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requirement. Similarly, the vertical protection level (VPL) provides a bound on the vertical position. If the computed HPL
exceeds the horizontal alert limit (HAL) for a particular operation, SBAS integrity is not adequate to support that operation.
The same is true for precision approach and APV operations, if the VPL exceeds the vertical alert limit (VAL).
6.3.5 One of the most challenging tasks for an SBAS provider is to determine UDRE and GIVE variances so that the
protection level integrity requirements are met without having an impact on availability. The performance of an individual
SBAS depends on the network configuration, geographical extent and density, the type and quality of measurements used and
the algorithms used to process the data. General methods for determining the model variance are described in Section 14.
23/11/06 ATT D-12
Attachment D Annex 10 — Aeronautical Communications
6.3.6 Residual clock and ephemeris error (σUDRE). The residual clock error is well characterized by a zero-mean,
normal distribution since there are many receivers that contribute to this error. The residual ephemeris error depends upon the
user location. For the precise differential function, the SBAS provider will ensure that the residual error for all users within a
defined service area is reflected in the σUDRE. For the basic differential function, the residual ephemeris error should be
evaluated and may be determined to be negligible.
6.3.7 Vertical ionospheric error (σGIVE). The residual ionospheric error is well represented by a zero-mean, normal
distribution since there are many receivers that contribute to the ionospheric estimate. Errors come from the measurement
noise, the ionospheric model and the spatial decorrelation of the ionosphere. The position error caused by ionospheric error is
mitigated by the positive correlation of the ionosphere itself. In addition, the residual ionospheric error distribution has
truncated tails, i.e. the ionosphere cannot create a negative delay, and has a maximum delay.
6.3.8 Aircraft element errors. The combined multipath and receiver contribution is bounded as described in Section 14.
This error can be divided into multipath and receiver contribution as defined in Appendix B, 3.6.5.5.1, and the standard
model for multipath may be used. The receiver contribution can be taken from the accuracy requirement (Appendix B, 3.5.8.2
and 3.5.8.4.1) and extrapolated to typical signal conditions. Specifically, the aircraft can be assumed to have σ2
air = σ2
receiver +
σ2
multipath, where it is assumed that σreceiver is defined by the RMSpr_air specified for GBAS Airborne Accuracy Designator A
equipment, and σmultipath is defined in Appendix B, 3.6.5.5.1. The aircraft contribution to multipath includes the effects of
reflections from the aircraft itself. Multipath errors resulting from reflections from other objects are not included. If
experience indicates that these errors are not negligible, they must be accounted for operationally.
6.3.9 Tropospheric error. The receiver must use a model to correct for tropospheric effects. The residual error of the
model is constrained by the maximum bias and variance defined in Appendix B, 3.5.8.4.2 and 3.5.8.4.3. The effects of this
mean must be accounted for by the ground subsystem. The airborne user applies a specified model for the residual
tropospheric error (σtropo).
6.4 RF characteristics
6.4.1 SBAS pseudo-random noise (PRN) codes. RTCA/DO-229C, Appendix A, provides two methods for SBAS PRN
code generation.
6.4.2 SBAS network time. SBAS network time is a time reference maintained by SBAS for the purpose of defining
corrections. When using corrections, the user’s solution for time is relative to the SBAS network time rather than core
satellite constellation system time. If corrections are not applied, the position solution will be relative to a composite core
satellite constellation/SBAS network time depending on the satellites used and the resulting accuracy will be affected by the
difference among them.
6.4.3 SBAS convolutional encoding. Information on the convolutional coding and decoding of SBAS messages can be
found in RTCA/DO-229C, Appendix A.
6.4.4 Message timing. The users’ convolutional decoders will introduce a fixed delay that depends on their respective
algorithms (usually 5 constraint lengths, or 35 bits), for which they must compensate to determine SBAS network time (SNT)
from the received signal.
6.5 SBAS data characteristics
6.5.1 SBAS messages. Due to the limited bandwidth, SBAS data is encoded in messages that are designed to minimize
the required data throughput. RTCA/DO-229C, Appendix A, provides detailed specifications for SBAS messages.
　
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