Contributed editorial appearing in
Scientific Computing & Instrumentation 18:4, March 2001, pg. 14.
Data acquisition is often thought as the handmaiden of automation. Existing experiments and manufacturing processes requiring the measurement of quantities including temperature, pressure, flow rate, and level can exhibit increased efficiency, throughput, and safety through the incorporation of an automated supervisory control and data acquisition (SCADA) system. SCADA improves the process “inside the box” by upgrading the communication of inputs and outputs between other components of the system existing “outside the box.” While automation and SCADA continue to benefit from evolutionary increases in analog-to-digital conversion rates and bandwidth, advances in data acquisition technology often culminate in the revolutionary creation of a new process, or, if I may, “a new box.”One such revolution is the measurement of windshear. A review of airplane incidents occurring between 1964 and 1985 attributed 500 fatalities and 200 injuries to windshear — a generic term used to describe any rapidly changing wind currents. Windshear most often results from a short-lived, powerful downdraft of air emitted from a low-altitude storm cloud. As this column of air strikes the ground, its energy radiates outward, similar to the splash of a pebble in a pond, creating peaks and troughs of alternating headwinds and tailwinds. Not recognizing the initial increase in headwinds as windshear, upon landing approach the unsuspecting flight crew decreases thrust to offset the increased lift experienced by the airplane. The subsequent wave of tail winds reduces the lift and the underpowered airplane literally falls out of the sky.
The Federal Aviation Administration (FAA) administers a ground-based SCADA system for windshear detection known as the Low Level Windshear Alert System (LLWAS). An array of 6 - 32 anemometers is deployed across the airport and each measurement of wind speed and direction is transmitted to a central processor in the control tower that scans the data for patterns of windshear. The system detects windshear below altitudes of 2,000 ft in proximity of the airport and air traffic controllers can advise pilots of the danger. While helpful, the LLWAS is not capable of windshear detection beyond the airport vicinity and every pilot in the area must be contacted in a manual process that can last two minutes.
Increases in radar (RAdio Distance And Ranging) technology, coupled with advanced data-acquisition (DAQ) hardware and digital signal processors (DSPs), have resulted in the development of real-time windshear detection systems that operate onboard the airplane. Moving beyond the automation of a previously manual process, forward-looking windshear radar (FLWR) systems are made possible by their DAQ and DSP components. Two popular commercial FLWR systems are the RDR-4B and WXR-700 systems, developed by AlliedSignal Aerospace (now Honeywell) and Rockwell - Collins, respectively.
These FLWR systems are based on Doppler radar using 9.3 - 9.4 GHz-, 125-Watt pulses, each lasting 1 - 20 microseconds. These pulses are emitted at a rate of 180 Hz to several kHz from a flat plate antenna mounted inside the nose of the airplane. Each pulse propagates away from the airplane and is partially reflected back to the antenna when it strikes the raindrops of a storm cloud. A cascade of echoes from as far as 320 nautical miles (593 km) is received from each pulse and the roundtrip travel times are used to calculate distances. In addition, the precise frequency of each echo is determined and its Doppler shift from the source frequency is used to calculate the speed of the raindrop toward or away from the airplane. The location and velocity of storm clouds is detailed in the cockpit on a color display, similar in fashion to the NEXRAD Doppler radar images appearing in the local television weather report. When flying below 2,300 ft, the FLWR automatically switches to “windshear mode” by increasing to the maximum pulse rate and decreasing the detection range to 5 nautical miles (9.3 km). While in this mode, a complete scan of the space lying ±40 degrees of the current heading is made every 3 seconds. When the velocity patterns are determined to indicate windshear, a visual and aural alarm declares, “Go around! Windshear ahead!”
A current disadvantage of the FLWR systems is their propensity to display stationary ground objects on approach to the runway. This “ground clutter” can be removed by adjusting the tilt of the radar antenna above the horizon while the airplane is nose down. To avoid the pilot distraction involved in manual tilt adjustment, the RDR-4B integrates with the Honeywell Enhanced Ground Proximity Warning System (EGPWS), an onboard worldwide database of terrain elevation data used to alert pilots of approaching mountainous terrain. Coupled with data acquired from global positioning satellites and heading information, the FLWR tilt is automatically adjusted to point above the horizon as determined by the EGPWS. Upcoming models of the Rockwell-Collins WXR series will utilize an array antenna that scans multiple elevations simultaneously. Each of these systems involves the real-time acquisition and processing of data streams with life-and-death consequences — a far cry from the routine digitization of a thermocouple.
