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Since the WAAS system utilizes stations at known, fixed locations on the ground to make determinations for positional correction, is there a loss in the precision or validity of that correction with an increase in altitude?

If there is, how does it scale? Is the loss in reliability of the correction linear, exponential, is it simply a negligible degradation, etc?

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  • $\begingroup$ It would help if you could explain why you think that distance from the ground-based reference point in the vertical direction would affect reliability more than distance from the same reference in the horizontal direction. After all, an airplane is generally a half-dozen miles above the station at most, but could be dozens or even hundreds of miles away laterally. If the latter distance doesn't affect reliability, why would the former? Are you really asking strictly about altitude, or about distance generally? $\endgroup$ – Peter Duniho Jun 2 at 22:27
  • $\begingroup$ The answer to that question is: because horizontal distance changes (on the surface) would tend to affect the straight line distance from a satellite less than moving upward but less as a satellite's orbit approaches the visible horizon from a given point on the surface. $\endgroup$ – Ryan Mortensen Jun 2 at 23:52
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Yes, but it is a fairly small error. Some of the major sources of GPS error are able to be "corrected" by WAAS. The WAAS stations are used to build a model to make these corrections. These corrections are most accurate near each station, and less so the further away from each station. A couple of the largest errors are due to Satellite clock error and ionosphere delay. Satellite clock error is "correctable" by WAAS and will only depend on the specific satellite so would not change with receiver location. (ie when using Satellite number 1, add 1 microsecond...). On the other hand the ionospheric delay is somewhat location specific (when receiving satellite number 1, while standing at position X, the atmosphere is delaying the signal by Y microseconds). For locations that are not at one of the WAAS ground stations an approximation for the the ionosphere delay must be used.
However, being at altitude is probably not going to change the error by much. I don't have data on how it scales.

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    $\begingroup$ A microsecond is 300 metres (~1000 ft), an insanely huge error for GPS. Normal error is in tens of nanoseconds at worst. $\endgroup$ – Jan Hudec Jun 2 at 21:39
  • $\begingroup$ This was meant simply as an example to explain the concept. An actual correction of 1 microsecond to GPS satellite number 1 should not be used for any air navigation. $\endgroup$ – Adam Jun 3 at 14:11

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