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Examining NMEA Sentences

Many applications enable you to look at, examine, and even save NMEA sentences from your GPS because this is the closest, purest way to work with the GPS data. However, most people don’t give them much thought because the sentences themselves seem to complex. Little information is provided as to what they contain and how the data is structured.

This is a real shame — with a bit of information and a little practice, reading NMEA can become quite easy as you learn what to look for and where to look for it.

The following sections examine some of the common NMEA sentences that you will come across if you view NMEA data. There are the main sentences, the ones that relate to navigation and signal quality.

RMB (Recommended Minimum Navigation Information)

RMB is the “recommended minimum navigation” sentence, and it is sent whenever a route or a goto (such as going to a waypoint) is set as active. Some systems are set to transmit this all the time, transmitting null data if no goto is selected, while others only send it when required.

$GPRMB,A,x.x,a,c--c,d--d,llll.ll,e,yyyyy.yy,f,g.g,h.h,i.i,j*kk

yyyyy.yyDestination waypoint longitude

g.gRange to destination (measured in nautical miles, maximum 999.9)

h.hBearing to destination, True degrees

RMC (Recommended Minimum Specific GPS/TRANSIT Data)

This is the NMEA equivalent of PVT (Position, Velocity, Time) data.

$GPRMC,hhmmss.ss,A,llll.ll,e,yyyyy.yy,f,x.x,y.y,ddmmyy,z.z,a*hh

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yyyyy.yyDestination waypoint longitude

y.yTrack made good (measured in degrees True)

z.zMagnetic variation (easterly variation is subtracted from True course)

GGA (Global Positioning System Fix Data)

This sentence provides 3D location and accuracy data.

$GPGGA,hhmmss.ss,llll.ll,e,yyyyy.yy,f,a,bb,x.x,y.y,M,z.z,M,s.s,

####*hh

yyyyy.yyDestination waypoint longitude

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s.sTime in seconds since last update

####DGPS station ID

VTG (Actual Track Made Good and Speed Over Ground)

This sentence provides the velocity made good information.

$GPVTG,t,T,?,??,s.ss,N,S.SS,K*hh

?Not used

??Not used

s.ssSpeed over ground (measured in nautical miles)

S.SSSpeed over ground (measured in kilometers/hour)

RMA (Navigation Data from Present Position)

This sentence provides navigational data based on current position.

$GPRMA,A,llll.ll,e,yyyyy.yy,f,?,??,ss.s,ccc,zz.z,a*hh

yyyyy.yyDestination waypoint longitude

??Not used

s.ssSpeed over ground (measured in nautical miles)

z.zMagnetic variation (easterly variation is subtracted from True course)

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GSA (GPS DoP and Active Satellites)

This sentence provides detailed information on the satellite fix. It includes the numbers of the satellites being used in the current solution and the dilution of precision (DoP). DoP is an indication of the effect of satellite geometry on the accuracy of the fix. DoP has no units of measure — it is a case of smaller is better.

$GPGSA,A,B,x1,x2,x3,x4,x5,x6,x6,x8,x9,x10,x11,x12,x,y,z*hh

AFix mode:

M = Manual (where the receiver is forced to work in 2D or 3D) A = Automatic

BFix mode:

1 = Fix not possible

2 = 2D

3 = 3D

x1 – x12 IDs of satellites (SVs) used for position fix

yHorizontal dilution of precision

zVertical dilution of precision

*hh Checksum

GSV (Satellites in View Data)

This is a very interesting and informative NMEA sentence. Satellites in view shows data about the satellites that the unit might be able to find based on its viewing mask and almanac data. It also shows the unit’s current ability to track this data.

One GSV sentence can provide data for up to four satellites, so three sentences may be needed for the full information. The GSV for all satellite sentences do not need to appear in sequence.

$GPGSV,A,B,C,D1,E1,Az1,SNR1,D2,E2,Az2,SNR2,D3,E3,Az3,SNR3,D4,E4, Az4,SNR4*hh

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WPL (Waypoint Location)

When a route is active, this NMEA sentence is sent once for each waypoint that the route contains, in sequence. When all waypoints have been sent, GPR00 is sent in the next data set to indicate the end.

In any block of NMEA sentences, only one WPL or GPR00 sentence will be sent.

$GPWPL,llll.ll,e,yyyyy.yy,f,####*hh

1111.11 Destination waypoint latitude

yyyyy.yyDestination waypoint longitude

####Waypoint ID

ZDA (Date and Time)

This sentence provides date and time information.

$GPZDA,hhmmss.ss,dd,mm,yyyy,xx,yy*hh

xxLocal zone hours (-13 to 13)

yyLocal zone minutes (00 to 59)

*hh Checksum

ALM (GPS Almanac Data)

The GPS almanac data sentence contains GPS week number, satellite health, and complete almanac data for one satellite. Multiple satellites mean that multiple messages may be transmitted, one for each satellite in the GPS constellation (up to a maximum of 32 messages).

This sentence breaks the 80-character rule.

$GPALM,A,B,C.D,E,F,G,H,J,K,L,M,T1,T2,*hh

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ATotal number of messages

BCurrent message number

CSatellite PRN number (01 to 32)

DGPS week ID (0 to 1023)

ESatellite health

FOrbital eccentricity

GAlmanac reference time

HSigma – Inclination angle

IOmegaDOT – Rate of right ascension

JSquare root of semi-major axis

KOmega – Argument perigee

LLongitude of ascension node

MMean anomaly

BOD (Bearing: Origin to Destination)

This NMEA sentence shows the bearing angle of a line calculated at the origin waypoint and extending to the destination waypoint for the active navigation leg of the journey.

$GPBOD,xxx,T,yyy,M,DESTID,STARTID*hh