GPS Receivers and NMEA Standards

GPS receivers receive almanac data from the satellite and also calculate their position by calculating its distance from then visible satellites and then by using triangulation method to calculate its position.

After the data has been received and position has been calculated, the data is configured according to standards set up by NMEA (National Marine Electronics Association) and is serially transmitted at a baud rate of 4800 bps.

The National Marine Electronics Association (NMEA) has developed standards that describe the interface between various marine electronic equipments. The standards allow marine electronics to send information to computers and to other marine equipments.

GPS receivers also work on these NMEA Standards. Most of the computer programs and devices which provide position and other related information expect the data to be in NMEA format.

The data given by the GPS receiver includes many information like position (latitude and longitude), altitude, speed, time etc. In its standards, NMEA has specified to send a series of data in a sentence. A particular sentence is totally self-reliant and is independent from other sentences. There are standard sentences for particular type of data and for various categories of devices. NMEA has also provided the functionality for individual companies to write their own sentences.

Fig. 1: Image showing working of GPS Receiver

All standard devices have a two letter prefix that defines the device for which it is being used, for GPS receivers the prefix is GP. The two letter prefix is then followed by three letters which represent the content of the sentence. The proprietor sentences allowed by the NMEA always start with P and are followed by a three letter sequence identifying manufacturer code and additional characters to define sentence type. For example a Garmin sentence would start with PGRM and Sony would begin with PSNY.

Every sentence begins with a ‘$’ sign, has about 80 characters and ends up with a carriage return/line feed sequence. Sentences are mostly framed in single lines (may run over to multiple lines sometimes) and the data items in each sentence are separated by commas.

The data received is just ASCII text and varies in precision. A sentence ends with checksum which consists of a ‘*’ and two hexadecimal digits. The checksum digits represent an 8 bit exclusive OR of all the characters between, but not including, the $ and *.

Fig. 2: Image showing format of NMEA Sentence

GPS units are made compatible to NMEA standards and are also compatible with serial ports using RS232 protocols. The serial configuration of a GPS receiver is summed as follows :

BAUD RATE	DATA bits	STOP bits	PARITY	HANDSHAKE
4800 bps	8	1	None	None

GPS Sentences

GPS Sentences
Some of the GPS sentences are explained below :

1) $GPGGA – Global Positioning System Fix Data

$GPGGA,132453.970,2651.0138,N,07547.7054,E,1,03,7.1,42.5,M,-42.5,M,,0000*45

where:

GGA

132453.970

2651.0138, N

07547.7054, E

7.1

42.5, M

46.9, M

Empty field

*45

Global Positioning System Fix Data

Fix taken at 13:24:53970 UTC

Latitude 26 deg 51.0138′ N

Longitude 07 deg 54.7054′ E

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)

7 = Manual input mode

8 = Simulation mode

Number of satellites being tracked

Horizontal dilution of position

Altitude, Meters, above mean sea level

Height of geoid (mean sea level) above WGS84 Ellipsoid

Time in seconds since last DGPS update

DGPS station ID number

The checksum data, always begins with *

2) $GPGSV – (Satellites in view)

GPGSV sentence shows data about the satellites that are in view of the receiver. Each GPGSV sentence can show data for maximum of four satellites, so three sentences are required represent full data. All 3 sentences need not appear in sequence as each sentence can be identified easily.

A GPGSV sentence can show more number of satellites than GPGGA sentence as it also shows the satellites which are not in solution. The SNR (Signal to Noise Ratio) can be used as raw signal strength and lies between 0 and 99. Zero being the signal strength for the satellite being viewed but not tracked.

$GPGSV,3,1,12,18,57,291,40,21,56,346,45,26,23,043,46,29,57,174,25*71  
$GPGSV,3,2,12,22,28,259,16,27,13,107,,09,11,130,,16,09,288,25*79  
$GPGSV,3,3,12,30,08,210,33,06,08,320,22,25,02,188,26,14,01,203,21*7B

where:

GSV

291

*75

Satellites in view

Number of sentences for full data

Sentence 1

Number of satellites in view

Satellite PRN(Pseudorandom Noise) number

Elevation, degrees

Azimuth, degrees

SNR (Signal to noise Ratio) – higher is better

For up to 4 satellites per sentence

The checksum data, always begins with *

1	Total number of messages of this type in this cycle
2	Message number
3	Total number of SVs in view
4	SV PRN number
5	Elevation in degrees, 90 maximum
6	Azimuth, degrees from true north, 000 to 359
7	SNR, 00-99 dB (null when SV is visible but not tracked)
8-11	Information about second SV, same as field 4-7
12-15	Information about third SV, same as field 4-7
16-19	Information about fourth SV, same as field 4-7

3) $GPRMC – Recommended minimum specific GPS/Transit data

$GPRMC,132455.970,A,2651.0145,N,07547.7051,E,0.50,342.76,301010,,,A*64

where:

RMC

132455.970

2651.0145, N

07547.7051, E

0.50

342.76

301010

Empty field (xxx.x, y)

*64

Recommended Minimum sentence C

Fix taken at 13:24:55.970 UTC

Status A=Active or V=Void.

