| Effects of orbital bias
|
- Errors resulting from the accuracy of the orbit
computation procedure itself
Data used are P code pseudo-ranges, and although the tracking geometry is not
strong (most of the tracking stations are in the equatorial belt), accuracies
better than 5 m are achievable.
- Errors resulting from unpredictable orbital motion
during the period since upload
These are essentially the prediction errors. Their magnitude can vary from a
few meters (close to the time of navigation message upload) to several tens of
meters.
- EE depends on:
- Number and location of tracking stations
- Orbital force model
- Satellite geometry
- EE are uncorrelated between satellites and affect both
code and phase measurements .
- EE produce equal error shifts in calculated absolute point
positions where height is a weakly determined component because
there are no satellites below the horizon . This component
is usually of the order of 2 or 3 times less accurate than
the horizontal components.
- EE of a particular satellite is identical to all users
world wide. However, different users see the same satellite
at different view angles, hence its effect on range measurement
and consequently on computed positions is different.
- Therefore, use of single receiver operation propagates
orbit error into the position results and results in amplification
of positional error. While using two receivers, both will
be in error by nearly the same amount (function of the distance
between the two receivers - the closer they are, the more
similar the error due to orbital bias). Use of Relative
or differential positioning (DGPS), therefore,
is an effective strategy for minimizing the effect of this
bias using differencing operation.
- Magnitude of error: usually in the order of 2 to 5 m,
can be up to 50 m under SA. Range error due to combined effect
of ephemeris and satellite clock ≈ 2.3 m (1 σ level).
|
| |
| |
|
