Radar space-time adaptative processings

Space Time Adaptive Processing (STAP) is a type of processing that exploits both the spatial and temporal dimensions of signals received on an antenna array, unlike conventional antenna processing, which exploits only the spatial dimension for filtering/separation. This processing structure makes it possible to take advantage of the specific two-dimensional space-time properties, or in the dual angle-frequency domain, of the signals received. This is particularly interesting in the case of an angle-frequency coupling property of the received signals, where, although the signals are extended in both spaces separately, they occupy only one 1D dimension in 2D space. Their filtering/separation then becomes possible with STAP processing, whereas it was not possible with mono-dimensional spatial or temporal processing.

This problem is particularly relevant to the filtering of ground echo signals received by airborne radar, for which there is a direct link between direction of arrival and Doppler frequency. These ground echoes, or clutter, are conventionally filtered in radar by spatial processing (spatial filtering by the antenna lobe) followed by Doppler processing (frequency filtering). These echoes, extended in both spatial and frequency domains, are therefore only imperfectly filtered, and their residuals still severely limit detection performance.

In this context, the use of STAP processing makes a major contribution, and its implementation in operational products is now made possible by the arrival of active antennas with multi-channel reception, combined with the increased computing capacity of on-board machines. The two main short- and medium-term applications for airborne radar are the detection of slow-moving ground targets in air-ground mode (target competing with clutter echoes entering through the antenna's main lobe), and improved detection of airborne targets on clutter-polluted areas of the range-Doppler radar map in air-air mode (target competing with clutter echoes seen through the antenna's secondary lobes). These two contexts will serve as the application framework for the article.

However, the choice of a specific processing architecture, in particular pre- or post-Doppler analysis, remains highly complex due to the variety of both possible architectures and operational contexts, as well as the sometimes contradictory arguments that govern this choice.

This article is intended for engineers who are not STAP specialists, but who are familiar with the basics of radar and signal processing. The aim is to explain the fundamental principles of the different STAP processing architectures, and the criteria for choosing one or the other for an airborne radar application. Indeed, there is no such thing as a...