Page 160 - Intro to Space Sciences Spacecraft Applications

DEPRESSION ANGLE Remote Sensing 147

TARGETS A&8 TARGETS C&D

Figure 6-5. Active sensor range resolution. Active sensors can improve on
resolution in the range direction by controlling the pulse length of the
transmitted energy.

mately the speed of light for most active sensor frequencies and space-to-
earth viewing scenarios. The relationship is given in equation 6-8.

This sensor resolution is achieved in the slant range plane and must be
translated into the ground plane as a function of the local target incident
angle (ei) as shown in equation 6-9.

Radars and lasers improve their spatial resolution in the range or beam
direction by using this technique. Active radar sensors can either stop at this
point or continue this spatial resolution improvement process by devising a
method of improving the along-track or azimuth spatial performance of the
sensor. The two basic types of active imaging radar systems are real-aper-
ture radar (RAR) and synthetic-aperture radar (SAR), which differ pri-
marily in the method of achieving a desired resolution in the azimuth direc-
tion. The RAR obtains its along-track resolution through the wavelength
scale physical dimensions of its antenna in the along-track direction. As a
result, the RAR uses an antenna of the maximum practical length to produce
a narrow, diffraction-limited, angular beam width in the azimuth direction.