(National Marine Electronics Association - Frequently Asked Questions)

Version 6.5  Mar 10, 2006
Additions, corrections, and comments should be emailed to the author,
Peter Bennett  peterbb4@interchange.ubc.ca
1.      What is NMEA?
1.1     What is an NMEA Standard
1.2     NMEA Address
2.      Electrical Interface
3.      NMEA-0180 and NMEA-0182
3.1     Simple Format
3.2     Complex Format
4.      NMEA-0183
4.1     General Sentence Format
4.2     Sentences sent by specific equipment
4.3     Sample Sentences Dissected
4.3.1   Standard Sentences
4.3.2   Garmin Proprietary Sentences
5.      RS-232 connections
6.      Troubleshooting
7.      About the author
7.1    Acknowledgements
1.      What is NMEA?
The National Marine Electronics Association is dedicated to the
education and advancement of the marine electronics industry and
the market which it serves.
It is a non-profit association composed of manufacturers,
distributors, dealers, educational institutions, and others
interested in peripheral marine electronics occupations
(quoted from a promo in "NMEA News")
1.1   What is an NMEA standard?
For the purposes of this article, an NMEA standard defines an
electrical interface and data protocol for communications
between marine instrumentation. (They may also have standards
for other things.)
1.2   NMEA Address
7 Riggs Avenue
Severna Park, MD 21146
Beth Kahr
Phone: 410-975-9425
email: director@nmea.org
Visit us at www.nmea.org
2.      Electrical Interface
These standards allow a single "talker", and several "listeners"
on one circuit.  The recommended interconnect wiring is a
shielded twisted pair, with the shield grounded only at the
talker.  The standards do not specify the use of any particular
The NMEA-0180 and 0182 standards say that the talker output may
be RS-232, or from a TTL buffer, capable of delivering 10 mA at
4 V.  A sample circuit shows an open collector TTL buffer with a
680 ohm resistor to +12 V, and a diode to prevent the output
voltage from rising above +5.7 V.
NMEA-0183 accepts this, but recommends that the talker output
comply with EIA-422.  This is a differential system, having two
signal lines, A and B. The voltages on the "A" line correspond
to those on the older TTL single wire, while the "B" voltages
are reversed (while "A" is at +5, "B" is at ground, and vice
In either case, the recommended receive circuit uses an
opto-isolator with suitable protection circuitry.  The input
should be isolated from the receiver's ground.
In practice, the single wire, or the EIA-422 "A" wire may be
directly connected to a computer's RS-232 input.
3.      NMEA-0180 and NMEA 0182
NMEA-0180 and 0182 are very limited, and just deal with
communcations from a Loran-C (or other navigation receiver,
although the standards specifically mention Loran), and an
From the information I have, it appears that 0180 and 0182 are
identical.  I suspect that equipment claiming to use NMEA-0180
will use the "simple" format described below, while those using
NMEA-0182 will use the "complex" format. (but this is really
just a guess... corrections??)
3.1   "Simple" data format
The simple format consists of a single data byte transmitted at
intervals of 0.8 to 5 seconds, at 1200 baud with odd parity.
Bits 5 - 0 give the cross-track error in units of 0.1 uS or 0.01
nautical mile.  The error is given in offset binary, with a
count of 1 representing full scale right error, 32 (hex 20) for
on course, and 63 (hex 3f) full scale left error.  Bit 6 is a 1
if the data is valid, and bit 7 is 0 to indicate the simple
data format.
3.2   "Complex" data format
The complex format consists of a data block of 37 bytes of
(mostly) readable ASCII text giving cross-track error, bearing
to waypoint, present Lat/Long, and a binary status byte.  The
data block shall be sent at intervals of 2 to 8 sec. All bytes
in the complex format have bit 7 = 1 to distinguish them from
the simple format.  It is permissible for a sending device to
send both simple and complex data, and even to send a "simple"
data byte in the middle of a "complex" data block.
Byte        Data
1          $
2          M                  | device
3          P                  | address
4          K = kilometres     | cross track
N = nautical miles | error
U = microseconds   | units
5 - 8      0 - 9 or .          cross track error value
9          L or R              cross track error position
10          T or M              True or Magnetic bearing
11 - 13     0 - 9               bearing to next waypoint
14 - 23     12D34'56"N or       present latitude
24 - 34     123D45'56"W or      present longitude
35          non-ASCII status byte
bit 0 = 1 for manual cycle lock
1 = 1     low SNR
2 = 1     cycle jump
3 = 1     blink
4 = 1     arrival alarm
5 = 1     discontinuity of TDs
6 = 1 always
36         "NUL" character (hex 80)(reserved status byte)
37         "ETX" character (hex 83)
Any unavailable data is filled with "NUL" bytes.
