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potentially suitable band sharing services based on analysis by experts from both fields.
This approach would also enable radar designs to incorporate protection mechanisms
taking account of the long lead times involved. Band sharing with safety of life services
can only be contemplated in a fully regulated environment.
3.2.8 Possible Overall Spectral Efficiency Improvements
3.2.8.1 The Concept of Spectral Efficiency
It is apparent from preceding sections that primary radar systems satisfy a wide range of
operational requirements and employ many different techniques to meet these
requirements. Furthermore, the primary determinant of spectrum utilisation is the required
radar resolution. The concept of spectral efficiency is therefore difficult to apply and
produces little benefit over and above the simple relationship between the required
resolution and bandwidth.
These issues are covered in a paper “Considering the Concept of Spectrum Efficiency as
applied to Radar” SE34(01)35. The paper considers the spectrum occupancy as inputs in
terms of polarisation, time, space and frequency. The performance achieved such as
probability of detection, update rate, volume of coverage and/or resolution can be
considered as outputs. The key input parameter is bandwidth which is directly related to
system resolution. The output side is much more difficult to define as all the parameters
are interrelated. If system resolution were adopted as the only output parameter than
virtually all radars would have similar efficiency.
The operational requirement for radar resolution is directly linked to aircraft separation
standards. The other critical factor is how the radar systems minimise out of band and
spurious emissions which are governed to some extent by the technology used. These
factors are covered in the earlier sections. However, the use of the term spectral
efficiency in radar creates an impression that the degree of control of spectrum usage is
more significant than it is for a given operational requirement. This view is echoed in the
SE34(01)35 paper.
The remainder of Section 3.2.8 therefore concentrates mainly on possible opportunities
for withdrawing or relocating services from existing band allocations.
3.2.8.2 CAA Frequency Assignment Process
The CAA was asked to outline their frequency assignment process. This is an outline of
their process in the context of S band:
A new assignment requires the following parameters:
1. Antenna gain characteristics;
2. Antenna location and height above ground;
3. Peak output power and feeder losses;
4. Instantaneous transmitted bandwidth;
5. Receiver IF bandwidth and sensitivity;
6. LNA (Low Noise Amplifier) saturation level;
7. PRF and pulse width;
8. Any frequency separation requirements.
The first step is to calculate the free space path loss between other civil radars in the
same band within a radius of 70 nm.
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