12-12-2012, 04:16 PM
Space-time adaptive processing (STAP)
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Introduction
Space-time adaptive processing (STAP) is a signal processing technique most commonly used in radar systems. It involves adaptive array processing algorithms to aid in target detection. Radar signal processing benefits from STAP in areas where interference is a problem (i.e. ground clutter, jamming, etc.). Through careful application of STAP, it is possible to achieve order-of-magnitude sensitivity improvements in target detection.
STAP involves a two-dimensional filtering technique using a phased-array antenna with multiple spatial channels. Coupling multiple spatial channels with pulse-Doppler waveforms lends to the name "space-time." Applying the statistics of the interference environment, an adaptive STAP weight vector is formed. This weight vector is applied to the coherent samples received by the radar.
In a ground moving target indicator (GMTI) system, an airborne radar collects the returned echo from the moving target on the ground. However, the received signal contains not only the reflected echo from the target, but also the returns from the illuminated ground surface. The return from the ground is generally referred to as clutter.
The clutter return comes from all the areas illuminated by the radar beam, so it occupies all range bins and all directions. The total clutter return is often much stronger than the returned signal echo, which poses a great challenge to target detection. Clutter filtering, therefore, is a critical part of a GMTI system.
INTRODUCTION TO RADAR
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detectaircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio waves ormicrowaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter.
Radar is microwave system for detecting object and determining their location by radiating electromagnetic energy and examining the reflected energy, the word radar is acronym of the RAdio Detection And Ranging . This energy normally travels through space in a straight line, at a constant speed, and will vary only slightly because of atmospheric and weather conditions. The electromagnetic waves are reflected if they meet an electrically leading surface. If this reflected wave is registered again at the place of the origin, this is a proof of this that an obstacle is in the propagation direction.
The electromagnetic energy is a signal from the transmitter and eventually propagates in the free space during transmission (RF signal). The RF system takes a signal from free space and passes it to the receiver during reception to be (IF signal). The RF signal is the Sendsignal and IF signal is the Echosignal shown in the figure
CLUTTER AND JAMMING
Jammers are sometimes used to prevent detection by radar. Jammers often emit a powerful signal over the entire frequency range of the radar. In other cases, a moving target has such a slow motion that Doppler processing is unable to detect against stationary background clutter – such as a person or vehicle moving at walking speed. A technique called space time adaptive processing (STAP) can be used to find targets that could otherwise not be detected.
Because the jammer is transmitted continuously, its energy is present in all the range bins. And, as shown in Figure 1, the jammer cuts across the all Doppler frequency bins due to its wideband, noise-like nature. It does appear at a distinct angle of arrival however. Figure 1 also depicts the ground degree of clutter in a side-looking airborne radar due to the Doppler of the ground relative to the aircraft motion. A slow moving target return can easily blend into the background clutter.
STAP radar processing combines temporal and spatial filtering that can be used to both null jammers and detect slow moving targets. It requires very high numerical processing rates as well as low latency processing, with dynamic range requirements that generally require floating-point processing.
Clutter and jammer effects in Doppler space
Clutter is a term used for unwanted echoes in electronic systems, particularly in reference to radars. Such echoes are typically returned from ground, sea, rain, animals/insects, chaff and atmospheric turbulences, and can cause serious performance issues with radar systems.
Radar returns are produced from nearly all surfaces when illuminated by a radar. Therefore, in competition with the return from an aircraft, there are many sources of unwanted signals. Unwanted signals in a search radar are generally described as noise and clutter. (Noise was discussed earlier in detail concerning the Radar Basics.) Clutter is the term used and includes ground returns, sea returns, weather, buildings, birds and insects. The definition of clutter depends on the function of the radar. Weather is not clutter in a weather detecting radar.
Since aircraft usually move much faster than weather or surface targets, velocity-sensitive radar can eliminate unwanted clutter from the radar indicator. Radar systems that detect and process only moving targets are called Moving-Target Indicators (MTI).
The basic types of clutter can be summarized as follows:Surface Clutter – Ground or sea returns are typical surface clutter.
Returns from geographical land masses are generally stationary, however, the effect of wind on trees etc means that the target can introduce a Doppler Shift to the radar return. This Doppler shift is an important method of removing unwanted signals in the signal processing part of a radar system. Clutter returned from the sea generally also has movement associated with the waves.Volume Clutter – Weather or chaff are typical volume clutter.
In the air, the most significant problem is weather clutter. This can be produced from rain or snow and can have a significant Doppler content.
Point Clutter – Birds, windmills and individual tall buildings are typical point clutter and are not extended in nature. Moving point clutter is sometimes described as angels. Birds and insects produce clutter, which can be very difficult to remove because the characteristics are very much like aircraft.Clutter can be fluctuating or non-fluctuating. Ground clutter is generally non- fluctuating in nature because the physical features are normally static. On the other hand, weather clutter is mobile under the influence of wind and is generally considered fluctuating in nature.
Clutter can be defined as homogeneous if the density of all the returns is uniform. Most types of surface and volume clutter are analysed on this basis, however, in practice this simplification does not hold good in all cases. Non-homogeneous clutter is non uniform clutter where the amplitude of the clutter varies significantly from cell to cell. Typically non-homogeneous clutter is generated by tall buildings in built up areas.
RADAR jammers are devices that make targets invisible to the radars ,which are tracking by the any use of any of the jamming techniques. Radar jammer work by mixing a portion of the radar signal with background clutter,then reflecting it back to the tracking radar with a special antenna.
ANTENNA ARRAYS
An antenna array is a group of isotropic or non-isotropic, identical or non-identical, similar or dissimilar radiators such that the currents running through them are of different amplitudes and phases to enhance the radiative properties in desired direction rather than in non-desired direction through ELECTRO MAGNETIC WAVE INTERFERENCE phenomenon. These are radiators of electromagnetic frequency and energy. Antenna arrays are the solution to the problem defined as the limitations of operating a single antenna. An example of the problem is that although a dipole antenna allows for better control of direction than an isotropic (omni-directional) antenna - as the length of the dipole increases, the control of direction decreases. Hence control by changing the length of a single antenna is very limited.