NMEA data

Table of Contents


The National Marine Electronics Association has developed a specification that defines the interface between various pieces of marine electronic equipment. The standard permits marine electronics to send information to computers and to other marine equipment. GPS receiver communication is defined within this specification. Most computer programs that provide real time position information understand and expect data to be in NMEA format. This data includes the complete PVT (position, velocity, time) solution computed by the GPS receiver. The idea of NMEA is to send a line of data called a sentence that is totally self contained and independent from other sentences. There are standard sentences for each device category and there is also the ability to define proprietary sentences for use by the individual company. All of the standard sentences have a two letter prefix that defines the device that uses that sentence type. (For gps receivers the prefix is GP.) which is followed by a three letter sequence that defines the sentence contents. In addition NMEA permits hardware manufactures to define their own proprietary sentences for whatever purpose they see fit. All proprietary sentences begin with the letter P and are followed with 3 letters that identifies the manufacturer controlling that sentence. For example a Garmin sentence would start with PGRM and Magellan would begin with PMGN.

Each sentence begins with a '$' and ends with a carriage return/line feed sequence and can be no longer than 80 characters of visible text (plus the line terminator). The data is contained within this single line with data items separated by commas. The data itself is just ascii text and may extend over multiple sentences in certain specialized instances but is normally fully contained in one variable length sentence. The data may vary in the amount of precision contained in the message. For example time might be indicated to decimal parts of a second or location may be show with 3 or even 4 digits after the decimal point. Programs that read the data should only use the commas to determine the field boundaries and not depend on column positions. There is a provision for a checksum at the end of each sentence which may or may not be checked by the unit that reads the data.

There have been several changes to the standard but for gps use the only ones that are likely to be encountered are 1.5 and 2.0 through 2.3. These just specify some different sentence configurations which may be peculiar to the needs of a particular device thus the gps may need to be changed to match the devices being interfaced to. Some gps's provide the ability configure a custom set the sentences while other may offer a set of fixed choices. Many gps receivers simply output a fixed set of sentences that cannot be changed by the user.

Hardware Connection

The hardware interface for GPS units is designed to meet the NMEA requirements. They are also compatible with most computer serial ports using RS232 protocols. The interface speed can be adjusted on some models but the NMEA standard is 4800 baud with 8 bits of data, no parity, and one stop bit. All units that support NMEA will support this speed. Note that, at a baud rate of 4800, you can easily send enough data to more than fill a full second of time. For this reason some units only send updates every two seconds or may send some data every second while reserving other data to be sent less often. In addition some units may send data a couple of seconds old while other units may send data that is collected within the second it is sent. Generally time is sent in some field within each second so it is pretty easy to figure out what a particular gps is doing. Some sentences may be sent only during a particular action of the receiver such as while following a route while other receivers may always send the sentence and just null out the values. Other difference will be noted in the specific data descriptions defined later in the text.

The NMEA standard has been around for many years (1983) and has undergone several revisions. The protocol has changed and the number and types of sentences may be different depending on the revision. Most receivers understand the latest standard which is called: 0183 version 2. This standard dictates a transfer rate of 4800 baud. Some receivers also understand older standards. The oldest standard was 0180 followed by 0182 which transferred data at 1200 baud. An earlier version of 0183 called version 1.5 is also understood by some receivers. Some Garmin units (and perhaps others) can be set to 9600 for NMEA output but this is only recommended if you have determined that 4800 works ok and then you can try to set it faster.

In order to use the hardware interface you will need a cable. Generally the cable is unique to the hardware model so you will need an cable made specifically for the brand and model of the unit you own. Some of the latest computers no longer include a serial port but only a USB port. Most gps receivers will work with Serial to USB adapters and serial ports attached via the pcmcia (pc card) adapter. For general NMEA use with a gps receiver you will only need two wires in the cable, data out from the gps and ground. A third wire, Data in, will be needed if you expect the receiver to accpet data on this cable such as to upload waypoints or send DGPS data to the receiver.

