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DO-253D Change 1
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data blocks in the Type 2 message, and unrecognized data blocks in the Type 4
message. Receiver designers must ensure that the presence of unrecognized
content on the datalink will not prevent normal processing of data in recognized
formats even if that data appears after the unrecognized content. For example, the
receiver should be able to find and use a FAS Data Block in a Type 4 message that
occurs after a data block describing another type of path definition in the Type 4.
Conditions for Use of Differential Corrections
Ephemeris CRC Conditions
A GPS ranging source shall [LAAS-117] not be used in the position solution until the
ephemeris data received from that source is verified by calculating the ephemeris CRC as
defined in the RTCA/DO-246() and comparing it to the Ephemeris CRC value broadcast
in the Type 1 message. For each ranging source used in the position solution, the calculated
CRC shall [LAAS-273] be compared to the broadcast CRC within 1 second after receiving
a new broadcast IOD. A new broadcast IOD is one whose value is different from the
broadcast IOD last received for the same ranging source. The equipment shall [LAAS-118]
cease using any satellite for which the computed and broadcast CRC values fail to match.
1. The LGF will not immediately base its corrections on the satellite clock and
ephemeris values in the GPS navigation message that has just changed, as
described in RTCA/DO-246().
2. The verification of the ephemeris CRC ensures that, for GBAS operations, there is
no possibility of mistaking one GPS satellite for another due to cross-correlation
during acquisition or reacquisition. Note that this ephemeris check can also be
used to satisfy the similar requirement in Section 22.214.171.124 of RTCA/DO-229().
Reference Time Conditions
All satellites used in the differential position solution shall [LAAS-120] use corrections
from the same ground station and with the same reference time as indicated by the modified
z-count, which is referred to as a set of differential corrections. For all equipment classes,
this set contains differential corrections for the 100 second smoothed pseudoranges (from
a Type 1 message or a linked pair of Type 1 messages). For GAEC D equipment when
operating to provide GAST D, this set also contains differential corrections for the 30
second smoothed pseudoranges (from a Type 11 message or a linked pair of Type 11
messages) in addition to the 100-second differential corrections. The most recently
received set of corrections shall [LAAS-121] be applied, including pseudorange
corrections, range rate corrections, refractivity index, and scale height. If the number of
measurements field in the Type 1 message indicates that there are no corrections, then the
equipment shall [LAAS-122] not apply any corrections.
Note: Differential corrections that do not belong to the same set cannot be used in the
position solution since the clock errors in the pseudorange corrections may be
relative to different sets of conditions.
Other Ranging Source Conditions
Ranging Source Conditions for All Equipment Classes
The following conditions apply to all equipment classifications.
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