VB6 NMEA Interpreter Class for Reading GPS

Comments

1. Alex Etchells
   20 Mar 2006 at 13:13

   As I delve deeper into NMEA sentences I am improving my understanding.
   The value that is extracted from the GGA string as 'altitude' is in fact the height
   above the WGS84 ellipsoid and not altitude.
   
   Alex
   
   
   
   
   
   

2. Alex Etchells
   28 Mar 2006 at 14:35

   The software in which I was using this class started throwing up the odd runtime error.
   To cut a long story short the GPS unit in question was sending truncated sentences and very occasionally the last 2 characters of the truncated sentence happened to equal the checksum resulting in an incomplete sentence being treated as valid.
   
   Here is the replacement IsValid function to prevent this from happening.

   Private Function IsValid(ByVal sentence As String) As Boolean
   ' Compare the characters after the asterisk to the calculation
   ' Returns True if a sentence's checksum matches the calculated checksum
   
   
   
   If Len(sentence) < 3 Then
       IsValid = False
       Exit Function
   End If
   
   
   If Mid$(sentence, Len(sentence) - 2, 1) <> "*" Then
       IsValid = False
       Exit Function
   End If
   
   
   
   If Right$(sentence, 2) = GetChecksum(sentence) Then
       IsValid = True
   Else
   
       IsValid = False
   End If
   
   End Function
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

   
   Alex
   
   
   
   

3. Ken Freudenberg
   29 May 2006 at 02:45

   Very useful code.
   However there is an a error which prevents displaying "Satellite time"
   
   The following references the "time" function instead of passing a variable:
   
   
   Private Sub gpsTimeChanged(ByVal Time As String)
   LabelTime.Caption = Time
   End Sub
   
   Accordingly the only time displayed is the system time, not the satellite time.
   
   Suggested simple change:
   
   Private Sub gpsTimeChanged(ByVal CTime As String)
   LabelTime.Caption = CTime
   End Sub
   

4. Alex Etchells
   16 Jun 2006 at 09:51

   Thank you for the compliment. =o)
   
   It's interesting that using Time causes an error for you.
   VB6NmeaInterpreter appends 'GMT' to the time string and this is what I see when I run the software, so I can be confident that I am seeing the satellite time.
   
   I wonder what is making things different for you.
   
   Certainly your suggested fix would avoid the issue in the test application.
   
   Changing
   
   Public Event TimeChanged(ByVal Time As String)
   
   to
   
   Public Event TimeChanged(ByVal satTime As String)
   
   in the VB6NmeaInterpreter class would prevent any future confusion, if using the class in another application.
   
   regards
   
   Alex
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

5. ra d
   16 Aug 2007 at 21:01
   I have a question regarding GPS bluetooth and programming in visual basic. Where and how do I specify to my VB project the port # of my GPS port? In other words my GPS's port is 41, in VB what do I need to do in order to connect to that port and get the info out of the GPS into strings in VB??
   
   Anyone's help is greatly appreciated..
   Thanks,
   R..