8 minute read
Connecting Australia to space with laser communications
By Dr David Gozzard, Forrest Fellow, International Centre for Radio Astronomy Research and the International Space Centre
Radio communications technology cannot meet the data transmission demands of future satellites and space missions. Laser communications hold the key to overcoming this data transfer bottleneck.
Laser Communications from Spacecraft
Our modern world relies on satellites and other spacecraft for communications, weather observations, crop monitoring, mapping, bushfire and disaster response, and a huge range of other applications. Thanks to ever improving cameras and sensors, newer generations of satellites are generating increasing amounts of data and, as the cost of building and launching satellites comes down, we are launching more of them. Radio communications technology does not have enough bandwidth (the amount of data that can be transmitted per second) to get all of that data down to the ground.
All communication systems are fundamentally limited in their bandwidth by the frequency they are transmitting at. Higher frequencies can carry more data per second and so have greater bandwidth. This is the main advantage of newer 5G mobile phone technology, which is currently being rolled out across the country, over older, lower-frequency 4G.
Laser communications could solve this data bottleneck. The much higher frequencies of lasers mean they have much greater bandwidths, which is why the internet uses laser transmitted through fibre optics cables to haul vast amounts of data across continents.
The lasers that will be used for communications with satellites operate at frequencies in the hundreds of terahertz (hundreds of trillions of hertz), meaning a satellite equipped with a laser transmitter will be able downlink its data to Earth at rates tens of thousands of times faster than it could with a radio transmitter. Laser communications will be critical to humanity’s sustainable use of space technology
Another advantage of laser communications is that the laser beam is a lot narrower than the beam from a radio transmitter. Even after travelling thousands of kilometres from space, the laser beam will only be a few tens of metres across at the ground. This means that, unlike radio communications, satellites in the same vicinity could transmit on the same frequency without interfering with each other, and the communications are much more secure, because it makes it extremely difficult for an eavesdropper to intercept the transmission. It also means that laser transmitters can be smaller and more energy efficient than radio transmitters, making the satellite that carries them smaller and cheaper.
However, to achieve internet-like data rates from satellites, we need to overcome atmospheric turbulence.
Atmospheric Turbulence
Pockets of air with slightly different temperature and density have different refractive indices, which means they deflect and distort the passing laser beam. At the receiver on the ground, the laser from the satellite twinkles like a star. This means that the laser beam is winking
in and out, and we lose data. This severely reduces the bandwidths and data rates that can be achieved with laser communications. Developing systems that are resistant to, or able to suppress, atmospheric turbulence is crucial to achieving high-speed satellite laser communications.
There are a variety of ways to overcome or suppress atmospheric turbulence, including methods that receive the distorted laser beam and recombine it into to one strong signal either optically or electronically.
One of the leading techniques is to use adaptive optics technology modified from astronomy. Twinkling stars are bad for astronomy, because atmospheric turbulence blurs the images of distant galaxies and nebulae they are trying to study. Large astronomical telescopes correct for atmospheric turbulence by detecting the distortion of the star light and physically deforming a mirror within
the telescope to compensate, creating a clearer image. The same technique applied to telescopes used as laser receivers will allow laser communications to achieve a reliable link through the turbulent atmosphere.
The Western Australian Optical Ground Station
In the Astrophotonics research group at the University of Western Australia (UWA), we are working to translate adaptive optics and other astronomy technologies to enable reliable space-to-ground laser links, and achieve high-speed laser communications to spacecraft around Earth, the Moon, and beyond.
We have been testing these technologies using smallscale optical terminals, and have achieved world-record stability of a high-precision laser signal bounced between buildings. (Because the atmosphere quickly gets thinner with increasing altitude, a horizontal link near the ground of only a few kilometres has the same amount of turbulence as a link all the way from the ground to space.) We are now working on demonstrating stable laser and data links to moving airborne targets, which is much harder than targeting a building that does not move.
