Call deep space: how NASA accelerates interplanetary communication

“There is practically nowhere to improve technology operating on radio frequencies. Simple solutions end there. "

On November 26, 2018 at 22:53 Moscow time, NASA did it again - the InSight probe successfully landed on the surface of Mars after entering the atmosphere, descent and landing maneuvers, which were later dubbed “six and a half minutes of horror”. A suitable description, because NASA engineers could not immediately find out whether the space probe successfully sat on the surface of the planet, due to the time delay in communications between the Earth and Mars, which amounted to approximately 8.1 minutes. During this window, InSight could not rely on its more modern and powerful antennas - everything depended on old-fashioned UHF communications (this method has long been used everywhere, from broadcasting and walkie-talkies to Bluetooth devices).

As a result, critical InSight data were transmitted on radio waves with a frequency of 401.586 MHz to two satellites - Cubsat, WALL-E, and EVE, which then transmitted data at 8 kbps to 70-meter antennas located on Earth. The Cubs were launched on the same rocket as InSight, and they accompanied him on a journey to Mars to observe the landing and immediately transmit data home. Other orbital Martian ships, for example, the Martian reconnaissance satellite (MRS), were in an uncomfortable position and could not at first provide exchange of messages with the landing module in real time. Not to say that the whole landing depended on two experimental Kubsats the size of a suitcase each, but the MPC could transmit data from InSight only after an even longer wait.

The InSight landing actually tested NASA’s entire communications architecture, the Mars Network. The signal from the InSight landing module transmitted to the orbiting satellites would have reached the Earth in any case, even if the satellites failed. WALL-E and EVE were needed for instant information transfer, and they coped with it. If these Kubsats didn’t work for some reason, the IFA was ready to play their role. Each of them worked as a node on a network similar to the Internet, sending data packets through different terminals, consisting of different equipment. Today, the most effective of them is the MPC, capable of transmitting data at speeds up to 6 Mbps (and this is the current record for interplanetary missions).


Like your ISP, NASA allows Internet users to check the connection with spacecraft in real time.

Deep space communications network

With the increasing presence of NASA in space, improved messaging systems are constantly appearing, covering more and more space: at first it was a low Earth orbit, then a geosynchronous orbit and the Moon, and soon communications went deeper into space. It all started with a rude portable radio with which telemetry from Explorer 1, the first satellite successfully launched by the Americans in 1958, was received at US military bases in Nigeria, Singapore and California. Slowly but surely, this basis has evolved into today's advanced messaging systems.

Douglas Abraham, head of strategic and systems forecasting at the NASA Interplanetary Network Directorate, highlights three independently developed networks for transmitting messages in space. Near Earth Network works with spacecraft in low Earth orbit. “This is a set of antennas, mostly from 9 to 12 m. There are several large ones, 15-18 m,” says Abraham. Then, above the Earth’s geosynchronous orbit, there are several data tracking and transmitting satellites (TDRS). “They can look down at satellites in low Earth orbit and communicate with them, and then transmit this information through TDRS to the ground,” Abraham explains. “This satellite data system is called NASA's space network.”

But even TDRS was not enough to communicate with a spaceship that went far beyond the orbit of the moon to other planets. “Therefore, we had to create a network covering the entire solar system. And this is the Deep Space Network, DSN, ”says Abraham. The Martian network is an extension of DSN .

Given the length and plans, DSN is the most complex of these systems. In fact, this is a set of large antennas, from 34 to 70 m in diameter. At each of the three DSN sites, several 34-meter antennas and one 70-meter antenna work. One site is located in Goldstone (California), another near Madrid (Spain), and the third in Canberra (Australia). These sites are located approximately 120 degrees apart around the globe, and provide round-the-clock coverage for all spaceships outside the geosynchronous orbit.

34-meter antennas are the main equipment of DSN, and there are two types: old antennas of high efficiency and relatively new waveguides. The difference is that the waveguide antenna has five accurate radio frequency mirrors that reflect the signals through the pipe to the underground operator room, where the electronics that analyze these signals are better protected from all sources of interference. 34-meter antennas, working individually or in groups of 2-3 plates, can provide most of the necessary NASA communications. But for special occasions when distances become too long even for several 34-meter antennas, DSN control uses 70-meter monsters.

“They play an important role in a few cases,” says Abraham about large antennas. The first is when the spacecraft is so far from Earth that it will be impossible to establish communication with it using a smaller plate. “Good examples are the New Horizons mission, which flew far beyond Pluto, or the Voyager spacecraft located outside the Solar System. Only 70-meter antennas are able to break through to them and deliver their data to Earth, ”explains Abraham.

