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Bluetooth GPS Receiver - NMEA Parsing

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The code snippets in this article show how to parse NMEA sentences from a connected Bluetooth GPS Receiver using Symbian C++.

Article Metadata
Article
Created: kiran10182 (23 May 2007)
Last edited: hamishwillee (31 May 2013)

Contents

Introduction

Getting GPS data is quite often needed by many location-based application with hand-held devices. In our case we can retrive GPS details such as Latitude, Logitude, Altitude, Speed etc... with Symbian devices. GPS receiver generally reads GPS data with the means of NMEA Sentences. One cannot retrieve straight away GPS details from these NMEA Sentences. NMEA sentences need to be parsed to get such GPS details. The following code snippets illustrate how to parse these NMEA sentences with Symbian C++ connecting to Bluetooth GPS Receiver.

Prerequisite

  • Bluetooth GPS Receiver
  • Here we are using btpointtopoint given in the SDK package to use Bluetooth functionalities.
  • S60 2nd Edition device.

NMEAParser.h

#ifndef __CNMEAParser_H__
#define __CNMEAParser_H__
 
#include <E32Base.h>
 
class CNMEAParser : public CBase
{
public:
 
// Set the NMEA Data
void SetData( TDesC8 & aNmeaMessage );
 
// Skip the next Token from NMEA sentence
void SkipNextTokenL( );
 
// Read the Next Token from NMEA sentence
void ReadNextTokenL( TDes8 & aBuffer );
 
// Read Next Token from NMEA sentecne as Latitude
TReal ReadNextTokenAsLatitudeL( );
 
// Read Next Token from NMEA sentecne as Longitude
TReal ReadNextTokenAsLongitudeL( );
 
// This will return Next Token from NMEA sentence as TInt
TInt ReadNextTokenAsIntL( );
 
// This will return Next Token from NMEA sentence as TReal
TReal ReadNextTokenAsRealL( );
 
// Clear the NMEA Data
void ClearData( );
 
private:
TDesC8 * iMessage;
TInt iParsePosition;
};
 
#endif


NMEAParser.cpp

#include <E32Math.h>
#include "NMEAParser.h"
 
// Set the NMEA Data
void CNMEAParser::SetData( TDesC8 & aNmeaMessage )
{
iMessage = &aNmeaMessage;
iParsePosition = 0;
}
 
 
// Read the Next Token from NMEA sentence
void CNMEAParser::ReadNextTokenL( TDes8 & aBuffer )
{
if ( iMessage == NULL )
User::Leave( KErrNotReady );
 
if ( iParsePosition >= iMessage->Length( ) )
return;
 
while ( iParsePosition < iMessage->Length( ) )
{
TUint8 nextChar = ( *iMessage )[ iParsePosition++ ];
if ( nextChar == ',' )
break;
aBuffer.Append( nextChar );
}
 
}
 
 
// Skip the next Token from NMEA sentence
void CNMEAParser::SkipNextTokenL( )
{
 
if ( iMessage == NULL )
User::Leave( KErrNotReady );
 
if ( iParsePosition >= iMessage->Length( ) )
return;
 
while ( iParsePosition < iMessage->Length( ) )
{
TUint8 nextChar = ( *iMessage )[ iParsePosition++ ];
if ( nextChar == ',' )
break;
}
}
 
 
// Read Next Token from NMEA sentecne as Latitude
TReal CNMEAParser::ReadNextTokenAsLatitudeL( )
{
TReal angle;
 
TBuf8< 15 > angleString;
ReadNextTokenL( angleString );
 
TBuf8< 5 > northSouth;
ReadNextTokenL( northSouth );
 
TLex8 latLexer( angleString );
TInt error = latLexer.Val( angle, '.' );
 
if ( error != KErrNone )
return 0.0;
 
TInt32 degrees;
Math::Int( degrees, angle / 100.0 );
 
TInt32 minutes;
Math::Int( minutes, angle - degrees * 100 );
 
TReal decimal;
Math::Frac( decimal, angle );
 
