Space-Time Adaptive Processing (STAP)
Figure 1: Space-Time Spectrum
Space-Time Adaptive Processing (STAP)
Advanced airborne radar systems are required to detect targets in the presence of both clutter and jamming. Ground clutter is extended in both angle and range, and is spread in Doppler frequency because of the platform motion.
Space-time adaptive processing (STAP) refers to the simultaneous processing of the signals from an array antenna during a multiple pulse coherent waveform. STAP can provide improved detection of very low velocity targets obscured by mainlobe clutter, sidelobe clutter, and jamming through two dimensional processing, that enhances the ability of radars to detect targets that might otherwise be obscured by clutter or by jamming. This approach uses processing in both the time and spatial domain. Till now the algorithms were based upon the first order statistical characteristics of the echo. But STAP uses the second order statistics. This is because the determination of a target in a particular cell is no longer confined to a look into a linear array of cells, rather the targets are determined using information about adjacent cells in both dimensions.
To implement STAP requires sampling the radar returns at each element of an antenna array, over a dwell encompassing several pulse repetition intervals. The output of STAP is a linear combination or weighted sum of the input signal samples.
What does STAP stand for?
- The „Adaptive” in STAP refers to the fact that STAP weights are computed to reflect the actual noise, clutter and jamming environment in which the radar finds itself.
- The „Space” in STAP refers to the fact that the STAP weights (applied to the signal samples at each of the elements of the antenna array) at one instant of time define an antenna pattern in space. If there are jammers in the field of view, STAP will adapt the radar antenna pattern by placing nulls in the directions of those jammers thus rejecting jammer power.
- The „Time” in STAP refers to the fact that the STAP weights applied to the signal samples at one antenna array element over the entire dwell define a system impulse response and, hence, a system frequency response. The clutter spectrum seen by ground based radars typically has a ridge at zero Doppler an can easily processed by pulse pair processing while the clutter spectrum seen by airborne radars is typically more complicated due to the combination of platform motion and antenna pattern.
- STAP processing adapts the radar frequency response to the actual clutter spectrum in which the radar finds itself so that the radar will preferentially admit signal power while simultaneously rejecting clutter power.
The adaptive weights used by STAP are computed using a clutter-plus-noise covariance matrix estimated from data collected at successive ranges. An accurate estimate of this matrix can be obtained only if the structure of the clutter spectrum remains unchanged over the range interval used for the estimation.
Figure 2: Illustration of the initial situation
As a starting point, we expect that the radar is a synthetic aperture radar. This works practically like a virtual radar which has a phased array antenna with the size of the synthetic aperture (often several kilometers long). Their single radiators do not work parallel and simultaneously but one after the other (according to the respective positions of the aircraft or satellite on its flight path). The raw data is buffered and only processed when all data is available.
This situation is shown in Figure 2.
The result in Figure 1 would only be the blue-colored diagram.
It is referred to as the normalized Doppler.
Normalized Doppler means that influences due to frequency changes in the transmitted signal have been deducted.
Somewhere inside there is the target of the moving target but at the wrong distance.
Its correct distance would be if it were exactly within the maximum of the diagram:
but then it would not have its motion and would also be a fixed target.
Figure 3: Intermediate step for understanding
Assume that, as shown in Figure 3, the entire synthetic antenna field is divided into two independent antennas. Now two radar images of the same situation on the ground are created one after the other. The fixed targets should still be in the same position and produce an echo signal with a constant phase. Only a slowly moving target has changed its position slightly. Often so slightly that it appears to be at the same location. But the phase of the calculated target has changed compared to the previous antenna position.
moving target indication
can be performed from this changed phase position, as already described in the chapter: “Coherent Radar Technology”.
However, with this assumed design, the usable synthetic aperture would decrease and leads to a significant deterioration of the angular resolution.
Figure 4: Two virtual antenna positions over the entire synthetic aperture
But since it is virtually a phased array antenna, a phase shift can be added to the raw data of each radiator position during further signal processing. This would allow the phased array antenna to electronically swivel its antenna pattern. Both virtual antenna positions can be calculated simultaneously now, allowing fixed target suppression at maximum synthetic aperture length.
The presented here as an example with only two virtual antenna positions can be performed with the same raw data source for each position of the single radiators shown in Figure 2. This procedure is possible for every pulse repetition period of the radar! Thus the second dimension in Figure 1 is created, which is denominated the sine of the azimuth.