Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Patent
1988-11-14
1991-01-15
Barron, Jr., Gilberto
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
G01S 1390
Patent
active
049857045
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a synthetic aperture processing system for processing data required from Spaceborne or Airborne sensors.
It has been known for many years that images of the earth's surface may be produced using microwave radiation by the technique known as Sideways-Looking Airborne Radar (SLAR). More recently radars carried in space satellites rather than in aircraft have used. However, the basic technique is the same.
SLAR in its simplest form is described by Jensen et al, Side-looking airborne radar, "Scientific American", (1977) and by Lillesand and Keifer, "Remote Sensing and Image Interpretation", Wiley, N.Y., 1979. A vehicle carrying a radar transmitter, e.g. an aircraft, is flown over the surface to be studied. Microwave radiation is emitted from the vehicle and the radiation reflected from the underlying surface is received by the vehicle. Two directions may be defined in relation to the vehicle. The azimuth direction is the direction in which the vehicle is moving. The range direction is the direction at right angles to the direction of movement of the vehicle.
The above mentioned publications describe the operation of SLAR in terms of the emission of pulses of microwave radiation and indicate that the resolution (i.e. the ability to separate two adjacent features on the ground) in the range direction is dependent on the length of the pulse. Commonly a relatively long duration frequency swept pulse is transmitted, in conjunction with a pulse correlation system to achieve an approximately equivalent effect to a short pulse. In the azimuth direction the resolution is dependent on the width of the beam emitted from the vehicle.
It is well known to increase the resolution to the azimuth direction by the technique of synthetic aperture radar. Originally this was done using optical techniques. However it is now possible to replace optical techniques by digital signal processing.
In synthetic aperture radar a vehicle, e.g. an aeroplane or a satellite, carrying a radar aerial or antenna is moved over a surface to be mapped. The radar beam illuminates the surface to be investigated and the movement of the vehicle results in an illuminated or irradiated swath being traced out. As the vehicle moves along its track it emits a succession of radar pulses, typically frequency swept, and receives back radar echoes from each pulse. If the echoes over a given period along track are recorded (sampled) at given time intervals they can be processed to give increased resolution in the azimuth direction. Instead of using a large physical antenna a synthetic aperture is produced. The amplitude of the radar echo at each sampling period is recorded. The echoes which are received after each pulse is emitted result from reflections from objects at successively greater distances from the vehicle. The individual sampled amplitudes can be considered as forming a matrix in which the rows in each column represent information about the illuminated surface in the azimuth direction and the columns represent information about the surface in the range direction. However as initially recorded information about any given object is distributed over the cells of the matrix and it is necessary to compress the data in range and in azimuth to concentrate the data about a given object in a picture cell (pixel) which will be used to represent it in the final image produced.
Synthetic aperture radar using digital signal processing techniques is disclosed in various references. Thus GB No. 1 540 950 discloses a synthetic aperture using an FFT signal processor. The specification explains that synthetic aperture radar may be used in either squinted or in side-looking mode and discusses the problems of maintaining the data from the radar in range and azimuth focus. GB No. 1 540 950 proposes to overcome the problem by using a motion compensation computer responsive to the velocity of the vehicle and pulse repetition frequency to maintain the data in range focus, and to vary the pulse rate frequency of the radar to maintain the data
REFERENCES:
patent: 4471357 (1984-11-01), Wu et al.
H. R. Anderson, "Digital Processing of Synthetic Array Radar Data", IEEE Symposium (1981 International), on Circuits and Systems Proceedings, Apr. 1981, Chicago, Ill., U.S.A., pp. 71-73.
Barron Jr. Gilberto
Scicon Limited
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