Real Aperture Radar: Range resolution

For the radar to be able to distinguish two closely spaced elements, their echoes must necessarily be received at different times.
In the upper part of the figure, the pulse of length L is approaching buildings A and B. The slant range distance between the two buildings is d.
Since the radar pulse must travel two ways, the two buildings lead to two distinguished echoes if:
d > L/2
The part of the pulse backscattered by building A is PA , and the part of the pulse backscattered by building B is PB .
It appears in the lower part of the figure that to reach the target and come back, PB has covered an extra distance 2d, and thus is at a slightly shorter distance than L behind PA..
Because of this, the end of PA and the beginning of PB overlap when they reach the antenna. As a consequence, they are imaged as one single large target which extends from A to B.
If the slant range distance between A and B were slightly higher than L/2, the two pulses would not overlap and the two signals would be recorded separately.
Range resolution (across track resolution) is approximately equal to L/2, i.e. half the pulse length.

Ground range resolution is:
where:
c speed of light
t pulse duration
q incidence angle

Incidence angle is the angle between the vertical to the terrain and the line going from the antenna to the object.
To improve range resolution, radar pulses should be as short as possible. However, it is also necessary for the pulses to transmit enough energy to enable the detection of the reflected signals.
If the pulse is shortened, its amplitude must be increased to keep the same total energy in the pulse.
One limitation is the fact that the equipment required to transmit a very short, high-energy pulse is difficult to build.
For this reason, most long range radar systems use the "chirp" approach which is an alternative method of pulse compression by frequency modulation.
In the case of the chirp technique, instead of a short pulse with a constant frequency, a long pulse is emitted with a modulated frequency.
The frequency modulation must be processed after reception to focus the pulse to a much shorter value. For the user, the result is the same as if a very short pulse had been used throughout the system.

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