Jump to navigation Jump to search Communication standard for marine electronics NMEA 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 by, and is controlled by, the National Marine Electronics Association. Although the standard calls for isolated inputs and outputs, there are various series of hardware that do not adhere to this requirement. The NMEA 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.
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Sample Streams 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I have been told by the NMEA folks that my information is old and out of date. 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. 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. 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. 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.
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.
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.
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. I have no specific information on this version, but I am not aware of any GPS products that require conformance to this version. They recommend conformance to EIA All units that support NMEA should support this speed. 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. 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 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: version 2. Some receivers also understand older standards. An earlier version of called version 1. Some Garmin units and other brands can be set to for NMEA output or even higher but this is only recommended if you have determined that 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. 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. 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. 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. 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.
GPS - NMEA sentence information
According to the NMEA website, the association was formed in by a group of electronic dealers to create better communications with manufacturers. The purpose of NMEA is to give equipment users the ability to mix and match hardware and software. Without a standard such as NMEA, it would be time-consuming and expensive to write and maintain such software. So, just like there are all kinds of GPS receivers with different capabilities, there are many different types of NMEA messages with different capabilities. MMMMM format. Decimal places are variable. N denotes north latitude.