Real Aperture Radar: Azimuth resolution

Azimuth resolution describes the ability of an imaging radar to separate two closely spaced scatterers in the direction parallel to the motion vector of the sensor
In the animation when two objects are in the radar beam simultaneously, for almost all pulses, they both cause reflections, and their echoes will be received at the same time.
However, the reflected echo from the third object will not be received until the radar moves forward. When the third object is illuminated, the first two objects are no longer illuminated, thus the echo from this object will be recorded separately.
For a real aperture radar, two targets in the azimuth or along-track resolution can be separated only if the distance between them is larger than the radar beamwidth. Hence the beamwidth is taken as the azimuth resolution depending also slant-range distance to the target for these systems.
For all types of radars, the beamwidth is a constant angular value with range. For a diffraction limited system, for a given wavelength l, the azimuth beamwidth b depends on the physical length dH of the antenna in the horizontal direction according to:
b = l/dH
For example, to obtain a beamwidth of 10 milliradians using 50 millimeters wavelength, it would be necessary to use an antenna 5 metres long. The real aperture azimuth resolution is given by:
raz = R* b
where:
raz azimuth resolution
R slant range
For example for a Real Aperture Radar of beamwidth 10 milliradians, at a slant range R equal to 700 kilometers, the azimuth resolution raz will be:
raz = 700 x 0.01
raz = 7 km
Real Aperture Radars do not provide fine resolution from orbital altitudes, although they have been built and operated successfully (for example COSMOS 1500, a spacecraft built by the former Soviet Union).

For such radars, azimuth resolution can be improved only by longer antenna or shorter wavelength. The use of shorter wavelength generally leads to a higher cloud and atmospheric attenuation, reducing the all-weather capability of imaging radars.

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry