Other hardware and software sources of phase error also are likely to be present even in a well-designed system. Without this location accuracy, phase errors will exist across the azimuth signal aperture and cause image distortion, defocus, and loss of contrast. Location accuracy on the order of a fraction of a wavelength is necessary, perhaps to a few millimeters in the case of X-band operation at 10-GHz center frequency. A major source of uncertainty in the relative phase among these signals is the exact location of the radar antenna at the time of transmission and reception of each pulse. The synthetic aperture achieves fine cross-range resolution by adjusting the relative phase among signals received from various pulses and coherently summing them to achieve a focused image. Ron Goodman, Walter Carrara, in Handbook of Image and Video Processing (Second Edition), 2005 3.1 Autofocus Algorithms L-band) are preferred for long-range and medium-range surveillance and the X-band is preferred for horizon surveillance.
In principle, therefore, low-frequency bands (e.g. Considering these real-world aspects, X-band operation is preferred for horizon surveillance. The frequency band, however, cannot be too high because the atmospheric attenuation and noise temperature of receivers increase as the frequency of operation increases and the transmitter power generated by solid-state transmitters decreases. If we adopt this criterion, higher frequencies are favored. For a given aperture, the radar's beamwidth should be as narrow as possible to avoid the problems associated with multiple paths considered in section 1.6.8. While maximizing the range is appropriate for long-range searches, other criteria have to be considered for horizon searches. It is therefore preferable for surveillance radars to operate at L-band, where the radar range can be maximized. Below L-band, however, the sky noise increases significantly and low-noise systems are not viable. The assumption is made that as the frequency of operation decreases, the transmitter power available increases and the noise temperature of receivers decreases (see Chapter 4). At frequencies below 3 GHz, α can be safely ignored and the lower the frequency the longer the range. However α, P av, and T are frequency dependent. At first sight the range attainable is not dependent on the wavelength of operation. The parameters in the bracket of equation (1.8) are either constants or given. In a third method, called conical scan, a single offset beam is rotated around the boresight direction, providing angle error information in both axes. Note that in both methods two offset beams are required for each of the two axes (azimuth and elevation). In a different approach, the two offset beams are created sequentially by alternating the direction of the beam every few pulses. Thus, angle measurement can be performed using a single reflected pulse. The name monopulse stems from the fact that the two offset beams are available simultaneously, due to the use of two feeds and two receiving channels. The two beams must be well balanced, so that the difference signal disappears exactly when the target is on the boresight line of the antenna. The difference signal, normalized with respect to a “sum” signal, is proportional to the angular error ( Fig. The received signals from the two beams are subtracted, creating a “difference” signal. Two beams in opposite offsets from boresight are created using two off-axis feeds. The most important is the monopulse technique. The various tracking methods differ in how they implement that subtraction. The derivative of a function is obtained by subtracting two slightly shifted copies of the function.
A derivative near a maximum is relative to the distance from the maximum (including the sign) and can serve as an error signal. All of them are based on effectively performing a derivative operation on the antenna pattern. There are several techniques to accomplish that. In high SNR it can be determined to within a very small fraction of the beamwidth. The angle error is the angle between the peak of the antenna beam and the direction to the target. Automatic tracking requires an ability to measure the angle error and a feedback loop that will use this error information to correct the antenna direction. In the tracking mode, the antenna is dedicated to one target and automatically follows its changing direction. In the scanning mode, all the targets in a section are illuminated periodically. Radar antennas operate in two modes: scannig and tracking.
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