FINAL REPORT - Stakeholders - Ofcom

R
max
⎡
= ⎢P
⎢⎣
2 2
G σλ
⎤
3
( 4 ) (min) ⎥ ⎥
p
π Pr
⎦
1/
4
Where: Pp is the peak power
Pr(min) is the Minimum Detectable Signal
σ is the RCS
and G is the antenna gain.
It will be noted that range is proportional to the fourth root of transmitted power and that
the longer the wavelength, the better the range performance. This is why 10cm radars
are less used for long range radars. Coupled with improved immunity to weather returns,
the lower frequencies are therefore attractive to system designers. The major
disadvantage is the cost of an antenna with the required resolution.
The minimum detectable signal is related to the probability of detection and probability of
false alarm. A high level of detection with low false alarms requires a high signal to noise
ratio. These closely interrelated aspects are also important in the context of band sharing
and are covered in more detail in the band sharing section.
3.2.2.3 Pulse Compression
The radar equation indicates that if all factors have been optimised, the peak power must
be increased by a factor of 16 to double the maximum radar range. Since the maximum
level of the instantaneous power is limited (which is the case for modern solid state
radars), an alternative (and usually easier approach) is to increase the pulse length.
Unfortunately a long pulse degrades the radar resolution. The requirements for increased
range and better range resolution clearly conflict for a conventional radar.
Typically, in pulse compression radar, a long frequency modulated pulse is amplified by
the transmitter output stage and passed to the antenna. The frequency increases linearly
over the duration of the pulse. This is often described as a chirp pulse. On reception, the
frequency modulated pulse is passed through a filter (the compressor) with a
characteristic which delays the higher frequencies less than the lower frequencies. The
net result is to compress the received pulse width and increase its effective peak power.
Note that the process can introduce sidelobes in the time domain. These sidelobes are
due to the non ideal characteristics of the compression filter and can generate false
alarms. The principles are shown on Figure 3-2.
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