Writing Your Own GPS Applications: Part 2

Enforcing Precision

Now that the precision needs of a GPS application can be determined, it's time to find out what source code is necessary to extract and enforce maximum DOP values. All DOP measurements are packaged into the $GPGSA sentence every few seconds. Here is a sample of a $GPGSA sentence:

$GPGSA,A,3,11,29,07,08,5,17,24,,,,,,2.3,1.2,2.0*30

A skillful GPS application developer could know if positional readings are precise enough to use just by looking at one $GPGSA sentence. Again, the best DOP ratings occur when there are several satellites involved in a fix and the satellites are evenly distributed throughout the sky and at separate elevations -– hitting the GPS device from all angles, so to speak.

The last three words of this sentence are 2.3, 1.2, and 2.0, representing mean, horizontal and vertical DOP, respectively. This sentence represents a high-precision environment (suitable enough for both golfing and driving). Using the final listing from part one of this article (Listing 1-8), a method called “ ParseGPGSA ” is added which extracts DOP values and reports them via three events: “ HDOPReceived ”, “ VDOPReceived ” and “ PDOPReceived ”.

C#

//*********************************************************************
//**  A high-precision NMEA interpreter
//**  Written by Jon Person, author of "GPS.NET" (www.gpsdotnet.com)
//*********************************************************************
using System;
using System.Globalization;
public class NmeaInterpreter
{
  // Represents the EN-US culture, used for numers in NMEA sentences
  public static CultureInfo NmeaCultureInfo = new CultureInfo("en-US");
  // Used to convert knots into miles per hour
  public static double MPHPerKnot = double.Parse("1.150779",
    NmeaCultureInfo);
  #region Delegates
    public delegate void PositionReceivedEventHandler(string latitude,
      string longitude);
    public delegate void DateTimeChangedEventHandler(System.DateTime dateTime);
    public delegate void BearingReceivedEventHandler(double bearing);
    public delegate void SpeedReceivedEventHandler(double speed);
    public delegate void SpeedLimitReachedEventHandler();
    public delegate void FixObtainedEventHandler();
    public delegate void FixLostEventHandler();
    public delegate void SatelliteReceivedEventHandler(
      int pseudoRandomCode, int azimuth, int elevation,
      int signalToNoiseRatio);
    public delegate void HDOPReceivedEventHandler(double value);
    public delegate void VDOPReceivedEventHandler(double value);
    public delegate void PDOPReceivedEventHandler(double value);
  #endregion
  #region Events
    public event PositionReceivedEventHandler PositionReceived;
    public event DateTimeChangedEventHandler DateTimeChanged;
    public event BearingReceivedEventHandler BearingReceived;
    public event SpeedReceivedEventHandler SpeedReceived;
    public event SpeedLimitReachedEventHandler SpeedLimitReached;
    public event FixObtainedEventHandler FixObtained;
    public event FixLostEventHandler FixLost;
    public event SatelliteReceivedEventHandler SatelliteReceived;
    public event HDOPReceivedEventHandler HDOPReceived;
    public event VDOPReceivedEventHandler VDOPReceived;
    public event PDOPReceivedEventHandler PDOPReceived;
  #endregion
 
