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A doppler shifted reflection requires a feature, significant in size to the wavelength, to reflect off of, somewhat perpendicular to the direction of movement. For RADAR, I don't think a smooth sand field would have such a thing. Doppler LIDAR could probably detect it, but I naively assume lidar is hard in dusty environments, without oodles of large moving optics required to penetrate a dusty, optically clear, window.

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To get a Doppler-shifted reflection you can also... move. Ground-based surface radars can work (somehow) with just no Doppler info, cataloguing, mapping fixed reflectors and very often just building ground maps. If your radar has multiple vertically and horizontally spaced receptors (eg. phased-array) or you can aim your beams, you can also somehow build a 3D map or at least map surface features/masks. This is before anything moves.

> Ground-based surface radars can work (somehow) with just no Doppler info > you can also somehow build a 3D map or at least map surface features/masks

The problem is this case was that there was nothing to reflect off of, except for a planar field of sand. You can't know how fast you're moving over a bare plane, without a feature being on that plane, that you can observe.

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