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DO-253D Change 1
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. Validation of ICAO SARPs ionosphere gradient monitor requirements
concluded that this monitor must be operating for at least 60 seconds prior to
reaching the LTP/FTP to ensure sufficient observation time to detect certain
ionosphere gradient conditions. The wait time in LAAS-420 represents a minimum
of 60 seconds of observation time after 140 seconds of PDIFF convergence time and
is not applied if the satellite is added outside the PAR.
2. Exclusion of satellites by this test is limited to a time window from the specified
time after liftoff 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. Time since liftoff should be reset to zero if the
airplane is on ground and the ground speed is below 30 knots. 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
3. These requirements were developed for fixed wing aircraft and were not validated
4. LAAS-424 only addresses the eligibility of a satellite when DSIGMA monitor
exclusion conditions clear. The satellite may remain unusable in the position
solution due to other conditions as described in this MOPS.
Position, Velocity, and Time (PVT) Outputs
When outputting the PVT based on GBAS differential corrections, the requirements in this
section and its subsections apply.
Note: The GBAS positioning service is a service that is distinct and separate from the
GBAS precision approach services categorized by GBAS Approach Service Types.
For the positioning service the PAN always computes a position solution based on
the 100 second smoothed pseudoranges and differential corrections supplied in
the current Type 1 message.
The PAN equipment shall [LAAS-278] verify the following before commencing the output
of differentially corrected position:
a) The ground station supports the differentially corrected positioning service (i.e.,
verify that RSDS is provided, and that it is not equal to ‘1111 1111’).
b) The distance (slant range) between the aircraft and the GBAS reference point is
less than the maximum GBAS usable distance (Dma x ), if Dma x is provided in the
Type 2 message being used.
The PAN equipment shall [LAAS-130] output differentially-corrected position based on
the 100-second smoothed pseudoranges and pseudorange corrections, in latitude, longitude
and height above the WGS-84 ellipsoid, three-dimensional velocity (east, north, and up),
and time at a rate of at least once per second.
1. A position output at 5 Hz or more may be required to support other applications.
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