Global Positioning System Reference
In-Depth Information
upon a radar screen, as we will soon see, thus providing navigators with a
lot of information in a form that they can rapidly assimilate and act upon.
15
BLIPS AND BEAMS
Radar is a remote sensor, and its operation di√ers significantly in one
respect from that of the other radio techniques that we have investigated
thus far. Radar equipment does not passively detect what is out there: it
transmits electronic signals into the outside world and then listens for the
echoes. This two-way process means that radar operation does not depend
upon a cooperative target. A civilian airliner transmits signals to aid in its
detection by an airport air tra≈c control (ATC) radar, whereas an enemy
fighter plane does not want to be detected, yet both may be seen by radar.
The basic principle of operation is simple. It depends upon directing a
radar signal and timing its echo. That is, a radar transmitter sends out a
signal (for simplicity, let us say that it is a single, brief ''blip'') in the
direction in which the transmitter antenna is pointing (let us say that the
beam is very narrow, so that the direction is well defined). A clock starts
when the blip is sent out and stops when its echo is received. Knowing the
speed of light—which is also the speed of radar microwaves—we can
calculate the target range. Thus the simplest radar set provides range and
bearing for an object of interest, a target.
To increase coverage area, the transmitter beam is rotated, so that, for
example, a beam sweeps 360\ around the horizon. The idea is sketched in
figure 8.8, where we see a terrestrial radar antenna beam sweeping over
the sea. Surface and airborne targets are
painted
(illuminated by the beam).
A transmitted blip is bounced o√ each and is reflected back toward the
radar, where a receiver antenna picks up the signal, processes it, and
displays the data on a screen, such as that shown in figure 8.9.
Between the simple diagram of figure 8.8 and the display of figure 8.9
lie 70 years of accumulated knowledge about how to filter, amplify, and
process radar signals. First the basics: we have seen how target bearing and
range can be determined through use of a directed beam and timing. It is a
straightforward matter to display this information on a radar screen that
shows a plan view of the radar coverage area. The screen shown in fig-
ure 8.9 has such a display, with range circles centered on the radar location
15. I worked for 20 years in the field of radar signal processing. To my surprise, I found
that there were remarkably few popular-level topics that sought to explain radar operation
in a nontrivial way, but without resorting to the heavy mathematics of dedicated textbooks,
and so I wrote one: see Denny (2007).
