Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Patent
1997-05-09
1999-01-19
Sotomayor, John B.
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
342196, G01S 1390
Patent
active
058618350
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Method to improve data obtained by a radar.
1. Field of the Invention
The present invention relates to a method of improving data obtained by radar.
2. Description of the Prior Art
A problem that may arise in the context of radars is that the reception of radar signals can be jammed by radio and television transmissions. Especially in wide band low-frequency radar of the type that is called CARABAS.RTM. (Coherent All Radio Band Sensing) and is the subject matter of Swedish Patent No. 8406007-8 (456,117) and European Patent 86900306.1 (202,320), this is a crucial problem. The signal intensity of at least local radio and TV stations is, at the frequencies thereof, much higher than the signal intensity of CARABAS at the corresponding frequencies. Moreover, it is impossible to increase the output power of a radar station to a sufficient degree to compete with the radio and TV signals. Besides, this is illegal. Instead it is convenient to exclude, if possible, such frequencies in the output signal or at least neglect the received signal at these frequencies.
SUMMARY OF THE INVENTION
To be able to perform the desired signal processing over the entire frequency interval that applies to the radar, it is desirable that narrow band interferences of the discussed type can be filtered off and replaced with an estimated value of the signal intensity. The invention has solved this problem.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail with reference to the accompanying drawings in which
FIG. 1 shows a first subroutine as used when implementing the invention in LabView2 called "Jungle Law",
FIG. 2 shows a second subroutine as used when implementing the invention in LabView 2, called "Mutation",
FIG. 3 shows a third subroutine as used when implementing the invention in LabView 2, called "Geometric Machine", and
FIG. 4 shows the main routine as used when implementing the invention in LabView 2, called "Darwinistic Relaxation".
DETAILED DESCRIPTION OF THE EMBODIMENTS
The present invention has been developed on the basis of the above-mentioned wide band radar equipment CARABAS, and in the following it will principally be described in view of this equipment. However, it is obvious to those skilled in the art that it may also be used in the context of other radars, which is consequently included in the scope of protection according to the claims.
Whether, when transmitting radar signals, certain frequencies have been excluded or not, the received radar data will be fragmented into a number of bands which are independent of each other and between which interference from radio transmissions etc. has caused gaps void of relevant radar data. In this context, there is no essential difference between the image spectrum in a SAR radar and general radar raw data, but by such an image spectrum is here meant radar data, from which the actual image is obtained by an inverse two-dimensional Fourier transform. In fact, the SAR image is obtained from radar raw data by a process which associates the transmitted frequency and the Doppler frequency of these data with the azimuth and radial frequencies of the image spectrum, respectively. Radio transmissions etc. will appear as concentric rings of destroyed data in this two-dimensional spectrum. The radial frequency of the rings will correspond to the transmitted frequencies at which interference occurs.
To evaluate the extent of degradation that occurs in the radar image owing to the degradation of radar data, assumptions must be made regarding the statistics of radar data and the radar image. For radar data, the statistics are highly constant and may be regarded as a spectrum of random phase, in which however there is little probability of great variations of amplitude (after adjusting for sensitivity fluctuations in the radar system).
On the other hand, the image is characterised by a highly non-uniform amplitude distribution, which in the ideal case consists of discrete, stronger reflectors against a background of
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