Calling Mars: How NASA Communicates With Curiosity

So how can you get in touch with a rover on Mars? Think about it - even when Mars is at the smallest distance from Earth, the signal needs to be overcome fifty-five million kilometers! This is a really great distance. But how does a small, lonely rover manage to transfer its scientific data and beautiful full-color images so far and in such numbers? In the very first approximation, it looks like this (I tried very hard, really):




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So, in the process of transmitting information, three key “figures” are usually involved - one of the centers of space communications on Earth, one of the artificial satellites of Mars, and the rover itself. Let's start with the old woman of the Earth, and talk about the centers of space communications DSN (Deep Space Network).

Space communications stations

Any of NASA’s space missions is designed to be able to communicate with the spacecraft 24 hours a day (or at least, whenever it can be possible in principle ). Since, as we know, the Earth rotates around its own axis rather quickly, to ensure the continuity of the signal, several points are necessary for receiving / transmitting data. These are the points and stations are DSN. They are located on three continents and are approximately 120 degrees apart from each other, which allows them to partially overlap each other's areas of operation, and, honoring this, “drive” the spacecraft 24 hours a day. For this, when the spacecraft leaves the zone of one of the stations, its signal is transferred to another.

One of the DSN complexes is located in the USA (Goldstone complex), the second is in Spain (about 60 kilometers from Madrid), and the third is in Australia (approximately 40 kilometers from Canberra).



Each of these complexes has its own set of antennas, but in terms of functionality all three centers are approximately equal. The antennas themselves are called DSS (Deep Space Stations), and have their own numbering - antennas in the USA are numbered 1X-2X, antennas in Australia - 3X-4X, and in Spain - 5X-6X. So, if you hear “DSS53” somewhere, you can be sure that this is one of the Spanish antennas.

To communicate with the rovers most often used complex in Canberra, so let's talk about it a little more.

The complex has its own website where you can find quite a lot of interesting information. For example, very soon, on April 13 of this year, the DSS43 antenna will be 40 years old.

In total, at the moment, the station in Canberra has three active antennas: DSS-34 (with a diameter of 34 meters), DSS-43 (an impressive 70 meters) and DSS-45 (again 34 meters). Of course, over the years of the center, other antennas were used, which for various reasons were decommissioned. For example, the very first antenna - DSS42 - was discontinued in December 2000, and DSS33 (11 meters in diameter) was decommissioned in February 2002, after which it was transported to Norway in 2009 to continue its work as an instrument for studying the atmosphere.

The first of the mentioned working antennas, DSS34 , was built in 1997 and became the first representative of the new generation of these devices. Its distinctive feature is that the equipment for receiving / transmitting and processing the signal is not located directly on the plate, but in the room under it. This made the plate much easier, and also made it possible to service the equipment without stopping the operation of the antenna itself. DSS34 is an antenna-reflector, the scheme of its work looks like this:


As you can see, under the antenna there is a room in which all the processing of the received signal is carried out. With a real antenna, this room is underground, so you will not see it in the photos.


^{DSS34, clickable}


DSS43 (which will soon have an anniversary) is a much older specimen, built in 1969-1973, and underwent modernization in 1987. The DSS43 is the largest mobile parabolic antenna in the southern hemisphere of our planet. A massive construction weighing more than 3,000 tons is rotated on an oil film about 0.17 mm thick. The surface of the plate consists of 1272 aluminum panels, and has an area of ​​4180 square meters.

^{DSS43, clickable}


DSS45 . This antenna was completed in 1986, and was originally designed to communicate with Voyager 2, who studied Uranus. It rotates on a circular base with a diameter of 19.6 meters, using 4 wheels for this, two of which are leading.

^{DSS45, clickable}


If we talk about the space communications station as a whole, then there are four main tasks that it should perform:
Telemetry - to receive, decode and process telemetry data from spacecraft. Usually this data consists of scientific and engineering information transmitted over the air. The telemetry system receives data, monitors their changes and compliance with the norm, and sends them to validation systems or research centers involved in their processing.
Tracking - the tracking system should provide the possibility of two-way communication between the Earth and the spacecraft, and carry out calculations of its location and velocity vector for the correct positioning of the torch.
Control - gives specialists the opportunity to transfer control commands to the spacecraft.
Monitoring and control - let you monitor and control the systems of the DSN itself

It is worth noting that the Australian station currently serves about 45 spacecraft, so the time schedule for its operation is clearly regulated, and it is not so easy to get additional time. Each antenna also has the technical ability to serve up to two different devices simultaneously.

