ISOFIT Atmospheric Correction — Arizona Area Trial

Scene Footprints

Geographic footprints of all 23 EMIT scenes used in this whitepaper, colored by the shared gta_quality tier. Click any footprint for its scene ID and QC reason; use the sliders to adjust per-tier fill opacity.

1. Why Atmospheric Correction Matters

An imaging spectrometer measures at-sensor radiance \(L(\lambda)\) — a tangled superposition of light scattered by atmospheric molecules and aerosols, light reflected from the surface that transits the atmosphere twice, and the spherical-albedo coupling between the two. Recovering surface reflectance \(\rho(\lambda)\) requires inverting that radiative transfer chain pixel-by-pixel.

The algorithms here apply to a family of NASA imaging spectrometers — AVIRIS-Classic, AVIRIS-NG, AVIRIS-3, and EMIT — that share a common forward-model architecture in ISOFIT. The 23 scenes presented here come from EMIT.

2. The ISOFIT Algorithm

2.1 The Forward Model

ISOFIT writes the at-sensor radiance as the composition of a surface model \(\rho(\mathbf{x}_s; \lambda)\) and an atmosphere model \(\mathcal{A}(\mathbf{x}_a; \lambda)\):

where \(\mathbf{R}\) applies the spectral response of the instrument and \(\mathbf{x}_s\), \(\mathbf{x}_a\) are the surface- and atmospheric-state vectors. The state \(\mathbf{x}_a\) is kept low-dimensional (column water vapor, AOT at 550 nm) because well-mixed gases can be held at climatology with negligible error in retrieved \(\rho\).

2.2 Accelerated Optimal Estimation

OE inverts the forward model under a Gaussian prior and Gaussian instrument noise. The cost is:

The Accelerated formulation collapses the >280-dimensional Levenberg–Marquardt iteration into a closed-form surface inversion wrapped in a small two-dimensional outer search over the atmospheric state, so retrievals are orders of magnitude faster while still reporting a full posterior covariance.

2.3 Radiative Transfer Engines and sRTMnet

The forward model needs \(\{L_a, T_\downarrow T_\uparrow, s\}\) at every wavelength for every plausible atmospheric state. ISOFIT supports MODTRAN 6 (operational JPL reference), LibRadTran (open-source), 6S (open-source, fast), and sRTMnet, a deep neural emulator trained on millions of 6S calls. We use sRTMnet for both EMIT and AVIRIS pipelines.

2.4 The Multicomponent Surface Prior

Without a prior, OE on a 285-channel reflectance spectrum is ill-posed. The multicomponent surface model partitions a global reflectance training set (vegetation, soils, water, mineral surfaces, snow) by k-means and assigns a Gaussian prior to each cluster; per pixel, ISOFIT picks the most likely component from a radiance-domain pre-classification, then uses that component's prior in the OE inversion.

3. The "Standard" Reflectance We Compare Against

We compare to the official EMIT L2A surface reflectance product as distributed by NASA's LP DAAC. Each scene's NetCDF header records its provenance:

title:        EMIT L2A Estimated Surface Reflectance 60 m V001
sensor:       EMIT (Earth Surface Mineral Dust Source Investigation)
platform:     ISS
institution:  NASA Jet Propulsion Laboratory / California Institute of Technology
software_build_version: 010621/010627
product_version: V001

The "standard" is the JPL operational ISOFIT pipeline (MODTRAN-based) running inside the EMIT Science Data System. Our local retrieval uses the open-source ISOFIT 3.7.x release with the sRTMnet neural emulator and a default multicomponent surface configuration:

description = L2A Analytical per-pixel surface retrieval
              (segmentation_size=40, engine=sRTMnet, isofit_version=3.7.x)
samples = 1242, lines = 1280, bands = 285
interleave = bil, no data value = -9999

Both retrievals operate on the same EMIT L1B radiance cube. Differences come from the radiative-transfer engine (full MODTRAN vs. sRTMnet emulator), the surface library configuration, and the segmentation size used to amortize OE across spatially coherent blocks.

