The Perseverance rover is already transmitting from Mars
Five spacecraft currently in orbit about the Red Planet make up the Mars Relay Network to transmit commands from Earth to surface missions and receive science data back from them. Clockwise from top left: NASA’s Mars Reconnaissance Orbiter (MRO), Mars Atmospheric and Volatile Evolution (MAVEN), Mars Odyssey, and the European Space Agency’s (ESA’s) Mars Express and Trace Gas Orbiter (TGO). Credits: NASA/JPL-Caltech, ESA.
A tightly choreographed dance between NASA’s Deep Space Network and Mars orbiters will keep the agency’s Perseverance in touch with Earth during landing and beyond.
The Arabian Hope and Chinese Tianwen-1 and NASA’s Perseverance successfully reached Mars. On this occasion, it is worth answering the question of what communication with these vehicles looks like. Because the delays are definitely very long.
We already have the first photos from the Perseverance rover that landed on Mars on February 18, 2021, as well as the first photo of the rover, not just from the rover’s cameras. These are technical photos taken just after landing, photos of the wheels, as well as a photo of the rover lowered down, taken by cameras installed in the rocket module. In other texts, you will find more information about the landing itself and the Mars 2020 mission.
Everything is nice, we are waiting for more, but by the way, you may be wondering how Perseverance, as well as other vehicles on Mars and in the solar system, communicate with the Earth. Here is the answer.
We have always waited for such a photo. Perseverance at the final stage of landing lowered to the surface of Mars
How far is Mars and what does it result from?
Mars, depending on the season, is on average between 55 and 401 million kilometers from Earth. And since the fastest form of communication is electromagnetic waves, the time it takes to send information to the Red Planet will be determined by the speed of light. Finished, so if we want to pass a command to such a rover or probe or receive data, we have to wait a bit.
Machines do not care about delays, just like a man who can throw even 60 ms lag out of balance in the game. And during this time, the radio signal will cover less than 18,000 kilometers. In the case of space vehicles, the negative side of this phenomenon is the need to give up the ability to control them in real-time. The only thing that remains is autonomy, which is widely used by Perseverance, and even more so, the Ingenuity helicopter, which is to begin its 30-day mission in the next several dozen days.
Who on Earth receives and sends data from and to Mars probes, rovers, and landers
For this purpose, a network of radio telescopes called the Deep Space Network (DSN) was prepared, which is part of an even larger structure called SCaN (Space Communications and Navigation). It connects all transmitters and receivers on Earth that are used to communicate with vehicles and astronauts in space. DSN is overseen by NASA JPL. Radio telescopes, the largest of which are 70 meters in diameter, are located near Madrid in Spain, Canberra in Australia, and in the USA in Goldstone in the Mojave Desert. Distribution around the Earth minimizes the risk of communication interruptions.
China, in order to become independent from other networks, built its own radio telescope, also about 70 m in size, through which it receives a transmission from Tianwen-1. Among other things, the first pictures of the planet were taken from this orbiter.
The power of transmitters and the strength of the received signal – a gigantic difference
The transmitters installed on these antennas and aimed at the Cosmos have a power of 20 kW in the X-band (frequencies from 8 to about 12 GHz), and even 400 kW (the use of power above 100 kW requires coordination with air and space traffic control) in the S-band ( frequencies from about 2 to 4 GHz, i.e. similar to home Wi-Fi or some cellular networks). For comparison, the power of the strongest 5G base station transmitters is 120W, but usually, it is much less, and the beam is formed differently than in the case of transmission to space vehicles.
When receiving the signal, the largest antennas of the DSN network are able to catch a data beam with a power of the order of attack (it is 10 ^ -18 W). On Earth, the signal currently received from Voyager 2 has this power. Besides, the signals from the Mararsian probes are also not very strong, given the distance and limited energy budget of the probes.
For example, the MRO (Mars Reconnaissance Orbiter) has two 100W signal amplifiers for the X band each. One of the other fails. It also has an experimental transmitter operating in the Ka-band (frequencies in the range of 26-40 GHz), which transmits with a power of 35 W, but only for testing purposes.
The farther into space, the slower it becomes
The DSN also talks to other probes, but you know the farther they are from Earth, the slower the data transmission rate. A lot also depends on the power of the transmitter on a given vehicle. Voyager 1, the furthest from Earth, transmits data at a rate of 160 bps, just slightly faster than the first modems from the 1950s. To open the benchmark.pl website with this text from such a distance, you would have to wait well over a day.
How much data does the Martian probe or rover transmit during its mission?
Missions to Mars typically take a basic two years plus the duration of an extended mission, which is the most common and can extend over a decade. Probes and instruments that perform visual observations require the most throughput. You know, photos are at least megabytes of data. The same volume can contain many more numbers that characterize other measurements, atmosphere, magnetic field, temperature, etc. In turn, the signal reaching the probe from Earth is stronger, but the Voyager antenna is only 3.7 meters in diameter, which makes the signal reception efficiency much weaker than if it was a 70-meter antenna.
