Star-tracking: experimental attitude control system to be tested next year

Two space services providers, the American Sidus Space, and Israeli Lulav, are teaming up for an innovative technology demonstration. The two companies will test a new star-tracking system for satellite attitude checking.

Sidus Space is a recently founded space company that started with system design, and payload deploying services back in 2019 and is now developing a completely new launch platform.

Its name is LizzieSat, a new generation partially 3D printed small satellite bus, complete with control and communication systems. Different companies can use this modular system as a testbed for their own projects in space. Sidus’ objective is to ultimately create a small constellation of these satellites. Sidus Space is targeting its first launch next October, aboard the SpaceX Transporter-9 mission. It has already finalized other 8 launches within the next 3 years.

Lulav, the Israeli start-up, is focusing on robotics and guidance, navigation, and control systems. It was founded in 2021 and some of its founders participated in the Beresheet-1 mission to the Moon in 2019. They are also involved in the Beresheet-2 mission.

In a statement released on June 9, 2023, the two companies announced they would be testing a new star-based navigation system on the fourth LizzieSat mission in June 2024.

How to navigate in space

Being able to execute precise attitude changes in space is of utmost importance for every mission. Hence, with decades of space exploration, different attitude control systems have been and are being developed.

However, to be able to maneuver in a certain direction, one must first know the current position, velocity, and attitude of the spacecraft. For Low Earth Orbit, position and velocity are obtained through ground tracking or other satellite networks. Instead, attitude is more complex to find.

Movement in space involves three dimensions, but unlike the flight of an airplane, there is no clear way to determine which direction is up or down. Because when orbiting Earth, or any other body, an object is “continuously falling”, the only acceleration that can be felt in space, is a change in the spacecraft’s velocity. So it is necessary to “look elsewhere” to determine one’s orientation in space.

You can determine your orientation by measuring the accelerations of your satellite from the moment you launch from Earth, and “keeping track” of them as time passes, using accelerometers or gyroscopes. This is known as “relative attitude determination”. However, it becomes more inaccurate as time passes and needs to be recalibrated, relying on other kinds of sensors.

The “absolute attitude sensors” are relying on outside phenomena, instead of considering the spacecraft itself. For instance, you can determine attitude by sensing the Earth’s magnetic field, or by looking for the Earth’s horizon. One of the most reliable and used systems is star tracking. By photographing stars with a CCD sensor, like one in cameras, and comparing it with a database of stars, you can estimate the satellite’s orientation.

Event-based star tracking

While current star tracking has become more refined with each iteration, it still has some limitations. First of all, its sensors are power-hungry. Secondly, their working speed is limited. Until now, these kinds of conditions lead to careful estimations to be performed, and have still some accuracy-related obstacles.

A developing technology of recent making is the Event-based Vision Sensor. It is been designed to only “register” changes related to the observed environment’s velocity. In the way it’s designed, changes are immediately highlighted, resulting in faster data analysis and lower power consumption. Possible applications are different: from road surveillance to manufacturing, vibration testing, or robotics. 

Lulav will instead try to use this sensor as a star tracker. Its capabilities should then make the estimation of a craft’s movement easier and more precise. A faster image acquisition becomes significant for those spacecraft that need to continuously rotate while functioning, such as LizzieSat. An event-based star tracker will make it easier to determine the spacecraft’s rotation axis, relative to the portion of the sky it’s seeing.

If their mission is successful, Sidus and Lulav will have demonstrated an all-new approach that has for now only been theorized.