4. Scene Comparisons 23 scenes

All 23 EMIT scenes are organized below by QC tier, following the Landsat Collection 2 convention: Tier 1 = best, Tier 4 = worst. Tier 1 (Excellent) and Tier 2 (Mild Cloud) scenes are processed and rendered with their full output set — RGB, spectra, PCA composites, eigenvalues, and the nested 15-map mineral-spectral-index accordion. Tier 3 (High Cloud) and Tier 4 (Missing Data) scenes are excluded from processing; only their RGB render is shown so you can see what was filtered. Click any processed scene to expand its full diagnostics; opening one collapses any other open scene.

2 Aug 2024 18:16 UTC — emit20240802t181618 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 2 Aug 2024 18:16 UTC — emit20240802t181618 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

6 Aug 2024 16:41 UTC — emit20240806t164107 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 6 Aug 2024 16:41 UTC — emit20240806t164107 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

26 Aug 2024 16:44 UTC — emit20240826t164408 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 26 Aug 2024 16:44 UTC — emit20240826t164408 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

29 Sep 2024 19:16 UTC — emit20240929t191631 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 29 Sep 2024 19:16 UTC — emit20240929t191631 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

2 Dec 2024 18:02 UTC — emit20241202t180206 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 2 Dec 2024 18:02 UTC — emit20241202t180206 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

31 Jan 2025 18:11 UTC — emit20250131t181145 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 31 Jan 2025 18:11 UTC — emit20250131t181145 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

26 Jul 2025 20:25 UTC — emit20250726t202545 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 26 Jul 2025 20:25 UTC — emit20250726t202545 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

2 Oct 2025 17:35 UTC — emit20251002t173514 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 2 Oct 2025 17:35 UTC — emit20251002t173514 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

2 Oct 2025 17:35 UTC — emit20251002t173526 Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 2 Oct 2025 17:35 UTC — emit20251002t173526 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

20 Apr 2024 19:25 UTC — emit20240420t192535Tier 2 — flagged: 12.4% cloud Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 20 Apr 2024 19:25 UTC — emit20240420t192535 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

20 Apr 2024 19:25 UTC — emit20240420t192547Tier 2 — flagged: 11.8% cloud Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 20 Apr 2024 19:25 UTC — emit20240420t192547 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

31 Jan 2025 18:11 UTC — emit20250131t181133Tier 2 — flagged: 14.1% cloud Click to expand spectra, PCA grid, eigenvalues, and mineral indices

Mean (black) and six PC-pure pixel spectra; shaded vertical bands at 1250–1500 nm and 1750–2000 nm mark the masked water regions.

PCA composites — six rows of (R,G,B) = (PCₖ, PCₖ₊₁, PCₖ₊₂) for k = 1, 4, 7, 10, 13, 16.

Eigenvalue spectrum in log scale.

Mineral spectral indices for 31 Jan 2025 18:11 UTC — emit20250131t181133 (15 maps)

FEAI — Ferric iron alteration

FEAI = R(2.165)/R(0.82) + R(0.56)/R(0.66) [µm]

FEI_simple — Ferrous iron

FEI_simple = R(2.165)/R(1.65) [µm]

CLAY_TM — Generic clay (TM5/TM7 analog)

CLAY_TM = R(1.65)/R(2.205) [µm]

CLAI — Argillic alteration (alunite/kaolinite)

CLAI = (R(1.65)+R(2.205))/R(2.165) [µm]

ALI — Alunite (Ninomiya)

ALI = R(2.260)²/(R(2.165)·R(2.330)) [µm]

KAI1 — Kaolinite slope

KAI1 = R(2.260)/R(2.165) [µm]

KLI — Kaolinite doublet

KLI = (R(1.65)/R(2.165))·(R(2.330)/R(2.205)) [µm]

KAI3 — Kaolinite (alt 3-band)

KAI3 = (R(1.65)+R(2.260))/R(2.205) [µm]

OHI — OH-bearing minerals

OHI = (R(2.260)/R(2.205))·(R(1.65)/R(2.205)) [µm]

OHI3 — OH near 2.17 µm

OHI3 = (R(1.65)·R(2.260))/R(2.165)² [µm]

CLI — Calcite

CLI = (R(2.205)/R(2.330))·(R(2.395)/R(2.330)) [µm]

