FINAL REPORT - Stakeholders - Ofcom

Co-ordination of PRF (pulse repetition frequency) on a national basis is required for
overlapping coverage areas of SSR. Essentially, this means tailoring the pulse rate to
factors such as: the technology of the SSR (Mode S, MSSR, Mode S Enhanced
Surveillance); the coverage required; and the SSRs in the vicinity. Allied to plot plan
processing techniques, this should assist in reducing the number of invalid responses
being processed by the ground system.
3.3.6 Possible Improvements to existing technology
The development of monopulse SSR (MSSR) has led to a reduction in the FRUIT levels
throughout Europe. Its implementation highlights an issue for the development of the
technology – many monopulse radars only supplemented their ‘sliding window’ equivalent
when the existing radar was due to be replaced.
Partly for this reason, mandates have been imposed by several States (Belgium, France,
Germany, Luxembourg, the Netherlands and Switzerland) for Mode S Elementary
Surveillance. The UK has plans for the implementation of Mode S SSR ground stations
across its coverage area in the period up to 2010. An AIC has been circulated advising
users of a possible mandate for Mode S Enhanced Surveillance in the 2005+ timeframe
(along with France and Germany) and the CAA has initiated separate Regulatory Impact
Assessments (RIAs) 3 for both the proposed implementation of Mode S Enhanced
Surveillance for IFR GAT flights in designated TMA and En Route airspace from 31
March 2005 and for the implementation of Mode S elementary surveillance for flights
elsewhere from 31 March 2008. These RIAs are now at the second stage of consultation.
This gradual shift from MSSR Mode A/C to Mode S should mean a reduction in the RF
interference level. The use of selective addressing (and lockout protocols) mean that
interrogations happen only once per cycle with Mode S (decreasing the second replies
and FRUIT). Also, use of the ICAO 24-bit address, which can be correlated with the Flight
ID, means that each aircraft is uniquely identified, leading to less confusion and possible
extra transmission of SPIs (special position indicator).
However, Mode S Enhanced Surveillance, as is planned in France, Germany and the UK
by 2005, will include a greater amount of data extracted from the transponder in the form
of Downlinked Aircraft Parameters (DAPs). The development of ADS-B (1090ES) may
make this redundant (as the information could be broadcast from the aircraft), but the
level of in-band interference will still be affected.
In summary, the technology in the short-term will improve, leading to a more efficient use
of the available spectrum. However, as extra data is transmitted over the frequency and
the traffic density increases in the medium-term, in-band efficiency may decrease to a
level unsustainable for some ‘safety-of-life’ applications.
3 An RIA is a policy tool that assesses the impact, in terms of costs, benefits and risks of
any proposed regulation. It is intended to improve the quality of advice to Ministers and
encourages informed debate. Part of the RIA process is to conduct full public
consultation.
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