WHAT IS NMEA?WHAT IS NMEA?NMEA is the acronyms stands for The National Marine Electronics Association (NMEA) which is a non-profit association of manufacturers, distributors, dealers, educational institutions, and others interested in peripheral marine electronics occupations. The National Marine Electronics Association (NMEA) has developed a standard over 20 years ago that defines the interface between various pieces of marine electronic equipment and navigational computers, allowing them to talk together and share vital information.

NMEA 0183 STANDARDSNMEA 0183 STANDARDSThe NMEA 0183 Interface Standards was first released in March of 1983. It has been updated from time to time, the latest release, currently (January 2002) Version 4.0. The NMEA 0183 is a combined electrical and data specification for communication between marine electronic devices.

The NMEA 0183 standard uses a simple printable ASCII form and serial communications protocol

The NMEA 0183 defines how data is transmitted in a "sentence" from one "talker" to multiple "listeners" at a time.

The data may include information such as position, speed, depth, frequency allocation, etc.

NMEA 0183 STANDARDSNMEA 0183 STANDARDSNMEA data is transmitted from an information source such as a GPS, echo sounder, or gyro compass etc. These data sending devices are called “Talkers”.

While the equipments receiving this information such as a chart-plotter, radar, PC or NMEA display is called a “Listener”. The NMEA 0183 defines the interface speed is 4800 b/s (bit per second rate) with 8 bits of data, no parity, one stop bit (or more) and none handshake. (means At 4800 b/s it can only send 480 characters in one second) NMEA output is EIA-422A but for most purposes it can consider RS-232 compatible

NMEA 0183 SENTENCESNMEA 0183 SENTENCESAll data is transmitted in the form of sentences. Only printable ASCII characters are allowed, plus carriage return (CR) and line feed (LF). Each sentence starts with a "$" sign and ends with <CR><LF> and can be no longer than 80 characters.  All of the standard sentences have a two letter prefix that defines the device that uses that ’sentence type’ and followed by a three letter sequence that defines the ’sentence contents’.

(E.g. GPGGA: GP is prefix for GPS receiver and GGA is prefix for GPS fix data)

NMEA 0183 SENTENCESNMEA 0183 SENTENCESAll data fields in the sentences are comma-delimited. And where data is unavailable, the corresponding field would contained NULL bytes.

(e.g., in "123,,456", the second field's data is unavailable). There is a requirement for checksum at the end of some sentence.

The checksum field consists of a '*' and two hex digits representing an 8 bit exclusive OR of all characters between the '$' and '*' sentence. For example:

($GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47)

NMEA 0183 SENTENCESNMEA 0183 SENTENCES

The Diagram shows the setting of NMEA default parameters and the NMEA Sentences

NMEA 0183 SENTENCESNMEA 0183 SENTENCES

There are three basic kinds of sentences: 1.Talker Sentences2.Query Sentences3.Proprietary Sentences

NMEA 0183 SENTENCESNMEA 0183 SENTENCES1. Talker Sentences

$ttsss,d1,d2,....<CR><LF>• The first two letters following the “$” are the talker identifier.• The next three characters (sss) are the sentence identifier,• followed by a number of data fields separated by commas, • followed by an optional checksum,• and terminated by carriage return/line feed.

 

$HCHDM,238,M<CR><LF>• "HC" specifies the talker as being a magnetic compass, • the "HDM" specifies the magnetic heading message follows.• The "238" is the heading value and,• "M" designates the heading value as magnetic.

NMEA 0183 SENTENCESNMEA 0183 SENTENCES2. Query sentences (means for a listener to request a particular sentence from a talker)

$ttllQ,sss,[CR][LF]

• The first two characters of the address field are the talker identifier of the requester. (tt)

• The next two characters are the talker identifier of the device being queried (listener). (II)

• The fifth character is always a "Q" defining the message as a query.

• The next field (sss) contains the three letter mnemonic of the sentence being requested.

 $CCGPQ,GGA<CR><LF>

• The "CC" device (computer) is requesting from the "GP" device (a GPS unit) the "GGA" sentence.

• The GPS will then transmit this sentence once per second until a different query is requested.

NMEA 0183 SENTENCESNMEA 0183 SENTENCES

3. Proprietary Sentences

The standard also allows individual manufacturers to define their proprietary sentence formats.

