Boeing ARINC 708 series radars, standard equipment on all new airplanes since 1980, are characterised by low power transmitters, flat plate antennas, digital processing, narrow beam width and colour screen imagery. These new technology, solid state radars offer greater reliability and new turbulence detection methods, but also require new operating techniques. This article describes the new features of the 708 series radar and recommended operating techniques.
With the advent of digital radar, most operators have made the switch from C-band (5400 MHZ) to X-band (9400 MEZ) in their new airplanes. Some basic properties of radar waves are: as frequency increases, reflectivity increases, attenuation increases, and beam width decreases.
Due to higher attenuation (signal weakening) rate, the X-band radar loses some ability to penetrate clouds with high levels of precipitation. However, radar manufacturers have added features for displaying areas of probable attenuation.
Reduction in beam width is significant in improving target resolution and definition. The best return is generated where the target fills the radar beam. For a storm at 60 mile range to fill the beam of a C-band radar, it would have to be 5 1/2 nm in diameter; while to fill the beam of an X-band radar, it would only have to be 3 nm in diameter.
The rainbow of colours on the new radar, representing variations in rainfall rate, create a display, which is easier to interpret than the older monochrome sets.
| Colour | Return Strength | Rainfall Rate |
|---|---|---|
| Black | Very Light or No Returns | Less than 0.7 mm/hr |
| Green | Light returns | 0.7-4 mm/hr. |
| Yellow | Medium returns | 4-12 mm/hr |
| Red | Strong returns | Greater than 12 mm/hr |
| Magenta (optional) |
Very strong returns | Greater than 25 mm/hr |
Another significant difference is the way the picture is painted on the screen. In the old analogue models, new returns were added by each sweep of the antenna, and the old returns gradually bled away depending on the level of persistence selected. The new radar display is generated in the same way as a TV screen; each new sweep is a totally new picture - the old picture is completely erased. Thus, colour changes can occur quickly if a return is close to the threshold between rainfall rate categories. The new digital radars incorporate hypersensitive receivers and sophisticated Sensitivity Time Control (STC) circuitry to present a true or calibrated image within a range of approximately 60 nm. Therefore a yellow storm return at 60 nm will still be yellow at 10 nm. The calibration accuracy is based on gain control being set at the automatic, preset, or calibrated position.
Some energy from the older parabolic antenna was lost in the side lobes resulting in more ground clutter at low altitudes and more close-range weather returns around the periphery of the main beam. The flat plate antenna transmits a narrow focus long range beam greatly reducing the side lobes and focusing much more energy into the main lobe. With loss of the side lobes, tilt control becomes more critical. As you approach storms or reduce the range, the tilt must be adjusted downward to avoid over-scanning significant returns.
Previous generation radars, the monochrome or green screen sets, used high power (60 KW) transmitters. By comparison, the new radar has a power output of about 125 watts. This large reduction in transmitter power has been made possible by improved receiver technology, such that the ability of the new radar to paint returns at any given range is comparable to the old high power radar. The most important benefit from this reduction in power output is the ability to accurately control frequency, which allows implementation of turbulence detection, ground clutter suppression and reduction in interference.
Turbulence detection in new generation radars is displayed directly and more accurately, eliminating interpretation of the weather display. However, rain is still a required ingredient.
The turbulence mode does not display clear air turbulence. By measurement of longitudinal velocity of rain, the radar displays those returns that are moving in line with the airplane's path faster than 11 mph. Due to the properties of the transmitted beam and pulse repetition frequency limits, this turbulence detection mode is only effective out to 50 mile range. Occasionally, turbulence may appear where there was no precipitation shown in the weather mode. This is because areas of light precipitation that are de-sensitised by the STC in the weather mode, but which have significant raindrop movement, are displayed in the turbulence mode where the STC is not functional.
Ground Clutter Suppression (GCS) in new generation radars eliminates any target that exhibits less than 2 mph longitudinal movement. Targets with speeds of 2-11 mph are considered precipitation returns and those over 11 mph as turbulent precipitation returns. Tilt angles of no more than minus 5 deg suppress ground returns well, but steeper tilt angles result in inadequate suppression of ground clutter. Since OCS “identifies” ground returns as anything that moves slower than 2 mph, it is possible that slow moving weather returns my also be filtered out. Thus, GCS should not be left on continuously, but rather used in quick analysis of returns, then turned off.
