We've got six BM-affiliated riders taking part in this weekend's MaXx Exposure night enduro, run by Trail Break and Exposure Lights. Five of them - Miles Hill, Tim Burden, Nigel Foskett, Jim Griffiths and Martin Wragg - won our recent competition to win event entries and Exposure lights. The sixth is our very own Dave Arthur, returning to the MaXx after taking part in the first one two years ago.

And to boost the interactivity of the whole event, we've got something very clever for you. As of 6pm on Saturday 22 September (assuming that everything works) the map below will show you Dave's exact location along the route so you'll be able to see just how well (or otherwise) he's doing. Before 6pm the map will be in "simulation mode" just so you can see what it'll look like when it's working.

How does this work?

We're using a tracking rig devised by Neil Newell and used by him on his recent 22h 20m South Downs Double ride. Neil has very generously loaned us the rig and spent some of his valuable time setting things up so that you'll be able to see Dave's exact position along the route on this very page.

The system is in two parts. Dave will have a Garmin eTrex GPS receiver on his bars. Using the eTrex's built-in RS232/power port, an external battery will be hooked up to give enough juice to allow the backlight to be left on continuously. Coming out of the RS232 port will be positional data. Then the clever stuff happens...

A seatpack under Dave's saddle will contain Neil's tracker box. That uses a little embedded microcomputer and a GSM cellphone module with GPRS support - effectively the guts out of a mobile phone. The microcomputer gets the current position from the GPS every minute and then feeds it to the GSM modem which sends it to Neil's webserver over GPRS.

Still with us? Good. The server writes the position to an XML file, and then a bunch of coding cleverness involving Google Maps and AJAX displays a map centred on the exact position, which is marked by, er, a marker. Every time there's a new update, the map shifts appropriately.

It's all clever stuff. Keep an occasional eye on the page - the riders will be setting off shortly after 6pm on Saturday 22 September and Dave's anticipating taking around seven hours to complete the 80 mile route. If you check in, post an update on the forum and we'll be able to see where he was, when, next week...

You can find out more about Neil's GPS tracker at http://www.hazeii.net/projects/gpstrack/gpstrack.html. Many thanks to Neil for offering to set all this up.