NMEA 0183

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NMEA 0183 (or NMEA for short) is a combined electrical and data specification for communication between marine electronic devices such as echo sounder, sonars, anemometer (wind speed and direction), 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.

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. Third-party switches are available that can establish a primary and secondary talker, with automatic failover if the primary fails.

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


Contents

[edit] Serial configuration (data link layer)

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

AIS units use a default baud rate of 38400.

[edit] Application layer protocol rules

  • Each message starting character is a dollar sign.
  • The next five characters identify the type of message (talker).
  • All data fields that follow are comma-delimited.
  • The first character that immediately follows the last data field character is an asterisk.
  • The asterisk is immediately followed by a two-digit checksum representing a hex number. The checksum is the exclusive OR 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

A compound string from the NR203 GPS Receiver containing multiple messages. The actual messages decoded include:

  • ZDA – Standard NMEA $..ZDA Time & Date message
  • GLL – Standard NMEA $..GLL Geographic Position – Latitude/Longitude message
  • NSV – NMEA message containing individual satellite information.
Typical ASCII string
$<CR><LF>
MRK,0<CR><LF>
ZDA,123336.8069,17,06,2001,13.0<CR><LF>
GLL,4916.45,N,12311.12,W,225444,A,*1D<CR><LF>[1]
VTG,218.7,T,2.38,H,0.18,V<CR><LF>
SGD,-1.0,G,-1.0,M<CR><LF>
SYS,3T,9<CR><LF>
ZEV,0.28745E-006<CR><LF>
NSV,2,00,000,00,0.0,00.0,00,00,D<CR><LF>
NSV,7,00,000,00,0.0,00.0,00,00,D<CR><LF>
NSV,28,00,000,00,0.0,00.0,00,00,N<CR><LF>
NSV,1,00,000,00,0.0,00.0,00,00,D<CR><LF>
NSV,13,00,000,00,0.0,00.0,00,00,D<CR><LF>
NSV,4,00,000,00,0.0,00.0,00,00,N<CR><LF>
NSV,25,00,000,00,0.0,00.0,00,00,N<CR><LF>
NSV,0,00,000,00,0.0,00.0,00,00,N<CR><LF>
NSV,11,00,000,00,0.0,00.0,00,00,D<CR><LF>
NSV,0,00,000,00,0.0,00.0,00,00,N<CR><LF>
&
Description Form Field
Start characters "$" 0
UTC of position fix ZDA,hhmmss.ssss,dd,mm,yyyy 1
Latitude in degrees/minutes llll.ll 2
Direction of latitude (N or S) 3
Longitude in degrees/minutes yyyyy.yy 4
Direction of longitude (E or W) 5
Number of SVs NSV 6
Satellite ID number NSV,n,… 7
Repeated for "n" sats
End character “&”


The new standard, NMEA 2000, accommodates several "talkers" at a higher baud rate, without using a central hub.

The NMEA standard is proprietary and sells for at least US$ 270 as of August 2007. [2] However, much of it has been reverse-engineered from public sources and is available in references like gpsd and Dale DePriest's.

[edit] Vendor extensions

Most GPS manufacturers include special messages 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.

[edit] Software compatibility

NMEA 0183 GPS compliant software

[edit] See also

[edit] References

  1. ^ gpsinformation.org. GLL. Retrieved March 10, 2009.
  2. ^ Publications and Standards from the National Marine Electronics Association (NMEA) / NMEA 0183

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/NMEA_0183"
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