Explanatory Statement
Civil Aviation Regulations 1988
Instructions — use of Global Positioning System (GPS)
Purpose
This instrument revokes its predecessor and remakes it with some modifications to reflect the introduction of a new air traffic control separation tolerance utilising global positioning systems. The new tolerance requires pilots to report to air traffic control if certain GPS equipment outage lasts for more than 5 minutes. The requirement in the predecessor instrument was for reporting if the outage lasted more than 10 minutes.
Legislation
Section 98 of the Civil Aviation Act 1988 (the Act) provides that the Governor-General may make regulations for the Act and in relation to the safety of air navigation.
Under subregulation 174A (1) of the Civil Aviation Regulations 1988 (CAR 1988), CASA may issue instructions specifying communications and navigation equipment that must be carried on, or installed in, an aircraft before it undertakes a flight under the Visual Flight Rules (VFR). Under subregulation 179A (1) of CAR 1988, CASA may issue instructions in relation to Instrument Flight Rules (IFR), specifying the method by which an aircraft is to be navigated and how a positive position fix is to be obtained.
Instructions
The instructions are directed to pilots in command of aircraft using a Global Positioning System (GPS) and equipment in aircraft, and set out procedures to be followed when GPS is used under different flight conditions.
GPS may be used as a navigation system in certain areas, flights, instrument approaches and departures and in VFR operations. Instructions are given for using GPS for these purposes. Table 1 in the instructions sets out the various GPS equipment standards and requirements for different phases of flight.
RAIM
GPS is now often referred to as Global Navigation Satellite Systems (GNSS). Receiver Autonomous Integrity Monitoring (RAIM) is a software algorithm in most GNSS receivers which gives an indication if the position accuracy of the receiver is suitable for use. A RAIM loss alarm is triggered if position accuracy is outside of limits or if there are insufficient satellites in view of the receiver to provide RAIM.
Instrument CASA 425/08 was the predecessor to the current instrument of instructions. Under paragraph 7 (2) (a) of CASA 425/08, if the pilot in command of an aircraft on a domestic en-route phase of flight is using GPS within an air traffic control area, he or she must, among other things, advise the air traffic service (ATS) provider (Airservices Australia) if there is RAIM loss or loss of integrity for more than 10 minutes.
Table 1 of CASA 425/08 specifies the “Loss of RAIM” procedures for IFR RNAV flight and includes the requirement to advise ATS if there is RAIM loss or loss of integrity for more than 10 minutes.
The MOS amendment
CASA 608/08 is consequential on the recent making of Manual of Standards Part 172 Amendment (No.1) 2008 (the MOS amendment) which was made to introduce a new air traffic control separation tolerance utilising GPS. This tolerance will be known as 7 CEP, and follows from CASA’s recently concluded public consultation for its introduction.
Chapter 10 of MOS Part 172 contains the Australian separation minima used by air traffic controllers (ATC). These separation minima include tables of Circular Error of Position (CEP) tolerances used to determine aircraft separation based on long-range navigation systems such as inertial navigation or reference systems (INS/IRS) and Global Navigation Satellite Systems (GNSS) (originally known as Global Positioning System [GPS]).
GNSS
When GNSS separation was first implemented in 1995, a 12 nautical mile (nm) CEP value was initially established. This CEP value was derived from modelling based on a 24-satellite constellation and the possible use by the US Government of Selective Availability (SA) — a system for deliberately degrading the accuracy of the GNSS signal.
Since then, the satellite constellation has increased with up to 32 satellites now available and SA is no longer used.
The 12 nm tolerance is quite conservative when compared to the demonstrated accuracy and reliability of GNSS in flight operations. For example, for an aircraft that meets the GPS RNAV or GPS OCEANIC capability (particular subtypes of GNSS navigation capability), a tolerance of 12 nm (i.e. radius 12 nm [22 km]) must be applied to the reported GNSS position. Separation must then be based on the extremities of this circular tolerance area or CEP.
Accordingly, for 2 aircraft with GPS RNAV or GPS OCEANIC capability reporting GNSS position, the minimum lateral separation must be the sum of both aircraft’s CEP plus a buffer factor of 1 nm [1.8 km], giving a lateral separation minimum of 25 nm [46 km].
More simply put, 2 aircraft at the same cruising level will only achieve minimum lateral separation if they are flying on different air routes spaced at least 25 nm [46 km] apart.
This lateral spacing requirement severely limits the options for efficient and fail-tolerant route structures between airports, particularly in the more remote areas of Australia that are not well provided with traditional navigation aids (for example, VHF Omni Range, non‑directional beacons etc), and which are outside surveillance coverage. One area particularly affected is regional Western Australia.
AA assessment
In light of improved GNSS performance and availability, Airservices Australia (AA) conducted an assessment of current and historical data relating to GNSS accuracy, possible failure and improved satellite coverage. This assessment, based on Australian and international research, found the current 12 nm CEP value could be reduced to 7 nm for aircraft that have GPS RNAV or GPS OCEANIC capability whilst maintaining appropriate safety levels.
