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Ground-tracking continues to be the best method for finding the positions of spacecraft in near-Earth space. Ground-tracking famously rendered the sextant aboard the Apollo spacecraft nearly obsolete even before the first lunar flight. In the forty years since then, space navigators have come up with a few new tricks.

As I'm sure we've all heard, a Chinese spacecraft, Chang'e 3, landed on the Moon yesterday. It's the first soft-landing since 1976. Naturally, mission controllers would love to know the exact position of the lander and its little robotic rover. At some point NASA's LRO spacecraft will probably photograph it from orbit. LRO has photographed all of the Apollo landing sites (except Apollo 16 --which was a hoax), and it has also photographed a number of unmanned spacecraft on the lunar surface including the large Soviet-era Lunokhod rovers. But with help from the ESA, China's mission control should have a very accurate fix on their spacecraft in just a few days.

The latest system of ground-based position-finding adds just a dash of celestial navigation to the recipe. They don't use the Sun or planets or even the stars. Instead they track distant quasars to remove uncertainties in the radio signals received from the spacecraft. The European Space Agency, ESA, is providing this quasar-based position-finding system for the Chang'e 3 mission. There's an introductory article here:
http://www.esa.int/Our_Activities/Operations/Helping_China_to_the_Moon. Follow the link within the article to "delta-DOR" for more details on this method of determining a spacecraft's position.

-FER

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