[dtengineering]

Here's a couple ideas to kick around... rather than using a radio signal, why not an ultrasonic signal? Since sound travels more slowly, it is a bit easier to measure the timing differences accurately. If there were four transmitters (one in each corner of the field) and each fired a slightly different length of noise at a pre-defined interval, it may be possible for the robot to determine its position with a fair bit of precision... barring reflections and other nastiness.

Or take a couple of CMUcams (or perhaps something with a bit higher resolution) and point them up at the ceiling... use some of the fixed field lighting as "stars" for an "astonomical" positioning system. It would require a fair bit of effort to deal with blinking and moving lights and such, let alone lens flares and other real-world weirdness, but in the happy, happy world of pure theory... it might work.

But anyways, that is off the point of the thread... my Garmin Etrex Legend has an RS-232 port and should be able to "talk" to a VEX. Although I've only used it for downloading track logs and uploading maps, I believe it IS capable of doing a real-time GPS location feed. The downside is that it has an older chipset... I've found the new SirfStar III devices to have much faster acquisition times and better reception. Some of them even work indoors.

There are also neat tricks for improving GPS accuracy. One of the less expen sive ones... although not a particularly useful one for real-time robotics applications... is to leave your GPS sitting in one place for an hour or so and record all the positional readings (they will fluctuate a bit). Then you compare them to the readings of a nearby receiver at a known location. Since some of the errors in your signal are caused by random atmospheric fluctuations, and since a nearby receiver will have similar fluctuations, then the errors between the two sets of readings should be similar. Since the reference receiver is at a known location, it will be possible to determine a known error, and modify the results for the mobile GPS unit. I suppose this technique could be modified to offer real-time benefits, but I don't know if I'd want to do that on a VEX platform.

Jason