FINAL REPORT - Stakeholders - Ofcom

primary radar service can be withdrawn. Secondary radar is now the principal civil ATC
surveillance tool with primary radar in a back up and safety net role. The use of low cost
transponders is a constructive development to increase SSR transponder fit and hence
the total population of transponder equipped aircraft. Subsequent sections describe the
issues concerning SSR and the use of ADS in the long term.
The parallel study “Techniques for improving radar spectrum utilization” is reviewing more
radical alternative technologies for radar. These technologies are likely to require
extensive trials in the context of design and operational proving before adoption by
equipment suppliers and air traffic service providers. This view was expressed by both
NATS and CAA.
3.2.7 Allocation Sharing
3.2.7.1 Current Situation
Allocation sharing is already a fact of life. In L band, the allocated bandwidth is already
shared with the military, amateurs (including wide band TV repeaters), earth exploration
satellites, and GNSS. Meteorological wind profiling systems also are in this band although
they are not an interference problem. L band is under consideration for sharing with
Galileo and two military developments are under review.
In S band, the allocated bandwidth is split between aeronautical radionavigation service
(mainly 2.7 to 2.9 MHz) and the maritime radionavigation service (mainly 2.9 to 3.1 MHz).
The military make extensive use of both the lower and upper parts of the band. Other
sharing possibilities are aircraft telemetry, ENG/OB and UAV (Unmanned Airborne
Vehicle) control signals.
Also other interference sources such as wind farms are degrading radar performance
particularly in terms of false alarms. The civil authorities do not have full control of the key
bands. DAP co-ordinate with the military regarding frequency allocation but are generally
not in a position to analyse the full impact of other services in the band.
Allocation sharing in the aeronautical services is a relatively new requirement and the
following sections are intended to highlight the technical feasibility and consider future
approaches.
3.2.7.2 Pulse Repetition Frequency Discrimination
Most primary radar systems use a technique known as pulse repetition frequency
discrimination to reduce the impact of adjacent radar interference. Essentially these
systems are designed to reduce the impact of interference which is pulsed in nature and
has an interpulse period which is not significantly shorter than the victim radar. This
technique enables much better spectrum utilisation than would otherwise be the case.
Note that it does not provide immunity to interference which raises the noise level for
extended periods of time because of high duty cycle.
3.2.7.3 Detection Thresholds
It has been noted in the earlier section on Primary Radar Technology that radar
performance (in terms of probability of target detection and probability of false alarms) is
heavily dependent on the characteristics of the background noise. Typical primary radar
systems use a detection threshold mechanism (constant false alarm rate detector –
CFAR) in which the threshold is controlled by the average of received amplitudes in cells
surrounding the test (target detection) cell, as a means of controlling the false-alarm rate.
A typical method uses the outputs of several range cells, which are summed and
multiplied by a constant to establish the detection threshold. The spacing of the cells is
equal to the radar range resolution. This form of detection is susceptible to interfering
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