understanding gps & nmea messages and algo to extract information from nmea

Hand to hand with GPS

Episode: 2

NMEA Messages

As we all know, what is GPS?

GPS is a system, widely used throughout the globe. It recites versatile superfluous

information including position, speed, time, altitude very precisely. Its having

diverse applications like navigation, tracking, tracing, location, timing and very soon

it will be very essential indigence for human being as same as the telephone.

Now briefly we see the constituents of GPS. It uses between 24 and 32 Medium

Earth Orbit satellites that transmit precise microwave signals. A GPS receiver

receives that microwave signals to determine the above mentioned information. It

was developed by United States for defence purpose, but now it can be accessed by

everyone freely.

The important questions are, how can we use GPS? , what information we can

get? So now a days it is very easy to use GPS, many of the electronic gazettes around

us are loaded with GPS and these are having very easy to use graphical user

interface. Tracking, navigation, mapping can be done with the help of these gazettes.

But to make customize systems that use GPS as per our requirement, we just require

a GPS receiver.

A GPS receiver is a chip that receives microwave signals from satellites, converts

that signals to a particular message protocol like NMEA and gives serial output. We

can use that serial output in our system. The canonical hassle is to understand the

protocol of NMEA. We are going to describe an algorithm that takes serial input,

analyze it and recites the superfluous information.

The algorithm is very easygoing, can be implemented to any programming

language, any embedded system’s compiler like Keil, AVR studio, winAVR, Keil for

ARM etc.

What is NMEA message format?, National Marine Electronics Association (NMEA)

is an organisation that is creating standards for marine electronics to unify the

systems. In GPS’s context NMEA standards are the communication protocol,

followed by GPS receiver to give serial output.

Fig 1 | Understanding NMEA messages.

The diagram shows that a GPS receiver receives microwave signals from

satellite system and gives output in NMEA messages. NMEA messages are

basically packets of data or information. GPS receiver delivers information

line by line, here we refer these lines as the packets of information. These

messages are classified in various types of packets. Each packet has its own

protocol to send the information. These packets are identified by their

names. GPS receiver sends these packets serially, every second it delivers a

packet of information.

Fig 2 | understanding of NMEA Packets

First five character of a packet represent the packet name, and other

superfluous information separated by comma.

Ie. $GPVTG,176.41,T,,M,0.00,N,0.00,K,A*38

Here ‘$GPVTG’ is packet name and it is followed by other information.

Fig 3 | Actual NMEA messages

Important NMEA message packets.

a. GGA – Global Positioning System Fix Data

b. GLL – Geographic Position – Latitude/Longitude

c. GSA – DOP and Active Satellites

d. GSV – Satellites in view

e. RMC – Recommended Minimum Specific GNSS Data

f. VTG – Course Over Ground and Ground Speed

a. $GPGGA - Global Positioning System Fix Data

Data/Information : Time, Position, Numbers of satellite used, altitude

etc.

Message format:

$GPGGA,hhmmss.ss,LLmm.llll,{N/S},yyymm.mmmm,{E/W},i,ns,h.d,a.h,M,

g.s,M,a.s,XXXX*CS

Index Message Discription

0 $GPGGA Packet name (Global Positioning System Fix Data)

1 hhmmss.ss UTC Time hh : hours mm : minutes ss.ss : seconds

2 LLmm.mmmm Latitude LL : Degree mm.mmmm = minutes

3 {N/S} North or South

4 yyymmm.mmmm Longitude yyy : Degree mmm.mmmm : minutes

5 {E/W} East or West

6 i Valid indicator (Fix value) 0: Not valid 1: Valid

7 ns Number satellites used

8 h.d HDOP : Horizontal Dilution Of Precision

9 a.h Altitude

10 M Antenna height Unit (meters)

11 g.s Difference between the WGS-84 reference ellipsoid surface and the mean-sea-level altitude.

12 M Unit of above parameter (meters)

13 a.s Age in seconds since last update

14 XXXX Reference station ID

15 CS Check Sum

Table 1 | GGA Packet format

b. $GPGLL – Geographic Position – Latitude/Longitude

Data/Information: Position, UTC time, and status etc.

Packet format:

$GPGLL,LLmm.mmmm,{N/S},yyymm.mmmm,{E/W},hhmmss.ss,i,m*CS


0 $GPGLL Packet name (Geographic Position – Latitude/Longitude)






6 i Status indicator A : valid V : invalid

7 m Mode indicator A: autonomous N: data not valid

8 CS Check sum

Table 2 | GLL Packet format

c. $GPGSA – DOP and Active Satellites

Data/Information : Operating mode, satellite used in fix position and DOP etc.

Packet format:

$GPGSA,i,j,aa,aa,aa,aa,aa,aa,aa,aa,aa,aa,aa,aa,p.d,h.d,v.d*CS


0 $GPGSA Packet name (DOP and Active Satellites)

1 i : Mode M : Manual, forced to operate in 2D or 3D A : Automatic, 3D/2D

2 J : Mode 1 : Fix not available 2 : 2D 3 : 3D

3 aa Ids of Satellites used in position fix , null for unused

4 p.d PDOP

5 h.d HDOP

6 v.d VDOP

7 CS Check sum

Table 3 | GSA packet format

d. $GPGSV : GPS Satellites in view

Data/information : Messages, messages id, satellite id, elevation, azimuth etc.

