FINAL REPORT - Stakeholders - Ofcom

3.3.4.2 Summary of operational requirements on spectrum
Even though there is some discussion over which technology will be implemented in-band
in the future, it is clear the frequency will continue to be congested for the foreseeable
future. Within the UK, NATS and the CAA do not foresee the decommissioning of SSR.
As the load on the frequency is in part proportional to the traffic density in the coverage
area, this will have a bearing on the frequency requirements. Traffic is expected to
increase steadily over the next 10-15 years, with a doubling of the current European
traffic levels predicted by 2015.
There may be a requirement for ground-based surveillance accuracies to be increased in
the future. In particular:
• There is a need for more consistent surveillance accuracy (currently, the impact
of a fixed azimuth accuracy leads to increased errors at long range);
• There is a potential need for increased accuracy should there be a need to
decrease separation minima.
These requirements may lead to further developments and implementation of SSR.
However, it is also possible that these requirements could be met by ADS systems based
on the superior accuracy available from GNSS. The downlinking of aircraft parameters
(via 1090ES or Mode S Enhanced Surveillance) may also play a vital role in this.
As such, the operational requirements on the spectrum will increase in the next few years,
mainly due to the increase in SSR replies.
The UK SSR environment is managed by the National IFF/SSR Committee which reports
to the UK Spectrum Strategy Committee. Modelling and interference investigations are
used to monitor interference. The IFF/SSR Committee reports to the UK Spectrum
Strategy Committee. According to the CAA, the results of the modelling and interference
investigations support the view that increased use of Mode A/C will adversely affect the
integrity of the SSR frequencies. This view is supported by work conducted in Germany
and other States. Due to the proliferation of SSR systems for ATC purposes and the
increasing use of ACAS systems, the SSR frequencies are in danger of becoming
saturated. This causes an increase in interference, which in turn is leading to a
consequent decrease in the probability of replies received. Through selective addressing,
Mode S reduces the number of replies being made and therefore reduces RF pollution.
Therefore, the drive within the UK is to implement Mode S as a means of ensuring that,
through the control of interference, future operational requirements can be met.
Furthermore, since the events of September 11 th , security issues have become more
important. In particular, the ongoing need for a secure surveillance system. In this
respect, SSR has an advantage over the alternative ADS-B systems, where surveillance
relies on self-location by the aircraft and, in some cases, air to ground communication
which may be more vulnerable than is the case for L band systems.
It is worth noting that the number of interrogators will not significantly change; the uplink
frequency of 1030MHz will therefore have scope for improvement (at present, it is
underused).
3.3.5 Regulatory and Standardisation Issues
3.3.5.1 In-Band Technical Regulation – ICAO SARPS – Annex 10 Vol IV,
Chapters 2, 3 and 4.
The ICAO SARPS are well developed for SSR, and include a range of regulatory features
designed to protect and enhance the frequency use. The following table presents a
Page 68