Radar principles and ground radar

Navigation · Chapter 41 · DGCA CPL and ATPL

How a pulse radar measures range and bearing, the maximum and minimum range sums, resolution, power, MTI and second-trace echoes.

Remember

Shouting at a cliff

An echo • Shout at a cliff and time the echo: the longer the wait, the further the cliff • Radar does that with a burst of radio energy: RAdio Detection And Ranging
Primary and secondary • primary radar: the target just reflects the pulse back; it does nothing itself • Secondary radar: the target answers, on another frequency (SSR, DME) • Some radars send a continuous wave instead (the radio altimeter): no blind zone close in
What it does • ATC: separating and vectoring aircraft, radar talk-downs (SRA, PAR), watching ILS approaches, storms • In the aircraft: weather radar (clouds, heavy rain, turbulence, ground mapping); DME and SSR use the same idea
Why VHF and up • Radar uses VHF and above: free of static and sky waves • Short waves make narrow beams and short pulses • Objects reflect best when they are big compared with the wavelength

Pulses and the echo

What a pulse is • A radar does not send a steady beam. It sends a very short burst of radio, then goes quiet • One burst holds thousands of waves
Pulse width • Pulse width: how long one burst lasts, in µs (millionths of a second) • Radio covers 300 m in 1 µs, so a 1 µs pulse is a 300 m slug of radio flying through the air
PRF • prf (pulse recurrence frequency): how many pulses leave each second, in pulses per second (pps) • Drag it and count the pulses in the bar
PRI • PRI (pulse recurrence interval): the time from one pulse to the next • PRI = 1 ÷ PRF: more pulses a second, less time between them
Mostly listening • The radar sends for 1 µs, then listens for over 1 000 µs. That quiet time is when echoes come home • At 800 pps it listens for more than 99.9 % of the time
See it fly • Gold: pulses leaving, one every PRI. Green: echoes coming home. The screen below traces both against time • Drag the PRF: the pulses bunch up in the air
Out and back • The pulse goes out and the echo comes back: the time covers the distance twice • Range = speed × time ÷ 2
The radar mile • Radio takes 12.36 µs to go out 1 NM and come back: the radar mile • So range (NM) = time (µs) ÷ 12.36
Bearing • The aerial makes a narrow beam and turns: whichever way it points when the echo comes back is the bearing (the searchlight idea) • The ppi (plan position indicator) screen sweeps round in step with it