Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Patent
1986-07-24
1989-09-12
Tarcza, Thomas H.
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
342201, G01S 1390
Patent
active
048664464
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method and a radar equipment.
One normally defines radar as working in the frequency interval about 220 MHz-35 GHz. The present invention, however, is intended to bring about an extremely wideband scanning at considerably lower frequencies. This is achieved, not by side bands of a carrier frequency but by using a series of discrete frequencies, distributed within the frequency band in question, which has an upper limiting frequency less than 300 MHz and the frequency band used is preferably 12.5-200 MHz.
For the invention the working name CARABAS ("Coherent All Radio Band Sensing") has been coined, which will be used in the following description of the invention.
The purpose for the invention is to bring about an exploration system, for which wavelengths of the emitted signal are equal to characteristic lengths of the structures to be classified by the system. It is thereby intended to achieve an optimal coupling between signal and structure, both in view of obtaining a good signal
oise-ratio and of suppressing ambiguities associated with the periodicity of the radar signal.
These and other advantages and characteristics are obtained according to the invention by a method and a radar equipment. The equipment is then especially suitable for airborne registration and information processing according to the principle of so-called synthetic aperture radar (SAR).
According to a preferred embodiment of the invention a distribution of radio frequencies within the band in question is used, that makes the distance between two adjacent frequencies equal to a constant quotient between the frequencies, that is in geometric progression, yet approximated by using as approximated values frequencies that are exact harmonics to a certain fundamental frequency, for instance 30 kHz. To use a determined fundamental frequency makes both the generation of the frequency signals and a detection by sampling easier and makes also the problem with the coherence easier. Radar coherence is here defined as the emitted signal being long term periodic. As by normal radar, this can be expressed such that the Fourier transform of the signal is dominated by a set of discrete spectral components, which is obtained in microwave radar with only one signal generator, that is locked to a strictly periodical mode of operation. In CARABAS the relative bandwidth is so large, that this form of signal generation is not possible. On the other hand the absolute frequencies are so low, that digital methods of synthesis could be used.
In the receiver part of the equipment the "chain" is ended for each of the N different frequencies by a low-pass filter (Doppler-filter) which in airborne SAR-mode filters out the signals that correspond to the Doppler-shifts. These are, as is explained below, dependent upon the velocity and the resp. frequency. As both these variables are known, it is possible to dimension the filters very narrow-banded and thereby to reduce noise. If the expected Doppler-shift is 10 Hz one can for instance place a pass-band with a band width of 20-40 Hz centered around this frequency. To determine the amplitude and the phase one samples with at least twice this frequency. Considering the great number of filter circuits it is suitable to sample in series, at which the following A/D-converter and data registration equipment can function according to a multiplex system.
It is previously known that it is possible with the SAR-method to produce air images, which accordingly are produced at normal radar frequencies. According to the invention it is possible to make a corresponding reproduction also with other frequencies that are more appropriately tuned to the objects.
Reference is made to the monograph Skolnik: Introduction to radar systems, 2nd edition, McGraw-Hill 1981, for a description of known technology within the radar field. This book is hereby incorporated by reference into this description.
In certain cases, for instance when there is a risk of interference with local short-waves communication, some frequ
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Hellner Mark
Tarcza Thomas H.
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