New Way to View Weather

Honeywell’s new RDR-4000 weather radar doesn’t just see storms ahead of the aircraft. It remembers what it’s seen, storing weather data for more than 1.9 million cubic miles. That’s a rough calculation based on scanning 90 degrees to the left and right of the aircraft’s nose, out ahead to 320 nautical miles (nm), and from the ground up to 60,000 feet.

The data in this aggregated 3D volumetric scan can be manipulated and viewed in many ways independently from the physical scan. The pilot and copilot, for example, can look at weather and ground map displays at the same time because they are viewing data, not moving the antenna. Data is stored in three short-term memory buffers, corresponding to different stages of processing, and each buffer is several gigabytes in size.

Honeywell asserts that its massive data scans capture a more complete picture of a weather cell than do multiscanning systems that rely on beam pairs. The company also says that the RDR-4000’s use of pulse compression and greater sensitivity mean that it does not have to place the radar beam tangential to the Earth’s surface, reducing the amount of ground clutter. It also uses an internal terrain database to extract ground returns.

"What’s important is that the processes for collecting data and pulling it out for display are completely independent," explains Stephen Hammack, manager of technical marketing-radar products with Honeywell Aerospace Electronic Systems. Today’s weather displays typically are wedded to the antenna scan. When the antenna swings in one direction, the screen is updated with new information that "wipes away" what was there before. However, by allowing data to accumulate, additional analytical and what-if presentations can be created.

The RDR-4000, Honeywell’s first all-new aviation weather radar in 25 years, will debut in the super jumbo Airbus A380 passenger jet and the U.S. Air Force C-17 airlifter, which will use the military RDR-4000M variant. American Airlines is expected to equip its fleet of Boeing 757s, B767s, B737NG and B777s with the sensor, and Honeywell is working on an agreement with a regional jet maker. The Japanese government also plans to use the new radar on the military cargo aircraft, the C-X. The military system recently received the designation of APS-150, which identifies it as a part of the U.S. inventory that can easily be purchased by U.S. forces and governments with foreign military sales funding.

Honeywell expects to begin producing the 4000M in March 2005 and to receive technical standard order (TSO) approval in July 2005. The civilian RDR-4000–to be installed as part of Honeywell’s integrated surveillance system on the A380–is to be TSO’d by November 2005. Airbus will begin test flying the RDR-4000 in 2005 en route to service introduction in 2006.

A380 pilots will enjoy simultaneous vertical and top-down weather displays. The vertical profile presents a cut-away view of storm cells, allowing pilots to quickly determine a storm’s height and locate its most reflective portion. A storm cell’s height is an indicator of its strength and danger. Honeywell claims its 320-nm detection range is the best in the industry.

Because the system can provide wind-shear alerting using 30-inch, 24-inch, 18-inch and 12-inch (76-cm, 61-cm, 45.7-cm and 30.5-cm) antennas, Honeywell targets business aviation and regional aircraft customers, as well as major airlines. Current Honeywell air transport weather radars, by contrast, use 24- and 30-inch (61- and 76.2-cm) antennas.

Pilots with displays that are not capable of a vertical profile presentation will be able to scroll up and down to see the weather on a planview display at altitudes up to 15,000 feet above or below cruise in 1,000-foot slices–the constant altitude mode. A380 pilots can view weather slices at absolute altitudes directly off of the display.

The radar also will improve the correlation between indicated turbulence areas and predicted G forces on the aircraft, Hammack says. And it will improve the display of turbulent areas with larger, more prominent icons. These icons–polygons of up to eight sides each–will differentiate between moderate and severe turbulence. The radar also will be able to detect turbulence with lower levels of reflectivity, he asserts.

In addition to weather, windshear and turbulence detection features, the military radar provides a high-resolution ground map mode and a "skin paint" mode–to better detect the returns from the skin of other aircraft. Both modes rely on pulse compression, a technique borrowed from military radar designs. The skin paint mode is used for formation flying, aerial rendezvous and refueling operations.

The military ground mapping mode, accurate to 60 feet (18.3 m), can be used to identify terrain features and runways during approach and landing or search and rescue missions. But because it is a "real-beam mode," painting the screen at the speed of the antenna, air crews can not simultaneously access modes relying on stored data. Commercial carriers will use the lower-resolution, "extended ground map mode," which provides enough resolution to identify features such as coastlines, large bodies of water and cities. Since this ground map mode uses data from the buffer, the pilot and copilot can access the other modes. "There are still areas in the world," Hammack says, "where even with triple inertial navigation systems, [a carrier] wants to make sure not to cross some border."

Listening Better

Older radars achieve longer range by "transmitting a huge power pulse," Hammack says. But higher resolution was sacrificed to get longer range. Newer radars like the RDR-4000 "listen better," using signal processing to optimize resolution.