  // Processes information from the GPS receiver
  public bool Parse(string sentence)
  {
    // Discard the sentence if its checksum does not match our
    // calculated checksum
    if (!IsValid(sentence)) return false;
    // Look at the first word to decide where to go next
    switch (GetWords(sentence)[0])
    {
      case "$GPRMC":
        // A "Recommended Minimum" sentence was found!
        return ParseGPRMC(sentence);
      case "$GPGSV":
        // A "Satellites in View" sentence was recieved
        return ParseGPGSV(sentence);
      case "$GPGSA":
        return ParseGPGSA(sentence);
      default:
        // Indicate that the sentence was not recognized
        return false;
    }
  }
  // Divides a sentence into individual words
  public string[] GetWords(string sentence)
  {
    return sentence.Split(',');
  }
  // Interprets a $GPRMC message
  public bool ParseGPRMC(string sentence)
  {
    // Divide the sentence into words
    string[] Words = GetWords(sentence);
    // Do we have enough values to describe our location?
    if (Words[3] != "" & Words[4] != "" &
      Words[5] != "" & Words[6] != "")
    {
      // Yes. Extract latitude and longitude
      // Append hours
      string Latitude = Words[3].Substring(0, 2) + "°";
      // Append minutes
      Latitude = Latitude + Words[3].Substring(2) + "\"";
      // Append hours
      Latitude = Latitude + Words[4]; // Append the hemisphere
      string Longitude = Words[5].Substring(0, 3) + "°";
      // Append minutes
      Longitude = Longitude + Words[5].Substring(3) + "\"";
      // Append the hemisphere
      Longitude = Longitude + Words[6];
      // Notify the calling application of the change
      if(PositionReceived != null)
        PositionReceived(Latitude, Longitude);
    }
    // Do we have enough values to parse satellite-derived time?
    if (Words[1] != "")
    {
      // Yes. Extract hours, minutes, seconds and milliseconds
      int UtcHours = Convert.ToInt32(Words[1].Substring(0, 2));
      int UtcMinutes = Convert.ToInt32(Words[1].Substring(2, 2));
      int UtcSeconds = Convert.ToInt32(Words[1].Substring(4, 2));
      int UtcMilliseconds = 0;
      // Extract milliseconds if it is available
      if (Words[1].Length > 7)
      {
        UtcMilliseconds = Convert.ToInt32(Words[1].Substring(7));
      }
      // Now build a DateTime object with all values
      System.DateTime Today = System.DateTime.Now.ToUniversalTime();
      System.DateTime SatelliteTime = new System.DateTime(Today.Year,
        Today.Month, Today.Day, UtcHours, UtcMinutes, UtcSeconds,
        UtcMilliseconds);
      // Notify of the new time, adjusted to the local time zone
      if(DateTimeChanged != null)
        DateTimeChanged(SatelliteTime.ToLocalTime());
    }
    // Do we have enough information to extract the current speed?
    if (Words[7] != "")
    {
      // Yes.  Parse the speed and convert it to MPH
      double Speed = double.Parse(Words[7], NmeaCultureInfo) *
        MPHPerKnot;
      // Notify of the new speed
      if(SpeedReceived != null)
        SpeedReceived(Speed);
      // Are we over the highway speed limit?
      if (Speed > 55)
        if(SpeedLimitReached != null)
          SpeedLimitReached();
    }
    // Do we have enough information to extract bearing?
    if (Words[8] != "")
    {
      // Indicate that the sentence was recognized
      double Bearing = double.Parse(Words[8], NmeaCultureInfo);
      if(BearingReceived != null)
        BearingReceived(Bearing);
    }
    // Does the device currently have a satellite fix?
