Man-made satellites getting smaller and more plentiful as scientists discover that groups of satellites working together can carry out many of the tasks currently performed by large, single devices more effectively and at lower cost.
A collection of small satellites working in unison is often called a distributed space system.
Basic forms are already in operation, such as constellations of satellites orbiting along similar trajectories, controlled centrally to provide constant communications coverage. There are also plans to develop them to perform complex tasks requiring groups of partially autonomous machines that can communicate with each other and fly in a precise formation to achieve a common goal. These could, for instance, deliver fuel or replacement parts to other satellites in orbit. More ambitiously, multiple small units could be launched into space separately and later autonomously assemble into a space station, or a craft designed for planetary exploration.
These smaller, distributed systems may be cheaper to launch, less prone to failure and easier to repair; but they require levels of navigational accuracy and positional responsiveness that are hard to achieve with current technology. “You need a higher degree of precision to enable these smaller satellites to interact, work closely together and possibly connect,” says Hesham Shageer, an assistant research professor at KACST. Shageer is also the principal investigator on a project working on navigation systems to control the next generation of miniaturized distributed space systems at the Centre of Excellence for Aeronautics and Astronautics (CEAA), a research collaboration between KACST and Stanford University in California.
The development of smaller electronic components and sensors have, in recent years, allowed satellite makers to pack greater functionality into increasingly smaller units. “Two key technologies are revolutionizing the way humans conduct spaceflight: the miniaturization of satellites and the distribution of payload tasks among multiple coordinated units,” says Shageer.
Keeping satellites on track
At the heart of the technology underpinning the group’s efforts is an instrument called the Modular Gravitational Reference Sensor (MGRS). Researchers at Stanford University first proposed the MGRS in 2004 as a way to achieve the precise position sensing and orbit stability a constellation of satellites would need to detect gravitational waves. These tiny ripples in space and time, caused by the movement of massive astronomical bodies, were first predicted by Einstein’s theory of general relativity, and detected for the first time in 2016 by ground-based detectors.
In essence, the MGRS is a box containing a metal sphere that, when on board a satellite in space, floats free at its centre, shielded from all non-gravitational forces, such as air resistance, solar wind, radiation pressure, and electro-magnetic forces. Within the box, optical sensors keep track of the precise position of the sphere. Its movement with respect to the box signals a shift in position of the satellite and allows for the precise measurement of gravitational forces. Thrusters can then be used to immediately counter the effects of gravity, and return the satellite to its orbit, and the sphere to its original central position.
Different forms of optical sensors can be used to keep track of the position of the sphere. The latest version of the MGRS developed by the Stanford University team uses a device called a Differential Optical Shadow Sensor (DOSS), in which pairs of laser beams are directed from the inner walls of the box at the outer edges of the sphere.
When the DOSS is in the centre of the box, the sphere’s edges block half of the light from each beam which would otherwise reach detectors positioned opposite their sources. Movement of the ball with respect to the box changes the intensity of the light reaching the detectors, allowing any change in position of the satellite to be measured. Four pairs of beams and detectors are positioned perpendicular to each other to track movement in three dimensions.
When a satellite moves through low-Earth orbit, it is affected by drag, which is the force generated by air resistance. The sphere at the centre of the DOSS is unaffected by drag, but the satellite itself is. The detection of this difference triggers the thrusters to amend the trajectory of the satellite in such a way that it can operate as if it was free from drag.
The latest prototype has also been redesigned to reduce noise and improve accuracy through the use of a fibre-coupled super luminescent light-emitting diode as the light source, a well-contained single structure and new amplifier electronics. The DOSS is able to check for small movements more frequently than other tools such as interferometers, which recombine beams of light after they have been split and made to travel along different optical paths. Satellites equipped with a DOSS can therefore make readjustments to their trajectories more quickly and achieve improved navigational precision.
“The position of the sphere within the laser beams provides a feedback loop to the satellite’s control thrusters which act in response to gravity to maintain the sphere in position,” says Shageer. “Existing technology allows the satellite navigation and control to be accurate to within a few metres, somewhere in the 3-5 metre range. With this newer technology, we could improve that to an accuracy of within a few centimetres.”
Working on resilience, accuracy and affordability
Plans to develop and test the sensor formed one of six collaborative research projects launched in 2014 with the creation of the CEAA. While Stanford University researchers had originally worked on the MGRS primarily for its potential as an astrophysics experimental tool, KACST scientists focused on demonstrating its capabilities and developing practical applications. Tests were carried out on a prototype form of the MGRS on board SaudiSat-4, a Saudi Arabian satellite launched in 2014. “It achieved all the performance requirements we had set for it,” says Shageer.
Shageer and his CEAA colleagues are working on new motion planning algorithms and software to allow the further development of distributed space systems with navigation capabilities that are accurate to within a few centimetres. They are using estimation theory, a branch of statistics designed to estimate variables when some parameters are missing, such as the location of a satellite that has stopped communicating. The group is also investigating the use of vision-based navigation, in which images collected by cameras on a satellite are used to amend its trajectory. They have been using Stanford University’s GNSS Navigation Testbed for Distributed Space Systems to test their novel algorithms.
The group aims to gather more data from the experiment on board SaudiSat 4 to further determine how it has performed over the past few years. Shageer also hopes CEAA’s work on motion planning algorithms and vision-based navigation will feed into NASA’s Restore-L mission, which aims to launch a robotic spacecraft in 2022 that can, for the first time, meet and refuel a satellite in low-Earth orbit. If the mission is a success, the spacecraft of tomorrow could be more affordable, sustainable, and resilient.
Zoellner, A, Hultgren, E and Sun, K. Integrated Differential Optical Shadow Sensor for Modular Gravitational Reference Sensor. 2013. 8th International LISA Symposium, Stanford University | article