The RDR-4000, in particular, is the first commercial air transport radar to use pulse compression, a signal processing technique that has been used by military radars for 35 years to increase resolution while maintaining range. The emergence of high-performance, off-the-shelf signal processors and digital synthesis chips has brought pulse compression to commercial customers.

With pulse compression, multiple frequencies are coded into a transmitted pulse. When the signal returns from the target, the frequencies are separated and the radio frequency (RF) energy adds up, giving you range. The end result is a very high peak power and very narrow pulse width, effectively compressing the pulse in time.

The greater the number of frequencies encoded in a pulse, the greater the resolution. Because the points of energy coded at different frequencies arrive at spatially separated targets at different times, signal processing is able to resolve these targets into different storm cells. This increases weather situational awareness in the cockpit and adds to flight safety. Different coding schemes exist for predictive windshear, weather and ground mapping.

The new antenna drive system boasts dual azimuth and elevation motors, making the system 600 times less likely to fail in flight, Honeywell claims. The drive motor works without gearing, which increases reliability and decreases antenna noise. (The drive uses low-RPM, high-torque, direct drive motors, so no gears are required.) The antenna also scans about twice as fast as existing antenna drives, Hammack adds. It covers about 90 degrees per second, with greater pointing accuracy. The memory buffer is filled in eight to 10 seconds.

Energy Saver

The RDR-4000 virtually eliminates the waveguide runs that typically are required to route RF signals between the transmitter–in the main or forward electronics bay–and the antenna in the nose. Waveguide adds weight, requires maintenance and attenuates the signal.

On the massive A380 a waveguide run from the EE-bay to the nose would have attenuated the signal so much that predictive windshear would have been impossible, Hammack explains. Honeywell engineers solved the problem by placing the transmitter, receiver, amplifier and RF electronics in the base of the antenna. Only about 1 foot (0.3 meter) of waveguide is required to bring the signal up to the radiating surface. This avoids the need to install waveguide and deal with its maintenance headaches. "It also saves a lot of energy, so you don’t need as much power," Hammack says.

The radar employs more reliable, solid state RF components, as well. A digital synthesis chip creates X-band waveforms directly, and the signals are amplified by a solid state gallium arsenide field effect transmitter. Key component mean times between failures (MTBFs) include:

Radar processor: 25,000 hours,

Transmitter/receiver: 60,714 hours,

Antenna drive: 64,000 hours, and

Control panel: 75,000 hours.

The design reduces the digital electronics package’s size from 8 modular concept units (MCUs) to 3 MCUs.

The A380 will feature a vertical profile display–shown in the lower portion of the main weather display. In this view altitude is marked off along the vertical axis and distance along the horizontal axis. Pilots can analyze the height of a storm by glancing at these "cut-away" images, shaded in the traditional red, yellow and green to indicate rainfall rate. The vertical information also helps a pilot thread a way between patches of bad weather. A storm cell along the current flight path might be at 35,000 feet, but one off to the left might be at only 15,000 feet.

The weather and terrain can be shown along the current aircraft track, projected out to 320 nm. The terrain depicted in the vertical profile view is derived from a version of the enhanced ground proximity warning system (EGPWS) database.

Pilots also can select a slice of airspace 40 degrees to the right or left of the nose and inspect the weather vertically within that span. And there is a third, "unwound flight plan" view that shows the weather along multiple flight legs, at different headings and altitudes.

The radar’s intrinsic terrain database–lower in resolution than the EGPWS database because it shows a wide swath of terrain at altitude–also is used to extract ground clutter. This retains more of the weather information than other filtering techniques do, claims Honeywell.

In the A380 the radar will be part of Honeywell’s aircraft environment surveillance system (AESS), combining processing for the radar, Mode S transponder, and traffic and terrain warning in a single box. AESS allows data to be shared back and forth–for prioritizing alerts–and greatly simplifies the configuration. Each aircraft has two AESS systems, and each enclosure has multiple Ethernet ports linking the functions to the avionics data network. If an element fails in one system, the function can be selected from the backup system.

Functional or physical integration of surveillance systems promises modes such as "smart alerting" via traffic alert collision avoidance system (TCAS) and EGPWS. TCAS alerts could be altered to warn against dangerous terrain during a descent, something that isn’t done today. Mountain wave turbulence warning is another possibility. "If you know the wind speed and direction from the flight management system, and you know there’s a mountain ahead [from the terrain database], you can predict mountain wave turbulence," Hammack states. Wake vortexes created by departing aircraft also could be predicted, he suggests, as TCAS will know that an aircraft has just left the runway.

Future versions of the RDR-4000 could allow Weather Channel-style presentations, as well, using new symbology to indicate a storm cell’s history, speed and direction. All that information is in the buffer, Hammack says. It just needs to be pulled out.

