FINAL REPORT - Stakeholders - Ofcom

The disadvantages can be summarised as follows:
• Increased complexity due to pulse compression;
• Requirement for separate pulse to meet minimum range requirements (separate
frequency but low amplitude);
• Potential for time sidelobes (=false alarms);
• Cost.
The first three disadvantages can generally be resolved by careful design. However, the
cost of a pulse compressed radar weighs heavily on the smaller ATC service operators
who often adopt low cost magnetron systems. Certainly they are likely to resist the early
replacement of serviceable equipment which fully satisfies their operational needs. The
introduction of incentives may bring forward replacement timescales.
The bandwidth benefits of pulse compression (relative to a non-pulse compressed radar)
depend directly on the radar parameters including the transmitter type. For example, an S
band solid state pulse compression radar may have a 20dB (necessary) bandwidth of
2MHz compared to a magnetron radar of 7MHz. There is, however, the need to provide
spectrum for an additional pulse to provide adequate short range performance (bandwidth
typically 4MHz). This pulse avoids the masking effect of the long pulse. Although the total
spectrum benefits seem marginal, it must be remembered that the short pulse has
significantly less energy. However, inappropriate choice of operating parameters could
lead to increased bandwidth requirements. Note that it is the combination of pulse
compression techniques and solid state technology (with slower rise times) that gives
optimum spectrum utilisation.
Pulse compression is the technology of choice because it provides good operational
performance and superior control of bandwidth requirements.
3.2.5.10 Self Oscillating Transmitters
Self oscillating transmitters mainly use magnetrons as the output tube. Transmitters
based on magnetrons are widely used in aeronautical approach radars (16 in S band and
7 in X band). Only one civil L band magnetron system exists and this is to be replaced in
the near future. They are also used in ASDE or Surface Movement Radars (SMR)
operating in X band and Ku Band. A total of eight airports are equipped with SMR
operating in these bands. They are cheap and produce the high levels of peak power
required for radar applications. However they suffer form a number of undesirable
characteristics from a spectrum utilisation point of view:
• Pulse rise time is not controllable and inherently fast causing undesirable levels
of out of band emissions;
• They suffer from frequency drift;
• They produce high levels of harmonics and spurious noise;
• Frequency drift and a deterioration of spectrum occur with age.
In addition the RF starting phase varies from pulse to pulse which limits the radar
performance in the area of the Doppler measurements necessary to distinguish between
fixed targets and aircraft.
There is scope to improve the OoB and spurious transmission levels from a magnetron
system by the use of low pass filters (see below).
The recommendations concerning magnetrons are as follows:
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