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include receiver noise, multipath, and errors in the calibration of the antenna phase centre. Receiver noise has a zero-mean,
normally distributed error, while the multipath and antenna phase centre calibration can result in a small mean error.
7.5.5 Residual tropospheric errors. Tropospheric parameters are broadcast in Type 2 messages to model the effects of
the troposphere, when the aircraft is at a different height than the GBAS reference point. This error can be well-characterized
by a zero-mean, normal distribution.
7.5.6 Residual ionospheric errors. An ionospheric parameter is broadcast in Type 2 messages to model the effects of
the ionosphere between the GBAS reference point and the aircraft. This error can be well-characterized by a zero-mean,
normal distribution.
7.5.7 Aircraft receiver contribution to corrected pseudo-range error. The receiver contribution is bounded as described
in Section 14. The maximum contribution, used for analysis by the GBAS provider, can be taken from the accuracy
requirement, where it is assumed that σreceiver equals RMSpr_air for GBAS Airborne Accuracy Designator A equipment.
7.5.8 Airframe multipath error. The error contribution from airframe multipath is defined in Appendix B, 3.6.5.5.1.
Multipath errors resulting from reflections from other objects are not included. If experience indicates that these errors are not
negligible, they must be accounted for operationally or through inflation of the parameters broadcast by the ground (e.g.
σpr_gnd).
7.5.9 Ephemeris error uncertainty. Pseudo-range errors resulting from ephemeris errors (defined as a discrepancy
between the true satellite position and the satellite position determined from the broadcast data) are spatially decorrelated and
will therefore be different for receivers in different locations. When users are relatively close to the GBAS reference point,
the residual differential error due to ephemeris errors will be small and both the corrections and uncertainty parameters σpr_gnd
sent by the ground subsystem will be valid to correct the raw measurements and compute the protection levels. For users
further away from the GBAS reference point, protection against ephemeris failures can be ensured in two different ways:
a) the ground subsystem does not transmit the additional ephemeris error position bound parameters. In this case, the
ground subsystem is responsible for assuring integrity in case of satellite ephemeris failures without reliance on the
aircraft calculating and applying the ephemeris bound. This may impose a restriction on the distance between the
GBAS reference point and the decision altitude/height depending upon the ground subsystem means of detecting
ranging source ephemeris failures. One means of detection is to use satellite integrity information broadcast by
SBAS; and
ATT D-23 23/11/06
Annex 10 — Aeronautical Communications Volume I
b) the ground subsystem transmits the additional ephemeris error position bound parameters which enable the airborne
receiver to compute an ephemeris error bound. These parameters are: coefficients used in the ephemeris error
position bound equations (Kmd_e_(), where the subscript () means either “GPS”, “GLONASS”, “POS, GPS” or “POS,
GLONASS”), the maximum use distance for the differential corrections (Dmax), and the ephemeris decorrelation
parameters (P). The ephemeris decorrelation parameter (P) in the Type 1 or Type 101 message characterizes the
residual error as a function of distance between the GBAS reference point and the aircraft. The value of P is
expressed in m/m. The values of P are determined by the ground subsystem for each satellite. One of the main
factors influencing the values of P is the ground subsystem monitor design. The quality of the ground monitor will
be characterized by the smallest ephemeris error (or minimum detectable error (MDE)) that it can detect. The
relationship between the P parameter and the MDE for a particular satellite can be approximated by Pi = MDEi/Ri
where Ri is the smallest of the predicted ranges from the ground subsystem reference receiver antenna(s) for the
period of validity of Pi. Being dependent on satellite geometry, the P parameters values are slowly varying.
However, it is not a requirement for the ground subsystem to dynamically vary P. Static P parameters could be sent
if they properly ensure integrity. In this latter case, the availability would be slightly degraded. Generally, as MDE
becomes smaller, overall GBAS availability improves.
7.5.10 Ephemeris error/failure monitoring. There are several types of monitoring approaches for detecting ephemeris
errors/failures. They include:
a) Long baseline. This requires the ground subsystem to use receivers separated by large distances to detect ephemeris
　
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