Having a 5Hz or 10Hz NMEA update rate does not make the systems more accurate. What you really need is either a dead reckoning system using a gyro and accelerometer to extrapolate the points in between the one second samples, or you need to process the RAW binary data from the GPS and write your own algorithms to process the data for better resolution. (or use a combination of RAW data and additional sensors. That's how we do it on the GPS products I work on.
And there is absolutely no reason that the GPS signal at Mosport would be weaker than anywhere else. If you want to read about the sort of module I use, here's a link: http://www.u-blox.com/images/downloa...W-10018%29.pdf
Just a module like that is more then the price of those external receivers that plug into your phone.
Here is one solution for an external GPS/GSM antenna that mounts on the inside of your windshield http://search.digikey.com/scripts/Dk...893cs926574116