Disclaimer

This site is based on personal research and is believed to be accurate but there is no guarantee that any of the information is correct or suitable for any purpose. I have been told by the NMEA folks that my information is old and out of date. The current version of NMEA at the time I wrote this is 3.01 which is not described here. This site is for historical information and is not intended to be used for any official purpose. For official data please contact the NMEA web site. Please see the bottom of this article for the sources of this data.

Introduction

The National Marine Electronics Association (NMEA) has developed a specification that defines the interface between various pieces of marine electronic equipment. The standard permits marine electronics to send information to computers and to other marine equipment. A full copy of this standard is available for purchase at their web site. None of the information on this site comes from this standard and I do not have a copy. Anyone attempting to design anything to this standard should obtain an official copy.

GPS receiver communication is defined within this specification. Most computer programs that provide real time position information understand and expect data to be in NMEA format. This data includes the complete PVT (position, velocity, time) solution computed by the GPS receiver. The idea of NMEA is to send a line of data called a sentence that is totally self contained and independent from other sentences. There are standard sentences for each device category and there is also the ability to define proprietary sentences for use by the individual company. All of the standard sentences have a two letter prefix that defines the device that uses that sentence type. (For gps receivers the prefix is GP.) which is followed by a three letter sequence that defines the sentence contents. In addition NMEA permits hardware manufactures to define their own proprietary sentences for whatever purpose they see fit. All proprietary sentences begin with the letter P and are followed with 3 letters that identifies the manufacturer controlling that sentence. For example a Garmin sentence would start with PGRM and Magellan would begin with PMGN.

Each sentence begins with a '$' and ends with a carriage return/line feed sequence and can be no longer than 80 characters of visible text (plus the line terminators). The data is contained within this single line with data items separated by commas. The data itself is just ascii text and may extend over multiple sentences in certain specialized instances but is normally fully contained in one variable length sentence. The data may vary in the amount of precision contained in the message. For example time might be indicated to decimal parts of a second or location may be show with 3 or even 4 digits after the decimal point. Programs that read the data should only use the commas to determine the field boundaries and not depend on column positions. There is a provision for a checksum at the end of each sentence which may or may not be checked by the unit that reads the data. The checksum field consists of a '*' and two hex digits representing an 8 bit exclusive OR of all characters between, but not including, the '$' and '*'. A checksum is required on some sentences.

There have been several changes to the standard but for gps use the only ones that are likely to be encountered are 1.5 and 2.0 through 2.3. These just specify some different sentence configurations which may be peculiar to the needs of a particular device thus the gps may need to be changed to match the devices being interfaced to. Some gps's provide the ability configure a custom set the sentences while other may offer a set of fixed choices. Many gps receivers simply output a fixed set of sentences that cannot be changed by the user. The current version of the standard is 3.01. I have no specific information on this version, but I am not aware of any GPS products that require conformance to this version.

Hardware Connection

The hardware interface for GPS units is designed to meet the NMEA requirements. They are also compatible with most computer serial ports using RS232 protocols, however strictly speaking the NMEA standard is not RS232. They recommend conformance to EIA-422. 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, and one stop bit. All units that support NMEA should support this speed. Note that, at a b/s rate of 4800, you can easily send enough data to more than fill a full second of time. For this reason some units only send updates every two seconds or may send some data every second while reserving other data to be sent less often. In addition some units may send data a couple of seconds old while other units may send data that is collected within the second it is sent. Generally time is sent in some field within each second so it is pretty easy to figure out what a particular gps is doing. Some sentences may be sent only during a particular action of the receiver such as while following a route while other receivers may always send the sentence and just null out the values. Other difference will be noted in the specific data descriptions defined later in the text.

At 4800 b/s you can only send 480 characters in one second. Since an NMEA sentence can be as long as 82 characters you can be limited to less than 6 different sentences. The actual limit is determined by the specific sentences used, but this shows that it is easy to overrun the capabilities if you want rapid sentence response. NMEA is designed to run as a process in the background spitting out sentences which are then captured as needed by the using program. Some programs cannot do this and these programs will sample the data stream, then use the data for screen display, and then sample the data again. Depending on the time needed to use the data there can easily be a lag of 4 seconds in the responsiveness to changed data. This may be fine in some applications but totally unacceptable in others. For example a car traveling at 60 mph will travel 88 feet in one second. Several second delays could make the entire system seem unresponsive and could cause you to miss your turn.

The NMEA standard has been around for many years (1983) and has undergone several revisions. The protocol has changed and the number and types of sentences may be different depending on the revision. Most GPS receivers understand the standard which is called: 0183 version 2. This standard dictates a transfer rate of 4800 b/s. Some receivers also understand older standards. The oldest standard was 0180 followed by 0182 which transferred data at 1200 b/s. An earlier version of 0183 called version 1.5 is also understood by some receivers. Some Garmin units and other brands can be set to 9600 for NMEA output or even higher but this is only recommended if you have determined that 4800 works ok and then you can try to set it faster. Setting it to run as fast as you can may improve the responsiveness of the program.

