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ATT D-13 23/11/06
Annex 10 — Aeronautical Communications Volume I
6.5.2 Data broadcast intervals. The maximum broadcast intervals between SBAS messages are specified in
Appendix B, Table B-54. These intervals are such that a user entering the SBAS service broadcast area is able to output a
corrected position along with SBAS-provided integrity information in a reasonable time. For en-route, terminal and NPA
operations, all needed data will be received within 2 minutes, whereas for precision approach operations, it will take a
maximum of 5 minutes. The maximum intervals between broadcasts do not warrant a particular level of accuracy
performance as defined in Chapter 3, Table 3.7.2.4-1. In order to ensure a given accuracy performance, each service
provider will adopt a set of broadcast intervals taking into account different parameters such as the type of constellations
(e.g. GPS with SA, GPS without SA) or the ionospheric activity.
6.5.2.1 For fast corrections, Table B-54 allows the maximum broadcast interval to be 60 seconds. Table B-56 gives
data time-out intervals for the various data messages with reference to Table B-57 which shows the fast corrections time-out
intervals as a function of the fast correction degradation factor indicator (aii). However, if the service provider chose
60 seconds for the fast corrections broadcast interval and the aii was 15, the user, due to data time-out prior to the next
broadcast, would not have current fast corrections for 42 seconds of every minute for NPA and APV-I and for 48 seconds of
every minute for APV-II and PA. In order to ensure that the aircraft element processes valid fast corrections without such
interruptions, the maximum broadcast interval for fast corrections has to be set at a value that is one third of the NPA and
APV-I time-out interval for fast corrections and one-half of the APV-II and PA time-out interval for fast corrections as
determined by aii.
Note.— Maximum fast corrections broadcast intervals for each aii can be found in RTCA/DO-229C, Table A-8.
6.5.3 Time-to-alert. Figure D-2 provides explanatory material for the allocation of the total time-to-alert defined in
Chapter 3, Table 3.7.2.4-1. The time-to-alert requirements in Appendix B, 3.5.7.3.1, 3.5.7.4.1 and 3.5.7.5.1 (corresponding to
the GNSS satellite status, basic differential correction and precise differential correction functions, respectively) include both
the ground and space allocations shown in Figure D-2.
6.5.4 Tropospheric function. Because tropospheric refraction is a local phenomenon, users will compute their own
tropospheric delay corrections. A tropospheric delay estimate for precision approach is described in RTCA/DO-229C,
although other models can be used.
6.5.5 Multipath considerations. Multipath is one of the largest contributors to positioning errors for SBAS affecting
both ground and airborne elements. For SBAS ground elements, emphasis should be placed on reducing or mitigating the
effects of multipath as much as possible so that the signal-in-space uncertainties will be small. Many mitigation techniques
have been studied from both theoretical and experimental perspectives. The best approach for implementing SBAS reference
stations with minimal multipath errors is to:
a) ensure that an antenna with multipath reduction features is chosen;
b) consider the use of ground plane techniques;
c) ensure that the antenna is placed in a location with low multipath effects; and
d) use multipath-reducing receiver hardware and processing techniques.
6.5.6 GLONASS issue of data. Since the existing GLONASS design does not provide a uniquely defined identifier for
sets of ephemeris and clock data, SBAS will use a specific mechanism to avoid any ambiguity in the application of the
broadcast corrections. This mechanism is explained in Figure D-3. The definitions of the latency time and validity interval
along with the associated coding requirements can be found in Appendix B, section 3.5.4. The user can apply the long-term
corrections received only if the set of GLONASS ephemeris and clock data used on board have been received within the
validity interval.
23/11/06 ATT D-14
Attachment D Annex 10 — Aeronautical Communications
6.6 SBAS final approach segment (FAS) data block
6.6.1 The SBAS final approach segment (FAS) data block for a particular approach procedure is as shown in Table D-1.
It is the same as the GBAS FAS data block defined in Appendix B, section 3.6.4.5, with the exception that the SBAS FAS
data block also contains the HAL and VAL to be used for the approach procedure as described in 6.3.4.
6.6.2 FAS data blocks for SBAS and some GBAS approaches are held within a common on-board database supporting
both SBAS and GBAS. Within this database, channel assignments must be unique for each approach and coordinated with
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