Ground Moving Target Indicator - GMTI
It is a well-known fact that high-frequency SAR technique can be modified to detect and reproduce moving objects by using an array of narrow-lobe antennae. By arranging the antennae so that the antenna lobes are displaced in parallel it is possible by using signal processing to essentially eliminate all influence on the radar signal deriving from stationary objects.
A conventional GMTI radar system utilizes multiple antennas to accomplish moving target detection. The number of antennas varies from 3 to 8. These antennas could be separate dish antennas, or different parts of a phased array antenna system. In the transmit mode, one of these antennas radiates from a transmit phase center. In alternative transmit modes, all or a selected group of the antennas radiate coherently together. In other words, they act as a single larger antenna with a single transmit phase center. The transmit phase center is the point from which the outward spreading electromagnetic pulses would seem to have originated, as seen by an observer from a distance. When the radar system radiates, there is a single transmit phase center, the location of which is controlled by electrical means and is not necessarily at the spatial center of the transmit antennas.
In the receive mode, the 3-8 antennas receive the reflected pulses as separate antennas. In other words, the antennas deliver separate outputs corresponding to different receive antenna phase centers. The separation of moving targets from stationary clutter is a key objective of a GMTI radar system. A variety of implementations have been demonstrated to meet this objective, and to meet a variety of other system requirements.
The problem of canceling clutter surrounding an object to be resolved in a radar image is a constant struggle for all radar applications. Specifically, clutter arises often times in space borne or in airborne radar systems when imaging the surface of the earth. In these applications the ground clutter must be suppressed for target detection, and the angular location of the object must be determined accurately to enable tracking of the object.
The GMTI function (ground moving target indication) can be implemented essentially in two different ways. Detection of moving targets requires maximization of the target signal compared to the clutter signal. In order to filter the strong clutter signal from stationary objects, the displaced-phase-center-antenna (DPCA) method was developed. This technique needs strict spatial alignment and system stability. In the extension of adaptive antenna technique the space-time adaptive processing (STAP) was found. The STAP is not only adaptive, in the space-time two-dimensional space the clutter spectrum is basically a narrow ridge, so that slow moving targets can be detected.
The first method is called DPCA (displaced phase centre antenna), which is used to eliminate stationary objects from the signals from two parallel-displaced antenna elements. This method utilises the fact that the signal, from all stationary objects, in the front and rear element, respectively, is repeated after a time interval in conformity with the platform moving the same distance as the element distance. After a delay, the signals from the stationary objects can thus be eliminated by subtraction. The drawback of this method is that it requires a calibrated and time-invariant radar system. A further problem with DPCA is that blind speeds arise, for which also moving objects are perceived to be stationary. The reason for this is that extinction also occurs when the phase change between the signals is a multiple of 2.pi.. In practice this involves a demand for maximum antenna separation, which thus affects the detectable minimum radial speed.
The second method is called STAP (space-time-adaptive filtering) and is based on the covariance properties of the time signals for the different elements in the array antenna. The covariance matrix for stationary and moving objects, respectively, is different which is used by linearly combining signals in time and space so that a maximum ratio of desired to undesired signal is obtained. In practice, the co-variance matrix is estimated by taking random samples of the undesired signal, which together with a model of the desired signal forms an adaptive signal-adjusted space-time filter. The STAP technique is not restricted to the elimination of stationary objects as is the DPCA technique. Essentially all forms of undesired signal can be processed in the same manner provided that the covariance matrix can be estimated and that it differs from the desired signal. For example, also intentional or unintentional interference signals can be eliminated by the same method.
Clutter suppression by GMTI filtering has in latest years developed in combination with SAR. In a SAR GMTI system the moving target will not only be detected, but also imaged to high resolution in its surroundings. Movement of a target will influence the focusing in the SAR process of the moving target compared to its surroundings. The moving target will be smeared and shifted in location. For SAR with a side-looking narrow beam antenna system these effects are known, and methods for detection of slow moving targets has also been proposed.
In the latest years there has been ongoing development to detect, focus and evaluate velocity components of a moving target in a SAR system. The idea is to use multi-channel antenna arrays to suppress the clutter signal from the stationary objects. The main detection scheme is to compare the likelihood ratio test with a threshold. Later experimental results have shown that SAR GMTI is a strong tool to detect and image moving targets in its surrounding. However, even in these narrow beam microwave systems, compensation in Doppler frequency is needed.
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