nmea.c

Construct NMEA packet in GPS module

/*extern char gSimulate;*/

void
SendNMEA (void)
{
  if (GPGGA != 0)
    NMEASendGPGGA ();           /* NMEA, use sentance options */
  if (GPGSA != 0)
    NMEASendGPGSA ();
  if (GPGSV != 0)
    NMEASendGPGSV ();
  if (GPRMC != 0)
    NMEASendGPRMC ();
  if (GPZDA != 0)
    NMEASendGPZDA ();
}

/**
 * NMEA Notes  
 *
 * Longitude and latitude can be expressed in several different
 * representations:
 *
 * dms (deg,min,sec) - Degrees, minutes and seconds format
 * (e.g. 5045'23.99")
 *
 * degrees - Decimal degrees (e.g. 50.7567) 
 *
 * radians - Radians (e.g. 0.88587090) 
 *
 * degrees, minutes - Degrees and minutes format (e.g. 5045.3998') 
 *
 * Select the appropriate representation found in the string files
 *
 * Converting Between Decimal Degrees, Degrees, Minutes and Seconds,
 * and Radians (dd + mm/60 +ss/3600) to Decimal degrees (dd.ff)
 *
 * dd = whole degrees, mm = minutes, ss = seconds
 *
 * dd.ff = dd + mm/60 + ss/3600
 *
 * Example: 30 degrees 15 minutes 22 seconds = 30 + 15/60 +
 * 22/3600 = 30.2561
 *
 * Decimal degrees (dd.ff) to (dd + mm/60 +ss/3600)
 *
 * For the reverse conversion, we want to convert dd.ff to dd mm
 * ss. Here ff = the fractional part of a decimal degree.
 *
 * mm = 60*ff
 *
 * ss = 60*(fractional part of mm)
 *
 * Use only the whole number part of mm in the final result.
 *
 * 30.2561 degrees = 30 degrees
 *
 * .2561*60 = 15.366 minutes
 *
 * .366 minutes = 22 seconds, so the final result is 30 degrees 15
 * minutes 22 seconds
 *
 * Decimal degrees (dd.ff) to Radians
 *
 * Radians = (dd.ff)*pi/180
 *
 * Radians to Decimal degrees (dd.ff)
 *
 * (dd.ff) = Radians*180/pi
 *
 * Degrees, Minutes and Seconds to Distance
 *
 * A degree of longitude at the equator is 111.2 kilometers. A minute
 * is 1853 meters.  A second is 30.9 meters. For other latitudes
 * multiply by cos(lat). Distances for degrees, minutes and seconds in
 * latitude are very similar and differ very slightly with latitude.
 * (Before satellites, observing those differences was a principal
 * method for determining the exact shape of the earth.)
 *
 *
 * D = Degrees
 * M = Minutes
 * S = Seconds
 * .m = Decimal Minutes
 * .s = Decimal Seconds
 *
 * DM.m = Degrees, Minutes, Decimal Minutes (eg. 45 22.6333)
 * D.d  = Degrees, Decimal Degrees (eg. 45.3772)
 * DMS  = Degrees, Minutes, Seconds (eg. 45 22'38")
 *
 * Process for Converting Latitude/Longitude Coordinates: 
 * 
 * 1) DMS --> DM.m  (45o22'38" --> 45o22.6333)
 *
 *  - Divide S by 60 to get .m (38/60=.6333)
 *  - Add .m to M to get M.m (22+.6333=22.6333)
 *
 * 2) DM.m --> D.d  (45o 22.6333 --> 45.3772)
 *
 *  - Divide M.m by 60 to get .d (22.6333/60=.3772)
 *  - Add .d to D to get D.d (45+.3772=45.3772)
 *
 * 3) D.d --> DM.m  ( 45.3772 --> 45 22.6333)
 *
 *  - Multiply .d by 60 to get M.m (.3772 * 60 = 22.6333)
 *
 * 4) DM.m --> DMS  (45o22.6333 --> 45 22'38")
 *
 *  - Multiply .m by 60 to get S(.6333*60=38)
 *
 *
 * Converting Degrees, Minutes, Seconds to Decimal Format
 *
 * latitude and longitude in a decimal format: 42.1361
 * 
 * latitude and longitude in degree, minute, second format: 42deg,
 * 08min, 10sec
 *
 * To convert coordinates from degrees, minutes, seconds format to
 * decimal format, use this easy formula:
 *
 * degrees + (minutes/60) +
 * (seconds/3600)
 *
 * The example coordinate above would be
 * calculated as:
 * 42 + (8/60) + (10/3600) = 42.1361
 * or
 * 42 + (.1333) + (.0028) = 42.1361
 *
 *
 **/

void
NMEAAddCRLF (char *TransmitBuffer, int *TXBufferIndex)
{
  char *TempPtr = TransmitBuffer + *TXBufferIndex;

  *TempPtr++ = 0x0D;
  *TempPtr++ = 0x0A;
  /**
   * Update the index.
   **/
  *TXBufferIndex += 2;
  *TempPtr++ = '\0';
}



void
NMEAAddField (char *TransmitBuffer, int *TXBufferIndex, char *StringToAdd)
{
  int NumberOfChars = 0;
  NumberOfChars = strlen (StringToAdd);

  /**
   * Copy the data.
   **/
  memcpy (&TransmitBuffer[*TXBufferIndex], StringToAdd, NumberOfChars);

  /**
   * Update the index.
   **/
  *TXBufferIndex += NumberOfChars;
}

double
ConvertLatLonFromDegreestoDegreesMinutes (double PositionDegrees)
{
  double Degrees = (int) PositionDegrees;
  double Minutes = PositionDegrees - Degrees;

  double DegreesMinutes = Minutes * 60 + (Degrees * 10 * 10);
  return DegreesMinutes;
}

/**
 * The checksum is the 8-bit exclusive OR (no start or stop bits) of
 * all characters in the sentence, including the "," delimiters,
 * between -- but not including -- the "$" and "*" delimiters.
 *
 * The hexadecimal value of the most significant and least significant
 * 4 bits of the result are converted to two ASCII characters (0-9,
 * A-F) for transmission. The most significant character is
 * transmitted first.
 **/
unsigned char
CalculateNmeaCheckSum (char *NmeaSentence)
{
  int Index;
  unsigned char NmeaCheckSum = 0;

  for (Index = 1; NmeaSentence[Index] != '*'; Index++)
    {
      NmeaCheckSum ^= NmeaSentence[Index];
    }

  return NmeaCheckSum;
}

void
NMEAAddCheckSum (char *TransmitBuffer, int *TXBufferIndex)
{
  unsigned char CheckSum;
  unsigned char TXCheckSum;
  char TempBuf[10];



  NMEAAddField (TransmitBuffer, TXBufferIndex, NMEA_SENTENCE_ASTERISK);

  CheckSum = CalculateNmeaCheckSum (TransmitBuffer);

  TXCheckSum = CheckSum & 0xf0;
  TempBuf[0] = (TXCheckSum >> 4) + '0';
  TempBuf[1] = '\0';

  if (TempBuf[0] > '9')
    {

      TempBuf[0] += 'A' - ('9' + 1);
      TempBuf[1] = '\0';
    }


  NMEAAddField (TransmitBuffer, TXBufferIndex, TempBuf);
  TXCheckSum = CheckSum & 0x0f;
  TempBuf[0] = (TXCheckSum & 0xf) + '0';
  TempBuf[1] = '\0';

  if (TempBuf[0] > '9')
    {

      TempBuf[0] += 'A' - ('9' + 1);
      TempBuf[1] = '\0';
    }

  NMEAAddField (TransmitBuffer, TXBufferIndex, TempBuf);
}

NMEA_STATUS
NMEAGetGPGGAData (pNMEA_GGA_DATA pNmeaGGAData)
{

  /**
   * Need to convert from degrees to degrees and minutes format.
   **/
  pNmeaGGAData->TimeOfFixUTC = GetTimeOfFixUTC ();
  pNmeaGGAData->Lat =
    ConvertLatLonFromDegreestoDegreesMinutes (GetLatitudeAbs ());
  pNmeaGGAData->NorthSouth = GetNorthSouth ();
  pNmeaGGAData->Long =
    ConvertLatLonFromDegreestoDegreesMinutes (GetLongitudeAbs ());
  pNmeaGGAData->EastWest = GetEastWest ();
  pNmeaGGAData->FixQuality = GetFixQuality ();
  pNmeaGGAData->NumberOfTrackingSV = GetNumberOfTrackingSVs ();
  pNmeaGGAData->HDOP = GetHDOP ();
  pNmeaGGAData->Altitude = GetAltitude ();
  pNmeaGGAData->HeightOfGeoid = GetHeightOfGeoid ();

  return NMEA_SUCCESS;
}

/**
 * GGA - essential fix data which provide 3D location and accuracy data. 
 *
 *  $GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47
 *
 * Where:
 *   GGA          Global Positioning System Fix Data
 *   123519       Fix taken at 12:35:19 UTC
 *   4807.038,N   Latitude 48 deg 07.038' N
 *   01131.000,E  Longitude 11 deg 31.000' E
 *   1            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) (2.3 feature)
 *           7 = Manual input mode
 *           8 = Simulation mode
 *   08           Number of satellites being tracked
 *   0.9          Horizontal dilution of position
 *   545.4,M      Altitude, Meters, 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
 *   *47          the checksum data, always begins with *
 *
 * If the height of geoid is missing then the altitude should be suspect. 
 *
 * Some non-standard implementations report altitude with respect to
 * the ellipsoid rather than geoid altitude. Some units do not report
 * negative altitudes at all.  This is the only sentence that reports
 * altitude.
 * 
 **/

NMEA_STATUS
NMEASendGPGGA (void)
{

  NMEA_GGA_DATA NmeaGGAData;
  char pFieldBuffer[MAX_NMEA_FIELD];
  int  TXBufferIndex = 0;
  char TransmitBuffer[MAX_NMEA_BUFFER];


  memset (TransmitBuffer, 0, MAX_NMEA_BUFFER);


  /**
   * Get the necessary data from the FW.
   **/
  NMEAGetGPGGAData (&NmeaGGAData);

  /**
   * Add the header
   **/
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_START);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_GPGGA);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 0 Time   
   *
   * The time field must be 6 digits long. If the hour is less than 10 you 
   * must pad it with an extra zero.
   **/
  sprintf (pFieldBuffer, "%06ld.00", (long) NmeaGGAData.TimeOfFixUTC);
  /*   ftod( NmeaGGAData.TimeOfFixUTC, pFieldBuffer  ); */
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);



  /**
   * Field 1 Latitude
   **/
  ftod4 (NmeaGGAData.Lat, pFieldBuffer);        /* 4 decimal places */
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 2 North/South 
   **/
  if (NmeaGGAData.NorthSouth == NMEA_SOUTH)
    {
      NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_SOUTH);
    }
  else
    {
      NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_NORTH);
    }

  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 3 Longitude
   **/
  ftod4 (NmeaGGAData.Long, pFieldBuffer);       /* 4 decimal places */
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);


  /**
   * Field 4 East/West
   **/
  if (NmeaGGAData.EastWest == NMEA_WEST)
    {
      NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_WEST);
    }
  else
    {
      NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_EAST);
    }

  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 5 Fix Quality
   **/
  sprintf (pFieldBuffer, "%d", NmeaGGAData.FixQuality);
  /* ftod( NmeaGGAData.FixQuality, pFieldBuffer ); */
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 6 Number of SV's used.
   **/
  sprintf (pFieldBuffer, "%02d", NmeaGGAData.NumberOfTrackingSV);
  /*itoa(  NmeaGGAData.NumberOfTrackingSV, pFieldBuffer, 10 );*/
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 7 HDOP
   **/
  ftodPrecision1 (NmeaGGAData.HDOP, pFieldBuffer);
  /* ftod( NmeaGGAData.HDOP, pFieldBuffer ); */
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 8 Altitude
   **/
  ftodPrecision1 (NmeaGGAData.Altitude, pFieldBuffer);
  /* ftod( NmeaGGAData.Altitude, pFieldBuffer ); */
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);


  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);
  /**
   * Field 9 Altitude Units
   **/
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_UNITS_METERS);



  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);
  /**
   * Field 10 Geoid Seperation ( Height of geoid above WGS84 ellipsoid )
   **/

  ftodPrecision1 (0.0, pFieldBuffer);
  NMEAAddField (TransmitBuffer, &TXBufferIndex, pFieldBuffer);

  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);
  /**
   * Field 11 Geoid Seperation units
   **/
  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_UNITS_METERS);

  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);

  /**
   * Field 12  DGPS Age
   **/

  NMEAAddField (TransmitBuffer, &TXBufferIndex, NMEA_SENTENCE_COMMA);
  /**
   * Field 12  DGPS StationID
   **/


  /**
   * Check Sum 
   **/
  NMEAAddCheckSum (TransmitBuffer, &TXBufferIndex);

  /**
   * CR/LF
   **/
  NMEAAddCRLF (TransmitBuffer, &TXBufferIndex);




  /**
   * Now send the sentence.
   **/
  if (!ComPortWrite ((unsigned char *) &TransmitBuffer[0], TXBufferIndex))
    {
      return NMEA_FAILED;
    }
  else
    {
      return NMEA_SUCCESS;
    }
}

NMEA_STATUS
NMEAGetGPGSAData (pNMEA_GSA_DATA pNmeaGSVData)
{
  int i;
  TRACKING_PRNs TrackingPRNs[12];

  memset (TrackingPRNs, 0, sizeof (TrackingPRNs));

  pNmeaGSVData->Mode = NMEA_MODE_A;     /* Cliff */

  if (GetNavStatus ())
    {
      pNmeaGSVData->FixMode = NMEA_FIX_MODE_3D_FIX;
    }
  else
    {
      pNmeaGSVData->FixMode = NMEA_FIX_MODE_NO_FIX;
    }

/*  if (gSimulate)
    {
      pNmeaGSVData->FixMode = NMEA_FIX_MODE_3D_FIX;
    } */

  GetTrackingPRNs (&TrackingPRNs[0]);