Implications for PBN

LNAV/VNAV

The estimated horizontal and vertical position is compared against the defined path created by the navigation computer.

The waypoints defining the path are normally extracted from the navigation database.

Lateral and Vertical guidance from the estimated position onto the defined path is called LNAV, VNAV respectively.

Vertical guidance is only active in the Final Approach segment.

Positioning Accuracy

Position estimation accuracy is related to the type of navigation sensor used; each sensor has its own error value, called the ‘Navigation Sensor Error’ (NSE).

• DOP is dependent upon the relative angle the signals subtend at the aircraft (angle of cut) and is used by the navigation computer to calculate the level of uncertainty in position estimation.

• NDB:
• is not considered in PBN as it is not accurate enough for position estimation.

• VOR:
• at long range is the least accurate of the ground-based Navaids used in PBN,
• it is too inaccurate for the more demanding lateral track accuracy requirements,
• the VOR only supports RNAV 5.

• DME:
• providing there are sufficient stations with appropriate geometry, supports a navigational performance down to 1NM,
• the accuracy of a DME/DME position estimation is too poor when the subtended angles (due to DOP) of the signals from a pair of stations are less than 30° and more than 150°,
• DME positions cannot support RNP APCH.

• GNSS:
• has the smallest error,
• with augmentation (integrity checking), provides a navigation solution for every Navigation Application.

The aircraft manufacturers and AOs decide which sensors are fitted to the aircraft.

On-board autonomous navigation capability

Although not a navigation sensor, position information can be provided by inertial platforms fitted to the aircraft.

Two types of inertial platforms are considered: Inertial Reference Systems and Inertial Navigation Systems.

Inertial capability is only a requirement for the most demanding Navigation Specification – RNP (AR) APCH.

Inertial Reference Systems (IRS):

• This positional information is derived autonomously, without reference to any external systems and can be used when no other position information is available.
• An IRS can provide short term accurate information that can be used with ground or space based navigation systems to enhance the position estimate. In addition, they can also be used to replace external sensors to cover short term outages.
• The limits on the their time of use are prescribed in the Navigation Specification. i.e. RNAV 5 - An Inertial platform can be used for 2 hours with no updating, but for an RNP approach only for 40 seconds.

An Inertial Navigation System (INS) is a standalone independent system. The inputs are fed from the Inertial Reference Unit and waypoints can be manually entered through the Inertial Navigation Control Display Unit (INCDU).

Integrity

Integrity is the degree of confidence that can be placed on the position estimation by the RNAV system.

For flight applications using RNP systems, failure to meet the integrity requirement should result in an alert to the pilot. This is also true for some RNAV systems including all those using GPS.

• The GPS constellation does not meet the civil aviation integrity requirements as there is the possibility of an undetected satellite failure every 100 000 hours of operation (10-5).
• To meet the civil aviation integrity requirement of one missed detection in 10 000 000 (10-7) per flight hour aircraft-based augmentation systems were developed.

ABAS provides integrity monitoring by:

• Aircraft Autonomous Integrity Monitoring (AAIM) links the GPS receiver to other aircraft systems, or
• Receiver Autonomous Integrity Monitoring (RAIM), which compares a series of position estimations within the GPS unit using redundant (extra) satellite signals.

TSO 129A receivers provide this functionality. All TSO 129A certified receivers are capable of Fault Detection (FD).

Most newer generation receivers are capable of performing Fault Detection and Exclusion (FDE). New ABAS receivers are qualified under TSO 196A.

Note: A TSO (Technical Standard Order) is issued by the FAA and stipulates the minimum performance standard for specified materials, parts and appliances used on civil aircraft. The European equivalent is called an ETSO (European Technical Standard Order) and is issued by EASA.

Integrity monitoring is provided on the flight deck by linking the GPS receiver with either an Inertial system or a Barometric altimeter.

RAIM:

This is the most common form of integrity monitoring. It is an algorithm integrated in the GPS receiver which compares a series of position estimations for internal consistency.

RAIM is based on the availability of additional satellites in view.

Using the extra satellite signals, the RAIM algorithm should detect a faulty satellite; this is known as Fault Detection (FD). If the receiver has extra functionality it may be able to perform Fault Detection and Exclusion (FDE).

The availability of integrity monitoring and FDE by RAIM is based on the number of visible, operational satellites.

Fault Detection (FD) requires at least 5 satellites:

• If the estimated positions start to spread out and exceed a preset value, then a fault is declared.

Fault Detection and Exclusion (FDE) requires at least 6 satellites:

• The receiver can detect which satellite is faulty and exclude any positional data received from it.

Most RAIM algorithms assume only one faulty satellite.

The probability of the RAIM algorithm failing to detect a faulty satellite is one time in 1000 (10-3).

SBAS:

Whilst ABAS provides a level of integrity monitoring on board the aircraft, other augmentation systems provide integrity using a ground-based infrastructure.

SBAS provides a higher level of integrity, meeting civil aviation requirements, by monitoring the GPS constellation and providing ‘use/do not use’ messages for each satellite in view of the ground system.

A 'do not use' flag can take as little as 6 seconds to be received by the aircraft.

By far the greatest use of SBAS systems today is the provision of a series of corrections to improve the lateral and vertical accuracy of the position solution.

To receive a SBAS signal the aircraft must be fitted with a specific type of receiver. Aviation certified SBAS receivers conform to TSO 145A/146A.

Integrity of the GPS signal from monitored satellites can be received anywhere within the footprint of the Geostationary satellite's transmission, which is very large and covers the whole of ECAC and beyond.

Whilst ABAS provides a level of integrity monitoring on board the aircraft, other augmentation systems provide integrity using a ground-based infrastructure.

SBAS provides a higher level of integrity, meeting civil aviation requirements, by monitoring the GPS constellation and providing ‘use/do not use’ messages for each satellite in view of the ground system.

A 'do not use' flag can take as little as 6 seconds to be received by the aircraft.

By far the greatest use of SBAS systems today is the provision of a series of corrections to improve the lateral and vertical accuracy of the position solution.

To receive a SBAS signal the aircraft must be fitted with a specific type of receiver. Aviation certified SBAS receivers conform to TSO 145A/146A.

Integrity of the GPS signal from monitored satellites can be received anywhere within the footprint of the Geostationary satellite's transmission, which is very large and covers the whole of ECAC and beyond.

Availability and continuity

To meet a specific navigation application both the signals-in-space and the aircraft systems must meet the required accuracy, integrity and continuity for that operation.

PBN requires that an aircraft and its systems should be able to perform for the whole of the defined operation, as long as it was operating correctly at the start of that operation.

Equally, the signals from the NAVAIDs should also be available for the required operation and once the particular phase of flight has begun, continue to function for the period of that operation.

The Service Provider will need to consider how to meet the appropriate requirement for signal availability and continuity. This is usually achieved through redundancy (additional capability to handle failures), or by the requirement for the aircraft to carry additional systems (for example, carriage of IRS/IRU).

The probability of failure and therefore being unable to complete an operation must be acceptably low.