Global Positioning System Reference
In-Depth Information
FIGURE 8.8.
Radar basics. A transmitter beam sweeps over an area of sea, ''painting''
the surface and the air above it. A blip, or pulse, of microwave energy is transmitted
along the beam; it reflects off all it encounters, and the reflected signal is picked up
by the radar receiver. Transmitted and reflected signals are here represented by
arrows. Signal processing enables the radar operator to discern useful targets—such
as a boat, a beacon, or an airplane—from clutter, such as the sea surface. Further
processing may yield the altitude of the airplane and the speed and heading of the
plane and boat. Even the most basic navigation radars can provide a reliable
estimate of a target range and bearing.
overlaid on the display, for easy reference. But how do we tell whether the
target is on the surface of the sea or above it? A plan view will show the
latitude and longitude of a target but not its height above the surface. More
importantly, the radar beam of figure 8.8 cannot distinguish between sur-
face targets and those at altitude; all are painted by each blip within the
beam. Another basic problem is the clutter: our transmitted blip will re-
flect o√ the sea surface as well as o√ the intended targets—and the sea
is much bigger than the targets, with a correspondingly overwhelming
radar signal.
In fact, all of these problems can be overcome. The radar beam can be
jittered
16
and, with appropriate processing, the airplane altitude extracted.
This altitude can then be appended as a number beside the signal plotted
on the radar display. Because sea clutter fluctuates in a characteristic man-
ner, its e√ects can be mitigated by filtering and signal integration. The
point is that modern radar systems are extremely capable as a result of the
advanced signal processing that manipulates the raw receiver signal data.
Target speed can be determined with Doppler radar. Many targets can be
tracked simultaneously on a display, as for the ATC radar of figure 8.9.
Another type of radar system—synthetic aperture radar, or SAR—exploits
16. ''Jitter'' in this context means rapidly oscillating the radar beam; the resulting data
can be processed to provide an improved target position estimate.
