The need for a Deep Space GPS

For this new article on Space Legal Issues, let us have a look at the need for a Deep Space GPS. Currently, spacecraft travelling beyond Earth rely on radio instructions from Earth stations, where large atomic clocks calculate the ideal trajectories for their journeys. Atomic clocks are the most precise timing devices ever invented to date.

Current navigation, however, has many limitations. One of them is to establish a constant dependence between the space object and the Earth. Another important limitation is to not allow deep space navigation. Thus, this navigation, when the range of action of the vessels will increase, communication times can be counted in minutes, even hours.

The issue is therefore clear, it is necessary to develop a Deep Space GPS system in the Solar System in order to allow probes and – possibly spacecraft with crew – to be guided autonomously to their destinations with a kind of Deep Space GPS. Not only will this allow robots to explore the outer reaches of the Solar System, but it will also ensure that astronauts on long-term space missions to Mars, or beyond, have a reliable navigation system with them.

The accuracy of the geolocation information is absolutely essential. Thus, on this precision depends your ability to find your way in outer space, having the main characteristics of being “big” and “empty”. According to NASA, “Accurately measuring billionths of a second could be the difference between a stable landing on Mars and missing the planet”.

There are few benchmarks to judge your position or speed, and most are too far to give accurate information. As a result, Jill Seubert from NASA explains that “Every decision to change direction begins with three questions: where am I? How fast am I moving? And in what direction?”. The best way to answer these questions is to examine objects for which the answers are already known, such as radio transmitters on Earth, or GPS satellites following known orbital tracks in outer space. “Send a signal at the speed of light with the precise time at point A and measure the time it takes to get to point B. This tells you the distance between A and B. Send two other signals from two other places, and you will have enough information to determine exactly where point B is in three-dimensional space” (this is how your phone’s GPS software works: by constantly checking the differences in minutes in the time signatures broadcast by different satellites in orbit).

Today, NASA relies on a similar but less precise system to navigate in outer space, said Jill Seubert. Most atomic clocks and broadcasting equipment are on Earth. They collectively form what is called the “Deep Space Network”. For example, NASA cannot generally calculate the position and speed of a spacecraft at once, with three sources of information. Instead, the American Space Agency uses a series of measurements given that the Earth and the spacecraft are constantly moving through outer space, in order to define the direction of the spacecraft and its position. For a spacecraft to know where it is, it must receive a signal from the Deep Space Network, calculate the time it took for the signal to arrive, and use the speed of light to determine a distance. “To define this very precisely, you have to be able to measure these times – the times of the sent signal and the times of the received signal”.

On the ground, when we send these signals from our Deep Space Network, we have atomic clocks that are very precise” says Jill Seubert. “Up to now, the clocks we have had, which are small enough and energy efficient enough to fly on a spacecraft, are called ultrastable oscillators; something which is completely wrong. They are not ultrastable and not precise enough”. If the location data on board the spacecraft is so unreliable, it is much more complicated to figure out how to navigate – when to turn on a thruster or change course, for example – and must be done on Earth. In other words, people on Earth are driving the spacecraft hundreds of thousands or millions of kilometers away.

Still according to Jill Seubert, “If you could record this time received by the signal on board with great precision thanks to an atomic clock, you would now have the possibility of collecting all the data allowing your computer and your on-board radio to drive independently of the spaceship”. So scientists hope to overcome this ineffective back-and-forth of information between Earth and spacecraft – by miniaturising atomic clocks while increasing their accuracy so that they can fit on space probes.

NASA and other space agencies have already put atomic clocks in outer space. In fact, the entire fleet of GPS satellites carries atomic clocks. “But, for the most part, they are too inaccurate and too heavy for long-term work” said Jill Seubert. The environment in outer space is much harsher than a research laboratory on Earth. Temperatures change depending on the exposure of the clocks to sunlight. Radiation levels go up and down. “This is a well-known problem in spaceflight, and we generally send radiation-hardened parts that have proven to work in different radiation environments with similar performance”. But radiation still alters the way electronics work. And these changes have an impact on the fragile equipment used by atomic clocks to measure time, threatening to introduce inaccuracies. Several times a day, stressed Jill Seubert, “The Air Force downloads corrections to the clocks of the GPS satellites to prevent them from drifting due to their offset from the clocks on the ground”.

NASA has deployed a new, highly accurate atomic space clock that the agency hopes will “One day help spacecraft to conduct themselves in deep space without relying on terrestrial clocks”. The Deep Space Atomic Clock or DSAC is an ultra-precise and miniaturised atomic clock with mercury ions for autonomous radio navigation in deep space. This technology works by measuring the behaviour of trapped mercury ions. This clock (or Deep Space GPS) has been in orbit since June 2019, but was successfully activated for the first time on August 23, 2019. “It’s not flashy at all – just a gray box the size of a grid – loaf of four slices and lots of threads” specified Jill Seubert.

Jill Seubert declared that this miniaturization is the key. The latter is now working with her colleagues on a project to create a clock small enough to be on board any spacecraft but at the same time precise enough to guide complicated maneuvers in deep space without any outside intervention. The goal of DSAC, she said, is to establish a system that is not only portable and simple enough to be installed on any spacecraft, but also robust enough to operate in deep space for long term without requiring constant adjustments on the part of ground teams. “In addition to allowing more precise navigation in deep space, such a clock could one day allow astronauts on distant outposts to move just like we do with our mapping devices on Earth” said Jill Seubert. “A small fleet of satellites equipped with DSAC devices could orbit the Moon or Mars, functioning like terrestrial GPS systems, and this network would not need corrections several times a day”. This Deep Space GPS would help a lot.

This article was written by Thomas DURAND (Paris-Saclay).