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system, which is usually co-mounted. They are normally integrated with a network of
similar installations to provide national or international coverage. The data is usually
provided to centralised air traffic control centres although in some cases the radar
information is supplied to a local airport for Terminal and Approach use. En route radars
are normally L band and provide good performance in adverse weather conditions.
En-route radars normally provide range coverage in excess of 80nm and more typically
160nm. Minimum range requirements are typically 1nm. Low coverage is usually more
generalised, for example, 5000ft throughout the coverage volume. High level coverage
requirements are typically 50000 ft. A rotation rate of 8 to 10 RPM is typical for a long
range radar providing services to aircraft flying in the en route phase of flight where a 5nm
separation standard is normal.
En route primary radars are required to detect a wide range of aircraft types, some of
which present a very small radar cross section. This means that a fairly stringent target
size (=RCS) criteria has to be applied (1 – 2 sq metres).
There has been significant debate regarding the need for en route primary radar for civil
ATC but there is reluctance to dispense with the only non co-operative service in a safety
of life environment. In addition, given the requirements of Military ATC, the basic
operational requirement for en route primary radar is likely to continue for the foreseeable
future. This view is supported by NATS current replacement plan for en route radar which
includes primary radar facilities. Finally, the need for primary radar has undoubtedly been
reinforced by the increased requirement to track non cooperative targets which may pose
a security threat. This view was confirmed by both CAA and NATS. The requirement for
primary radar is also covered in the section on Regulatory and Standardisation issues.
The CAA noted the close relationship with the military in terms of frequency allocations for
en route radar services. However, the CAA is not party to the overall military requirements
for frequencies in the L and S bands.
3.2.3.6 Coverage
The general policy adopted by CAA and NATS is to provide coverage from two
independent radar sources. This is required to satisfy the overall availability requirements
noting that a given service can be interrupted as a result of equipment failure or planned
maintenance. For example in the case of en route radar, normal practice is to provide
overlapping cover at 5000 ft. Note that this results in an apparent over provision of
services because several radars may provide coverage at higher levels.
Coverage is usually referenced to a probability of detection. The quoted probabilities of
detection are typically 90%. This figure appears low; however this is the level achieved at
the edges of cover and is really a performance yardstick. Within the normal operational
service areas much higher figures are expected.
3.2.3.7 False Alarms
Maintaining the required level of false alarms can be technically challenging and it is
possible that the operational requirement for reduced false alarm performance will
become even more exacting. This situation is one of the reasons why air traffic service
providers are particularly sensitive to band sharing where an increased false alarm rate is
a possible risk.
3.2.3.8 Sector Blanking
The use of sector blanking as a means of limiting radar interference is technically possible
and is considered subject to the operational requirements for coverage. In practice, given
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