Millimeter-Wave Radar Targets and Clutter (Artech House Radar Library)

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This is done using the Doppler-frequency shift of the received signals. The clutter has only small radial velocity components due to the motion of vegetation or waves, while moving targets are likely to have larger radial velocities. Limitations of MTI processing result from the stability of the radar components and the velocity spread of the clutter. MTI processors may be simply characterized by the minimum detectable target velocity MDV , and the clutter cancellation ratio see Sections 9.

In pulse-Doppler radar [8], a coherent burst of pulses is transmitted. Coherency implies that the phases of the individual pulses are derived from a continuous stable signal, that is also used in processing the received signals. The returned signals are processed using a Fourier-transform-type algorithm to divide the received signal into a series of spectral bands.

The pulse-Doppler band in which a target is detected also gives a measure of its Doppler-frequency shift, and hence its radial velocity. Pulse-Doppler processing is often used in airborne and space-based radar see Section With these moving platforms, the radar returns from terrestrial clutter may have a large Doppler frequency spread, due to the spread of angles at which the clutter is viewed, both in the main radar beam and through the antenna sidelobes see Section Thus, cancellation of the clutter by MTI techniques is often not effective.

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Pulse-Doppler processing, however, allows rejection of bands having large clutter components, detection of targets in bands clear of clutter returns, and setting of detection thresholds over the clutter signal return in bands, where target returns may exceed clutter returns. Pulse-Doppler processing may be simply characterized by the velocity resolution corresponding to the processed Doppler frequency bands, and the suppression of clutter not in a band. Synthetic-aperture radar SAR processing [9] is used by moving radar e. Radar may achieve good range resolution by using short pulses or employing pulse compression.

For example, with a beamwidth of 10 mR 0. With an X-band wavelength of 0. An SAR radar processor may be simply characterized by the resolution it provides, 1.

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SAR processing is based on Fourier transforms, but corrections are made for factors such as changes in target range and platform path perturbations during the processing time. In a strip-mapping mode, a SAR collects and processes data at a fixed angle relative to the platform as it moves along its path. This technique, often referred to as side-looking radar, generates a continuous strip map. In a spotlight SAR, the radar beam is scanned relative to the platform to keep the desired target region in coverage. This allows increased processing time, and hence greater angular resolution for the region imaged.

Some modern airborne multifunction radars may employ both conventional radar modes using MTI and pulse Doppler to detect moving targets, and SAR modes to image terrain, as discussed in Chapter This is referred to as monostatic radar. Radars often use the same antennas for transmitting and receiving, and so is monostatic by definition. Other radars have their transmitting and receiving antennas close together, compared with the target range. These generally have the same characteristics as monostatic radar, and are included in that class.

Advantages of monostatic radar are the common use of radar hardware at a single site, illumination of the same region of space by the transmit and receive antennas, and simplified radar coordination. With bistatic radar, the transmitting and receiving antennas are separated, as shown in Figure 2.

With bistatic radar, it is usually necessary to coordinate operation of the transmitting and receiving sites, to provide multiple receive beams to cover the transmitted beam region, and to take the bistatic geometry into account in the signal processing. What is the range to the radar horizon from a radar having a height of 20m above a smooth earth? What is the range if the height in the above example is increased to 2,m?

Are signal losses from the atmosphere and rain more severe at higher or lower radar frequencies? What type of reflector antenna is well suited for tracking a single target? What is the maximum range that a radar 25m above a smooth earth can observe a target having an altitude of 5 km?

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At what radar frequencies can the signal be distorted by the ionosphere? What type of radar antenna allows rapid electronic scanning of the radar beam? Brookner, ed. Cantafio, ed. Skolnik, ed. Skolnik ed. Selected Bibliography A general discussion of radar configurations is given by Skolnik in his original book, which was updated in Further information on airborne radar, including pulse-Doppler operating modes, can be found in Morchin, Morris, and Brookner. Design and operation of SAR is treated in Hovanessian. Brookner, E. Cantafio, L. J, ed. Hovanessian, S.

Morchin, W. Morris, G. Analysis and modeling of radar at the system level utilizes parameters of the radar components and operating modes, and of the environment in which the radar operates. Parameters of the major radar components and of radar targets are described in this chapter.


