4. GNSS receiver

4.1. In this chapter

This chapter provides information about the GNSS receiver. Topics include:

  1. Overview
  2. GNSS configuration
  3. Obtaining system time from the GNSS receiver

4.2. Overview

The GNSS receiver is used for streaming location information from the node (for example, for vehicle position information) or for querying GNSS information on the node.

4.3. GNSS configuration

GNSS services are enabled in the CLI under the cellular MDA (mda 1/1). Use the CLI for the following:

  1. enabling or disabling GNSS
  2. configuring the GNSS satellite constellation
  3. configuring NMEA parameters
  4. displaying GNSS location information and satellite information

4.3.1. Enabling or disabling GNSS

GNSS services are enabled using the config>card>mda>gnss no shutdown command. When GNSS services are enabled, the GNSS receiver begins acquiring GPS and/or GLONASS satellite signals and determines the position of the system. The GPS LED on the chassis blinks green during this process. The GPS LED is lit solid green when the GNSS receiver has determined the position of the node. The GPS LED is unlit when the GNSS receiver is disabled.

When NMEA services are also enabled, NMEA sentences are streamed according to the parameters associated with that service. See Configuring NMEA parameters for information.

GNSS services are disabled using the shutdown command. When GNSS services are disabled, the GNSS receiver is disabled and satellite information is reset. The GPS LED is unlit when the GNSS receiver is disabled.

GNSS services are disabled by default.

The GNSS receiver generates a logging event when it starts to acquire a position fix and when it has acquired a position fix.

4.3.2. Configuring the GNSS satellite constellation

The constellation of the GNSS receiver can be set to either GPS (gps) or GPS and GLONASS (gps-glonass). The constellation can be modified only when the GNSS service is shut down. The default constellation setting is gps.

4.3.3. Configuring NMEA parameters

The node can be configured to send position, velocity, and time information at regular intervals to servers that can process the data. When the data is formatted as an ASCII string according to National Marine Electronics Association (NMEA) standards, it is called an NMEA sentence. The node uses an IP transport service to send NMEA sentences to remote hosts. For information about enabling IP transport for NMEA sentences, refer to the 7705 SAR-Hm and SAR-Hmc Main Configuration Guide, “GNSS NMEA Data IP Transport Service”.

NMEA data streaming is enabled on the node when the IP transport ipt-id parameter is configured as gnss and the nmea no shutdown command is issued.

The following NMEA parameters must be configured on the node when streaming is enabled:

  1. sentence-type
  2. sentence-interval

The NMEA defines a number of sentence types for streaming. The node supports the following sentence types:

  1. GGA — this sentence is for time, position, and fix-related data for a GNSS receiver
  2. RMC — this sentence is for time, date, position, course, and speed data provided by the GNSS receiver
  3. VTG — this sentence is for vector track and speed relative to the ground
  4. GNS — this sentence is for time, position, and fix-related data for single or combined constellations for a GNSS receiver.

For information about the sentence types, refer to NMEA 0183, Standard For Interfacing Marine Electronic Devices.

The sentence interval specifies the frequency with which NMEA sentences are sent from the GNSS receiver. The interval can be set from 1 s to 3600 s. Different sentence types can be enabled concurrently so that multiple sentences can be streamed per sentence interval.

4.3.4. Displaying GNSS location and satellite information

The following GNSS location information can be displayed in the CLI:

  1. latitude of the last position fix
  2. longitude of the last position fix
  3. time at which the last position fix was taken
  4. altitude at which the last position fix was taken
  5. heading and speed of the system

If a GNSS fix is acquired and then subsequently lost by the node, the last known GNSS fix data continues to be displayed in the CLI. When a GNSS fix is reacquired, the GNSS data will then update to match the new fix.

The last GNSS fix data is not preserved after a power-cycle, admin reboot, clear mda 1/1, or shutdown command is issued for GNSS.

The following satellite information can be displayed in the CLI for up to 30 satellites:

  1. the satellite NMEA identifier—for GPS, the range is from 1 to 32; for GLONASS, the range is from 65 to 96
  2. the elevation of the satellite relative to the node, from 0° to 90°
  3. the azimuth relative to the node position, from 0° to 360°
  4. the signal-to-noise ratio (SNR), from 0 to 99 dB

4.4. Obtaining system time from the GNSS receiver

The GNSS receiver can obtain the system time when the config>card>mda>gnss>sync-system-time command is enabled. When the command is enabled and the GNSS receiver establishes a lock, system time is retrieved and set on the system.

Note:

If NTP or SNTP is configured when the sync-system-time command is enabled, there is no time source precedence and either process can update the system time at its own discretion. Do not enable NTP or SNTP when the sync-system-time command is enabled unless NTP (or SNTP) and the GNSS receiver are using the same time source.

The cellular interface and the GNSS receiver can be configured concurrently to obtain the system time. When the sync-system-time command is enabled concurrently on the cellular interface and on the GNSS receiver, the GNSS receiver takes priority when it establishes a lock.

When the sync-system-time command is enabled, the system time cannot be set manually.

The sync-system-time command works in conjunction with the following commands:

  1. config>system>time>dst-zone
  2. config>system>time>prefer-local-time
  3. config>system>time>zone

For information about the dst-zone, prefer-local-time, and zone commands, refer to the 7450 ESS, 7750 SR, 7950 XRS, and VSR Classic CLI Command Reference Guide.