These sentences start with "$P", then a 3 letter manufacturer ID, followed by whatever data the manufacturer wishes, following the general format of the standard sentences. Such an examples:

NMEA 0183 SENTENCESNMEA 0183 SENTENCESMagellanMagellan uses proprietary sentences to do all of their waypoint and route maintenance. They use the MGN prefix for their sentences.

$PMGNST,02.12,3,T,534,05.0,+03327,00*40• The first character defines it is a proprietary sentences (P)• The next three character is refer to the manufacturer ID (MGN)• ST is status information• 02.12 is a Version number• 3 means 3D fix (x, y, z) or otherwise 2D (x, y)• T is for True if it has a fix or F for False otherwise• 534 numbers change – unknown• 05.0 is time left on the GPS battery in hours• +03327 numbers change (freq. compensation)• 00 is Pseudo Random Noise (PRN)[1] number receiving current focus• *40 checksum 1. Pseudo Random Noise (PRN) is a signal similar to noise which satisfies one or more of the standard tests for statistical randomness.

NMEA 0183 TALKERS IDENTIFIERSNMEA 0183 TALKERS IDENTIFIERS

NMEA 0183 SENTENCE NMEA 0183 SENTENCE FORMATTERSFORMATTERS

List of the three formatters which related to GPS

DECODE OF THE NMEA 0183 SENTENCESDECODE OF THE NMEA 0183 SENTENCES

1. GGA – GPS Fix Data that provides Time, Position (3D location) etc for GPS receiver.

$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47

1. Time (UT) – Fix taken at 12 hours 35 minutes and 19 seconds 2. Latituted – Latitude at 4807.0383. Northing or Southing – Northing 4. Longitude – Longitude at 1131.0005. Easting or Westing – Easting 6. GPS Quality Indicator: 1 – GPS fix 0 – Fix is not available 4 – Real Time Kinematic fix 8 – Simulation Mode 1 – GPS fix 5 – Float RTK fix 2 – Differential GPS fix 6 – Estimated (dead reckoning) 3 – Precise Positioning Services fix 7 – Manual Input Mode

7. Number of Satellite in View (00 to 12) – 8 satellites available8. Horizontal Dilution of Precision (HDOP) – 0.99. Antenna Altitude above / below Mean Sea Level (MSL) (Geoid) – 545.4 10. Unit of Antenna Altitude – Meters11. Geoid Separation - 469.9 (means MSL above the ellipsoid. If negative means otherwise)12. Unit of Geoid Separation – Meters 13. Time (in Seconds) the Last DGPS Updates – Null means DGPS is not been used14. DGPS Station ID number – not available15. Checksum – total of the characters of the sentence is 47

DECODE OF THE NMEA 0183 SENTENCESDECODE OF THE NMEA 0183 SENTENCES

2. GSA – GPS dilution of precision and active satellites

$GPGSA,A,3,19,28,14,18,27,22,31,39,,,,,1.7,1.0,1.3*35 1. Selection Mode – Automatic (2D or 3D) otherwise is Manual (force to operate in 2D or 3D)2. Mode: – 3 means 3D 1= Fix not available, 2 = 2D, 3 = 3D3. ID of 1st Satellite Used For Fixto 14. ID of 14th Satellite Used For Fix15. PDOP in Meters – 1.716. HDOP in Meters – 1.017. VDOP in Meters – 1.318. Checksum – Total of characters in the sentence is 35

DECODE OF THE NMEA 0183 SENTENCESDECODE OF THE NMEA 0183 SENTENCES

3. GSA – Satellites in View

$GPGSV,4,1,13,02,02,213,,03,-3,000,,11,00,121,,14,13,172,05*67 1. Total Number of Messages – 42. Message Number – 13. Total Number of Satellite in View – 13 Satellites available4. Satellite PRN number – 025. Elevation in Degrees – 02 Degree (90 Degree is maximum)6. Azimuth, Degrees From True North, 000 to 360 – 213 Degree7. SNR in dB, 00 to 99 – Null means no tracking8. to 11. Information about 2nd Satellite (same as No. 4 – 7) 12. to 15. Information about 3rd Satellite (same as No. 4 – 7)16. to 19. Information about 4th Satellite (same as No. 4 – 7)20. Checksum – Total of characters of the sentence is 67 Signal-to-noise ratio (SNR) is a measure used in science and engineering to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to the noise power corrupting the signal.

Decibel (dB) is a logarithmic unit of measurement in acoustics and electronics

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