NMEA message definitions
NMEA 0183 sentences carry time, position, navigation, and satellite-status data. The three-letter sentence type identifies the content; for example, RMC identifies a navigation message. The definitions below describe common sentence contents. Available fields and sentence versions depend on the device and its configuration.
| Type | Definition and data |
|---|---|
| ZDA | Time and date. UTC time, day, month, year, and local time-zone offset fields. Use this sentence when the receiving application needs date and time information. |
| GSA | Active satellites and dilution of precision. Selection mode, fix type (no fix, 2D, or 3D), identifiers of satellites used in the solution, and position, horizontal, and vertical dilution of precision (PDOP, HDOP, and VDOP). |
| GSV | Satellites in view. The visible satellite count and each satellite’s identifier, elevation, azimuth, and signal-to-noise ratio. Multiple numbered sentences can carry a complete satellite list. Satellites in view are not necessarily satellites used in the fix. |
| RMC | Recommended minimum navigation data. UTC time and date, validity status, latitude, longitude, speed over ground in knots, course over ground, and magnetic variation where provided. |
| GGA | Position fix data. UTC time of the fix, latitude, longitude, fix-quality indicator, number of satellites used, HDOP, altitude relative to mean sea level, and geoid separation. Differential correction age and station ID may also be present. |
| VTG | Course and speed over ground. Course relative to true north and, where provided, magnetic north; speed in knots and kilometers per hour. Course over ground describes direction of travel, not necessarily the direction the equipment is facing. |
| GLL | Geographic position. Latitude, longitude, UTC time of the position fix, and validity status. Some sentence versions also include a mode indicator. |
RMC output rate and telemetry updates
RMC can be output five times per second (5 Hz), while telemetry is updated only once per second (1 Hz). The higher message rate does not provide five independent telemetry updates per second. Receiving applications should distinguish the sentence transmission rate from the rate at which new telemetry becomes available.
Time messages and pulse alignment
An NMEA time message does not by itself provide a precise top-of-second marker. In a supported setup, a separate pulse input provides the timing edge used to align the received time message. See Two NMEA-referenced devices report time slightly out of sync for the existing GMR procedure.
Related articles
Article ID: KB-GPS-001
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