Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix L
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DO-253D Change 1
fault free RR and the fault is no longer present in the broadcast. If the secondary
RR is faulted then a fault on one RR in use would occur. If one of the other primary
RRs had a fault at this time, the secondary is then assumed to be fault free and
there is no change (still one faulted RR).
2. After the primary RRi has been replaced by a secondary RRk (k is 5,6,... or K) the
memorized information (calibrated error states) in the inertial integration tied to
the primary RRi (in case of a fault) will eventually expire as the fault condition is
removed. This means that the primary RRi could be reused in B-value slot j ≠i. If
there was a fault present in RRi it will now move to B-value slot j but there is no
memory of RRi in slot i so there is only one faulted RR affecting the integration.
Note: As soon as a secondary RR has been used to replace a primary RR the secondary
RR must be treated as a primary RR (i.e., it may have a fault in it and it cannot
immediately be moved to another B-value slot).
The relaxation time needed for the memorized information to expire was set to 5 minutes.
The second principle supersedes the first principle since all backup RRs are a subset of all
RRs that have not been used in the last 5 minutes.
The SARPs requirement (ICAO Annex 10, Appendix B, Section 18.104.22.168.4) states that
during continuous operation the indices 1-4 must correspond to the same physical reference
receiver for every epoch transmitted from a given ground subsystem but adds an exception
that allows a physical reference receiver to be replaced by any other physical reference
receiver that has not been used for transmissions during the last 5 minutes.
1. A physical reference receiver is a receiver with antenna at a fixed location.
2. A physical reference receiver that has not been used for transmissions during the
last 5 minutes may for instance be a primary receiver temporarily not used or a
secondary reference receiver intended as a backup receiver.
Expiration Time Determination
The determination of the expiration time assumed the performance of a GAD C ground
station. An integrated GPS/inertial system using a GAD A or GAD B station may have
different performance. The Type 2 message Ground Accuracy Designator letter can be
used to differentiate station performance if required.
The 5 min expiration time has been determined based on simulation of GBAS faults.
Appendix R in RTCA/DO-316 (and RTCA/DO-229) includes a set of baseline inertial
sensor parameters used to predict expected typical performance. This set of parameters
(providing approximately 1.0 nmi/h 95%) lies somewhere in between what is required in
14 CFR Part 121 Appendix G (traditional standard 2 nmi/h 95%) and the ITAR limit
(corresponding approximately to 0.5 nmi/h 95%). The objective would be to make sure
the fault induced contamination would ring out for the Appendix R baseline type
integrations. In the simulation setup validating the 5 min expiration time it is assumed the
ground station will remove the faulted receiver if the range error exceeds 2.5 m. The total
range error will be limited to 2.5 m but the ramp magnitude and duration may vary. The
expected coasting performance in a good geometry (at Newark with the Optimal 24
Satellite Constellation) is around 1 m (2-sigma) over 2 min (north, east or down). The
objective is to determine the relaxation time necessary to regain that capability after the
fault has been removed. In the simulation a slow 0.01 m/s ramp is applied to the worst
case satellite. The range error grows to 2.5 m during 250 seconds and then the ramp is
removed and a wait period occurs (relaxation time) before coasting is attempted. A
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