More Ac­cu­rate Than Ev­er – New Clocks for Fu­ture Satel­lite Nav­i­ga­tion

COMPASSO Project (Credit: DLR)

OBERPFAFFENHOFEN, Germany (DLR PR) — There are no easy comparisons to show how accurate the clocks on the Galileo satellites are. Is it a matter of fractions of seconds, or of milliseconds, perhaps? That is far too imprecise. The Galileo system uses atomic clocks that are accurate to the nanosecond. One nanosecond is a billionth of one second.

And there is more: “The iodine laser clocks being developed at the DLR Institute of Quantum Technologies will be many times more accurate than other systems,” says Felix Huber of the Galileo Competence Center at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). The more accurately time can be determined, the more accurate this can make navigation on Earth, for example.

The Galileo Competence Center in Oberpfaffenhofen was founded in 2019 and the development phase is almost complete. The Competence Center works continuously on improving technologies for the Galileo navigation system. To this end, innovations devised by DLR institutes are advanced in cooperation with industry so that they can be used for the satellites and ground systems. The DLR Institute of Quantum Technologies has contributed the iodine laser clocks to the COMPASSO project, which is being led by the Galileo Competence Center.

These iodine laser clocks are now being qualified for use in space; for this, they have to be particularly compact, robust and durable. For the COMPASSO project, the DLR Institute of Communications and Navigation has also worked with partners in industry to a develop laser terminal that transmits the data, synchronises the satellite clocks and determines distances with the utmost precision. It has also developed a frequency comb and other instruments that support experiments in space. The frequency comb transmits the optical signals in the frequency range used for satellite navigation.

The DLR Institute for Software Technology is supplying the operating software for the computer that controls the experiments. DLR Space Operations supports and oversees the preparation and implementation of the overall operation.

Minimal deviation with major impact

The Galileo satellite navigation system already offers extremely high positional accuracy and precise timing information. For navigation purposes, the satellites constantly transmit data that allow users to determine their location. The correct interpretation of the transit times between transmitter and receiver is crucial.

“An inaccuracy of a nanosecond in the time measurement would, for example, correspond to an error of 30 centimeters in the distance measurement,” explains Huber. This might not seem like much; after all, the Galileo satellites orbit Earth at an altitude of approximately 23,000 kilometers. However, this would be unacceptable in the case of road vehicles that are driving automatically.

“The atomic clocks in the satellites must match so precisely that they allow positional accuracy in the range of a few centimetres in real time,” says Huber.

The iodine laser clocks of the COMPASSO project are based on the principles of quantum mechanics. This describes physical processes at an atomic level – that is, the world of the very smallest objects. In addition to the DLR Institute of Quantum Technologies in Ulm and the DLR Institute for Satellite Geodesy and Inertial Sensing in Hanover, the Galileo Competence Center plays an important role in terrestrial and space-based quantum innovations. Research and development work for future quantum computers is also a primary focus.

High value for users

How should future systems be designed in order to achieve the greatest benefit? Which technologies have the capacity to make a difference? And which ones have market potential? The Galileo Competence Center is also exploring these questions in the Robust Precise Timing Facility (RPTF) project. Unlike COMPASSO, this does not involve qualifying technologies for space, but instead further developing the ground systems necessary for Galileo operations. Here, the focus is on determining the hardware and software for perfect time distribution in the Galileo system.

The measurement instruments on Earth can be expanded as required for this purpose. They act as a ‘team’ and are able to deliver perfect timing even if some of them fail or need to be replaced. “The reference time on the ground must always be reliable for the satellites,” explains Huber. One side-effect of the Robust Precise Timing Facility is that its stability is such that it can be used in other systems for maintenance and troubleshooting.

“The research comes from the institutes,” says Huber. In the case of the RPTF, this is mainly the DLR Institute of Communications and Navigation. The Galileo Competence Center ensures that new ideas are acted upon. It supports technology transfer so that research findings can be implemented together with partners in industry.

How does satellite navigation work?

The satellites continuously transmit data about their on-board time and their orbit. The receiver calculates the distance to the satellite by determining how long the signal has been in transit. The position is determined by receiving signals from three satellites at the same time. A fourth satellite is also required to ensure that the receiver clock runs synchronously with the satellite clocks. In turn, the satellite clocks have to be synchronised as perfectly as possible. This accuracy is not only important for transport, but also for financial transactions, energy supply and agriculture.

What is Galileo?

Galileo is the European satellite navigation system. It makes Europe independent of the satellite systems of other nations, such as GPS (USA) or GLONASS (Russia). However, Galileo can work together with other systems, and the different systems can also complement one another. At present, Galileo consists of a global network of 22 operational satellites, occupying three orbital planes. The civilian Galileo system provides navigation signals of unprecedented accuracy. The satellites are operated from two control centres, one in Fucino, Italy, and the other at the DLR site in Oberpfaffenhofen.