How MERIDIAN Works
MERIDIAN tackles the simulation in two sequential stages: first it establishes which parts of the Earth disk are geometrically visible from the selected lunar site; then it computes the infrared radiance arriving from each visible sector and integrates the result into a final output spectrum.
The engine is implemented in MATLAB, using SPICE (NAIF/JPL) libraries in C for high-precision celestial ephemerides. Both stages run on a server backend, so no local installation or specialised hardware is required on the user’s side.
Stage 1. Visibility Simulation
The fraction of the Earth disk visible from a lunar site changes continuously as the Sun–Earth–Moon geometry evolves. MERIDIAN computes the elevation and azimuth of Earth in a topocentric reference frame centred on the selected lunar site, evaluating both full and partial disk visibility through ray-tracing on a grid wrapping the Earth’s atmosphere.
Lunar topography is incorporated using altimetric data from the LOLA instrument aboard NASA’s Lunar Reconnaissance Orbiter, so local horizon obstructions are taken into account. The output of this stage is a time-resolved visibility map: a precise description of what fraction of the Earth disk is exposed at each moment during the simulation window.
Stage 2. Radiative Tranfer
For each visible sector of the Earth disk, MERIDIAN computes the outgoing spectral radiance using sigma-FORUM, a fast parametrised radiative transfer model covering the full atmospheric spectrum between 10 and 2760 cm⁻¹ (Masiello et al., Journal of Quantitative Spectroscopy & Radiative Transfer, 312, 2024).
The computation proceeds pixel by pixel across the visible disk:
- Atmospheric and surface parameters are extracted from the ERA5 (ECMWF) and CMIP6 databases, with spectral soil emissivity provided by the Huang et al. (2016) dataset.
- sigma-FORUM computes the spectral radiance for each pixel along the line of sight, using the zenith angle relative to the outgoing surface normal and accounting for surface type, multiple cloud scattering (ice and liquid phases), and viewing geometry.
- Per-pixel radiances are weighted by the cosine of the zenith angle and integrated over the full visible disk to produce the final output spectrum.
Input and Output
Main input parameters:
- LETO position on the lunar surface (latitude, longitude)
- Temporal parameters (date, time, simulation duration)
- Infrared spectral band of interest (within 100–1600 cm⁻¹ / 6–100 μm)
Outputs:
- Spectral radiance dataset (downloadable)
- Spectral plot of the received infrared flux
- Visibility conditions report for the selected site and time window
- Saved session file for deferred or sequential analyses
Processing time ranges from a few tens of minutes to several hours depending on the spatial and temporal resolution requested.
Technical Specifications
| Parameter | Value |
|---|---|
| Spectral range | 100–1600 cm⁻¹ (6–100 μm) |
| Field of view | 2.3° (matched to LETO) |
| Ephemeris library | SPICE (NAIF/JPL) |
| Radiative transfer model | sigma-FORUM |
| Atmospheric database | ERA5 (ECMWF), CMIP6 |
| Surface emissivity | Huang et al. (2016) |
| Topography | LOLA/LRO altimetric data |
| Processing time | Tens of minutes to hours |
| Output formats | Dataset + spectral plot |
| Session management | Saveable and reloadable |