radtrax - radtrax

Differentiable radiative transfer in JAX and Equinox, for methane and multi-gas remote sensing.

radtrax (formerly rtmx) is the GeoStack’s radiative transfer package. It computes a forward model $y = F(x)$ from an atmospheric state to what a sensor measures, and gets every Jacobian a retrieval needs, $K = \partial F / \partial x$ , from automatic differentiation rather than hand-derived code.

Status

radtrax moved the code that already existed elsewhere in the stack, then rebuilt it as one layered forward model; all three stages are on main, and the first release is pending:

Stage	What lands	State
Port	plumax’s band-integrated Beer–Lambert models, HITRAN look-up tables, matched filter and 3D-Var retrieval, ported with their tests	on `main`
One of each	one matched filter, one SRF integration, one air-mass factor; GeoTensor operators (from geotoolz) behind `radtrax[geo]`	on `main`
v0	the layered forward model (atmosphere → opacity → Beer–Lambert → instrument) and matched-filter targets from autodiff	on `main`; gate V2 against HITRAN pending (#89)
v1	a spherical air mass, kernel BRDFs and spectral albedo, a calibrated instrument and noise model, and optimal estimation on vardax, gated on EMIT, Sentinel-2 and Landsat 8/9	planned: Wave 4

The plan is tracked in one epic per wave, the next being Wave 4; the design is radtrax · 3 in the GeoStack docs, and the v0 notebook, Why layers, shows what the layered model changes.

Installation¶

pip install radtrax            # JAX, Equinox, lineax, optimistix, gaussx
pip install "radtrax[geo]"     # + geotoolz and georeader (GeoTensor operators)
pip install "radtrax[hitran]"  # + HAPI, to build cross-section tables

Rules the package keeps¶

radtrax never imports plumax. Plume physics uses radiative transfer, not the other way round.
Geo carriers arrive through extras. The base install stays JAX-only.
Float64 for spectroscopy. In float32 the wavenumber grid near 4300 cm⁻¹ is coarser than 7 % of a Doppler half-width.