Threading the needle

Looking to the Stars

the Instrument Landing System (Ils) That Has Safely Guided Millions of Flights to a Safe Arrival Was Developed More Than 80 Years Ago and First Delivered Commercial Passengers to Their Destination in 1938. On That Occasion the Classic Twin-engined Boeing 247d Made Its Timely Arrival Amid a Pittsburgh Snow-storm. While the Ils System Has Continued to Be Developed and Refined Over Successive Decades, It Has Remained a Terrestially-based System and Consequently Carries With It the Limitations of Its Fixed Footings.

In Comparison, the 21st Century Has Seen the Growth of Satellite-based Navigation Systems That Look Toward the Stars and Offer an Extremely High Degree of Accuracy. Furthermore, the Limitations of Terrain and Buildings Don’t Affect the Orbiting Satellites in the Same Way as Their Ground-dwelling Predecessors. The Supporting Infrastructure of Satellite-based Systems is Not as Cumber-some or as Susceptible to the Myriad Forms of Interference. In the Case of Required Navigation Performance (Rnp) Approaches, All Hardware is on Board the Aircraft and the Flightpath is Free to Wind Its Way Through Terrain Before Culminating in a Short Runway-aligned Segment.

Yet Even With a Degree of Accuracy in the Realm of Mere Three Metres, Rnp Alone Cannot Offer the Recognised Level of Preci-sion Required to Provide Landing Guidance. For This to Take Place, the Satellite Signal Must Be Augmented. In the Case of Gls It is a Ground-based Augmentation System, or Gbas. With Such Augmentation, the Level of Accuracy Generally is Less Than One Metre and Sufficiently Precise to Allow Auto-coupled Landings Where the Autopilot Flies the Aircraft Onto the Runway. This Capabil-ity is Available on Some Ils Approaches in Australia and a Key Element in Low Visibility Operations. However, Gls Has the Poten-tial to Achieve ‘autolands’ With a Far Greater Degree of Flexibility Than Ils.

Making It Work

at the Heart of Gbas is a Relatively Simple Principle of Operation and an Equally Straightforward Combination of Air and Ground-based Equipment. The Existing Ils Approach System Calls for a Combination of Antennae Arrays to Emit a Series of Signals to Provide Both Lateral and Vertical Guidance Down to the Runway. The Aircraft Receives These Signals Through Its Onboard Equipment and Conveys Its Posi-tion Relative to the Ils ‘beams’ Through a Cockpit Instrument That Guides the Pilot With a Crosshair-like Display. The Objective is to Keep the Needles Centred, Resulting in the Aircraft Flying Down the Centre of the Lateral and Vertical Beams With the Highest Degree of Precision.

While Ils Has Proven a Tremendous Precision Navigation Aid for Decades, It Does Come With Limitations. Firstly, Ils Calls for a Significantly Straight-in Long Final Approach Leg to the Runway, Which Can Often Be Precluded by Surrounding Terrain. Additionally, Each Runway to Be Serviced Requires Its Own Ils Installation and This Relatively Cumbersome Equipment is Both Expensive to Install and Maintain. In the Case of Sydney Airport, It Calls for Six Independent Ils Units to Be Maintained to a High Degree of Operational Precision.

Furthermore, When Installing the Ground-based Hardware There is a Geo-metrically Limited Range of Sites on the Airport From Which the Vertical and Lateral Guidance Signals for the Ils Approach Can Be Emitted. And Even When the Ils is in Place, It is Still Sensitive to Interference From Terrain, Buildings and Aircraft Move-ments. This Requires Interference-sensitive Areas to Be Protected During Low-visibility Operations and Effectively Slows the Rate of Flow of Aircraft Arrivals and Departures and Increases Aircraft Taxi Times.

The Gls Escapes Many of the Limita-tions of the Ils by Its Principle of Opera-tion and the Way in Which the Signal is De-rived. Fundamentally, Before Augmentation by Gbas, the Satellite-based Navigation Can Bring the Aircraft Close to the Runway’s End With an Accuracy of About Three Me-tres Without the Need for Any Supporting Airport Infrastructure. The Core Positioning Information and Subsequent Guidance is Gleaned From Satellites in Medium Earth Orbit (Meo), So on a Gls Approach the Task of the Ground-based Equipment is Not to Emit a Guidance Signal but Merely Refine It. It Achieves This Through a Relatively Straightforward Process.

The Gbas Ground Station’s Location is Not Critical and Can Be Situated on or Near the Airport. It Then Compares Its Known, Fixed Position on the Ground to the Position Calculated by the Individual Satellites Orbit-ing Overhead and Continually Translates Any Slight Difference Into a Factor of Correction. This Correction Factor is Then Processed and the Approach Definition or ‘path’ is Then Transmitted to the Aircraft via Vhf Data Broadcast (Vdb) Datalink With the High Degree of Accuracy Required for Precision Approaches and Low-visibility Operations.

In the Air, the Vdb Datalink is Received by the Aircraft Through a Multi-mode Receiver (Mmr). The Representation of the Gls Signal and the Method of Flying the Centred Cross-hairs is Identical to Exist-ing Ils Displays. All the Crew Has to Do is Dial Up the Appropriate Channel for the Runway, Just as With an Ils Frequency. The Instrument Approach is Not Held by the Aeroplane’s Flight Management Com-puter Database for Anything Other Than the Instrument Presentation and Guidance for the Go-around – the Actual Approach Guid-ance is Uplinked via the Gbas System. This Simplicity of Operation Offers a Seamless Transition for Crew Training. However, the Advantages of the Gls Extend Far Beyond the Flightdeck.

More on Offer

the Simplicity of Gbas Opens Up a Range of Advantages, Opportunities and Efficiencies That Ils Cannot Offer. Firstly, the Ground-based Gbas Hardware Con-sists of Gps Reference Receiver Antennas to Receive the Satellite Information, Which Are Linked to a Processor and in Turn a Vhf Datalink Antenna to Broadcast the Vdb Signal to the Aircraft. On Any Airport They Represent an Inconspicuous Series of Aerials and a Lone, Small Processor a Little Taller Than a Person.

Furthermore, This Network Covers All of the Runways at the Airport and There is No Need for Runway-specific Equipment as Gbas is About Refining Position Accuracy, Not Emitting Narrow Beams. In Fact, One Gbas Installation Has the Ability to Service Other Nearby Airports Should They Have the Need. And Up to 26 Approach Options Can Be Serviced Within a 20nm Radius From a Single Gbas Installation.

Gbas Effectively Opens the Way for a Precision Approach Capability Where It Has Previously Not Been Available. A Gls Does Not Require a Long, Straight Final Approach Like the Ils So It is Foreseeable That Satel-lite-based Rnp Approaches Will Transition Smoothly Into the Short Final Approach of a Gls. This Flexibility Will Open Up Opera-tions at Airfields That Have Previously Been Limited by Terrain or Financial Resources to Host Such High-precision Approaches and the Safety and Schedule Integrity They Offer.

The Gls Infrastructure Costs Less to Both Install and Maintain and Covers Multiple Runways. Its Signal is More Solid and Accu-rate Than an Ils and Free From the Interfer-ence of Aircraft Movements Adjacent to the Runway. Coupled With Rnp Approaches It Offers More Efficient and Community-friendly Flightpaths With Reduced Track Miles and Lower Emissions and Yet Does Far More Than Mimic the Ils.

The Single Unit Offers Multiple Approach-es to Multiple Runways and Can Be Re-con-figured to Still Provide Precision Approach Guidance When Displaced Thresholds Are in Place Due to Runway Maintenance. The Ap-proaches Can Vary in Their Slope as Required by Terrain, Aircraft Performance or Noise Abatement. And the Gbas Signal is Equally Valid for Takeoff and Departures as It is for Approaches and Landings. Gbas Opens Up a Whole New World of Options.

