Early on Monday July 13th, researchers at the NERC-funded SGF located the NASA Lunar Reconnaissance Orbiter (LRO) to within 10cm as it orbited the Moon, by aiming powerful pulses of laser light at the far-off satellite and timing how long they took to reach their target.

In doing so they were contributing to NASA’s Robotic Lunar Exploration Program (RLEP), which could ultimately prepare the ground for a possible return to the Moon by people. The LRO is a key part of the project; among its tasks is accurately measuring the Moon’s complex gravity field and mapping its surface at high resolution.

‘If we can redefine the Moon’s gravity field, it could show where mass is concentrated and also enable spacecraft to visit or return to a particular location, or even suggest where to look for lunar resources like water or minerals,’ says Dr Graham Appleby, head of service at SGF. ‘Along with the mapping and other remote-sensing work being done by LRO, this could influence the decision of where to land a manned expedition in the future.’

Researchers at the SGF, based at Herstmonceux in East Sussex, routinely use lasers to measure the positions of satellites orbiting the Earth to precisions of a few millimetres. These observations then allow very accurate orbits of the satellites to be determined, which in turn enable accurate analyses of the satellites’ observations of, for example, sea-level changes or ice-sheet dynamics. But SGF instruments have never reached as far as a lunar orbit before; their typical targets are between 400km and 20,000km away in orbit around the Earth.

The scientists at Herstmonceux used a different technique for this project than is usual for Earth-orbiting satellites. These satellites typically feature a reflector that simply deflects incoming photons of laser light from their ground-based source back to a receiver on the ground, where their times of flight are recorded and converted to ranges, using the known value of the speed of light.

This means satellites can use a simple reflector rather than having to be equipped with a transmitter of their own, so they can be lighter and have fewer potentially fragile complex electronic systems.

The LRO satellite, by contrast, used its own on-board photon detector for a short period to pick up the incoming light. It then transmitted the data back to NASA using radio waves.

SGF technicians had to work closely with NASA to ensure their laser was pointed exactly at the satellite’s receiver – and that its pulses were properly timed. The LRO normally uses the receiver for its own laser rangefinder, which lets it measure its constantly-changing distance from the lunar surface. But for brief intervals, it is turned away from the Moon to pick up ranging signals from Earth.

The hour-long session observing the LRO yielded 2195 seconds’ worth of position data, out of a possible 3600 – a success rate of around 60 per cent. The data was relayed back to the observer at SGF, Matthew Wilkinson, via a NASA website which displayed it in graphical form with only a slight time-lag. He was also able to take the reproduced photograph, showing laser pulses being beamed from the SGF telescope towards the Moon.

The LRO was launched from Cape Canaveral in June 2009, and a few days later reached an elliptical orbit that takes it from 30 to 200km above the lunar surface in a period of about two hours. This is the first RLEP mission, and it aims to conduct research targeted specifically at preparing for and supporting future human exploration of the Moon.

Accurately determining a spacecraft’s range and position relative to the Earth and the Moon using lasers helps scientists plot its orbit more precisely than is possible with earlier methods, such as tracking using radio waves. In turn, this more precise orbit allows researchers to map the Moon’s terrain and gravity field more accurately. SGF is the first facility outside the US to be asked to join the NASA research program.

As well as its work on this project, SGF forms part of the International Laser Ranging Service, a global network of very precise satellite tracking stations, operates two geodetic Global Navigational Satellite System (GNSS) receivers that, again, are part of a large global network, and manages an absolute gravimeter, a sensitive instrument to measure local variations in the Earth’s gravity.

Posted by: Soderman/NLSI Staff
source: Planet Earth


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