Tough time, safe landing

Results of simulated landings using the global positioning system show that planes can now land safely even in zero visibility

Published: Wednesday 31 July 1996

-- (Credit: Vishwajyoti)over the last few years, the military's network of 24 global positioning satellites has emerged as a promising technology for landing planes in bad weather. Compared with the existing system of poor-visibility landings, the global positioning system (gps) is cheaper, more versatile, and could drastically reduce weather related airport delays.

But one question remained unanswered: can gps reliably land planes in near zero visibility weather category -- also known as 'Category-3' conditions? Now the gps has answered decisively ( Popular Science , Vol 248, No 4).

In the us Federal Aviation Administration (faa)-sponsored simulations of Category-3 approaches in California, a gps-based landing system successfully brought down a 737 aircraft in 110 out of 111 tries -- the 111th was aborted after the satellite was abruptly shut down for maintenance. A second set of trials landed a 757, 50 out of 50 times. According to faa administrator David Hinson, "These tests were a landmark for gps development."

Both the trials relied on a refinement known as differential gps to improve accuracy to within a few feet. gps satellites' intrinsic accuracy is about one metre. But for security reasons, military degrades the accuracy of gps signals for non-military users to several hundred feet -- not good for precision landings. In differential gps, ground-based reference stations beam a signal that allows aircrafts to correct the gps transmission.

The California tests used the most accurate differential gps system to date, invented by Clark Kohn of Stanford University. Two credit card sized transcievers or 'integrated beacons' were mounted on poles a few miles from the runway. During approach, the plane received signals from both the gps satellites and the ground receivers -- boosting accuracy of the position read out to less than a foot.

If gps is approved, planes could fly the same curved approaches in bad weather that they often use in good weather, as against the current instrument-landing systems, where jets have to fly long, straight approaches in poor visibility, causing extensive delays.

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