Demo — Range-Doppler Map

Demo — Range-Doppler Map#

The range-Doppler map is what a pulse-Doppler radar actually produces: a 2D picture with clutter piled up at zero Doppler and movers standing out in their own cells. This demo lets you move a target, toggle MTI, and find the blind speed where the target vanishes into the clutter notch.

The picture#

A target’s Doppler shift is \(f_d = 2 v_r / \lambda\). MTI cancels the zero-Doppler clutter but creates blind speeds at

\[ v_{\text{blind},n} = \frac{n\,\lambda\,\text{PRF}}{2}. \]

A target whose velocity lands on a blind speed is cancelled along with the clutter.

Interactive demo#

Open in full screen

Walkthrough#

  1. Look at the default map. A bright vertical ridge sits at zero Doppler (clutter); a single bright cell off the ridge is the moving target.

  2. Slide the target velocity. The target cell slides along the Doppler axis while the clutter ridge stays fixed at zero — that separation is what makes the target detectable.

  3. Toggle MTI on. The zero-Doppler column is nulled: the clutter ridge collapses and the target pops out against a clean background.

  4. Read the blind-speed readout. With MTI on, set the target velocity to the first blind speed \(v_{\text{blind},1}\). The target lands in the notch and disappears — cancelled with the clutter.

  5. Raise the PRF. The blind speeds move higher; nudge the target off the notch and it reappears. This is exactly why pulse-Doppler radars favor higher PRF.

  6. Change the wavelength (band). The Doppler-to-velocity scaling changes; the same Doppler shift now corresponds to a different velocity.

Key observations#

  • Clutter is rejected by Doppler, not amplitude. It is far stronger than the target but lives at zero Doppler, so MTI nulls it without touching the mover.

  • Blind speeds are MTI’s cost. A target on a notch is invisible; PRF choice manages where the notches fall, trading against range ambiguity.

  • The B-21’s motion is hard to hide. Once moving, its Doppler signature stands off the clutter ridge — defeating this picture is what Block 2’s EA techniques are for.

Source#

MATLAB · code/L5_DopplerMTI.m↓

The in-class script plots the single-delay MTI response \(|H(f)| = 2|\sin(\pi f T)|\), marks the blind speeds, and builds a synthetic range-Doppler map before and after MTI.

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