In August 2021, we installed a 0.7 m robotic telescope on the roof of the UWA physics building. This telescope is the core of the Western Australian Optical Ground Station (WAOGS), the first laser communications ground station in the southern hemisphere. Since then, we have been working hard to commission the WAOGS, which involves programming it to track satellites by first slewing to their predicted orbital position, homing in on them using computer vision, and then locking onto the narrow laser beam using adaptive optics.
Currently, we are testing the complete integration and effectiveness of the systems by establishing robust laser links to a drone making simulated satellite passes, and measuring the stability and data transfer performance of the link. Following these tests, we aim to receive our first transmissions from a laser-equipped satellite in low Earth orbit early next year.
An Australian Optical Ground Station Network
The WAOGS is the first of a larger Australasian Optical Ground Station Network (AOGSN), which will eventually comprise four ground station in Western Australia, South Australia, the ACT, and New Zealand. The stations will work together to “handover” a satellite from one to another as it passes over Australia, increasing the opportunity to downlink valuable data. Having multiple ground stations also provides resilience against adverse weather. Thanks to Australasia’s generally clear skies, if one or more of the stations are obscured by cloud, the AOGSN will still be able to establish a link to a satellite more than 90% of the time.
The AOGSN will be used to test and develop advanced laser communications, and promises to benefit telecommunications in all areas of Australia, for example, by providing higher bandwidths to regional and remote areas, and providing greater reliability for disaster and emergency response.
Benefits to Science and Industry
Australia has a long history of supporting spacecraft communications. During the 1960s, ground stations at Carnarvon and Honeysuckle Creek, and the Parkes radio telescope in NSW, tracked and received images and data from satellites and crewed space missions. Neil Armstrong’s moonwalk during the Apollo 11 mission was famously received and relayed around the world by Honeysuckle Creek and Parkes.
Today, Australia hosts and operates deep space tracking stations for NASA in Tidbinbilla, and ESA in New Norcia, as well as other spacecraft communication and tracking stations for public and private organisations.
Australia's share of the global space economy market grew by 30 per cent from 2019 to 2020 and we hope that, with the AOGSN, we will see this trend continue. The aim of the AOGSN is to make Australia the southern hemisphere hub for advanced laser communications from Earth orbit and deep space.
Robust and stable laser links to spacecraft also have a wide range of benefits beyond communications, including fundamental and applied science.
The most precise measurements, in any field of science, are currently provided by atomic clocks, which use ultrastable frequencies of light to make ultra-precise timing measurements. Einstein’s General Theory of Relativity, which explains gravity, black holes, and the evolution of the universe shows that spacetime (and, therefore, the time counted by an atomic clock) is affected by gravity. A clock ticks more slowly on the ground than it does in space, where Earth’s gravitational field is weaker.
Using stable laser links to compare atomic clocks in space against atomic clocks on the ground will allow us to measure this time dilation with unparalleled precision, which will help physicists to refine the Theory, and build a better understanding of our universe.
Because the time measured by these clocks is affected by gravity, it means that, via these stable laser links, we can use them to make very precise measurements of the gravity of the Earth below. The Earth’s gravitational field is not uniform, and changes due to movements such as the flow of water and ice around the planet. Precise measurements of the Earth’s gravity field will be enormously beneficial to geodesy and geoscience, and the sustainable use of resources.
These measurements, and the ability to accurately compare atomic clocks over vast distances will also benefit positioning, navigation and timing. GPS satellites use an older generation of less precise atomic clocks. As newer generations, compared using stable laser links, come online, the improved navigation and timing accuracy these systems would offer will benefit a huge range of industries, from robotics to finance.
Small steps in laser communications technology will lead to giant leaps in humanity’s use and exploration of space. With the WAOGS, the AOGSN, and the advanced laser systems being developed and tested with these facilities, Australia will be at the forefront of spacecraft communications and support for a long time to come.
