Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix L
DO-253D Change 1
© 2019, RTCA, Inc.
APPENDIX L: RULES FOR B-VALUE USE IN GBAS GROUND STATIONS WHEN
SUPPORTING GBAS AIDED INERTIAL SYSTEMS
There are 4 B-values per satellite in the Type 1 message. There are at least 3 reference
receivers (RRs) in the ground station. For GPS aided inertial systems it makes a difference
in what way the RRs (RR1,...RRK) are assigned to the B-values. An integrated GPS/inertial
system that provides a position bounded by VPL and HPL relies on the standard
assumption that only one of the reference receivers used for broadcast RRi would generate
misleading information (fault). The integration has memory since it calibrates a set of error
states based on the satellite measurements. The typical integration capable of integrity
based coasting will associate a set of error states with each reference receiver RRm tied to
the broadcast B-value Bm (m=1 -4).
Note: The integration has no access to the individual differential corrections in each RR
but these can be reconstructed via the B-values.
As the differentially corrected pseudoranges are smoothed they will exhibit some small
errors that are changing slowly over time due to the smoothing. The integration filter
assumes these do not change abruptly which would be the case if a reference receiver
suddenly was associated with a different B-value. This is one reason the association
between a reference receiver and the B-value should remain fixed over time. For GAD C
reference receivers the pseudorange errors are much smaller than the airborne pseudorange
errors caused by multipath. Consequently, if there were no faults in any of the ground
receivers the fixed association would not be critical for GAD C.
Now assume there is a fault in reference receiver RRi (i is 1, 2, 3 or 4). The inertial error
states associated with RRi will now be miscalibrated. This is expected and the integration
algorithms will be capable of dealing with that as long as there is not a fault occurring in
another RRj (j is 1, 2, 3 or 4) at the same time. Further assume that the ground station
decides to change the association between RRi and the B-value Bi so that RRi now is
associated with another B-value Bj ≠ Bi. The integration has no information that this
happens so the fault now moves away from error set i to instead affect error set j which
was tied to Bj where the fault now resides. Since the integration has memory via the
calibrated error states the error will continue to impact the error states tied to Bi but the
fault is now also impacting the states tied to Bj. If there are 3 RRs operating in this scenario,
then this could lead to the situation where the 3rd correct RR is excluded since the other
two agreed. By holding the association between RRs and B-values fixed this situation can
Ground Facility Requirement Discussion
A requirement to hold the association between RRs and B-values fixed would assure the
inertial integration could successfully use the broadcast. However, when considering the
continuity and availability impact of a faulted or jammed RR, the GAD C ground system
would benefit from a less rigid requirement so that additional backup RRs could
immediately replace a lost RR. It would then also be beneficial if a previously jammed RR
could be reinstated to free up backups for subsequent use.
There are two principles that were used to support developing the current ground
requirement for the association between RRs and B-values.
1. A backup receiver (secondary) that has not been used can always replace any one
of the used RRs (primary) at any time since only one of the available receivers
would be assumed faulted. If the primary RR is faulted it will be replaced by a
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