Table 12 1.  Typical Range of Integrity Parameters

Integrity Parameter

Typical Range

  Acceptable Risk of HMI

10

 5

 to 10

 7

/hr

  Enroute Alarm Threshold

2 to 7 nmi

  Terminal Area Threshold

1 to 3.5 nmi

  Nonprecision Approach Threshold

0.3 to 1 nmi

  Time to Alarm

6 30 sec

  Availability of the Integrity Decision

95% to 99.999%

  Acceptable False Alarm Rate

0.0003 to 0.00001/hr

  Assumed Inherent Integrity of GPS

10

 4

 to 10

 5

/hr

  Required Fault Detection Rate

99.9%

It should be noted, that when integrity is a prime consideration, estimates of system

accuracy and availability become dependent on the integrity methodology.  The

accuracy of a system becomes dependent on the accuracy estimate developed by

the integrity methodology and used to compare against the accuracy threshold

requirement.  If the accuracy estimate is conservative, there will be a consequent

loss of availability, since the system will be given less credit for accuracy than truly

exists, and the comparisons against the accuracy thresholds will fail more often. 

Similarly, if additional measurements are required to make an integrity decision, the

availability of the integrity decision can be significantly less than the availability of a

navigation solution.  In addition, to maximize availability and minimize prolonged

periods of unavailability, if a faulty measurement is detected that affects integrity, it

should then be excluded from the position solution, so that navigation can continue

whenever possible using the remaining valid measurements.

12.4.2  Required Navigation Performance

As of this writing, there is a significant change beginning in the way navigation

systems will be approved in the future.  Historically, approvals to operate in a

particular airspace or on a particular route or approach have been based on

requirements to carry and operate specific types of equipment.  The Required

Navigation Performance (RNP) concept is to define navigation performance

requirements for airspace, routes, or approaches and let the user demonstrate that

equipment is provided aboard the aircraft that meets the applicable requirements. 

This is sometimes described as a "tunnel" concept, that is, RNP for a route would

be defined in terms of an inner tunnel defined around the route centerline

consisting of a 95% accuracy standard and an outer tunnel consisting of a 99.99%

or higher accuracy standard.  The outer tunnel is sometimes described as the

"containment" tunnel, because that is a threshold beyond which a hazard is

assumed to exist, typically described as a collision risk with another aircraft or the

ground.

The parameters being considered to define RNP are accuracy, integrity, continuity,

and availability.  Continuity is defined as the probability that the navigation

accuracy within a containment threshold will continue to be provided once an

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