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    NMEA - Norma 0183


NMEA 0183

In 1980, a group of professionals from the industry met to develop a standard "language" for marine interfaces. The result was the National Marine Electronics Association (NMEA) 0180 standard. It addressed one problem, that of making lorans and autopilots work together, and it was a success. In the following years this standard, revised, broadened, then totally revamped to take into account the wide range of electronics appearing on boats, led to the current NMEA 0183 standard used on almost all equipment today. The NMEA 0183 standard calls for data communication in the form of coded "sentences." Each sentence begins with the character "$" and ends with a carriage return and line feed (). These last two characters are "control" characters and are not normally printed (for this reason they are customarily shown enclosed in brackets). Between the beginning and end of each sentence are "fields" of data, a comma separating each field. The first field in any sentence (field 0) begins with the two letter talker mnemonic code ("talkers" are devices that send out information, "listeners" take it in) followed by the three letter code for the sentence. Data follows in the standard format for that sentence. Here's an example:

field #: 0 1 2 3 4

sentence: $LCGLL,4001.74,N,07409.43,W<CR><LF>

What is being said here? Let's look at it field by field. The sentence begins with the start character "$." Next comes the talker identifier and sentence format code. LC stands for Loran-C, GLL for present position in Lat/Lon. Field 1 contains the set's current latitude. Field 2 is either N or S for North or South. Field 3 is the set's current longitude. Field 4 is W or E for West or East. Thus, this sentence reads "Loran-C present position in Lat/Lon: 40 degrees 1.74 minutes North, 74 degrees 9.43 minutes West." A carriage return and line feed close the sentence. The GLL sentence is always displayed in this format. Each type of sentence, and there are many, has its own specific standard format (for a brief catalog of formats, see sidebar: Deciphering the Code). Standard sentences, each in a standard format. This is how NMEA 0183 is intended to work. But compliance with the standard is voluntary. It is up to the manufacturers to decide how they will implement the standard, and to what extent they will comply with it. Because the standard is so broad, there is often more than one way to express the same information, and this can cause problems. There are many different sentences, and few if any talker devices say them all, or adhere strictly to the standard format of those they do. Does your autopilot need information in a form that your loran isn't sending out? How can you tell? Don't count on there being an adequate explanation in the manuals, because often it isn't there. The owners manual for my loran describes the workings of the interface this way. "The extra connector on the rear panel is a serial data output interface which can provide loran data to an autopilot, track plotter, or fishfinder. The serial data is in NMEA 0183 format and is transmitted continuously." That's it; the manual makes no other mention of the interface anywhere. Other manuals I have seen are about as descriptive. In a follow-up bulletin nearly a year after I bought it, the manufacturer of my loran mentioned what sentences the data output sent (there were four) and what the pin assignments for the output connector were. This section was labeled "for techies only." These interfaces are touted in advertising copy, but it's clear the manufacturers don't really expect you to use them. In the end, you probably will have to call the manufacturer for a proper explanation of their interface, and ask specifically whether they have tested the devices you have in mind for compatibility. So far we've discussed only software incompatibilities, but there can be physical ones, too. NMEA 0183 specifies no standard connector, and no two manufacturers seem to use the same one. Usually, the owner will have to buy plugs from the manufacturers and make his own cable.


NMEA 0183 STANDARD NMEA-0183 is a standard for interfacing marine electronic devices. This standard defines the electrical signal requirements, data transmission protocol, data transmission timing, and specific message formats for a 4800 baud serial data bus. Each bus may have only a single talker, but may have multiple listeners. The latest version of the document describing this standard may be obtained by contacting:

NMEA Executive Director

PO Box 50040

Mobile, AL 36605

TEL: (205)473-1783

Fax: (205)473-1669


The following summarizes the NMEA 0183 Standard for use by the general programmer.

NMEA 0183 devices employ an asynchronous serial interface with the following parameters:

Baud Rate 4800

Data Bits 8(d7=0)

Parity None

Stop Bits One(or more)

Devices are designated as either talkers or listeners with some devices being both. A talker is any device which sends data to other NMEA 0183 devices. A listener is any device which receives data from other NMEA 0183 devices. All data either transmitted or received is interpreted as 8 bit ASCII (d7=0).



