Soaring Safe

August 1, 2005 By: Alan Cameron GPS World

Lightweight Collision-Avoidance System

Quick — how many life-saving decisions can you make in 18 seconds? Pilots often must take such rapid action, especially when other airborne bodies suddenly come too close. This can happen in an eyeblink aloft, as pilots must concentrate on a multitude of sensory data, instrument readings, and other data.

Photo courtesy of FLARM

Commercial aviation requires large aircraft to carry air collision avoidance systems (ACAS), based on powerful onboard primary radar interrogating nearby transponders similar to ground-based radars. Aircraft in the General Aviation category — all flights other than scheduled airlines and military planes — typically do not carry such large, heavy, and expensive systems. So pilots of small planes, helicopters, and gliders, all of whom operate under Visual Flight Rules (VFRs) live by their eyes only.

Now a newly designed Swiss system uses GPS to add a significant measure of safety to such airborne adventures.

The FLARM System mounted atop a glider pilots instrument console, with the man serving as a second pilot or passenger

Each year, about 20 mid-air collisions occur in General Aviation, half of them fatal to pilots. One-third of these collisions involve gliders. There is an acute need for a reasonably priced system with a small footprint and low energy consumption, that effectively and efficiently warns pilots of dangerous traffic in the area and thus optimally supports them in airspace surveillance.

To Market

Drawing from their professional backgrounds, electrical engineer Urs Rothacher and physicist Andrea Schlapbach, both experienced glider pilots, designed a lightweight GPS-based flying alarm system dubbed FLARM (see Figure 1).

Figure 1 depicts how FLARM hardware components integrate.

"These were our challenges," said Schlapbach. "First, understand the problem and the users in detail — largely a non-technical issue. Then, define the requirements. This includes aircraft movement models, behavior patterns of aircraft and pilots, such as local aircraft clusters, frequent exchange between potential and kinetic energy, frequent speed and track changes, team flying, and ridge-soaring dynamics. Also, installation factors such as vibration, size, power consumption, and voltage spikes, and environmental aspects such as sunlight, heat, and reflections.

"Finally, choose appropriate technologies to meet our requirements: radio band and frequency, range considerations, radio protocol layers, transceiver, GPS Kalman filters, and more," he added.

Rothacher and Schlapbach introduced their first product in early 2004. Within six months, half of the Swiss glider aircraft population had been equipped with FLARM. By spring 2005, more than 2,500 FLARM devices were in use in Germany, France, Switzerland, Austria, New Zealand, and South Africa. Recently the Swiss Air-Rescue (Rega) adopted the FLARM anti-collision device for the 10 rescue helicopters it operates from several bases in Switzerland transporting medical assistance to the scene of an accident.

How It Works

The device utilizes position and movement information obtained from an integrated WAAS-enhanced GPS receiver and an embedded barometric sensor.

Internal algorithms predict a glider's future flight path and the unit transmits it over low-power, short-range radio as a very short digital message once per second. Other compatible units within range receive these messages and compare them with their own predicted flight paths.