The NMEA Sentence

The NMEA sentence is a comma separated list of values which contains information on a single GPS reading. There are a number of sentence types, but the most common is a GPRMC sentence – the “Recommended Minimum” data that a sentence must contain.

Example

$GPRMC,084752.000,A,5152.4175,N,00832.5083,W,11.90,62.01,231110,,*12

Definition

Sentence Type, Satellite Date & Time, Signal Status, Latitude, Latitude Hemisphere, Longitude, Longitude Hemisphere, Speed in Knots, Bearing, Local Time,, Check Sum

Sentence Type ($GPRMC)
This indicates the type of NMEA sentence.

Satellite Date & Time (084752.000)
This indicates the date and time (UTC, HHMMSS.XXX) during a reading. As the reading is calculated using four satellites this date and time is usually quite accurate.

Satellite Status (A)
This indicates if a signal is active (A – active) or not (V – invalid).

Latitude (5152.4175)
This indicates the distance from the equator, north or south.

Latitude Hemisphere (N)
This indicates if the distance is North (N) or South (S) of the equator.

Longitude (00832.5083)
This indicates the distance from the Prime Meridian, east or west.

Longitude Hemisphere (W)
This indicates if the distance is West (W) or East (E) of the Prime Meridian.

Speed in Knots (11.90)
This indicates the speed that the device is currently travelling at.

Bearing (62.01)
This indicates the direction that the device is currently moving in, from 0 to 360, measured in “azimuth”.

0 – North
90 – East
180 – South
270 – West

Local Time (231110)
This indicates the local date on the device, in DDMMYY format.

Check Sum
The checksum is used to indicate any errors in the rest of the sentence.

Latitude readings

The Latitude component of the NMEA sentence is stored in the HHMM.M format (Hours, Minutes).

In order to convert the Latitude to the correct format, the minutes component needs to be converted to decimal format. Therefore, 52.4175 must be divided by 60, giving us 0.873625.

5152.4175 becomes 51.873625.

The Latitude Hemisphere indicates if this should be a positive or negative number – if the Hemisphere is North (N), then it is positive, if not, it becomes negative.
Longitude readings

Longitude readings

Similar to the Latitude component of the NMEA sentence, the Longitude component – 00832.5083,W – is in the HHHMM.M format (Hours, Minutes).

In order to convert the Longitude to the correct format, the minutes component needs to be converted to decimal format. Therefore, 32.5083 must be divided by 60, giving us 0.541805.

00832.5083 becomes 8.541805.

The Longitude Hemisphere indicates if this should be a positive or negative number – if the Hemisphere is West (W), then it is negative, if not, it becomes positive.

Last edited Jan 17, 2011 at 11:32 PM by DamienDennehy, version 2

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