Let us study for this new article the need to establish a communication network in deep space. Currently, communications between Earth and outer space or between satellites are by radio waves. A signal is emitted by probes, the higher the frequency of the wave, the more it is possible, at equal energy, to transmit a large amount of data. The Ka band (a portion of the microwave part of the electromagnetic spectrum) for example which operates frequencies between twenty-seven and thirty-one GHz. These waves travel at the speed of light (around three hundred thousand kilometers per second). On a spatial scale, the distances are so great that radio communications are not instantaneous. Thus an order sent to the “Opportunity” rover on Mars takes between three minutes and twenty seconds and twenty-two minutes to reach the rover, depending on the distance from Earth to Mars.
However, it is necessary to manage to pick up this tiny signal: some stations have a satellite dish seventy meters in diameter. And these are extraordinarily sensitive since they can detect a fraction of a billionth of a billion watt. However, as the distance increases, the amount of noise on the signal becomes more and more important. So much so that parasitic noise can be deafening compared to the tiny signal sent by the probes. “The only way to counter this phenomenon is to decrease the bitrate, that is to say the amount of information (or number of bits) sent per second via the high frequency electromagnetic wave” explains CNES Francis Rocard. We therefore see that beyond a certain distance, it will no longer be possible to lower the bitrate and that the parasitic noise will cover all communications. However, the problem is rather to look for the source of energy in the case of the “Opportunity” rover on Mars.
Indeed, the nuclear energy source of the NASA rover is depleted over time. When it comes to an end and will no longer be able to recharge properly, it will no longer be able to supply energy to the vital functions of the spacecraft. In particular, the stabilisation and precise pointing of the Earth with its antenna. As a result, any communication with the sensors, which consumes a lot of energy, will then be impossible. The laser and photonics may well be a game-changer, with much faster exchanges, there may be a need to establish a communication network in deep space. Optical space telecommunications is a category of space telecommunications based on the use of lasers for data transmission. This technique makes it possible to considerably increase the speed compared to radio links while reducing the electrical power required. Indeed, space laser communication technology has the potential to provide data rates ten to one hundred times higher than traditional radio-frequency systems for the same mass and the same power.
The use of the laser in this context, however, comes up against the need for extremely precise pointing from a support possibly moving at high speed relative to the receiver, and when the latter is on the ground with problems of transparency of the atmosphere. Optical space telecommunications have been the subject of numerous experiments since the early 2000s. The use of lasers aims to respond to different contexts: growth in the volume of data transmitted by instruments on board increasingly powerful satellites, distance from Earth’s spacecraft, increasing demands from consumers. The main advantages are, on the one hand, the small divergence of the signal. This is one thousand times less important than a radio link which greatly reduces the power required to transmit the same amount of data. On the other hand, the optical frequency makes it possible to transmit a large amount of data.
As for the main drawbacks, this first requires a very fine pointing of the laser transmitter which is particularly difficult to obtain when the distance and the direction of the receiver change rapidly. But also, there is a sensitivity to the optical transparency of the atmosphere when it is a link with the Earth. Two missions are intended to test the use of the laser for space communications.
The first, LCRD (Laser Communications Relay Demonstration) is a NASA technology demonstrator designed to test the technique of optical communications between a geostationary satellite and a ground station. The transmitter must be on board an experimental Air Force satellite to be placed in orbit in 2020. Two telescopes will be mobilised on the ground to receive the data: the OCTL of the JPL center located in Table Mountain in California, with a one meter aperture telescope, and a station in Hawaii with a sixty centimetres aperture telescope. Both stations are also equipped with a laser transmitter. After that, NASA plans to launch in 2022 the Psyche space probe equipped with an optical telecommunications system DSOC (Deep Space Optical Communication). The space probe must be placed in orbit around an asteroid which circulates at a distance from the Sun of between two and a half and three and a half Astronomical Units. This time, it will be a question of testing laser communication over an even greater distance: it will be a delicate operation since it will take into account the rotation of the Earth and the proper movements of the probe and the asteroid.
“High data rates will allow scientists to gather research data faster, study sudden events like dust storms and spacecraft landings, and even send videos from the surface of others planets” says NASA. In short, very high speed in space will change the lives of astronomers.
However, NASA does not intend to get rid of radio waves anytime soon. Because these propagate in all weathers, unlike the laser which risks being blocked by the clouds or disturbed by the atmosphere of a planet. In addition, the light beams have no interest in functions that do not require a large flow, such as piloting instructions. In addition, it is also necessary to adapt the current installations: “Lasers need to have ground infrastructures that do not yet exist. NASA’s Deep Space Network, a system of antenna networks spread across the globe, is based entirely on radio technology”. It will therefore be necessary to build new stations where the sky is clear.
In addition, NASA seeks to develop photonic telecommunications by equipping its satellites with photonic modems, that is to say, which study photons either as waves or as particles, in a classical or quantum approach. The field of study of photonics covers the entire light spectrum from terahertz to X-rays. This spectrum therefore includes lasers but also goes beyond them. NASA explains “Integrated photonics looks like an integrated circuit, except that it is light that is used in place of electrons, to take advantage of the wide variety of uses offered by optics”. The advantage of this device is above all its very small size, but also its lower energy cost. NASA intends to use this experience to improve its knowledge of photonics and to extend its research in other fields. This is what can be said concerning the need to establish a communication network in deep space.
This article was written by Solène FAUQUEUX (Paris-Saclay).