Writing GPS Applications in .NET: Part 1

International readers may have spotted a subtle problem early on that was not handled in the listings – numbers were being reported in the numeric format used in the United States! Countries like Belgium and Switzerland which use different formats for numbers, require adjustments to the interpreter in order to work at all. Fortunately, the .NET framework includes built-in support for converting numbers between different cultures, so the changes to the interpreter required are straightforward. In the interpreter, the only fractional value is speed, so only one change is necessary. The NmeaCultureInfo variable represents the culture used for numbers within NMEA sentences. The Double.Parse method is then used with this variable to convert speed into the machine’s local culture. Listing 1-8 shows the completed interpreter, now ready for use internationally.

Listing 1-8: The completed interpreter, suitable for use anywhere in the world.

'**  Listing 1-8.  Adding support for international cultures
'*************************************************************
Imports System.Globalization
Public Class NmeaInterpreter
  ' Represents the EN-US culture, used for numers in NMEA sentences
  Private NmeaCultureInfo As New CultureInfo("en-US")
  ' Used to convert knots into miles per hour
  Private 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)
  ' 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"
        Return ParseGPGSV(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
      Dim Latitude As String = Words(3).Substring(0, 2) & "°"  ' Append hours
      Latitude = Latitude & Words(3).Substring(2) & """"    ' Append minutes
      Latitude = Latitude & Words(4)    ' Append the hemisphere
      Dim Longitude As String = Words(5).Substring(0, 3) & "°" ' Append hours
      Longitude = Longitude & Words(5).Substring(3) & """"    ' Append minutes
      Longitude = Longitude & Words(6)    ' Append the hemisphere
      ' 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)
      End If
      ' 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 = CType(Words(8), Double)
      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
  ' 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

Final Thoughts

You should now have a good understanding that an NMEA interpreter is all about extracting words from sentences. You can harness the power of satellites to determine your location, synchronize your computer clock, find your direction, watch your speed, and point to a satellite in the sky on a cloudy day. This interpreter will also work with the .NET Compact Framework without any modifications. If sentences were also stored in a file, the interpreter can be used to play back an entire road trip. These are all great features, especially considering the small size of the class, but is this interpreter ready to navigate your car and help your golf game? Not quite yet. There is one important topic remaining which is required to make GPS applications safe for the real world: precision.
GPS devices are designed to report any information they find, even if the information is inaccurate. In fact, information about the current location can be off as much as half a football field, even when devices are equipped with the latest DGPS and WAAS correction technologies! Unfortunately, several developers are not aware of this problem. There are some third-party components out there which are not suitable for commercial applications that require enforcing a minimum level of precision. Keep this article handy, however, because in part two of this series, I will explain precision enforcement in detail and take the interpreter even further to make it suitable for professional, high-precision applications!

(To be continued...)

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