Radio Altimeter and radar beams

[akafrank07]

Can anyone explain exactly why you can go down to 0ft with a continuous-wave radar beam and only 50ft a for pulse radar beam?

Thanks

[Lightning Mate]

Because the minimum range of a pulse radar is governed by the pulse length.

[keith williams]

PULSED RADARS
Primary Radars using the pulsed transmission system are able to calculate the range of targets by measuring the time of flight of each pulse of the signal going to and from the target. But this system imposes a minimum target range because the transmission of each pulse must end before its reflected energy completes the round trip back to the reaches the antennae. To achieve very short minimum ranges each pulse must be extremely short. This means that the pulse length limits the minimum target detection range.

In order to produce unambiguous range information, the time delay between transmitting each pulse must be sufficient to permit the energy to move out to maximum range and return to the aerial. This means that the maximum theoretical range is determined by the pulse repetition frequency PRF.

To obtain unambiguous range information for targets at minimum range they must be far enough from the transmitter to enable an entire pulse to be transmitted before its leading edge reaches the target. This means that the pulse width (or duration of the pulse) determines the minimum range.


CONTINUOUS WAVE RADARS
In continuous wave radars the energy is not transmitted in pulses, but in the form of a constant wave. The frequency of this wave is modulated in a set pattern. This enables the system to determine range by calculating the time of flight of a signal to and from the target.

Instead of using discrete pulses, it uses the frequency difference between the transmitted and reflected signals to determine the time of flight of the signal and hence the range of the target. This process eliminates the minimum target reception range that applies to pulsed radars.

[akafrank07]

Thanks for that clear and easy to understand explanation Keith :ok: