12.4  PRIMARY CIVIL AVIATION CONCERNS WITH GPS

The primary civil aviation concerns with GPS are availability, accuracy, integrity,

and liability.  As mentioned above, a GPS signal loss or severe accuracy

degradation can affect large areas and large numbers of aircraft simultaneously. 

For this reason civil aviation organizations have been strong advocates for

maximizing the number of active GPS satellites in order to minimize the effect of

losing any particular satellite signal, supplementing GPS with the other components

of the GNSS, and providing an independent monitoring and warning system.

Integrity is defined in most references as the ability of a system to provide timely

warnings to users when the system should not be used for navigation.  From an

operational point of view, the primary purpose of an integrity function is to detect

navaid signal failures that would otherwise result in hazardously misleading

navigation information (HMI) being displayed to a pilot or transmitted to an

autopilot.  Existing ground based radio navigation aids continuously monitor their

output signals and typically shut down when a significant error is detected. 

Although the Control Segment and each satellite monitors GPS signal performance,

the response time and fault monitoring has not been proved sufficient for civil

aviation purposes.  In addition, for SPS users, the accuracy degradation due to SA

can occasionally result in position errors that are significant to some aviation uses,

such as nonprecision approach.  For these reasons an additional system or

technique is required to assure GPS integrity.

12.4.1  Integrity Requirements

Integrity requirements for aviation are based on the general requirement to maintain

safe navigation and avoid hazardous situations.  Consequently, integrity

requirements can be different for equipment with different purposes and can vary

with phase of flight as the proximity to potential hazards changes.  The key integrity

parameters are the acceptable probability of a hazardous event, the navigation

accuracy threshold that defines a hazard, and the allowed time delay before a

warning must be issued.  Requirements are most stringent when GPS is intended

as the primary means of navigation under instrument flight rules (IFR). 

Requirements are correspondingly less stringent when GPS is intended as a

supplementary aid to IFR flight, or a supplementary aid to visual flight rules (VFR)

flight.  In all cases, requirements also change based on the phase of flight. 

Requirements are most stringent when GPS is used for approach and landing, and

correspondingly less stringent for terminal area and enroute flight, depending

primarily on the dimensions of individual air traffic routes and/or aircraft spacing

requirements.  The allocation of integrity requirements to GPS can also vary if GPS

is used in a hybrid system that performs automatic switching and/or cross checking

between different navigation sources.  The following table shows an example of the

ranges of integrity parameters.  However, the promulgation of actual figures shall

be given by ICAO.

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