Goldstone antenna

The NASA Deep Space Network (DSN) is an international network of antennas that provide the communication links between the scientists and engineers on Earth to the missions in space and on Mars.

The DSN consists of three deep-space communications facilities placed approximately 120 degrees apart around the world: at Goldstone, in California's Mojave Desert; near Madrid, Spain; and near Canberra, Australia. This strategic placement permits constant observation of spacecraft as the Earth rotates on its own axis.

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Size and Strength of the DSN Antennas

The DSN antennas are extremely large: 34 meters (about 37 yards) and 70 meters (about 76 yards). These enormous antennas enable humans to reach out to spacecraft millions of miles away. The larger the antenna, the stronger the signal and greater the amount of information the antenna can send and receive. The Mars Science Laboratory, while in its cruise stage configuration, communicated through low and medium-gain antennas. While Curiosity is roving on the planet, it is communicating with the Mars Reconnaissance Orbiter via its UHF antenna and to the DSN on Earth by way of its high-gain antenna.

Preventing Busy Signals

The Deep Space Network (DSN) communicates with nearly all spacecraft flying throughout our solar system. Many spacecraft are cruising in space, observing Saturn, the sun, asteroids and comets. In addition, the Mars Exploration Rovers are still busy on the surface of Mars and NASA's Mars Reconnaissance Orbiter has joined the other martian orbiters. The DSN antennas are extremely busy trying to track all of these space missions at once. The Mars Science Laboratory spacecraft must therefore share time on the DSN antennas. A sophisticated scheduling system with a team of hundreds of negotiators around the world ensures that each mission's priorities are met.

During critical mission events, such as landing on Mars, multiple antennas on Earth and the Mars Reconnaissance Orbiter track the signals from the spacecraft to minimize risk of loss of communication. During the landed operations phase on the martian surface, the Mars Science Laboratory utilizes the Multiple Spacecraft Per Aperture (MSPA) capability of the DSN, which allows a single DSN antenna to receive downlink from up to four spacecraft simultaneously, as well as using the relay capabilities of the Mars Odyssey (ODY) and Mars Reconnaissance Orbiter (MRO) spacecraft.

The rover's downlink sessions (when the rover sends information back to Earth) are generally roughly 15 minutes each, with usually two downlink sessions per relay orbiter (ODY, MRO) per martian day (sol), with two sessions overnight and two sessions in the late martian afternoon. MSPA allows only one spacecraft at a time to have the uplink, and Curiosity commands early in each sol (martian day) for roughly 30 minutes to provide the instructions for that sol's activities.


How the DSN helps engineers navigate the spacecraft during cruise

During cruise, the Deep Space Network antennas pick up signals from the spacecraft that tell navigators where the spacecraft are located. Engineers cannot physically see the spacecraft with the naked eye or a telescope, and they rely on radio "tracking" to know where the spacecraft are at any given time. Like a game of "Marco-Polo," the DSN listens for signals from outer space and can detect where the spacecraft is from where the sound comes from.

This navigation service is called "tracking coverage" and it includes Doppler, ranging and delta differential one-way ranging, or "Delta DOR."

Doppler Data

In order to calculate the speed that a spacecraft is flying, engineers use Doppler data to plot velocity along the line of sight between Earth and the spacecraft.

Most people are familiar with the phenomenon of a car horn or train whistle changing its frequency as it moves towards or away from them. Electromagnetic radiation (e.g. light waves or radio signals) also experience this effect. The size of the frequency shift, or "Doppler shift," depends on how fast the light source is moving relative to the observer. Astronomers often refer to the "redshift" and "blueshift" of visible light, where the light from an object coming towards us is shifted to the blue end of the spectrum (higher frequencies), and light from an object moving away is shifted towards the red (lower frequencies).

The Mars Science Laboratory spacecraft commmunicates with controllers on the ground by radio signals. Ground controllers know the frequency of the signal that is emitted from the spacecraft. However, since the spacecraft is moving away from (or towards) us, this frequency is being Doppler shifted to a different frequency. So, engineers (or, more accurately, computers) compare the received frequency with the emitted frequency to get the Doppler shift. It's then straightforward to find the velocity that would cause the resulting Doppler shift.


Ranging is sending a code to the spacecraft, having the spacecraft receive that code and immediately send it back out the spacecraft's own antenna, and finally receiving that code back on Earth. The time between sending the code and receiving the code, minus the delay in turning the signal around on the spacecraft, is twice the light time to the spacecraft. So that time, divided by two and multiplied by the speed of light, is the distance from the DSN station to the spacecraft. This distance is accurate to about five to ten meters (16-33 feet), even though the spacecraft may be 200 billion meters away!

Delta Dor

Delta DOR is similar to ranging, but it also takes in a third signal from a naturally occurring radio source in space, such as a quasar, and this additional source helps scientists and engineers gain a more accurate location of the spacecraft.

Quasars are a few billion light years away and a few billion years in the past. Quasars are used as extremely well known positions in the sky to provide a calibration for the same measurements made within a few tens of minutes of each other on a spacecraft. Being able to do quasar and spacecraft ranging near the same time and subtracting the answers cancels a lot of errors that are the same in both measurements from the atmosphere and the equipment.

The "ranging" is not really ranging, but differenced ranging. What is measured is the difference in the distance to the source between two complexes on Earth (for example, Goldstone and Madrid or Goldstone and Canberra). From that an angle in the sky can be determined relative to the stations. The angle for the quasar is subtracted from the angle of the spacecraft, giving the angular separation of the quasar and the spacecraft. That angle is accurate to about five to ten nanoradians, which means when the spacecraft is near Mars, say 200 million kilometers away, it can determine the position of the spacecraft to within one kilometer (0.6 miles).


Special signal tones the DSN received during entry, descent, and landing

During the entry, descent and landing phase of the Mars Exploration Rover mission, engineers listened anxiously for 128 distinct tones that indicated when steps in the process were activated; one sound indicated the parachute deployed, while another signaled that the airbags had inflated. These sounds were a series of basic, special individual radio tones.

The Mars Science Laboratory spacecraft transmitted in X-band during its entry, descent and landing process, which was the expected path for confirmation of the initial events in the process. Due to signal strength constraints, these transmissions were simple tones, comparable to semaphore codes, rather than full telemetry. The Deep Space Network listened for these direct-to-Earth transmissions. However, Earth went out of view of the spacecraft, “setting” below the Martian horizon, partway through the descent, so the X-band tones were not available for confirming the final steps in descent and landing. By then, the bent-pipe relay via Odyssey had begun.


How the rover can communicate through Mars-orbiting spacecraft

Not only does the rover send messages directly to the DSN stations, but it is also able to uplink information to other spacecraft orbiting Mars, utilizing mainly the Mars Reconnaissance Orbiter and Mars Odyssey (if necessary) spacecraft as messengers that pass along news to Earth for the rover. The respective spacecraft mainly "talk" via their UHF antennas. The Mars Reconnaissance Orbiter carries an Electra UHF payload with the capability of helping navigate the Mars Science Laboratory safely toward Mars. The Ka-Band package aboard the Mars Reconnaissance Orbiter can serve as another possible pipeline to "talk" to the Mars Science Laboratory (read more about the Mars Reconnaissance Orbiter Engineering Instruments).

The benefits of using the orbiting spacecraft are that the orbiters are closer to the rover than the DSN antennas on Earth and the orbiters have Earth in their field of view for much longer time periods than the rover on the ground.

Because the orbiters are only between 160 and 250 miles (257 and 400 kilometers) above the surface of Mars, the rover doesn’t have to "yell" as loudly (or use as much energy to send a message) to the orbiters as it does to the antennas on Earth.

X-band Radio Waves

X-band radio waves used by the rover to communicate

The rover communicates with the orbiters and the DSN through radio waves. They communicate with each other through X-band, which are radio waves at a much higher frequency than radio waves used for FM stations.

X-band radio waves
Curiosity Spotted on Parachute by Orbiter
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The radio waves to and from the rover are sent through the orbiters using UHF antennas, which are close-range antennas that are like walkie-talkies compared to the long range of low-gain and high-gain antennas. All three orbiters active at Mars — NASA’s Mars Odyssey and Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express — were at positions where they could receive transmissions from the Mars Science Laboratory spacecraft during its entry, descent and landing. Only Odyssey relayed the information immediately, however. The other two orbiters recorded Mars Science Laboratory data from the Mars Science Laboratory spacecraft, holding it onboard, and sending it to Earth hours later. Mars Reconnaisance Orbiter even captured images of the spacecraft on its parachute during entry, descent and landing.

The cruise stage had two antennas that were used to communicate with the Earth. The low-gain antenna was omni-directional and was used when the spacecraft was near the Earth. Because it radiated in all directions, the low-gain antenna did not need to be pointed at the Earth to enable a communications link. The medium-gain antenna was a directional antenna that had to be pointed toward the Earth for communications, but had more power to communicate when the spacecraft was farther away from the Earth. The medium-gain antenna acted like a floodlight and could direct the energy into a tighter beam to reach Earth. Just like a floodlight directs more light into a focused area than a normal light bulb does out of a lamp, the medium-gain antenna could direct the data from the spacecraft into a tighter beam than the low-gain antenna.

When the rover speaks directly to Earth (from the surface of Mars), it sends messages via its high-gain antenna (HGA). The high-gain antenna can send a "beam" of information in a specific direction and it is steerable, so the antenna can move to point itself directly to any antenna on Earth. The benefit of having a steerable antenna is that the entire rover doesn't necessarily have to change positions to talk to Earth. Like turning your neck to talk to someone beside you rather than turning your entire body, the rover can save energy by moving only the antenna.

Data Rates/Returns

The data rate direct-to-Earth varies from about 500 bits per second to 32,000 bits per second (roughly half as fast as a standard home modem). The data rate to the Mars Reconnaissance Orbiter is selected automatically and continuously during communications and can be as high as 2 million bits per second. The data rate to the Odyssey orbiter is a selectable 128,000 or 256,000 bits per second (4-8 times faster than a home modem).

An orbiter passes over the rover and is in the vicinity of the sky to communicate with the rover for about eight minutes at a time, per sol. In that time, between 100 and 250 megabits of data can be transmitted to an orbiter. That same 250 megabits would take up to 20 hours to transmit direct to Earth! The rover can only transmit direct-to-Earth for a few hours a day due to power limitations or conflicts with other planned activities, even though Earth may be in view much longer.

Mars is rotating on its own axis so Mars often "turns its back" to Earth, taking the rover with it. The rover is turned out of the field of view of Earth and goes "dark," just like nighttime on Earth, when the sun goes out of the field of view of Earth at a certain location when the Earth turns its "back" to the sun. The orbiters can see Earth for about 2/3 of each orbit, or about 16 hours a day. They can send much more data direct-to-Earth than the rover, not only because they can see Earth longer, but also because they have a lot of power and bigger antennas than the rover.