4.      NMEA-0183
4.1     General Sentence Format
Under the NMEA-0183 standard, all characters used are printable
ASCII text (plus carriage return and line feed).  NMEA-0183 data
is sent at 4800 baud.
The data is transmitted in the form of "sentences".  Each
sentence starts with a "$", a two letter "talker ID", a three
letter "sentence ID", followed by a number of data fields
separated by commas, and terminated by an optional checksum, and
a carriage return/line feed.  A sentence may contain up to 82
characters including the "$" and CR/LF.
If data for a field is not available, the field is simply
omitted, but the commas that would delimit it are still sent,
with no space between them.
Since some fields are variable width, or may be omitted as
above, the receiver should locate desired data fields by
counting commas, rather than by character position within the
The optional checksum field consists of a "*" and two hex digits
representing the exclusive OR of all characters between, but not
including, the "$" and "*".  A checksum is required on some
The standard allows individual manufacturers to define
proprietary sentence formats.  These sentences start with "$P",
then a 3 letter manufacturer ID, followed by whatever data the
manufacturer wishes, following the general format of the
standard sentences.
Some common talker IDs are:
GP      Global Positioning System receiver
LC      Loran-C receiver
OM      Omega Navigation receiver
II      Integrated Instrumentation
(eg. AutoHelm Seatalk system)
4.2   Sentences sent by specific equipment
This section lists the sentence types used by various equipment.
The format and data included in each sentence type is given in
section 4.3.
Eagle AccuNav
Standard: RMB, RMC, GLL, APB
Proprietary: PSLIB
It also pretends it's a Loran, sending LCGLL, as well as GPGLL
Garmin 12XL, NMEA-0183 V 1.5
Standard: RMB, RMC, WPL
Proprietary: PGRMM (map datum), PGRMZ (altitude), PSLIB (DGPS ctrl)
Garmin 12XL, NMEA-0183 V 2.0
Proprietary: PGRME (estimated error) PGRMM, PSLIB
Garmin GPS-38, NMEA-0183 V. 1.5 mode
Proprietary: PGRMM (map datum), PGRMZ (altitude), PSLIB (dgps ctrl)
Garmin GPS-38, NMEA-0183 V. 2.0 mode
Proprietary: PGRME (estimated error), PGRMM, PGRMZ, PSLIB
Garmin GPS-45 (and probably GPS-40 and GPS-90)
Proprietary: PGRME, PGRMM, PGRMZ
Garmin GPS-65 (and probably GPS-75)
Standard: BWC, GLL, RMB, RMC, R00, WPL, XTE, VTG
Proprietary: PGRMM, PGRMZ, PSLIB
Lowrance Global Map 100
Standard: GLL, RMC, RMB, APB, GGA, GSV
Proprietary: PSLIB
Magellan Trailblazer
Trimble Ensign XL
Standard: APA, BWC, BWR, GGA, GLL, RMB
Trimble Flightmate Pro and Scoutmaster
Autohelm Seatalk
Autohelm Seatalk is a proprietary bus for communications
between various intruments.  Some of the instruments can act
as NMEA-0183 talkers or listeners.  Data received from an
external NMEA-0183 device will, if Seatalk understands the
sentence, be re-transmitted, but not necessarily in the same
sentence type.
The specific sentences sent will depend on the data
available on the Seatalk bus (i.e. sentences containing wind
speed and direction will only be sent if the system includes
a wind instrument)  Note that NMEA data can only be sent to,
or received from, a SeaTalk system using AutoHelm's
NMEA<->SeaTalk interface box, or those instruments that provide
an NMEA-0183 interface.  SeaTalk itself is not compatible
with NMEA, and cannot be read with a normal PC serial port.
Seatalk output:
Seatalk input:
4.3   Sample Sentences Dissected
4.3.1  Standard Sentences
A talker typically sends a group of sentences at intervals
determined by the unit's update rate, but generally not more
often than once per second.
Characters following the "*" are a checksum.  Checksums are
optional for most sentences, according to the standard.
