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The Structure of NMEA Sentences

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The Structure of NMEA Sentences

NMEA data is rather straightforward to interpret and sentences follow the same structure regardless of the GPS device's manufacturer.

$GPGSA,A,2,29,19,28,,,,,,,,,,23.4,12.1,20.0*0F

• A dollar sign
• Command word
• Data words
• An asterisk
• A checksum

Command Word

This word is the most important in the sentence because it indicates the structure of the remaining data to follow. Standard NMEA sentences begin with "GP," but some manufacturers will make their own command words. Typically, manufacturer-specific command words have an abbreviated form of the manufacturer's name. For example, one Garmin® sentence is "$PGRMZ".

Data Words

The number of data words in a sentence varies depending on the command word. When a GPS device does not have any information, such as during the period before a fix is obtained, words may be completely blank.

The Checksum

The checksum is a two-character hexadecimal value calculated by the device which is used to detect corrupt information. NMEA interpreters will typically calculate their own checksum.

If you need help with this page, please feel free to contact Jon Person at info@gpsdotnet.com.
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