NMEA 0183 (or NMEA for short) is a combined electrical and data specification for communication between marine electronics and also, more generally, GPS receivers.
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.
By Dale DePriest
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 ‘$’ and ends with a carriage return/line feed sequence and can be no longer than 80 characters of visible text (plus the line terminators). The data is contained within this single line with data items separated by commas. The data itself is just ascii text and may extend over multiple sentences in certain specialized instances but is normally fully contained in one variable length sentence. The data may vary in the amount of precision contained in the message. For example time might be indicated to decimal parts of a second or location may be show with 3 or even 4 digits after the decimal point. Programs that read the data should only use the commas to determine the field boundaries and not depend on column positions. There is a provision for a checksum at the end of each sentence which may or may not be checked by the unit that reads the data. The checksum field consists of a ‘*’ and two hex digits representing the exclusive OR of all characters between, but not including, the ‘$’ and ‘*’. A checksum is required on some sentences.
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.
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.
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
- GSA - Overall Satellite data
- 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
- VTG - Vector track an Speed over the Ground
- WCV - Waypoint closure velocity (Velocity Made Good)
- WPL - Waypoint information
- XTC - cross track error
- XTE - measured cross track error
- ZTG - Zulu (UTC) time and time to go (to destination)
- ZDA - Date and Time
In addition some gps receivers with special capabilities output these special messages.
- HCHDG - Compass output
- PSLIB - Remote Control for a DGPS receiver
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. The sentences sent by some typical receivers include:
|GPAPB||N||Y||Y||Auto Pilot B|
|GPBOD||Y||N||N||bearing, origin to destination - earlier G-12’s do not transmit this|
|GPGLL||Y||Y||Y||Lat/Lon data - earlier G-12’s do not transmit this|
|GPGSA||Y||Y||Y||overall satellite reception data, missing on some Garmin models|
|GPGSV||Y||Y||Y||detailed satellite data, missing on some Garmin models|
|GPRMB||Y||Y||Y||minimum recommended data when following a route|
|GPRMC||Y||Y||Y||minimum recommended data|
|GPRTE||Y||U||U||route data, only when there is an active route. (this is sometimes bidirectional)|
|GPWPL||Y||Y||U||waypoint data, only when there is an active route (this is sometimes bidirectional)|
|GPAPA||N||Y||Automatic Pilot A|
|GPBOD||Y||N||bearing origin to destination - earlier G-12’s do not send this|
|GPBWC||Y||Y||bearing to waypoint using great circle route.|
|GPGLL||Y||Y||lat/lon - earlier G-12’s do not send this|
|GPRMC||Y||N||minimum recommend data|
|GPRMB||Y||N||minimum recommended data when following a route|
|GPVTG||Y||Y||vector track and speed over ground|
|GPWPL||Y||N||waypoint data (only when active goto)|
|GPXTE||Y||Y||cross track error|
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)
Note that Garmin converts lat/lon coordinates to the datum chosen by the user when sending this data. This is indicated in the proprietary sentence PGRMM. This can help programs that use maps with other datums but is not an NMEA standard. Be sure and set your datum to WGS84 on Garmin units when communicating to other NMEA devices.
Magellan also converts lat/lon coordinates to the datum chosen on the receiver but do not indicate this in a message. Magellan units use proprietary sentences for waypoint maintenance and other tasks. They use a prefix of PMGN for this data.
Most other units always output NMEA messages in the WGS84 datum. Be sure and check the user documentation to be sure.
Some units also support an NMEA input mode. While not too many programs support this mode it does provide a standardized way to update or add waypoint and route data. Note that there is no handshaking or commands in NMEA mode so you just send the data in the correct sentence and the unit will accept the data and add or overwrite the information in memory. If the data is not in the correct format it will simply be ignored. A carriage return/line feed sequence is required. If the waypoint name is the same you will overwrite existing data but no warning will be issued. The sentence construction is identical to what the unit downloads so you can, for example, capture a WPL sentence from one unit and then send that same sentence to another unit but be careful if the two units support waypoint names of different lengths since the receiving unit might truncate the name and overwrite a waypoint accidently. If you create a sentence from scratch you should create a correct checksum. Be sure you know and have set you unit to the correct datum. Many units support the input of WPL sentences and a few support RTE as well.
On NMEA input the receiver stores information based on interpreting the sentence itself. While some receivers accept standard NMEA input this can only be used to update a waypoint or similar task and not to send a command to the unit. Proprietary input sentences could be used to send commands. Since the Magellan upload and download maintenance protocol is based on NMEA sentences they support a modified WPL message that adds comments, altitude, and icon data.
Decode of selected position sentences
The most important NMEA sentences include the GGA which provides the current Fix data, the RMC which provides the minimum gps sentences information, and the GSA which provides the Satellite status data.
GGA - essential fix data which provide 3D location and accuracy data.
$GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47 Where: GGA Global Positioning System Fix Data 123519 Fix taken at 12:35:19 UTC 4807.038,N Latitude 48 deg 07.038' N 01131.000,E Longitude 11 deg 31.000' E 1 Fix quality: 0 = invalid 1 = GPS fix (SPS) 2 = DGPS fix 3 = PPS fix 4 = Real Time Kinematic 5 = Float RTK 6 = estimated (dead reckoning) (2.3 feature) 7 = Manual input mode 8 = Simulation mode 08 Number of satellites being tracked 0.9 Horizontal dilution of position 545.4,M Altitude, Meters, above mean sea level 46.9,M Height of geoid (mean sea level) above WGS84 ellipsoid (empty field) time in seconds since last DGPS update (empty field) DGPS station ID number *47 the checksum data, always begins with *
If the height of geoid is missing then the altitude should be suspect. Some non-standard implementations report altitude with respect to the ellipsoid rather than geoid altitude. Some units do not report negative altitudes at all. This is the only sentence that reports altitude.
GSA - GPS DOP and active satellites. This sentence provides details on the nature of the fix. It includes the numbers of the satellites being used in the current solution and the DOP. DOP (dilution of precision) is an indication of the effect of satellite geometry on the accuracy of the fix. It is a unitless number where smaller is better. For 3D fixes using 4 satellites a 1.0 would be considered to be a perfect number, however for overdetermined solutions it is possible to see numbers below 1.0.
There are differences in the way the PRN’s are presented which can effect the ability of some programs to display this data. For example, in the example shown below there are 5 satellites in the solution and the null fields are scattered indicating that the almanac would show satellites in the null positions that are not being used as part of this solution. Other receivers might output all of the satellites used at the beginning of the sentence with the null field all stacked up at the end. This difference accounts for some satellite display programs not always being able to display the satellites being tracked. Some units may show all satellites that have ephemeris data without regard to their use as part of the solution but this is non-standard.
$GPGSA,A,3,04,05,,09,12,,,24,,,,,2.5,1.3,2.1*39 Where: GSA Satellite status A Auto selection of 2D or 3D fix (M = manual) 3 3D fix - values include: 1 = no fix 2 = 2D fix 3 = 3D fix 04,05... PRNs of satellites used for fix (space for 12) 2.5 PDOP (dilution of precision) 1.3 Horizontal dilution of precision (HDOP) 2.1 Vertical dilution of precision (VDOP) *39 the checksum data, always begins with *
GSV - Satellites in View shows data about the satellites that the unit might be able to find based on its viewing mask and almanac data. It also shows current ability to track this data. Note that one GSV sentence only can provide data for up to 4 satellites and thus there may need to be 3 sentences for the full information. It is reasonable for the GSV sentence to contain more satellites than GGA might indicate since GSV may include satellites that are not used as part of the solution. It is not a requirment that the GSV sentences all appear in sequence. To avoid overloading the data bandwidth some receivers may place the various sentences in totally different samples since each sentence identifies which one it is.
The field called SNR (Signal to Noise Ratio) in the NMEA standard is often referred to as signal strength. SNR is an indirect but more useful value that raw signal strength. It can range from 0 to 99 and has units of dB according to the NMEA standard, but the various manufacturers send different ranges of numbers with different starting numbers so the values themselves cannot necessarily be used to evaluate different units. The range of working values in a given gps will usually show a difference of about 25 to 35 between the lowest and highest values, however 0 is a special case and may be shown on satellites that are in view but not being tracked.
$GPGSV,2,1,08,01,40,083,46,02,17,308,41,12,07,344,39,14,22,228,45*75 Where: GSV Satellites in view 2 Number of sentences for full data 1 sentence 1 of 2 08 Number of satellites in view 01 Satellite PRN number 40 Elevation, degrees 083 Azimuth, degrees 46 SNR - higher is better for up to 4 satellites per sentence *75 the checksum data, always begins with *
RMC - NMEA has its own version of essential gps pvt (position, velocity, time) data. It is called RMC, The Recommended Minimum, which will look similar to:
$GPRMC,123519,A,4807.038,N,01131.000,E,022.4,084.4,230394,003.1,W*6A Where: RMC Recommended Minimum sentence C 123519 Fix taken at 12:35:19 UTC A Status A=active or V=Void. 4807.038,N Latitude 48 deg 07.038' N 01131.000,E Longitude 11 deg 31.000' E 022.4 Speed over the ground in knots 084.4 Track angle in degrees True 230394 Date - 23rd of March 1994 003.1,W Magnetic Variation *6A The checksum data, always begins with *
Note that, as of the 2.3 release of NMEA, there is a new field in the RMC sentence at the end just prior to the checksum. For more information on this field see 2.3.
GLL - Geographic Latitude and Longitude is a holdover from Loran data and some old units may not send the time and data active information if they are emulating Loran data. If a gps is emulating Loran data they may use the LC Loran prefix instead of GP.
$GPGLL,4916.45,N,12311.12,W,225444,A,*31 Where: GLL Geographic position, Latitude and Longitude 4916.46,N Latitude 49 deg. 16.45 min. North 12311.12,W Longitude 123 deg. 11.12 min. West 225444 Fix taken at 22:54:44 UTC A Data Active or V (void) *31 checksum data
Note that, as of the 2.3 release of NMEA, there is a new field in the GLL sentence at the end just prior to the checksum. For more information on this field see 2.3.
VTG - Velocity made good. The gps receiver may use the LC prefix instead of GP if it is emulating Loran output.