Gps receivers may be used to interface with other NMEA devices such as autopilots, fishfinders, or even another gps receivers. They can also listen to Differential Beacon Receivers that can send data using the RTCM SC-104 standard. This data is consistent with the hardware requirements for NMEA input data. There are no handshake lines defined for NMEA.

NMEA sentences

NMEA consists of sentences, the first word of which, called a data type, defines the interpretation of the rest of the sentence. Each Data type would have its own unique interpretation and is defined in the NMEA standard. The GGA sentence (shown below) shows an example that provides essential fix data. Other sentences may repeat some of the same information but will also supply new data. Whatever device or program that reads the data can watch for the data sentence that it is interested in and simply ignore other sentences that is doesn't care about. In the NMEA standard there are no commands to indicate that the gps should do something different. Instead each receiver just sends all of the data and expects much of it to be ignored. Some receivers have commands inside the unit that can select a subset of all the sentences or, in some cases, even the individual sentences to send. There is no way to indicate anything back to the unit as to whether the sentence is being read correctly or to request a re-send of some data you didn't get. Instead the receiving unit just checks the checksum and ignores the data if the checksum is bad figuring the data will be sent again sometime later.

There are many sentences in the NMEA standard for all kinds of devices that may be used in a Marine environment. Some of the ones that have applicability to gps receivers are listed below: (all message start with GP.)

In addition some gps receivers with special capabilities output these special messages.

The latest version of the NMEA standard is 2.3. It adds a mode indicator to several sentences which is used to indicate the kind of fix the receiver currently has. This indcation is part of the signal integrity information needed by the FAA. The value can be A=autonomous, D=differential, E=Estimated, N=not valid. This mode character has been added to the RMC, RMB, VTG, and GLL, sentences (and perhaps some others).

If you are interfacing a GPS unit to another device, including a computer program, you need to ensure that the receiving unit is given all of the sentences that it needs. If it needs a sentence that your GPS does not send then the interface to that unit is likely to fail. Here is a Link for the needs of some typical programs. The sentences sent by some typical receivers include:

NMEA 2.0
Name Garmin Magellan Lowrance Notes:
GPAPB N Y Y Auto Pilot B
GPBOD Y N N bearing, origin to destination - earlier G-12's do not transmit this
GPGGA Y Y Y fix data
GPGLL Y Y Y Lat/Lon data - earlier G-12's do not transmit this
GPGSA Y Y Y overall satellite reception data
GPGSV Y Y Y detailed satellite data
GPRMB Y Y Y minimum recommended data when following a route
GPRMC Y Y Y minimum recommended data
GPRTE Y U U route data (this is sometimes bidirectional)
GPWPL Y Y U waypoint data (this is sometimes bidirectional)

NMEA 1.5 - some units do not support version 1.5. Lowrance units provide the ability to customize the NMEA output by sentences so that you can develop your own custom sentence structure.
Name Garmin Magellan Notes:
GPAPA N Y Automatic Pilot A
GPBOD Y N bearing origin to destination - earlier G-12's do not send this
GPBWC Y Y bearing to waypoint using great circle route.
GPGLL Y Y lat/lon - earlier G-12's do not send this
GPRMC Y N minimum recommend data
GPRMB Y N minimum recommended data when following a route
GPVTG Y Y vector track and speed over ground
GPWPL Y N waypoint data (only when active goto)
GPXTE Y Y cross track error

The Trimble Scoutmaster outputs: APA, APB, BWC, GGA, GLL, GSA, GSV, RMB, RMC, VTG, WCV, XTE, ZTG.

The Motorola Encore outputs: GGA, GLL, GSV, RMC, VTG, ZDA and a proprietary sentence PMOTG.

Garmin receivers send the following Proprietary Sentences:

Note that Garmin converts lat/lon coordinates to the datum chosen by the user when sending this data. This is indicated in the proprietary sentence PGRMM. This can help programs that use maps with other datums but is not an NMEA standard. Be sure and set your datum to WGS84 on Garmin units when communicating to other NMEA devices.

Magellan units use proprietary sentences for waypoint maintenance and other tasks. They use a prefix of PMGN for this data.

