Inverse synthetic aperture radar
Encyclopedia
Inverse synthetic aperture radar (ISAR) is a technique to generate a two-dimensional high resolution image of a target.
ISAR technology utilizes the movement of the target rather than the emitter to create the synthetic aperture. ISAR radars have a significant role aboard maritime patrol aircraft to provide them with radar image of sufficient quality to allow it to be used for target recognition purposes. In situations where other radars display only a single unidentifiable bright moving pixel, the ISAR image is often adequate to discriminate between various missiles, military aircraft, and civilian aircraft.
of the scattering centers. If the target rotates in azimuth at a constant rate through a small angle, scatters will approach or recede from the radar at a rate depending only on the cross range position- the distance normal to the radar line of sight with the origin at the target axis of rotation. The rotation will result in the generation of cross range dependent doppler frequencies
which can be sorted by a Fourier transform
. This operation is equivalent to the generation of a large synthetic aperture phased array
antenna
formed by the coherent summation of the receiver outputs for varying target / antenna geometries. For small angles, an ISAR image is the 2 dimensional Fourier transform of the received signal as a function of frequency and target aspect angle.
If the target is rotated through large angles, the doppler frequency history of a scatter becomes non linear, following a sine-wave trajectory. This doppler history can not be processed directly by a Fourier transform because of the smeared doppler frequency history resulting in the loss of cross range resolution. The maximum rotation angle which can be processed by an unmodified Fourier transform is determined by the constraint that the aperture phase error across the synthesized aperture should vary by less than a specified arbitrary amount, usually 45 degrees. This occurs when the synthetic aperture to the target range is less than required by the 2D2/lambda limit where D is the required lateral extent of the target. At this point the synthetic aperture is within the target nearfield region and requires focusing. The focusing is accomplished by applying a phase correction to the synthetic aperture.
Research has also been done with land based ISAR. The difficulty in utilizing this capability is that the object motion is far less in magnitude and usually less periodic than in the maritime case.
Errors in the 2D planar Inverse ISAR transform include:
ISAR technology utilizes the movement of the target rather than the emitter to create the synthetic aperture. ISAR radars have a significant role aboard maritime patrol aircraft to provide them with radar image of sufficient quality to allow it to be used for target recognition purposes. In situations where other radars display only a single unidentifiable bright moving pixel, the ISAR image is often adequate to discriminate between various missiles, military aircraft, and civilian aircraft.
Radar Cross Section (RCS) Imaging
Images of the target region produced by ISAR can be a useful tool in locating scattering regions on the target. ISAR images are produced by rotating the target and processing the resultant doppler historiesDoppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
of the scattering centers. If the target rotates in azimuth at a constant rate through a small angle, scatters will approach or recede from the radar at a rate depending only on the cross range position- the distance normal to the radar line of sight with the origin at the target axis of rotation. The rotation will result in the generation of cross range dependent doppler frequencies
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
which can be sorted by a Fourier transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
. This operation is equivalent to the generation of a large synthetic aperture phased array
Phased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
formed by the coherent summation of the receiver outputs for varying target / antenna geometries. For small angles, an ISAR image is the 2 dimensional Fourier transform of the received signal as a function of frequency and target aspect angle.
If the target is rotated through large angles, the doppler frequency history of a scatter becomes non linear, following a sine-wave trajectory. This doppler history can not be processed directly by a Fourier transform because of the smeared doppler frequency history resulting in the loss of cross range resolution. The maximum rotation angle which can be processed by an unmodified Fourier transform is determined by the constraint that the aperture phase error across the synthesized aperture should vary by less than a specified arbitrary amount, usually 45 degrees. This occurs when the synthetic aperture to the target range is less than required by the 2D2/lambda limit where D is the required lateral extent of the target. At this point the synthetic aperture is within the target nearfield region and requires focusing. The focusing is accomplished by applying a phase correction to the synthetic aperture.
ISAR Applications
ISAR is utilized in maritime surveillance for the classification of ships and other objects. In these applications the motion of the object due to wave action often plays a greater role than object rotation. For instance a feature which extends far over the surface of a ship such as a mast will provide a high sinusoidal response which is clearly identifiable in a two dimensional image. Images sometimes produce an uncanny similarity to a visual profile with the interesting effect that as the object rocks towards or away from the receiver the alternating doppler returns cause the profile to cycle between upright and inverted. ISAR for maritime surveillance was pioneered by Texas Instruments in collaboration with the Naval Research Laboratory and became an important capability of the P-3 Orion and the S-3B Viking US Navy aircraft.Research has also been done with land based ISAR. The difficulty in utilizing this capability is that the object motion is far less in magnitude and usually less periodic than in the maritime case.
