Home' RTCA Documents for Review : DO-253D_Change1_FRAC Contents Appendix K
© 2019, RTCA, Inc.
DO-253D Change 1
stages of approach flies over or near the VOR prior to the runway threshold. It could also
apply where a VOR is located near the runway.
Summary of Continuity Allocations
The signal-in-space (SIS) continuity allocations in the LAAS MASPS RTCA/DO-245A
(Table D-8) include a continuity allocation for the “Continuity VDB Channel Reception” .
The GBAS Service Level (GLS) defined in DO-245A that corresponds with GAST C is
GLS C, which in DO-245A Table D-8 had an allocated continuity for the VDB Channel
Reception of 1x10-7
per 15 seconds exposure time, which is written as 1x10-7
After RTCA DO-245A was completed, GAST D service was defined to have more
stringent integrity requirements, but the same continuity as the GAST C service at and
above 200 feet above the runway threshold. Thus, the 1x10-7
per 15 seconds also applies
to the GAST D service at and above 200 feet above the Landing Threshold Point (LTP).
In the DO-245A, the most stringent allocations for the SIS continuity of VDB channel
reception in this table is for GSL F, which was intended to support Category III operations
without inertial augmentation, and it must also meet the continuity allocations for lower
level GSLs. This includes 3x10-8 /15 seconds (for GSL E) and 6x10-8/30 seconds (for GSL
F). These allocations were intended to apply when the aircraft position is in the PAR and
below 200 feet above the LTP. Prior to the aircraft position being below 200 feet above
the LTP, the GLS C (aka GAST C) continuity applies for precision approach operations
being supported above 200 feet. For GAST C, in DO-245A the VDB channel reception
continuity allocation is 1x10-7
/15 seconds. The rationale for the specified nominal MFR
of 0.001 is to ensure meeting the GLS F continuity allocation.
The GAST C and D continuity allocations (in the RTCA DO-245A MASPS, Table D-8)
were based on specific risk, assuming 10 critical satellites (which is excessively
conservative). GAST D continuity is still considered as specific risk; however, GAST C
continuity has since been changed to average risk in the ICAO Annex 10 GBAS SARPs.
The nominal number of critical satellites is less than 1. A more practical upper limit on the
number of critical satellites is 5. Such a change in analysis assumption, if needed, allows
for a reassessment of some of the continuity allocations.
Loss of Continuity (LOC) due to Loss of Multiple Messages
Given that the probability of the loss of a single message is defined by the Message Failure
Rate (e.g., 0.001), the probability of the loss of X consecutive messages in a single trial
(assuming message loss independence among messages) can be calculated as follows:
P(Single_Message_Loss) = MFR
P(X_Consecutive_Losses_One_Trial) = [P(Single_Message_Loss)]X
Where: MFR = Message Failure Rate (or Messages Failure Rate in case of set of Type
1 and Type 11 Messages with the same modified z-count)
X = Number of consecutive relevant missed message opportunities that result
However, when the exposure time period for loss of continuity (LOC) is specified to be a
time duration that is longer than the time period for the loss of X consecutive messages,
Links Archive DO-246_revE_Change1-FRAC ED-ABC_DO-XYZ_OC_FRAC_Profiles_SC223_WG108 Navigation Previous Page Next Page