70-meter plates are also used when a spacecraft cannot work with an amplifying antenna, either because of a planned critical situation such as going into orbit, or because something goes completely wrong. A 70-meter antenna, for example, was used to safely return Apollo 13 to Earth. She also adopted Neil Armstrong's famous phrase, “A small step for man, a giant step for mankind.” And even today, DSN remains the most advanced and sensitive communications system in the world. “But for many reasons, she has already reached her limit,” warns Abraham. “There is practically nowhere to improve technology operating on radio frequencies.” Simple solutions end there. ”


Three ground stations 120 degrees apart


DSN plates in Canberra


DSN complex in Madrid


DSN Goldstone


Camera Room at the Jet Propulsion Laboratory

Radio, and what will happen after it

This story is not new. The history of long-distance space communications consists of a constant struggle to increase frequencies and shorten wavelengths. Explorer 1 used frequencies of 108 MHz. Then NASA introduced large antennas with better gain, supporting frequencies from the L-band, from 1 to 2 GHz. Then came the turn of the S-band, with frequencies from 2 to 4 GHz, and then the agency switched to the X-band, with frequencies of 7-11.2 GHz.

Today space communication systems are changing again - now they are moving to a range of 26-40 GHz, K _and A range. “The reason for this trend is that the shorter the wavelength and the higher the frequency, the greater the data transfer speed you can get,” says Abraham.

There are reasons for optimism, given that historically the speed of development of communications in NASA has been quite high. A 2014 research study from the Jet Propulsion Laboratory provides the following bandwidth data for comparison: if we used Explorer 1 communications technology to transfer a typical iPhone photo from Jupiter to Earth, it would take 460 times more time than the current age The universe. For Pioneers 2 and 4 of the 1960s, this would take 633,000 years. Mariner 9 from 1971 would have dealt with this in 55 hours. Today, the IFA will take three minutes to do this.

The only problem, of course, is that the amount of data received by spacecraft is growing just as fast, if not faster than the growth of transmission capabilities. Over the 40 years of operation, Voyagers 1 and 2 have produced 5 TB of information. The NISAR Earth Science satellite, scheduled to launch in 2020, will produce 85 TB of data per month. And if the satellites of the Earth can do it, the transfer of such a volume of data between the planets is a completely different story. Even a relatively quick MRS will transmit 85 TB of data to Earth for 20 years.

“The estimated data transfer rate during the exploration of Mars in the late 2020s and early 2030s will be 150 Mbps or higher, so let's calculate,” says Abraham. - If an MPC class spacecraft at a maximum distance from us to Mars can send about 1 Mbit / s to a 70-meter antenna on Earth, then an array of 150 70-meter antennas will be required to establish communication at a speed of 150 Mbit / s. Yes, of course, we can come up with ingenious ways to slightly reduce this absurd amount, but the problem obviously exists: the organization of interplanetary communication at a speed of 150 Mbit / s is extremely complicated. In addition, we are ending the spectrum of allowed frequencies. ”

As Abraham demonstrates, working in the S or X band, one mission with a bandwidth of 25 Mbps will occupy the entire available spectrum. In K_a -band is larger, but only two Mars satellites with a bandwidth of 150 Mbps will occupy the entire spectrum. Simply put, the interplanetary Internet will require more than just radio — it will rely on lasers.

The advent of optical communications

Lasers sound futuristic, but the idea of ​​optical communications can be traced back to the patent filed by Alexander Graham Bell in the 1880s. Bell developed a system in which sunlight, focused to a very narrow beam, was directed to a reflective diaphragm that vibrated due to sounds. Vibrations caused variations in the light passing through the lens into a coarse photodetector. Changes in the resistance of the photodetector changed the current passing through the phone.

The system was unstable, the volume was very low, and Bell eventually abandoned the idea. But, after almost 100 years, armed with lasers and optical fiber, NASA engineers returned to this old concept.

“We knew about the limitations of RF systems, so the Jet Propulsion Laboratory in the late 1970s and early 1980s started discussing the possibility of transmitting messages from deep space using space lasers,” said Abraham. To better understand what is possible and what is not in optical communications in deep space, the laboratory in the late 1980s organized a four-year study, the Deep Space Relay Satellite System (DSRSS), the Deep Space Relay Satellite System (DSRSS). The study was supposed to answer critical questions: what about the weather and visibility problems (after all, radio waves can easily pass through the clouds, while lasers can not)? What if the angle of the Sun-Earth-probe becomes too sharp? Does a detector on Earth distinguish a weak optical signal from sunlight? And finally how much will it all cost and will it be worth it? “We are still looking for answers to these questions,” Abraham admits. “However, the answers increasingly confirm the possibility of optical data transmission.”

DSRSS suggested that a point located above the Earth’s atmosphere is best suited for optical and radio communications. It was stated that the optical communications system installed on the orbital station would work better than any terrestrial architecture, including the iconic 70-meter antennas. In near-Earth orbit, it was supposed to deploy a 10-meter plate, and then raise it to geosynchronous. However, the cost of such a system — consisting of a satellite with a dish, a launch rocket, and five user terminals — was excessive. Moreover, the study did not even lay down the cost of the necessary auxiliary system, which would come into operation in the event of a satellite failure.

As this system, Laboratory experts began looking at the ground-based architecture described in the Ground Based Advanced Technology Study (GBATS) analytical report, conducted at the Laboratory at about the same time as DRSS. The people working on GBATS have put forward two alternative suggestions. The first is the installation of six stations with 10-meter antennas and one-meter spare antennas located 60 degrees from each other across the equator. Stations had to be built on the mountain peaks, where at least 66% of the days in the year are clear weather. Thus, 2-3 stations will always be visible to any spaceship, and they will have different weather. The second option is nine stations, grouped in groups of three, and located 120 degrees apart.

Both GBATS architectures were cheaper than the space approach, but they also had problems. Firstly, since the signals needed to pass through the Earth’s atmosphere, reception in the daytime will be much worse than at night because of the lighted sky. Despite its ingenious location, ground-based optical stations will depend on the weather. The spacecraft that directs the laser to the ground station will eventually have to adapt to bad weather conditions and re-establish communication with another station that the clouds do not block.

However, regardless of the problems, the DSRSS and GBATS projects laid the theoretical foundation for optical systems for long-distance space communications and modern developments of engineers at NASA. It only remained to build such a system and demonstrate its operability. Fortunately, only a few months remained.

Project implementation

By that time, optical data transmission in space had already taken place. The first experiment was conducted in 1992, when the Galileo probe was heading towards Jupiter, and deployed its high-resolution camera to the Earth to successfully receive a set of laser pulses sent from the 60-cm telescope of the Table Mountain Observatory and from 1.5 m of the USAF Starfire Optical telescope Range in New Mexico. At this point, Galileo was 1.4 million km from Earth, however, both laser beams hit his camera.

The Japanese and European space agencies have also been able to establish optical communications between ground stations and satellites in orbit around the Earth. Then they were able to establish a connection at a speed of 50 Mbps between two satellites. A few years ago, the German team established a 5.6 Gbps coherent optical bidirectional communication between the NFIRE satellite in low Earth orbit and the ground station in Tenerife (Spain). But all these cases were associated with Earth orbit.

The very first optical link connecting a ground station and a spacecraft in orbit near another planet of the solar system was established in January 2013. A black and white image of Mona Lisa measuring 152x200 pixels was transmitted from the next-generation satellite ranging laser station, located at the Goddard Space Flight Center in NASA, to the Lunar Reconnaissance Orbiter (LRO) at a speed of 300 bps. The connection was one-way. LRO sent the image received from Earth back over conventional radio communications. The image needed a little software error correction, but without this encoding it was easy to recognize. And at that time, the launch of a more powerful system to the moon was already planned.


From the “Lunar Reconnaissance Orbital Vehicle” project in 2013: to clear information from transmission errors introduced by the Earth’s atmosphere (left), scientists from the Goddard Space Flight Center applied Reed-Solomon error correction (right), which is actively used in CD and DVD. Typical errors include missing pixels (white) and false signals (black). A white bar indicates a short pause in the transmission.

" Researcher of the lunar atmosphere and dust environment"(LADEE) entered the orbit of the moon on October 6, 2013, and just a week later launched its pulsed laser for data transfer. This time, NASA tried to arrange two-way communication at a speed of 20 Mbps in that direction and a record speed of 622 Mbps the only problem was the mission’s short lifetime. LRO optical communication worked for only a few minutes. LADEE exchanged data with his laser for 16 hours in a total of 30 days. This situation should change when the Laser Communications Demonstration Satellite (LCRD) is launched ), on the tagged for June 2019. Its task is to show how future communication systems in space will work.

LCRD is being developed at the Jet Propulsion Laboratory at NASA in conjunction with the Lincoln Laboratory at MIT. He will have two optical terminals: one for communication in low Earth orbit, the other for deep space. The first will have to use Differential Phase Shift Keying (DPSK). The transmitter will send laser pulses with a frequency of 2.88 GHz. According to this technology, each bit will be encoded by the phase difference of consecutive pulses. It will be able to work at a speed of 2.88 Gbit / s, but this will require a lot of energy. Detectors are able to recognize the difference between pulses only in high-energy signals, because DPSK works perfectly with near-Earth communications, but this is not the best method for deep space, where it is problematic to store energy. A signal sent from Mars will lose energy,


NASA engineers prepare LADEE for testing


In 2017, engineers tested flight modems in a thermal vacuum chamber

“In fact, this is photon counting,” explains Abraham. - The short period allocated for communication is divided into several time periods. To get the data, you just need to check if the photons in each of the gaps collided with the detector. So the data is encoded in FIM ". This is similar to Morse code, only at ultrafast speed. Either there is a flash at some point or not, and the message is encoded by a sequence of flashes. “And although this is much slower than DPSK, we can still arrange optical communications at speeds of tens or hundreds of Mbps at a distance to Mars,” adds Abraham.

