The performance of the radar
The primary goal of the RADARS AT mission is to generate economic
benefits from the use of the radar data. For this reason when the mission
requirements for RADARSAT were being defined careful consideration was
given to the selection of the radar frequency. Experimental data was gathered
from a wide variety of sources, including an ongoing multi-frequency airborne
SAR experimental program. It was quickly discovered that on the one hand
the range of possible applications of SAR data was far wider than had
previously been expected but on the other it would require a very versatile
SAR to provide data suitable to meet all these needs.
This requirement has led to a design for a single frequency SAR which can
function in a number of different modes under the control of an on-board
computer. C-band was selected as the optimum frequency because it enables
the greatest number of application needs to be satisfied. Each day ground
control will transmit to the satellite a schedule of the day’s activities listing
times at which the SAR is to be turned on and off corresponding to the
geographic locations where the data are to be acquired. Associated with each
SAR turn-on is a file of radar parameters specifying the required SAR mode.
The SAR antenna consists of an array of emitting elements each fed with a
constant amplitude signal thus defining the shape of the radar beam. The
phase of these signals is programmable, however, and this permits the width
and pointing direction of the beam to be selected at will in the range
direction. As the satellite moves along its track the data from which the radar
image is created are collected continuously. In this way, controlling the beam
width, in effect, controls the swath width of the resulting picture. Associated
with these variable swath widths are corresponding variations in the size of
the resolution elements on the ground.
Control of the beam direction enables the incidence angle to be varied. This
allows a given site on the ground to be imaged from different directions thus
permitting additional information to be obtained for features in the scene
which are sensitive to the incidence angle. It also allows stereo pairs of
images to be collected which can be used jointly with radar shadow
information for geomorphological studies or alone to produce topographic
maps. Where the scene parameters of interest are not sensitive to incidence
angle the variable incidence angle can be used to increase the frequency of
opportunities for obtaining images of a given site.
