Side looking airborne radar

The range resolution (the ability to separate the pixels of the image perpendicular to the direction of flight) of a SLAR depends on the length of the transmitted pulse. At the ground of Earth the range resolution has an inverse relationship with the depression angle:

${\displaystyle \delta _{g}={\frac {\tau c_{0}}{2\cos \gamma }}}$

${\displaystyle \tau }$ = duration of the (may be compressed in matched receiver) radar pulse

${\displaystyle c_{0}}$ = speed of light

${\displaystyle \gamma }$ = depression angle

The pulse width ${\displaystyle \tau }$ is typically 0.4 ... 1 μs, i.e. ${\displaystyle \delta _{g}}$ = 8 ... 200 m. The shorter the pulse width ${\displaystyle \tau }$ the lower ${\displaystyle \delta _{g}}$ and the highest the range resolution, but the lower the echo signal. This limitation can be overcome using intra-pulse modulation. Using a step-frequency waveform of bandwidth B the range resolution is ${\displaystyle \delta _{r}=c_{0}/2B}$.

The azimuthal resolution (better known as crossrange resolution) depends on the beamwidth of the radar antenna. It is derived from the ratio of the physical size of the antenna (the real aperture) to the wavelength used. By the spreading of the beam it is also dependent on the slant range.

${\displaystyle \delta _{Az}={\frac {\rho \lambda }{2L}}={\frac {H\lambda }{L\sin \gamma }}}$

${\displaystyle \lambda }$ = wavelength

${\displaystyle L}$ = antenna length (in direction of flight)

${\displaystyle \rho }$ = slant range

${\displaystyle H}$ = height of the platform

It is apparent that SLAR antennas as real aperture could not be built large enough to achieve the desired azimuth resolution. In fact, SLAR was never feasible to be used in space because the antennas would be too large and their launch in space too expensive. Synthetic aperture radar refers to a method for improving the azimuth resolution (not range resolution).

• Radar imaging

• Radartutorial
• "A Sideways Glance" a 1966 Flight article on SLAR