GPS Time and Leap Seconds

Time is an important component of any satellite navigation system, and it is essential that any receiver attached to the system has a clock that is fully up to date. The current GPS system uses its own timescale, which is closely linked to (but not completely in sync with) Co-ordinated Universal Time (or UTC). And to allow GPS receivers to give users the precise time according to UTC, the precise value of the current offset between the two clocks is broadcast by the satellite system.

While UTC is maintained centrally using super-precise atomic clocks, it does have to be adjusted occasionally to keep in sync with the earth's changing rotation and to reflect mean solar time. And just as our calendar is periodically adjusted by the addition of a day each leap year, UTC is periodically adjusted by the addition (or subtraction) of a leap second. These events will usually take place on the last day of June or December. But they are relatively rare, amounting to approximately 0.6 seconds per year.

These leap second events are virtually imperceptible as far as telling the time goes, but are broadcast when they occur to enable clocks around the globe to stay synchronized with UTC. But for satellite navigation systems they are essential information. And so the leap second event is broadcast to each receiver as part of the navigation data message.

Clearly, a receiver's response to the arrival of such a message is critical. And that is why Spirent's GNSS simulators offer facilities for testing the response of receivers to leap second events.

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