MSL Picture of the Day: T-3 Days: Communications

MSL Picture of the Day: T-3 Days: Communications

Without communications no mission; it is a simple as that.

Like all of NASA’s interplanetary missions, the Mars Science Laboratory will rely on the agency’s Deep Space Network to communicate with the spacecraft and to track it during flight. The network has groups of antennas at three locations: at Goldstone in California’s Mojave Desert; near Madrid, Spain; and near Canberra, Australia.
These locations are about one-third of the way around the world from each other. That assures, whatever time of day it is on Earth, at least one of them will have the spacecraft in view during its trip from Earth through landing. Also at least one location will have Mars in view at any time during the rover’s Mars-surface operations.

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. Each complex is equipped with one antenna 70 meters (230 feet) in diameter, at least two antennas 34 meters (112 feet) in diameter, and smaller antennas. All three complexes communicate directly with the control hub at NASA’s Jet Propulsion Laboratory, Pasadena, California.

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 will track the signals from the spacecraft to minimize risk of loss of communication. During the landing operations phase on the martian surface, the Mars Science Laboratory is expecting to utilize the Multiple Spacecraft Per Aperture (MSPA) capability of the DSN, which allows a single DSN antenna to receive downlink from up to two spacecraft simultaneously.

The rover’s downlink sessions (when the rover sends information back to Earth) will generally be limited to a couple of hours at a stretch, with perhaps two downlink sessions per martian day (sol). MSPA allows only one spacecraft at a time to have the uplink, and it is expected that the rover will command early in each sol (martian day) for roughly an hour to provide the instructions for that sol’s activities.

As the spacecraft travels from Earth to Mars during the cruise and approach phases of the mission, it communicates directly with Earth in the X-band portion of the radio spectrum (at 7 to 8 gigahertz). For this, the spacecraft uses a transponder and amplifier in the spacecraft’s descent stage and two antennas.

One of the antennas, the parachute low-gain antenna, is on the aeroshell’s parachute cone, which is exposed through the center of the cruise stage.

The other, the medium-gain antenna, is mounted on the cruise stage. The parachute lowgain antenna provided communications during the early weeks of the cruise to Mars, and will do so again starting shortly before cruise-stage separation. For most of the voyage, the job switched to the medium-gain antenna, which provides higher data rates but requires more restrictive pointing toward Earth.

The telecommunications system provides position and velocity information for navigation, as well carrying data and commands. Communication during atmospheric entry, descent and landing is a high priority. Landings on Mars are notoriously difficult. If this landing were not successful, maintaining communications during the entry, descent and landing would provide critical diagnostic information that could influence the design of future missions.

All three orbiters currently active at Mars — NASA’s Mars Odyssey and Mars Reconnaissance Orbiter and the European Space Agency’s Mars Express — will be at positions where they can receive transmissions from the Mars Science Laboratory spacecraft during its entry,descent and landing.

These transmissions to the orbiters use the ultra-high frequency (UHF) portion of the radio spectrum (at about 400 megahertz) from three different UHF antennas. The parachute UHF antenna, mounted on the back shell, transmits information from a few minutes before atmospheric entry until the rover and descent stage separate from the back shell. At that point, the descent UHF antenna on the descent stage takes over. When the rover drops away from the descent stage on its sky-crane bridle, the rover UHF antenna is exposed to begin transmissions that continue through landing.

The orbiters relay to Earth via X-band the information they receive from the Mars Science Laboratory during this critical period. Only Odyssey relays the information immediately, however. The other two orbiters record data from the Mars Science Laboratory spacecraft, hold it onboard, and send it to Earth hours later.

The Odyssey relay, called “bent pipe,” is what the flight team and the public will rely upon on landing day for step-by-step information about the latter part of the descent and landing. Odyssey will not begin receiving transmissions from Mars Science Laboratory until about two minutes after atmospheric entry. After first acquisition of signal, the orbiter may lose and regain the signal more than once as the descending spacecraft goes through changes in configuration. Odyssey will be in position to continue receiving and relaying information from Curiosity for about half a minute to more than two minutes after the rover lands.
Then the orbiter will drop below the horizon from the landing-site perspective.