Packet format:

$GPGSV,N,M, ns,si,se,saz,sn,…………. , ns,si,se,saz,sn *CS


0 $GPGSV Packet name (GPS Satellites in view)

1 N Number of messages

2 M Message Number

3 ns Number of satellites in view

4 si Satellites ID

5 Se Satellites Elevation

6 saz Satellite azimuth

7 sn SNR number

8-19 3 set of 4-7 Satellites information

20 CS Checksum

Table 4 | GSV packet format

e. $GPRMC – Recommended Minimum Specific GNSS Data

Data/information : Time, Date, Position, speed and status etc.

Packet Format:

$GPRMC,hhmmss.ss,i,LLmm.mmmm,{N/S},yyymm.mmmm,{E/W},s.n,t.c,DDMMYY,m.v,

{E/W},

m*CS<CR><LF>


0 $GPRMC Packet name (Recommended Minimum Specific GNSS Data)


2 i Valid indicator 0: Not valid 1: Valid





7 s.n Speed in Knots

8 t.c True Course/ Heading

9 DDMMYY Date

10 m.v Magnetic variation



13 CS Check Sum

Table 5 | RMC packet format

f. $GPVTG – Course Over Ground and Ground Speed

Data/Information: Speed and course.

Packet format:

$GPVTG,m.v,T,m.h,M,s.n,N,s.k,K,m*CS<CR><LF>


0 $GPVTG Packet name (Course Over Ground and Ground Speed)

1 m.v Magnetic Variation / heading

2 T Fixed text (Heading unit degree)

3 m.h Magnetic heading

4 M Degree, magnetic heading unit

5 s.n Speed in Knots

6 N Speed unit Knots

7 s.k Speed in KM per Hour

8 K Speed unit KM / hr


10 CS Check sum

Table 4 | GSV packet format

Algo for extracting GPS data

form NMEA messages

Step 1. Create connection with GPS receiver.

Step 2. While RX: go to Step 3 –

Step3. Message = Read Line (Rx data)

Step4. if ( message [0:2] ! = ‘$GP’ ) – we are not getting NMEA messages / packets

{

Break – go to step 3.

}

. else – we are getting NMEA messages.

{

Step 5. if (message [0:5+ = = ‘$GPGGA’ )

{

Fragments [] = explode(“,” message)

if (Fragments [2] == ‘’” OR Fragments [4] == “‘’)

{

latitude or longitude can’t be determined

Break loop – go to step 3

}

UTCTime = Fragments[1]

– Format :- hhmmss.ss

UTCTime = UTCTime *0:1+ . “:”.UTCTime *2:3+ . “:”.UTCTime *4:5+

– Format :- hh:mm:ss

Latitude = Fragments [2]

-Format :- xxmm.dddd

Latitude = Latitude[0:1] + Latitude[2:8] / 60

-Format :- Decimal format

Loc_N/S = Fragments[3]

Longitude = Fragments[4]

-Format:- yyymm.dddd

Longitude = Longitude[0:2] + Longitude [3:9] / 60


Loc_E/W = Fragments[5]

Fix = Fragments[6]

Nos_satellite = Fragments[7]

HDOP = Fragments[8]

Altitude = Fragments[9]


}

Step6. if (message *0:5+ = = ‘$GPGLL’ )

{

Fragments *+ = explode(“,” message)

if (Fragments *1+ == ‘’” OR Fragments *3+ == “‘’)

{



}















Status = Fragments[6]

Mode = Fragments [7]


}

Step7. if (message *0:5+ = = ‘$GPGSA’ )

{


Mode_a = Fragments[1]

Mode_b = Fragments[2]

Operating_setellites = Fragments[3]

PDOP = Fragments[4]

HDOP = Fragments[5]

VDOP = Fragments[6]


}

Step8. if (message *0:5+ = = ‘$GPGSV’ )

{


Nos_messages = Fragments[1]

Message_No = Fragments[2]

Nos_setellites = Fragments[3]

Satellite_ID = Fragments[4]

Satellite_elevation = Fragments[5]

Satellite_azimuth = Fragments[6]

SNR_No = Fragments[7]


}

Step 9. if (message *0:5+ = = ‘$GPRMC’ )

{


if (Fragments *3+ == ‘’” OR Fragments *5+ == “‘’)

{



}





Status = Fragments[2]











Speed = Fragments[7]

Heading = Fragments[8]

Date = Fragments[9]

-format :- ddmmyy

Date = Date*0:1+. “-”. Date*2:3+. “-”. Date*4:5+

-Format:- dd-mm-yy

Magnetic_variation = Fragments[10]

Mode = Fragments[12]


}

Step10. if (message *0:5+ = = ‘$GPVTG’ )

{


Heading = Fragments[1]

Heading_unit = Fragments[2]

Magnetic_heading = Fragments[3]

Magnetic_heading_unit= Fragments[4]

Speed_knots= Fragments[5]

Speed_unit_knots = Fragments[6]

Speed_KM = Fragments[7]

Speed_unit_KM = Fragments[8]

Mode = Fragments[9]

Break -go to step. 3

}


}

Explanation for some functions used in algo :

Message[i:j] : here message in a string that stores a packet in it.

Message[i:j] : represent a substring of message from index i to j elements (note i

and j both are included).

Explode : it splits the string in to fragments by a specific character, here we are splitting

message by comma “,”, and it stores all the fragments in to an array named

fragments[].

Other notations are common notations for many programming languages like C, C++.

User can use either full or partial algorithm according to requirement.

understanding gps & nmea messages and algo to extract information from nmea

Education

gps receiver

nmea packets

packets of information

protocol of nmea

gps episode

constituents of gps

packet of information

understanding nmea messages