Under the latest version of the standard, NMEA 0183 talkers are supposed to be differential drive compatible with EIA RS-422. Differential drive signals have no reference to Ground and are more immune to noise. Earlier versions of the NMEA 0183 standard allowed single-ended drive circuitry for talker(s) (i.e., 0 to +15VDC). Therefore, there are still instruments around which are not differential drive output. A single talker can drive multiple listeners within drive current limitations.


NMEA 0183 listeners are supposed to have input isolation from the ships Ground. Opto-isolation, which is generally used to meet this requirement, limits the input impedance. The standard specifies a minimum input resistance for listeners of 500 ohms and most devices will probably be close to this value.


The general NMEA 0183 message format is shown below:


All NMEA 0183 messages start with a "$" and are terminated by a [Carriage Return][Line Feed]. The five characters immediately after the "$" are the address field. The address field is interpreted based on the type of sentence (talker, query or proprietary). Multiple data fields follow the address field and are delimited by commas.

There are three basic sentence structures defined by the standard;

talker sentence

query sentence

proprietary sentence

talker Sentence

The general format for a talker (instrument) message is:


The address field of a talker sentence contains a five character string immediately following the "$" sign. The first two characters are the talker identifier while the next three are the sentence identifier. The sentence identifier defines the remaining data fields. Under the NMEA 0183 Standard, the data fields are uniquely defined for each sentence type.

An example talker sentence is:


where the "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.

query Sentence

A query sentence is a means for a listener to request a particular sentence from a talker. For example, a query message might be sent to a GPS receiver to request "DISTANCE-TO-WAYPOINT" information be transmitted.

The general format of a query sentence is:


The first two characters of the address field are termed the talker identifier of the requester and the next two characters are the talker identifier of the device being queried (listener). 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:


where the "CC" device (computer) is requesting from the "GP" device (GPS) the "GGA" sentence. The GPS will then transmit this sentence once per second until a different query is requested.

Proprietary Sentence

The proprietary sentence is a means for manufacturers to use special sentences which are not pre-defined by the standard.

The general format of a proprietary message is


Where the "P" indicates that it is a proprietary message and that the data fields which follow do not necessarily correspond to any approved sentence structure. The "mmm" is defined as the manufacturers message code. The fifth character of the address field is a letter (A-Z) which defines the specific message type.


The NMEA 0183 Standard allows for an optional checksum. The checksum is preceeded by the asterisk (*) character placed after the last data field. It is a two-character field equal to the hex value obtained by XOR'ing all the character bytes of the sentence.


The "language" of NMEA 0183 is extensive but not difficult to understand. The ground rules are these: maximum sentence length is 80 characters including the starting "$," the terminating , and everything in between. Minimum number of data fields is two, including the address field (talker identifier and sentence format). If data for any field is unavailable, a "null" field, two commas with nothing between them (",,") can be sent instead. Here is a sampling of two letter talker identifier codes. There are others, for everything from satellite communications to atomic clocks, but these are some of the most familiar to the recreational sailor.

LC Loran-C



AP Autopilot (magnetic)

HC Magnetic heading compass

RA Radar

SD Depth sounder

VW Mechanical speed log

Now for some of the more common sentences that might be spoken by your LORAN, SATNAV, or GPS. This is not a complete listing, but should illustrate how the system works. In the following examples, the character "#" will denote some number (0-9). Remember that each of these sentences would be preceded by the start character "$" and the two letter talker ID. Each also would be followed by the sentence terminator, <CR><LF>.


Version 6.3 April 25, 2000

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.)

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 connector.

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 versa) 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 autopilot.
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 sentence.
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 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.

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)

4.3 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 different.)

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 ellipsoid (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 baud.

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 disclaimer:

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.


(Address Characters 1 and 2)



