Use of the Instrument Landing System (ILS) has been widespread for around three-quarters of a century. However, winds of change are blowing with the adoption of Global Navigation Satellite Systems (GNSS) in precision approach systems that provide both lateral (heading) and vertical (glidepath) guidance. The radio navigation-based ILS uses a complex system of signals and an antenna array making it the go-to short-range guidance system for aircrafts to approach runways at night or in inclement weather.
Most recently, Airbus has integrated new satellite navigation technologies with an ILS look-alike interface which benefits from the ILS operational experience. The most recent of these to be rolled out is a new certified cockpit avionics function for operators of the A320 and A330 families, for line-fit and retrofit – referred to as a satellite-based/augmented landing system (SLS).
While the operation is largely seamless for the pilots, behind the scenes a variety of enablers had to be created in order for SLS to exist. Upgraded avionics on board the aircraft are primary enablers in this respect. These need to embody the new functions to support SLS. GNSS coverage is a prerequisite, as is Satellite-Based Augmentation Systems (SBAS) coverage. SBAS satellites improve the accuracy and reliability of GNSS information by correcting any ionosphere-induced signal errors and by providing information about the integrity of its signals.
With SLS, pilots do not have to change the way they fly the aircraft because the guidance symbology presented in the cockpit looks just the same as what they are already used to for an ILS approach. Through the aircraft’s GNSS antenna, its multi-mode receiver (MMR) receives data from satellites. In addition to those positioning sources, the MMR also receives overlay corrections from SBAS, which transmit their signal via Geostationary Satellites.
GNSS-based precision approach systems are a more cost-efficient alternative to existing radio navigation-based approach systems. They facilitate precision approach access to secondary airports where ILS might not be deployed. Besides, SLS might be also deployed at main runways as a backup to ILS, say, during maintenance.
“It is often the ‘secondary airports’ which are less likely to have sufficient throughput of traffic to justify the cost of installing radio-based ILS equipment on the ground. By contrast, ILS is usually present at primary airports to allow precision tracking of the glidepath down to the runway decision height, and is especially useful in bad weather or when there are obstacles below the approach. Now, with the combination of SLS capability in the aircraft and SBAS augmentation coverage in the sky, the aircraft is able to perform accurate ILS-like CAT1 approaches in low-visibility conditions into the secondary airports too,” explained Mathieu Hialé-Guilhamou, Avionics Engineer, Airbus France S.A.S.
He notes that conventional “non-precision approaches” are affected by variations in barometric altitude and temperature, whereas using SLS has the advantage of not relying on conventional barometric altitude nor temperature calibration.
SLS first entered service in Europe with the A350 in 2015 after Airbus pioneered its development and introduction with the support of the EU Agency for the Space Programme (EuSPA), formerly known as the European GNSS Agency (GSA), and the European Commission.
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