Basic Use of GPSgraf

The GPSgraf program runs 24 hours a day, all the time. It reads in the Garmin GPS II+ GPS receiver output through the modem serial port. If the GPS unit is broadcasting correct absolute time, GPSgraf sends a message to SeismoGraf telling it the precise time (and location of the seismometer). It also writes files into the local directory GPSview, which allow for some detailed analysis for absolute time locks. However, for most of the stations, most of the time, SeismoGraf will be locked onto absolute time, and it will note this in the seismogram graphics window.

The default time setting for the SeismoGraf software is "GPS time". This means that SeismoGraf looks for data from the companion program GPSgraf. If it does not receive these data, SeismoGraf will continue to run using the Macintosh clock as its time reference. SeismoGraf "stamps" the seismogram files with a YES or NO for the absolute time lock.

Before starting up GPSgraf the GPS receiver needs to be connected to the computer, turned on, and configured properly. If this has not been done go to the Connecting the Garmin GPS II+ Receiver and Garmin GPS II+ Receiver Setup sections. Also, it is very important the Macintosh clock is set as a 24 hour clock on Coordinated Universal Time (UTC) and precisely synchronized to the GPS clock (see Setting the Macintosh Clock to Coordinated Universal Time).

• The GPSgraf program should reside in the "xxxx_Station" directory. If not place it there. Double-click on the GPSgraf icon.
   
• When GPSgraf starts up two windows will open up, (1) a message window, and (2) a graphics SkyPlot window. The message window will be in front, and can be re-sized to make it easier to see more of the messages. The message window has two columns for messages that scroll up as new messages appear at the bottom, focus your attention on the right-hand column. GPSgraf reads the incoming signal on the modem serial port three times each and every minute; it reads at 20, 30, and 40 seconds of each minute. After it reads the incoming signal, it writes to the right-hand message column.
   
  If GPSgraf has read "0" number of bytes from the serial port, then there is a problem with the connection from the GPS receiver. Check all the hardware connections, and that the GPS "Interface" setting is "NMEA/NMEA 4800 Baud." If a string of "gibberish" goes across the entire column, followed by many rows of cryptic messages, then everything is working OK.
   
• As GPSgraf receives proper absolute time from the GPS receiver, it compares that time to the Macintosh clock. GPSgraf will reset the Mac clock to absolute time when needed, but only in small increments. This is why the Mac clock needs to be manually syncronized to the GPS receiver within a second of the correct absolute time. It has been the experiance of the Ohio Seismic Network that if the Mac clock is off by more than a second GPSgraf will "crash" at this point.
   
• Once GPSgraf is reading the serial output from the GPS receiver, it will show the GPS satellite tracks in the SkyPlot window. It may take a minute or two before the satellite tracks are plotted. At this point, if desired, the message and SkyPlot windows can be closed (click once in the window close box, upperleft corner). GPSgraf should be left running in the background, 24 hours a day, each and every day.
   
• In detail, GPSgraf will send messages to SeismoGraf about the status of time and the corrections made to the Mac clock. When SeismoGraf has shifted the seismogram recording to within 0.1 second of proper absolute time, the graphics window will say "TimeLock=Yes". It also puts this information into the seismogram file so that SeismoView will know this when it reads in the seismogram.