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Airlines estimated that it would save US$238,000 per month, per aircraft, flying on North Pacific
routes by flying user preferred routes utilising ADS and GNSS. Therefore, to increase capacity in
these types of operating environments, the limitations of the line of sight restrictions of radar and
VHF voice communications must be overcome to provide a more flexible and efficient ATC
service.
The FANS committee sought to use a number of emergent technologies that have reached a
mature stage in their development to address these problems.
ADS: - The Concept
The most notable enabling technology is the use of satellites, in two forms –communications (or
SATCOM) and satellite navigation (or SATNAV). It is important to distinguish between the two.
Other technologies that will be employed will be the use of digital data communications and
networks, on a global scale - the ATN or Aeronautical Telecommunication Network. This uses
protocols and an architecture that is able to meld data in an operable way through several levels
or 'layers'. Airlines are already well advanced in utilising digital data communications datalink
such as AIRCOM, ACARS, and AVPAC, based upon ARINC 622 and 722 protocols.
" ADS is a service for use by air traffic services in which aircraft automatically provide, via a
datalink, data derived from on board navigation and position fixing systems." One way of
considering ADS is to think of it in terms of a communications system. The aircraft
communicates its position, derived from on board systems, to ATC automatically. That is all that
ADS is. It is automatic because position reports and other routine messages are transmitted
automatically by equipment on board the aircraft. The rates at which these are sent is
established prior to entry into ADS airspace, by forming a contract between the ATSU and the
aircraft. If the ATSU wants a report at a different interval then this can be obtained. It is
dependent because the position of an aircraft that is presented to ATC is derived solely from
aircraft on board systems. There is no independent determination or corroboration of the
aircraft's position, such as radar provides, carried out by ATC.
10 IFATCA technical manual Section 8 / Chapter 1
Document prepared by EVP Europe, August 1999 Page 13 of 14
To turn ADS into an operable system it requires a lot more than just the communications links. It
needs a system: or ADS-ATC system components. An ADS-ATC system is purely an ATC
system that uses ADS, wholly or in part as its surveillance medium.
It is important to understand that the way that FANS and the CNS/ATM has evolved is such as to
give a 'menu' of tools with which to mix and match systems to suit the operational requirements
of particular ATS providers. However, given the flexible approach that the FANS committee
developed, there is still a basic level of system components that will be required for operational
use. Therefore the composition of an ADS-ATC system might take the form depicted and
described in Section F.
D. IFATCA policy11
When any failure to the GNSS occurs, it is to be expected that some aircraft will fail to
meet the RNP (Required Navigation Performance) and require special action to be taken, while
others will be able to continue to meet the RNP. It is important that controllers and pilots are
made aware of any degradation in a timely manner.
IFATCA policy is:
"A monitoring and interpretation service should be established to monitor the status of all
elements of the GNSS and interpret this information in a manner that provides relevant
information to pilots and ATC. The information disseminated from the monitoring service or
displayed at controllers' positions must be expressed in operational terms. ATC procedures
must be established for the use of GNSS and must cover the failure or degradation of the
system. When ATC is informed of a change in the status of the GNSS by the monitoring service
or by display equipment, specific procedures associated with that change must be implemented.
Should it not be possible to achieve the RNP in an airspace, an alternative RNP should be
declared."
IFATCA requires that the safety, integrity, and reliability of GNSS be guaranteed before GNSS
achieves a “sole means” status of navigation. One important benefit of CNS is the declaration
that GNSS becomes the “sole means” of navigation and communication, which will make the
terrestrial navaids obsolete. Therefore, many of the existing ground aids could be removed
eradicating the major cost of their maintenance and replacement.
E. References
Galileo Global Satellite Navigation Services for Europe – Meeting of the Council of EU
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