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{
{
char *mystring= "GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A";
char *mystring= "GPRMC,092751.000,A,5321.6802,N,00630.3371,W,0.06,31.66,280511,,,A";
int lengthofstring, checksum, i=0;
int lengthofstring = strlen(mystring);
int checksum=0, i;
lengthofstring = strlen(mystring);

⚫
printf("The string:\n%s\nis %d chars long",mystring,lengthofstring);
while (i<=lengthofstring-2)
for( i=0; i<lengthofstring; i++)
⚫
checksum ^= mystring[i];
{

if (i==0)
⚫
printf("\nchecksum of the string\n\t%s\nis %02X",mystring,checksum);
{
checksum = mystring[i]^mystring[i+1];
}
else
{
⚫
checksum = checksum^mystring[i+1];
}
i++;
}
printf("\nchecksum of the string is 0x%X",checksum);
return 0;
return 0;
}
}

Revision as of 07:47, 4 October 2012

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 is 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.

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

  • 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 contains NUL bytes (e.g., in "123,,456", The double comma between 3 and 4 is telling the listener the second field's data is unavailable).[clarification needed]
  • 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).
  • <CR><LF> ends the message.

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

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

NMEA 0183 GPS compliant software
  • AggreGate Device Management Platform
  • Master Navigator Software (MNS) - Map and Navigation Software
  • seaPro Navigation Software (www.euronav.co.uk)
  • OrbitGPS
  • ElectricCompass
  • Telogis GeoBase
  • NetStumbler
  • Nimble Navigator - Marine Navigation and Charting Software
  • Rand McNally StreetFinder
  • Coastal Explorer by Rose Point Navigation Systems[3]
  • Magic e-Map
  • GPS 2 IP - an iPhone NMEA server
  • NemaTalker NMEA instrument simulation utility by Sailsoft
  • Microsoft Streets & Trips
  • Microsoft MapPoint
  • Serotonin Mango M2M[4] (suitable for NMEA compliant weather stations)
  • MapKing
  • gpsd - Unix GPS Daemon
  • GPSy X for Mac OS X[5]
  • Turbo GPS PC/PPC/Android[6]
  • GRLevelX Weather Suite[7]
  • Google Maps Mobile Edition[8]
  • JOSM - OpenStreetMap Map Editor
  • PolarCOM - a set of digital and analog NMEA instruments[9]
  • Avia Sail - PC instruments for both NMEA 0183 and NMEA 2000[10]
  • VisualGPS - A free NMEA Monitoring utility for NMEA 0183 GPS devices[11]
  • DeLorme Street Atlas
  • GPS TrackMaker[12] - A Brazilian solution which supports many GPS brands
  • Java Marine API[13] - Open-source NMEA library for Java
  • Vox Maris GMDSS Simulator[14] - A GMDSS Simulator that is able to receive NMEA Data from external sources
  • C_GPS2KML - A tool to convert logged data to KML/KML for Google Earth supporting NMEA (Including Glonass)

Sample file

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

C implementation of checksum generation

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;
}

See also

References

  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://marineapi.sourceforge.net/
  14. ^ http://www.voxmaris.com.ar
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