Errors in ISAR
Errors in the ISAR imaging process generally result in defocusing and geometry errors in the image. ISAR transform errors include:- Unknown target or antenna motion: Unmodeled motion will cause the target image to defocus and be at an incorrect location. This error is controlled by suitable mechanical design or by the use of auto-focus techniques. This error can be measured by the analytic signal phase measurement method described earlier.
- Vertical nearfield errors: Unless 3D ISAR is performed, the vertical target extent at right angles to the horizontal synthetic aperture must fit within the vertical far field limit. Tall targets will defocus and move to incorrect positions. The 2D ISAR representation of a target region is a planar surface.
- Integrated sidelobe return: ISAR image quality is degraded by rangeRange-In mathematics:* Range , the set of all output values produced by a function* Interval , also called a range, a set of real numbers that includes all numbers between any two numbers in the set...
and azimuth compression side lobeSide lobeIn antenna engineering, side lobes or sidelobes are the lobes of the far field radiation pattern that are not the main lobe....
s. The sidelobes are due to data truncationTruncationIn mathematics and computer science, truncation is the term for limiting the number of digits right of the decimal point, by discarding the least significant ones.For example, consider the real numbersThe result would be:- Truncation and floor function :...
and can be reduced by the application of appropriate window functions. The sidelobes can cause significant image degradation. First, the peaks of the stronger sidelobes may cause a string of progressively weaker targets to appear on either side of a strong target. Second, the combined power of all sidelobes tends to fog or washout detail in low RCS areas. The integrated sidelobe level can under poor conditions reach a level 10 dBDecibelThe decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
below the peak target return. - Frequency and azimuth sampling errors: Incorrectly selected frequency or aspect deltas will result in aliased images, creating spurious targets. The SIM program described earlier specifically monitors for aliening errors effectively eliminating this error source.
- Antenna aberrations: Aberrations in the geometry result when the antenna phase center position is dependent upon the antenna aspect or RF frequency. This error source is normally controlled by using small, simple antennas over narrow frequency bands at long ranges. First order corrections to frequency dispersive antennas such as log periodic can be handled by phase correcting the received signal. Full correction of the aberrations can be accomplished by a direct integration of the ISAR transform using the aberrated geometry.
- Target dispersion: Dispersive targets have a non-minimum phase response, appearing to shift in position with RFRadio frequencyRadio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
frequency. Examples of dispersive targets include RF absorbers in which the absorption depth is a function of frequency and various antenna in which the phase center position is frequency dependent. CW ISAR imaging or in some cases preprocessing prior to a FMCW ISAR transform an eliminate dispersive defocusing of the target image. - Multipath: Multiple reflections can result in ISAR imaging distortions such as the classic ghost image trails from jet aircraftJet aircraftA jet aircraft is an aircraft propelled by jet engines. Jet aircraft generally fly much faster than propeller-powered aircraft and at higher altitudes – as high as . At these altitudes, jet engines achieve maximum efficiency over long distances. The engines in propeller-powered aircraft...
tail pipes.
Errors in the 2D planar Inverse ISAR transform include:
- Image blocking modeling errors: The Inverse ISAR transform currently assumes that scatters are on a planar surface and cannot block other scatters.
- Image multipath modeling errors: The Inverse ISAR transform currently does not model the multipath environment. Note the current ISAR transforms also do not correctly process multipath.
See also
- Synthetic aperture radarSynthetic aperture radarSynthetic-aperture radar is a form of radar whose defining characteristic is its use of relative motion between an antenna and its target region to provide distinctive long-term coherent-signal variations that are exploited to obtain finer spatial resolution than is possible with conventional...
- aperture synthesisAperture synthesisAperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection...
- beamformingBeamformingBeamforming is a signal processing technique used in sensor arrays for directional signal transmission or reception. This is achieved by combining elements in the array in a way where signals at particular angles experience constructive interference and while others experience destructive...
- phased arrayPhased arrayIn wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
- Optical heterodyne detectionOptical heterodyne detectionOptical heterodyne detection is an important special case of heterodyne detection. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" that is set at a close-by frequency...