Magnetic AP

COMMUNICATIONS: Digital Selective Calling (DSC) *CD

Satellite *CS

Radio-Telephone (MF/HF) *CT

Radio-Telephone (VHF) *CV

Scanning Receiver *CX

DECCA Navigation DE

Direction Finder *DF

Electronic Chart Display & Information System (ECDIS) EC

Emergancy Position Indicating Beacon (EPIRB) *EP

Engineroom Monitoring Systems ER

Global Positioning System (GPS) GP

HEADING SENSORS: Compass, Magnetic *HC

Gyro, North Seeking *HE

Gyro, Non-North Seeking HN

Integrated Instrumentation II

Integrated Navigation IN


Loran-C LC

OMEGA Navigation System OM

Proprietary Code P

Radar and/or ARPA *RA

Sounder, depth *SD

Electronic positioning system, other/general TR

Sounder, scanning SS

Turn Rate Indicator *TI

TRANSIT Navigation System TR

VELOCITY SENSORS: Doppler, other/general *VD

Speed Log, Water, Magnetic VM

Speed Log, Water, Mechanical VW



Chronometer ZC

Quartz ZQ

Radio Update, WWV or WWVH ZV

Weather Instruments WI


* Designated by I.E.C. for use with I.M.O. marine electronic devices. This is the minimum requirement for equipment that is specified by I.M.O. to meet S.O.L.A.S. regulations.

6.2 Field Definitions



AAM - Waypoint Arrival Alarm............................... 22

ALM - GPS Almanac Data..................................... 22

APB - Autopilot Sentence "B"............................... 23

*ASD - Autopilot System Data................................ 23

BEC - Bearing & Distance to Waypoint, Dead Reckoning....... 23

BOD - Bearing, Origin to Destination....................... 24

BWC - Bearing & Distance to Waypoint, Great Circle......... 24

BWR - Bearing & Distance to Waypoint, Rhumb Line........... 24

BWW - Bearing, Waypoint to Waypoint........................ 24

DBT - Depth Below Transducer............................... 24

DCN - Decca Position....................................... 25

*DPT - Depth................................................ 25

*FSI - Frequency Set Information............................ 25

GGA - Global Positioning System Fix Data................... 26

GLC - Geographic Position, Loran-C......................... 26

GLL - Geographic Position, Latitude/Longitude.............. 27

GSA - GPS DOP and Active Satellites........................ 27

GSV - GPS Satellites in View............................... 27

GXA - TRANSIT Position..................................... 28

*HDG - Heading, Deviation & Variation....................... 28

*HDT - Heading, True........................................ 28

HSC - Heading Steering Command............................. 28

LCD - Loran-C Signal Data.................................. 29

MTW - Water Temperature.................................... 29

*MWV - Wind Speed and Angle................................. 29

OLN - Omega Lane Numbers................................... 29

*OSD - Own Ship Data........................................ 30

RMA - Recommend Minimum Specific Loran-C Data.............. 30

RMB - Recommend Minimum Navigation Information............. 31

RMC - Recommend Minimum Specific GPS/TRANSIT Data.......... 31

*ROT - Rate of Turn......................................... 32

*RPM - Revolutions.......................................... 32

*RSA - Rudder Sensor Angle.................................. 32

*RSD - RADAR System Data.................................... 32

RTE - Routes............................................... 33

*SFI - Scanning Frequency Information....................... 33

STN - Multiple Data ID..................................... 34

TRF - TRANSIT Fix Data..................................... 34

*TTM - Tracked Target Message............................... 35

*VBW - Dual Ground/Water Speed.............................. 35

VDR - Set and Drift........................................ 35

VHW - Water Speed and Heading.............................. 35

VLW - Distance Traveled through the Water.................. 36

VPW - Speed, Measured Parallel to Wind..................... 36

VTG - Track Made Good and Ground Speed..................... 36

WCV - Waypoint Closure Velocity............................ 36

WNC - Distance, Waypoint to Waypoint....................... 36

WPL - Waypoint Loacation................................... 36

XDR - Transducer Measurements.............................. 37

XTE - Cross-Track Error, Measured.......................... 37

XTR - Cross-Track Error, Dead Reckoning.................... 38

ZDA - Time & Date.......................................... 38

ZFO - UTC & Time from Origin Waypoint...................... 38

ZTG - UTC & Time to Destination Waypoint................... 38

AAM - Waypoint Arrival Alarm Status of arrival (entering the arrival circle, or passing the perpendicular of the course line) at waypoint c--c.


| | | | |

| | | | +---------------Waypoint ID

| | | +------------------Units of radius, nautical miles

| | +---------------------Arrival circle radius

| +------------------------Status: A = perpendicular passed at waypoint

+--------------------------Status: A = arrival circled entered

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.


| | | | | |

| | | | | +-------e, eccentricity [3]

| | | | +-----------SV health, bits 17-24 of each almanac page [2]

| | | +--------------GPS week number [1]

| | +------------------Satellite PRN number, 01 to 32

| +---------------------Message number

+-------------------------Total number of messages



| | | | |

| | | | +--------Omega, argument of perigee [3]

| | | +---------------SQRT(A), root of semi-major axis [3]

| | +---------------------OMEGADOT, rate of right ascension [3]

| +--------------------------(sigma) index i, inclination angle [3]