“Time is in favor of space probes. They don’t broadcast too quickly, but they do it persistently for years”
The MRO probe, which has been photographing Mars since 2005, has already taken over 50,000 Mars revolutions and more than 90,000 photos covering 99% of the planet’s surface (data from 2017). It also transmits transmissions from Mars rovers. Curiosity itself has already taken nearly a million raw photos (not all of them are processed into characters we like to admire). The volume of collected data on Earth received from the MRO is approaching 0.5 petabytes (estimated data for 2021).
However, MRO is a mission focused on photos and data transfer. For comparison, the Cassini orbiter, which has been studying Saturn and its moons for several years, sent to Earth only 635 GB of data, which included 453,000 photos. In turn, the Opportunity rover, which has traveled around Mars for 15 years, sent to Earth by 2018 (soon after we lost contact with it for good) over 225,000 photos.
The amount of data that is sent towards Mars is much smaller. Because these are mainly commands and commands, or software fixes (these weigh the most), the transfer of which does not require even particularly powerful transmitters.
How do a Martian probe, lander, and rover talk to the Earth?
We already know how data from Mars is received on Earth, but how is a connection from a vehicle on the Red Planet initiated? Probes are the easiest to orbit because they can conveniently call Earth directly and send large amounts of data. Such communication uses the X band most often mentioned. Two transmitters (low and high power) working on this band are also used by the Perseverance rover, just like the Curiosity.
With their help, the rover is able to call home on its own, but the data transfer rate from the high-power transmitter is 800 bps at most when the signal is received by a 70-meter antenna, or 160 bps when it is a 34-meter antenna. The low-power transmitter is only the last resort because it is a 10-bit link for transmission or 30 bit for data reception.
That is why today Curiosity and Perseverance landers and rovers usually first connect in the UHF band to their “base station” in Mars orbit, probes that have much larger transmitting antennas. For this purpose, MRO, MAVEN (Mars Atmospheric and Volatile EvolutioN), Mars Odyssey and European Mars Express, and TGO (Trace Gas Orbiter) are used. They form a network called MRN (Mars Relay Network).
Before such a network of transmitters was created, landers such as the Viking 1 and 2 had to rely on accompanying orbiters. For direct communication with Earth, transmitters with a power of 20 W and the S-band were used. With the orbiter, communication was carried out at the frequency of 381 MHz (UHF band), similar to rovers on Mars today.
The maximum speed of the Mars-Earth link
Perseverance will first send images and other data to probes in orbit using a frequency of 400 MHz, using an antenna located at the rear of the vehicle, next to the shield of a radioisotope thermoelectric generator. The link capacity from the surface to the orbit of the Red Planet is up to 2 Mbps. The efficiency of the connection with Mars orbit depends on its distance from Earth, and this, as we know, varies widely.
The maximum connection speed varies from 500 kbps when Mars is farthest from Earth to over 3 Mbps when Mars comes very close to our planet. Typically 34 meter DSN antennas are used, approximately 8 hours a day. This does not mean, however, that data is constantly transmitted at the maximum speed, which can be seen in the summary of DSN antennas.
You can also call Perseverance from Earth
You can also try to connect directly to Earth with rovers and landers on Mars. These are emergency connections or for sending only simple control commands. They use the same antennas on Earth as on the rover, and the X-band. The bandwidth to Mars is 3-4 times greater than for direct transmission from the Mars surface.
Communication breaks over which we have no influence today
Their cause is the sun. The Sun itself can interfere with data transmission from probes passing near it, but the Red Planet is simply obscured by them from time to time. And since we do not yet have a well-developed communication network in the solar system, let alone further delay from relays, every two years it takes about 10 days for Mars to slide past the sun’s disc.
Sometimes, however, there is no way out, you have to work hard and wait for the data for days or even months
Fortunately, we did not have such problems in the case of Mars, but if any of you remember the Galileo probe from the 1990s, you know what concerns it provided to ground control. Only the partially deployed transmitting antenna was not able to achieve the assumed throughput of 134 kbps. Scientists had to develop new data compression techniques in order not to lose the probe’s potential. They managed to increase the performance of the second low gain antenna from only 8-16 bps (yes, bits per second only) to 160 bps, then to around 1 kbps. It was still very little, but it was enough to save the mission.
In turn, very distant vehicles have no choice. They would have to either be equipped with very large transmitting antennas and powerful energy sources, or we accept that the transfer takes a long time. In the case of the New Horizons probe, whose transmitting antenna has a power of 12 W, after its flight near Pluto, we waited for months for the complete data sent.
Source: inf. Benchmark, NASA, ESA