A list of symbols used for the parameters is provided in Appendix A. The peak power may be maintained for some period, usually determined by the heating of small components of the transmitter tube or solid-state device used to generate the power. The ratio of average to peak power is called the transmitter duty cycle: 3. Solid-state devices are usually limited in their power capabilities by heating in the semiconductor junctions. Since the thermal mass of these junctions is very small, the peak and average power of solid-state devices are often nearly the same.

The efficiency of a radar transmitter is defined as the ratio of the average RF power produced to the prime power supplied to the transmitter: Y] T 3. The transmitter power levels are usually specified at the transmitter output. Tube transmitters often use a single tube to generate the RF power for a radar, but in other cases the outputs of two or more tubes are combined to produce the power needed. Solid-state transmitters for large radar combine the outputs from several solid-state devices.

When the power is combined in the transmitter, the resulting transmitter output power is usually specified. This output power is normally less than the sum of the powers generated by the individual devices, due to losses in the combining network. The combining losses are usually included in the transmitter efficiency. In these cases, the transmitter power is usually specified as the sum of the powers generated by the individual transmitters see Section 5.

The RF power radiated by the antenna is usually less than that generated by the transmitter, due to losses in the microwave circuits between the transmitter and the antenna, and losses in the antenna: 3. Note that the portion of the prime power that is not radiated as RF power creates heat in the radar. Most modern radar uses coherent transmitters.

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This means that the phases of the transmitted waveforms are derived from a stable reference signal that is also used by the receiver. This allows the received signals to be processed coherently to measure Doppler-frequency shift see Section 1. The phase stability of the transmitter determines the degree to which these functions may be performed, and the time interval that may be used. Some older radars were limited in the coherent processing that could be employed, or were not coherent at all [1, pp.

The most important characteristics are beamwidth, gain, and sidelobe levels. These are illustrated in Figure 3. The antenna pattern is usually defined in the far field of the antenna. This is a range at which the rays from the antenna are essentially parallel also called the Fraunhofer region. The antenna beamwidth is related to the antenna size, w, and radar wavelength by: w 3. For square or circular antennas, the two orthogonal beamwidths are equal; for rectangular or oval shapes, they are unequal.

Many radars use the same antenna to transmit and receive. These radars use microwave switching devices to switch the antenna between the transmitter and receiver. Gain is expressed in decibels relative to isotropic dBi , which is the gain of a lossless source radiating uniformly over 4jt sterradians. Sidelobe levels are usually expressed in decibels relative to the gain negative values.

This means that the RF current density and signal phase are constant across the antenna face. The level of the first sidelobe in the principal angular coordinates those parallel to the edges of the rectangle , for such antennas is For circular antennas with uni- 3. The sidelobe levels decrease with distance from the mainbeam. Lower close-in sidelobe levels are often desired to allow viewing of closely- spaced targets of different sizes or to reject interfering clutter or jamming signals.

Close-in sidelobes, those within a few beamwidths of the mainbeam, may be reduced by varying the RF current density, called aperture illumination, across the antenna face, providing lower current density near the edge of the antenna than at the center.

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This is called tapering or weighting the antenna illumination. In general, the power density pattern in the far field of the antenna is given by a Fourier- transform function of the aperture illumination function [2, pp. The relationship in 3. By carefully controlling the illumination weighting, the close-in sidelobes may be reduced to dB or lower.

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  • This improvement in close-in sidelobes comes at the cost of reduced antenna efficiency, reduced the gain, and increased beamwidth. For example, a cos 3 illumination weighting produces a maximum sidelobe level of dB, but the antenna efficiency is reduced from 1. The antenna beamwidth coefficient, k A , increases from 0.

    When aperture weighting is applied in both antenna dimensions, the aperture efficiency and efficiency loss are the product of the factors for the two weighting functions used. Sidelobes may also be produced in reflector antennas by energy spillover from the feedhorn illuminating the reflector, and by reflections off structural elements. These reduce the aperture efficiency and increase the aperture-efficiency losses.

    The transmit antenna in such cases often uses uniform illumination in order to maximize its radiation efficiency. The needed taper is applied to the receive antenna. Phased-array antennas are often designed to use dif- 28 Radar Analysis Parameters Table 3. Reflector antennas that use feedhorns and perform both transmit and receive functions usually are configured to use the same illumination pattern for both functions.