FINAL REPORT - Stakeholders - Ofcom

Figure 3-5: Probability of detection as a function of signal to noise ratio
Figure 3-5 is for illustrative purposes only and is not intended to support calculations. It
shows two different types of target model: non-fluctuating and fluctuating (Swerling 1). It
will be noted that the relationship between probability of detection and changes in signal
to noise ratio is heavily dependent on the target model chosen.
For a non-fluctuating target and a given false alarm rate, a one dB change in signal to
noise ratio can reduce the probability of detection by about 28%. In the case of a
fluctuating target, a one dB change in signal to noise ratio can reduce the probability of
detection by some 8%.
If the protection level is set at an interference level to noise ratio (INR) of –6dB, then the
impact on overall signal to noise ratio is about 1dB. If the INR is set at –9dB, then the
impact on overall signal to noise ratio is about 0.5 dB.
In general, primary radar systems are very sensitive to interfering signals unless they
have radar like characteristics i.e. a pulsed waveform with a low duty cycle.
3.2.7.5 Pulse Compression
It has already been noted that the use of pulse compression improves immunity to
interference provided that the encoding regime is significantly different. This underlines
the potential benefits of planning band sharing to use differences in characteristics which
optimise the immunity between services. For example it may be possible to adopt a long
term policy to standardise ATM services on the use of pulse compression including
guidelines on the encoding techniques. This may improve the probability of band sharing
with other services which use different encoding techniques.
3.2.7.6 Allocation Sharing Summary
The points in the preceding paragraphs are intended to illustrate that extreme caution
must be observed in the evaluation of band sharing with safety of life primary radar
services. Band sharing may be a possibility but it is likely to require a number of
changes to the installed radar systems, the application of standards to primary radar
systems, the identification of the necessary characteristics of potential band sharing
services and full analysis of their suitability. All this must be achieved with the support of
the ATM equipment and service suppliers, candidate sharing system and service
suppliers and regulators. It is proposed that this is developed as a band sharing strategy
for the specific bands concerned.
These points enable some observations to be drawn with regard to opportunities for band
sharing:
• Band sharing incurs the risk of increasing the false alarm rate and/or a reduction
in probability of detection.
• The CAA are acutely aware of the need for the efficient use of the spectrum
including band sharing if the analysis can demonstrate no risks to the primary
radar service.
• There is a lack of primary radar standards which would assist the assessment of
band sharing proposals.
• The protection ratios (interference to noise ratio) need to be further developed.
An agreed method for assessing the feasibility of band sharing should be developed as a
first step. Such a methodology would then permit the development of an agreed strategy
for band sharing. This strategy would take into consideration the characteristics of
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