why is the vertical error likely to be greater than the horizontal error?

Hi,

I've just been looking through some of my lecture notes on GPS systems
for air traffic navigation, and at one point it says that the vertical
erroe can be up to 4 times the magnitude of horizontal error.

Could anybody tel me why this is?

The only sources of error I can think of would be anomilies in the
ionosphere or water vapour in the troposhpere "bending" the EM waves
from satellites.

I wouldn't think that the location of antennas on the aircraft, because
it just seems like an unacceptable design limitation to me.

Any help would be greatly appreciated.

Thanks,
Adam

Adam Chapman wrote:

I guess if i imagined a satellite above the aircraft and a straight
line betwee the two, and imagined a standard error along that vector i
could translate the vector in to horizontal and verticle components.
if the ratio between verticle and horizontal error is 4:1, the angle fo
the satellite above the aircraft's forward flight vector would be about
76 degrees.

Am I on the right track?

Hi Adam,
In the case of the horizontal plane the trigometrical calculation is
in two dimensions whereas with vertical (height) you are calculating
in 3 dimensions.
If the satellites could transmit their signals through earth there
would not be a problem.

On 20 Jan 2007 09:07:17 -0800, "Adam Chapman"


I guess the lack of angular diversity between the locked satellites. In
the horizontal plane you can track satellites in up to 360 degrees of sky
but in a vertical plane you are restricted to just up to 180 degrees
because a certain planet is obscuring satellites in the other 180 degrees.

Tony