APB - Autopilot format B
A            Loran-C blink/SNR 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
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 be given in True as 033,T)
(note: some pilots, Roberston in particular, misinterpret "bearing
from origin to destination" as "bearing from present position to
destination".  This apparently results in poor performance if the
boat is sufficiently off-course that the two bearings are
BOD - Bearing - origin to destination waypoint
045.,T       bearing 045 True from "START" to "DEST"
023.,M       breaing 023 Magnetic from "START" to "DEST"
DEST         destination waypoint ID
START        origin waypoint ID
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
BWR - Bearing and distance to waypoint - rhumb line
(format same as BWC)
DBT - Depth below transducer
0017.6,f     17.6 feet
0005.4,M     5.4 Metres
GGA - Global Positioning System Fix Data
GGA,123519,4807.038,N,01131.324,E,1,08,0.9,545.4,M,46.9,M, , *42
123519       Fix taken at 12:35:19 UTC
4807.038,N   Latitude 48 deg 07.038' N
01131.324,E  Longitude 11 deg 31.324' E
1            Fix quality: 0 = invalid
1 = GPS fix
2 = DGPS fix
08           Number of satellites being tracked
0.9          Horizontal dilution of position
545.4,M      Altitude, Metres, above mean sea level
46.9,M       Height of geoid (mean sea level) above WGS84
(empty field) time in seconds since last DGPS update
(empty field) DGPS station ID number
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 valid
(Garmin 65 does not include time and status)
GSA - GPS DOP and active satellites
A            Auto selection of 2D or 3D fix (M = manual)
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)
DOP is an indication of the effect of satellite geometry on
the accuracy of the fix.
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           Signal strength - higher is better
<repeat for up to 4 satellites per sentence>
There my be up to three GSV sentences in a data packet
HDM - Heading, Magnetic
HDM          Heading, Magnetic
235.,M       Heading 235 deg. Magnetic
(HDG, which includes deviation and variation, is recommended
HSC - Command heading to steer
258.,T       258 deg. True
236.,M       136 deg. Magnetic
MTW - Water temperature, Celcius
11.,C        11 deg. C
R00 - List of waypoint IDs in currently active route
(This sentence is produced by a Garmin 65, but is not listed
in Version 2.0 of the standard.  The standard lists RTE for
this purpose.)
RMB - Recommended minimum navigation information (sent by nav.
receiver when a destination waypoint is active)
A            Data status A = OK, V = warning
0.66,L       Cross-track error (nautical miles, 9.9 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
052.5        True bearing to destination
000.5        Velocity towards destination, knots
V            Arrival alarm  A = arrived, V = not arrived
*0B          mandatory checksum
RMC - Recommended minimum specific GPS/Transit data
225446       Time of fix 22:54:46 UTC
A            Navigation receiver warning A = OK, V = warning
4916.45,N    Latitude 49 deg. 16.45 min North
12311.12,W   Longitude 123 deg. 11.12 min West
000.5        Speed over ground, Knots
054.7        Course Made Good, True
191194       Date of fix  19 November 1994
020.3,E      Magnetic variation 20.3 deg East
*68          mandatory checksum
RTE - Waypoints in active route
2            two sentences for full data
1            this is sentence 1 of 2
c            c = complete list of waypoints in this route
w = first listed waypoint is start of current leg
0            Route identifier
W3IWI...     Waypoint identifiers
VHW - Water speed and heading
259.,T       Heading 259 deg. True
237.,M       Heading 237 deg. Magnetic
05.00,N      Speed 5 knots through the water
09.26,K      Speed 9.26 KPH
VWR - Relative wind direction and speed
148.,L       Wind from 148 deg Left of bow
02.4,N       Speed 2.4 Knots
01.2,M       1.2 Metres/Sec
04.4,K       Speed 4.4 Kilometers/Hr
VTG - Track made good and ground speed
054.7,T      True track made good
034.4,M      Magnetic track made good
005.5,N      Ground speed, knots
010.2,K      Ground speed, Kilometers per hour
WCV - Waypoint Closure Velocity
WDC - Distance to Waypoint
WDR - Waypoint Distance, Rhumb Line
WPL - waypoint location
4917.16,N    Latitude of waypoint
12310.64,W   Longitude of waypoint
003          Waypoint ID
When a route is active, this sentence is sent once for each
waypoint in the route, in sequence. When all waypoints have
been reported, GPR00 is sent in the next data set. In any
group of sentences, only one WPL sentence, or an R00
sentence, will be sent.
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
XTR - Cross-Track Error - Dead Reckoning
0.67         cross track error distance
L            Steer left to correct error (or R for right)
N            Distance units - Nautical miles
4.3.2  Proprietary Sentences
The following are Garmin proprietary sentences.  "P" denotes
proprietary, "GRM" is Garmin's manufacturer code, and "M" or "Z"
indicates the specific sentence type.