The mathematical analysis has also been reviewed by the International Civil Aviation Organization (ICAO) Separation and Safety Panel and found satisfactory.
Subsequently, AA presented a concept and design safety case to CASA arguing the intrinsic safety of the revised tolerance, and asked for approval to use the tolerance for ATC separation. CASA assessed both the design and an implementation safety case and accepted the proposal.
The AA assessment found that the 10-minute reporting requirement would be too long a delay to preserve the integrity of the 7 nm CEP tolerance, and considered that a 5 minute delay would be more appropriate. A key enabler for a 7 nm CEP tolerance, therefore, would be to change the reporting requirement for RAIM loss or outage to 5 minutes.
CASA 608/08
CASA 608/08, therefore, revokes CASA 425/08 and remakes it changing only the references above in section 7 and Table 1 to 10 minutes so that they read 5 minutes. Thus, when using GPS, RAIM loss or loss of integrity for more than 5 minutes must now be reported to the ATS.
The opportunity has also been taken to correct a typographical where a reference in section 10 to “GPS navigation equipment standards in section 10” should correctly read “GPS navigation equipment standards in section 11”.
Timing
The 7 CEP tolerance is a critical enabler for the introduction of a revised air route structure for Western Australia. This new route structure is needed for the airways system to better cope with current and projected traffic levels. The implementation date for the new structure, and therefore for the MOS amendment, is 20 November 2008.
CASA has, therefore, decided to introduce the revised GNSS CEP value for use with GNSS‑equipped aircraft from that date. While the main reason for the revised CEP tolerance is the new Western Australian airspace structure, the tolerance will be introduced throughout all domestic Australian controlled airspace. Accordingly, the revised RAIM outages/failures reporting requirement for pilots will be introduced throughout all Australian administered airspace on and from 20 November 2008.
Legislative Instruments Act
Under subregulations 174A (1C) and 179A (3A) of CAR 1988, a VFR or IFR flight instruction that is not in the form of a Civil Aviation Order (CAO) is a disallowable instrument for the purposes of section 46A of the Acts Interpretation Act 1901.
Under subparagraph 6 (d) (i) of the Legislative Instruments Act 2003 (the LIA), an instrument is a legislative instrument for section 5 of the LIA if it is declared to be a disallowable instrument under legislation in force before the commencement of the LIA. The instruction instrument is, therefore, a legislative instrument and it is subject to tabling and disallowance in the Parliament under sections 38 and 42 of the LIA.
Consultation
Consultation under section 17 of the LIA has taken place as part of the process of consultation for the MOS amendment.
Consultation for the MOS amendment was in the usual way under the Notice of Proposed Change process and in accordance with the requirements for making a MOS under Subpart 11.J of CASR 1998.
Notice of Proposed Change (NPC) 172/03 was released for public consultation on 16 September 2008 and placed on the Safety Consultative Committee (SCC) website. The period for comment closed on 20 October 2008. The NPC related to both the 7 nm CEP tolerance and the change to RAIM reporting requirements (CASA 608/08).
There was little public comment on the NPC, and none that requires CASA to reconsider the proposal. This may be a consequence of AA’s extensive consultation undertaken for the development of their safety case. AA conducted several hazard identification workshops and industry briefings with major and regional airlines, and industry groups. CASA is, therefore, satisfied that industry is aware of the change and that there are no significant industry concerns.
Office of Best Practice Regulation (OBPR)
A preliminary assessment of business compliance costs indicates that the amended instructions will have no or low cost impact on business. The change to RAIM loss reporting is not expected to significantly increase operational workload. AA intends to track pilot reported RAIM failures based on the 5 minute reporting requirement, and will assess and address any adverse outcomes in a Post Implementation Review. CASA will consult with AA on this review.
The MOS amendment, for which the instructions are considered to be an indispensible support, will have the potential for more efficient use of available airspace under ATC, the measure will be advantageous to industry.
The 7 nm CEP tolerance will enable the existing airspace to be more efficiently utilised, and will provide greater capacity for aircraft to achieve their preferred levels and tracking. Additionally, new routes can be designed (with CASA approval) based on the amended tolerance providing more efficient use of airspace.
Industry can expect significant safety and efficiency benefits due to decrease in delays at interim levels and nose-to-nose traffic outside controlled airspace. The reduced delays will have a positive effect on the environment with the subsequent reduction in fuel burn.
Making and commencement
CASA 608/08 commences on 20 November 2008 after it is registered.
Under subregulations 174A (1B) and 179A (3) of CAR 1988, if an instruction is not in the form of a CAO, it does not bind a person unless served on them or published in NOTAMS or AIP. For this purpose, therefore, CASA 608/08 is published in AIP.
The instrument has been made by the Director of Aviation Safety, on behalf of CASA, in accordance with subsection 84A (2) of the Act.
[Instrument number CASA 608/08]