Performance Assessment of Dual FrequencyGBAS Protection Level Algorithms using a Dual Constellation and Non-Gaussian Error Distributions
2009
The landing of aircraft under low visibility conditions has
always been a challenge even with conventional
navigation systems like ILS (Instrument Landing
System). The requirements for CAT III can not be
reached with Ground Based Augmentation Systems
(GBAS) for single frequency GPS only without relaxing
the alarm limits and continuity requirements of air
navigation. Large delay gradients between the GBAS
ground station and the user caused by ionospheric
anomalies, remain the main threat for GBAS.
Using GBAS with both GPS and Galileo in a combined
constellation will increase the robustness of the complete
system. Galileo is providing promising features like the
possibility offered to the aviation community to acquire 3
frequencies: L1, E5a and E5b in the Aeronautical Radio
Navigation Service (ARNS) band. The consideration of
phase observations allows the use of efficient smoothing
techniques: the ionosphere free and the divergence free
dual frequency smoothing algorithms which have been
defined in [1], allow to mitigate or even to cancel the
ionosphere gradient. Due to the different geometry
characteristic of the extended constellation the Geometry
Dilution of Precision (GDOP) is reduced. The low
probability of satellite outages combined with the number
of additionally available satellites will dramatically
improve the availability of the combined GPS and
Galileo system.
The objective of this work is to analyze the impact of
Galileo through the use of a combined constellation on
the performance of GBAS under severe ionospheric
gradients. The errors experienced by a user with a spatial
separation of 5km and 20NM respectively to the GBAS
ground station are evaluated. The simulation scenario
considers an ionosphere anomaly with a gradient of
420mm per km between the ground station and the user –
a value which has turned out to be a worst-case
assumption as explained before in several publications
[2]. The dual frequency smoothing techniques mentioned
above are applied.
The simulation is performed over a period of several days
to account for the effects of the changing satellite
geometry.
The models of the GBAS residual errors used in the
preceding work [3] were considered to be Gaussian
distributed individual errors. To give a more realistic
representation of the individual errors used in the
simulation, we use distributions of errors which are, in
general not Gaussian. The distributions are derived from
measurements or theoretical considerations pertaining to
the origin of the error. Here, we consider the four major
individual sources of pseudorange error in GBAS
systems: receiver noise, ionosphere and troposphere,
multipath.
For this work, the impact of applying smoothing filter,
averaging and position calculation are taken into account.
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