Building Autonomous Spacecraft: A Guide to Self-Flying Space Vehicles
Introducing An Autonomous Board For Avoiding Collisions (ABACO)
There are now more than 7,000 objects in space that can be considered active.
While historically Satellites were launched into orbit and largely stayed there throughout their useful life, the invention of cheap and efficient electric propulsion has introduced unprecedented degrees of freedom as now these spacecraft can reach primary orbits from their launch service providers (like Exolaunch), actively keep station, and also perform in-space logistics and de-orbit actively (see D-Orbit).
As a result, space is becoming increasingly more congested. And the likelihood of a conjunction events, i.e., a scenario where two or more spacecraft come close to each other, increases. While that can be intentional, i.e. in case of a docking maneuver, it also can be unintentional leading to a collision. A collision then leading to a loss of payload operations. I.e., your internet goes down or your autonomous car stops working.
Even in 2024, these events are not that rate anymore. A typical Starlink satellite, experiences up to  5 conjunction events per day. Suppose that each event is discovered, on average, 7 days in advance, then a total of up to 35 events is expected to be processed with a typical 24h workday.
If we assume that the traditional approach of collision avoidance takes several hours, we will soon reach a point where conjunction processing becomes unsustainable. And that does not even take into consideration new spacecraft that are increasingly launched nor debris and other objects in space.
Therefore, there is a clear operational need for constellation operators to protect their investments and allow their spacecraft to navigate autonomously in space.
Benefits of such a system
Reduce the ground station’s workload for conjunction assessment and risk analysis
Reduce the need to operate collision avoidance maneuvers manually
Detect risks of in-space collusions with higher accuracy and react faster
Funded by the Italian Space Agency, Project ABACO aims to create these benefits through combining the operational and physical architecture of an onboard satellite subsystem with a sophisticated software solution. ABACO is a system that is integrated into the hosting spacecraft and is designed to be invisible to the spacecraft operators unless they want to intervene. This autonomy a huge advantage compared to traditional methods that rely on ground-station staff to plan and execute the maneuver.
Physical architecture
ABACO’s main functionality is derived from two physical architecture features.
A navigation unit (NAV), which orchestrates the autonomous (yeah!), Global Navigation Satellite System (GNSS)-based, orbit determination and prediction for the spacecraft, and
A collision avoidance unit (COLA), that provides risk assessment and mitigation functionalities and can compute evasive maneuvers.
Operational architecture
ABACO implements an automatic collision avoidance process, that allows the system to be able to estimate the risk of collision, make a decision on it, interface with a coordinating agent, and compute and program a collision avoidance maneuver.
CAS Service. A service that provides Collision Avoidance Analysis services.
TLE. Two Line Element data. Explained here already.
However, TLEs predictions worked well in the past, but because they experience errors in the order of tens of kilometers for propagation horizons of a few days from epoch, using them for precise conjunction assessments is increasingly difficult.
CDM. Conjunction Data Messages. A standard data format created to track spacecraft and predict through orbit propagation potential conjunction point. One provider CelesTrak’s SOCRATES Plus
With this information provided, the spacecraft might be able to perform an automated collision avoidance maneuver.
The spacecraft then needs to execute three operational functions.
GNSS positioning. The GNSS positioning function provides both the internal risk assessment function and the host platform with the precise ephemeris of the hosting spacecraft. I always understood ephemeris as a kind of "schedule" or "map" that tells the position and velocity among other data-points about a spacecraft.
Risk Assessment (RA). The RA function is triggered from ground through the upload of a Conjunction Data Messages (CDM) for any conjunction. Once the primary assessment is completed using onboard data, a decision about the event is made.
Collision Avoidance (COLA). Automating COLA onboard reduces the time between maneuver decision and close approach, because there is no need for the ground station to become visible. As you can see below, the number of spacecraft that are visible from a given ground-station can vary.
The benefit of this is that it reduces the number of false alerts the ground-team has to react on. This is because the system is much more accurate compared to the ground station measurement.
The system calculates and proposes the maneuver autonomously and only sends it to the ground-station for approval. I believe that this governance approach is necessary especially in the beginning of autonomous spacecraft operations.
ABACO’s decision making algorithm
In this diagram we can see the different hardware components mentioned prior in action and how they guide the spacecraft to make a maneuver decision.
The typical and effective action to avoid a collision with another spacecraft is changing the primary’s trajectory by thrusting. Thrusting is commonly performed by onboard propulsion systems. Historically, satellites have not been equipped with propulsion systems in the past, but with the dawn of electric propulsion (FEEP and Hall Thrusters), this technology has become much more affordable and low-risk. That of course means that tradtional orbit propagation techniques that update three times per day are not working anymore!
Nevertheless, the decision to execute evasive maneuver shall be made very carefully to avoid unnecessary loss of payload operations and propellant.
We don’t have fuel-stations in space just yet ;-)
Conclusion
The goal of this post was to provide an overview of the components that are necessary for a spacecraft to fly autonomously in space. Given the rapidly increasing number of spacecraft in orbit, existing methods are assumed to be soon outdated.
ABACO provides an inexpensive and low risk solution that if commercially developed can lay the ground work for a multitude of high-value activities in space. And I think that the problem of outdated technology will be even further amplified by in-orbit logistics, assembly, and manufacturing.
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