Aircraft Operations. Volume II - Construction of Visual and Instrument ...

III-1-2-2 Procedures — Aircraft Operations — Volume II
23/11/06
2.3 SYSTEM USE ACCURACY FOR BASIC GNSS RNAV PROCEDURES
2.3.1 General
2.3.1.1 Despite the inherent accuracy of the GNSS space segment position, the usability of a fix is also affected by
the number of satellites available and their orientation with respect to the GNSS receiver. These factors vary from place
to place and time to time. The ability of a receiver to detect and alert the pilot to these factors when they are
unfavourable is a measure of the navigation system’s operational capability.
2.3.1.2 To qualify for use as a non-precision approach navigation system, GNSS receivers must incorporate an
integrity monitoring routine which alerts the pilot when the fixing information does not meet the required level of
confidence. For integrity monitoring alarm limits see 2.3.3.2, “Integrity monitoring alarm limits”.
2.3.2 Horizontal accuracy
The agreed level of horizontal accuracy of the GNSS space segment is assumed to be 100 m (328 ft) at the 95 per cent
confidence level.
2.3.3 Navigation system accuracy/tolerances
2.3.3.1 The factors on which the navigation system accuracy of GNSS RNAV depends are:
a) inherent space segment accuracy;
b) airborne receiving system tolerance;
c) system computational tolerance; and
d) flight technical tolerance.
See Table III-1-2-2 for values.
2.3.3.2 Integrity monitoring alarm limits. The values of the space elements (including control element) and the
airborne system tolerances (including system computation tolerance) are taken into account within the integrity
monitoring alarm limits for basic GNSS systems. See Table III-1-2-2 for values.
2.4 FLIGHT TECHNICAL TOLERANCE (FTT)
FTT defines the total system cross-track tolerance (XTT). The FTT will vary with the type of position indicator used in
the cockpit instrumentation. FTT contributions to cross-track tolerance are listed in Table III-1-2-2.
2.5 XTT, ATT AND AREA SEMI-WIDTH
The values specified in 2.3.3.1 and 2.4 define the total system ATT and XTT according to the following equations: