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Figure 1: mean time between failures

time to failure
time between
time to
system failure
next system failure

Figure 1: mean time between failures


Mean Time Between Failures (MTBF) is the predicted elapsed time (usually in hours) between inherent failures of a system. It is the average time between inherent system failure during normal operation, in each case taking the time when a failure is reported. Between actual failure and the reporting of the failure is the time of alerting the maintenance personnel. (Mean Time To Acknowledge, MTTA).

The definition of MTBF depends on the definition of what is considered a failure. In the case of radar equipment, faults can also occur that do not lead to a system failure because redundant assemblies continue to ensure operation (so-called soft error management). Here, only those conditions that put the system out of service are considered (critical) failures. For example, the failure of one of perhaps 16 amplifier modules in the solid-state transmitter has only a few percent loss of range and repair can be postponed until the next scheduled maintenance. This is why the term Mean Time Between Critical Failures (MTBCF) is used. Mean time to acknowledge (MTTA) is the average time between an incident’s detection and the beginning of assistance or “acknowledgement” to resolve the issue.

Time to Repair (MTTR)

Mean Time To Repair (MTTR) is the average pure repair time until readiness for use is established, and if necessary spare parts are immediately available. A possible time until procuring a spare part does not count to the repair time! For this total time, there is the term Mean Time to Recovery (MTTR), the mean time to recovery of operational readiness.


The Built-In Test Equipment (BITE) is a test and verification hardware system, which is always built into complex systems such as radar sets. It is a powerful tool for troubleshooting and repair. The BITE continuously checks all signal paths for function and performance and immediately reports a possible fault. It can also sometimes make an independent decision to use a redundant system.

However: the BITE is also a vulnerable system. With a certain probability, the BITE itself can also be defective and either report no error or a wrong cause of the error. There is even the possibility of a false alarm.