Communications: directive radio wave systems and devices (e.g. – Clutter elimination
Patent
1999-05-12
2000-09-19
Sotomayor, John B.
Communications: directive radio wave systems and devices (e.g.,
Clutter elimination
342128, 342196, G01S 1334
Patent
active
061219185
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention concerns a procedure for the elimination of interferences of short duration, such as pulses, in a radar unit of the FMCW type with linear frequency sweep, where the transmitted and received signals are combined to form a useable signal in the form of a difference signal, the beat signal, with a wave for each target, where the frequency, amplitude and phase of the wave contain the information about the target and the beat signal is sampled. The procedure within the field of mobile radars, but can also be used for other FMCW radar applications.
BACKGROUND OF THE INVENTION
The principle for linear FMCW radar is well-known, see for example Skolnik, Introduction to Radar Systems, 2nd Ed., McGraw-Hill 1980, Chapter 3. Technical advances have in recent years resulted in an increased use of FMCW radar units, which will not be considered further here. A linear FMCW (Frequency Modulated Continuous Wave) radar unit works in principle as follows:
A frequency sweep controls an oscillator with a variable frequency so that the transmitted frequency varies periodically. Each period has principally three parts, namely a constant base frequency, a linear frequency sweep and a rapid return to base frequency. The linear frequency sweep is the time when the radar unit is "carrying out useful work" and often constitutes 70-80% of the total time (work factor 0.7-0.8).
For the sake of simplicity in the discussion below the radar unit and its target are stationary. In the case of moving targets or moving radar units the Doppler effect also comes into play. For most actual FMCW systems however, the Doppler effect only involves a minor correction to the following.
The propagation time from the radar unit to a target and back again is typically a few microseconds. A signal received from a target has therefore the frequency that was transmitted a certain time before. As the frequency is swept this is not the same frequency that is being transmitted. The received frequency also has a linear frequency sweep. As the received frequency sweep and the transmitted frequency sweep are parallel with a time-displacement equal to the propagation time, as a result for a fixed target the difference in frequency between the transmitted and received signal will be constant. This constant frequency difference is given by the product between the propagation time to the target and the gradient of the frequency sweep expressed as frequency per unit of combining time.
The signal processing in a linear FMCW radar unit consists principally of the transmitted and received signals, so that the difference signal (the beat signal) is generated. This signal is the sum of a number of sine waves, where each sine wave represents a radar target. The sine waves have different frequencies, amplitudes and phase positions in accordance with the principle that large amplitude corresponds to large target, high frequency corresponds to target at a great distance. The Doppler effect (due to the relative speed) mainly affects the phase positions.
In order to determine what targets are being observed and what are their sizes and relative speeds, the difference signal is frequency-analyzed. The frequency analysis is best carried out digitally by passing the difference signal through an anti-alias filter and sampling at a constant sampling rate, after which the sampled signal is multiplied by a window function to reduce the amplitude of the signal at the start and end of the sampling period and is sent to a signal processor that carries out a Discrete Fourier Transform, DFT, usually with a fast algorithm, known as an FFT, Fast Fourier Transform. The Fourier Transform is generally complex but for a real time signal (difference signal) it has a certain degree of symmetry. In order to be able to use FFT algorithms the number of samples is usually selected as a power of two (256, 512, 1024, . . . . ). 256 samples give 256 FFT coefficients, but if the signal is real the symmetry means that of these 256 values only 128 (actually 129) are ind
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Celsiustech Electronics AB
Sotomayor John B.
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