and when you factor in the constantly changing Bias the american GPS system uses i don't trust it 100% my Garmin hand held GPS unit i use for walking and Mountain bike comes up with some real strange readings (even when standing still) and that can track 12 satellites and in clear skys still register's a 20 meter inaccuracy
this is done to prevent you using a satnav to pilot a plane into a building (only encrypted military units can decode it correctly)
Uh oh!!! You appear to be talking about Selective Availability. This contantly changing code was used during certain times to reduce the accuracy of civilian GPS systems, it could be limited down to something in the region of 100 metres but was typically around 10 metres of added inaccuracy. This was used to stop the enemies having accurate targeting systems based on it. SA was permanantly turned off in 2000 under an agreement with the Federal Aviation Administration to allow more accurate and reliable systems so planes can land automatically using GPS.
Military GPS is more accurate as it uses a completely separate encrypted signal on a different frequency that can be compared with the standard signal to reduce path errors and interference.
The civilian equavalents, satellite based Wide Area Augmentation Systems, available in the united states, europe and japan, allow accuracy of greater than 7.5 metres 95% of the time and less than 1 metre 50% of the time and is available for 99.999% of the year. However their geostationary orbits can make them difficult to pick up on the ground due to a low level on the horizon.
The system will use doppler shifts as well as location to produce a velocity, however, it must be remembered that this can only be considered accurate when travelling on a completely flat surface with not inclines or descents. The way it is measured is the same as the way radar guns work, and is extrememly accurate with the biggest failing being the quality of the hardware.
I have tested 3 cars with both a laptop connected GPS and a Sony Sat-Nav unit. And guess what!! They both gave different readings!! The failing is the quality of the chips in the sat nav units. They are mass produced and cheaply manufactured.
Both units despite differences between the two gave steady readings indicating that the speedos on all 3 cars overread by around 4-8%. This means that if your actual speed was 50 mph, the indicated speed would be 52 to 54 mph.
'Your Mileage May Vary' due to tyre sizes, pressures and other variables, however I think most manufactured tend to produce cars with speedos calibrated to around this area.
Wow, theres my essay for the night
No offence Andy or anyone else, im just rediculously bored and like to lecture. I think i should become a very boring and angry teacher :devil:
Wow, just realised how off topic I am, Sorry
