The UK Civil Aviation Authority has investigated the reliability of the global- positioning system (GPS), and found it wanting. GPS, it says, is not reliable enough, in its current form, to be used as a sole means of navigation. In this, the CAA is at odds with the single most powerful civil-aviation authority of all, the US Federal Aviation Administration - but is it also at odds with common sense?
It has long been the dream of aviators to establish their positions in space and time with absolute accuracy - and, with the GPS, that dream has come tantalisingly (some would say deceptively) close to reality. In fact, in most branches of aviation other than commercial air-transport, the GPS has become accepted as the single most important and accurate tool of navigation, whether or not it has been granted that status by sceptical national authorities.
The arguments against an unconditional acceptance of the GPS are well-rehearsed: the satellites are the property of the US Government, which can down grade (or even switch off) the service without notice; the reliability of individual spacecraft within the constellation cannot be guaranteed; their orbits are such that the quality of the positioning service deteriorates dramatically near the geographic poles.
The CAA's latest research seems to prove that such misgivings have a basis in truth. In a sample of 34,419 test points, outages detected by the onboard avionics were experienced in 759 (2.21%) cases and, more worryingly, there were undetected outages in another 487 (1.41%). According to this test, only 96.38% of the GPS position reports were correct. The results suggested that a receiver would go fewer than 60h between outages.
The last time that the air-transport industry relied on a piece of technology which was apparently that unreliable was in the 1950s, when an airliner powered by four of the powerful but complex Wright Turbo-Compound piston engines could virtually be guaranteed to suffer an engine failure in the course of a return transatlantic trip.
The failure of a sole-means navigation system could be a traumatic event - but would it really be so? For a start, not since the dawn of aviation, when an aviator was lucky to get safely airborne at all, far less worry about doing anything more sophisticated in navigation than use the Mk1 eyeball, has the average pilot relied solely on one means of navigation. Even when using primitive radio-navigation aids, the pilot would check back on visual cues, and no transatlantic pilot today would rely on a single inertial-navigation system. Now, it seems, he could be asked to rely on one system - the GPS.
Does this make sense? The military itself tends to use one or more inertial-navigation systems (INS) as a back-up. The concept of using a single system without a back-up seems alien to an industry which has adopted caution as a watchword. It is not impossible to conceive a navigation system which does not rely entirely on satellite-derived data as its primary feed, but to to check and augment accuracy. A system combining an INS with the GPS should be inherently more reliable than one which relies on the GPS alone.
The industry does need the GPS, or some other ultra-accurate navigation system, sole-means or otherwise, to meet its demands for greater use of the airspace available. Without positional accuracy measured in a few metres, rather than hundreds or even thousands of metres, the concept of vertical and horizontal separations reduced to hundreds of metres can never be realised. The only available sources of such data are the satellite-based systems, a fact which those who would question the reliability of GPS would do well to remember. The GPS is not perfect - and it will not remain unaugmented in accuracy, availability and integrity for long - but it is much closer to perfection than anything which has gone before.
"Pilots would check back on visual cues, and no transatlantic pilot today would rely on a single inertial-navigation system."
Source: Flight International
