SUG514 - Hydrographic Surveying - NMEA

MARA UNIVERSITY OF TECHNOLOGY Jack Ruzaini ([email protected]) BACHELOR OF GEOMATIC AND SURVEYING SCIENCE (AP220) ASSIGNMENT 3 HYDROGRAPHIC SURVEYING (SUG514) NMEA _________________________________________________________________________________________________________

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1.0 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.

2.0 NMEA 0183 STANDARDS

The NMEA 0183 Interface Standards is a voluntary industry standard, 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 that 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 data is transmitted from an information source such as a GPS, echo sounder, or gyro compass

etc. These data sending devices are called “Talkers”. Equipment receiving this information such as a

chart-plotter, radar, PC or NMEA display is called a “Listener”.

The NMEA 0183 standard defines the interface speed can be adjusted on some models but the NMEA

standard is 4800 b/s (bit per second rate) with 8 bits of data, no parity, one stop bit (or more) and

none handshake. (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


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1.0 WHAT IS NMEA?














allocation, etc.









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1.0 WHAT IS NMEA?














allocation, etc.









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3.0 NMEA SENTENCES

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

All 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).

All data fields that follow are comma-delimited. Where data is unavailable, the corresponding field would

contained NUL 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.

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


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There are three basic kinds of sentences:

1. Talker Sentences

2. Query Sentences

3. Proprietary Sentences

3.1 Talker Sentences

The general format for a talker sentence is: $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.

An example talker sentence is: $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.

3.2 Query sentences

A query sentence is a means for a listener to request a particular sentence from a talker. The general

format is: $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.

An example query sentence is: $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.


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3.3 Proprietary Sentences

The standard allows individual manufacturers to define 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:

Magellan

Magellan 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.


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4.0 NMEA TALKER IDENTIFIERS

AG Autopilot General IN Integrated Navigation

AP Autopilot Magnetic LC Loran C

CR Communications (Receiver / Beacon Receiver) P Proprietary

CD Communications (Digital Selective Calling) (DSC) RA RADAR and / or ARPA

CS Communications (Satellite) SD Sounder, Depth

CT Communications (Radio-Telephone) (MF/HF) SN Electronic Positioning System or other

CV Communications (Radio-Telephone) (VHF) SS Sounder, Scanning

CX Communications (Scanning Receiver) TI Turn Rate Indicator

DF Direction Finder VD Velocity Sensor, Doppler or other

EC Electronic Chart Display & Information System (ECDIS) DM Velocity Sensor, Speed Log, Water (Magnetic)

EP Emergency Position Indicating Beacon (EPIRB) VW Velocity Sensor, Speed Log, Water (Mechanical)

ER Engine Room Monitoring Systems WI Weather Instruments

GP Global Positioning System (GPS) YX Transducer

HC Heading (Magnetic Compass) ZA Timekeeper (Atomic Clock)

HE Heading (North Seeking Gyro) ZC Timekeeper (Chronometer)

HN Heading (Non North Seeking Gyro) ZQ Timekeeper (Quartz)

II Integrated Instrumentation ZV Timekeeper (Radio Update, WWV or WWVH)

5.0 NMEA SENTENCE FORMATTERS (Version 3.0 Approved)

Below is the list of the three formatters relating to GPS

ALM GPS Almanac Data

DTM Datum Reference

GGA GPS Fix Data

GRS GPS Range Residuals

GSA Overall Satellite Data

GST GPS Pseudorange Noise Statistics

GSV Detailed Satellite data in View


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6.0 DECODE OF SELECTED NMEA GPS SENTENCES

6.1 GGA – Global Positioning System Fix Data that provides Time, Position (3D location) and fix

related data for a 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 4807.038

3. Northing or Southing – Northing

4. Longitude – Longitude at 1131.000

5. 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 available

8. Horizontal Dilution of Precision (HDOP) [2] – 0.9

9. Antenna Altitude above / below Mean Sea Level (MSL) (Geoid) – 545.4

10. Unit of Antenna Altitude – Meters

11. Geoid Separation [3] - 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 (Type 1 – 9) – Null means DGPS is not been used

14. DGPS Station ID number – not available

15. Checksum – total of the characters of the sentence is 47

2. Horizontal Dilution of Precision – The horizontal dilution of precision (HDOP) allows one to more precisely estimate the

accuracy of GPS horizontal (latitude/longitude) position fixes by adjusting the error estimates according to the geometry of

the satellites used.

3. Geoid Separation – the height difference between the WGS84 ellipsoid and MSL geoid


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6.2 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 = 3D

3. to 14. ID of 1st to 14

th Satellite Used For Fix

15. PDOP in Meters – 1.7

16. HDOP in Meters – 1.0

17. VDOP in Meters – 1.3

18. Checksum – Total of characters in the sentence is 35

6.3 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 – 4

2. Message Number – 1

3. Total Number of Satellite in View – 13 Satellites available

4. Satellite PRN number – 02

5. Elevation in Degrees – 02 Degree (90 Degree is maximum)

6. Azimuth, Degrees From True North, 000 to 360 – 213 Degree

7. SNR [4] in dB

[5], 00 to 99 – Null means no tracking

8. 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

4. 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.

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

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SUG514 - Hydrographic Surveying - NMEA