Eclipse-Free-Time Assessment Tool for IRIS

can be used to perform an Eclipse-Free-Time (EFT) assessment of IRIS (Infrared Imaging Surveyor) mission orbits. EFT is defined to be those time intervals longer than one day during which the IRIS spacecraft is not in the Earth’s shadow. Program IRIS_EFT implements a special perturbation of orbital motion to numerically integrate Cowell’s form of the system of differential equations. Shadow conditions are predicted by embedding this integrator within Brent’s method for finding the root of a nonlinear equation. The IRIS_EFT software models the effects of the following types of orbit perturbations on the long-term evolution and shadow characteristics of IRIS mission orbits:

• Non-spherical Earth gravity,
• Atmospheric drag,
• Point-mass gravity of the Sun, and
• Point-mass gravity of the Moon.

The objective of this effort was to create an in-house computer program that would perform eclipse-free-time analysis of candidate IRIS spacecraft mission orbits in an accurate and timely fashion. The software is a suite of Fortran subroutines and data files organized as a “computational” engine that is used to accurately predict the long-term orbit evolution of IRIS mission orbits while searching for Earth shadow conditions.

The core algorithms of this software product have been used to solve a variety of unique orbital mechanics and targeting problems. Past applications include lunar shadow requirements for Chandra, perigee decay of geosynchronous transfer orbits due to third-body point-mass perturbations, and prediction of orbital lifetime and decay of Earth satellites.

This work was done by David Eagle of a.i. solutions Inc. for Kennedy Space Center. For additional information, contact David Eagle at (321) 867-8913. KSC-13519

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