The fan beam used in old generation radars for low to intermediate altitude mapping has been eliminated in the new generation radars. However, improvements in airborne and ground based navigation aids over the years have made this map mode a secondary function is most areas of the world. The mapping function from high altitude is quite effective if, as in weather avoidance, the tilt control is understood and used properly. For instance, at FL330 with 5 degrees of down tilt, the beam sweeps the ground 50 to 190 nm in front of the airplane, whereas with 10 degrees of down tilt, the beam sweeps a narrow band 25 to 40 nm in front of the airplane. Since the STC circuit is deactivated in the map mode, adjustment to the gain is necessary to “break out” terrain features, especially for closer range targets. Good practice in radar mapping can be had on clear days when mountains, coast lines, cities, rivers, etc., can be seen visually and compared to radar screen images.
Takeoff and Climb. Prior to takeoff, it is desirable to tilt the antenna up to scan for weather along the departure path. During initial climb, the antenna should remain tilted up to avoid ground clutter and to coincide with the airplane's climb angle. The antenna stabilisation, controlled by the airplane gyro or IRU, is referenced to the horizon, not to the longitudinal axis of the airplane. Range selection should be appropriate for airplane speed and location of weather returns. As the airplane continues climbing, the tilt should be gradually decreased to aim at the regions of maximum precipitation while avoiding ground clutter. Tilt angles below approximately +4 deg will pick up some ground clutter below 5,000 ft AGL.
Cruise. For cruise, recommended tilt settings vary from one manufacturer to another, but generally speaking, the tilt should be adjusted so that ground returns are barely visible at the outer edge of the screen. Ground returns are displayed in arcs, parallel to range marks. They merge together as the tilt is brought down and cause shadowing behind prominent features. They are generally smaller, sharper, and more angular than weather returns. The tilt will have to be adjusted more frequently as storms are approached or range is changed to avoid over-scanning. Having once adjusted the tilt setting, the flight crew should not be content with just an occasional glance at the screen. Failure to periodically down tilt leads to “disappearing” targets.
The narrow beam width of the radar presents only a two dimensional cross section of the storm. Setting the tilt near zero at cruise altitude can degrade the usefulness of the radar significantly. This radar detects only liquid moisture in the form of raindrops, wet hail, or wet snowflakes. Unless the beam is aimed at or below the freezing level of weather cells, there may not be sufficient moisture to paint a return on the radar.
The National Severe Storm Laboratory in Oklahoma City tells us that thunderstorms to 60,000 ft show little variation in turbulence intensity with altitude. Further, they state that strong vertical drafts and large hail exists to within several thousand feet of the tops of these storms. Strong returns may be received from rain water at lower altitudes, but as the antenna is tilted up the return will tend to weaken and disappear as the water becomes ice. Remember, while the returns may diminish at the higher altitudes the turbulence might not.
Any storm return with more than one colour is bound to have turbulence, even in the green area. Steep gradients, that is, thin lines of colour, are areas of greatest turbulence. A return that is changing shape or size over a short period of time is potentially very turbulent. Other prime indicators of severe activity and hail such as fingers, hooks, scalloped edges and horseshoe shapes require avoidance, and should not be ignored even if they are green in colour. Tornadoes frequently have very little moisture and paint as green narrow curved fingers or figure 6's. Shape is just as important as colour in determining the intensity and turbulence of a storm.
For normal cruise altitudes, it will be more difficult to detect cells lower than the cruise altitude inside of 40 mile range due to ground clutter, particularly over land. For this reason, a diversion route should be initiated before the target is inside of 40 nm.
Descent. Antenna tilt will have to be raised approximately one degree per 10,000 ft of descent down to 15,000 ft, then one degree per 5,000 ft below 15,000 ft. Range should be adjusted as necessary to scan the arrival route adequately. In heavy weather, the longest appropriate range should be used to plan a safe storm avoidance route; then selection of shorter ranges will show greater detail as you enter the affected area. Remember that more tilt adjustment will be required each tine the range is switched.
Boeing Flight Operations Review 03, 30th May 1986