Are We Fixed Yet?

The $GPRMC sentence includes a value which indicates whether or not a “fix” has been obtained. A fix is possible when the signal strength of at least three satellites is strong enough to be involved in calculating your location. If at least four satellites are involved, altitude also becomes known. The third word of the $GPRMC sentence is one of two letters: “A” for “active,” where a fix is obtained, or “V” for “invalid” where no fix is present. Listing 1-6 includes code to examine this character and report on fix status.

Listing 1-6: The interpreter now knows when the device has obtained a fix.

'*******************************************************
'**  Listing 1-6.  Extracting satellite fix status
'*******************************************************
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()
  Public Event FixObtained()
  Public Event FixLost()
  ' 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
    ' 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
  ' 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

As you can see, a whole lot of information is packed into a single NMEA sentence. Now that the $GPRMC sentence has been fully interpreted, the interpreter can be expanded to support a second sentence: $GPGSV. This sentence describes the configuration of satellites overhead, in real-time.