Technologies for space surveillance and tracking
Space debris is a major threat to active satellites. With more than 750,000 debris in orbit larger than 1 cm, space debris objects can severely damage or end a satellite’s operational capacity. In the most populated and congested orbits –LEO and GEO primarily-, the risk of mission failure due to space debris is beaten out only by the risks during launch and deployment in orbit. In order to ensure the safety of the mission and avoid collisions, space debris must be detected, tracked, and catalogued through different technologies on the ground. Space Surveillance and Tracking (SST) systems rely on several technologies, each with advantages and limitations.
Radar systems are particularly useful for space surveillance and tracking objects in LEO, having the capability to operate regardless of weather conditions and time of day. One such example is the US Air Force Space Fence, declared operational in March 2020 and able to track objects below sizes of 10cm through an S-band radar system. While radars could be used beyond LEO to cover medium Earth orbits and GEO, this technology becomes expensive to operate for higher altitudes as the power required to track an object scales up bi-quadratically with the distance. Different types of radar systems have been employed for space surveillance, with tracking and immobile antennae working in bistatic mode (where one antenna emits a pulse and another receives the return), as well as phased-array antennae tracking multiple satellites simultaneously with no moving mechanical parts. These are composed of thousands of small phased elements used to electronically steer the system.
Exhibit 1: Distribution of the systems contributing to the EU Space Surveillance and Tracking capabilities
Optical telescopes can be used to supplement radar systems in order to more efficiently track spacecraft in MEO and GEO. Optical systems can provide satellite and debris tracking at lower power expenditures and cost compared to radars, though the short observability of objects in LEO makes these systems less suitable for low Earth orbit tracking. As with any optical system, telescopes used for SST can only operate during the night and are restricted by cloud cover. Despite these constraints, optical telescopes continue to be paramount in the cataloguing of defunct spacecraft and smaller space objects; both survey telescopes, with a large field of view, and tracking systems, with a narrower field of view but higher accuracy, can be used to detect and track space debris.
Third, laser equipment is frequently used to track space debris in orbit. While past iterations of this technology required retro-reflectors to be fitted on-board a satellite in order for it to be tracked, thus significantly limiting its range of use, recent developments have allowed laser tracking to be reliably used on “non-cooperative” objects. This category necessarily includes space debris, whose position can be tracked by illuminating the object with a laser beam and using receivers on the ground to track it. Measurement campaigns at Graz observatory in Austria have been recently carried out to evaluate the reliability of this system: using receivers at three different sites enabled a success rate of 60% in tracking the object’s orbit, pinpointing non-cooperative debris or spacecraft to one order of magnitude more precisely compared to radar. In addition, while many of the limitations of optical systems apply to laser tracking, powerful lasers can be used for day-time operations, although further research and development activities are required to increase the system’s reliability.
Exhibit 2: Example systems feeding data into EUSST
The final system comprising effective space surveillance capabilities involves Space-based Space Surveillance (SBSS). SBSS are satellites carrying sensors usually placed in dawn-dusk sun-synchronous orbits (SSO) in LEO, allowing for the tracking of the population of space debris in GEO without the limitation imposed by the day cycle, range or weather conditions. The US Space Force has begun deploying satellites as part of their SBSS constellation in order to support space situational
awareness activities. Spacecraft supporting ground infrastructure to detect and track space debris offer significant advantages in terms of operability, though of course require launching campaigns with the inherent risks of placing satellites in orbit.
All of the data collected from the above systems are then processed and catalogued, providing consistent SST activities. In practice, this data enables a multitude of services ensuring the safety of space assets; conjunction prediction, risk assessments and collision avoidance manoeuvres all necessarily rely on SST data. Further, other analyses enabled by SST such as in-orbit fragmentation, orbital lifetime prediction, and re-entry monitoring are necessary to protect the outer space environment and ensure the sustainability of operations.
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 | This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101004319. This article reflects the author’s view and not necessarily the views of the European Commission or of the European Health and Digital Executive Agency |