Takuro Tsuchikawa, Mitsubishi Electric Corporation; Takao Endo, Mitsubishi Electric Corporation
Keywords: GEO, satellite characterization, light curve, polarimetry, multi-band photometry
Abstract:
As space development is now actively progressing, the characterization of satellites and space debris is becoming important to ensure their sustainability. In particular, GEO satellites are widely used for observation, communication, and military applications because they are located at fixed points as seen from the earth. On the other hand, GEO satellite characterization is still under development since it is difficult to understand satellite characteristics from limited ground-based observations.
Spatially-resolved optical imaging is a simple method to characterize the satellite’s attitude and rotation. However, the method cannot be applied to GEO satellites because they are too far from the earth compared to LEO satellites to spatially resolve them unless an extremely large-aperture telescope and a highly precise adaptive optics system can be used. Optical light curve is easy to observe since the resolving power is not needed. The lighting source of GEO satellites in night is only the sun, and therefore light curve data with simple and wide range of lighting conditions of solar phase angle can be obtained by continuous observation during half a day. Thus we can barely estimate the attitude and surface material by comparing the observed optical light curves with an lighting simulation.
In this study, we characterize GEO satellites with light curves observed by multi-band polarimetry.
The observations were performed in Ofuna Space Monitoring and Communications center (OSMC) in our institute located in Kamakura, Japan. An 11 inch-aperture optical telescope, a polarization image sensor and the Johnson-Cousins BVRI photometric filters were used for the observations. We can obtain four directional polarization images in one shot (0/45/90/135 degrees) with the polarization sensor, and therefore can calculate the degree of linear polarization (DoLP) and the polarization angle for each shot. 55 GEO satellites are observed in about one year from December 2023, with the most frequently observed satellite being observed a total of 17 times. Since the SNR is significantly lower compared to non-polarized photometry, we enabled observations with a maximum exposure time of 60 seconds.
Aperture photometry was applied for all the polarized frames, and the light curves and the variations of DoLP and polarization angle were calculated. In all the data, the DoLP tends to approach 0% when the solar phase angle approaches 0 degrees. This is a reasonable trend since the illumination source for GEO satellites is the sun. On the other hand, the light curves differ greatly among satellites. For example, some satellites show peak structures in their light curves, likely due to specular reflection, while others show asymmetric dependences of the light curve on solar phase angle with respect to 0 degrees. For communications and SIGINT satellites which are expected to have large deployable antennas, the DoLP of B-band light curves tend to be significantly larger overall than those of the other photometric bands. The overall high DoLP of B band is considered to be due to the blue SAP rotating in sync with the solar phase angle, while the lower DoLP of the other photometric bands due to the larger contribution of the large deployable antenna.
In the future, by accumulating these data and knowledge, it will be possible to monitor the attitude of each component, and multi-band polarimetry is expected to become important for GEO satellite characterization.
Date of Conference: September 16-19, 2025
Track: Satellite Characterization