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DO-253D Change 1
For GAEC D equipment, when the active Approach Service Type is GAST D and the
aircraft is inside the PAR, the PAN shall [LAAS-353] perform fault detection on the
precision approach position solution using redundant corrected pseudorange measurements
if more than four satellites with associated differential corrections are available. The test
statistic and threshold shall [LAAS-434] be computed based on the GAST D projection
matrix (S) defined in Section 22.214.171.124.3 applied to the 100 second smoothed pseudoranges
defined in Section 126.96.36.199.1 and GAST D 100 second smoothed sigmas (σi_x) as defined in
The fault detection algorithm shall [LAAS-354] be performed at a rate of at least once per
15 seconds, as well as within 2 seconds of the most recent satellite geometry change or
achieved service type change to GAST D. A fault shall [LAAS-355] be declared as detected
when the test statistic exceeds the threshold.
The probability of missed detection has no requirement since no protection level is
computed for this monitor. The fault detection threshold shall [LAAS-356] be set to
provide a probability of false alert equal to 2 × 10-8 per sample.
1. This fault detection algorithm is intended to provide additional means for detection
of rare ionosphere gradient anomalies and position errors that may go undetected
by other methods required by this MOPS.
2. The intent of requiring this specific false alert rate is to provide the best detection
possible while also limiting the probability of fault-free detection, regardless of
the rate that the fault detection is performed.
3. The failure response for this fault detection algorithm is given in Section
188.8.131.52.2.1.2 for loss of approach guidance for GAST D.
Fault Detection for Satellite Addition
GAEC D equipment, when the active Approach Service Type is GAST D, shall [LAAS-
357] perform fault detection as specified in this section at least once after the airborne Code
Carrier Divergence monitor output has converged below its threshold (see Section
The equipment shall [LAAS-358] form a test statistic and compare it to a threshold set to
provide a Probability of False Alert of no greater than 0.001 per sample. In addition, a
vertical protection level, VPLFD, and a lateral protection level, LPLFD, shall [LAAS-397] be
computed based on the detection thresholds and a probability of missed detection of 0.001
per sample. The test statistic, the threshold, LPLFD and VPLFD shall [LAAS-398] be
computed based on the hypothesis that there is a bias (due to a stationary ionosphere
gradient) affecting only the satellite measurement to be added (as opposed to the hardest
to detect satellite measurement), and with all other measurement errors defined by their
expected distributions. The test statistic and threshold shall [LAAS-435] be computed
based on the GAST D projection matrix (S) defined in Section 184.108.40.206.3 applied to the 100
second smoothed pseudoranges defined in Section 220.127.116.11.1 and GAST D 100 second
smoothed sigmas (σi_x) as defined in Section 18.104.22.168.2.1.4.
The satellite measurement shall [LAAS-359] be considered vindicated for the purpose of
defining CCD monitor history window (See Section 22.214.171.124), when all of the following
a) VPLFD + DV is less than FASVAL (as defined in RTCA/DO-246), and
b) LPLFD + DL is less than FASLAL (as defined in RTCA/DO-246), and
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