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Why Is My PXI-6682 Timestamp Off When Using GPS Protocol?

Hardware: PXI/CompactPCI>>Controllers

Problem:
When I use the PXI-6682 module with a GPS (Global Positioning System) time reference the time is off by several seconds when compared to a secondary GPS device. Why is this occurring?

Solution:
GPS Time is not corrected to match the rotation of the Earth and currently there is a 19 second offset with International Atomic Time (TAI). GPS time does not take into account leap seconds or other factors which are periodically added to Coordinated Universal Time (UTC). The atomic clocks on satellites are set to GPS time, which is where this offset can be seen. Some specifications of the two timescales are shown below.

International Atomic Time (TAI) Scale:

  • Constant Timescale
  • Employed by network standards like IEEE 1588

Global Positioning System (GPS) Timescale:

  • Constant Timescale
  • The GPS epoch was set from UTC in 1980
  • 19 seconds off TAI time


When using a GPS time reference the NI-Sync driver returns the current time using the TAI timescale. This allows PXI-6682 to be used with IEEE 1588, IRIG-B, and GPS time protocols. Using the TAI timescale gives the PXI-6682 the ability to drive a IEEE-1588 network with GPS time and  it will be the correct timescale.  Because of the multipurpose nature of the PXI-6682 it will not automatically correct the PXI-6682 time reference to match the GPS time. 

If the TAI time scale does not work for your application the time source can be manually adjusted by using the NI-Sync property node and selecting "Clock Adjustment Offset".  Reference the following screenshot. 


Selecting this property will allow you to offset the time source by a fixed amount from the time reference being used.  This property is in nanoseconds and will allow you to offset the time by up to 2.3 seconds using NI-Sync 3.0.  To use a larger offset, use LabVIEW to add or subtract the time manually.

To adjust the TAI timescale to UTC time, you can get the current GPS to UTC offset using the UTC Offset Property Node. 



Related Links:
Products & Services: NI PXI-6682
Developer Zone Tutorial: GPS Synchronization Architecture for DSA Devices

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Report Date: 12/28/2007
Last Updated: 04/16/2009
Document ID: 4GRC2OQX

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