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DO-253D Change 1
b) 5 minutes after liftoff
c) Time satellite was vindicated through fault detection for satellite addition per
A satellite shall [LAAS-325] be excluded from the GAST D precision approach position
solution within 2 seconds from the onset of all of the following conditions being met
a) a CCD failure condition presently exists; and
b) the elapsed time since liftoff is greater than 5 minutes; and
c) the computed ground speed is greater than 30 knots; and
d) the computed position is inside the PAR.
Note: The elapsed time since liftoff should be reset to zero if the airplane is on ground
and the ground speed is below 30 knots. The elapsed time since liftoff can be
assumed to have occurred greater than 5 minutes prior to power up or selection
of a new approach, when the aircraft is in the air at the time of power up or
selection of a new approach. Aircraft in air status may be determined using height
above the selected LTP/FTP or other available information such as ground speed.
Once a satellite has been excluded by this monitor, it shall [LAAS-425] be considered
eligible for reinclusion in the GAST D precision approach position solution if one or more
of the above conditions “a” through “d” are no longer met.
1. The purpose of this monitor is to detect abnormally large gradients in the
ionospheric delay that could cause unacceptable errors in the differential position
solution. The purpose for requiring a CCD statistic that has been below the
threshold for a history window of up to 20 minutes is to ensure that the airplane
does not steadily track the motion of an ionospheric gradient just prior to landing
and thereby experience a "stationary front" scenario.
2. Exclusion of satellites by the CCD filter threshold test is limited to the PAR in a
time window from 5 minutes after takeoff until the landing operation is completed
to avoid nuisance exclusions. The intent is to limit the inappropriate exclusion of
satellites affected by multipath, due to low or no aircraft speed and the proximity
to potential reflectors such as buildings.
3. LAAS-425 only addresses the eligibility of a satellite when CCD monitor exclusion
conditions clear. The satellite may remain unusable in the position solution due
to other conditions as described in this MOPS.
4. These requirements were developed for fixed wing aircraft and were not validated
Message Processing Function
The PAN equipment for all equipment classifications shall [LAAS-107] be capable of
processing LAAS Message Types 1, 2, and 4 as defined in the RTCA/DO-246().
The PAN equipment for equipment classification D shall [LAAS-326] be capable of
processing LAAS Message Type 11 as defined in the RTCA/DO-246().
When processing Type 2 messages, the PAN equipment shall [LAAS-277] utilize the
message length parameter so that it can decode Type 2 messages without additional
message blocks, as well as Type 2 messages with one or more additional message blocks.
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