13 May 2016, © Leeham Co: Last week we started to describe what is necessary to make a precision approach after a flight. We described the rather elaborate installations needed for the classical precision approach with an ILS system. It requires two transmitters and large antennae installations for each runway.
We will now describe the system which will replace ILS as worldwide instrument landing system, an augmented Global Navigation Satellite System (GNSS), where GPS is the variant provided by the US Department of Defence, Figure 1. Other GNSS are Russia’s GLONASS and Europe’s Galilleo.
The problem with a non-augmented GPS is the precision. Classically the accuracy was worst case any where in the World around 100m horizontally and 150m vertically, but that was when the US military deliberately reduced the accuracy for civil use (Selective Availability). Today this deliberate reduction has stopped and the accuracy is 25m horizontally and 43m vertically worst case.
This is not enough for a precision approach. We will now describe what is done to bring the accuracy to a level where precision approaches can be flown with GPS.
A GPS position fix by a receiver can be made more accurate if the GPS signal contains good correction information for the different atmospheric disturbances the signal can endure. The way to provide this information is straight forward. One puts up a fixed GPS receiver at a known location and observes what the GPS signal says for that location. The difference between the true location and what is given by the GPS is the correction.
There are principally two ways to measure and deliver this correction:
Wide area augmented GPS
The wide area augmented GPS for the North American continent is called WAAS (Wide Area Augmented System). It uses 38 fixed position receivers placed over the continent to collect GPS accuracy data and then sends these corrections to the satellites. The WAAS correction can be received with GPS receivers that need the accuracy such as aeronautical receivers. All modern aeronautical GPS receivers can handle WAAS signals as well as the more exact local correction, see below.
Figure 2 shows the coverage of the WAAS . The yellow line puts the border for ILS Cat I accuracy which requires minimum 16m laterally and 4m vertically. The WAAS performance in this area is better than what is required for the ILS Cat 1 system.
The WAAS system also provides the Integrity (warning for bad signals) and surveys the Availability required for the precision approaches. WAAS means an approved GPS receiver can provide precision approaches with “Localiser Performance with Vertical Guidance” to 200ft minimum (LPV200) over the whole of North America.
It’s pretty impressive that with no ground-based equipment, LPV procedures can be made and published for this area. To get to the lowest minimum, which is around 200ft, the runway must have adequate approach lighting and a parallel taxiway but no other installation will be necessary at the airports.
For simpler airports, less demanding procedures with higher minimums can be published (LP, LNAV/VNAV..)
Further, WAAS also improves the accuracy and check integrity and availability for the underway GPS-based RNAV, allowing tighter routing with lower minimums.
Over 3,500 WAAS-based approaches (LPV, LP, LNAV/VNAV) have been approved and published in the area during 2015, serving over 1,700 airports. LPV approaches now outnumber the ILS systems in this area. Europe (EGNOS) and Asia (India GAGAN, Japan MSAS) have also implemented wide area augmentation systems.
Local augmentation systems
The local augmentation systems are called Ground Based Augmentation Systems, GBAS. Here the correction and integrity/availability checks are valid for an airport. Figure 3 shows a system which has three GPS receivers and a VHF transmitter to send the correction signals as a broadcast data-link message to the VHF COM receiver of the aircraft.
The GBAS correction requires certain ground based equipment and a COMs receiver in the aircraft that can decode the GBAS corrections. But this is nothing in terms of installations compared to ILS. Imagine an airport like Nice with two runways and four approaches. Instead of four complete ILS installations, all to CAT IIIc standard, you need 3 GPS receivers and an updated COMs transmitter in the tower.
Coverage for a GBAS is 20nm, well beyond what is needed for precision approaches with the GBAS Landing System, GLS. GLS approaches are operational at Newark and Houston airports in the US; at Bremen, Frankfurt, Zurich, Malaga and Toulouse in Europe and several other airports around the world.
The GLS system is in its infancy but is spreading fast, both to airports and aircraft. The Airbus A350 that I piloted a year ago had GLS. One of my approaches at the Toulouse Airport was a GLS approach. The dialing in of the system was different to an ILS Cat III (channel number instead of frequency) but the actual flying experience was identical.
WAAS and GBAS will revolutionize aerial navigation. The only infrastructure needed for a World class underway and precision approach system will be one WAAS style installation per continent and GBAS installations for the airports which want to offer ILS CAT II and III level approaches. All other airports will be ILS CAT I level capable with no local system installations needed.