Leading the Way

Australia Has Been at the Forefront of Previous Navigation Innovations Such as the Future Air Navigation System (Fans) in the 1990s and Rnp Through the First Decade of This Century – and This is the Case Again With Gbas. Nick Welch is the Man-ager of Gbas Development With Airser-vices and Has Seen the Process Grow From Concept to Reality. There Were Various Play-ers in the Market to Introduce the Technol-ogy but Honeywell Has Become the Leader Evolving Its Original Sls-1000 Technology to the Latest Generation Sls-4000 ‘smart-path’ System That Can Be Found at Sydney Airport. As He Explains, “honeywell Was the Only Supplier That Effectively Had a Road Map for Development. ” Implementation of New Technology is Never an Overnight Sensation, the Founda-tion Stones Being Laid Back in 2006 When Airservices Began Working With Honeywell. In October of That Year Casa Approved the Use of the System in Visual Conditions to Gather Data. Since That Time, Qantas A380s and 737-800s Have Flown Nearly 3,000 Ap-proaches Including Autolands, Utilising the Now Superseded Sls-3000 System. In 2009, Us Approval for the System Was Granted and Casa Was Actively Involved as the Faa Undertook an Audit to Examine Gbas.

For Airservices, It Called for a New Perspective as It Had to Consider All Issues Associated With Maintaining a Signal in Space Rather Than a Transmitter Box on the Ground. Mr Welch Explains That New Engi-neering Documentation Had to Be Drafted and Implemented and Staff Trained and Educated on the System. Threat Assessments Had to Be Made in the Case of Atmospheric Ionospheric Events and Were Proven in Early 2012 to Have No Effect on the System. Working Groups Needed to Be Established to Gather Data and Monitor the System in an Asia Pacific Regional Sense While Icao Requirements Also Had to Be Met in the Case of This Brave New World.

But the Process Has Not Been Without Challenges. An Sls-4000 Installation in Newark, Us Had a Hiccup Involving Illegal Chinese Gps Jamming Equipment Used by Truck Drivers to Avoid Their Management Knowing Their Precise Location. Since the Newark Facility is Very Close to a Major Freeway, the Gbas Picked Up the Anomaly and It Tripped the System Into Shutdown. With the Problem Now Understood a Soft-ware Upgrade is Being Rolled Out to Correct the Sensitivity Without Compromising Sys-tem Integrity by Making It More Resistant to Isolated Events Like These.

The Sls-4000 System Already is Used to a Degree in Bremen, Germany and Newark, Usa but Australia Will Be the First Nation to Adopt the System in Large Numbers on Regular Operations. In Fact, Qantas Took Delivery of the World’s First Gls-capable Production Aeroplane in May 2005 With the Arrival of 737-800 Vh-vyd and the First Gls-capable Airbus With the Arrival of A380 Vh-oqa. On November 23 2006 It Was a Qantas 737-800 That Made the World’s First Gls Approach. Today, the Entire Qantas A380 Fleet and 50 737-800s Are Gls-capable With the Remaining Ngs to Be Progressively Modified by June 2013.

Sydney Airport Will Be the First to Host Multiple Airlines and Aircraft Types Equipped for Gls. Virgin Australia is Now Equipping Its 737ng Fleet as Are Emirates for Its A380s While Cathay Will Operate the Boeing 747-8f. In the Future, Gls Will Come as Standard on the Next Generation of 787s and A350s. For Airservices, the Focus Definitely is on the Safe Implementation of Gls at Sydney Airport and Its Use Down to Category I Minimas. Although Airser-vices Concedes That There is the Possibility of the System Appearing at Other Airports in Australia, Sydney is the Most Immediate Step in This Meticulous, Measured Path to the Future.

Threading the Needle

One Can Only Imagine What the 10 or So Passengers Thought as the Ground Grew Closer Through the Snowstorm Outside Their Window in 1938 During That First Ils Landing. Now, Nearly 75 Years Later, the First Major Revolution in Precision Approach Guidance is Set to Take Place at Sydney Airport. Ils Has Been and Will Continue to Be a Tremendous Navigational Aid but as Technology Moves Forward the Efficiencies of Gbas Will Introduce a New World of Arrivals and Departures in Places They Have Never Been Seen Before.

It is an Ongoing Challenge to Safely Implement Change in Aviation and Gbas and Gls is No Different. Now, After Years of Research, Study and the Accumulation of Data, Gls is Set to Become a Way of Life in Australian Air Transport. And Just as That Boeing 247-d Sought to Weave Its Flight-path Toward the Runway in 1938, We’re Still Trying to Find the Best Way of Threading the Needle.