service reliability. Reliability needs to be
considered in conjunction with the backup systems
and alternative methods of achieving the operational objectives.
on
In the radar environment, reliability is dependent
- aircraft avionics and supporting
systems
systems
- ground sensors and supporting
In the ADS-B environment, reliability is dependent on
(ie: GPS satellite availability)
- reliability of the GPS signals in space
- aircraft avionics and supporting
systems
- ground sensors and supporting
systems
Reliability engineering considers the mean time between failure (MTBF) of various system
elements to arrive at overall system reliability. The
US Department of Defence standards define the procedures and accepted MTBF values for system
components (resistors, capacitors etc).
Manufacturers usually provide MTBF values for system elements based on the DoD methodology.
These methods do not account for software
failures, nor do they account for system events
such as human error and for example lightning. In the past two decades, human error and
correlated events such as lightning have tended
to become dominant. Protection against
correlated events, software error and human error are normally provided through ensuring
minimal exposure to single points of failure.
However, comparison of systems by examining
the reliability (rates of random failure) of each
system remain a valid measure of performance between the systems.
When comparing radar and ADS-B environments,
reliability can be considered as a number of series
elements, namely
a) Space segment : The availability of
GPS signals (in the ADS-B case)
b) Avionics segment : The reliability of
avionics and aircraft power systems
c) The surveillance ground system : The radar ground station or the ADS-B
ground station
d) The link between the ATC system and
the ground sensor
e) The ATC display system
For the purposes of this assessment, the ground
system, links and display system will be calculated together.
4 Avionics
4 Avionics
Based on a mean time between unit removal
ADS-B Extended Squitter and modern Mode
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