It is possible to just view the information presented on the NMEA interface using a simple terminal program. If the terminal program can log the session then you can build a history of the entire session into a file. More sophisticated logging programs can filter the messages to only certain sentences or only collect sentences at prescribed intervals. Some computer programs that provide real time display and logging actually save the log in an ascii format that can be viewed with a text editor or used independently from the program that generated it.

NMEA input

Some units also support an NMEA input mode. While not too many programs support this mode it does provide a standardized way to update or add waypoint and route data. Note that there is no handshaking or commands in NMEA mode so you just send the data in the correct sentence and the unit will accept the data and add or overwrite the information in memory. If the waypoint name is the same you will overwrite existing data but no warning will be issued. The sentence construction is identical to what the unit downloads so you can, for example, capture a WPL sentence from one unit and then send that same sentence to another unit but be careful if the two units support waypoint names of different lengths since the receiving unit might truncate the name and overwrite a waypoint accidently. If you create a sentence from scratch you should create a correct checksum. Be sure you know and have set you unit to the correct datum.

On NMEA input the receiver stores information based on interpreting the sentence itself. While some receivers accept NMEA input this can only be used to update a waypoint or similar task and not to send a command to the unit. Since Magellan upload and download maintenance protocol is based on NMEA sentences they support a modified WPL message that adds comments, altitude, and icon data.

Some marine units may accept input for alarms such as deep or shallow water based on the DPT sentence. Other units may use NMEA input to provide initialization data via proprietary sentences, or to select which NMEA sentences to output.

Decode of selected position sentences

The most important NMEA sentences include the GGA which provides the current Fix data, the RMC which provides the minimum gps sentences information, and the GSA which provides the Satellite status data.

GGA - essential fix data which provide 3D location and accuracy data.


     GGA          Global Positioning System Fix Data
     123519       Fix taken at 12:35:19 UTC
     4807.038,N   Latitude 48 deg 07.038' N
     01131.000,E  Longitude 11 deg 31.000' E
     1            Fix quality: 0 = invalid
                               1 = GPS fix
                               2 = DGPS fix
                               6 = estimated (2.3 feature)
     08           Number of satellites being tracked
     0.9          Horizontal dilution of position
     545.4,M      Altitude, Meters, above mean sea level
     46.9,M       Height of geoid (mean sea level) above WGS84
     (empty field) time in seconds since last DGPS update
     (empty field) DGPS station ID number
     *42          the checksum data, always begins with *

GSA - GPS DOP and active satellites. This sentence provides details on the nature of the fix. It includes the numbers of the satellites being used in the current solution and the DOP. DOP (dilution of precision) is an indication of the effect of satellite geometry on the accuracy of the fix. It is a unitless number where smaller is better. For 3D fixes using 4 satellites a 1.0 would be considered to be a perfect number. For overdetermined solutions it is possible to see numbers below 1.0. There are differences in the way the PRN's are presented which can effect the ability of some programs to display this data. For example, in the example shown below there are 5 satellites in the solution and the null fields are scattered indicating that the almanac would show satellites in the null positions that are not being used as part of this solution. Other receivers might output all of the satellites used at the beginning of the sentence with the null field all stacked up at the end. This difference accounts for some satellite display programs not always being able to display the satellites being tracked.


     GSA      Satellite status
     A        Auto selection of 2D or 3D fix (M = manual) 
     3        3D fix - other values include: 1 = no fix
                                             2 = 2D fix
     04,05... PRNs of satellites used for fix (space for 12) 
     2.5      PDOP (dilution of precision) 
     1.3      Horizontal dilution of precision (HDOP) 
     2.1      Vertical dilution of precision (VDOP)
     *42      the checksum data, always begins with *

GSV - 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 current ability to track this data. Note that one GSV sentence only can provide data for up to 4 satellites and thus there may need to be 3 sentences for the full information.