    if (Words[2] != "")
    {
      switch (Words[2])
      {
        case "A":
          if(FixObtained != null)
            FixObtained();
          break;
        case "V":
          if(FixLost != null)
            FixLost();
          break;
      }
    }
    // Indicate that the sentence was recognized
    return true;
  }
  // Interprets a "Satellites in View" NMEA sentence
  public bool ParseGPGSV(string sentence)
  {
    int PseudoRandomCode = 0;
    int Azimuth = 0;
    int Elevation = 0;
    int SignalToNoiseRatio = 0;
    // Divide the sentence into words
    string[] Words = GetWords(sentence);
    // Each sentence contains four blocks of satellite information. 
    // Read each block and report each satellite's information
    int Count = 0;
    for (Count = 1; Count <= 4; Count++)
    {
      // Does the sentence have enough words to analyze?
      if ((Words.Length - 1) >= (Count * 4 + 3))
      {
        // Yes.  Proceed with analyzing the block. 
        // Does it contain any information?
        if (Words[Count * 4] != "" & Words[Count * 4 + 1] != ""
          & Words[Count * 4 + 2] != "" & Words[Count * 4 + 3] != "")
        {
          // Yes. Extract satellite information and report it
          PseudoRandomCode = System.Convert.ToInt32(Words[Count * 4]);
          Elevation = Convert.ToInt32(Words[Count * 4 + 1]);
          Azimuth = Convert.ToInt32(Words[Count * 4 + 2]);
          SignalToNoiseRatio = Convert.ToInt32(Words[Count * 4 + 2]);
          // Notify of this satellite's information
          if(SatelliteReceived != null)
            SatelliteReceived(PseudoRandomCode, Azimuth,
            Elevation, SignalToNoiseRatio);
        }
      }
    }
    // Indicate that the sentence was recognized
    return true;
  }
  // Interprets a "Fixed Satellites and DOP" NMEA sentence
  public bool ParseGPGSA(string sentence)
  {
    // Divide the sentence into words
    string[] Words = GetWords(sentence);
    // Update the DOP values
    if (Words[15] != "")
    {
      if(PDOPReceived != null)
        PDOPReceived(double.Parse(Words[15], NmeaCultureInfo));
    }
    if (Words[16] != "")
    {
      if(HDOPReceived != null)
        HDOPReceived(double.Parse(Words[16], NmeaCultureInfo));
    }
    if (Words[17] != "")
    {
      if(VDOPReceived != null)
        VDOPReceived(double.Parse(Words[17], NmeaCultureInfo));
    }
    return true;
  }
  // Returns True if a sentence's checksum matches the
  // calculated checksum
  public bool IsValid(string sentence)
  {
    // Compare the characters after the asterisk to the calculation
    return sentence.Substring(sentence.IndexOf("*") + 1) ==
      GetChecksum(sentence);
  }
  // Calculates the checksum for a sentence
  public string GetChecksum(string sentence)
  {
    // Loop through all chars to get a checksum
    int Checksum = 0;
    foreach (char Character in sentence)
    {
      if (Character == '$')
      {
        // Ignore the dollar sign
      }
      else if (Character == '*')
      {
        // Stop processing before the asterisk
        break;
      }
      else
      {
        // Is this the first value for the checksum?
        if (Checksum == 0)
        {
          // Yes. Set the checksum to the value
          Checksum = Convert.ToByte(Character);
        }
        else
        {
          // No. XOR the checksum with this character's value
          Checksum = Checksum ^ Convert.ToByte(Character);
        }
      }
    }
    // Return the checksum formatted as a two-character hexadecimal
    return Checksum.ToString("X2");
  }
}

