FINAL REPORT - Stakeholders - Ofcom

There is little cost difference between the various VDL modes; hence there is no airborne
operator marginal cost. However, the ground network will need to support two modes,
probably via multimode ground equipment located at the same ground station. It will be
assumed that ground station conversion costs are similar to those quoted above for
conversion to 8.33 kHz, namely £75,000. Assuming that there are 30 ground stations
providing network services in the UK, the total cost is therefore £2.25M. Assuming that
the network supports 4 channels and that a threefold increase in efficiency is obtained,
the marginal cost table becomes (Table 5-15):
Cost element Scenario A
ATC station upgrade + Network conversion N/A
Non-ATC station upgrade £2.25m
Aircraft station upgrade 0
Total cost £2.25m
Annualised total £240k
Cost per MHz £2.4m
Cost per 25 kHz £60k
Table 5- 15 Revised Marginal Cost Table
Although the cost per 25 kHz channel is relatively high, the revenues that could be
supported on the channels made available are also potentially high. Also the cost per 25
kHz channel decreases as the number of channels used for data increases (since the
same equipment can be used to support a large number of channels).
In order to encourage the adoption of more efficient VDL solutions, it may be necessary to
offer incentives or disincentives for aircraft to equip noting that there is little inherent
marginal cost differential between the airborne equipment.
Recommendation 5.3: It is proposed that new VDL services will occupy the spectrum
between 136 and 137 MHz (i.e. that spectrum newly assigned to aeronautical
communications). However this spectrum is already in use for analogue services which
will have to be found new frequencies below 136 MHz. One of the problems with the band
136 to 137 MHz revolves around industrial, scientific and medical (ISM) heating
machines. Such machines operate at frequencies around 13 and 27 MHz at very high
powers for the purposes of drying (biscuits and paint commonly). Whilst these devices are
not intended to radiate, the high powers involved mean that even small amounts of
harmonic distortion can produce signals at higher frequencies that can cause interference
to other services. Unfortunately the 10 th harmonic of the 13 MHz band and the 5 th
harmonic of the 27 MHz band fall into the aeronautical communications band. This
causes particular concerns for airborne receivers which have a very large radio line-ofsight.
It was common practise, prior to the opening of 136 – 137 MHz for aeronautical
communications that these devices, if found to be radiating excessively on harmonic
frequencies, were re-tuned such that the harmonics fell above 136 MHz and hence
outside the aeronautical bands. This now means, however, that there are a large number
of interfering carriers present on frequencies between 136 and 137 MHz. It will take
further work by the relevant national administrations to clear these bands further so that
they are suitable for the introduction of digital services (to which they cause greater
problems than for analogue services). A first stage in the movement to data is to use
136MHz+ for initial data services – Ofcom should work with the CAA to ensure that this
part of the spectrum is used efficiently and that sources of interference are removed as
soon as is practical.
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