Traditional radar sends directional pulses of electromagnetic energy and detects the presence, position and motion of an object (such as an aircraft) by analyzing the portion of the energy reflected from the object back to the radar station. Imaging radar attempts to form an image of the object as well, by mapping the electromagnetic scattering coefficient onto a two-dimensional plane. Objects with a higher coefficient are assigned a higher optical reflective index, creating an optical image.
Several techniques have evolved to do this. Generally they take advantage of the Doppler shift caused by the rotation or other motion of the object and by the changing view of the object brought about by the relative motion between the object and the back-scatter that is perceived by radar of the object (a plane) flying over the earth. Through recent improvements of the techniques, this can be precisely calculated. Imaging radar has been used to map the Earth, other planets, asteroids, other celestial objects and to categorize targets for military systems.
Imaging radar
An imaging radar is a kind of radar equipment which can be used for imaging. A typical radar technology includes emitting radio waves, receiving their reflection, and using this information to generate data. For an imaging radar, the returning waves are used to create an image. When the radio waves reflect off objects, this will make some changes in the radio waves and can provide data about the objects, including how far the waves traveled and what kind of objects they encountered. Using the acquired data, a computer can create a 3-D or 2-D image of the target.
Imaging radar has several advantages. It can operate in the presence of obstacles that obscure the target, and can penetrate ground (sand), water, or walls.
Applications
- military: tracking, early warning, targeting
- surface topography, crustal change
- speed monitoring (police radar)
- commercial aviation, navigation, collision-avoidance
- land use monitoring, agricultural monitoring, ice patrol, environmental monitoring
- weather radar: storm monitoring, wind shear warning
- search and rescue
- Through wall radar imaging
- medical microwave tomography
Technique and methods
Current radar imaging techniques rely mainly on synthetic aperture radar (SAR) and inverse synthetic aperture radar (ISAR) imaging. Emerging technology utilizes monopulse radar 3-D imaging.
Synthetic aperture radar (SAR)
Synthetic-aperture radar (SAR) is a form of radar which moves a real aperture or antenna through a series of positions along the objects to provide distinctive long-term coherent-signal variations. This can be used to obtain higher resolution.
SARs produce a two-dimensional (2-D) image. One dimension in the image is called range and is a measure of the "line-of-sight" distance from the radar to the object. Range is determined by measuring the time from transmission of a pulse to receiving the echo from a target. Also, range resolution is determined by the transmitted pulse width.The other dimension is called azimuth and is perpendicular to range. The ability of SAR of producing relatively fine azimuth resolution makes it different from other radars.To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam. The sharpness of the beam defines the azimuth resolution. An airborne radar could collect data while flying this distance and process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beamwidth results from the relatively long synthetic aperture, which gets finer resolution than a smaller physical antenna.
Inverse aperature radar (ISAR)
Inverse synthetic aperture radar (ISAR) is another kind of SAR system which can produce high-resolution on two- and three-dimensional images.
An ISAR system consists of a stationary radar antenna and a target scene that is undergoing some motion. ISAR is theoretically equivalent to SAR in that high-azimuth resolution is achieved via relative motion between the sensor and object, yet the ISAR moving target scene is usually made up of non cooperative objects.
Algorithms with more complex schemes for motion error correction are needed for ISAR imaging than those needed in SAR. ISAR technology uses the movement of the target rather than the emitter to make the synthetic aperture. ISAR radars are commonly used on vessels or aircraft and can provide a radar image of sufficient quality for target recognition. The ISAR image is often adequate to discriminate between various missiles, military aircraft, and civilian aircraft.
Disadvantages of ISAR
- The ISAR imaging cannot obtain the real azimuth of the target.
- There sometimes exists a reverse image. For example the image formed of a boat when it rolls forwards and backwards in the ocean.
- The ISAR image is the 2-D projection image of the target on the Range-Doppler plane which is perpendicular to the rotating axis. When the Range-Doppler plane and the coordinate plane are different, the ISAR image can not reflect the real shape of the target. Thus, the ISAR imaging can not obtain the real shape information of the target in most situations.
Monopulse radar 3-D imaging technique
Monopulse radar 3-D imaging technique uses 1-D range image and monopulse angle measurement to get the real coordinates of each scatterer. Using this technique, the image doesn’t vary with the change of the target’s movement. Monopulse radar 3-D imaging utilizes the ISAR techniques to separate scatterers in the Doppler domain and perform monopulse angle measurement.
Monopulse radar 3-D imaging can obtain the 3 views of 3-D objects by using any two of the three parameters obtained from the azimuth difference beam, elevation difference beam and range measurement, which means the views of front, top and side can be azimuth-elevation, azimuth-range and elevation-range, respectively.
Monopulse imaging generally adapts to near-range targets, and the image obtained by monopulse radar 3-D imaging is the physical image which is consistent with the real size of the object.
See also
- DSMAC
- Automatic target recognition
References
External links
- What is imaging radar?
- radar imaging