I read the title and assumed it would be some sort of sensor fusion with accelerometers. Anyone know what kind of price/accuracy you get with that? My experience says a good 9-axis IMU costs $1000 which is a lot more than they're aiming for, but for differential movement it seems to come down more to a high-frequency hardware integrator. I've seen some compelling things just using cellphone hardware.
I've delved into this question some time ago and it's hard to make it work. First of all, the IMU measures the second derivative of your position/orientation, so you get double integration error, which blows up really quickly. Second, since it is completely relative, your accuracy is only as good as the best static accuracy of your GPS. So the conclusion is it's usefulness would be limited to filling in gaps in the GPS while at movement, decreasing latency, and improving velocity measurement; nice, but not game changing. Lastly, I believe the greatest problem is the diversity of IMUs out there: you need a lot of consistency out of the IMU to get anything done at all (this may be solvable if you settle on a single manufacturer).
Lately I've put more faith in using cameras to get good positioning everywhere (as good as you can get your GPS accuracy, it's not going to work under a canopy or inside a building, and because of double drift IMU's just cannot solve it either for more than a short time span).