So, the data that should be transmitted to the rover, send to the DSN station, from where they go on their short (5 to 20 minutes) space trip to the Red Planet. Let's now move on to the rover itself. What means of communication does he have?

Curiosity

Curiosity is equipped with three antennas, each of which can be used to receive and transmit information. These are UHF antenna, LGA and HGA. [1] All of them are located on the “back” of the rover, in various places.

[one]
HGA - High Gain Antenna
MGA - Medium Gain Antenna
LGA - Low Gain Antenna
UHF - Ultra High Frequency
Since the abbreviations HGA, MGA and LGA already have the word antenna in themselves, I will not reassign this word to them, unlike the abbreviation UHF.


^{We are interested in RUHF, RLGA, and High Gain Antenna}

UHF antenna is used most often. With its help, the rover can transmit data via the MRO and Odyssey satellites (which we will discuss later) at a frequency of about 400 megahertz. The use of satellites for signal transmission is preferable because they are in the field of view of DSN stations much longer than the rover itself, sitting alone on the surface of Mars. In addition, since they are much closer to the rover, the latter needs to expend less energy to transfer data. The transfer rate can reach 256kb / s for Odyssey and up to 2 Mb / s for MRO. Most of the information coming from Curiosity passes through the MRO satellite. The UHF antenna itself is located at the rear of the rover, and looks like a gray cylinder.

Curiosity also has HGA, which it can use to receive commands directly from Earth. This antenna is movable (it can be sent to the side of the Earth), that is, to use it, the rover does not have to change its location, simply turn the HGA in the right direction, and this allows you to save energy. HGA is mounted approximately in the middle of the left side of the rover, and is a hexagon with a diameter of about 30 centimeters. HGA can transmit data directly to Earth at a speed of about 160 bps to 34-meter antennas, or at speeds up to 800 bps to 70-meter.

Finally, the third antenna is the so-called LGA.
It sends and receives signals in all directions. LGA works in the X-band (7-8 GHz). Nevertheless, the power of this antenna is quite small, and the transmission rate leaves much to be desired. Because of this, it is mainly used to receive information, and not to transmit it.
In the photo, the LGA is a white turret in the foreground.
In the background is a UHF antenna.

It is worth noting that the rover generates a huge amount of scientific data, and not always all of them manage to send. NASA specialists prioritize importance: the information with the highest priority will be transferred first, and the information with the lower priority will wait for the next communication window. Sometimes a part of the least important data does have to be deleted.

Odyssey and MRO satellites

So, we figured out that usually in order to communicate with Curiosity you need an “intermediate link” in the form of one of the satellites. Due to this, it is possible to increase the time during which communication with Curiosity is possible at all, as well as to increase the transmission speed, since the more powerful satellite antennas are capable of transmitting data to Earth at a much higher speed.

Each of the satellites has two communication windows with a rover in each sol. Usually these windows are quite short - just a few minutes. In case of emergency, Curiosity can also contact the Mars Express Orbiter satellite of the European Space Agency.

Mars odyssey

^{Mars odyssey}
The Mars Odyssey satellite was launched in 2001 and was originally intended to study the structure of the planet and search for minerals. The satellite has a size of 2.2x2.6x1.7 meters and weighs more than 700 kilograms. The height of its orbit varies from 370 to 444 kilometers. This satellite was actively used by previous rovers: about 85 percent of the data received from Spirit and Opportunity were transmitted through it. Odyssey can communicate with Curiosity in the UHF range. As for the means of communication, it has HGA, MGA (medium gain antenna), LGA and UHF antenna. Basically, HGA with a diameter of 1.3 meters is used to transmit data to Earth. Transmission is carried out at a frequency of 8406 MHz, and data is received at a frequency of 7155 MHz. The angular size of the beam is about two degrees.