TReal latitude = degrees + ( minutes + decimal ) / 60.0;
 
if ( northSouth[ 0 ] == 'S' )
latitude = -latitude;
 
return( latitude );
}
 
 
// Read Next Token from NMEA sentecne as Longitude
TReal CNMEAParser::ReadNextTokenAsLongitudeL( )
{
TReal angle;
 
TBuf8< 15 > angleString;
ReadNextTokenL( angleString );
 
TBuf8< 5 > northSouth;
ReadNextTokenL( northSouth );
 
TLex8 latLexer( angleString );
TInt error = latLexer.Val( angle, '.' );
 
if ( error != KErrNone )
return 0.0;
 
 
TInt32 degrees;
Math::Int( degrees, angle / 100.0 );
 
TInt32 minutes;
Math::Int( minutes, angle - degrees * 100 );
 
TReal decimal;
Math::Frac( decimal, angle );
 
TReal longitude = degrees + ( minutes + decimal ) / 60.0;
 
if ( northSouth[ 0 ] == 'W' )
longitude = -longitude;
 
return( longitude );
 
}
 
 
// This will return Next Token from NMEA sentence as TInt
TInt CNMEAParser::ReadNextTokenAsIntL( )
{
TInt result = 0;
 
TBuf8< 10 > intString;
ReadNextTokenL( intString );
 
TLex8 lexer( intString );
 
TInt ret = lexer.Val( result );
if( ret == KErrNone)
return( result );
else
return 0.0;
 
}
 
 
// This will return Next Token from NMEA sentence as TReal
TReal CNMEAParser::ReadNextTokenAsRealL( )
{
TReal result = 0;
 
TBuf8< 10 > realString;
ReadNextTokenL( realString );
 
TLex8 lexer( realString );
TInt ret = lexer.Val( result );
if( ret == KErrNone)
return( result );
else
return 0.0;
}
 
 
// Clear the NMEA Data
void CNMEAParser::ClearData( )
{
iMessage = NULL;
iParsePosition = 0;
}


messageclient.h

  • You can find messageclient.h in the btpointtopoint example given in the SDK package.
  • Include NMEAParser.h in the messageclient.h
#include "NMEAParser.h" //For NMEA sentence Parsing


Declare the following list of functions:
//Following functions are declared for NMEA sentence parsing purpose
public:
void WriteToFile(TBuf8<100> aData);
void NMEAUpdate( const TDes8 & aNMEABuffer );
 
void HandleMessageL( TDesC8 & aCommand );
void HandleGPGGAMsgL( TDesC8 & aCommand );
void HandleGPGLLMsgL( TDesC8 & aCommand );
void HandleGPVTGMsgL( TDesC8 & aCommand );
void HandleGPRMCMsgL( TDesC8 & aCommand );


  • Declare the following list of variables and object.
	TBuf8 <1024>  iDummyBuffer
TBuf8 <1024> iTempBuffer;
TBuf8 <256> iFileData;
TBuf8 <512> iMessageBuffer;
TReal speedInKMH;
 
CNMEAParser iNmeaParser;


messageclient.cpp

  • Change EConnceted case like:


case EConnected:
iTempBuffer.Zero();
iTempBuffer.Copy(iDummyBuffer);
if( iTempBuffer.Length() > 200 )
NMEAUpdate(iTempBuffer);
// Just dump data
iDummyBuffer.Zero();
// Catch disconnection event
// By waiting to read socket
WaitOnConnectionL();
break;


  • Implement functions to process NMEA Sentences:


//Following functions are implemeted for NMEA sentence parsing purpose 
void CMessageClient::NMEAUpdate( const TDes8 & aNMEABuffer )
{
 
TBool inMessage = ( iMessageBuffer.Length( ) > 0 ) ? ETrue : EFalse;
 
for ( TInt Index = 0; Index < aNMEABuffer.Length( ); Index++ )
{
TUint8 next = aNMEABuffer[ Index ];
if ( inMessage == EFalse && ( next == '$' ) )
inMessage = ETrue;
 
if ( inMessage )
iMessageBuffer.Append( aNMEABuffer[ Index ] );
 
if ( inMessage && ( next == 13 ) )
{
inMessage = EFalse;
HandleMessageL( iMessageBuffer );
iMessageBuffer.SetLength( 0 );
}
}
}


void CMessageClient::HandleMessageL( TDesC8 & aCommand ) 
{
TBuf8< 32 > temp;
 
