Some Links...
http://vancouver-webpages.com/peter/ - look under "wiring diagrams and antenna construction"http://vancouver-webpages.com/peter/eagle.gifI was researching this a couple of years ago, and last year I put something together. Basically, the signal levels of the NMEA interface are very close to RS232, and in most cases will work if you wire a serial connector on to your GPS data out cable. I used 3 RS232 to USB adapters because we had instruments on different "networks", and I was able to read data from all of them with no problems. Section 5 here http://www.eoss.org/pubs/nmeafaq.htm has the pinout.An alternative is to buy one of these:http://www.usglobalsat.com/item.asp?itemid=60then you have two separate GPS sources.Kevin