+------------------------------t index OA, almanac reference time [3]



| | | |

| | | +-----------a index f1, clock parameter [3]

| | +---------------a index f0, clock parameter [3]

| +---------------------M index O , mean anomaly [3]

+----------------------------(OMEGA) index O, longitude of ascension node[3]

[1] Variable lengh integer, 4-digits maximum. Converted from (10) most significant binary bits of Subframe 1, Word 3. Reference Table 20-I, ICD-GPS-200, Rev. B.

[2] Reference paragraph, Table 20-VII and Table 20-VIII, ICD- GPS-200, Rev. B.

[3] Reference Table 20-VI, ICD-GPS-200, Rev. B for scaling factors and units.

APB - Autopilot Sentence "B"

Commonly used by autopilots this sentence contains navigation receiver warning flag status, cross-track-error, waypoint arrival status, initial bearing from origin waypoint to the destina tion, continuous bearing from present position to destination and recommended heading-to-steer to destination waypoint for the active navigation leg of the journey.


| | | | | | | | | |

| | | | | | | | | +---Destination waypoint ID

| | | | | | | | +-----\M/T Magnetic or True

| | | | | | | +--------/Bearing origin to destination

| | | | | | +------------Status: A = perpendicular passed at waypoint

| | | | | +--------------Status: A = arrival circle entered

| | | | +----------------XTE units, nautical miles

| | | +------------------L/R Direction to steer

| | +---------------------Magnitude of XTE (cross-track-error)

| +------------------------Status: V = Loran-C Cycle Lock warning flag

| A = OK or not used

+--------------------------Status: V = Loran-C Blink or SNR warning

V = general warning flag for other navigation systems when a reliable fix is not available



| | | |

| | | +-------------------\M/T Magnetic or True

| | +----------------------/Heading-to-steer to destination waypoint

| +-------------------------\M/T Magnetic or True

+----------------------------/Bearing, Present position to destination


*ASD - Autopilot System Data

I.M.O. Ref. A342 (IX). Autopilot operating parameters, alarm status commanded course and vessel heading.


BEC - Bearing & Distance to Waypoint - Dead Reckoning Time (UTC) and distance & bearing to, and loacation of, a specified waypoint from the dead-reckoned present position.


| | |

| | +-------\N/S North or South

| +--------------/Waypoint Latitude

+------------------------UTC of observation



| | | | | | | |

| | | | | | | +--\nautical miles

| | | | | | +-----/Distance

| | | | | +--------\degrees Magnetic

| | | | +-----------/Bearing

| | | +--------------\E/W East or West

| | +-----------------/Waypoint longitude

| +--------------------\N/S North or South

+---------------------------/Waypoint latitude




+-----------------------------Waypoint ID


BOD - Bearing - Origin to Destination 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.


| | | | | |

| | | | | +--------Origin waypoint ID

| | | | +-------------Destination waypoint ID

| | | +---------------\degrees Magnetic

| | +------------------/Bearing

| +---------------------\degrees True




BWC - Bearing & Distance to Waypoint - Great Circle

BWR - Bearing & Distance to Waypoint - Rhumb Line

Time (UTC) and distance & bearing to, and location of, a specified waypoint from present position. '$--BWR' data is calculated along the rhumb line from present position rather than along the great circle path.


| | |

| | +-------\N/S North or South

| +--------------/Waypoint latitude

+------------------------UTC of observation



| | | | | | | |

| | | | | | | +--\nautical miles

| | | | | | +-----/Distance

| | | | | +--------\degrees Magnetic

| | | | +-----------/Bearing

| | | +--------------\degrees True

| | +-----------------/Bearing

| +--------------------\E/W East or West




+-----------------------------Waypoint ID




| | |

| | +-------\N/S North or South

| +--------------/Waypoint latitude

+------------------------UTC of observation



| | | | | | | |

| | | | | | | +--\nautical miles

| | | | | | +-----/Distance

| | | | | +--------\degrees Magnetic

| | | | +-----------/Bearing

| | | +--------------\degrees True

| | +-----------------/Bearing

| +--------------------\E/W East or West




+-----------------------------Waypoint ID



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