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Planetary News: Mars (2004)

Mars Express Links with Mars Exploration Rovers in Interplanetary Network Experiment

The European Space Agency's Mars Express orbiter joined forces with the Mars Exploration Rovers -- Spirit and Opportunity -- for a series of telecommunications tests this month that demonstrated an international, interplanetary network at the Red Planet.

In an experiment showing capabilities envisioned long before either the orbiter or the rovers launched, Mars Express linked with Spirit and Opportunity on four different occasions between August 3 and August 13, proving the interoperability of UHF radio relay systems between the European orbiter and American rovers.

"These tests explored three different features or modes of telecommunication, exercised higher data rates and longer communication ranges and included image data," says Brad Arnold, a telecom systems engineer in the Mars Networks Office at the Jet Propulsion Laboratory (JPL) who helped design the experiment. The three modes demonstrated how the network could be used to locate another spacecraft during critical events; transfer data at the highest possible rate; and collect Doppler phase data to pinpoint the location and velocity of a spacecraft.

Mars Express and MER have linked once before. The first trial test of an international, interplanetary communiqué took place back in February when Mars Express received engineering data from Spirit at 32 kilobits per second (KBPS) verifying communication compatibility with the rovers, The experiment conducted this month builds on that.

"The capabilities that our international teamwork is advancing this month will be important in future exploration of Mars -- and of other planets, including our own Moon," suggests JPL engineer Gary Noreen of the Mars Network Office, who worked on the protocol used in the experiment.

On August 3 and 6, Mars Express passed over Gusev Crater and tracked a beacon from Spirit while high in its orbital ellipse, at about 3728 miles [6000 kilometers] above the surface, demonstrating the 'Canister Mode' that can be used to locate another spacecraft during critical events, like the descent to a planet's surface, or for orbital rendezvous maneuvers. Because of the geometry of Mars Express' orbit and the location of the rovers during August, both the Spirit passes were 'Canister Mode' tests taken with Mars Express in the higher altitude in its orbital ellipse "to simulate data collection during an EDL event," says Arnold. The data from both of these passes was successfully relayed by Mars Express back to Earth and on into the Jet Propulsion Laboratory, where the rover team is based.

The term 'Canister Mode' refers to the means by which the data was collected, Arnold notes. "Digital data is coded prior to transmission. During reception, an orbiter radio will normally digitize the analog transmission, and then decode the data prior to retransmission by the orbiter's Earth link. In 'Canister Mode,' the received signal is digitized, but not decoded. The raw data is then relayed back to Earth where the decoding is performed as desired. This provides opportunities to evaluate characteristics of the original data that may have been lost during the orbiter's decoding process," he explains.

Given that NASA already has Mars Odyssey and Mars Global Surveyor (MGS) in orbit around the Red Planet, why would we need Mars Express?

Arnold explains: "Mars Express is in an elliptical orbit about Mars. A feature of this is that the spacecraft dwells over portions of the Mars surface for longer durations. This is well-suited for telecommunication events during entry, descent, and landing (EDL) of landers, such as the Mars Exploration Rovers (MER). In contrast, it is much more difficult to coordinate a landing during the overpass of the Odyssey orbiter, which has a low altitude circular orbit. Odyssey typically passes over a location on the Mars surface in 15 minutes, whereas Mars Express may be suited to observe the landing site for over an hour."

On August 4, Mars Express received data from Opportunity, which the rover had previously collected and stored, from much lower in its orbital ellipse, just 870 miles [1,400 kilometers] above the Martian surface or about four times closer than when it tracked Spirit's beacon. During this session, Mars Express received a hefty 42.6 megabits of data from Opportunity in about six minutes, setting a new record for international networking at another planet. The orbiter then, in another first-time demonstration, relayed that data -- which included 15 images from the rover's nine cameras and engineering telemetry -- to the European Space Operations Centre, in Darmstadt, Germany, where controllers received and directed it on to JPL in Pasadena, California, to meet the experimental objective of "a reliable transfer" of a significant amount of data.

"This test exercised the Mars Express UHF receiver at the highest possible data rate -- 128,000 bits per second," says Arnold. Not surprisingly, the team dubbed it the 128 KBPS test. This mode allows telecom systems engineers "to validate the models of the UHF relay system," he adds, and "to demonstrate the suitability of using Mars Express as a relay asset to the MER program and to future landers."

Although Mars Express and Opportunity had another 128 KBPS test 'date' August 10, that link-up was canceled by the MER team, because, says Arnold, of conflicting science and onboard power supply constraints.

In the final session of the series, on Friday, Aug. 13, Mars Express linked with Opportunity to demonstrate a mode for garnering navigational information from the Doppler shift in the radio signal. "Scientists use radio signals to evaluate the position and velocities of spacecraft such as the rovers and Mars Express -- this is crucial to successful navigation," says Arnold. "The Mars Express UFH radio has the ability to collect Doppler phase data from the telecom link, and relay that data back to Earth. The Mars Express radio was configured specifically for that measurement during the August 13 pass with Opportunity. This mode had not previously been tested on Mars Express in situ. We're still analyzing the data. Success of this test will demonstrate suitability of the Mars Express platform to support current on future navigation requirements on Mars."

As seamless and easy as these telecom link-ups between Mars Express and the rovers this month may sound, the tests that proved the protocol and the ability for a European mission and a NASA mission to successfully communicate with each other at Mars required years of groundwork on Earth.

Back in 1998, while Mars Odyssey was still in development, ESA, France's National Centre for Space Studies (CNES), and NASA began work on developing a protocol for standardizing communications techniques used for handling space data. These agencies cooperated under the auspices of the Consultative Committee for Space Data Systems (CCSDS), an international partnership of national space agencies. "We jointly developed a protocol called Proximity-1," recalls Noreen. "We first put it on Odyssey, but the Europeans jointly developed the protocol with us and they also put it on Mars Express." Using the same communications protocol, means, basically, that whatever spacecraft has that protocol -- "whether it be a rover or any other user of relay services at Mars, the rover, such as an aerobot (a robotic airplane or balloon) or a lander like Beagle 2," says Noreen, "is now possible because we worked together to establish this protocol early on."

Once spacecraft with relay radios supporting the Proximity-1 protocol reached Mars, the best times for link-ups had to be determined. In April of this year, Arnold began the process of figuring out the best times for these events to occur. "These demonstrations had to be coordinated within the existing science activities of all three platforms," he points out. "I used software tools and a precise description of the Mars Express orbit to evaluate when the best telecom events would occur to meet our test objectives. Then, in a series of conference calls and emails with both JPL and ESA, this analysis was used to narrow down the actual candidate telecom events that were performed." From there, it pretty much went like clockwork.

The importance of establishing an interplanetary communications network at Mars extends well beyond one country's occasional assistance during entry, descent and into our near-term and long-term future in space. "The interplanetary network is an extension of Earth's satellite network," Arnold posits. "As a simple example, an orbiter around Mars may allow a lander to transmit valuable data during periods it cannot see Earth. This may be desirable due to mission constraints, such as power usage. Due to the relative close proximity to the orbiter, the lander can send data at high data rates, and therefore for shorter periods of time. This saves battery power, which is a precious asset aboard a spacecraft that relies on solar energy alone."

In the future, Arnold continues, "[t]he interplanetary network will become necessary to provide reliable and efficient relay of data between robotic and manned missions and scientists back on Earth. It is analogous to existing communications satellites that support many relay functions in Earth orbit today, including satellite phones, satellite radio and satellite television."

For the short term, now that ESA and NASA have demonstrated that this protocol works, NASA could call upon Mars Express, theoretically, for some of our future missions, such as the Phoenix mission, says Noreen. "Similarly, the Europeans are now confident that they would be able to use our relay service because we have demonstrated the interoperability between the rovers and Mars Express, and the interoperability between the agencies."

Looking into the not too distant future, Noreen adds: "We would eventually like to do the same thing at the Moon. There are lots of missions going to the Moon soon from many countries, including Japan, Indian, China, as well as the United States and Europe. Goddard Space Flight Center is considering adding a communications relay to Lunar Reconnaissance Orbiter, the first component of NASA's new Vision for Space Exploration. We'd like to see other agencies do that as well, so that we can also have a compatible network at the Moon to support missions going into areas not in view of Earth, like the lunar polar regions. If a lander or rover lands in a permanently shadowed crater, it can't see Earth. But if all these agencies sending orbiters to the Moon each have relay radios that are compatible, it would easy to use a relay network of these orbiters around the Moon to provide communications whenever Earth is not in view."

An interplanetary telecommunications network will also ultimately serve to make the transfer of data more cost-efficient as our planetary explorations continue. "Emphasis is constantly being placed on driving the costs of exploration down," Arnold notes. "One means to explore the surface of a planet is to place a large number of small science packages about the surface. Suppose the number is 20. These are small, inexpensive landers, which are solar powered and use low-power transmitters. Signals from these landers would not be able to relay back to Earth directly at high data rates. A single orbiter about the planet would pass over each of these landers at regular intervals. Being in relatively close proximity, this link would be robust and be capable of high data rates. The landers would 'time share' with the orbiter, sending their valuable data one at a time, or perhaps several at a time. The data would be stored locally on the orbiting satellite and then relayed back to Earth at an opportune time. The orbiter would feature much larger antennas, solar arrays, onboard memory, and transmitters to relay the data back to Earth, making it the single high-cost asset of the telecom link. This asset would be available to all 20 landers, and any future missions as well."

"Spirit and Opportunity sent most of their data to Earth to Mars orbiters," reminds Noreen. "NASA will build on the international Mars network, currently consisting of Odyssey, MGS, and Mars Express, with relay radios on Mars Reconnaissance Orbiter (MRO), to be launched in 2005, and on Mars Telecommunications Orbiter (MTO), to be launched in 2009. MTO will be the first orbiter sent to another planet primarily to provide relay services." And, he adds, "MTO will be able to relay one to two orders of magnitude more data from rovers on Mars to Earth than any other Mars orbiter."

A key feature of the Proximity 1 system -- or any cost-effective communications protocol, Arnold points out, is the capacity to store data on the satellite for relay at some future time. "This allows data to be transmitted between the landers and orbiters regardless of the geometry to Earth," he explains. "These types of data relay scenarios play out in both robotic and manned missions, both near and far. They insure robust and capable telecom links required for integrity of the missions, as well as safety as man considers leaving Earth orbit once again."