Le radar aéroporté à balayage latéral (2/3)(en anglais)

The Side-looking Airborne Radar (2/3)

Principle of operation

Zoom Sign
Side-looking airborne radar principle
Geometry of SLAR operation and resolved surface area. The SLAR antenna (shown in red colour) with length is mounted to the lower side of the aircraft at flight altitude H. A section of the beam emitted at nadir angle θ is considered. The element shown in red colour denotes an irradiated volume at distance R. Other parameters are explained in the text.

Another parameter of radar instruments is the polarisation of emitted radar pulses and received signals. Due to the linear shape and along-track alignment of the SLAR antenna, electromagnetic waves are emitted with the electric field vector in a vertical orientation with respect to the plane of incidence to the ground. In the same way, backscattered waves are registered with the same orientation of the electric field vector. These characteristics are referred to as VV polarisation, i.e., vertical for both the emitted and received waves. With other instruments, e.g., radar in space, a HH polarisation (horizontal-horizontal) and a HV or VH polarisation (i.e., cross-polarisation of the emitted and received waves) can be used for specific applications.



The pixel dimensions are functions of the beamwidth in the along-track direction and the pulse length in the slant range direction. For the beamwidth angle θa it follows from the principles of diffraction of electromagnetic waves:

θ a λ

where λ is the radar wavelength and is the antenna length.

Synthetic antennas

The along-track resolution is then:

r a =R θ a = H θ a cosθ

The along-track resolution is therefore a function of the slant range R and is not the same in different parts of the image.

The slant range resolution is:

r R = cτ 2

where τ is the radar pulse length, c is the velocity of light, and θ is the nadir angle of the considered radar pulse section. The factor of two in the denominator is due to the fact that the radiation propagates forth and back the distance interval rR to achieve this resolution. The horizontal resolution on the ground is then:

r ρ = cτ 2sinθ

The pixel dimensions ra and rρ depend on the angle of incidence θ and hence on the distance to the target sidewise of the flight track.

Exercise: Radar pixel size