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3.2.5.7 Emission Masks
The ITU and CEPT have developed emission masks for radars that are detailed in
Recommendation ITU-R SM.1541 and ERC Recommendation (02)05 respectively. The
current unwanted emission mask is shown in Fig 3-3 below.
Figure 3-3: Current Out of Band Mask for Primary Radars
3.2.5.8 Pulse Rise and Fall Time
The pulse width and rise times have been noted as having a direct effect on the overall
spectral requirements of the radar. Radar pulses are typically derived from transmitters
where there is comparatively little control of the pulse shape. Furthermore, the most
efficient way of transmitting energy is to generate a pulse with a rectangular shape.
However, there are significant benefits to be obtained in terms of out of band and
spurious emissions if the pulse rise times are controlled. In practice, the use of such
techniques is likely to be essential if the recommendations of the emission masks are to
be achieved. The use of solid state transmitters provides superior out of band spectrum
control. This is because the rise times associated with solid state transmitters are typically
in the range 100 to 200ns compared with 15 to 60 ns for a magnetron. Superior control of
pulse rise and fall times resulting in reduced bandwidth requirements can be achieved by
utilising modern solid state transmitters.
3.2.5.9 Pulse Compression
The concept of pulse compression as applied to primary radar has been outlined earlier.
For ground based ATC radars, this technology is now considered optimal and all the
prime suppliers base their main product lines on this approach. The basic advantages
are as follows:
• Good range performance from long pulses;
• High resolution equals compressed pulse equivalent;
• Low peak powers – compatible with solid state transmitters;
• No need for high voltages, waveguide pressurisation and dehydration;
• Reduced overall bandwidth requirements (depending on the choice of operating
parameters);
• Reduced susceptibility to interference utilising different encoding principles.
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