Taking Out the Garbage

A checksum is calculated as the XOR of bytes between (but not including) the dollar sign and asterisk. This checksum is then compared with the checksum from the sentence. If the checksums do not match, the sentence is typically discarded. This is okay to do because the GPS devices tend to repeat the same information every few seconds. With the ability to compare checksums, the interpreter is able to throw out any sentence with an invalid checksum. Listing 1-3 expands the interpreter to do this.

(Listing 1-3: The interpreter can now detect errors and parse only error-free NMEA data.)

'*******************************************************
'**  Listing 1-3.  Detecting and handling NMEA errors
'*******************************************************

Public Class NmeaInterpreter

  ' Raised when the current location has changed
  Public Event PositionReceived(ByVal latitude As String, _                                ByVal longitude As String)

  ' 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
    ' 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