15.0,M       Estimated horizontal position error in metres (HPE)
45.0,M       Estimated vertical error (VPE) in metres
25.0,M       Overall spherical equivalent position error
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.
$PGRMM,NAD27 Canada*2F
Currently active horizontal datum
Proprietary sentences to control a Starlink differential beacon
receiver. (I assume Garmin's DBR is made by Starlink)
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
blank = tuning message
When the GPS receiver is set to change the DBR frequency or
baud rate, the "J" sentence is replaced (just once) by (for
example): $PSLIB,320.0,200*59 to set the DBR to 320 KHz, 200
5.      RS-232 connections
Although this is not really related to NMEA, many people want to
connect a GPS to a computer, so need to know about the RS-232
serial ports on a computer.
The RS-232 standard defines two classes of devices that may
communicate using RS-232 serial data - Data Terminal Equipment
(DTE), and Data Communication Equipment (DCE).  Computers and
terminals are considered DTE, while modems are DCE.  The
standard defines pinouts for DTE and DCE such that a "straight
through" cable (pin 2 to pin 2, 3 to 3, etc) can be used between
a DTE and DCE.  To connect two DTEs together, you need a "null
modem" cable, that swaps pins between the two ends (eg. pin 2 to
3, 3 to 2).  Unfortunately, there is sometimes disagreement
whether a certain device is DTE or DCE, hence my standard RS-232
if it doesn't work, swap pins 2 and 3!
The standard RS-232 connector is a 25 conductor DB-25, although
many PCs (and some other equipment) now use a 9 pin DE-9 (often
incorrectly called DB-9)
Serial Port Connections
Computer (DTE)                              Modem
DB-25   DE-9   Signal          Direction    DB-25
2      3    Tx Data             ->          2
3      2    Rx Data             <-          3
4      7    Request to send     ->          4
5      8    Clear to send       <-          5
6      6    Data Set Ready      <-          6
7      5    signal ground                   7
8      1    Data CarrierDetect  <-          8
20      4    Data Terminal Ready ->          20
22      9    Ring Indicator      <-          22
For NMEA-0183 interfacing, we are only concerned with Rx Data,
signal ground (and possibly Tx Data, if we want the computer to
talk to the GPS)
NMEA-0183 data is sent at 4800 baud.
6.  Troubleshooting
First check that the talker (usually GPS or Loran) can send
NMEA-0183, and determine what sentences it sends.  Also, verify
that the listener understands NMEA-0183, and that it understands
the sentences the talker is sending.  In some cases the same
information may be sent in two or more different sentences.  If
the talker and listener don't both use the same sentences, there
will be no communication.  It may be possible to change the
sentences sent by the talker, to match those understood by the
Next, check that the talker is indeed set to send NMEA-0183
data.  Some talkers may have provision to send NMEA-0180 or
0182, or some proprietary format.
A computer, using any convenient terminal program (Telix,
Procomm, Windows Terminal, etc.) set to 4800 baud, can be used
to monitor the NMEA data, and confirm what sentences are sent,
and that the data is in the correct format.
Verify that the wiring is correct - that the talker data output
is connected to the listener data input, and that a signal
ground line is connected between the two pieces of equipment.
If you have multiple listeners connected to a single talker, you
may be overloading the talker port.  Try connecting only one
listener at a time.
On any NMEA-0183 circuit, there can _only_ be one talker.  If
you must have more than one talker, and one of the talker
devices can also act as a listener, you may be able to connect
things "in series", so a talker-only output is connected to a
listener/talker input, and the listener/talker output is
connected to other listeners.  However, some listener/talker
devices may reformat the data, or only pass data they
understand. (The Autohelm Seatalk system does this, and claims
the data as it's own, starting all output sentences with "$II".)
Particularly with older equipment, the equipment may claim to
comply with NMEA-0183, but in fact have an error in the data
format.  (My Kings 8001 Loran-C claims to send an APB sentence,
but gets some of the fields in the wrong order, so my autopilot
can't understand it.)  This sort of problem can be verified by
capturing the NMEA-0183 data on a computer, and comparing the
data formats with those given above.
7.  About the author
This FAQ was written by:
Peter Bennett
I have an Web site containing this file, a GPS FAQ, and other
NMEA information files and PC programs for capturing and
displaying NMEA data, and related things: