The Possibilities and Limits of Space-Based Radio and Optical Communications

For nearly 60 years, communication with a spacecraft has been through radio waves. As technology and the aerospace industry evolve, new methods of communication, like lasers, are being used.

The State of the Art of Communications

Today (and for nearly 60 years), the most common way to communicate with a spacecraft is through radio waves, as they’re the most effective for long-distance communications.

Let’s take the example of the ISS (“International Space Station”). Located at an altitude of approximately 36,000 km (22,000 miles) in a geostationary orbit, relay satellites provide communication with the space station. This orbit actually allows satellites to stabilize on a fixed-line above the Earth’s surface.

There are three types of radio communication in space: communication between astronauts, communication between astronauts and the Earth and finally communication between the ship itself and the Earth. The latter requires the most advanced technology, being an essential factor for the success of the mission.

As for the broadcast to the general public, although the images we receive from the station are transmitted live, they still have a (slight) delay of about 30 seconds – due to signal processing and transmission. This signal, once emitted, is actually processed via tracking and data relay satellites (more commonly referred to as “TDRS”, for “Tracking & Data Relay Satellite”), and goes down directly to Houston.

IoT Constellations

There are today dozens of satellite constellations (several satellites operating synchronously together) including positioning systems, telecommunications services and remote sensing. These constellations all belong to major players in the aerospace sector, and more recently to political powers; China plans to launch 72 nanosatellites dedicated to IoT within 3 years, in the same way as Russia and Europe. However, after the land deployment of LoRa, Sigfox or even the most advanced cellular technologies, operators are looking to the stars. Several companies are now considering launching their own satellites into orbit.

Among these operators are Objenious and Sigfox. In 2018, Sigfox signed a partnership agreement with Eutelsat to take advantage of the nanosatellites already launched by the latter and develop its own network called “0G”. Objenious, on the other hand, intends to create a hybrid network combining terrestrial and satellite networks.

Be aware that the idea isn’t to create a new IoT protocol, but to make existing protocols compatible with spatial frequency bands. Objenious doesn’t intend to put aside the development of its LoRa module but wants to have, ultimately, only one module that supports several technologies.

Ambitious, isn’t it? By doing so, the operators intend to cover the entire territory, in particular, the areas not covered by the current solutions. Among these areas, we find a large part of the maritime territory, for which the deployment of a satellite network would be less expensive than installing antennas. If we add to this objective the advantageous taxation associated with space exploitation, the entry of operators into this 21st-century space race seems to be one of the major trends of the years to come.

How Far Will We Go?

Have you ever heard about the Voyager program? It’s a space program set up by NASA to explore the most remote planets (such as Saturn, or Jupiter). Voyager 1 is one of the two probes sent into space in 1977. It left the solar system in 2012. Even today, and more than 20 billion kilometers from Earth, the probe is still able to communicate some information with us. How is that possible? The communication system is enabled by a parabolic antenna more than 3.7m in diameter that supports both the transmission and reception of radio waves from the Earth.

Today, the probe only transmits a very small amount of information, which takes more than 20 hours to reach Earth.

The Optical Solution

Last year, the European Space Agency announced the deployment of a telecommunications satellite to become the main relay between low orbits and the Earth. Until then, these satellites in low orbit (below 36,000 km from the Earth’s surface) had to be over a relay point on Earth to communicate. This new satellite would retrieve this information and send it to ground relay points more easily accessible from its high geostationary orbit.

While communications between space and Earth are usually by radio waves, the innovative feature of the project is the use of a laser, allowing a much higher throughput than the technology used until now (10 to 100 times faster), as well as lower energy consumption. But NASA isn’t left out, and also wants to deploy a new network by 2020 by installing a photonic modem (allowing the use of an optical solution) on existing satellites and the ISS. NASA would then benefit from a smaller and less expensive solution.

Optical communications have a bright future ahead of them because, in addition to allowing a dialogue between space objects and the Earth, they also help to advance scientific research in several fields (especially meteorology), and an immediacy that’s still impossible today. NASA has even announced the possibility of sending videos from the surface of other planets!

The End of Radio Waves?

However, the radio waves haven’t had their last word yet. They still offer many advantages, such as better resistance to climate conditions. Indeed, light beams are currently not trustworthy enough in snow or rainy weather and can be disturbed by a single cloud (not to mention the atmospheric conditions of other planets). Moreover, this technology is only relevant when communications require high throughput.

And the installation is expensive! All the ground infrastructure has to be built up, as lasers have completely different technology than radio; the latter is based on NASA’s “Deep Space Network” (an international antenna network). In order to be able to extend the use of optical communication, there’s no choice but to build new stations in areas subject to good weather. In other words, there’s still a long way to go.

PS: Ever Heard of Space Hackers?

In the 1940s the US Army wanted to take advantage of the ionized trails left by meteorites when they entered the atmosphere, to perform long-distance communications. These ionized trails have the ability to bounce some radio waves.

It should be pointed out that space isn’t only the playground of the great powers! Since 1953, space hackers, those passionate about radio communication, became interested in the topic. Most communications are possible when stations “make an appointment” to transmit and receive waves. But since it’s almost impossible for them to define whether the meteor track is in the appropriate location for radio communication, the same messages are usually sent continuously until the receiving station confirms receipt of the same message. However, protocols have been established to regulate these transmissions.

By using the ionized trails of the meteors, space hackers were able to propagate radio messages without waiting for the receiving stations to be prepared in advance. By voice or Morse code, countless messages were exchanged until the 2000s, when computer programs replaced these methods (which were considered archaic) with ever more sophisticated transmission systems.