$GPVTG,054.7,T,034.4,M,005.5,N,010.2,K*33 where: VTG Track made good and ground speed 054.7,T True track made good (degrees) 034.4,M Magnetic track made good 005.5,N Ground speed, knots 010.2,K Ground speed, Kilometers per hour *33 Checksum
Note that, as of the 2.3 release of NMEA, there is a new field in the VTG sentence at the end just prior to the checksum. For more information on this field see 2.3.
Decode of some Navigation Sentences
WPL - Waypoint Location data provides essential waypoint data. It is output when navigating to indicate data about the destination and is sometimes supported on input to redefine a waypoint location. Note that waypoint data as defined in the standard does not define altitude, comments, or icon data. When a route is active, this sentence is sent once for each waypoint in the route, in sequence. When all waypoints have been reported, the RTE sentence is sent in the next data set. In any group of sentences, only one WPL sentence, or an RTE sentence, will be sent.
$GPWPL,4807.038,N,01131.000,E,WPTNME*5C With an interpretation of: WPL Waypoint Location 4807.038,N Latitude 01131.000,E Longitude WPTNME Waypoint Name *5C The checksum data, always begins with *
AAM - Waypoint Arrival Alarm is generated by some units to indicate the Status of arrival (entering the arrival circle, or passing the perpendicular of the course line) at the destination waypoint.
$GPAAM,A,A,0.10,N,WPTNME*43 Where: AAM Arrival Alarm A Arrival circle entered A Perpendicular passed 0.10 Circle radius N Nautical miles WPTNME Waypoint name *43 Checksum data
APB - Autopilot format B is sent by some gps receivers to allow them to be used to control an autopilot unit. This sentence is commonly used by autopilots and contains navigation receiver warning flag status, cross-track-error, waypoint arrival status, initial bearing from origin waypoint to the destination, continuous bearing from present position to destination and recommended heading-to-steer to destination waypoint for the active navigation leg of the journey.
Note: some autopilots, Robertson in particular, misinterpret “bearing from origin to destination” as “bearing from present position to destination”. This is likely due to the difference between the APB sentence and the APA sentence. for the APA sentence this would be the correct thing to do for the data in the same field. APA only differs from APB in this one field and APA leaves off the last two fields where this distinction is clearly spelled out. This will result in poor performance if the boat is sufficiently off-course that the two bearings are different.
$GPAPB,A,A,0.10,R,N,V,V,011,M,DEST,011,M,011,M*82 where: APB Autopilot format B A Loran-C blink/SNR warning, general warning A Loran-C cycle warning 0.10 cross-track error distance R steer Right to correct (or L for Left) N cross-track error units - nautical miles (K for kilometers) V arrival alarm - circle V arrival alarm - perpendicular 011,M magnetic bearing, origin to destination DEST destination waypoint ID 011,M magnetic bearing, present position to destination 011,M magnetic heading to steer (bearings could True as 033,T)
BOD - Bearing - Origin to Destination shows the bearing angle of the line, calculated at the origin waypoint, extending to the destination waypoint from the origin waypoint for the active navigation leg of the journey.
$GPBOD,045.,T,023.,M,DEST,START*01 where: BOD Bearing - origin to destination waypoint 045.,T bearing 045 True from "START" to "DEST" 023.,M bearing 023 Magnetic from "START" to "DEST" DEST destination waypoint ID START origin waypoint ID *01 checksum
BWC - Bearing & Distance to Waypoint using a Great Circle route. Time (UTC) and distance & bearing to, and location of, a specified waypoint from present position along the great circle path.
$GPBWC,225444,4917.24,N,12309.57,W,051.9,T,031.6,M,001.3,N,004*29 where: BWC Bearing and distance to waypoint - great circle 225444 UTC time of fix 22:54:44 4917.24,N Latitude of waypoint 12309.57,W Longitude of waypoint 051.9,T Bearing to waypoint, degrees true 031.6,M Bearing to waypoint, degrees magnetic 001.3,N Distance to waypoint, Nautical miles 004 Waypoint ID *29 checksum
RMB - The recommended minimum navigation sentence is sent whenever a route or a goto is active. On some systems it is sent all of the time with null data. The Arrival alarm flag is similar to the arrival alarm inside the unit and can be decoded to drive an external alarm. Note the use of leading zeros in this message to preserve the character spacing. This is done, I believe, because some autopilots may depend on exact character spacing.
$GPRMB,A,0.66,L,003,004,4917.24,N,12309.57,W,001.3,052.5,000.5,V*20 where: RMB Recommended minimum navigation information A Data status A = OK, V = Void (warning) 0.66,L Cross-track error (nautical miles, 9.99 max), steer Left to correct (or R = right) 003 Origin waypoint ID 004 Destination waypoint ID 4917.24,N Destination waypoint latitude 49 deg. 17.24 min. N 12309.57,W Destination waypoint longitude 123 deg. 09.57 min. W 001.3 Range to destination, nautical miles (999.9 max) 052.5 True bearing to destination 000.5 Velocity towards destination, knots V Arrival alarm A = arrived, V = not arrived *20 checksum
RTE - RTE is sent to indicate the names of the waypoints used in an active route. There are two types of RTE sentences. This route sentence can list all of the waypoints in the entire route or it can list only those still ahead. Because an NMEA sentence is limited to 80 characters there may need to be multiple sentences to identify all of the waypoints. The data about the waypoints themselves will be sent in subsequent WPL sentences which will be sent in future cycles of the NMEA data.
