Olli Wilkman, Finnish Geospatial Research Institute
Keywords: Modeling, radiation pressure, photometry, satellite laser ranging
Abstract:
We present a simulation tool developed to study the interaction of visible/NIR radiation and space objects, with a variety of applications.
Besides its core geodetic work, the Space Geodesy research group at the Finnish Geospatial Research institute is involved in space debris and Space Situational Awareness research. Topics in current research projects include: observations of space debris with a modern satellite laser ranging (SLR) system, SLR observations of cubesats, optical observations of large debris and satellites with small telescopes, the effects of radiation pressure forces on orbital and rotational dynamics of space objects.
A simulation tool has been developed for modeling the interaction between light and objects in space. It is based on a ray-tracing framework, computing the flow of radiative energy in a system consisting of an object and various light sources. The object is defined as a collection of elementary shapes with varying material properties, while light sources are mainly the Sun, the Earth and laser ranging systems. The software is modular on many levels. The core ray-tracing simulation can be used with different output modules to compute e.g. a visual image, integrated (non-resolved) photometry, radiation pressure forces and torques, or the pulse shapes of reflected laser ranging pulses. Inside the simulation, different shape models and surface scattering models can be plugged in easily. The software will be released as open-source once it reaches a stable development state.
The first main application of the software is the simulation of an SLR laser pulse reflected by a space debris object. Laser ranging can be performed even on objects without retro-reflectors, as long as they are large and near enough. However, the reflected pulse is not as well-defined, and depends on the shape and material properties of the object, affecting the observed photon statistics. Currently no good methods exist to infer the rotational state from such observations. The aim of the work is to model the pulse shapes and study the link between the photon statistics and the rotational state.
The second application is the modeling of Earth radiation pressure on space objects with complex shapes and realistic material properties. Radiation pressure forces caused by the flux of electromagnetic radiation from the Sun and the Earth affect the orbits of satellites. In theory, it is therefore possible to invert the problem and extract information about the radiation fluxes on a satellite based on observations of the orbits. Combined observations of a a large number of satellites with well-known orbits, such as a constellation of GNSS satellites, could then be used as an instrument for measuring the outgoing radiation flux of the Earth, an important parameter in climate models. The approach would give very low spatial resolution of the Earth radiation, but potentially a high temporal resolution, which could serve as an independent measurement to compare against established satellite-based measurements.
The radiation pressure forces also change the rotational states of space objects that are not actively controlled. Their rotation causes systematic variations in their photometric lightcurves, as well as distances measured with laser ranging. Knowledge of the rotation state is important particularly in the case of large objects re-entering the atmosphere. The aerodynamic drag forces depend strongly on the attitude of the object, leading to uncertainty of the re-entry trajectory.
A third application is simulating non-resolved photometry of space debris when observed with small telescopes. Optical photometry is the only way to get information about the rotational state of space objects that are small or in high orbits. Long-term monitoring of rotational states can be used to verify models of radiation pressure torques, as well as learn about events such as meteor or debris impacts, which suddenly change the rotational state.
Date of Conference: September 11-14, 2018
Track: Space Situational Awareness