knpjdad,

You're close, the acceleration due to gravity is approximately 32.2 ft per second squared. This does not take into account drag or any other opposing force.

ptebbe,

I don't think the accuracy of the GPS unit (with regards to velocity) are a function of velocity, rather a function of the distance traveled in which the velocity is measured. Remember, velocity is the distance traveled divided by the time it takes to travel that distance. The farther the subject travels, the more accurate the indicated velocity. This is because the error in the GPS unit becomes very tiny compared to the total distance traveled. It's a calculus problem Take care!