Radar and Satellite Communications

A technical paper presentation on….

ATC AND WIND SHEER DETECTION

THROUGH

TDW RARDAR

Abstract

This paper deals with Terminal Doppler Weather Radar (TDWR) installed in airports to provide wind shear detection services and precipitation reflectivity data to controllers and supervisors. The TDWR’s narrow beam and aggressive ground clutter suppression algorithms provide excellent data on boundary layer reflectivity and winds – in particular the locations of thunderstorm outflow boundaries. These data are known to be essential for providing high resolution convective weather forecasts out to two hours. Similarly, its narrow beam could be useful for detection of severe weather signatures (e.g., tornado vortices) with small azimuth extent. Relative to the Weather Service Radar 88-D (NEXRAD) it scans rapidly (e.g., surface updates once per minute), facilitating monitoring of rapidly evolving low altitude wind shear hazards. It is typically located near to population centers and congested airspace, so that it is well situated for supporting weather services for operationally important areas.

As the name suggests, the Terminal Doppler Weather Radar (TDWR) was purposely built to serve the terminal area of

The airport. Its mission is to detect wind shear and microburst associated with convective storms, so as to enhance the safety of aircraft landing and taking off. It is located near the airport at a distance of 12km so that it has a clear view of the runways, airport approach and departure zones.

The TDWR is specially designed to operate in a high clutter environment normally present in the vicinity of airports. It makes use of a variety of methods to minimize clutter and to eliminate the influence of such moving targets as birds, aircraft and automobiles. In this way the TDWR can accurately measure the radial wind speed and its fluctuation from which low level wind shear can be computed. Equipped with sophisticated computer programs, the TDWR is able to automatically detect thunderstorm-induced wind shear phenomena.

TDWR ARCHITECTURE

The TDWR system was designed and built in the late 1980’s, and is encountering issues related to parts obsolescence. To ensure that the system continues to be maintainable, the FAA has commenced a Service. Life Extension Program (SLEP) to improve supportability and, where appropriate, introduce improved capability.

A simplified block diagram of the TDWR is shown in Figure. The RPG subsystem, shown in green on the upper left, has recently been re-hosted from a Harris Nighthawk UNIX system to one based on a pair of redundant SGI Origin computers. The next major digital subsystem to be addressed,

and the focus of this paper, is the RDA, which includes the receiver and DSP subsystems.The existing TDWR DSP subsystem hardware consists of a mixture of COTS and custom cards, installed in a single 19” Multibus system chassis. The COTS boards include a 68020-based single-board computer (SBC), and a SCSI and serial controller. The custom components include five boards to handle the A/D interface and timing needs, eight boards to perform clutter filtering, and six boards to handle the generation of moment’s data.

ANALYSIS OF ATC

Air Traffic Control, management of aircraft proceeding along civil airways, including airport arrivals and departures.

The minimum instruments required under VFR include an airspeed indicator, altimeter, and magnetic direction indicator. Minimum flying conditions in radar-controlled airspace in transition areas specify a cloud ceiling about 215 m (700 ft) above ground level and 1.6 km (1 mi) visibility. Other VFR requirements for visibility and distance from clouds depend on altitude and whether operation is in controlled or uncontrolled airspace. VFR flight is not permitted in all airspaces, and terminal control areas sometimes require positive (radar) air traffic control. Airport traffic areas typically encompass a radius of 8 km (5 mi) and are extended laterally for the control of instrument-dependent departures and landings. Control zones around airports extend upwards with no limit.

UPGRADATION OF RADARS

WEATHER SERVICE RETIRES LAST OF OLD RADARS

The National Weather Service turned off the last of its 1957-model weather radars on Dec. 2, 1996. The radar, at the Charleston, S.C., Airport went into service in 1959, the 16th of 66 WSR-57 radars the NWS eventually installed around the USA.

WSR-57 stood for Weather Surveillance Radar-57 for 1957, the year it was designed. The old radars were replaced by 88-D radars, which stand for a 1988-design Doppler radar. The new radars are more sensitive, which means they can detect more weather details than the old ones could. The Doppler capabilityalso means they can detect wind speeds and directions, giving a much better picture of bad weather.

The new radars make extensive use of computers, which means they can be programmed to sound an alarm when weather patterns are beginning to appear dangerous. With the old radars, someone had to watch the screen constantly when storms were possible to make sure nothing important was missed.