^{Satellite Instrument Location}

Communications with rovers are carried out using a UHF antenna at frequencies of 437 MHz (transmission) and 401 MHz (reception), the data exchange rate can be 8, 32, 128 or 256 kb / s.

Mars Reconnaissance Orbiter

^Mro

In 2006, MRO joined the Odyssey satellite, the Mars Reconnaissance Orbiter, which today is the main interlocutor of Curiosity.
However, in addition to the work of the signalman, the MRO itself has an impressive arsenal of scientific instruments, and, most interestingly, is equipped with a HiRISE camera, which is essentially a reflecting telescope. Being at an altitude of 300 kilometers, HiRISE can take pictures with a resolution of up to 0.3 meters per pixel (for comparison, satellite images of the Earth are usually available with a resolution of about 0.5 meters per pixel). The MRO can also create stereo pairs of the surface with an accuracy of up to 0.25 meters. I highly recommend that you familiarize yourself with at least a few pictures that are available, for example, here . What is, for example, this image of Victoria crater (clickable, the original is about 5 megabytes):


I suggest that the most attentive find on the image of the rover Opportunity;)


Note that most color shots are made in an extended range, so if you stumble upon a picture on which part of the surface is bright blue-greenish, don't rush to conspiracy;) But you can be sure that in different pictures the same breeds will have the same color. However, back to the communication systems.

The MRO is equipped with four antennas that coincide in purpose with the antennas of the rover - a UHF antenna, an HGA, and two LGAs. The main antenna used by the satellite - HGA - has a diameter of three meters, and works in the X-band. That it is used to transmit data to Earth. The HGA is also equipped with a 100-watt signal amplifier.


^{1 - HGA, 3 - UHF, 10 - LGA (both LGA are mounted directly on the HGA)}

Curiosity and MRO communicate using a UHF antenna, the communication window opens twice in a salt, and lasts about 6-9 minutes. The MRO allocates 5 GB per day for data obtained from rovers, and stores them until it is in view of one of the DSN stations on Earth, after which it transmits the data there. Data transfer to the rover is carried out on the same principle. For storage of commands that must be transferred to the rover, allocated 30 MB / sol.

DSN stations conduct MROs 16 hours a day (the remaining 8 hours the satellite is located on the back of Mars, and cannot exchange data because it is closed by the planet), 10-11 of which it transmits data to Earth. Usually, the satellite works with a 70-meter DSN antenna for three days a week, and twice with a 34-meter antenna (unfortunately it is not clear what he does in the remaining two days, but he is unlikely to have a weekend). The transfer rate can vary from 0.5 to 4 megabits per second - it decreases with the distance of Mars from the Earth and increases with the approach of two planets. Now (at the time of publication of the article) Earth and Mars are almost at the maximum distance from each other, so the transmission rate is most likely not very high.

NASA claims (there is a special widget on the satellite’s website) that during the entire period of operation, the MRO transmitted more than 187 terabits (!) Of data to Earth - this is more than all the devices sent into space before it, taken together.

Conclusion

So let's summarize. When transmitting control commands to the rover, the following happens:

• JPL specialists send commands to one of the DSN stations.
• During a session with one of the satellites (most likely it will be an MRO), the DSN station transmits a set of commands to it.
• The satellite stores the data in the internal memory, and waits for the next communication window with the rover.
• When the rover is in the access zone, the satellite transmits control commands to it.

When transferring data from the rover to Earth, it all happens in the reverse order:

• Rover stores its scientific data in the internal memory and expects the nearest communication window with the satellite.
• When the satellite is available, the rover transmits information to it.
• The satellite receives data, stores it in its memory, and waits for the availability of one of the DSN stations
• When a DSN station becomes available, the satellite sends the received data to it.
• Finally, after receiving the signal, the DSN station decodes it, and sends the received data to those for whom it is intended.

I hope I was able to more or less briefly describe the process of communication with Curiosity. All this information (in English; plus a huge pile of additional information, including, for example, fairly detailed technical reports on the principles of operation of each of the satellites) is available on various JPL sites, it is very easy to find if you know exactly what interests you.

^{Please report all errors and typos in PM!}