Radar Corner

Here is what a windshear looks like in the Doppler domain when it is picked up in the main beam and the sidelobes. The raindrops coming towards the aircraft look faster (light colors) and those moving away from the aircraft look slower (darker colors). Of course, the system must remove any aircraft movement (groundspeed) from the data.

The picture below shows Kansai airport and the orientation of the Sky Gate Bridge.

Finally, here is a look at what happens when the sidelobes hit the Sky Gate Bridge. There are several things at play here. The large metal bridge dominates the radar environment at Kansai. The orientation of the bridge and the runways causes returns to be picked up in the sidelobes. Even though the bridge is stationary, there appears to be movement because it is picked up in the sidelobes. And finally, the bridge is the same length as a typical windshear.

And the big finale… if the two Doppler signatures are placed side by side, it is apparent that the signature of a typical microburst is almost identical to the signature from the Skygate bridge. Fortunately, now that the problem is known, it can be addressed. With the RDR-4B, engineers had several options. They ended up using what they called an “upper bar scan” to validate that there was a rain shaft above the outflow causing the microburst. Because the airport is on an island, it won’t ever have a dry microburst scenario, so engineers could also look at the location and adjust the system sensitivity.

One might ask what the wind has to do with all of this. It turns out that on calm wind days, the radar energy bounced off the smooth water, hit the bridge, and was reflected back to the radar which caused the nuisance alert. On days when the wind was stronger, it caused waves in Osaka bay. The waves scattered the radar’s energy, which meant much less of it reflected off the bridge.

Fast forward a few more years to Sydney Kingsford Smith airport in Australia. A major airline based in Singapore was getting frequent nuisance windshear system failures while performing the windshear system test before departure. Their gate is tucked into a corner and speculation is that the failures are caused by radar reflections from the corner walls, interference from cell phones, hand held radios, etc.

Honeywell offered to take test equipment to the airport and determine the root cause of the interference, but was turned down multiple times because security wouldn’t allow it. This went on for quite a while and caused considerable pain for the airline. Flying between Australia and Singapore means crossing the Inter-tropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ), both which are very active weather producers. So, a failed radar test is a no-go item and the airline was required to stock additional spares and delay flights while the units were replaced. This became unacceptable and the airport authority “hired” a firm to come in and determine the source of the interference.

After multiple days of testing and analysis the source was determined. On the other side of one of the walls was a men’s bathroom. The toilets, like most, had motion detectors that sense motion and flush the toilet automatically. These sensors used radio frequency to detect motion, but unfortunately they created harmonic frequencies in the same frequency band used for windshear detection. So if by chance the windshear test was being run at the same time the sensor was transmitting, pilots would get a windshear fault.

Honeywell’s new RDR-4000 IntuVue weather radar is becoming available on more and more business and commuter aircraft. In addition to providing windshear detection for this market, it introduces many other significant advancements in radar technology including automatic modes, vertical profile of weather and terrain, enhanced long-range turbulence detection, hail and lightning prediction, analysis modes, ground clutter elimination, and much more.

Previously only available on military radars costing millions of dollars, the RDR-4000 also includes Pulse Compression. Pulse Compression provides both longer range and higher resolution that were exclusive on legacy radars. In addition, there are different types of Pulse Compression used. One type reduces azimuth sidelobes, making the RDR-4000 less susceptible to nuisance alerts. The picture below shows Honeywell’s RDR-4000-equipped B757 doing data collection at Kansai International Airport.

This concludes Direct-TO's three part series on understanding airborne windshear detection systems. The entire series is now available on the Honeywell Pilot Gateway, and will be stored in the Direct-TO Newsletter archive.

Program Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached at Stephen.Hammack@Honeywell.com.