Use the following techniques to troubleshoot errors you might encounter while using GPS as an external time reference:

  • Ensure that the module has a GPS receiver and a properly connected GPS antenna.
  • Ensure that the GPS self survey is completed using the Percent Complete of Self Survey property. GPS is most accurate after the self survey.
  • Ensure that the GPS receiver is not in mobile mode if your antenna is in a fixed position. Devices in mobile mode are not as accurate as stationary devices. You can return the status of mobile mode or activate mobile mode using the Mobile Mode property.
  • Ensure that your GPS receiver is in mobile mode if you are trying to retrieve velocity and location information. Velocity and position information are only available while your device is in mobile mode.
  • Ensure that there are at least four satellites visible from your GPS antenna. The more satellites that are visible, the more accurate GPS is. GPS does not function if less than four satellites are visible. You can confirm the number of visible satellites using the Satellites Available property.
  • Ensure that you are within the maximum cable length for accurate GPS performance. Maximum cable length depends on the GPS antenna gain and the cable's loss per unit of distance. NI recommends a GPS signal strength of between -135 dBm and -120 dBm at the device's SMB input. Targeting a signal strength of -125 dBm at the SMB input, you can compute the maximum cable length as:

    Max_cable_loss = -130 dBm + antenna_gain - (-125 dBm)

    Max_cable_length = Max_cable_loss / (loss_per_unit_of_distance)

    If you are not within the maximum cable length, the self-survey may not complete due to poor signal integrity.