ExoMars uses new navigation technique to tack for the Red Planet

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The Trace Gas Orbiter and Schiaparelli vehicle that form ExoMars 2016(Credit: ESA)

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ESA's ExoMars 2016 Trace Gas Orbiter (TGO) spacecraft zeroed in on Mars today as it successfully conducted a 53-minute course correction burn of its main engine. According to the space agency, the probe used a new ultra-precise navigation technique that fixed its position to within a thousand meters (3,300 ft), allowing it to make a much more efficient and precise change to its trajectory.

ESA says that the telemetry from ExoMars received by its ground station in New Norcia, Western Australia confirmed that the burn began at 09:30 GMT (11:30 pm CEST). The engine firing was carried out automatically by the TGO using commands uploaded on Tuesday.

This was the fourth and largest of the last series of burns before reaching Mars, with smaller burns scheduled for August 11, September 19, and October 14. It was also a mild cliffhanger because a test burn on July 18 failed to go to plan and had to be repeated on July 21. The latter "ran perfectly," but it still introduced an element of suspense in today's maneuver.

According to ESA, ExoMars is using a new navigation technique called Delta-Differential One-Way Ranging (delta-DOR), which uses quasars to calibrate navigational fixes on deep-space probes. Quasars are the most luminous objects in the Universe and are billions of light years from Earth. They can generate a thousand times as much energy as our entire galaxy and their great distance from us means that they act like fixed lighthouses in the sky.

For delta-DOR, radio signals from the TGO are received by two deep-space tracking stations on Earth that are widely separated on different parts of the globe to create a long baseline. By measuring the difference between the times the signals from the spacecraft arrive at each station, the spacecraft's position can be calculated.

This should be a very accurate method of fixing a spacecraft's position if it weren't for interference from the Earth's atmosphere throwing off the timing. However, by simultaneously tracking radio signals from a quasar, the position of which is very precisely known, ESA engineers can correct for the error and achieve a much more accurate fix.

"The current set of delta-DOR observations will enable us to locate the spacecraft to less than 1,000 m when it's near Mars, a distance of slightly more than 150 million km from Earth," says Mattia Mercolino, who's responsible for delta-DOR activities at ESOC, ESA's operations center in Darmstadt, Germany. "This is comparable to detecting the location of an object in Singapore from Darmstadt, to about five cm (two in) precision. In future, with currently planned technology improvements, we should be able to get the accuracy down to just 150 m (500 ft) at 150 million km."

A joint mission with Russia's Roscosmos, ExoMars, is halfway through its journey to Mars. Launched on March 14, the TGO carrying the Schiaparelli entry, descent, and landing demonstrator will arrive in Mars orbit on October 19. On that day Schiaparelli, deployed three days previously, will enter the Martian atmosphere and land. Meanwhile, the TGO will go into an elliptical orbit around Mars, then use the Martian atmosphere to bring it into a circular orbit at an altitude of 400 km (250 mi) from which it will begin its five-year scientific mission in December 2017.

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