Collins’ MultiScan Radar

The heart of Rockwell Collins’ WXR-2100 radar is MultiScan technology.

The antenna transmits two successive beams at angles slightly offset from each other vertically. Because ground features reflect the two beams differently, Collins’ algorithms compare and contrast returns in the upper and lower beams to remove ground clutter. The system even extracts clutter from high sea states, the company claims.

Radar data from multiple scans, stored in the system’s 64-Mbyte memory area, is massaged, filtered and presented as a single, constantly updated picture. Collins says its filtering allows the radar to safely look down into the most reflective part of a thunderstorm.

The company contends that its "sensor-based" filtering technique removes spurious radar returns more effectively than is possible by using a terrain database to help control antenna tilt, says Tom Yerke, avionics marketing manager. Because of its confidence in the MultiScan approach, Collins does not intend to undertake Honeywell-style volumetric scans, he says. And if you do a volumetric scan, it’s harder to detect storm trends, he adds.

The WXR-2100 manages radar gain, automatically adjusting the sensor’s sensitivity to the airplane’s location, says Steve Paramore, manager of systems marketing. Collins discovered that storms vary according to their location. A storm in the South Pacific, for example, picks up less moisture than one in the Midwest. And the reflectivity of the equatorial region differs from that of other land areas.

But the WXR-2100’s strongest-selling feature is "overflight protection," Paramore says. This warns crews of hard-to-detect, glaciated storm tops located at or near a plane’s altitude, as the aircraft closes in on them. It keeps a storm’s image on the planview screen after the image would have "fallen off" a typical display, when the storm’s most reflective, moisture-laden portion is below the radar beam.

Collins’ plan for the WXR-2100 parallels the Honeywell sensor’s evolution, but with key differences. Like Honeywell, Collins plans a new radar packaged in an integrated surveillance system. Like Honeywell, Collins will combine processing for the weather radar, Mode S transponder, and traffic and terrain warning. Like Honeywell’s system, the initial hardware configuration will be loosely integrated. Collins is developing the communications and surveillance system for the Boeing 7E7, and its surveillance function is widely assumed to follow an integrated approach.

The next step will be to marry MultiScan software to new radar hardware. Like Honeywell, Collins plans to package the new receiver/transmitter in the antenna base, virtually eliminating waveguide runs. The antenna drive will stay the same, as its mean time between failures is an impressive 40,000 hours.

The MultiScan software is being upgraded as well, although it’s too early to announce a release date. Vertical analysis will be a major thrust, which will be enabled by interrupting the horizontal scan with a more detailed vertical scan. The vertical scan will use the beam edge–which can be accurately located in space–rather than several full-beam scans. This technique, along with background analysis, will enable the radar to detect storm tops at up to 80-nm ranges much more accurately than other methods do, according to Collins.

The upper, "God’s eye" area of the prospective next-generation display, shown here, presents splotches of green, yellow and red in relation to the flight path, but each area is tagged with altitude. The lower portion of the display shows side views of storm cells projected along the aircraft’s current heading. A magenta line indicates the aircraft’s current altitude, and storm tops are identified at 20,000 and 42,000 feet. Terrain is sketched in white below the storm cell cut-aways. A "predictive overflight" feature–indicated by yellow hash marks on the planview screen–alerts the pilot to the storm cell boiling up rapidly from 20,000 feet. A storm that far below current altitude would not normally be shown on a radar screen. But if it is growing at 6,000 feet a minute, it will reach 30,000 feet in about 90 seconds, and the pilot needs warning. Background processing also would indicate clear air turbulence above the storm—not shown here–as the rapidly growing weather cell displaces the air above it.

Collins plans an "enhanced turbulence" software upgrade in the nearer term, possibly available by late next year. Developed under a NASA program, "E-Turb" will enable the radar to see turbulence with moisture levels as low as 0.0014 inch per hour at ranges up to 40 nm. Delta Air Lines installed the software on a B737-800 last July and will evaluate it in revenue service through September 2005.

The enhanced turbulence detection and presentation feature currently uses a "solid" magenta to warn of severe weather–a greater than 1-G bump–vs. a "speckled" magenta for less severe conditions.

Why Weather Radar?

Adverse weather is a factor in one-third of all aircraft accidents. Even if it doesn’t cause an accident, turbulence is the leading cause of in-flight injuries to passengers and cabin crews.

Weather conditions also cause up to 75 percent of all flight delays, according to Delta Air Lines. That wasted time cost airlines $6.5 billion in 2000, alone. With the air traffic forecast to increase by 65 percent between 2003 and 2015, the flying public is expected to zoom from 641 million in 2003 to more than 1 billion in 2014.

Statistics like these are driving the development of more precise, accurate and intuitively understandable radar systems.

RDR-4000 At a Glance