(MTBUR) of transponder – range is 1370 to
S transponders – MTBUR is approximately
3700 hours (as determined by one State)
3000 hours.
4 Space
4 Space
In the context of ADS-B, a failure is an unexpected loss of positional data.
No equivalent
To date, ADS-B ATM systems are exclusively reliant on GNSS systems,
specifically GPS, for aircraft positional data.
The US Government has published a “Global Positioning System Standard Positioning
Service Performance Standard October 2001.
The “availability” standard in this document is the only documentation giving a hint of
unexpected failure rates. This standard
provides a “Service Availability Standards” for an average location on the earth as
>=99%. This standard defines the expected
time that the GPS service can deliver a 36 metre signal in space solution. It includes
both planned and unplanned outages. It also
includes the predictable effect of
constellation geometry. If the predictable failures are excluded, the service availability
can be expected to be significantly higher
than 99%. Given the difficulties in providing logistic support in space, exceptionally high
satellite reliability is required to achieve this
service availability..
The US Government also says :
In support of the service availability standard, 24 operational satellites must be
available on orbit with 0.95 probability
4 Ground station
MTBF calculated by one State as between 25 and 30 years based on MIL Standards using figures provided by the manufacturers. In practical terms these figures are not achievable. Non random failures such as lighting and human error swamp these figures.
(averaged over any day). At least 21 satellites in the 24 nominal plane/slot positions must be set healthy and transmitting a navigation signal with 0.98 probability (yearly averaged)
This seems to state that the chance of having 21 satellites radiating is only 98%. This seems grossly underestimated. Field experience in the last 10 years has recorded no event whereby a satellite has unexpectedly failed leaving less than 24 satellites (refer analysis at para Error! Reference source not found. Error! Reference source not found. which shows an analysis of actual GPS system performance derived from GPS NANU data over the period 1996-2002 ).
It is concluded that no announced design
system reliability figure is available which
can be compared to radar.
4 Ground station + Space
MTBF calculated by one State as approximately 30 years based on MIL Standards using figures provided by the manufacturers. The space component MTBF figure is based on Significant Outage (individual satellite) MTBF of 124 Months [with MTTR of 1 month], and Short term failure (individual satellite) MTBF of 7300 Hours [with MTTR 36 hours]. Also a constellation MTBF of 81 years based on 26 available satellites and 24 required.(J Durand & A Caseau, “GPS Availability, Part II:
Evaluation of state probabilities for 21 and 24 satellite constellations,” NAVIGATION: Journal of the ION, Vol 37, No. 3, Fall 1990”) and the “GRAS Service availability Study Final Report” prepared by ITT Industries/AES Communication Systems Integration Group).
Integrity. The integrity of data is the probability that the data, when not indicated to be in error, is in fact correct.
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