Primary radarHow a pulse of radio energy becomes a target on your screen

Seeing with radio waves

RADAR stands for Radio Detection And Ranging. It uses radio waves to find the range and bearing of objects, and in some systems their height.

A torch beam is a good way to picture it. Light shone on an object scatters in many directions, and some of it reaches your eye, so the object is visible. Move the beam away and nothing reflects: the object is effectively invisible.

Primary radar only “sees” what its beam is pointing at, and only if enough energy comes back.

Pulse out, echo back

The radar doesn’t transmit continuously. It fires a short pulse, then switches to receive and listens. If the pulse hits something, a little energy reflects back.

Radio waves travel at the speed of light, so the time between sending the pulse and hearing the echo tells us how far away the target is:

We divide by 2 because the measured time covers the trip out and back, but we only want the distance out.

The strength of the echo depends on the target: its size and what it’s made of. Try a light aircraft against an airliner.

Range and bearing make a position

Range comes from the echo timing you just saw.

Bearing is simply the direction the aerial is pointing when the echo comes back. As the aerial rotates, the sweep line on the display rotates with it.

Put the two together and the radar can paint a return at the right place on the display. With primary radar that return is just an unlabelled blip: the radar knows where something is, not what it is.

Tap anywhere on the display to put an aircraft there, then watch the sweep find it.

Four components

A basic Primary Surveillance Radar system has four parts:

• Aerial
• Transmitter (TX)
• Receiver (RX)
• Display

Follow the energy round the loop: the transmitter makes the pulse, the aerial sends it out and collects the echo, the receiver picks it up and the display shows it. Select a component to see what it does.

Beam width: telling two aircraft apart

An ATC radar needs to pin down bearing accurately, so it uses a narrow beam in azimuth (horizontally).

Every target is painted across the whole width of the beam. If two aircraft at the same range are inside the beam at the same time, their returns run together and the radar can’t separate them.

Widen the beam until the two returns merge into one.

Vertical coverage and lobes

Vertically, ATC wants the opposite: a wide beam, so high and low aircraft are caught by the same sweep.

But the aerial sits on or near the ground. Some energy goes straight to the aircraft and some bounces off the surface first. The two combine, strengthening the signal in some directions and cancelling it in others, so the beam splits into lobes with gaps between them.

An aircraft flying through a gap can fade from the display at certain levels and ranges. The number and shape of lobes depends on wavelength, aerial design and how close the aerial is to the ground. Aerials on towers or hilltops are affected less.

Mountains cast radar shadows

Radar energy travels in straight lines. A hill or mountain between the aerial and the aircraft blocks the beam, leaving a shadow behind it where nothing is detected.

These fade areas come from terrain shadowing, as well as from the coverage gaps you saw in the lobes.

Lower aircraft stay in the shadow longer. Raise the aircraft and see how much sooner it appears.

The fix is overlapping coverage: a second radar site that looks into the shadow from another direction.

The radar overhead

There’s a blind area directly above the radar site. The aerial is designed to look outwards, not straight up, so an aircraft passing overhead drops out of coverage.

The size of the overhead depends on aerial design, the aerial’s tilt and the aircraft’s height. The blind area widens as you go up, so the higher the aircraft, the longer it stays in the overhead.

At 30,000 ft an aircraft flying directly over a radar site may be out of coverage for as much as 20 NM.

Overlapping coverage from another site solves this too.

Height versus range

Detection depends on transmitter power, aerial location and the aircraft’s height and reflective properties. In civil aviation most of those are fairly constant, which leaves two main variables:

• the aircraft’s altitude
• its distance from the radar aerial

Rule of thumb: 100 ft of height for every nautical mile of range (1,000 ft per 10 NM). Aircraft below this line are expected to be below radar coverage.

Drag the sliders, or tap the picture to place the aircraft.

Anomalous propagation

Sometimes the atmosphere lets radio waves travel much further than usual. This is anomalous propagation, or anaprop.

It needs temperature increasing with height (an inversion) and humidity decreasing with height. Together these give a sharp drop in density with height, often over large bodies of water next to hot, dry land.

The energy gets trapped in a duct and bent back down, hopping along the surface well beyond the normal radar horizon.

An echo from that far away can arrive after the next pulse has already gone out. The radar then times it from the wrong pulse and shows the target at the wrong range: an overreach.

Clutter, moving targets and the tangential problem

Clutter is unwanted echoes: ground, sea, rain, birds and insects, chaff and turbulence. It can swamp the picture.

Fixed-wing aircraft must keep moving to stay airborne, so a Moving Target Indicator shows only returns that are moving like aircraft. A Moving Target Detection system goes further, using filters to remove weather, birds and stationary objects.

These rely on the Doppler effect, which needs the target to have some speed towards or away from the aerial.

An aircraft flying tangentially, across the radar’s line of sight, has little or no radial speed. The filters treat it like a stationary object and it isn’t displayed.

Switch the filter on and watch aircraft B as it crosses the top of the display.