Differential GPS - Understanding GPS: Principles and Applications - page 448

Performance Requirements
The LAAS performance requirements are described in [59] and define navigation
performance parameters such as accuracy, integrity, continuity, and availability.
Furthermore, [59, 61] define the coverage area within which the LAAS service
should be available. Table 8.6, based on [59, 61], assigns values to the navigation
performance parameters of accuracy and integrity. Table 8.6 identifies three LAAS
performance types that correspond to the three categories of precision approach and
landings: CAT I, CAT II, CAT IIIa, and CAT IIIb. Each of these landing categories is
defined by the decision height (DH) at which the pilot or aircraft must make the
decision to either continue or abort the landing. This decision depends on the run-
way visual range (RVR) at the corresponding decision height. Table 8.7 shows the
DH and RVR for each of the categories.

Integrity Monitoring
LAAS includes an integrity monitoring function that determines, with a certain level
of probability, that the code and carrier phase corrections do not contain misleading
information. The integrity monitor function can be subdivided into multiple moni-
tors: a signal quality monitor (SQM) to detect anomalous behavior in the satellite
and pseudolite signals, a data quality monitor (DQM) to check if the satellite navi-

Table 8.6 LAAS Performance Requirements

Performance Type
1 2 3

Category supported CAT I CAT II/IIIa CAT IIIb

Decision Height* >200 ft >100/50 ft >50 ft

Vertical position
Accuracy, 95%

Accuracy

4.4m (NSE) 2.0/2.0m (NSE) 2.0m (NSE)

Lateral position
accuracy, 95%

16.0m (NSE) 6.9/6.2m (NSE) 6.2m (NSE)

10m (200-ft
HAT*)

5.3m (100-ft
HAT)

5.3m (100-ft
HAT)

Vertical alert limit (VAL)

Integrity (SIS)

40m (200-ft
HAT)

17.3 (100-ft
HAT)

15.5m (100-ft
HAT)

Lateral alert limit (LAL)

Time to alert 2 seconds 1 second 1 second

Lateral: 15
seconds
Vertical: 30
seconds

Exposure time 150 seconds 15 seconds

×
10 −7 /approach

2

Allowable integrity risk 10 −9 /approach 10 −9 /approach

Table 8.7 DH and RVR for LAAS

DH RVR

CAT I

200 ft HAT > 2,400 ft

CAT II

100 ft HAT > 1,200 ft

CAT IIIa

< 100 ft HAT > 700 ft

CAT IIIb

< 50 ft HAT > 150m