, amplitude and DOA information to discriminate between emitters, cleverly optimizing the search strategies and the setting of the Comm ESM algorithms and receivers according to the characteristics of the emitters to be detected and characterized.

The “parameters measurement” is a memoryless algorithm which works on single data acquisitions, while the “emitter separation” stage which concludes the de-interleaving block involves also tracking over time and reconstruction of the time activity of each emitter.