Long recognised overseas for its contribution to aviation safety, the Hong Kong Observatory (HKO) finally won local accolade at Hong Kong ICT Awards 2009 for its LIDAR Windshear Alerting System (LIWAS), which beat more than 600 other entries to win the Award of the Year and also clinched the Best Innovation and Research Grand Award and Gold Award for the Open stream.

LIWAS is an automated system developed by HKO to issue windshear alerts at the airport to ensure flight safety. It combines the use of LIDAR (for "LIght Detection And Ranging") - essentially a laser beam - to measure wind speed variations along flight paths with a sophisticated computer algorithm to automatically generate windshear alerts. It is the first operational windshear alert system in the world to use LIDAR technology for automatic detection of windshear, thus making HKO a world leader in aviation weather services.

According to HKO scientific officer Li Ping-wah, development of the system started at the former Kai Tak airport in 1979. There are two main types of LIWAS: one designed to detect terrain-induced windshear, and one designed to detect windshear associated with thunderstorm-induced gust fronts and microbursts.

Whereas many airports in the US, for example, are subject to thunderstorm-induced windshear, in Hong Kong the terrain and sea breeze account for 90% of episodes; the system at Hong Kong International Airport, therefore, was designed to detect windshear in fine weather conditions. HKO began to study wind conditions at Chek Lap Kok in the 1980s, long before the relocation of the international airport. The data gathering continues today.

Windshear is caused by a change in wind direction/speed that leads to a change in the headwind or tailwind encountered by an aircraft. A decreased lift will cause an aircraft to dip below its flight path; conversely, an increase will lift an aircraft above it. LIDAR works by shining an infrared beam on moving suspended particles and calculating changes in the light reflected from these particles to detect sudden changes in headwind speed. A change that exceeds 15 nautical miles is classified as a windshear event which will automatically trigger the issue of an alert. Without LIDAR, windshear detection would have to rely on wind gauges on the ground.

There are two LIDARs at the airport and each system is installed on a swivel base with two cameras, one for the laser and one for a videocam that helps the operator in the control room position the equipment. The laser can rotate the full 360 deg and tilt vertically from the horizontal to 6 deg - corresponding to the glide paths of arriving and departing aircraft, which typically range from 3 deg to 6 deg. This helps the system follow the ascents and descents of aircraft. The two lasers work in concert to detect windshear in eight directions but one will cover all of them alone when the other's under annual maintenance.

One LIDAR is installed on the roof of the fire station at the end of the south runway while the other is installed on the roof of the fire station at the end of the north runway. The systems should ideally be installed as close to the runways as possible so that the laser beams can be shone directly over them, but aviation requirements as well as maintenance considerations make their present locations preferable as they are easily accessible.

"When angle of the laser shone over the runway exceeds 15 deg, the data start to become meaningless," said Dr Li.

The concept of LIDAR was still an emerging technology when HKO conducted tests at Chek Lap Kok in 1995-6, before the reclamation for the new airport began, to determine whether it could be used to measure wind changes. In 2001 a pulsed Doppler LIDAR system that could operate round-the-clock was purchased from the US and installed at the north runway.

"But even in the US none of the systems operated 24 hours a day, so we didn't know how well it would work," Dr Li said.

LIWAS was developed by HKO based on glide path scans conducted by the LIDARs, the first of which began operation in 2002, initially collecting data for internal reference only. It began full operation in 2003 when it demonstrated an accuracy rate of above 90%. It now has the distinction of being the oldest-operating LIDAR unit in the world. With radial velocity data gathered from the unit, HKO then developed the world's first LIDAR-based windshear alerting system and put it into operation in 2005. The second LIDAR was installed near the north runway in October 2006 to serve as a backup for the first LIDAR. When not required to serve as a backup, it is configured to scan more frequently over the north runway, over which most of the approaches are made.

HKO has also improved the LIWAS algorithm to better detect rapid headwind changes in terrain-induced windshear. Spring, with northeasterly winds predominant, is the time of year with the largest number of windshear alerts; June accounts for 40% of the alerts issued in a year. During the typhoon season of July and August as well as other wet days, LIDAR is combined with radar detection to cover all weather conditions.

According to Dr Li, the accuracy of the system is dependent on pilots' report of windshear encounters after alerts have been issued. HKO therefore works with both the Civil Aviation Department (CAD) and airlines in order to gather as much data as possible through CAD querying pilots about any such encounter every time LIDAR detects windshear. On average, pilots report 700-800 windshear encounters per year, with about one in 500 flights encountering an episode.

Interestingly, analysis of pilots' report and black box data indicates that not all windshear epiodes are reported, with a 40% difference between pilots' report and figures obtained from aircraft black boxes. This is put down to the pilots' experience: less experienced pilots may report marginal cases while more experienced colleagues, having handled so many cases in their careers, sometimes do not even consider them to be windshear episodes.

Dr Li paid tribute to the two local airlines, Cathay Pacific and Dragonair, for their willingness to cooperate with HKO. "Most airlines consider black box data as a commercial secret," he observed.

As a result of this level of cooperation, HKO has become the world's top meteorological service when it comes to cumulative windshear data and has been complimented by the International Federation of Airline Pilots' Association for being a "centre of excellence within the aviation meteorological community".

HKO's work is also referenced by international standards on aviation covering the use of LIDAR for wind shear and turbulence detection. Its data is especially valued because of the effort put in to ensure the precision of alerts. According to Dr Li, windshear episodes may last half an hour to a whole day, as when there is a storm, and it is possible to 'cheat' by issuing an alert for the whole day regardless of the length of the actual episode so that every pilot that reports encountering windshear will score a hit. To address this potential problem, another indicator is used to plot the number of short alerts that score.

For the past three years HKO has been developing an enhanced version of LIWAS, a fully automated service that sends alerts directly to pilots from the airlines' servers, via satellite. Chek Lap Kok is the only airport in the world to offer this service and HKO hopes to make it more widely available by persuading more airlines to collaborate with it.

Windshear may affect a critical 200 m of a takeoff or landing so the alert system plays a vital role in aviation. By improving aviation safety and efficiency, LIWAS is esimated to provide social economic benefit that averages an estimated HK$0.05 per passenger per year, assuming it has a ten-year life span.

LIDAR works by shining an infrared beam on suspended particles and calculating changes in the light reflected

The LIDARS scan the runways at different angles

Separate alerts are issued for the eight runway corridors

HKO has developed the world's only automated service, which sends alerts directly to pilots via satellite