Takuro Tsuchikawa, Mitsubishi Electric Corporation; Takao Endo, Mitsubishi Electric Corporation; Hidenobu Tsuji, Mitsubishi Electric Corporation
Keywords: light curve, satellite’s rotation, CubeSat
Abstract:
The growing number of Low Earth Orbit (LEO) objects requires monitoring of their physical status, such as their positions, velocities, and orientations. Ground-based observations using small optical telescopes are effective methods for monitoring the status of LEO objects, with low installation cost compared to radio observations.
In particular, it is likely to be difficult to observe CubeSats, which have been launched frequently in these days, due to their small size.
In this study, we aim to investigate the capability of optical tracking observations to understand the physical states of CubeSats.
A 3U CubeSat whose telemetry data is available was selected as the target for the optical tracking observation. We utilized an optical telescope with an aperture diameter of 36 cm and a 2k x 2k CMOS array detector. While tracking the satellite based on the azimuth and elevation angles calculated from Two Line Elements (TLE), we obtained 449 frames of optical image data with an exposure time of 100 ms and a frame rate of 9 fps. Prior to the observation, it was expected that the satellite is considerably faint since it is a 3U CubeSat with dimensions of 10 cm x 10 cm x 34 cm. Despite the small size, it was detected in all the frames with sufficient signal-to-noise ratios and an averaged magnitude of 9.
We performed aperture photometry on all the frames to obtain a light curve, the total duration of which is over 60 s. The light curve thus obtained shows a brightness oscillation with a period shorter than 1 s in addition to a global brightening trend. The global brightening trend and the brightness oscillation are likely to originate from a variation of the solar phase angle and a rotation of the satellite itself, respectively.
In order to confirm that a rotational motion causes the brightness oscillation, we perform a Fourier analysis on the light curve in a 20 s epoch during which image data was acquired at equal time intervals. The Fourier analysis estimates the angular velocity to be 1100 ± 50 deg/s, which is consistent with the estimate from telemetry data of 1074 deg/s. On the other hand, the error of the angular velocity was large at 50 deg/s. To improve the accuracy, we perform an epoch folding analysis using the entire data acquired over 60 s. The resultant angular velocity was estimated to be 1074.6±1.0 deg/s, which strongly supports that the oscillation of the light curve originates from a rotation of the satellite itself.
The folded light curve, which the epoch folding analysis yields by folding the data in a precise period, shows a weak but statistically significant scatter. Hence, we divide the light curve into three parts and performing an epoch folding analysis of each part of the light curve. The result indicates that the oscillation profile has time variability, which causes the weak scatter of the folded light curve. This is likely to imply the slight change of the attitude of the 3U CubeSat in the second observation. Furthermore, we compare the folded light curve with a simple sunlight illumination model to obtain additional information other than the angular velocity. The best fitted model parameters suggest that the 3U CubeSat does not rotate around its long axis but its short axis and the structure of each surface of the 3U CubeSat is slightly different. Hence, we can confirm that the optical observation of the CubeSats is effective in detecting and characterizing them, which can contribute to ensuring safety in orbit as well as to the sustainable and secure use of outer space.
Date of Conference: September 17-20, 2024
Track: Satellite Characterization