Sentinel-2 equal-area HEALPix power-spectrum outcome
The angular power spectrum of a Sentinel-2 scene can be computed end to end on an equal-area HEALPix grid on the WGS84 ellipsoid with the GRID4EARTH stack (healpix-geo + healpix-analyse). Because every HEALPix cell covers the same surface area, the resulting spectrum carries none of the latitude-dependent area weighting that a lon/lat grid would impose — supporting the claim by construction. The spectrum shows the expected monotonic power-law fall-off with spatial frequency, the signature of a natural surface-reflectance field.
Configuration and result recorded in results/powerspectrum.json: - scene S2A_22WEV_20190723, band B04 (red, 10 m), 256×256 px window (~2.6 km) - equal-area HEALPix at depth 18 (nside = 262144), ellipsoid = WGS84 - field sampled at equal-area cell centres, then radially-averaged isotropic power spectrum computed with healpix-analyse `ps` - the spectrum (results/powerspectrum.png) falls off monotonically with spatial frequency on a log–log axis (power-law behaviour), with no latitude-dependent amplitude term, because the cells are equal-area by construction. Github repository: https://github.com/annefou/healpix-analyse-s2-powerspectrum
The repo demonstrates the equal-area pipeline rather than quantifying the lon/lat bias it avoids: no side-by-side lon/lat baseline spectrum is computed here, so the bias magnitude is argued by construction, not measured. Single scene, single band, single date; no atmospheric, cloud, or coastal effects; generalisation to other sensors and to a controlled multi-latitude comparison is left to follow-up.