DOLI — Dolomite

DOLI = (R(2.205)+R(2.330))/R(2.260) [µm]

MGAI — Mg-OH/chlorite/epidote

MGAI = (R(2.395)+R(2.205))/R(2.330) [µm]

MONI — Montmorillonite

MONI = (R(1.65)+R(2.205))/R(2.260) [µm]

PRAI — Propylitic alteration

PRAI = (R(2.260)+R(2.395))/R(2.330) [µm]

29 Jan 2024 19:59 UTC — emit20240129t195908Tier 3 — Not Included: 24.8% cloud

Reason: cloud+cirrus+dilated = 24.8% (> 15%)

26 Jun 2024 16:52 UTC — emit20240626t165222Tier 3 — Not Included: 16.2% cloud

Reason: cloud+cirrus+dilated = 16.2% (> 15%)

2 Dec 2024 18:01 UTC — emit20241202t180154Tier 3 — Not Included: 37.6% cloud

Reason: cloud+cirrus+dilated = 37.6% (> 15%)

6 Apr 2025 16:33 UTC — emit20250406t163321Tier 3 — Not Included: 22.6% cloud

Reason: cloud+cirrus+dilated = 22.6% (> 15%)

3 Jun 2025 17:35 UTC — emit20250603t173505Tier 3 — Not Included: 44.3% cloud

Reason: cloud+cirrus+dilated = 44.3% (> 15%)

27 Jun 2025 15:59 UTC — emit20250627t155944Tier 3 — Not Included: 16.7% cloud

Reason: cloud+cirrus+dilated = 16.7% (> 15%)

18 Jul 2025 23:39 UTC — emit20250718t233916Tier 3 — Not Included: 59.1% cloud

Reason: cloud+cirrus+dilated = 59.1% (> 15%)

11 Aug 2025 22:02 UTC — emit20250811t220237Tier 3 — Not Included: 19.5% cloud

Reason: cloud+cirrus+dilated = 19.5% (> 15%)

11 Aug 2025 22:02 UTC — emit20250811t220249Tier 3 — Not Included: 15.4% cloud

Reason: cloud+cirrus+dilated = 15.4% (> 15%)

2 Feb 2024 18:24 UTC — emit20240202t182442Tier 4 — Not Included: 95.0% dead rows

Reason: 95.0% dead rows (> 10%)

7 Jun 2025 15:58 UTC — emit20250607t155817Tier 4 — Not Included: 224 interior dead rows

Reason: interior gap(s) totaling 224 rows AND 20.0% dead rows (> 10%)

5. Discussion

Across all scenes the true-color renders are visually indistinguishable: both retrievals flatten the atmospheric path radiance and recover surface color contrast. The PC-pure spectra agree closely on broad continuum shape but show systematic differences at the edges of the masked water regions — exactly the wavelengths where MODTRAN and the sRTMnet emulator differ most. The eigenvalue spectra are nearly parallel in their leading components but diverge in the tail, where atmospheric residuals live.

That divergence is informative. A perfect atmospheric correction would push residual covariance toward isotropic noise — a flat tail in log-eigenvalue space. Curvature in the tail indicates structured residuals that PCA can latch onto. We use that curvature, together with the higher-order PC composites, as a quick visual diagnostic of atmospheric-correction quality without ever needing ground truth.

6. Reproducibility

All figures here are generated by a single self-contained Python script (digital_whitepaper/build_whitepaper.py) that loads the standard L2A from the LP DAAC NetCDF, the local ISOFIT cube via the spectral ENVI reader, applies the water-band mask, fits PCA on a 80,000-pixel subsample, picks PC-pure pixels, and renders all comparison figures. The local ISOFIT runs are produced by:

# EMIT scene → ISOFIT 3.7.x + sRTMnet
from isofit.utils.apply_oe import apply_oe
apply_oe(
    input_radiance     = "<flight>_rdn",
    input_loc          = "<flight>_loc",
    input_obs          = "<flight>_obs",
    sensor             = "emit",
    surface_path       = "data/surface/emit_surface.mat",
    emulator_base      = ".../sRTMnet_v100.h5",
    segmentation_size  = 40,
    presolve           = True,
    analytical_line    = True,
)