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Concepts

This page covers the key ideas behind the S2GOS Observation Simulator: spectral modes, how atmosphere interacts with measurements, and the available sensor types.

Spectral Modes

The simulator uses Eradiate as its radiative transfer backend. Eradiate operates in a spectral mode that determines how wavelength integration and gas absorption are handled. The mode is set via backend_hints in the simulation config:

backend_hints={"mode": "ckd"}

Sensors

The simulator supports the following platform types, each with different instrument options and viewing geometries.

Satellite

Satellite sensors produce 2D imagery at a specified ground resolution. Two main instrument categories:

  • MSI (MultiSpectral Instrument) — Few broad bands. Sentinel-2A/2B. Each band is simulated independently with its spectral response function.
  • HSI (HyperSpectral Instrument) — Hundreds of narrow contiguous channels such as the future CHIME mission. Gaussian SRF post-processing simulates the instrument's spectral response.

Built-in satellite platforms: Sentinel-2A/2B, Sentinel-3A/3B, CHIME (approximation), EnMAP. Custom platforms are supported with user-provided SRFs.

CKD mode is recommended for all satellite instrument simulations.

Ground

Ground-based sensors are placed at low heights for in-situ validation measurements:

  • HYPSTAR — HYPERNETS network sensor. Circular FOV, approximate SRF
  • Perspective camera — Standard camera with configurable FOV
  • Radiancemeter — Directional radiance measurement

See the Sensors API reference for full configuration details and helper functions like create_chime_sensor() and create_hypstar_sensor().

Level 2 Products

Every simulation includes top of atmosphere solar irradiance automatically. All other quantities are BOA (bottom-of-atmosphere) measurements and must be explicitly requested by listing them under measurements in SimulationConfig.

Quantity Illumination Outgoing geometry
Irradiance — Downwelling flux at the surface (BOA)
BRF Direct beam only Single direction
HDRF Direct + diffuse (atmosphere) Single direction
HCRF Direct + diffuse (atmosphere) Finite cone (camera FOV)
BHR Isotropic All directions (albedo)

PixelBRFConfig, PixelHDRFConfig, and PixelBHRConfig compute the same quantities at locations derived from a satellite sensor's pixel footprint.

See the Measurements API reference for full configuration details.