FINAL REPORT - Stakeholders - Ofcom

5.8.2.2 SDLS
SDLS is a proposed new satellite system designed to provide safety-related aeronautical
communications. The system is being developed by ESA 23 . The idea is to use existing
satellite and communications infrastructure as far as possible. SDLS represents a
potential NGSS.
It should also be noted that some of the characteristics designed here are design goals
and may be different in a final system.
The concept behind the design of the Satellite Data Link System (SDLS) is to provide
improved satellite communications and surveillance services to offer:
• Circuit Mode Voice for AOC.
• ATS Specific Voice Service – replication of partyline and quasi-instant access
by dedication of forward and return channels to each ATC sector.
• All tuned receivers will constantly monitor traffic on the channel.
• The partyline capability can be emulated by re-broadcasting the voice
transmissions from aircraft via the forward channel.
• Point-to-Point Data – ATN Compliant sub-network.
• Polling Service - This ‘Polling Service’ allows the transfer between aircraft and
ground of repetitive data and is similar to Automatic Downlinking of Aircraft
Parameters (ADAP) services. It is proposed that the Polling Service be used to
send:
• Basic information - Position (latitude, longitude, altitude); Time; Figure of Merit;
Flight ID.
• Extended information - Ground Vector; Weather information; Projected profile;
Short term intent; Intermediate intent.
The SDLS system could use any of the satellite bands, but the L-band seems the most
probable option. Currently there is no exclusive allocation, but priority access for
AMS(R)S to a 10 MHz sub-band of the MSS band is granted through a footnote in the
Radio Regulations. This could require the use of priority and pre-emption rules against
other MSS uses of the band.
Alternatively, dedicated usage of a sub-band could be decided. These issues are under
consideration in ITU fora. By developing a system dedicated to AMS(R)S, the case for
dedicated spectrum is likely to be easier to make and defend. The amount of spectrum
needed for AMS(R)S is also under consideration, and is dependent on the range of
applications to be supported, the final design of the system and the frequency re-use
achievable.
The SDLS system is still being defined. However it is proposed that SDLS would use
QPSK modulation. The data rate would be 6.8 kbits/s per channel, and the vocoder rate
for voice services would be 4.8 kbits/s per channel. These baseband channels would then
be mapped onto a 1 MHz Code Division Multiple Access (CDMA) channel using a
spreading code.
23 EUROCONTROL have started the process of standardising an NGSS called NexSat
which re-uses some elements of SDLS.
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