Sarah Caddy, University of Melbourne, Australia; Lee Spitler, Macquarie University; Sam Kirkwood, HEO; James Allworth, HEO; Stuart Bartlett, HEO
Keywords: off-the-shelf, non-Earth imaging, space assets, Huntsman Telescope, light curves
Abstract:
Satellite-based Non-Earth Imaging (NEI) of Resident Space Objects (RSO) is a compelling way to obtain timely information about objects of interest. Simultaneous ground-based observations of a RSO at the same instant NEI is captured provides additional leverage to better characterise an RSO. For example, the longer observational opportunity from a ground-based site allows for the possibility of characterising active manoeuvres or tumbles. In turn, the satellite attitude information from simultaneous, resolved NEI allows for the possibility of deriving attitude information from unresolved, ground-based light curves.
In this work, we present the first simultaneous observations of RSOs from both the ground and space. We detail the results from 4 successful simultaneous observations of Starlink satellites, including calibrated daytime optical light curves and resolved NEI of our targets.
To accomplish the simultaneous observing we had to overcome scheduling challenges that impacted previous attempts.
From the ground, we overcame the scheduling issue by utilizing daytime observations, which opens the opportunities of a particular site by a factor of x2-3 times compared to traditional Low Eart Orbit (LEO) observations conducted during terminator conditions. We leveraged new findings that LEO satellites appear ~10 times brighter from the ground during the daytime compared to twilight. This is due to previously not appreciated contribution from Earthshine at visible wavelengths of light.
Ground-based observations were acquired from an optical setup resembling the Macquarie University’s Huntsman Telescope, a Canon-lens telescope array that has been configured for daytime observations of LEO satellites. The commercial lenses present a low-cost sensor that can detect at least 90% of Starlink satellites passing over a site during the daytime. Observations for this study were calibrated so that high-quality light curves could be derived and compared to model predictions.
From space, we overcame the scheduling problem by taking advantage of the growing space sensor network that makes up the HEO Inspect service. HEO provides commercial access to over 46 sensors that deliver NEI and analytics like attitude information and satellite configuration.
We utilized the attitude estimated from HEO imagery to provide context to the light curves generated from the unresolved ground-based data. In one instance, for example, the Inspect data strongly favored “open book” configuration for the Version 1.5 Starlink satellite that was observed. This configuration provides a significantly better match to the ground-based light curves compared to the nominal “shark fin” configuration. Generally, we find good agreement between the predicted light curves and our ground observations.
Our work shows that coordinated space and ground-based observations are now possible with single ground sites if the targets are abundant, like the Starlink constellation. The consistency between ground light curves and predictions using space-based attitude determination demonstrate the strong potential for using space-based NEI to calibrate light curve data that is more readily available, but harder to interpret.
Date of Conference: September 16-19, 2025
Track: Space-Based Assets