FINAL REPORT - Stakeholders - Ofcom

• Use of the band 1544-1545 MHz is limited to distress and safety operations,
including feeder links of satellites needed to relay the emissions of satellite
emergency position-indicating radio beacons to earth stations and narrow-band
(space-to-Earth) links from space stations to mobile stations.
• Use of the band 1 645.5-1 646.5 MHz is limited to distress and safety operations,
including transmissions from satellite EPIRBs and the relay of distress alerts
received by satellites in low polar Earth orbits to geostationary satellites.
5.3 Current technology description
5.3.1 DSB/AM (analogue) voice
VHF DSB/AM voice is available throughout the world in 25 kHz channels. Recently, 8.33
kHz channelisation has been introduced in Europe in high density areas, although not yet
in the UK. The transceiver power usually takes values between 40 and 60W.
In the UK, the “Climax system” uses offset carrier techniques to increase regional
coverage. The principals behind Climax are described below.
Obtaining the required wide area coverage given local line of sight constraints requires
the use of more than one ground transmitter/receiver site. If all of these stations operated
on the same frequency, interference would result in low frequency beat frequencies which
would not be a problem for aviation since they could be filtered out (a similar effect is
found in FM broadcasting but it has a low impact because of filtering). However, Doppler
effects will cause frequency shifting depending on the location of transmitters relative to
the aircraft path. This results in variable audio frequency beating which cannot be filtered
without detriment to the desired audio communication.
Climax operates by introducing significant transmission offsets between transmitter sites
which, by careful planning, produces acceptable performance. A key part of the system is
the narrower bandwidth for the transmission compared with the receiver performance –
hence the receiver can receive a wide range of offset transmissions (wide receiver
bandwidth) and interference between offset transmissions is minimised (narrow
transmission bandwidth).
The application of Climax to 8.33 kHz channel spacing systems is described in section
5.6.1.
The safety requirement for party-line communications (leading to situational awareness
for all airborne and ground-based users) leads to a wide geographic coverage for each
channel (and thus very poor spectrum efficiency).
5.3.2 HF voice
High frequency (HF) is extensively used in remote and oceanic areas, due to the ability of
HF waves (3-32MHz) to reflect off the ionosphere and thus work over significant ranges
(typically between 1000 and 3000 kilometres). As it is reliant on the characteristics of the
ionosphere (and propagating atmosphere), the integrity and reliability of the signal can
vary according to season, time of day, geographical location, and solar fluctuations.
Transmitter powers are usually in the range of 10-200W (peak envelope performance) –
and approximately 100W on average.
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