GNSS (Global Navigation Satellite System)

Overview

GNSS is a general term which covers satellite systems used for navigation, like GPS (Global Positioning System). GNSS services are usually accessed through GNSS modems which receive and process GNSS signals to determine their position, or more specifically, their antennas position. They usually additionally provide a precise time synchronization mechanism, commonly named PPS (Pulse-Per-Second).

Subsystem support

The GNSS subsystem is based on the Modem modules. The GNSS subsystem covers everything from sending and receiving commands to and from the modem, to parsing, creating and processing NMEA0183 messages.

Adding support for additional NMEA0183 based GNSS modems requires little more than implementing power management and configuration for the specific GNSS modem.

Adding support for GNSS modems which use other protocols and/or buses than the usual NMEA0183 over UART is possible, but will require a bit more work from the driver developer.

Power management

GNSS receivers typically begin acquiring and tracking GNSS signals as soon as they are powered on, unless configured otherwise. In order to preserve power, an application can either remove power from the GNSS receiver or stop the internal GNSS engine from actively acquiring and tracking signals. The latter option halts position computation while keeping the receiver powered with reduced power consumption and the communication link to the host application available for other operations or for resuming the GNSS engine.

In the GNSS subsystem, this can be done using the gnss_stop() API call. This is distinct from Zephyr’s device power management (suspend/resume): the receiver remains powered and able to communicate with the host application, but GNSS tracking is stopped.

GNSS tracking can be resumed using gnss_start(). The application can specify a gnss_start_mode, which affects time-to-first-fix. A hot start preserves navigation data and allows the receiver to reacquire signals quickly, while warm and cold starts discard some or all navigation data, respectively, resulting in longer acquisition times.

GNSS device drivers must ensure the GNSS modem starts tracking when resumed from a suspended or powered-off state, without requiring the application to call gnss_start() explicitly. Most receivers will end up performing a cold start in this situation, simply because they lost their tracking data while unpowered, but a driver may instead attempt a warm or hot start if the modem retained enough state to do so faster. The requirement is that tracking resumes automatically on power up; clearing navigation data is only a side effect of the modem having been unpowered, not an intended part of resuming from suspend.

Configuration Options

Related configuration options:

GNSS API Reference

GNSS

Related code samples

  • GNSSConnect to a GNSS device to obtain time, navigation data, and satellite information.