Accuracy of differential GPS
I feel that the accurate statements previously made about GPS absolute accuracy give a misleading impression of the utility of present GPS systems for measuring civilian aircraft attitude.
There are lots of causes of GPS inaccuracy; such as deliberate encoding, atmospheric effects, and engineering imperfections. However these effects are similar for two GPS readings taken at positions close in space and time.
This effect is used in the various differential GPS systems in operation, in which base stations broadcast correction factors. For measuring aircraft attitude its even easier -- the error is surely [almost entirely] canceled when you take the difference between two GPS readings; e.g. the difference in altitude of the two wingtips.
Wikipedia suggests that differential GPS -- with all the bells and whistles -- can provide accuracies of the order of 10 cm. I cannot see why local differencing should not be at least as accurate.
I'm not advocating such a system, just commenting on its engineering feasibility.
Regards Peter
Unsettling idea of the week.
Some years ago I read in the popular scientific press of an idea for increasing runway throughput. Basically you bolt two wide-body jets together. They share a wing and of necessity fly in close formation. You only have pilots in one body; and rely on a data link between the cockpits and GPS sensors all over the place to ensure that the autopilot in the second body "doesn't overstress anything".