VB.NET

' *********************************************************************
' **  A high-precision NMEA interpreter
' **  Written by Jon Person, author of "GPS.NET" (www.gpsdotnet.com)
' *********************************************************************
Imports System
Imports System.Globalization
Public Class NmeaInterpreter
  ' Represents the EN-US culture, used for numers in NMEA sentences
  Public Shared NmeaCultureInfo As New CultureInfo("en-US")
  ' Used to convert knots into miles per hour
  Public Shared MPHPerKnot As Double = Double.Parse("1.150779", _
    NmeaCultureInfo)
  ' Raised when the current location has changed
  Public Event PositionReceived(ByVal latitude As String, _
  ByVal longitude As String)
  Public Event DateTimeChanged(ByVal dateTime As DateTime)
  Public Event BearingReceived(ByVal bearing As Double)
  Public Event SpeedReceived(ByVal speed As Double)
  Public Event SpeedLimitReached()
  Public Event FixObtained()
  Public Event FixLost()
  Public Event SatelliteReceived(ByVal pseudoRandomCode As Integer, _
  ByVal azimuth As Integer, _
  ByVal elevation As Integer, _
  ByVal signalToNoiseRatio As Integer)
  Public Event HDOPReceived(ByVal value As Double)
  Public Event VDOPReceived(ByVal value As Double)
  Public Event PDOPReceived(ByVal value As Double)
  ' Processes information from the GPS receiver
  Public Function Parse(ByVal sentence As String) As Boolean
    ' Discard the sentence if its checksum does not match our
    ' calculated checksum
    If Not IsValid(sentence) Then Return False
    ' Look at the first word to decide where to go next
    Select Case GetWords(sentence)(0)
      Case "$GPRMC"
        ' A "Recommended Minimum" sentence was found!
        Return ParseGPRMC(sentence)
      Case "$GPGSV"
        ' A "Satellites in View" sentence was recieved
        Return ParseGPGSV(sentence)
      Case "$GPGSA"
        Return ParseGPGSA(sentence)
      Case Else
        ' Indicate that the sentence was not recognized
        Return False
    End Select
  End Function
  ' Divides a sentence into individual words
  Public Function GetWords(ByVal sentence As String) As String()
    Return sentence.Split(","c)
  End Function
  ' Interprets a $GPRMC message
  Public Function ParseGPRMC(ByVal sentence As String) As Boolean
    ' Divide the sentence into words
    Dim Words() As String = GetWords(sentence)
    ' Do we have enough values to describe our location?
    If Words(3) <> "" And Words(4) <> "" _
    And Words(5) <> "" And Words(6) <> "" Then
      ' Yes. Extract latitude and longitude
      ' Append hours
      Dim Latitude As String = Words(3).Substring(0, 2) & "°"
      ' Append minutes
      Latitude = Latitude & Words(3).Substring(2) & """"
      ' Append the hemisphere
      Latitude = Latitude & Words(4)       
      ' Append hours
      Dim Longitude As String = Words(5).Substring(0, 3) & "°"
      ' Append minutes
      Longitude = Longitude & Words(5).Substring(3) & """"
      ' Append the hemisphere
      Longitude = Longitude & Words(6)
      ' Notify the calling application of the change
      RaiseEvent PositionReceived(Latitude, Longitude)
    End If
    ' Do we have enough values to parse satellite-derived time?
    If Words(1) <> "" Then
      ' Yes. Extract hours, minutes, seconds and milliseconds
      Dim UtcHours As Integer = CType(Words(1).Substring(0, 2), Integer)
      Dim UtcMinutes As Integer = CType(Words(1).Substring(2, 2), Integer)
      Dim UtcSeconds As Integer = CType(Words(1).Substring(4, 2), Integer)
      Dim UtcMilliseconds As Integer
      ' Extract milliseconds if it is available
      If Words(1).Length > 7 Then UtcMilliseconds = _
        CType(Words(1).Substring(7), Integer)
      ' Now build a DateTime object with all values
      Dim Today As DateTime = System.DateTime.Now.ToUniversalTime
      Dim SatelliteTime As New System.DateTime(Today.Year, Today.Month,
        Today.Day, UtcHours, UtcMinutes, UtcSeconds,
        UtcMilliseconds)
      ' Notify of the new time, adjusted to the local time zone
      RaiseEvent DateTimeChanged(SatelliteTime.ToLocalTime)
    End If
    ' Do we have enough information to extract the current speed?
    If Words(7) <> "" Then
      ' Yes.  Parse the speed and convert it to MPH
      Dim Speed As Double = Double.Parse(Words(7), NmeaCultureInfo)
          * MPHPerKnot