Of course, the LCRD project is not only these two terminals. It should also work as an Internet site in space. Three stations will work on land with LCRD: one at White Sands in New Mexico, one at Table Mountain in California, and one on the island of Hawaii or Maui. The idea is to check the switch from one ground station to another in case of bad weather at one of the stations. The mission will also verify the operation of the LCRD as a data transmitter. An optical signal from one of the stations will be sent to the satellite and then transmitted to another station - all through optical communication.

If the data transfer fails immediately, the LCRD will store it and transmit it when the opportunity arises. If the data is urgent, or if there is not enough space in the storage on board, LCRD will send it immediately through its antenna K_a -band. So, the predecessor of future satellite transmitters, LCRD will be a hybrid radio-optical system. It is such a NASA unit that needs to be placed in orbit around Mars in order to organize an interplanetary network that supports the study of deep space by humans in the 2030s.

Bringing Mars Online

Over the past year, the Abraham team has written two works describing the future of long-distance space communications, which will be presented at the SpaceOps conference in France in May 2019. One describes long-distance space communications in general, and the other ( Martian Interplanetary Network for the era of human exploration - potential problems and solutions ") a detailed description of the infrastructure that can provide an Internet-like service for astronauts on the Red Planet is proposed.

Estimates of the peak average data transfer rate were obtained in the region of 215 Mbit / s for downloading and 28 Mbit / s for downloading. The Martian Internet will consist of three networks: WiFi, covering the area of ​​research on the surface, a planetary network that transmits data from the surface to Earth, and the Earth network, a space communications network with three sites responsible for receiving this data and sending responses back to Mars.

“There are a lot of problems when developing such an infrastructure. It must be reliable and stable, even at a maximum distance of 2.67 AU to Mars. during the periods of the upper solar conjunction, when Mars hides behind the Sun, ”says Abraham. Such a connection occurs every two years and completely disrupts communication with Mars. “Today, I can’t cope with this. All landing and orbital stations that are on Mars simply lose contact with the Earth for about two weeks. With optical communications, communications losses due to solar connections will be even longer, from 10 to 15 weeks. ” For robots, such gaps are not particularly scary. Such isolation does not cause them problems, because they do not begin to get bored, do not feel loneliness, they do not need to see their loved ones. But for people, this is completely wrong.

“Therefore, we theoretically allow the commissioning of two orbital transmitters placed in a circular equatorial orbit 17300 km above the surface of Mars,” continues Abraham. According to the study, they should weigh 1,500 kg, and have on board a set of terminals operating in the X-band, K _a -band, and optical band, and be powered by solar panels with a capacity of 20-30 kW. They must support the Delay Tolerant Network Protocol - essentially TCP / IP, designed to handle the large delays that will inevitably occur on interplanetary networks. The orbital stations participating in the network should be able to communicate with astronauts and vehicles on the surface of the planet, with ground stations and with each other.

“This crossover is very important because it reduces the number of antennas required to organize data transfer at 250 Mbps,” says Abraham. His team estimates that in order to receive data at a speed of 250 Mbps transmitted from one of the orbital transmitters, an array of six 34-meter antennas will be required. This means that NASA will need to build three additional antennas at the sites of long-distance space communications, but their construction takes years, and they are extremely expensive. “But we think that two orbital stations can share data with each other and send them simultaneously at a speed of 125 Mbps, when one transmitter will send one half of the data packet and the other the other,” says Abraham. Even today, 34-meter long-range space communications antennas can simultaneously receive data from four different spaceships at once, as a result of which three antennas will be required to complete the task. “To receive two transmissions at a speed of 125 Mbps from one and the same section of the sky, as many antennas are required as to receive one transmission,” Abraham explains. “More antennas are needed only if you need to establish communication at a higher speed.”

To cope with the problem of solar connection, the Abraham team proposed launching a satellite transmitter at the L4 / L5 points of the Sun-Mars / Sun-Earth orbit. Then during periods of connection it can be used to transmit data around the Sun, instead of sending signals through it. Unfortunately, during this period, the speed will drop to 100 Kbps. Simply put, it will work, but it's bad.

In the meantime, future astronauts on Mars will have to wait a little more than three minutes to get a photo of the kitten, not counting the delays, which can be up to 40 minutes. Fortunately, until the ambitions of mankind drive us even further than the Red Planet, the interplanetary Internet will work quite well most of the time.