speedometer inaccuracy

Psymon wrote: Just about every geostationary satalite (tv and phone, weather, mapping) that goes up has a GPS module on it. They use it keep the satalite in place, becase they can drift about a fair bit, normally up to a kilometer off target, the gps is locked on to a ground target and uses thrusters to get back into postion, after about 10 years all the fuel is used up and the satalite is allowed to burn up in our atmosphere, there's hundrends of them all together, its getting pretty busy up there, the astra and hotbird satallites (you get sky and freesat from these) have to use thrusters to avoid each other.

GPS is accurate at the magic number of 7 locks at sea level traveling dead flat, soon as you travel up hill or down hill it can't cope. first generation couldn't cope with different altitudes either but current generation are lot better on level travel at different altitudes.

As broon says the more locks you have the greater the accuracy you have.
We are talking a variance of half a foot and 0.5 mph here, its good enough for me.

Arent we due for a serious accuracy soon once the european GPS system is up and running (galileo). was meant to be 2012 , but now more like 2018. Should get down to sub-metre accuracy