$GPRTE,2,1,c,0,W3IWI,DRIVWY,32CEDR,32-29,32BKLD,32-I95,32-US1,BW-32,BW-198*69 Where: RTE Waypoints in active route 2 total number of sentences needed for full data 1 this is sentence 1 of 2 c Type c = complete list of waypoints in this route w = first listed waypoint is start of current leg 0 Route identifier W3IWI,... Waypoint identifiers (names) *69 checksum
XTE - Measured cross track error is a small subset of the RMB message for compatibility with some older equipment designed to work with Loran. Note that the same limitations apply to this message as the ones in the RMB since it is expected to be decoded by an autopilot.
$GPXTE,A,A,0.67,L,N*6F Where: XTE Cross track error, measured A General warning flag V = warning (Loran-C Blink or SNR warning) A Not used for GPS (Loran-C cycle lock flag) 0.67 cross track error distance L Steer left to correct error (or R for right) N Distance units - Nautical miles *6F checksum
Other sentences that may be useful
ALM - GPS Almanac Data contains GPS week number, satellite health and the complete almanac data for one satellite. Multiple messages may be transmitted, one for each satellite in the GPS constellation, up to maximum of 32 messages. Note that these sentences can take a long time to send so they are not generally sent automatically by the gps receiver. (Sorry I don’t have an exact example of the sentence.) Note that this sentence breaks the 80 character rule.
$GPALM,A.B,C.D,E,F,hh,hhhh,... Where: ALM Almanac Data being sent A Total number of messages B Message number C Satellite PRN number D GPS week number (0-1023) E Satellite health (bits 17-24 of message) F eccentricity hh t index OA, almanac reference time hhhh sigma index 1, inclination angle ... OMEGADOT rate of right ascension SQRA(A) root of semi-major axis Omega, argument of perigee Omega index 0, longitude of ascension node M index 0, mean anomaly a index f0, clock parameter a index f1, clock parameter
HCHDG - Compass output is used on Garmin etrex summit, vista , and 76S receivers to output the value of the internal flux-gate compass. Only the magnetic heading and magnetic variation is shown in the message.
$HCHDG,101.1,,,7.1,W*3C where: HCHDG Magnetic heading, deviation, variation 101.1 heading ,, deviation (no data) 7.1,W variation
ZDA - Data and Time
$GPZDA,hhmmss.ss,dd,mm,yyyy,xx,yy*CC $GPZDA,201530.00,04,07,2002,00,00*6E where: hhmmss HrMinSec(UTC) dd,mm,yyy Day,Month,Year xx local zone hours -13..13 yy local zone minutes 0..59 *CC checksum
MSK - Control for a Beacon Receiver
$GPMSK,318.0,A,100,M,2*45 where: 318.0 Frequency to use A Frequency mode, A=auto, M=manual 100 Beacon bit rate M Bitrate, A=auto, M=manual 2 frequency for MSS message status (null for no status) *45 checksum
MSS - Beacon Receiver Status
$GPMSS,55,27,318.0,100,*66 where: 55 signal strength in dB 27 signal to noise ratio in dB 318.0 Beacon Frequency in KHz 100 Beacon bitrate in bps *66 checksum
Proprietary sentences can either be output from the gps or used as input to control information. They always start with P which is followed by a 3 character manufactures code and additional characters to define the sentence type.
The following are Garmin proprietary sentences. “P” denotes proprietary, “GRM” is Garmin’s manufacturer code, and “M” or “Z” indicates the specific sentence type. Note that the PGRME sentence is not set if the output is set to NMEA 1.5 mode.
$PGRME,15.0,M,45.0,M,25.0,M*1C where: 15.0,M Estimated horizontal position error in meters (HPE) 45.0,M Estimated vertical error (VPE) in meters 25.0,M Overall spherical equivalent position error $PGRMZ,93,f,3*21 where: 93,f Altitude in feet 3 Position fix dimensions 2 = user altitude 3 = GPS altitude This sentence shows in feet, regardless of units shown on the display. Note that for units with an altimeter this will be altitude computed by the internal altimeter. $PGRMM,NAD27 Canada*2F Currently active horizontal datum
Proprietary sentences are used to control a Starlink differential beacon receiver. (Garmin’s DBR is Starlink compatible as are many others.) When the GPS receiver is set to change the DBR frequency or b/s rate, the “J” sentence is replaced (just once) by (for example): $PSLIB,320.0,200*59 to set the DBR to 320 KHz, 200 b/s.
$PSLIB,,,J*22 Status request $PSLIB,,,K*23 configuration request
These two sentences are normally sent together in each group of sentences from the GPS. The three fields are: Frequency, bit Rate, Request Type. The value in the third field may be: J = status request, K = configuration request, or null (blank) = tuning message. The correct values for frequency range from 283.5-325.0 KHz while the bit rate can be set to 0, 25, 50, 100 or 200 bps.
