ESA - European Space Agency emblem.
May 20, 2021
In recent years, ESA has designed some of the most advanced spacecraft ever built to reach exotic locations such as the Sun, Mercury, Mars, Jupiter and the Didymos asteroids – a trend that will continue into the years ahead. As missions voyage further from Earth, it is important to consider how we can continue to communicate with them and how they will navigate through space when they are so far from home.
Deep space communication and navigation
To communicate effectively with spacecraft, we need to send and receive status, navigation and scientific data. This is achieved using ground stations on Earth. ESA operates a sophisticated system of ground stations, including three Deep Space Antennas (DSA) located around the world, providing continuous coverage as Earth rotates.
To ensure that missions meet their science objectives, ESA continues to develop technologies to communicate with them more effectively. This includes the technologies on board spacecraft, as well as those on the ground.
Tracking spacecraft deep across the void
What is Discovery & Preparation doing in this area?
Discovery & Preparation lays the groundwork for ESA's short- to medium-term future activities. The Preparation element recently ran the Open Space Innovation Platform (OSIP) Campaign 'What's Next – new ideas for space missions and concepts'. A number of ideas for new deep space missions were addressed, especially supporting future human interplanetary spaceflight, Mars exploration and missions to near-Earth objects.
As part of its provision for future deep space missions, Discovery & Preparation has conducted several investigations into future ESA's space science missions. A study that finished in 2009 developed a system to enhance the operation of these missions – which typically travel relatively far from Earth – through a flexible planning, scheduling and optimisation process. A more recent study proposed an end-to-end mission simulator to improve their efficiency.
Discovery & Preparation has also contributed significantly to ESA's Proba missions, which test new technologies in space. A 2009 study envisaged an interplanetary Proba mission – Proba-IP – to travel to a near-Earth object and validate autonomous onboard guidance, navigation and control technologies.
On top of these general investigations, Discovery & Preparation has carried out more specific studies that focus on individual deep space communication and navigation technologies.
Image above: ESA ground station network, with the three deep space tracking stations labelled with yellow boxes.
Communication – making long-distance relationships work
Communicating with distant spacecraft is difficult. The signals that pass between the spacecraft and ground stations are very weak and because of the large distances, it takes them a long time to travel between the two. It can take up to 24 minutes for a signal to travel between Earth and Mars, for example, and almost an entire day to receive a signal sent by NASA's Voyager 1 – a spacecraft that has travelled beyond the edge of the Solar System.
As there are strong constraints for equipment on board spacecraft, a lot of the more complex communications technologies are incorporated into the ground stations. Many Discovery & Preparation studies have contributed to developing such technologies.
A study that finished in 2012 explored the feasibility of developing klystrons entirely within Europe. These devices transform electrical power into amplified radio signals to send commands from ground stations. The study set the requirements and objectives for the development of such a device, as well as defining an industrial environment and potential roadmap for the future. Klystrons are now used in ESA's network of ground stations; you can discover more about them in the video at the beginning of this article.
ESA’s New Norcia station (Deep Space Antenna-1) in Australia
Another study focused on selecting the best ground station architecture for future deep space missions. By gathering data about the current performance of ESA's Deep Space Antennas, as well as collecting the needs and characteristics of future missions, the study calculated the characteristics of the ground system that would be required to meet these needs. The study noted that communicating with optical frequencies is more efficient than the more traditional radio frequencies.
To reach distant spacecraft or for intra-satellite communication, optical communication is becoming an interesting alternative to radio communication because it allows more data to be transmitted; this maximises scientific return and could enable new types of missions. However, optical signals are affected more by Earth's atmosphere.
Given the increasing demand on downlink data rate to increase science return, in 2016 the Deep Space Optical Communications Architecture Study (DOCOMAS) presented how technology needs to evolve in the future to enable optical communication between a deep space probe and Earth. The study focused on the ground segment, including cloud mitigation strategies. It identified that the key enabling technologies are dedicated optical ground antennas, novel photon detectors and a generic design approach to an optical payload terminal. The concept design was tailored for ESA's Asteroid Investigation Mission (AIM), which has evolved into the Hera mission.
