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Spirent KB Article
Doc ID: FAQ18452
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Positioning Application: Satellite Clock Issues
PosAPP - All Versions
The theoretical basis underpinning GNSS is the trilateration (similar to triangulation but with distances rather than angles) of satellite positions whose range is derived by measuring, as precisely as possible, their signal propagation times to the receiver. Thus, accurate timing and clocks are a fundamental component of GNSS; they are used in the calculation of the propagation time, or equivalently, the pseudo-ranges. Since the positioning solution performance fully relies on clock measurements, receiver clocks are aligned as closely as possible to the GNSS system time.
Whilst GPS was initially conceived to aid navigation, globally synchronized time is now used in various applications such as:
Electrical power grids
The finance sector
Computer network synchronization
Digital television and radio
Doppler radar weather reporting
Because of the importance of timing anything that impacts this can have an adverse impact on device performance. As an example, there is a recorded instance of a failure in a GPS satellite clock that caused some issues detailed below.
SVN23 Timing Anomaly
On January 1st, 2004, a GPS clock failure occurred suddenly onboard the GPS satellite PRN23 at around 18:30 UTC with the signal being transmitted for hours after that. The Unhealthy flag of its navigation message, used by the receivers to discard its measurements from the navigation position computation, was raised a few hours after the occurrence of this event. It was observed that the estimated user position was impacted in different manners depending on the different technology/firmware embedded in the receivers, and on the integrity monitoring information available to the receiver. Indeed, depending on the technology/ firmware used to track the signals and produce the raw measurements from the received GPS signal, the measurements and thus the position is affected differently.
This anomaly affected time and positioning over a wide area, illustrated below.
The anomaly was caused by a failure of the atomic clock on satellite PRN 23 and induced errors on the pseudo-range of this satellite.
This pseudo-range error induced latitude, longitude, altitude and potion errors on some receivers over the period of the anomaly
Spirent offers scenarios to allow users to test the effects of this and other events to ensure their receivers/systems are capable of handling various eventualities.
GPS satellite outages can be obtained from sites such as -
Spirent software offers the capability of replicating a range of satellite clock/outage issues for the purpose of receiver testing.
Contact Spirent Global Services for further information by raising an SR case or emailing firstname.lastname@example.org.
Note: Information for this article taken from the paper -
Effect of a GPS Anomaly on Different GNSS Receivers
Product : Positioning