Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Patent
1987-02-12
1988-11-22
Blum, Theodore M.
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
342 43, 342192, 342196, G01S 1390, G01S 1380
Patent
active
047869062
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method of phase compensation in synthetic aperture radar target imaging.
In a synthetic aperture radar, the synthetic aperture is produced periodically by sampling the radar doppler signal from the target during its motion. This can be done with a pulse radar and a CW radar. Where a pulse radar is used, the scanning frequency will be equal to the pulse repetition frequency (PRF) of the radar. Where a CW radar is used, the sampling frequency may be selected as desired.
When a target is illuminated with a radar, the reflected signal contains doppler information which is caused by all the motion components of the target. These are mainly a translatory motion and a rotary motion. If it is possible to compensate for the part of the doppler shift which is caused by the translatory motion, the target, as seen from the radar, will be stationary and rotate about an axis through a given point of reference. This means that it will be possible to measure the distribution of radar cross-section across the target by determining the doppler spectrum with a sufficiently good resolution or for a sufficiently long time.
To measure the distribution of the radar cross-sections across the target, it is necessary, as mentioned above, to correct the measured doppler shift for the part caused by the translatory motion of the target. This may be done e.g. by determining the path and velocity of the target and calculating on the basis of this the doppler shift caused by the translatory component of the motion. However, this is subject to great uncertainty, and moreover it is not possible to compensate for doppler shift which is caused by the influence of the propagation path on the signals, or which is caused by slight target deviation from the measured/calculated course.
The invention is based on the finding that when placing a modulating, coherent transponder on the target it is possible to produce a reference signal which is coherent with the illuminating radar signal and which can be used for compensating for the translatory motion of the target.
This compensation provides the additional advantage that the reference signal used for the compensation propagates through the same propagation path as the illuminating signal, so that also the influence of the propagation path on the illumination signal is compensated.
The invention will be described more fully below with reference to the drawing, in which
FIG. 1 shows how a radar and a target, here an aircraft, move with respect to each other,
FIG. 2 shows an imagined situation where a radar and a target perform the same translatory motion,
FIG. 3 is a simplified block diagram showing the means necessary for performing the method of the invention,
FIG. 4 is a block diagram of a digital processing unit for processing the signals received from the radar, and
FIGS. 5-10 are examples of the spectral distribution of the radar signals received, and of how these spectral distributions change when the signal is processed in accordance with the present method.
FIG. 1 shows the relation between target and radar in the ordinary situation where the radar is stationary and the target moves. Below, it is generally assumed that the target is an aircraft. However, the present invention may also be applied to determination of radar reflection distributions of all types of vehicles. But the aircraft is here taken as an example since the method is of special interest for the identification of aircraft by their radar reflection distribution. As will be seen from FIG. 1, the motion of the target may be resolved into a translatory and a rotary part. The rotary motion entails that the individual parts of the aircraft contribute with different doppler shifts, and the reflected radar signal thus contains information on the distribution of reflection cross-sections across the target. FIG. 2 shows the imagined situation: the radar follows the translatory motion of the target. This provides the effect that the target, as seen from the radar, now performs a purely rotary motion w
REFERENCES:
patent: 3522602 (1970-08-01), Ver Planck
patent: 4134113 (1979-01-01), Powell
Blum Theodore M.
Forsvarets Forskningstjeneste
Gregory Bernarr Earl
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