Magellan uses proprietary sentences to do all of their waypoint and route maintenance. They use the MGN prefix for their sentences. This use is documented in their interface specification and will not be repeated here. However, they also send proprietary sentences to augment the gps data just like Garmin does. Here is an example of a sentence sent by the GPS Companion product:
$PMGNST,02.12,3,T,534,05.0,+03327,00*40 where: ST status information 02.12 Version number? 3 2D or 3D T True if we have a fix False otherwise 534 numbers change - unknown 05.0 time left on the gps battery in hours +03327 numbers change (freq. compensation?) 00 PRN number receiving current focus *40 checksum
A tracklog on a Meridian is made up of propretary sentences that look like:
$PMGNTRK,4322.061,N,07948.473,W,00116,M,173949.42,A,,020602*67 $PMGNTRK,4322.058,N,07948.483,W,00090,M,174202.45,A,,020602*69. where TRK Tracklog 4322.071 Latitude N North or South 07948.473 Longitude W East or West 00116 Altitude M Meters or Feet 173949.42 UTC time A Active or Void ,, Track Name 020602 date *67 checksum
$PMOTG,xxx,yyyy where: xxx the sentence to be controlled yyyy the time interval (0-9999 seconds) or $PMOTG,FOR,y where: y MPB=0, GPS=1, Loran=2
The Rockwell chipset is used on a number of gps receivers. It outputs some proprietary sentences with the PRWI prefix and accepts input from some special sentences similar to the approach used by Magellan. It can also be switched to a separate binary mode using a proprietary sentence. The input sentence most used to initialize the unit is $PRWIINIT and one output sentence is $PRWIRID
$PRWIRID,12,01.83,12/15/97,0003,*42 where: $PRWIRID 12 12 channel unit 01.83 software version 12/15/97 software date 0003 software options (HEX value) Bit 0 minimize ROM usage Bit 1 minimize RAM usage *42 checksum
An input sentence that will define which NMEA sentences are to be output from the Rockwell unit is:
$PRWIILOG,GGA,A,T,1,0 where: $PRWIILOG GGA type of sentence A A=activate, V=deactivate T cyclic 1 every 1 second 0 ??
The initialization sentence which can be input to speed up acquisition looks like:
$PRWIINIT,V,,,4308.750,N,07159.791,W,100.0,0.0,M,0.0,T,175244,230503*77 where: $PRWIINIT INIT = initialization V V = reset, A = no reset ,, Reserved for future use 4308.750 Latitude N N = North, S = South 07159.791 Longitude W W = West, E = East 100.0 Altitude in meters 0.0 Speed M M = m/s, N = knots, K = km/hr 0.0 Heading T T = True, M = Magnetic 175244 UTC time (hour, min, sec) 230503 UTC date (day, month, year) *77 Checksum
Note: Commas may be used to signify using existing data. If units are supplied then the data must be present. Speed and direction must be supplied together. Lat/Lon must be supplied together. UTC time and date must be supplied together. If heading is magnetic then lat/lon needs to be supplied along with UTC time and date.
The sentences available for the Rockwell Jupiter chipset are: GGA, GSA, GSV, VTG, RMC and some proprietary sentences.
The SiRF line of chips support several input sentences that permit the user to customize the way the chip behaves. In addition SiRF has a binary protocol that is even more powerful permitting different implementations to behave entirely differently. However, most applications do not attempt to customize the behavior so a user will need to make sure that the any customization is compatible with the application they are planning to use. There are 5 input sentences defined that begin with $PSRF which is followed by three digits. Each sentence takes a fix amount of input fields which must exist, no null fields, and is terminated with the standard CR/LF sequence. The checksum is required.
The sentences 100 and 102 set the serial ports. 100 sets the main port A while 102 sets the DGPS input port B. 100 has an extra field that can be used to switch the interface to binary mode. Binary mode requires 8 bits, 1 stop bit, no parity. There is a command in binary mode that will switch the interface back to NMEA. Do not use the NMEA command to switch to binary mode unless you have the ability to switch it back. You could render your gps inoperative.
$PSRF100,0,9600,8,1,0*0C $PSRF102,9600,8,1,0*3C where $PSRF100 0 0=SiRF, 1=NMEA - This is where the protocol is changed. 9600 b/s rate 4800, 9600, 19200, 38400 8 7, 8 Databits 1 0, 1 Stopbits 0 0=none, 1=odd, 2=even Parity *0C checksum
The sentences 101 and 104 can be used to initialize values to be used by the gps. Supplying these values can shorten the initial lock time. If the clock offset is set to 0 then an internal default will be used. Sentence 101 supplies data in the internal ECEF (Earth centered, Earth Fixed) format in meters while sentence 104 supplies the data in the traditional Lat / Lon format.
$PSRF101,-2686700,-4304200,3851624,95000,497260,921,12,3*22 $PSRF104,37.3875111,-121.97232,0,95000,237759,922,12,3*3A where $PSRF104 37.3875111 Latitude in degrees -121.97232 Longitude in degrees 0 Ellipsoid Altitude in meters 95000 Clock offset 237759 GPS Time of Week in seconds 922 GPS Week Number 12 Channel count (1 to 12) 3 Reset config where 1 = warm start, ephemeris valid 2 = clear ephemeris, warm start (First Fix) 3 = initialize with data, clear ephemeris 4 = cold start, clear all data 8 = cold start, set factory defaults *3A checksum
The sentence 103 is used to control which NMEA sentences are to be sent and how often. Each sentence type is controlled individually. If the query bit is set then the gps responds by sending this message in the next second no matter what the rate is set to. Note that if trickle power is in use (can only be set in binary mode) then the actual update rate will be the selected update rate times the trickle rate which could mean that the data will be sent less frequently than was set here.
