WRC-2003 Agenda Item 1.28
"to permit the use of the band 108-117.975 MHz for the transmission of
radionavigation satellite differential correction signals by ICAO standard groundbased systems;"
Issue
A new aviation requirement has emerged for the transmission of augmentation data for
GNSS, to be used by aircraft receivers to satisfy the stringent accuracy and integrity
requirements for GNSS applications. ICAO plans to operate the new ground-based
augmentation systems (GBAS) in the present VOR/ILS band at 108-117.975 MHz
ICAO also wants this band to be used for flight surveillance system. The recommended ICAO
system (VHF Digital Link Mode 4, VDL Mode4) transmits from the aircraft its position and
other relevant data either automatically or in response to ground station requests (Automatic
Dependent Surveillance, ADS). This information is to be received by other aircraft and/or by
ground stations and is used to establish safe separation between aircraft.
The appropriate regulatory environment to facilitate this new use of the band needs to be
established, but in such a way that existing use by the standard aeronautical radionavigation
systems remains fully protected.
Background
Landing facilities at most major airports now employ the Instrument Landing System (ILS),
which is based on intersecting radio beams. The Category III ILS configuration can provide
sufficient accuracy for zero visibility landings provided that the high level of accuracy needed
is maintained through constant monitoring and regular calibration.
Satellite radionavigation systems are sufficiently accurate for navigating from place to pace
but not enough for zero-visibility landings. Their accuracy can however be brought up to the
required level in the vicinity of an airport by installing a Ground-Based Augmentation System
(GBAS). This will allow any landing area to be adapted for zero visibility operations without
the cost and complication of installing a full specification Category III ILS.
The band 108-117.975 MHz is currently allocated to the aeronautical radionavigation service.
Footnote 5.197 gives an additional allocation for secondary mobile service subject to
agreement under 9.21 and other restrictions in several countries (Japan, Pakistan, and Syria).
Because the new systems envisaged communicate data directly, and do not provide
navigational information from the direction or timing of the signals, they do not fall within the
definition for a radionavigation service (i.e. using the property of the propagation
characteristics of radio waves). The Radio Regulations therefore need to be amended to
permit the transmission of the supplementary information in the aeronautical radionavigation
band 108-117.975 MHz.
ICAO has adopted relevant Standards and Recommended Practices for all these new
systems, which have been incorporated these in Annex 10 to the Convention on International
Civil Aviation.
At the May 2001 meeting of WP-8B, draft CPM text was established on the basis of UK input
that referenced GBAS only. CPG also accepted that this was a straightforward regulatory
issue. Within CEPT, the issue was then assigned to CPG/PT1 on the basis that this should be
a purely regulatory issue.
Since then, ICAO and Eurocontrol, have been promoting other aeronautical applications via
WP-8B and CPG PT1. This would involve a considerable extension of the agenda item
because airborne transmissions are used in the additional systems envisaged. Moreover, the
problem of compatibility with the broadcasting service in the adjacent band below 108 MHz
could become a significant factor, requiring further technical studies.
Significant Developments
CPG has now decided to support the introduction of systems additional to GBAS. In
particular, CEPT now supports the introduction of systems introducing surveillance functions
in air traffic management (VHF Digital Link Mode 4, VDL Mode4) subject to demonstration
that compatibility can be achieved without placing any additional constraints on other
services. CEPT will take a final decision when the result of the compatibility study is available.
That CPG is also reluctant to impose any additional constraints on the broadcasting in the
adjacent band, 87.5 – 107.9 MHz could lead to a difficult point of decision. Interference to
receivers in the band 108-117.975 MHz is the result of a cumulative effect of all broadcasting
transmitters in sight. Because such interference is the result of interactions between an
ensemble of broadcasting emissions, the addition of any additional service or station in the
band 108-117.975 MHz will require assessment of additional incompatibilities which, in
principle, imposes further constraints on the broadcasting service. This will be the case even
if the interference criteria are no more stringent than before.
Because GBAS was designed to higher immunity standards than ILS/VOR systems and the
development of GBAS is expected to result in a rationalisation of existing VOR/ILS use, it is
not expected that GBAS will place any additional constraints to FM broadcast service in
practice. Also, GBAS installations will only be located at airports, which means that
compatibility and frequency management problems between the various aeronautical
applications themselves are kept to a minimum. However, the other systems envisaged by
ICAO introduce novel interference scenarios that are not covered by existing studies or ITU-R
Recommendations.
A multiplicity of airborne transmissions has the potential to degrade ILS/VOR operation. The
current aeronautical/broadcasting compatibility analysis assesses potential degradation to
ILS/VOR operation on the basis of a power sum of all radiated and in-receiver intermodulation products linked to broadcasting emissions. The power sum from in-band airborne
transmitters could be comparable to the extraneous effects from broadcasting transmitters
operating in the lower adjacent band. This may mean that the aeronautical systems will
become more susceptible to powerful broadcasting transmitters triggering improper operation
of ILS or VOR receivers, or that in-band interference will have to be considered as the more
important factor. Either way, the basis of the existing treatment of compatibility in
Recommendation ITU-R IS.1009 will be compromised.
Opinions in ITU-R have oscillated between concentrating all action at WRC-03 on GBAS,
leaving the wider possibilities till the next conference, or allowing all the new systems to be
introduced at WRC-03 before resolving all the compatibility issues involved.
Neither outcome was wholly satisfactory and WP-8B went on, at its May 2002 meeting, to
develop draft CPM text on three possible methods to meet the objectives of agenda item
1.28:
Method 1:
The adoption of a footnote that would enable the use of the band 108 –
117.975 MHz on a global basis for the transmission of radionavigation differential
correction signals by international aeronautical standard ground-based systems.
Since this would enable only the use of the GBAS system, as standardized by ICAO,
and would not include provisions enabling the introduction of VDL Mode 4 in this
band, the agenda for WRC [2006] should include an agenda item which addresses
expanded aeronautical surveillance application in this frequency band.
Method 2:
The adoption of a footnote that will permit the use of the band 108 – 117.975
MHz on a global basis by ICAO standard systems that support navigation and
surveillance functions through a data link, on the condition that priority and protection
be given to the aeronautical radionavigation service. This footnote would reference a
Resolution that lays down the minimum criteria for such systems operating in this
band in terms of compatibility requirements with, and the protection of, the
broadcasting service.
Method 3:
The adoption of a footnote that will permit the use of the band 108 – 117.975
MHz on a global basis by ICAO standard systems that support navigation and
surveillance functions through a data link, on the condition that priority and protection
be given to the aeronautical radionavigation service.
Method 2 was introduced, with UK support, as a third method (chronologically) with the
intention to provide a compromise option that would avoid revisiting the issue again at the
next WRC.
Nevertheless, there was no great move to a common view within WP-8B, and it was evident
that the aeronautical and radiocommunications administrations from several countries did not
agree with each other. This as partly due to a realisation that even if the protection criteria
need for the new aviation systems are no more stringent than the current radionavigation
systems, additional constraints would still be placed on the deployment and development of
the broadcasting service in the adjacent band.
UK Considerations
Having promoted the agenda item as originally intended for the purpose of GBAS only, the
UK now finds itself lobbying for the compromise option – Method 2 – in order to satisfy all
views while ensuring that compatibility issues with the broadcasting service are addressed
correctly.
Action for WRC-03
Promote the compromise route of Method 2 and support the additional compatibility studies
required.