[Paper Review] Planning a Reference Constellation for Radiometric Cross-Calibration of Commercial Earth Observing Sensors

Sreeja Nag, Philip W. Dabney+2 more|arXiv (Cornell University)|Oct 20, 2020

Calibration and Measurement Techniques15 references4 citations

TL;DR

This paper proposes a planner for designing a 4-satellite reference constellation to enable radiometric cross-calibration of commercial Earth-observing smallsats. By integrating orbit propagation, Earth reflectance modeling, and instrument performance simulation, the planner identifies calibration opportunities within a half-day horizon, showing that 4 satellites can provide multiple viable cross-calibration pairs within 5° of solar/view angle differences and under one hour for TOA calibration.

ABSTRACT

The Earth Observation planning community has access to tools that can propagate orbits and compute coverage of Earth observing imagers with customizable shapes and orientation, model the expected Earth Reflectance at various bands, epochs and directions, generate simplified instrument performance metrics for imagers and radars, and schedule single and multiple spacecraft payload operations. We are working toward integrating existing tools to design a planner that allows commercial small spacecraft to assess the opportunities for cross-calibration of their sensors against current satellite to be calibrated, specifications of the reference instruments, sensor stability, allowable latency between calibration measurements, differences in viewing and solar geometry between calibration measurements, etc. The planner would output cross-calibration opportunities for every reference target pair as a function of flexible user-defined parameters. We use a preliminary version of this planner to inform the design of a constellation of transfer radiometers that can serve as stable, radiometric references for commercial sensors to cross-calibrate with. We propose such a constellation for either vicarious cross-calibration using pre-selected sites, or top of the atmosphere (TOA) cross-calibration globally. Results from the calibration planner applied to a subset of informed architecture designs show that a 4 sat constellation provides multiple calibration opportunities within half a day planning horizon, for Cubesat sensors deployed into a typical rideshare orbits. While such opportunities are available for cross calibration image pairs within 5 deg of solar or view directions, and with-in an hour (for TOA) and less than a day (vicariously), the planner allows us to identify many more by relaxing user-defined restrictions.

Motivation & Objective

• Address the growing need for accurate radiometric calibration of commercial smallsats with limited on-board calibration resources.
• Overcome challenges in achieving consistent, traceable calibration due to varying viewing and solar geometry across sensors.
• Design a reference constellation that enables both vicarious and top-of-atmosphere (TOA) cross-calibration with minimal latency.
• Provide a flexible planning tool that identifies viable calibration opportunities based on user-defined constraints like angular differences and time windows.
• Optimize constellation architecture to maximize calibration frequency and reliability for CubeSats in rideshare orbits.

Proposed method

• Integrate existing tools for orbit propagation, Earth reflectance modeling, and instrument performance simulation into a unified planner.
• Model Earth reflectance across multiple spectral bands and viewing geometries using customizable sensor and target parameters.
• Simulate sensor stability and radiometric performance to assess calibration feasibility.
• Use the planner to evaluate candidate constellation architectures, focusing on temporal and geometric alignment between reference and target satellites.
• Apply constraints such as ≤5° solar/view angle differences and ≤1-hour time lag for TOA calibration, or ≤1-day lag for vicarious calibration.
• Generate cross-calibration opportunity schedules as a function of flexible user-defined parameters like orbit inclination, altitude, and sensor characteristics.

Experimental results

Research questions

• RQ1 How many viable cross-calibration opportunities can be identified for a 4-satellite reference constellation within a half-day planning window?
• RQ2 What are the optimal orbital configurations that maximize calibration frequency while maintaining acceptable angular and temporal constraints?
• RQ3 How do differences in solar and viewing geometry affect the feasibility of radiometric cross-calibration between smallsats?
• RQ4 To what extent can relaxing angular and temporal constraints increase the number of available calibration opportunities?
• RQ5 Can a reference constellation enable both TOA and vicarious cross-calibration with comparable reliability and frequency?

Key findings

• A 4-satellite constellation provides multiple cross-calibration opportunities within a half-day planning horizon for CubeSats in typical rideshare orbits.
• Calibration opportunities are feasible when solar and viewing angles differ by less than 5°, and time lags are within one hour for TOA calibration.
• For vicarious calibration, viable pairs are found within a 24-hour window, with angular differences constrained to ≤5°.
• Relaxing user-defined constraints—such as angular differences and time lags—significantly increases the number of available calibration opportunities.
• The planner successfully identifies a range of viable calibration pairs across different constellation architectures, demonstrating its utility in mission design.
• The results validate the feasibility of using a dedicated reference constellation for radiometric cross-calibration, improving sensor accuracy and consistency across constellations.