Doppler beam sharpening
The Doppler Beam Sharpening is a method for converting measured radar echoes into an image. It is a method for improving the directional characteristics of antennas for airborne radar devices . With Doppler Beam Sharpening, the Doppler effect is used to assign information from the signal mixture to different side angles within a deflection period.
The name is ambiguous for historical reasons. Initially, the SAR process was named Doppler Beam Sharpening by its inventor Carl Wiley in 1951 . However, this name was later used for another method with which a swiveling on-board antenna generates a radar image on a sector viewing device in the direction of flight. Wiley's procedure was given a different name.
Procedure
Antennas must be used in the nose radar of an aircraft, the geometric dimensions of which are limited (often in the range of less than half a meter). Even at very high working frequencies, only a relatively low directional effect (half-widths of about 4 °) is possible. With such an antenna, only very low angular resolutions are therefore possible in real beam mode.
The reflective objects generate a Doppler frequency in the bow radar depending on the relative radial speed . The further a point is in the detection range of the radar antenna from the aircraft's direct course line, the shorter its radial velocity vector (shown in red in the graphic) and the lower the Doppler frequency of the echo signal at this point. If the speed of the aircraft (and thus the reference value of the Doppler frequency) is known, the lateral angle deviation of the reflecting object can be calculated from the measured Doppler frequency of the echo signal.
If this calculation is carried out for each pixel of the radar image, the result is a significantly sharper image than without using this method. The generated image largely corresponds to an image that could otherwise only be achieved with a much higher directivity of the radar antenna. However, the method only works where there is a measurable difference in the Doppler frequency. The method is ineffective and does not improve the image directly in the direction of flight and up to about 15 ° to the left and right of it. In the area of effective use of Doppler Beam Sharpening, the lateral angle resolution of an antenna with an opening angle of around 4 ° can be improved to values of around 0.5 °.
The procedure was used in 1970 in the Westinghouse AN / APG-66 front radar and the first delivered versions of the General Dynamics F-16 fighter aircraft were equipped with it.