      GSV          Satellites in view
      2            Number of sentences for full data
      1            sentence 1 of 2
      08           Number of satellites in view
      01           Satellite PRN number
      40           Elevation, degrees
      083          Azimuth, degrees
      46           Signal strength - higher is better
           for up to 4 satellites per sentence
      *75          the checksum data, always begins with *

RMC - NMEA has its own version of essential gps pvt (position, velocity, time) data. It is called RMC, The Recommended Minimum, which might look like:


     RMC          Recommended Minimum sentence C
     123519       Fix taken at 12:35:19 UTC
     A            Status A=active or V=Void.
     4807.038,N   Latitude 48 deg 07.038' N
     01131.000,E  Longitude 11 deg 31.000' E
     022.4        Speed over the ground in knots
     084.4        Track angle in degrees True
     230394       Date - 23rd of March 1994
     003.1,W      Magnetic Variation
     *43          The checksum data, always begins with *
Note that, as of the 2.3 release of NMEA, there is a new field in the RMC sentence at the end just prior to the checksum. The value of the entry is A=autonomous, D=differential, E=estimated, N=Data not valid.

GLL - Geographic Latitude and Longitude is a holdover from Loran data and some old units may not send the time and data valid information if they are emulating Loran data. If a gps is emulating Loran data they may use the LC Loran prefix instead of GP.


     GLL          Geographic position, Latitude and Longitude
     4916.46,N    Latitude 49 deg. 16.45 min. North
     12311.12,W   Longitude 123 deg. 11.12 min. West
     225444       Fix taken at 22:54:44 UTC
     A            Data valid or V (void)
     *23          checksum data

Decode of some Navigation Sentences

WPL - Waypoint Location data provides essential waypoint data. It is output when navigating to indicate data about the destination and is sometimes supported on input to redefine a waypoint location. Note that waypoint data as defined in the standard does not define altitude, comments, or icon data. When a route is active, this sentence is sent once for each waypoint in the route, in sequence. When all waypoints have been reported, the RTE sentence is sent in the next data set. In any group of sentences, only one WPL sentence, or an RTE sentence, will be sent.


With an interpretation of:

     WPL         Waypoint Location
     4807.038,N  Latitude
     01131.000,E Longitude
     WPTNME      Waypoint Name
     *31         The checksum data, always begins with *

AAM - Waypoint Arrival Alarm is generated by some units to indicate the Status of arrival (entering the arrival circle, or passing the perpendicular of the course line) at the destination waypoint.


    AAM    Arrival Alarm
    A      Arrival circle entered
    A      Perpendicular passed
    0.10   Circle radius
    N      Nautical miles
    WPTNME Waypoint name
    *43    Checksum data

APB - Autopilot format B is sent by some gps receivers to allow them to be used to control an autopilot unit. This sentence is commonly used by autopilots and contains navigation receiver warning flag status, cross-track-error, waypoint arrival status, initial bearing from origin waypoint to the destination, continuous bearing from present position to destination and recommended heading-to-steer to destination waypoint for the active navigation leg of the journey.

Note: some autopilots, Robertson in particular, misinterpret "bearing from origin to destination" as "bearing from present position to destination". This is likely due to the difference between the APB sentence and the APA sentence. for the APA sentence this would be the correct thing to do for the data in the same field. APA only differs from APB in this one field and APA leaves off the last two fields where this distinction is clearly spelled out. This will result in poor performance if the boat is sufficiently off-course that the two bearings are different.


    APB     Autopilot format B
    A       Loran-C blink/SNR warning, general warning 
    A       Loran-C cycle warning 
    0.10    cross-track error distance 
    R       steer Right to correct (or L for Left) 
    N       cross-track error units - nautical miles (K for kilometers) 
    V       arrival alarm - circle 
    V       arrival alarm - perpendicular 
    011,M   magnetic bearing, origin to destination 
    DEST    destination waypoint ID 
    011,M   magnetic bearing, present position to destination 
    011,M   magnetic heading to steer (bearings could True as 033,T) 

BOD - Bearing - Origin to Destination shows the bearing angle of the line, calculated at the origin waypoint, extending to the destination waypoint from the origin waypoint for the active navigation leg of the journey.


        BOD          Bearing - origin to destination waypoint
        045.,T       bearing 045 True from "START" to "DEST"
        023.,M       bearing 023 Magnetic from "START" to "DEST"
        DEST         destination waypoint ID
        START        origin waypoint ID
        *18          checksum

BWC - Bearing & Distance to Waypoint using a Great Circle route. Time (UTC) and distance & bearing to, and location of, a specified waypoint from present position along the great circle path.

        BWC          Bearing and distance to waypoint - great circle
        225444       UTC time of fix 22:54:44
        4917.24,N    Latitude of waypoint
        12309.57,W   Longitude of waypoint
        051.9,T      Bearing to waypoint, degrees true
        031.6,M      Bearing to waypoint, degrees magnetic
        001.3,N      Distance to waypoint, Nautical miles
        004          Waypoint ID
        *29          checksum

RMB - The recommended minimum navigation sentence is sent whenever a route or a goto is active. On some systems it is sent all of the time with null data sent.


           RMB          Recommended minimum navigation information
           A            Data status A = OK, V = warning
           0.66,L       Cross-track error (nautical miles, 9.9 max.),
                                steer Left to correct (or R = right)
           003          Origin waypoint ID
           004          Destination waypoint ID
           4917.24,N    Destination waypoint latitude 49 deg. 17.24 min. N
           12309.57,W   Destination waypoint longitude 123 deg. 09.57 min. W
           001.3        Range to destination, nautical miles
           052.5        True bearing to destination
           000.5        Velocity towards destination, knots
           V            Arrival alarm  A = arrived, V = not arrived
           *0B          checksum

RTE - RTE is sent to indicate the names of all of the waypoints used in an active route. There may need to be multiple sentences to identify all of the waypoints. The data about the waypoints themselves will be sent in subsequent WPL sentences. These will be sent in future cycles of the NMEA data.

           RTE          Waypoints in active route
           2            two sentences for full data
           1            this is sentence 1 of 2
           c            c = complete list of waypoints in this route
                        w = first listed waypoint is start of current leg
           0            Route identifier
           W3IWI...     Waypoint identifiers
           *69          checksum

VTG - Velocity made good. The gps receiver may use the LC prefix instead of GP if they are emulating Loran output.


        VTG          Track made good and ground speed
        054.7,T      True track made good
        034.4,M      Magnetic track made good
        005.5,N      Ground speed, knots
        010.2,K      Ground speed, Kilometers per hour
Note that, as of the 2.3 release of NMEA, there is a new field in the VTG sentence at the end just prior to the checksum. The value of the entry is A=autonomous, D=differential, E=estimated, N=Data not valid.

XTE - Measured cross track error.


           XTE          Cross track error, measured
           A            General warning flag V = warning
                                (Loran-C Blink or SNR warning)
           A            Not used for GPS (Loran-C cycle lock flag)
           0.67         cross track error distance
           L            Steer left to correct error (or R for right)
           N            Distance units - Nautical miles
           *07          checksum

Other sentences that may be useful

ALM - GPS Almanac Data contains GPS week number, satellite health and the complete almanac data for one satellite. Multiple messages may be transmitted, one for each satellite in the GPS constellation, up to maximum of 32 messages. Note that these sentences can take a long time to send so they are not generally sent automatically by the gps receiver. (Sorry I don't have an exact example of the sentence.) Note that this sentence breaks the 80 character rule.


       ALM   Almanac Data being sent
       A     Total number of messages
       B     Message number
       C     Satellite PRN number
       D     GPS week number (0-1023) 
       E     Satellite health (bits 17-24 of message)
       F     eccentricity
       hh    t index OA, almanac reference time
       hhhh  sigma index 1, inclination angle
       ...   OMEGADOT rate of right ascension
             SQRA(A) root of semi-major axis
             Omega, argument of perigee
             Omega index 0, longitude of ascension node
             M index 0, mean anomaly
             a index f0, clock parameter
             a index f1, clock parameter

HCHDG - Compass output is used on etrex summit and vista receivers to output the value of the internal flux-gate compass. Only the magnetic heading and magnetic variation is shown in the message.


     HCHDG    Magnetic heading, deviation, variation
     101.1    heading
     ,,       deviation (no data)
     7.1,W    variation

ZDA - Data and Time


	hhmmss    HrMinSec(UTC)
        dd,mm,yyy Day,Month,Year
        xx        local zone hours -13..13
        yy        local zone minutes 0..59
        *CC       checksum

Proprietary Sentences


The following are Garmin proprietary sentences. "P" denotes proprietary, "GRM" is Garmin's manufacturer code, and "M" or "Z" indicates the specific sentence type. Note that the PGRME sentence is not set if the output is set to NMEA 1.5 mode.


     15.0,M       Estimated horizontal position error in meters (HPE)
     45.0,M       Estimated vertical error (VPE) in meters
     25.0,M       Overall spherical equivalent position error


      93,f         Altitude in feet
      3            Position fix dimensions 2 = user altitude
                                           3 = GPS altitude
   This sentence shows in feet, regardless of units shown on the display.

  $PGRMM,NAD27 Canada*2F
     Currently active horizontal datum

Proprietary sentences are used to control a Starlink differential beacon receiver. (Garmin's DBR is Starlink compatible) When the GPS receiver is set to change the DBR frequency or baud rate, the "J" sentence is replaced (just once) by (for example): $PSLIB,320.0,200*59 to set the DBR to 320 KHz, 200 baud.

      $PSLIB,,,J*22   Status request
      $PSLIB,,,K*23   configuration request

These two sentences are normally sent together in each group of sentences from the GPS. The three fields are: Frequency, bit Rate, Request Type. The value in the third field may be: J = status request, K = configuration request, or blank = tuning message. The correct values for frequency range from 283.5-325.0 KHz while the bit rate can be set to 0, 25, 50, 100 or 200 bps.


Magellan uses proprietary sentences to do all of their waypoint and route maintenance. They use the MGN prefix for their sentences. This use is documented in their interface specification and will not be repeated here. However, they also send proprietary sentences to augment the gps data just like Garmin does. Here is an example of a sentence sent by the GPS Companion product:


      ST      status information
      02.12   Version number?
      3       2D or 3D
      T       True if we have a fix False otherwise
      534     numbers change - unknown
      05.0    time left on the gps battery in hours
      +03327  numbers change (freq. compensation?)
      00      PRN number receiving current focus
      *40    checksum


The PMOTG is used by Motorola Oncore receivers to send a command to the receiver. This command is used to set the output of the sentence to a particular frequency in seconds (or to 0) or to switch the output formula to motorola binary, gps, or loran.

      xxx    the sentence to be controlled
      yyyy   the time interval (0-9999 seconds)


      y    MPB=0, GPS=1, Loran=2

Rockwell International

The Rockwell chipset is used on a number of gps receivers. It outputs some proprietary sentences and accepts input from some special sentences similar to the approach used by Magellan. It can also be switched to a separate binary mode using a proprietary sentence. The input sentence most used to initialize the unit is $PRWIINIT and one output sentence is $PRWIRID

     12         12 channel unit
     01.83      software version
     12/15/97   software date
     0003       software options (HEX value)
                Bit 0 minimize ROM usage 
                Bit 1 minimize RAM usage
     *42        checksum
An input sentence that will define which NMEA sentences are to be output from the Rockwell unit is:

   GGA        type of sentence
   A          A=activate, V=deactivate
   T          cyclic
   1          every 1 second
   0          ??
The sentences available for the Rockwell Jupiter chipset are: GGA, GSA, GSV, VTG, RMC and some proprietary sentences.

Sample Streams

Garmin g12 sentences for version 4.57
$PGRMM,WGS 84*06
Here are some observations: Garmin etrex summit output
Some observations as compared to the G-12: Magellan GPS companion sentences
Some observations: Raytheon RN300 sentences:
Some observations:

Still more to come.

Peter's and Joe's web sites were used as primary sources for data in this article as well as some personal research. Some data was obtained from the Garmin product manuals. All rights to this presentation are reserved.

Dale DePriest