$PSRF103,05,00,01,01*20 where $PSRF103 05 00=GGA 01=GLL 02=GSA 03=GSV 04=RMC 05=VTG 00 mode, 0=set rate, 1=query 01 rate in seconds, 0-255 01 checksum 0=no, 1=yes *20 checksum
The 105 sentence controls a debug mode which causes the gps to report any errors it finds with the input data. $PSRF105,1*3E would turn debug on while $PSRF105,0*3F would turn it off.
The old Magnavox system used mostly proprietary sentences. The Magnavox system was acquired by Leica Geosystems in 1994. Information on this system can be found at this site. They all use the MVX prefix and include:
Control Port Input sentences
- $PMVXG,000 Initialization/Mode Control - Part A
- $PMVXG,001 Initialization/Mode Control - Part B
- $PMVXG,007 Control Port Configuration
- $PMVXG,023 Time Recovery Configuration
- $CDGPQ,YYY Query From a Remote Device / Request to Output a Sentence
Control Port Output Sentences
- $PMVXG,000 Receiver Status
- $PMVXG,021 Position, Height, Velocity
- $PMVXG,022 DOPs
- $PMVXG,030 Software Configuration
- $PMVXG,101 Control Sentence Accept/Reject
- $PMVXG,523 Time Recovery Configuration
- $PMVXG,830 Time Recovery Results
The Sony interface uses a proprietary sentence that looks like:
$PSNY,0,00,05,500,06,06,06,06*14 where PSNY 0 Preamp (external antenna) status 0 = Normal 1 = Open 2 = shorted 00 Geodesic system (datum) 0-25, 0 = WGS84 05 Elevation mask in degrees 500 Speed Limit in Km 06 PDOP limit with DGPS on 06 HDOP limit with DGPS on 06 PDOP limit with DGPS off 06 HDOP limit with DGPS off *14 Checksum
These streams will be modified when a route is active with the inclusion of route specific data.
Garmin g12 sentences for version 4.57
$GPRMC,183729,A,3907.356,N,12102.482,W,000.0,360.0,080301,015.5,E*6F $GPRMB,A,,,,,,,,,,,,V*71 $GPGGA,183730,3907.356,N,12102.482,W,1,05,1.6,646.4,M,-24.1,M,,*75 $GPGSA,A,3,02,,,07,,09,24,26,,,,,1.6,1.6,1.0*3D $GPGSV,2,1,08,02,43,088,38,04,42,145,00,05,11,291,00,07,60,043,35*71 $GPGSV,2,2,08,08,02,145,00,09,46,303,47,24,16,178,32,26,18,231,43*77 $PGRME,22.0,M,52.9,M,51.0,M*14 $GPGLL,3907.360,N,12102.481,W,183730,A*33 $PGRMZ,2062,f,3*2D $PGRMM,WGS 84*06 $GPBOD,,T,,M,,*47 $GPRTE,1,1,c,0*07 $GPRMC,183731,A,3907.482,N,12102.436,W,000.0,360.0,080301,015.5,E*67 $GPRMB,A,,,,,,,,,,,,V*71
Here are some observations:
- Notice the complete cycle shows an update interval of 2 seconds which is caused by the fact that there is too much data to fit in one second at 4800 b/s.
- Upping the b/s rate to 9600 will cause an update every second.
- Notice that the samples are in real time for each sentence because the GGA sentence shows an update in the time of 1 second.
- It would be possible to provide update data every second by parsing more sentences since the data is adjusted every second.
- Notice the gaps in the GSA message where the satellites in use are shown in a there slots as compared to the GSV locations. Some tools do not decode this configuration correctly.
- Note the GGA sentence starts the sequence every two seconds.
- This sample is similar for other Garmin receivers designed in the same time frame as the G-12.
Garmin etrex summit outputs
$GPRMC,002454,A,3553.5295,N,13938.6570,E,0.0,43.1,180700,7.1,W,A*3F $GPRMB,A,,,,,,,,,,,,A,A*0B $GPGGA,002454,3553.5295,N,13938.6570,E,1,05,2.2,18.3,M,39.0,M,,*7F $GPGSA,A,3,01,04,07,16,20,,,,,,,,3.6,2.2,2.7*35 $GPGSV,3,1,09,01,38,103,37,02,23,215,00,04,38,297,37,05,00,328,00*70 $GPGSV,3,2,09,07,77,299,47,11,07,087,00,16,74,041,47,20,38,044,43*73 $GPGSV,3,3,09,24,12,282,00*4D $GPGLL,3553.5295,N,13938.6570,E,002454,A,A*4F $GPBOD,,T,,M,,*47 $PGRME,8.6,M,9.6,M,12.9,M*15 $PGRMZ,51,f*30 $HCHDG,101.1,,,7.1,W*3C $GPRTE,1,1,c,*37 $GPRMC,002456,A,3553.5295,N,13938.6570,E,0.0,43.1,180700,7.1,W,A*3D
Some observations as compared to the G-12:
- Information is buffered. It is all for the same second.
- Information is only updated every two seconds at 4800 b/s.
- Lat/Lon numbers have an extra digit.
- This is NMEA 2.3 data as indicated by the extra A at the end of RMC, RMB and GLL.
