SouthPAN: The Next Big Leap in Satellite Navigation (Part 2) - by Andrew Andersen

SouthPAN will bring an evolution in satellite navigation for our region. In the second of two articles, Andrew Andersen, a passionate advocate for general aviation and expert in Australian surveillance and navigation technology policy, discusses the implications of SBAS on instrument approaches and vertical guidance.

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Approaches with Vertical Guidance

By Andrew Andersen

Last time, we looked at integrity, one of four critical criteria for the certification of aircraft navigation systems, and why it is so important when satellite navigation is to be used for vertical guidance.  SouthPAN, the Australia-New Zealand SBAS currently being developed, will provide an ICAO-compliant solution for aircraft equipped with TSO C146 GNSS avionics.

In this article, let’s consider the different types of instrument approaches that can utilise satellite-based procedures with vertical guidance.

A safe way to get down

Instrument flying began in the 1920s, often as quite a risky affair.  As so often happens in aviation, technology came to the rescue, and in 1938 the first passenger aircraft landed in a snowstorm using the newly authorised Instrument Landing System, or ILS. 

The ILS brought a whole new level of safety to civil aviation, which in turn enabled more reliable operations, even in poor weather.  It didn’t take long for ILS to be a must-have for major every major airport.

Unfortunately, the ILS has its drawbacks, too.  The biggest hurdle is the cost of buying and installing the equipment, which these days runs into millions at each runway end.  There are critical areas near the ILS localiser antenna which must be kept clear of vehicles and taxiing aircraft when it is in operation.  ILS also requires stand-by power sources, precise calibration, regular checking and monitoring during use.

In regional areas of geographically large countries, particularly Australia, it is impossible to justify the cost of installing and operating ILS at most regional locations.

One question, many answers

Recognising the need to find another way, in May 2000, ICAO adopted the concept of Approaches with Vertical Guidance, or APV, to provide the basis for a stable descent whilst avoiding the cost associated with ILS precision approaches.

APV improves safety by providing pilots with a glidepath indication, even at airports without ground-based navigation aids.  As we learnt last time, satellite-based navigation, on its own, is capable of adequate accuracy and integrity laterally, but not vertically.

Initially, Australia chose to implement APV only for aircraft equipped with Baro-VNAV technology.  Baro-VNAV utilises a sensitive altimetry system interconnected with a flight management system (FMS) to enable the display of deviations from a glidepath defined in the FMS database by comparing the barometric altitude of the aircraft with its satellite-derived lateral horizontal position along the approach path.  Because of its complexity and the need for a multi-sensor FMS, Baro-VNAV equipment is mostly confined to turbine-powered aircraft, although it is available as an option for recent Cirrus SR-series and Piper Seneca V piston engine aircraft.

Conversely, however, Baro-VNAV equipment is not available for around 85% of the Australian IFR aircraft fleet.

To enable the use of Baro-VNAV equipment, a new standard for landing minima was created, known as LNAV/VNAV.  Take a look at the RNP approach chart for runway 23 at Dubbo, for example, which lists the LNAV/VNAV minima as 1,280 feet and 2.1 kilometres visibility, compared to the LNAV’s 1,500 feet and 3.4 kilometres visibility. 

Baro-VNAV requires an accurate landing QNH, and is subject to temperature limitations, which do not apply to the use of SBAS for APV.

In addition to SBAS, a ground-based system known as GBAS has been implemented at Sydney and Melbourne airports.  GBAS generates GNSS corrections for use at only one location.  GBAS equipment is economic only for larger airline aircraft.

SBAS for APV

At the same time as APV was being defined by ICAO, the FAA was working on WAAS, which was to become the world’s first SBAS.  With its corrections to improve accuracy and integrity, it was determined that SBAS would provide a compliant alternate means, in addition to Baro-VNAV, to enable APV, not just for LNAV/VNAV minima, but to a new standard for lines of minima known as Localizer Precision with Vertical Guidance, or LPV.  LPVs are able to support landing minima lower than LNAV/VNAV, and utilise procedures designed with converging lateral and vertical path limits, just like an ILS.

The FAA has already implemented more than 4,000 LPVs in the USA, serving more than 2,000 airports, and their use in Canada and Europe continues to expand.  Airbus and Boeing both are delivering or have plans for SBAS-capable airliners.

Once SouthPAN makes certified SBAS available in Australia and New Zealand, appropriately rated GA pilots of IFR aircraft with TSO C146 avionics will be able to fly RNP approaches to LNAV/VNAV and LPV minima.

Do you have questions, or would like to learn more about satellite navigation?  Ask your SFT instructor, or suggest some topics for future articles.