# 10.- Reliability

Reliability describes the ability of a system or component to function under stated conditions for a specified period of time. Reliability refers to the consistency of the results provided by a system; internal and external reliability are, respectively, the ability to detect gross errors and the effect of an undetected blunder on the solution.

Reliability is about the overall consistency of a measure. It is a concept that encompasses service continuity, and thus it is related to satellite availability, to those indicators described at availability, accuracy and repeatability, and to integrity. The latter requires the definition, for each measurement of interest, of:

• an alert limit, defined as the error tolerance not to be exceeded without issuing an alert,
• a time to alert, defined as the maximum allowable time elapsed from the onset of the navigation system being out of tolerance until the equipment enunciates the alert,
• the corresponding integrity risk, defined as the probability that, at any moment, the position error exceeds the alert limit, and
• a protection level, defined as the statistical bound error computed so as to guarantee that the probability of the absolute position error exceeding said number is smaller than or equal to the target integrity risk.

## Indicators of Reliability

It follows a list of possible reliability indicators for a software-defined GNSS receiver:

• Percentage of false and missed alerts.
• Availability of receiver autonomous integrity monitoring (RAIM) mechanisms:
• Fault detection (requires $\geq 5$ in-view satellites).
• Fault detection and exclusion (requires $\geq 6$ in-view satellites).
• RAIM prediction tools.
• Horizontal / Vertical Protection Limits (HPL / VPL): radius of circles which are centered on the GNSS position solution and are guaranteed to contain the true position of the receiver to within the specifications of the RAIM scheme (i.e., which meets specified false alarm and missed detection probabilities).

• Availability of mechanisms providing robustness against RF interferences and multipath:
• Out-of-band rejection of RF interferences (see ETSI EN 303 413 Standard).
• In-band rejection techniques for continuous wave, pulsed, and wideband interferences.
• Countermeasures against spoofing, meaconing, and fake assisted and differential data.
• Spatial diversity: Fixed / Controlled Reception Pattern Antennas1.
• Deployment of network security and data integrity mechanisms.
• Availability of GNSS signal authentication mechanisms.
• Probability of failure.
• Time to authentication.
• Safety-critical software certifications (e.g., DO–178B).

## References

1. C. Fernández-Prades, J. Arribas and P. Closas, Robust GNSS Receivers by Array Signal Processing: Theory and Implementation, Proceedings of the IEEE, Vol. 104, No. 6, pp. 1207 - 1220, June 2016.

Updated: