NMEA 0183

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NMEA 0183 is a combined electrical and data specification for communication between marine electronic devices such as echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the U.S.-based National Marine Electronics Association. It replaces the earlier NMEA 0180 and NMEA 0182 standards.[1] In marine applications, it is slowly being phased out in favor of the newer NMEA 2000 standard.

The electrical standard that is used is EIA-422, although most hardware with NMEA-0183 outputs are also able to drive a single EIA-232 port. Although the standard calls for isolated inputs and outputs, there are various series of hardware that do not adhere to this requirement.

The NMEA 0183 standard uses a simple ASCII, serial communications protocol that defines how data are transmitted in a "sentence" from one "talker" to multiple "listeners" at a time. Through the use of intermediate expanders, a talker can have a unidirectional conversation with a nearly unlimited number of listeners, and using multiplexers, multiple sensors can talk to a single computer port.

At the application layer, the standard also defines the contents of each sentence (message) type, so that all listeners can parse messages accurately.

Contents

Serial configuration (data link layer) [edit]

Typical Baud rate 4800
Data bits 8
Parity None
Stop bits 1
Handshake None

There is a variation of the standard called NMEA-0183HS that specifies a baud rate of 38,400. This is in general use by AIS devices.

Application layer protocol rules [edit]

  • Each message's starting character is a dollar sign.
  • The next five characters identify the talker (two characters) and the type of message (three characters).
  • All data fields that follow are comma-delimited.
  • Where data is unavailable, the corresponding field remains blank (it contains no character before the next delimiter – see Sample file section below).
  • The first character that immediately follows the last data field character is an asterisk, but it is only included if a checksum is supplied.
  • The asterisk is immediately followed by a checksum represented as a two-digit hexadecimal number. The checksum is the bitwise exclusive OR of ASCII codes of all characters between the $ and *. According to the official specification, the checksum is optional for most data sentences, but is compulsory for RMA, RMB, and RMC (among others).

As an example, a waypoint arrival alarm has the form:

$GPAAM,A,A,0.10,N,WPTNME*32

where:

GP Talker ID (GP for a GPS unit, GL for a GLONASS)
AAM Arrival alarm
A Arrival circle entered
A Perpendicular passed
0.10 Circle radius
N Nautical miles
WPTNME Waypoint name
*32 Checksum data

The new standard, NMEA 2000, accommodates several talkers at a higher baud rate, without using a central hub, or round-robin packet buffering.

The NMEA standard is proprietary and sells for at least US$ 325 as of June 2010.[2] However, much of it has been reverse-engineered from public sources, and is available in references like gpsd and Dale DePriest's.

Vendor extensions [edit]

Most GPS manufacturers include special messages in addition to the standard NMEA set in their products for maintenance and diagnostics purposes. These extended messages are not standardized at all, and are normally different from vendor to vendor.

Software compatibility [edit]

NMEA 0183 GPS compliant software

Sample file [edit]

A sample file produced by a Tripmate 850 GPS logger. This file was produced in Leixlip, County Kildare, Ireland. The record lasts two seconds.

$GPGGA,092750.000,5321.6802,N,00630.3372,W,1,8,1.03,61.7,M,55.2,M,,*76
$GPGSA,A,3,10,07,05,02,29,04,08,13,,,,,1.72,1.03,1.38*0A
$GPGSV,3,1,11,10,63,137,17,07,61,098,15,05,59,290,20,08,54,157,30*70
$GPGSV,3,2,11,02,39,223,19,13,28,070,17,26,23,252,,04,14,186,14*79
$GPGSV,3,3,11,29,09,301,24,16,09,020,,36,,,*76
$GPRMC,092750.000,A,5321.6802,N,00630.3372,W,0.02,31.66,280511,,,A*43
$GPGGA,092751.000,5321.6802,N,00630.3371,W,1,8,1.03,61.7,M,55.3,M,,*75
$GPGSA,A,3,10,07,05,02,29,04,08,13,,,,,1.72,1.03,1.38*0A
$GPGSV,3,1,11,10,63,137,17,07,61,098,15,05,59,290,20,08,54,157,30*70
$GPGSV,3,2,11,02,39,223,16,13,28,070,17,26,23,252,,04,14,186,15*77
$GPGSV,3,3,11,29,09,301,24,16,09,020,,36,,,*76
$GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A*45

Note some blank fields, for example:

  • GSV records, which describe satellites 'visible', lack the SNR (signal–to–noise ratio) field for satellite 16 and all data for satellite 36.
  • GSA record, which lists satellites used for determining a fix (position) and gives a DOP of the fix, contains 12 fields for satellites' numbers, but only 8 satellites were taken into account—so 4 fields remain blank.

C implementation of checksum generation [edit]

Persons looking to implement NMEA 0183 are often stumped by the checksum part. The following C code generates a checksum for the string entered as "mystring" and prints it to the output stream. In the example, a sentence for the sample file is used.

#include <stdio.h>
#include <string.h>
int main()
{
    char *mystring= "GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A";
    int lengthofstring = strlen(mystring);
    int checksum=0, i;
 
    for( i=0; i<lengthofstring; i++)
        checksum ^= mystring[i];
 
    printf("\nchecksum of the string\n\t%s\nis %02X",mystring,checksum);
    return 0;
}

Which can be simplified to function form as follows:

#include <stdio.h>
#include <string.h>
 
int checksum(char *s) {
    int c = 0;
 
    while(*s)
        c ^= *s++;
 
    return c;
}
 
int main()
{
    char mystring[] = "GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A";
 
    printf("String: %s\nChecksum: 0x%02X\n", mystring, checksum(mystring));
 
    return 0;
}

See also [edit]

References [edit]

  1. ^ http://www.kh-gps.de/nmea.faq
  2. ^ Publications and Standards from the National Marine Electronics Association (NMEA) / NMEA 0183
  3. ^ http://rosepointnav.com
  4. ^ http://mango.serotoninsoftware.com/
  5. ^ http://www.gpsy.com/
  6. ^ http://www.turboirc.com/tgps/
  7. ^ http://www.grlevelx.com/
  8. ^ http://www.google.com/mobile/default/maps.html
  9. ^ http://www.polarnavy.com/main/prod2
  10. ^ http://www.aviasail.com
  11. ^ http://www.visualgps.net/VisualGPS
  12. ^ http://www.gpstm.com
  13. ^ http://seesea.sourceforge.net/datalogger
  14. ^ http://marineapi.sourceforge.net/
  15. ^ http://www.voxmaris.com.ar

External links [edit]

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