DOCMAS built on the results of an earlier study that focused on developing technologies for communicating with interplanetary missions, including investigating the required optical technologies. With communications being a notorious bottleneck in interplanetary science and exploration missions, the goal was to propose a technology development roadmap to enhance communication capabilities.
Another study explored how ESA's Optical Ground Station (OGS) – usually used for communicating with nearby spacecraft – could be used to communicate with deep space missions.
Navigation – turning time into distance
Good communication is vital not only for collecting science and status data, but also for navigating spacecraft through the Solar System. To navigate spacecraft we need to know their position, which is no easy feat when they are so far away. But by measuring three parameters – distance, velocity and the angle at which a spacecraft is located in the sky – it is possible to calculate a satellite’s position down to a small box-shaped region of space.
One element that is essential to navigating deep space is timing, in particular ensuring that the time on board spacecraft is synchronised with the time on the ground. To calculate where a spacecraft is in the Solar System, we precisely measure the time it takes for electromagnetic waves to travel between the spacecraft and an antenna on Earth. Navigators on Earth then transmit course adjustments. A 2007–2009 study explored forward thinking techniques to synchronise time on board deep space probes for accurate navigation, in particular looking into low-cost options. A parallel study found that an accuracy of ten nanoseconds for a signal passing from a spacecraft to Earth is possible without using an onboard atomic clock.
Navigating with pulsars
Navigating a spacecraft to distant locations requires a team of scientists and engineers using sophisticated radios, large antennas, computers, and precise timing equipment. Whilst the DSAs have been the standard tool for navigating spacecraft in the past, the network comes with limitations and partial autonomous navigation is becoming more common. One method that has been explored more over the past decade is to navigate using pulsars – magnetised, swiftly rotating, dying stars that emit beams of electronic radiation out of their magnetic poles.
Millisecond pulsars – which have rotation periods of less than ten thousandths of a second – offer the most precise timing standard known. In a kind of celestial GPS, spacecraft can measure the time between receiving each pulse of radiation from three different pulsars, looking for tiny changes in the arrival times to pinpoint its location.
This was still a very novel idea between 2012 and 2014, when Discovery & Preparation supported two studies that explored the feasibility of deep space navigation with X-ray pulsars. The first was carried out by the UK’s National Physics Laboratory and University of Leicester, and the second by the University of Helsinki. Among other discoveries, the research found that the benefits of such a technique include increased spacecraft autonomy, improved position accuracies and much lower mission operating costs due to the substantial reduction in the use of the associated ground-based systems.
Preparing the practical test of the navigation of Hera around Didymos
Pulsars are not the only astronomical objects with the potential to be used for navigation. A 2016 study investigated the feasibility of an onboard visual navigation system for ESA’s Hera mission (then AIM), which will visit double asteroid Didymos later this decade. The system laid the path towards developing such a system; Hera will use its onboard camera to determine the position of the asteroids with respect to the background stars. Hera will also demonstrate communication with a ground station via an optical link as well as communication between the main spacecraft and two CubeSats.
What about making use of Global Navigation Satellite Systems that enable Earth-based navigation to negotiate our way further afield? Navigation satellites orbit around 22 000 kilometres above Earth's surface. As they point down towards Earth, any spacecraft below them are served well by the signals they send out. But around ten years ago, engineers started demonstrating that spacecraft outside the orbit of navigation satellites could also navigate in space using their 'spill over' signal.
