Common GPS Queries

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Is the GPS time signal the same as the GPS positioning signal?

Yes. The signals that are broadcast by GPS satellites contain time information and the position of the satellite it came from (and its velocity). The timing information is generated by an onboard caesium atomic clock. It is this information used by satellite navigation devices (sat navs) that enables global positioning. Sat Navs use these signals from multiple satellites to triangulate a position.

How accurate is GPS positioning?

Because the time signal generated by GPS comes from an atomic clock it is accurate to within 16 nanoseconds (16 billionths of a second). As light travels nearly 186 000 miles in a second this equates to around 16 feet (5+metres) which means a GPS positioning system is usually accurate to this much.

Is GPS time the same as UTC?

No. GPS time, like UTC (Coordinated Universal Time)is based on International Atomic Time (TAI) – the time told by atomic clocks. However as the GPS system was developed several decades ago it is now 14 seconds (and soon to be 15) behind UTC because it has missed out on the Leap Seconds added to UTC to calibrate for the Earth’s slowing rotation.

How can I use GPS as a source of UTC then?

Fortunately a GPS time server will convert GPS to the current UTC time, which as od 1 January 2009 will mean it has to add exactly 15 seconds.


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Richard N Williams is a technical author and a specialist in the NTP Server and Time Synchronisation industry. Richard N Williams on Google+