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 Radar
(radio detecting and ranging) is a device for detecting and tracking the
motion of an object from its reflected electromagnetic signature. A radar
transmits a pulse at a given frequency over a given time duration. The
reflected pulse fro the targeted object is correlated with the transmitted
pulse and the round-trip travel time is computed. A range estimate is
determined by dividing the round-trip travel time in half. The ability
to detect an object is influenced by many factors including the average
transmitter power, pulse duration, range to the object, background noise,
effective size of the receiver antenna, and the material composition of
the target (radar cross section). Pulse duration is related to range resolution
in an obvious manner: the shorter the pulse, the better your estimate
of range since there is less smearing in time. Of course, this has the
undesirable effect of increasing the power at the transmitter since average
power is inversely proportional to pulse duration. Fortunately, some clever
people devised a way to generate waveforms that are not subject to these
limitations. These extended pulses, when correlated with themselves, mimic
pulses of much shorter time duration. This formed the basis for pulse
compression radar.
The astute reader may be bothered by the fact that our ability to estimate
an object's speed is at odds with our ability to estimate its range. It
appears that in order to estimate an object's speed, we need to estimate
frequency. Why? For the same reason we are able to determine if a train
is approaching or receding from us based on the Doppler-shifted frequency
of its whistle. For many years, long-time duration sinusoidal-like waveforms
were used to estimate an object's speed while relying on more traditional
approaches for extracting an object's range. Again, some clever people
realized that if the radar is periodically pulsed, the corresponding broadband
frequency spectrum (average waveform power per unit frequency) develops
discrete line components indicative of sinusoidal-like waveforms that
could be used to estimate Doppler frequency. This formed the basis for
pulse Doppler processing, generally known as Moving Target Indication
(MTI). Both target range and target speed could be estimated simultaneously.
Our ability to resolve a target in azimuth (cross range) is related to
the effective size of the receiving antenna (or aperture); that is, the
larger the aperture, the better the angular resolution. One way to produce
a large aperture is to couple a number of antennas together coherently
into a large array. Large baseline arrays are commonly used to image distant
objects in astronomy. Another way to generate a large aperture is to do
it synthetically. Picture an antenna moving along a path where its position
is known as a function of time. We can record the data at the antenna
and compensate for the fact that the antenna is moving. As far as the
radar is concerned, it sees a much larger antenna aperture since the movement
of the antenna forms a synthetic aperture. This formed the basis for synthetic
aperture radar (SAR).
The "alphabet soup" of radar designations can appear confusing
to the uninitiated but they merely reflect the frequency at which the
radar operates. For example, X-band radar operates at a frequency of 9375
MHz, L-band radar operates at a frequency of 1250-1350 MHz, and S-band
radar operates at a frequency of 2700-2900 MHz.
For more information, please contact Garry
Jacyna using the employee directory.
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