Radar Principles and Weather Radar - ATPL 062 — Radio Navigation

Radar is the aid that asks nothing of its target. Its applications in aviation run through air traffic control (ATC), weather observations, and the airborne weather radar (AWR) in the aircraft's nose: three jobs, one principle underneath.

The pulse and the echo

That principle is the pulse technique: primary radar transmits a short, powerful pulse and then listens; whatever reflects it returns an echo, and the time between transmission and echo gives the range, while the antenna's pointing direction at that moment gives the bearing. This is the pulse modulation of the propagation lesson put to work: the carrier exists only in bursts, and the silence between bursts is where the listening happens. How far such a radar can see is not one number but a balance. The range of a radar depends on all of the following:

pulse repetition frequency (PRF) — sets how long the radar may listen before the next pulse goes out;
pulse length and pulse power — decide how much energy each echo can bring home;
height of the aircraft and height of the antenna — together set the radio horizon;
frequency used — decides how the wave propagates on the way.

Ground radar

On the ground, primary radar provides the bearing and distance of its targets, drawn as raw position on the controller's screen, and it keeps one irreplaceable job: primary ground radar is used to detect aircraft that are not equipped with a secondary radar transponder: whatever flies, cooperative or not, reflects. The screen itself has outgrown any single sensor: modern ATC systems use inputs from various sensors to generate the display, fusing primary and secondary radar and other sources into one synthetic picture.

The weather radar

The AWR in the nose has two main tasks: weather detection and navigation, and its design follows the first. Modern weather radars employ frequencies that give wavelengths of about 3 cm, which reflect best on wet hailstones: 3 cm sits in the SHF (super high frequency) band of the band ladder, and the wavelength is chosen so that precipitation-sized water reflects strongly. Water is the operative word: wet hail, rain and wet snow return echoes; dry ice crystals barely do. The antenna is the one the propagation lesson promised to this radar: the parabolic dish, or in more modern sets the slotted planar array, and it sends its energy in a cone-shaped pencil beam of about 3 to 5 degrees beam width, narrow enough to place an echo precisely, and the antenna is stabilised in the horizontal plane with signals from the aircraft's attitude reference system, so that a banked turn does not swing the beam into the ground or the sky.

The control panel and the picture

Four controls on the radar control panel run the show:

off/on switch — the master, and on the ground a deliberate act (more below);
function switch — with WX (weather), WX plus T (weather plus turbulence) and MAP modes, choosing what the processing looks for;
gain control (auto/manual) — the receiver's sensitivity: auto for calibrated weather colours, manual for deliberate exploration;
tilt/autotilt switch — the beam's elevation above or below the horizontal plane, the single most important knob in the lesson.

The picture the radar paints uses colour gradations of green, yellow, red and magenta, indicating the increasing intensity of precipitation area by area: green the lightest returns, magenta the heaviest. Position is read off the screen furniture:

azimuth-marker lines — give the relative bearing to a thunderstorm on the screen;
range lines — give the distance to it.

The same picture rides on the ND's weather overlay in MAP mode (ND: navigation display), drawn over the route.

Working the tilt

In use, weather detection and mapping are worked with three handles: range, tilt and, where available, gain. Range scales the picture; gain sets what counts as an echo; tilt aims the pencil beam, and aiming is everything. The appropriate tilt setting depends on altitude and on the thunderstorms being examined: at altitude the beam is tilted down toward the storm's rain-bearing middle, and adjusted so the returns come from the precipitation, not from the ground below it. Aim badly and the storm vanishes: a thunderstorm may not be detected when the tilt is set too high, because the beam then passes through the storm's upper levels, where the water is frozen dry and returns almost nothing, while the reflective wet core sails underneath the beam.

Dangers

Two warnings own this section. First, on the ground: the AWR should be used with extreme caution when on the ground, because at close range the transmitting antenna's radiation can harm ground personnel and equipment near the nose; it stays off or standing by until the aircraft is clear of people. Second, in the air, the trap in the picture: behind heavy rain lies a shadow area into which no radar waves penetrate. The heavy cell absorbs and scatters the pulse so completely that nothing is left to probe what stands behind it: the blackness behind a red cell is not clear air but no information, and some of the worst weather ever flown into has hidden there.

Radar as navigator and guard

The second of the radar's two tasks: in the mapping mode, the radar serves navigation: the beam is tilted down deliberately so the ground paints, and coastlines, islands, big rivers and cities return recognisable shapes to fix position against. Back in the weather modes, the radar is the instrument for avoiding a thunderstorm (Cb) (cumulonimbus): the cell is spotted early on a long range scale, its bearing and distance read from the screen furniture, and the track bent around it with a healthy margin, never through the shadow behind it.

Modern sets add two guards built on the Doppler principle: turbulence (though not clear-air turbulence, CAT) can be detected by a modern weather radar, because turbulent air throws its raindrops about, and the spread of Doppler shifts in their echoes betrays the churning: no droplets, no detection, which is why clear-air turbulence stays invisible. And wind shear can be detected by a modern weather radar the same way: the radar measures the along-beam motion of precipitation ahead and below, and a sudden organised change in that motion close to the ground is the signature of shear, announced before the aircraft flies into it.

What the exam asks

The range factors, the 3 cm/wet-hail pairing, the colour ladder, the tilt traps and the shadow area carry the marks. The overview, following the learning objectives:

Name the radar applications (ATC, weather observations, AWR); describe the pulse technique and echo principle; state the six range factors

Explain primary radar's bearing and distance, its role against non-transponder aircraft, and the multi-sensor ATC display

List the AWR's two tasks; state the 3 cm wavelength reflecting best on wet hailstones; describe the 3-5 degree pencil beam and the attitude-stabilised antenna

Explain the panel controls (off/on, WX / WX plus T / MAP, gain, tilt); name the colour gradations; state the use of azimuth-marker and range lines

Explain the use of range, tilt and gain; describe appropriate tilt settings; explain why too-high tilt misses a thunderstorm

Explain the extreme caution on the ground and the danger of the shadow area behind heavy rain

Describe the mapping-mode navigation function and Cb avoidance; explain the detection of turbulence (not CAT) and of wind shear