Pulsed RADAR and RADAR Clutter

Pulsed RADAR

• Radar transmits pulsed radio frequency signals

• In receiver a clock measures the time for pulse to hit target and return

• This time is given by 𝝉_𝑫 = 2𝑅c

• Hence target range is R = c 𝝉_𝑫 / 2

PRF and PRI

Pulse repetition frequency = 1 / (Pulse repetition interval)

PRF determines the maximum unambiguous range R_max

If the time taken for an echo pulse to return is greater than the time between transmit pulses (1/PRF) then the return is ambiguous

Pulse Width

Pulse width determines range resolution

RADAR returns from the 2 targets are separated in time by t = 𝟐∆R / 𝒄

Unambiguous range

Can avoid range ambiguities by using:

• Multiple PRFs

• Pulse coding (e.g. phase coding)

Pulsed Doppler RADAR

Provides range and speed

Receiver is a sampled system that uses In-Phase and Quadrature detectors

• Ratio of I and Q gives phase of received signal

• Successive samples gives rate of change of phase which gives speed

Ambiguous pulsed doppler RADAR

For doppler shift to not be ambiguous ∆𝒇 ≤ PRF / 2

Doppler resolution

Synthesise a wider pulse in the time domain so that we have a narrower pulse in the frequency domain

• Width of Doppler spectrum from a single pulse of length 𝜏 𝑖𝑠 ≈ 1/𝜏 Hz

• But if have N pulses then can integrate N pulses to give a Doppler spectrum pulse width of ≈ 1/𝑁𝜏 Hz

Doppler resolution = 1/ Integration time

RADAR Clutter

• All unwanted radar returns

• Most common type is ground or surface clutter (including sea)

• Best described as RCS per unit area 𝜎₀

• Typical 𝜎₀ for land is -20dbm²m^-2

Simplified geometry

• Clutter return is often much larger than the target return

• Ground area illuminated by radar pulse 𝐴𝐺 = 𝑅∆∅∆𝑅