- Note that the satellites in use have been shoved to the left of the GSA message instead of the slot location.
- The RMC sentence starts the sequence.
- Note the HCHDG sentence for the built in compass.
- Except for the compass output this sentence list is similar for most Garmin units designed around the time of the Summit receivers, beginning with the emap.
Garmin etrex Vista release 2.42 outputs
$GPRMC,023042,A,3907.3837,N,12102.4684,W,0.0,156.1,131102,15.3,E,A*36 $GPRMB,A,,,,,,,,,,,,A,A*0B $GPGGA,023042,3907.3837,N,12102.4684,W,1,04,2.3,507.3,M,-24.1,M,,*75 $GPGSA,A,3,04,05,,,09,,,24,,,,,2.8,2.3,1.0*36 $GPGSV,3,2,11,09,47,229,42,10,04,157,00,14,00,305,00,24,70,154,33*79 $GPGLL,3907.3837,N,12102.4684,W,023042,A,A*5E $GPBOD,,T,,M,,*47 $GPVTG,156.1,T,140.9,M,0.0,N,0.0,K*41 $PGRME,8.4,M,23.8,M,25.7,M*2B $PGRMZ,1735,f*34 $PGRMM,WGS 84*06 $HCHDG,,,,15.3,E*30 $GPRTE,1,1,c,*37 $GPRMC,023044,A,3907.3840,N,12102.4692,W,0.0,156.1,131102,15.3,E,A*37
Some observations as compared to the Summit:
- Output still repeats at a rate of once every 2 seconds and is NMEA 2.3 Data
- The satellite status sentences are interleaved. This GSV sentences are only sent one in each two second group. Note the example shows sentence two of three. Thus the complete cycle would take 6 seconds.
- New sentence VTG.
- The altitude in PGRMZ is from the altimeter while the altitude in the GGA is from the gps computation.
- Note the HCHDG sentence for the built in compass and is missing for the Legend.
Garmin basic yellow etrex European version
$GPRMC,152926,V,6027.8259,N,02225.6713,E,10.8,0.0,190803,5.9,E,S*22 $GPRMB,V,,,,,,,,,,,,A,S*0E $GPGGA,152926,6027.8259,N,02225.6713,E,8,09,2.0,44.7,M,20.6,M,,*79 $GPGSA,A,3,07,08,09,11,18,23,26,28,29,,,,6.6,2.0,3.0*38 $GPGSV,3,1,09,07,29,138,44,08,22,099,42,09,30,273,44,11,07,057,35*75 $GPGSV,3,2,09,18,28,305,43,23,14,340,39,26,64,222,49,28,60,084,49*7E $GPGSV,3,3,09,29,52,187,48*4E $GPGLL,6027.8259,N,02225.6713,E,152926,V,S*48 $GPBOD,,T,,M,,*47 $PGRME,15.0,M,22.5,M,15.0,M*1B $PGRMZ,147,f,3*19 $GPRTE,1,1,c,*37 $GPRMC,152928,V,6027.8319,N,02225.6713,E,10.8,0.0,190803,5.9,E,S*29
- The sentence sequence starts with RMC and repeats every 2 seconds.
- The PGRMM sentence is missing so the datum is not identified.
Magellan GPS companion sentences
$GPGGA,184050.84,3907.3839,N,12102.4772,W,1,05,1.8,00543,M,,,,*33 $GPRMC,184050.84,A,3907.3839,N,12102.4772,W,00.0,000.0,080301,15,E*54 $GPGSA,A,3,24,07,09,26,05,,,,,,,,03.6,01.8,03.1*05 $PMGNST,02.12,3,T,534,05.0,+03327,00*40 $GPGLL,3907.3839,N,12102.4771,W,184051.812,A*2D $GPGGA,184051.81,3907.3839,N,12102.4771,W,1,05,1.8,00543,M,,,,*34 $GPRMC,184051.81,A,3907.3839,N,12102.4771,W,00.0,000.0,080301,15,E*53 $GPGSA,A,3,24,07,09,26,05,,,,,,,,03.6,01.8,03.1*05 $GPGSV,3,1,08,07,57,045,43,09,48,303,48,04,44,144,,02,39,092,*7F $GPGSV,3,2,08,24,18,178,44,26,17,230,41,05,13,292,43,08,01,147,*75 $GPGSV,3,3,08,,,,,,,,,,,,,,,,*71 $GPGLL,3907.3840,N,12102.4770,W,184052.812,A*21
- Complete cycle takes two seconds.
- RMC, GGA, GSA, and GLL are update every second.
- GSV data is swapped with MGNST data every other second.
- Time is shown to .xx and for GLL .xxx precision but the unit output is not that accurate. Data seems asynchronous and not tied to top of any particular second.
- Lat/Lon has an extra digit as compared to the Garmin G-12.
- There is a third GSV sentence that is technically not required.
- Notice that all the satellites used are shoved to the left in the GSA message.
- No geoid corrections are shown in the GGA message. This indicates that altitude is shown with respect to the ellipsoid instead of MSL.
Magellan 315 shown in simulation mode.
$GPAPB,A,A,0.0,L,N,,,1.1,M,SIM002,1.1,M,,,*21 $GPGSA,A,3,01,02,03,04,,,,,,,,,2.0,2.0,2.0*34 $GPGSV,3,1,11,01,77,103,,13,53,215,,04,47,300,,20,47,090,*76 $GPGSV,3,2,11,19,24,158,,07,21,237,,25,16,039,,24,11,315,*73 $GPGSV,3,3,11,11,08,149,,27,00,179,,30,00,354,,,,,*46 $GPGLL,5100.2111,N,00500.0006,E,104715.203,A*37 $GPGGA,104715.20,5100.2111,N,00500.0006,E,1,04,2.0,-0047,M,,,,*39 $GPRMB,A,0.00,L,SIM001,SIM002,5102.6069,N,00500.0000,E,002.4,000.,021.7,V*0D $GPRMC,104715.20,A,5100.2111,N,00500.0006,E,21.7,003.0,140801,01.,W*70 $GPAPB,A,A,0.0,L,N,,,1.1,M,SIM002,1.1,M,,,*21 $GPGSA,A,3,01,02,03,04,,,,,,,,,2.0,2.0,2.0*34
- This listing shows navigation sentences simulating a route between two locations, SIM001 and SIM002.
- GLL starts the sequence and time stamp in the GLL message shows more precision.
- Update is every 2 seconds.
- NMEA data is only transmitted in simulation mode or you have an actual fix.
Raytheon RN300 sentences:
$GPGGA,171537,3350.975,N,11823.991,W,2,07,1.1,-25.8,M,,M,1.8,,D*17 $GPGLL,3350.975,N,11823.991,W,171537,A,D*50 $GPRMC,171537,A,3350.975,N,11823.991,W,0.0,096.5,060401,013.0,E,D*07 $GPVTG,096.5,T,083.5,M,0.0,N,0.0,K,D*22 $GPGSA,A,2,04,09,07,24,02,05,26,,,,,,,1.1,*3C $GPGSV,2,1,07,04,62,120,47,09,52,292,53,07,42,044,41,24,38,179,45*7B $GPGSV,2,2,07,02,34,101,43,05,18,304,40,26,09,223,36,,,,*48 $PRAYA,6,1,122,0,0,2,36,1,1,,,,,*5A $GPDTM,W84,,0.000000,N,0.000000,E,0.0,W84*6F $GPGGA,171538,3350.974,N,11823.991,W,2,07,1.1,-25.8,M,,M,1.8,,D*19
- Complete cycle every second triggered off of GGA.
- Date is NMEA 2.3 with integrity value added.
- The proprietary raytheon sentences seems to be for WAAS SV #122.
- Note the new DTM sentences that permits conversion of NMEA datum being used to WGS84.
- The satellites are listed in an arbitrary order, stacked to the left.
NavMan 3400 (SiRF chipset sentences)
$GPGGA,230611.016,3907.3813,N,12102.4635,W,0,04,5.7,507.9,M,,,,0000*11 $GPGLL,3907.3813,N,12102.4635,W,230611.016,V*31 $GPGSA,A,1,27,08,28,13,,,,,,,,,21.7,5.7,20.9*38 $GPGSV,3,1,10,27,68,048,42,08,63,326,43,28,48,239,40,13,39,154,39*7E $GPGSV,3,2,10,31,38,069,34,10,23,282,,03,12,041,,29,09,319,*7C $GPGSV,3,3,10,23,07,325,,01,05,145,*7E $GPRMC,230611.016,V,3907.3813,N,12102.4635,W,0.14,136.40,041002,,*04 $GPVTG,136.40,T,,M,0.14,N,0.3,K*66 $GPGGA,230612.015,3907.3815,N,12102.4634,W,0,04,5.7,508.3,M,,,,0000*13
- A cycle is every second triggered off of GGA.
- The GSA, GSV sentences are only sent every 4 seconds or so. The actual sentences and rate is adjustable using proprietary NMEA commands.
- Altitude is based on the ellipsoid model and is not corrected for geoid. Note that no geoid corrections are shown in GGA.
- All headings are stated as true direction. There are no magnetic direction outputs.
- The ,0000 at the end of GGA is non standard.
- Lat/Lon has an extra digit as compared to the Garmin G-12.
- The clock is shown with millisecond precision.
- The Navman sends 10 lines of non-nmea ascii data when it is first turned on. Each line does begin with a $.
- This is a sample sentence sequence. The Navman can be programmed to send less sentences or sentences at a different rate.
- The Navman uses the SiRF chipset, see above for more data on this chipset.
- Sentences are stated to be NMEA 2.2 based on documentation.
Earhmate with SiRF chipset (firmware 2.31)
$GPGGA,120557.916,5058.7456,N,00647.0515,E,2,06,1.7,108.5,M,47.6,M,1.5,0000*7A $GPGSA,A,3,20,11,25,01,14,31,,,,,,,2.6,1.7,1.9*3B $GPGSV,2,1,08,11,74,137,45,20,58,248,43,07,27,309,00,14,23,044,36*7A $GPGSV,2,2,08,01,14,187,41,25,13,099,39,31,11,172,37,28,09,265,*71 $GPRMC,120557.916,A,5058.7456,N,00647.0515,E,0.00,82.33,220503,,*39 $GPGGA,120558.916,5058.7457,N,00647.0514,E,2,06,1.7,109.0,M,47.6,M,1.5,0000*71
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