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b) Current year in common form is calculated by the following formula:
Y = 1996 + 4 (N4 – 1) + (J – 1).
c) Current day and month (dd/mm) are extracted from the reference table stored in user equipment ROM. The table
interrelates NT parameter and common form dates.
4.2.8 GLONASS coordinate system. The GLONASS coordinate system is PZ-90 as described in Parameters of Earth,
1990 (PZ-90), published by the Topographic Service, Russian Federation Ministry of Defence, Moscow.
4.2.8.1 PZ-90 parameters include fundamental geodetic constants, dimensions of the common terrestrial ellipsoid, the
characteristics of the gravitational field of the earth, and the elements of the Krasovsky ellipsoid (coordinate system 1942)
orientation relative to the common terrestrial ellipsoid.
4.2.8.2 By definition, the coordinate system PZ-90 is a geocentric Cartesian space system whose origin is located at the
centre of the earth’s body. The Z-axis is directed to the Conventional Terrestrial Pole as recommended by the International
Earth Rotation Service. The X-axis is directed to the point of intersection of the earth’s equatorial plane and zero meridian
established by the Bureau International de l’Heure. The Y-axis completes the right-handed coordinate system.
4.3 Dilution of precision
Dilution of precision (DOP) factors express how ranging accuracy is scaled by a geometry effect to yield position accuracy.
The optimal geometry (i.e. the lowest DOP values) for four satellites is achieved when three satellites are equally spaced on
the horizon, at minimum elevation angle, and one satellite is directly overhead. The geometry can be said to “dilute” the
range domain accuracy by the DOP factor.
4.4 GNSS receiver
4.4.1 The failures caused by the receiver can have two consequences on navigation system performance which are the
interruption of the information provided to the user or the output of misleading information. Neither of these events are
accounted for in the signal-in-space requirement.
4.4.2 The nominal error of the GNSS aircraft element is determined by receiver noise, interference, and multipath and
tropospheric model residual errors. Specific receiver noise requirements for both the SBAS airborne receiver and the GBAS
airborne receiver include the effect of any interference below the protection mask specified in Appendix B, 3.7. The required
performance has been demonstrated by receivers that apply narrow correlator spacing or code smoothing techniques.
5. Aircraft-based augmentation system (ABAS)
5.1 ABAS augments and/or integrates the information obtained from GNSS elements with information available on
board the aircraft in order to ensure operation according to the values specified in Chapter 3, 3.7.2.4.
5.2 ABAS includes processing schemes that provide:
ATT D-9 23/11/06
Annex 10 — Aeronautical Communications Volume I
a) integrity monitoring for the position solution using redundant information (e.g. multiple range measurements). The
monitoring scheme generally consists of two functions: fault detection and fault exclusion. The goal of fault
detection is to detect the presence of a positioning failure. Upon detection, proper fault exclusion determines and
excludes the source of the failure (without necessarily identifying the individual source causing the problem),
thereby allowing GNSS navigation to continue without interruption. There are two general classes of integrity
monitoring: receiver autonomous integrity monitoring (RAIM), which uses GNSS information exclusively, and
aircraft autonomous integrity monitoring (AAIM), which uses information from additional on-board sensors (e.g.
barometric altimeter, clock and inertial navigation system (INS));
b) continuity aiding for the position solution using information of alternative sources, such as INS, barometric altimetry
and external clocks;
c) availability aiding for the position solution (analogous to the continuity aiding); and
d) accuracy aiding through estimation of remaining errors in determined ranges.
5.3 Non-GNSS information can be integrated with GNSS information in two ways:
a) integrated within the GNSS solution algorithm (an example is the modelling of altimetry data as an additional
satellite measurement); and
b) external to the basic GNSS position calculation (an example is a comparison of the altimetry data for consistency
with the vertical GNSS solution with a flag raised whenever the comparison fails).
5.4 Each scheme has specific advantages and disadvantages, and it is not possible to present a description of all
potential integration options with specific numerical values of the achieved performance. The same applies to the situation
when several GNSS elements are combined (e.g. GPS and GLONASS).
　
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