| Space
Time Adaptive Processing |
Definition
Space-Time Adaptive Processing (STAP) refers to a class of signal processing
techniques used to process returns of an antenna array radar system. It enhances
the ability of radars to detect targets that might otherwise be obscured by clutter
or jamming.
. The output of STAP is a linear combination or weighted sum
of the input signal samples .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 STAP 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 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 element over the entire dwell define
a system impulse response and hence a system frequency response. STAP
is a multi-dimensional adaptive signal processing technique over spatial and temporal
samples. In this approach, the input data collected from several antenna sensors
has a cubic form. Depending on how this input data cube is processed, STAP is
classified into Higher Order Post-Doppler (HOPD), Element Space Pre-Doppler, Element
Space Post-Doppler, Beam Space Pre-Doppler, and Beam Space Post-Doppler. STAP
consists of three major computation steps. First, a set of rules called the training
strategy is used to select data which will be processed in the subsequent computation.
The second step is weight computation. It requires solving a set of linear equations.
This is the most computationally intensive step. Finally, thresholding operation
is performed after applying the computed weights. In HOPD processing, Doppler
processing (FFT computations) is followed by solving least square problems (QR
decompositions). Introduction
To Radar
Radar is an electromagnetic
system for the detection and location of objects. RADAR is nothing but Radio Detection
And Ranging. It operates by transmitting a particular type of waveform and detects
the nature of the echo signal. An elementary form of radar consists of a transmitting
antenna emitting electromagnetic radiation generated by an oscillator of some
sort, a receiving antenna, and an energy detecting device or receiver .A portion
of the transmitted signal is intercepted by a reflecting object (target) and is
reradiated in all directions. It is the energy reradiated in the back direction
that is of prime interest to the radar. The receiving antenna collects the returned
energy and delivers it to a receiver, where it is processed to detect the presence
of the target and to extract its location and relative velocity.
The transmitter may be an oscillator such as magnetron, which is pulsed by the
modulator to generate a repetitive train of pulses. The waveform generated by
the transmitter travels via a transmission line to the antenna where it is radiated
into space.
A single antenna is generally used for both transmitting and receiving.
The receiver must be protected from damage caused by the high power of the transmitter.
This is the function of the duplexer. The duplexer also serves to channel the
returned echo signals to the receiver and not to the transmitter. The receiver
is usually of the superhetrodyne type.
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