GPS Timestamp Considerations for PXI and PXI Express


Logging data for extended periods of time can result in inaccurate timestamp information due to clock drift. The NI PXI-6682 and Trimble® Thunderbolt® can provide accurate, GPS-based timestamping information that ensures correct correlation of data to time.


In this article we will cover how to correlate acquired data with a GPS time reference. The following documents will also assist the reader in gaining a greater understanding of GPS time synchronization:

GPS Time Fundamentals

Global Positioning System, GPS, consists of 24 satellites revolving around the earth every 12 hours. Each of these satellites has an atomic clock onboard with an accuracy of 10-13 seconds. The GPS satellites continuously transmit their coordinates in space along with a time message on a 1.575 GHz carrier frequency.

Trigger and clock signals can be derived from this atomic clock using special GPS receivers such as the PXI-6682 and Trimble® Thunderbolt®. Exact time references can also be marked on a receiver from various trigger sources.


Marking Initial Time Reference

In order to correlate data with time we need an initial time reference, or t0 (pronounced Tee-not), to mark the beginning of the acquisition. Once absolute initial time is marked,  the absolute time of any acquired sample can be derived from absolute initial time (t0), and the number of sample clock periods from t0 to that sample.  The accuracy of this measurement method is limited to the stability of the sample clock timebase. We will discuss maximizing this accuracy in greater detail in the next section.

There are three methods for using a PXI-6682 to mark a GPS timestamp for a PXI data acquisition device. First, the PXI-6682 can generate a start trigger and send it to the data acquisition device on the PXI backplane. This introduces a 2-3 nanosecond propagation delay per slot and 20-30 nanosecond delay per PCI-PCI bridge the trigger crosses on the chassis. The PXI STAR line can be used by the PXI-6682 if multiple data acquisition devices are present in the system to avoid slot to slot propagation delay.

Figure 1: Marking initial timestamp and exporting start trigger signal to PXI data acquisition device.

It is also possible to use an external trigger to have the PXI-6682 mark a GPS timestamp and simultaneously trigger a PXI data acquisition device to start acquiring data. The connections should be made using equal-length cables to avoid any propagation delay.

Figure 2. Marking initial timestamp from external trigger which also serves as start trigger for PXI data acquisition device.

Alternately, the PXI data acquisition device can export a trigger signal to the PXI-6682 when it begins an acquisition. This introduces a 2-3 nanosecond propagation delay per slot and 20-30 nanosecond delay per PCI-PCI bridge the trigger crosses on the chassis, similar to the first scenario.

Figure 3. Marking initial timestamp from exported start trigger sent by PXI data acquisition device.


Reducing Clock Drift

After having established an initial time reference, data points can be referenced using the time delta, or dt (pronounced Dee-Tee) of the sample clock. However if this clock is not aligned with a GPS time reference the accuracy of the timestamp data will degrade over time as the clock skews. 

Suppose the master clock timebase of a PXI data acquisition device has an stability of 25 ppm. After one hour the clock drift will be the following:

0.000025 drift error * 3600 seconds per hour =  .090 drift error seconds per hour

So in the worst scenario our timestamps will be inaccurate by a factor of 90 ms per hour. By continuously phase-aligning the sample clock of the data acquisition device with a GPS-derived clock we ensure the accuracy of the timestamps over long acquisition periods.

There are a two methods to phase-align the sample clock to the GPS-derived clock. In a PXI or PXI Express system, you have the option of using the NI PXI Multi-Device Clock Disciplining Bundle or the NI PXI Express Multi-Device Clock Disciplining Bundle. Additionally, a PXI system has the option of using the Trimble® Thunderbolt®.

The first option for a PXI system is to use the NI PXI Multi-Device Clock Disciplining Bundle. The Clock Disciplining Bundle utilizes the PXI-6682H and PXI-6653 timing and synchronization modules to discipline the 10 MHz backplane reference clock to the 10 MHz clock on the PXI-6653. The NI PXI-6653 offers a controllable 10 MHz high-stability oscillator, and the NI PXI-6682H monitors the timebase. The PXI-6653 10 MHz clock is kept phase-aligned to the GPS-derived 10 MHz reference clock on the PXI-6682H using the Clock Disciplining Bundle software. 

For a PXI Express system, NI offers the NI PXI Express Multi-Device Clock Disciplining Bundle. This bundle works in the same manner as the PXI Multidevice Clock Disciplining except that the PXI-6653 is replaced with the PXIe-6674t. The backplane reference clock of the PXI Express system is replaced with the high-stability oscillator of the PXIe-6674t. The oscillator of the PXIe-6674t is phase-aligned to the GPS-derived clock from the PXI-6682H. 

By disciplining the 10 MHz clock in multiple systems using the NI Multi-Device Clock Disciplining Bundle, you can align each system to remove drift and keep phase aligned to less than 6 ns standard deviation between systems.

The other option for a PXI system utilizes the Trimble® Thunderbolt® connected to the backplane of the PXI system, replacing the 10 MHz reference clock. This clock allows the sample clocks of the devices in the system to phase align themselves to a GPS time reference assuming they have the capability. Since each rising edge of the 10 MHz clock from the Thunderbolt is within 15 ns of the GPS atomic clock, we eliminate the errors due to drifting and significantly improve the accuracy of our data relative to GPS time.

Figure 4. Using a 10 MHz GPS-derived reference clock and GPS-based initial time ensures accurate data to time correlation.

The accuracy of the timestamps is now a function of the accuracy the hardware and the GPS satellites. Though the atomic clock on the GPS satellites is accurate to 10-13 seconds, the accuracy of the time signals they send for civilian use is only guaranteed to be within ±170 ns of UTC. As the propagation delays in the PXI chassis are within this limit the time accuracy of the acquired data should be within ±170 ns of UTC.

Real-world testing of multi-chassis systems using the synchronization techniques described above achieved results of synchronization between chassis to approximately ; where 0 ns would be perfect synchronization. 


Configuring The PXI System

Now that we have discussed why GPS-based time synchronization is needed for accurate timestamping, let us now configure our PXI or PXI Express system.

If you are using the Trimble® Thunderbolt® in a PXI system, the PXI-6682 should be placed in slot 2 of the PXI chassis in order to drive the 10 MHz reference clock and access the PXI STAR line if necessary. The 10 MHz Out terminal from the Thunderbolt® should be connected to the Clk In terminal on the PXI-6682. A GPS antenna should be connected to both the PXI-6682 and the Thunderbolt®.

Using the Clock Disciplining Bundle, the PXI-6653 in a PXI system or the PXIe-6674t in a PXI Express system should be placed in the system timing slot in the chassis. The PXI-6682H should be placed in the peripheral slot directly right of the system timing slot. In a PXIe-1082 chassis, the PXIe-6674t is placed in slot 4 and the PXI-6682H is placed in slot 5. A GPS antenna should be connected to the PXI-6682H. Please refer to the Multi-Device PXI_Clk10 Disciplining Software User Guide for more detailed instructions.

The recommended antenna for the PXI-6682 is the Trimble Bullet™ III. If a splitter is needed it is possible to use the ALDCBS1X2 from GPS Networking to avoid using a second antenna.

For information on how to configure and program the PXI-6682 to mark a timestamp from a trigger and pass along the 10 MHz reference clock see the NI-Sync User Manual. Once you have installed NI-Sync you can also use the examples located at <root>:\Documents and Settings\All Users\Start Menu\Programs\National Instruments\NI-Sync\Examples for application development in LabVIEW, CVI and C++.


Trimble® and Thunderbolt® are registered trademarks of Trimble Navigation Limited.

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