In order to use the hardware interface you will need a cable. Generally the cable is unique to the hardware model so you will need an cable made specifically for the brand and model of the unit you own. Some of the latest computers no longer include a serial port but only a USB port. Most gps receivers will work with Serial to USB adapters and serial ports attached via the pcmcia (pc card) adapter. For general NMEA use with a gps receiver you will only need two wires in the cable, data out from the gps and ground. A third wire, Data in, will be needed if you expect the receiver to accept data on this cable such as to upload waypoints or send DGPS data to the receiver.

GPS receivers may be used to interface with other NMEA devices such as autopilots, fishfinders, or even another gps receivers. They can also listen to Differential Beacon Receivers that can send data using the RTCM SC-104 standard. This data is consistent with the hardware requirements for NMEA input data. There are no handshake lines defined for NMEA.

NMEA sentences

NMEA consists of sentences, the first word of which, called a data type, defines the interpretation of the rest of the sentence. Each Data type would have its own unique interpretation and is defined in the NMEA standard. The GGA sentence (shown below) shows an example that provides essential fix data. Other sentences may repeat some of the same information but will also supply new data. Whatever device or program that reads the data can watch for the data sentence that it is interested in and simply ignore other sentences that is doesn't care about. In the NMEA standard there are no commands to indicate that the gps should do something different. Instead each receiver just sends all of the data and expects much of it to be ignored. Some receivers have commands inside the unit that can select a subset of all the sentences or, in some cases, even the individual sentences to send. There is no way to indicate anything back to the unit as to whether the sentence is being read correctly or to request a re-send of some data you didn't get. Instead the receiving unit just checks the checksum and ignores the data if the checksum is bad figuring the data will be sent again sometime later.

There are many sentences in the NMEA standard for all kinds of devices that may be used in a Marine environment. Some of the ones that have applicability to gps receivers are listed below: (all message start with GP.)

• AAM - Waypoint Arrival Alarm
• ALM - Almanac data
• APA - Auto Pilot A sentence
• APB - Auto Pilot B sentence
• BOD - Bearing Origin to Destination
• BWC - Bearing using Great Circle route
• DTM - Datum being used.
• GGA - Fix information
• GLL - Lat/Lon data
• GRS - GPS Range Residuals
• GSA - Overall Satellite data
• GST - GPS Pseudorange Noise Statistics
• GSV - Detailed Satellite data
• MSK - send control for a beacon receiver
• MSS - Beacon receiver status information.
• RMA - recommended Loran data
• RMB - recommended navigation data for gps
• RMC - recommended minimum data for gps
• RTE - route message
• TRF - Transit Fix Data
• STN - Multiple Data ID
• VBW - dual Ground / Water Spped
• VTG - Vector track an Speed over the Ground
• WCV - Waypoint closure velocity (Velocity Made Good)
• WPL - Waypoint Location information
• XTC - cross track error
• XTE - measured cross track error
• ZTG - Zulu (UTC) time and time to go (to destination)
• ZDA - Date and Time

Some gps receivers with special capabilities output these special messages.

• HCHDG - Compass output
• PSLIB - Remote Control for a DGPS receiver

In addition some GPS receivers can mimic Loran-C receivers by outputing the LC prefix in some of their messages so that they can be used to interface to equipment that is expecting this prefix instead of the GP one.

The last version 2 iteration of the NMEA standard was 2.3. It added a mode indicator to several sentences which is used to indicate the kind of fix the receiver currently has. This indication is part of the signal integrity information needed by the FAA. The value can be A=autonomous, D=differential, E=Estimated, N=not valid, S=Simulator. Sometimes there can be a null value as well. Only the A and D values will correspond to an Active and reliable Sentence. This mode character has been added to the RMC, RMB, VTG, and GLL, sentences and optionally some others including the BWC and XTE sentences.

If you are interfacing a GPS unit to another device, including a computer program, you need to ensure that the receiving unit is given all of the sentences that it needs. If it needs a sentence that your GPS does not send then the interface to that unit is likely to fail. Here is a Link for the needs of some typical programs. The sentences sent by some typical receivers include:

NMEA 2.0

NMEA 1.5 - some units do not support version 1.5. Lowrance units provide the ability to customize the NMEA output by sentences so that you can develop your own custom sentence structure.

The NMEA 2.3 output from the Garmin Legend, Vista, and perhaps some others include the BWC, VTG, and XTE sentences.

The Trimble Scoutmaster outputs: APA, APB, BWC, GGA, GLL, GSA, GSV, RMB, RMC, VTG, WCV, XTE, ZTG.

The Motorola Encore outputs: GGA, GLL, GSV, RMC, VTG, ZDA and a proprietary sentence PMOTG.

Units based on the SiRF chipset can output: GGA, GLL, GSA, GSV, RMC, and VTG. What is actually output is based on which sentences are selected by the user or application program. See below for more details. Some implementations have enhanced the SiRF capabilities with other sentences as well by changing the firmware. For example, the u-blox receivers add ZDA and some proprietary sentences to the above list of sentences. Check your documentation for more details.

Garmin receivers send the following Proprietary Sentences:

• PGRME (estimated error) - not sent if set to 0183 1.5
• PGRMM (map datum)
• PGRMZ (altitude)
• PSLIB (beacon receiver control) </