FINAL REPORT - Stakeholders - Ofcom

• A long term policy to replace magnetron based transmitters (L and S band) with
solid state transmitters should be adopted by service providers. Solid state
technology is becoming available at X band.
• In the short term, coaxial magnetrons with low pass filters should be adopted to
meet the current unwanted emission mask.
3.2.5.11 Driven Transmitters
Driven transmitters provide a much higher degree of control of the transmitted waveforms.
Until recently, klystrons and travelling wave tubes were the main output devices used in
“driven” civil ATC radars. Klystrons are capable of high peak powers but with a
restricted bandwidth which limits their use in pulse compression systems. They can suffer
from a relatively high harmonic content in their output. Travelling wave tubes have a
higher bandwidth capability but can only provide relatively low peak power outputs. They
are ideally suited to pulse compression systems and are widely used by National Air
Traffic Services in the current long range network (L Band) and as approach/TMA radars
(S band). However, they suffer from relatively fast rise times thus limiting spectral
efficiency.
Most ATC radars employing valves require dual channels to achieve the specified level of
system availability. Dual channel systems require two sets of operating frequencies.
Solid state technology is now the preferred option for new L and S band systems although
currently they represent only a small percentage of the installed systems in the UK. NATS
has a long term radar replacement programme which will adopt solid state technology
including pulse compression.
Individual devices provide only low mean and peak powers. The low mean power
limitation is overcome by using the appropriate number of devices in parallel while the low
peak power limitation is overcome by utilising pulse compression. The output of individual
devices can be combined in a combiner network and the output coupled to a conventional
antenna in the normal way. Alternatively the individual devices can be placed on a
radiating surface and the output combined in space. This is known as a phased array and
represents a combination of transmitter and antenna technology. Solid state devices are
ideally matched to the use of pulse compression to achieve overall spectrum benefits.
Solid state technology can be designed to be inherently fail-soft. A fail soft capability
means that a system element can fail without an interruption to the service. This is a
consequence of using large numbers of modules in parallel and having some redundancy
in the number of modules necessary to satisfy the operational requirement. This may
obviate the requirement to have two channels of equipment for availability reasons. This
approach may partially offset the higher initial cost of solid state equipment and provide
savings in frequency allocations.
Solid state technology (coupled with pulse compression) represents the optimum
approach to the provision of ATC approach and long range radar services in terms of
radar performance and controlled spectrum utilisation. Initial costs can be partially offset
by single channel configurations and reduced operating costs. Significant progress has
been made by service providers in the adoption of this technology.
3.2.5.12 Antenna Design and Coverage
Careful design of the antenna is necessary to avoid unnecessary illumination outside the
areas of required coverage. This approach maximises the performance of the radar
system and minimises the potential for interference to and from other systems. Generally
radar systems utilise a narrow beam in the horizontal plane to provide adequate azimuth
accuracy and resolution. In the vertical plane, a fan beam is used to provide adequate low
level and high level coverage. These parameters are therefore directly linked to the
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