  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.
