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GPS

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Last Modified: September 15, 2017

Describes the GPS timing protocol.

GPS is a system of 24 triangulation satellites funded and controlled by the U.S. Department of Defense. The global positioning satellites (GPS) continually transmit their coordinates in space along with a time message on a 1.5 GHz carrier frequency. A time-based timing and synchronization module that has a GPS receiver can use this global timebase to precisely correlate, trigger, and timestamp measurement data. Each global positioning satellite contains multiple atomic clocks, which are controlled and referenced by the Master Clock (MC) at the United States Naval Observatory (USNO), called UTC (USNO).

Synchronizing to GPS Time

NI time-based synchronization modules can use GPS technology as a time reference. If you set GPS as the time reference for a module, the module uses the time updates received by the onboard GPS receiver every second, derives from it the current TAI time, and sets this time as the current board time.

When you initially connect a GPS antenna to a time-based synchronization module or set GPS as the time reference, the onboard GPS receiver searches for visible satellites. After detecting at least four satellites, the GPS receiver performs a self survey, which is the process of performing measurements of the visible satellites once per second and averaging those measurements to determine the receiver's current position as accurately as possible. During a self survey, you can use GPS as a time reference, but it is less accurate than after the self survey is finished. Once the GPS unit completes the self survey, you can precisely apply the time data received from GPS satellites.

If you configure the time-based synchronization module for mobile mode, a self-survey does not apply. If the antenna is moving and mobile mode is not enabled, you may get unexpected and invalid timing results. However, using mobile mode degrades the accuracy of the onboard GPS receiver, so you should not use it unless the antenna is moving.

Factors Affecting GPS Synchronization Accuracy

You can obtain the best GPS timing results by having an ideally located, long-term, stable GPS antenna installation. Ideally, you should mount the GPS antenna in a location where it has an unobstructed, clear view of the entire sky. In this orientation, the GPS receiver can detect additional satellites and perform additional averaging while discarding the worst signals and alleviating the effects of multipath, which is when the GPS antenna receives a signal reflected off an object or surface instead of a signal directly from the satellite(s).

Completing a self survey will also improve accuracy by performing long-term averaging of location. It is best to ensure the antenna is in a fixed location throughout the self survey process and throughout use, because any small movement of the antenna reduces accuracy. For this reason, you should also attempt to minimize the movement of a GPS antenna caused by wind or vibration.

Antenna cable latency also adds constant error. For maximum accuracy, you can calculate the latency of the GPS antenna cable in use and apply a correction. You can use the Time Reference Correction property to remove the source of this error. For example, if the antenna cable in use has a published latency of 5 ns/m, and the antenna installation uses 30 m of cable, the total delay that the antenna installation causes is 150 ns, so you can set the Time Reference Correction property to 150 to correct this. While not necessary in all cases, you can improve the accuracy of GPS by accounting for all sources of delay in the GPS installation, including cabling lightning arresters, and amplifiers.

You can also use the niSync Property Node to query the number of visible satellites using the Satellites Available property and determine if any fatal GPS errors are present using the Status parameter. A minimum of four satellites should be visible for stable GPS operations, and GPS clock accuracy and stability increase as the number of visible satellites increase.


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