Monitoring satellites and space debris is more important than ever today because more and more objects are orbiting Earth. In order to improve the measurements and make them more efficient, Graz researchers have now developed a new laser-based measuring system. The use of a megahertz laser makes it possible to set up highly precise measuring routes to satellites and to locate space debris – without the previously necessary switching of laser strength and pulse duration. With the help of this laser technology, around 40 stations around the world that were previously only used for satellite monitoring could be used to detect space debris in the future, as the team explains. This would help predict collisions between satellites and space debris in good time and initiate appropriate avoidance maneuvers.
In the last ten years, the number of satellites orbiting the Earth has increased rapidly, tripling to around 10,000 active satellites. The commercial mega-constellations for orbital Internet such as Starlink, Kuiper and Co. have particularly contributed to this. In addition, there is a growing amount of space junk: around 40,000 pieces of scrap measuring more than ten centimeters in size and around a million scrap objects measuring more than one centimeter in size are known to date. Despite their small size, these extremely fast-flying fragments can cause severe damage to active satellites in the event of a collision. A collision with larger pieces of junk can even completely destroy a satellite.
How satellites and space debris are measured
It is therefore important to monitor as closely as possible. It takes place using laser-based measuring stations on the earth’s surface. However, the focus so far has been on measuring the distance to satellites, because it is used, for example, to measure the earth’s gravity field, the earth’s topography or the earth’s magnetic field. “Around 40 laser ranging stations (SLR) worldwide deliver these distance measurement data to several data centers, which then determine the high-precision trajectories and orbit predictions,” explain Michael Steindorfer and his colleagues from the Institute for Space Research at the Austrian Academy of Sciences in Graz. To do this, the stations use lasers with pulse durations of just ten picoseconds and powers of 0.8 watts. These laser pulses are reflected back to the satellite by small reflectors and picked up by the measuring stations. The distance and position of the satellite are then determined from the transit time. The common measuring systems enable up to 2,000 individual measurements per second and a positioning accuracy of around three millimeters, as Steindorfer and his team explain.
However, this is different with space debris: Because the debris particles are much smaller than satellites and have no reflectors, they can only be located using stronger lasers with around 16 watts of laser power. These usually have a longer pulse duration of around three nanoseconds and can therefore only carry out around 200 individual measurements per second. This reduces the precision of position determination to just around one meter. The problem, however: “So far there are only a few stations worldwide that can monitor space debris using lasers,” explain the researchers. As a result, there is also a lack of measuring stations for globally coordinated space debris monitoring. Steindorfer and his colleagues may now have found a solution to this. They have developed and tested a method that can detect satellites and space debris using the same laser and with greater precision than before.
Megahertz lasers can do both
The basis of the new monitoring system is a megahertz laser that combines high power and short pulse durations. “The megahertz laser works with repetition rates between 0.05 and ten megahertz, a central wavelength of 532.15 nanometers and a pulse duration of ten picoseconds,” report Steindorfer and his team. The high power of this laser allows the measurement of space debris, while the short pulse duration makes it possible to carry out high-precision measurements of satellites using the same system. For initial tests, the researchers installed their new laser at the Lustbühel Observatory near Graz, which has already monitored satellites and space debris, but requires two different laser setups. In the first test of the new megahertz system, they used a so-called monostatic measurement mode. The measuring laser and receiving telescope are directly adjacent. “With the exception of the megahertz-capable laser, this setup is identical to the systems for standard geodetic observations,” said the team. In order to avoid disturbing effects caused by atmospheric backscattering of the laser light, the laser measurement must be carried out in monostatic mode at intervals. However, this reduces the usable laser power, as the Steindorfer and his colleagues explain.
A more effective alternative is therefore to use the megahertz laser in a so-called bistatic measurement. The receiving telescope for the laser signals reflected from Earth’s orbit is positioned at a greater distance from the laser transmitter. “With the help of a second telescope on the roof of the Lustbühel Observatory, we were able to prove that a distance of around ten meters between the transmitter and receiver is sufficient to avoid atmospheric scattering,” reports Steindorfer. “This enabled us to use the full laser power and measure satellites and space debris at a repetition rate of one megahertz for the first time.” During the test, the highly precise positioning and measurement of several pieces of scrap, including disused satellites and rocket upper stages, was achieved. The test measurements also showed that the megahertz laser also increases the accuracy of the measurements on satellites. The team managed to carry out up to two million successful distance measurements to the former research satellite Jason-2, which was equipped with reflectors, within 15 seconds. This caused a significant reduction in the so-called normal point accuracy. “Our system has succeeded in reducing this normal point accuracy to a few micrometers,” says Steindorfer. Previously, this value was in the millimeter range.
According to the researchers, their new laser process opens up the opportunity to use the existing network of laser measuring stations for satellites to monitor space debris in the future. “By upgrading to a megahertz-capable system, such stations could measure space debris in regular observation operations without having to make ongoing adjustments to the system,” explains Steindorfer. “In this way, the stations could work together to improve the orbit accuracy of high-risk objects, for example, without reducing the regular observations from research satellites.”
Source: Michael Steindorfer (Institute for Space Research, Austrian Academy of Sciences, Graz) et al., Nature Communications, doi: 10.1038/s41467-024-55777-8