OK. How hard would it be to just stick a GPS in an Airbus?
The Airbus has GPS. At least two, probably three. (I've never flown the Airbus other than commuting in the jumpseat)
A modern airline has a flight management system (FMS). The names may differ, but the functions are all very similar.
Two, or three, Inertial Reference systems are the heart of the FMS. They need data from their inertial platforms to the FMS. Each inertial reference unit is aligned prior to flight and each then maintains a position (where it thinks the airplane is) based on the accelerations each IRU senses when the airplane is in motion. Inertial platforms aren't perfect so, as the flight progresses, the IRS positions drift away from the aircraft's actual position.
The FMS takes inputs from the IRUs as well as from one of several other inputs. These inputs include DME/DME and Rho/DME from VORs, TACANS, and DME ground stations. GPS updating was added in the 1990s. The FMS calculates the accuracy of each input based on its capabilities and uses the most accurate updating to keep the FMS position as accurate as possible. Since the airplane has two, often three, FMSs, the airplane has two, often three, FMS positions.
The combined accuracy of the FMS systems is displayed to the pilots as ANP or Actual Navigation Performance. The ANP in the 737s (two IRU/two FMS/ two GPS) is typically 0.02 to 0.04 while in flight. The procedures and airspace in which we fly has established RNP or Required Navigation Performance. RNP can be as high as 10.0 in oceanic airspace to as low as 0.11 on the RNP approaches with the lowest landing minimums. If our ANP exceeds the RNP for the airspace or procedure we are flying, we receive alerts which tell us that our RNAV is no longer able to provide the required navigation performance.
RNP approaches are available at a lot of airports and allows for very accurate curved paths, including radius-to-fix or RF legs, and vertical paths. Eugene, OR has a nice RNP approach that leads you down a curved path between mountain peaks to land north (Rwy 32?). There is no ILS to that runway because of the terrain.
GLS is a form of Local Area Augmentation System (LAAS) which uses a single ground station which transmits a correction value, to the nearby airplanes flying one of many GLS approaches that can be available at the airport, that improves the accuracy of the GPS position in that local area. GLS provides minimums similar to CAT I ILS approaches and is intended to eventually provide for autoland.
But, we don't do LPV.