Latitude 26 deg 51.0145′ N

Longitude 075 deg 47.7051′ E

Speed over the ground in knots

Track angle in degrees True

Date : 30th of October 2010

Magnetic Variation

The checksum data, always begins with *

4) $GPVTG –

$GPVTG,054.7,T,034.4,M,005.5,N,010.2,K*48

where:

VTG

054.7, T

034.4, M

005.5, N

010.2, K

*48

Track made good and ground speed

True track made good (degrees)

Magnetic track made good

Ground speed, knots

Ground speed, Kilometres per hour

The checksum data, always begins with *

1	Track made good
2	Fixed text ‘T’ indicates that track made good is relative to true north
3	Magnetic track made good
4	Fixed text ‘N’ indicates degrees magnetic
5	Speed over ground in knots
6	Fixed text ‘N’ indicates that speed over ground in knots
7	Speed over ground in kilometres/hour
8	Fixed text ‘K’ indicates that speed over ground is in kilometres/hour
9	Checksum

5) $GPGSA –

This sentence shows the dilution of precision (DOP) and PRN of active satellites. DOP shows the effect of satellite geometry on the accuracy of the fix. DOP is a number and for 3D fix using four satellites a DOP of 1.0 is perfect.

$GPGSA,A,2,26,21,18,,,,,,,,,,7.1,7.1,1.0*3C

where:

GSA

26, 21, 18…

7.1

1.0

*3C

Satellite status

Auto selection of 2D or 3D fix (M = manual)

2D fix

where:

1 = no fix

2 = 2D fix

3 = 3D fix

PRNs of satellites used for fix (space for 12)

PDOP (dilution of precision)

Horizontal dilution of precision (HDOP)

Vertical dilution of precision (VDOP)

The checksum data, always begins with *

Proprietary Sentences

Proprietary Sentences
NMEA allows proprietary sentences for private companies and they can be used to control information or can be used as output from GPS. Proprietary sentences always start with P followed by a three character manufacturers’ code and additional characters to determine sentence type.

Some examples of proprietary sentences are as follows :

S.No.	Company	Sentence
1	Garmin	$PGRME,15.0,M,45.0,M,25.0,M*1C
2	Magellan	$PMGNST,02.12,3,T,534,05.0,+03327,00*40
3	Rockwell International	$PRWIRID,12,01.83,12/15/97,0003,*42
4	Sony	$PSNY,0,00,05,500,06,06,06,06*14
5	Sirf	$PSRF103,05,00,01,01*20

All the GPS sentences with a brief description are tabulated below :

S.No.	Sentence	Description
1	$GPAAM	Waypoint Arrival Alarm
2	$GPALM	GPS Almanac Data
3	$GPAPA	Autopilot Sentence “A”
4	$GPAPB	Autopilot Sentence “B”
5	$GPASD	Autopilot System Data
6	$GPBEC	Bearing & Distance to Waypoint, Dead Reckoning
7	$GPBOD	Bearing, Origin to Destination
8	$GPBWC	Bearing & Distance to Waypoint, Great Circle
9	$GPBWR	Bearing & Distance to Waypoint, Rhumb Line
10	$GPBWW	Bearing, Waypoint to Waypoint
11	$GPDBT	Depth Below Transducer
12	$GPDCN	Decca Position
13	$GPDPT	Depth
14	$GPFSI	Frequency Set Information
15	$GPGGA	Global Positioning System Fix Data
16	$GPGLC	Geographic Position, Loran-C
17	$GPGLL	Geographic Position, Latitude/Longitude
18	$GPGSA	GPS DOP and Active Satellites
19	$GPGSV	GPS Satellites in View
20	$GPGXA	TRANSIT Position
21	$GPHDG	Heading, Deviation & Variation
22	$GPHDT	Heading, True
23	$GPHSC	Heading Steering Command
24	$GPLCD	Loran-C Signal Data
25	$GPMTA	Air Temperature (to be phased out)
26	$GPMTW	Water Temperature
27	$GPMWD	Wind Direction
28	$GPMWV	Wind Speed and Angle
29	$GPTTM	Tracked Target Message
30	$GPVBW	Dual Ground/Water Speed
31	$GPVDR	Set and Drift
32	$GPROO	Waypoint active route (not standard)
33	$GPOLN	Omega Lane Numbers
34	$GPVHW	Water Speed and Heading
35	$GPVLW	Distance Travelled through the Water
36	$GPVPW	Speed, Measured Parallel to Wind
37	$GPVTG	Track Made Good and Ground Speed
38	$GPWCV	Waypoint Closure Velocity
39	$GPWNC	Distance, Waypoint to Waypoint
40	$GPTRF	Transit Fix Data
41	$GPSTN	Multiple Data ID
42	$GPRMA	Recommended Minimum Specific Loran-C Data
43	$GPRMB	Recommended Minimum Navigation Information
44	$GPRMC	Recommended Minimum Specific GPS/TRANSIT Data
45	$GPROT	Routes
46	$GPSFI	Scanning Frequency Information
47	$GPOSD	Own Ship Data
48	$GPROT	Rate of Turn
49	$GPRPM	Revolutions
50	$GPRSA	Rudder Sensor Angle
51	$GPRSD	RADAR System Data
52	$GPWPL	Waypoint Location
53	$GPXDR	Transducer Measurements
54	$GPXTE	Cross-Track Error, Measured
55	$GPXTR	Cross-Track Error, Dead Reckoning
56	$GPZDA	Time & Date
57	$GPZFO	UTC & Time from Origin Waypoint
58	$GPZTG	UTC & Time to Destination Waypoint