Mission engineers are uncertain how soon after landing the signal will be lost, because of uncertainty in duration of the entry, descent and landing process, and the possibility that Curiosity could land where a hill or other obstruction blocks the line of sight between the rover and the orbiter.

The Mars Science Laboratory spacecraft will also transmit in X-band during its entry, descent and landing process. This is the expected path for confirmation of the initial events in the process. Due to signal strength constraints, these transmissions will be simple tones, comparable to semaphore codes, rather than full telemetry. The Deep Space Network will listen for these direct-to-Earth transmissions.

However, Earth will go out of view of the spacecraft, “setting” below the Martian horizon, partway through the descent, so the X-band tones will not be available for confirming the final steps in descent and landing. The X-band antenna in use from cruise-stage separation until atmospheric entry is the parachute low-gain antenna located on the back shell. Then, transmissions are shifted to a tilted low-gain antenna, also on the back shell. This tilted antenna will transmit tones during the banking manoeuvres of the guided entry.

About five minutes after the spacecraft enters the atmosphere, possibly shortly after the parachute opens, Earth will set, ending receipt of X-band tones. By then, the bent-pipe relay via Odyssey may have begun.

Radio transmissions travel at the speed of light. The distance between Mars and Earth on Curiosity’s landing day, 248 million kilometres (154 million miles), means the signal takes 13.8 minutes to cross at light speed.

The whole process of entry, descent and landing takes about seven minutes. By the time any transmissions could be reaching Earth with confirmation of the first events of that process, Curiosity will actually already be on the surface of Mars, whether the landing was successful or not.

The communication links are not necessary for a successful landing. Under some scenarios of communication difficulties, the flight team on Earth could have no confirmation of safe landing for a day or more and still recover a successful mission after regaining communication.
During Mars surface operations, the rover Curiosity has multiple options available for receiving commands from mission controllers on Earth and for returning rover science and engineering information.

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.

During the landing of the Mars Science Laboratory rover similare tones will aid engineers in keeping track of their spacecraft’s status during entry, descent and landing. The mission will always have coverage through the Mars Reconnaissance Orbiter, and the Mars Odyssey and, for a few minutes at the start of the EDL, real-time direct-to-Earth coverage with tones.

Curiosity has the capability to communicate directly with Earth via X-band links with the Deep Space Network. This capability will be used routinely to deliver commands to the rover each morning on Mars. It can also be used to return information to Earth, but only at relatively low data rates — on the order of kilobits per second — due to the rover’s limited power and antenna size, and to the long distance between Earth and Mars.

Curiosity will return most information via UHF relay links, using one of its two redundant Electra-Lite radios to communicate with a Mars orbiter passing overhead. In their trajectories around Mars, the Mars Reconnaissance Orbiter and Mars Odyssey orbiter each fly over the Curiosity landing site at least twice a day: once each afternoon and once each morning before dawn.

While these contact opportunities are short in duration, typically lasting only about 10 minutes, the proximity of the orbiters allows Curiosity to transmit at much higher data rates than the rover can use for direct-to-Earth transmissions. 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 will only be between 257 and 400 kilometers (160 and 250 miles) above the surface of Mars, the rover won’t have to “yell” as loudly (or use as much energy to send a message) to the orbiters as it will to the antennas on Earth.

The rover can transmit to Odyssey at up to about 0.25 megabit per second and to the Mars Reconnaissance Orbiter at up to about 2 megabits per second. The orbiters, with their higher-power transmitters and larger antennas, then take the job of relaying the information via X-band to the Deep Space Network on Earth. Mission plans call for the return of 250 megabits of Curiosity data per Martian day over these relay links. The links can also be used for delivering commands from Earth to Curiosity.

While not planned for routine operational use during Curiosity’s surface mission, the European Space Agency’s Mars Express orbiter will be available as a backup communications relay asset should NASA’s relay orbiters become unavailable for any period of time.

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