Direction & Speed Alerts

GPS devices analyze your position over time to calculate speed and bearing. The $GPRMC sentence at the beginning of this article also includes these readings. Speed is always reported in knots and bearing is reported as an “azimuth,” a measurement around the horizon measured clockwise from 0° to 360° where 0° represents north, 90° means east, and etc. A little math is applied to convert knots into miles per hour. The power of GPS is again demonstrated with one line of code in listing 1-5 which figures out if a car is over the speed limit.

(Listing 1-5: This class can now tell you which direction you're going and help prevent a speeding ticket.)

'**  Listing 1-5.  Extracting speed and bearing
'*******************************************************
Public Class NmeaInterpreter
  ' 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()
  ' 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 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)
      ' 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.  Convert it into MPH
      Dim Speed As Double = CType(Words(7), Double) * 1.150779
      ' If we're over 55MPH then trigger a speed alarm!
      If Speed > 55 Then RaiseEvent SpeedLimitReached()
      ' Notify of the new speed
      RaiseEvent SpeedReceived(Speed)
    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
    ' 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