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, distinctly fainter cell off the ridge is the moving target. The ridge is drawn about 12 dB over the target — that gap is exactly why amplitude cannot separate them.

  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. The demo opens at X-band with PRF = 10 kHz, so the first blind speed is 150 m/s. Set the target velocity to 150 and leave MTI on: the target folds onto the zero-Doppler column, lands in the notch, and disappears — cancelled with the clutter. Watch the target attenuation readout drop to “nulled” as you close the last few m/s.

  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 — the map draws it about 12 dB up, and real ground clutter runs 40–60 dB over a small target — but it 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 bundle · L5_DopplerMTI.m↓

The companion script plots the single-delay MTI response \(|H(f)| = 2|\sin(\pi f T)|\), prints the blind speeds, and builds a synthetic range-Doppler map before and after MTI — applying the same \(|H(f)|\) weighting the demo uses.

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