      ' Notify of the new speed
      RaiseEvent SpeedReceived(Speed)
      ' Are we over the highway speed limit?
      If Speed > 55 Then RaiseEvent SpeedLimitReached()
    End If
    ' Do we have enough information to extract bearing?
    If Words(8) <> "" Then
      ' Indicate that the sentence was recognized
      Dim Bearing As Double = Double.Parse(Words(8), NmeaCultureInfo)
      RaiseEvent BearingReceived(Bearing)
    End If
    ' Does the device currently have a satellite fix?
    If Words(2) <> "" Then
      Select Case Words(2)
        Case "A"
          RaiseEvent FixObtained()
        Case "V"
          RaiseEvent FixLost()
      End Select
    End If
    ' Indicate that the sentence was recognized
    Return True
  End Function
  ' Interprets a "Satellites in View" NMEA sentence
  Public Function ParseGPGSV(ByVal sentence As String) As Boolean
    Dim PseudoRandomCode As Integer
    Dim Azimuth As Integer
    Dim Elevation As Integer
    Dim SignalToNoiseRatio As Integer
    ' Divide the sentence into words
    Dim Words() As String = GetWords(sentence)
    ' Each sentence contains four blocks of satellite information. 
    ' Read each block and report each satellite's information
    Dim Count As Integer
    For Count = 1 To 4
      ' Does the sentence have enough words to analyze?
      If (Words.Length - 1) >= (Count * 4 + 3) Then
        ' Yes.  Proceed with analyzing the block. 
        ' Does it contain any information?
        If Words(Count * 4) <> "" And Words(Count * 4 + 1) <> "" _
        And Words(Count * 4 + 2) <> "" And Words(Count * 4 + 3) <> "" Then
          ' Yes. Extract satellite information and report it
          PseudoRandomCode = CType(Words(Count * 4), Integer)
          Elevation = CType(Words(Count * 4 + 1), Integer)
          Azimuth = CType(Words(Count * 4 + 2), Integer)
          SignalToNoiseRatio = CType(Words(Count * 4 + 2), Integer)
          ' Notify of this satellite's information
          RaiseEvent SatelliteReceived(PseudoRandomCode, Azimuth, _
            Elevation, SignalToNoiseRatio)
        End If
      End If
    Next
    ' Indicate that the sentence was recognized
    Return True
  End Function
  ' Interprets a "Fixed Satellites and DOP" NMEA sentence
  Public Function ParseGPGSA(ByVal sentence As String) As Boolean
    ' Divide the sentence into words
    Dim Words() As String = GetWords(sentence)
    ' Update the DOP values
    If Words(15) <> "" Then
      RaiseEvent PDOPReceived(Double.Parse(Words(15), NmeaCultureInfo))
    End If
    If Words(16) <> "" Then
      RaiseEvent HDOPReceived(Double.Parse(Words(16), NmeaCultureInfo))
    End If
    If Words(17) <> "" Then
      RaiseEvent VDOPReceived(Double.Parse(Words(17), NmeaCultureInfo))
    End If
    Return True
  End Function
  ' Returns True if a sentence's checksum matches the calculated checksum
  Public Function IsValid(ByVal sentence As String) As Boolean
    ' Compare the characters after the asterisk to the calculation
    Return sentence.Substring(sentence.IndexOf("*") + 1) = _
      GetChecksum(sentence)
  End Function
  ' Calculates the checksum for a sentence
  Public Function GetChecksum(ByVal sentence As String) As String
    ' Loop through all chars to get a checksum
    Dim Character As Char
    Dim Checksum As Integer
    For Each Character In sentence
      Select Case Character
        Case "$"c
          ' Ignore the dollar sign
        Case "*"c
          ' Stop processing before the asterisk
          Exit For
        Case Else
          ' Is this the first value for the checksum?
          If Checksum = 0 Then
            ' Yes. Set the checksum to the value
            Checksum = Convert.ToByte(Character)
          Else
            ' No. XOR the checksum with this character's value
            Checksum = Checksum Xor Convert.ToByte(Character)
          End If
      End Select
    Next
    ' Return the checksum formatted as a two-character hexadecimal
    Return Checksum.ToString("X2")
  End Function
End Class

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About the author

Jon Person

Jon Person United States

Jon Person is the author of the award-winning “GPS.NET” software now in use in over two thousand GPS applications in all industries, from geocaching to disease outbreak prevention. Jon runs his...

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