NMEA 0183 is a combined electrical and data specification for communication between marine electronics such as echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined and is controlled by the National Marine Electronics Association (NMEA). It replaces the earlier NMEA 0180 and NMEA 0182 standards. In leisure marine applications it is slowly being phased out in favor of the newer NMEA 2000 standard, though NMEA 0183 remains the norm in commercial shipping.
Details
The electrical standard that is used is EIA-422, also known as RS-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.
While NMEA 0183 only defines an RS-422 transport, there also exists a de facto standard in which the sentences from NMEA0183 are placed in UDP datagrams (one sentence per packet) and sent over an IP network.
The NMEA standard is proprietary and sells for at least US$2000 (except for members of the NMEA) as of September 2020. However, much of it has been reverse-engineered from public sources.
UART settings
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.
Message structure
All transmitted data are printable ASCII characters between 0x20 (space) to 0x7e (~) Data characters are all the above characters except the reserved characters (See next line) Reserved characters are used by NMEA0183 for the following uses:
Messages have a maximum length of 82 characters, including the $ or ! starting character and the ending <LF> The start character for each message can be either a $ (For conventional field delimited messages) or ! (for messages that have special encapsulation in them) 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 *, not inclusive. According to the official specification, the checksum is optional for most data sentences, but is compulsory for RMA, RMB, and RMC (among others). ends the message.
As an example, a waypoint arrival alarm has the form: $GPAAM,A,A,0.10,N,WPTNME*32
Another example for AIS messages is: !AIVDM,1,1,,A,14eG;o@034o8sd<L9i:a;WF>062D,0*7D
NMEA sentence format
The main talker ID includes:
BD or GB - Beidou
GA - Galileo
GP - GPS
GL - GLONASS.
NMEA message mainly include the following "sentences" in the NMEA message:
One example, the sentence for Global Positioning System Fixed Data for GPS should be "$GPGGA".
Vendor extensions
Most GPS manufacturers include special messages in addition to the standard NMEA set in their products for maintenance and diagnostics purposes. Extended messages begin with "$P". These extended messages are not standardized.
Software compatibility
NMEA 0183 is supported by various navigation and mapping software. Notable applications include: DeLorme Street Atlas ESRI Google Earth Google Maps Mobile Edition gpsd - Unix GPS Daemon JOSM - OpenStreetMap Map Editor MapKing Microsoft MapPoint Microsoft Streets & Trips NetStumbler OpenCPN - Open source navigation software OpenBSD's hw.sensors framework with the nmea(4) pseudo-device driver OpenNTPD through sysctl API Rand McNally StreetFinder ObserVIEW