"It took considerable skill to determine storm intensities from green blotches on the radar scope," said Steve Rich, meteorologist-in-charge of the Charleston office. "It took even greater skill to tell if a storm had tornado characteristics."

Like the user-friendliness built into much of today's technology, the new Doppler radars produce highly accurate storm signatures. "For the first time in history, we are now able to broadcast a tornado warning for a given area before a tornado is formed," Rich said. "This is a remarkable technological achievement and has already saved many lives.”

THE DOPPLER EFFECT

"The observed change in the frequency of sound or electromagnetic waves due to the relative motions of the source and observer."

The Doppler Effect (or Doppler shift), named after Austrian physicist Doppler discovered in 1842. Doppler worked out his ideas using sound waves, long before radio, much less radar, was invented. But the same principle applies to radar's radio waves and to light arriving from distant stars.

The Doppler Effect is best described by the change in pitch of a train’s whistle as the train passes by your position. The pitch of the train’s whistle changes because the frequency/wavelengths of the sound increases/decreases as the train moves toward you and decreases/increases as the train moves away from you. The same concept applies to meteorological targets. Doppler radar is able to detect the changes in frequency/wavelength resulting from storm motions toward or away from the radar. This concept is the basis for all velocity products.

DOPPLER RADAR AND KEY FACTORS FOR UNDERSTANDING WEATHER RADAR

All weather radars send out radio waves from an antenna. Objects in the air, such as rain drops, snow crystals, hail stones or even insects and dust, scatter or reflect some of the radio waves back to the antenna. All weather radars, including Neared, electronically convert the reflected radio waves into pictures showing the location and intensity of precipitation. Doppler radars also measure the frequency change in returning radio waves. The computer that's a part of a Doppler radar uses the frequency changes to show directions and speeds of the winds blowing around the rain drops, insects and other objects that reflected the radio waves.

Scientists and forecasters have learned how to use these pictures of wind motions in storms, or even in clear air, to more clearly understand what's happening now and what's likely to happen in the next hour or two.

MOBILE RADAR CHASE TARNADOES

Tornado researchers used two portable Doppler radars on the Plains this year to obtain even better pictures of what happens inside tornadoes. This complete look at tornado-generated winds should allow meteorologists to fine tune their conceptual model of tornado structure. Such a better understanding could improve tornado forecasting. In 1995, scientists used one Doppler radar, nicknamed Doppler on Wheels (DOW) because it was mobile, during the Verification of the Origins of Rotation in Tornadoes Experiment. At a distance of less than two miles from a tornado, the Doppler radar sends out a beam of radio waves to scan the tornado's wind field. But, with only one radar, researchers can measure only the speeds of winds going toward or away from the radar.

APPLICATION OF TERMINAL DOPPLER WEATHER RADAR

Wind versus height

Meteorological Doppler radars generally detect precipitation particles, insects and refractive index gradients. These scatterers move with the wind. Thus Doppler radar can measure the wind component in the direction in which the radar beam is pointing. Lhermitte and Atlas (1961) first described how a single Doppler radar could be used to measure vertical profiles of wind velocity in widespread precipitation; it can also be used in clear-air.

Front location

Sharp wind shifts, which are frequently associated with frontal boundaries, are usually easily identified on the Doppler velocity display. Thus, fronts can be precisely located and their movement closely monitored on low-level wind shift associated with a cold front approaching the coast of Washington state. In this geographic region accurate frontal locations are rarely known because data is sparse.

Radar Sees Rain

Rain or snow that evaporates as it falls into a layer of dry air near the earth’s surface is called virga. While virga can happen at any time during the year, it is most common during winter, when low-level air is very dry. As falling precipitation evaporates it moistens the dry air from the top down. If the precipitation lasts, the air becomes moist enough to block evaporation and the rain or snow reaches the ground.

CONCLUSION

The early experience of operational forecasters as the WSR-88D's are being installed at Weather Forecast Offices across the U.S is that they are able to improve their abilities to understand and forecast severe thunderstorms, blizzards, windshifts, thunderstorm initiation and to monitor precipitation accumulations. . It is expected that as more and more operational Doppler radars are installed and a large number of forecasters results will be more better Its analysis, forecasting and warning applications possible with Doppler radar, however, the radar should not be considered as a stand alone tool.