Europe's constellation of Galileo GNSS satellites
In 2012, two Discovery & Preparation studies kicked off to investigate a seemingly radical question: could this spill over signal even be used to navigate our way around the Moon, and if so, what kind of receiver would we need to build to be able to use these signals? The studies – led by Deimos and Joanneaum Research – found that indeed, the signal from navigation satellites orbiting Earth could be used to navigate the Moon's surface. But with the signal being so weak, they concluded that a new type of receiver would need to be built. ESA has now invested in the development of such a receiver, and is exploring whether it could be demonstrated on the Lunar Pathfinder mission.
What else is ESA doing?
ESA has several missions already working in deep space, including Solar Orbiter, ExoMars and BepiColombo. Next year will see the launch of the JUpiter ICy moons Explorer (JUICE), which will spend at least three years observing Jupiter and three of its largest moons. In 2024, ESA’s planetary defence mission, Hera, will set off to visit an asteroid, in the process discovering more about these rocky objects and finding out if we could deflect an asteroid on collision course with Earth.
ESA's ambitious plans for the next decade of human and robotic space exploration will take us from the ISS to the Moon, a deep-space gateway and a Mars landing. Concrete steps are already being taken towards exploring the Moon; NASA's new Orion vehicle, with a European service module at its core, will build bridges to Moon and Mars by sending humans further into space than ever before.
Destination: Moon
For all robotic and human missions to the Moon, asteroids, Mars or beyond, at least one DSA is essential for communications. ESA's Operations directorate controls spacecraft – including those voyaging deep into the Solar System – and develops and manages the necessary ground infrastructure. Prior to every mission launch, Operations teams carefully design and build the ground segments that enable engineers to control satellites in space and receive and distribute their data.
ESA Operations oversees ESA's tracking station network, Estrack, the core of which comprises seven stations in seven countries, including the three DSAs. Furthermore, the directorate is currently operating the tiny OPS-SAT satellite, which is devoted to testing and validating drastically improved mission control capabilities.
ESA’s Malargüe tracking station supports many deep space missions
In addition to the daily operation of spacecraft exploring space hundreds of millions of kilometres away, ESA's operations teams continually work to develop new capabilities to support future missions, including flight-dynamics techniques, delay-tolerant networks, deep-space communication technologies and innovative satellite control software and systems.
What are other space agencies doing?
ESA shares Estrack capacity with other space agencies, who in return provide tracking services to ESA missions under a number of resource-sharing agreements. These include networks and stations operated by ASI (Italy), CNES (France), DLR (Germany), NASA's Deep Space Network and Goddard Space Flight Center and JAXA (Japan).
For example, NASA's Deep Space Network stations routinely support Mars Express (as well as other, now-completed missions such as Rosetta, Huygens and Venus Express), while Estrack is supporting Japan's Hayabusa-2 mission. In recent years, Estrack has provided support to missions operated by China and Russia, as well as tracking the descent of NASA rovers to the surface of Mars.
Other space agencies are also developing their own technologies for communicating with and navigating deep-space spacecraft. For example, NASA has developed a deep space atomic clock and an X-ray navigation device that determines the position of a spacecraft anywhere in the Solar System, and JAXA has worked on a navigation system using highly accurate 3D radar and navigation guidance control technology for rendezvous and orbit transition to the neighbourhood of the Moon.
Related links:
ESA's Operations directorate: https://www.esa.int/Enabling_Support/Operations/About_Operations
ESA's Optical Ground Station (OGS): https://nebula.esa.int/content/optical-ground-station-optical-data-links-and-deep-space-missions
Deep Space Optical Communications Architecture Study (DOCOMAS): https://nebula.esa.int/content/future-deep-space-optical-communication-architecture
Open Space Innovation Platform (OSIP): https://ideas.esa.int/servlet/hype/IMT?userAction=BrowseCurrentUser&templateName=MenuItem
Enabling & Support: https://www.esa.int/Enabling_Support
Images, Videos, Text, Credits: ESA/Sacha Berna/S. Marti/G. Ortega/P. Carril/D. Pazos, CC BY-SA 3.0 IGO/MPE.
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