_LIT( KGlobalPositioning, "$GPGGA" );
temp.Copy( KGlobalPositioning );
 
TInt foundGPGGA = aCommand.Find( temp );
if ( foundGPGGA == 0 )
HandleGPGGAMsgL( aCommand );
 
_LIT( KGeographicPosition, "$GPGLL" );
temp.Copy( KGeographicPosition );
 
TInt foundGPGLL = aCommand.Find( temp );
if ( foundGPGLL == 0 )
HandleGPGLLMsgL( aCommand );
 
_LIT( KCourseData, "$GPVTG" );
temp.Copy( KCourseData );
 
TInt foundGPVTG = aCommand.Find( temp );
if ( foundGPVTG == 0 )
HandleGPVTGMsgL( aCommand );
}


void CMessageClient::HandleGPGGAMsgL( TDesC8 & aCommand ) 
{
 
iNmeaParser.SetData( aCommand );
iNmeaParser.SkipNextTokenL( ); // skip message id
 
 
TReal utcTime = iNmeaParser.ReadNextTokenAsRealL( );
TReal Latitude = iNmeaParser.ReadNextTokenAsLatitudeL( );
TReal Longitude = iNmeaParser.ReadNextTokenAsLongitudeL( );
TInt fixValid = iNmeaParser.ReadNextTokenAsIntL( );
TInt satellites = iNmeaParser.ReadNextTokenAsIntL( );
TReal hdop = iNmeaParser.ReadNextTokenAsRealL( );
TReal altitude = iNmeaParser.ReadNextTokenAsRealL( );
 
//Added for storing values in File
TBuf8<100> iData;
iData.Format(_L8("Lat=%f\nLon=%f\nAlt=%f\n"),Latitude,Longitude,altitude);
WriteToFile(iData);
//Code ends
 
iNmeaParser.ClearData( );
}



void CMessageClient::HandleGPGLLMsgL( TDesC8 & aCommand ) 
{
iNmeaParser.SetData( aCommand );
iNmeaParser.SkipNextTokenL( ); // skip message id
 
TReal Latitude = iNmeaParser.ReadNextTokenAsLatitudeL( );
TReal Longitude = iNmeaParser.ReadNextTokenAsLongitudeL( );
 
//Added for storing values in File
TBuf8<100> iData;
iData.Format(_L8("Lat=%f Lon=%f\n"),Latitude,Longitude);
WriteToFile(iData);
//Code ends
 
iNmeaParser.ClearData( );
}


void CMessageClient::HandleGPVTGMsgL( TDesC8 & aCommand ) 
{
iNmeaParser.SetData( aCommand );
iNmeaParser.SkipNextTokenL( ); // skip message id
 
TReal heading = iNmeaParser.ReadNextTokenAsRealL( );
iNmeaParser.SkipNextTokenL( ); // skip heading unit
iNmeaParser.SkipNextTokenL( ); // skip magnetic heading
iNmeaParser.SkipNextTokenL( ); // skip magnetic heading unit
 
TReal speedInKnots = iNmeaParser.ReadNextTokenAsRealL( );
 
//Convertion from speed in Knots into speed in KiloMeters/Hour
speedInKMH = speedInKnots * 1.85 ;
//Conversion ends
 
iNmeaParser.SkipNextTokenL( ); // skip speed unit
iNmeaParser.SkipNextTokenL( ); // skip speed in km/h
iNmeaParser.SkipNextTokenL( ); // skip speed unit
 
iNmeaParser.ClearData( );
 
//Added for storing values in File
TBuf8<100> iData;
iData.Format(_L8("VTG Speed=%f\n"),speedInKMH);
WriteToFile(iData);
//Code ends
}


Related Links

GPS - NMEA sentence information

  

This page was last modified on 31 May 2013, at 02:27.
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