5G Signals to Track Space Trash?

Paper Title: Space debris detection over intersatellite communication signals

Authors: Antti Anttonen, Markku Kiviranta, Marko Höyhtyä

All Authors’ Institution: VTT Technical Research Centre of Finland Ltd, P.O. Box 1100, FI-90571 Oulu, Finland

Status: Published in Acta Astronautica open access

Space Debris

Over the past several decades, as humans have launched more artificial objects into orbit around the Earth, the amount of debris in space has increased dramatically. Over the many years of human orbital activity, in-orbit satellites occasionally collide with each other, creating clouds of debris. These clouds of debris stay in orbit, colliding with functional satellites and creating even more debris. Over time, this has become an exponential problem, famously called the Kessler syndrome. In short, we have now gotten to the point where it is absolutely imperative that we clean up our orbital space by removing debris from orbit (through a process called active debris removal, or ADR for short).

Finding the Space Debris…from Space?

A crucial and difficult element of ADR is finding the debris that needs to be removed problem is to find the debris that needs to be removed. Before knowing which pieces of debris need to be removed, it is important to first know which pieces of debris actually exist in orbit and track them. Today, we do most debris tracking from the ground. We use radar to track debris that’s nearby in Low-Earth Orbit (LEO, see Figure 1) and optical imaging to track farther away debris that’s in Geostationary Earth Orbit (GEO, see Figure 1). However, tracking from the ground has its drawbacks. Distance to the debris from the ground, atmospheric distortions of telescope observations, and day-night cycles all complicate the prospect of ground-based tracking.  Today’s paper talks about a special way to track debris from space using telecommunications satellites. They specifically ask whether it would be possible to track space debris using the many planned and current satellite constellations. Instead, if we use existing satellites to help with tracking from space, at least some of these problems can be solved.

This figure shows the broadly three altitudes where we place our satellites: low Earth orbit (LEO), medium Earth orbit (MEO), and geosynchronous orbit (GEO). LEO refers to all altitudes within ~2000 km of the Earth’s surface, and GEO is at 35,786 km above the surface, where the orbital period is equal to the Earth’s spin period. MEO broadly refers to all altitudes between LEO and GEO.
Figure 1: There are broadly three altitudes where we place our satellites: low Earth orbit (LEO), medium Earth orbit (MEO), and geosynchronous orbit (GEO). LEO refers to all altitudes within ~2000 km of the Earth’s surface, and GEO is at 35,786 km above the surface, where the orbital period is equal to the Earth’s spin period. MEO broadly refers to all altitudes between LEO and GEO. Cite: Sedrubal on Wikimedia Commons, https://commons.wikimedia.org/wiki/File:Earth_Orbits.svg

Using 5G Signals to Track Debris

The image is a cartoon showing that when two satellites communicate with each other, signals sent between each other can are shown to reflect off nearby pieces of debris and come back to the original satellite which emitted the signal and the receving satellite.
Figure 2: A simplified schematic of using existing communications between existing satellites to track nearby pieces of debris. Adopted from figure 1 of today’s paper

One solution would be to actively use each satellite as a radar detector, so each satellite sends out a radio signal into its surroundings to specifically search for debris. However, this would require reserving a special band of radio frequencies only for the process of debris detection, which would further use up an already crowded radio spectrum. Instead, is it possible to passively use the already existing 5G signal that satellites use to communicate with each other as a radar sensor to find nearby objects? Figure 2 shows a schematic of this idea from the paper.The benefits of such a system are that a separate space-born radar infrastructure is not necessary, the weight of a satellite could be reduced, and extra spectral bands don’t need to be reserved specifically just for tracking.

This method is already widely used here on Earth (e.g. autonomous vehicles), but using it to track space debris is more complicated. However, using it to track space debris is more complicated. An important difference between terrestrial and satellite objects is the relative speed between the detector and the object being tracked. In orbit, this relative speed can be up to 15 km/s, which is many orders of magnitude faster than in terrestrial applications.

Speed Considerations and Doppler

To understand why exactly the relative speed between the satellite and debris are such problems, let’s understand what the 5G signal is in more detail. When a signal is sent between satellites, a complex algorithm is used to maximize the available bandwidth, so there is no wastage of electronics. In short, when radio signals are sent and received over some channel, the signals are sent over different frequencies and are wrapped together using orthogonal frequency-division multiplexing, which is really just a fancy way to send multiple messages over multiple different frequencies using the same transmitter. The receiver then does a Fourier transform to reconstruct all of the messages by reading over a predefined range of frequencies. 

As the satellites use this system to communicate with each other, reflections of radio signals from this system off nearby debris will also need to be detected and understood to be coming from debris. However, because of the relative speed between debris and satellites, the reflected signal will be Doppler shifted to a different frequency. Because the satellite system only communicates over a narrow band of frequencies, if the reflected signal is not within that band, it will not be detected by the receiver.

Results

This is a simplified cartoon showing many satellites tracking the same pieces of debris, and pooling measurements to reduce uncertainty in the position and speed of the piece of debris as it passed by many satellites
Figure 3: Schematic of satellite tracking by combining observations to minimize uncertainties in debris position and velocities (figure 4 from the paper)

The authors present a new mathematical model to estimate the strength of the shifted frequencies, even if they don’t fall within the band of the received signal. This is possible because within the electronics that take the Fourier transform, there’s an oscillating circuit. When this oscillating circuit is presented with a radio signal with a frequency different from the oscillation, the signal that the satellite ends up receiving becomes a jumbled and messy combination of many frequencies. This combination can be estimated using their mathematical model, and therefore one can estimate the probability of the presence of shifted frequencies in the received signal even if they are not within the observed spectral band. Furthermore, by having multiple satellites tracking the same pieces of space debris, they can combine observations to reduce uncertainty in both the velocity and location of the piece of debris (see Figure 3). Overall, the authors find that constellation sizes of about 10,000 satellites provide sufficient detection performance for debris in the size range of 1-10mm. Upcoming satellite constellations plan for up to 40,000 satellites, so it’s looking like debris detection using existing satellites is possible!

Future Avenues

This paper presented one specific way to track debris, using existing and planned satellite constellations. However, tracking and remediating space debris will require a multi-pronged effort from both ground-based and space-based observations, along with better understanding of how space debris evolves over time. In addition, though satellites can potentially be used to track more debris, they do still contribute to the overall amount of debris in space, and already are making ground-based astronomical observations more difficult. Overall, we are still in the nascent phase of both understanding and remediating the problem of space debris! How well we can deal with it and protect our orbital space is still up to us!

Astrobite edited by Megan Masterson

Featured image credit: Karthik Yadavalli, with help from public domain images 

About Karthik Yadavalli

Hello! I am a third year graduate student at Harvard University. I primarily work on supernova modeling, focusing specifically on stripped envelope supernovae. I am also super interested in space sustainability and cleaning up space debris!

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