NASA's first laser ranging effort to track a spacecraft beyond low-Earth orbit on a daily basis is producing distance measurements accurate to about 4 inches; by comparison, the microwave stations tracking the Lunar Reconnaissance Orbiter measure its range to a precision of about 65 feet.
The precise laser measuring of the LRO's location is a project of NASA's Goddard Space Flight Center in Greenbelt, Md., where scientists fire a laser beam every 28 seconds approximately 250,000 miles into space to hit the minivan-sized LRO as it orbits the moon at nearly 3,000 miles an hour.
The primary objective of LRO is to conduct investigations preparing for future exploration of the moon. Specifically, LRO will scout for safe and compelling lunar landing sites, locate potential resources with special attention to the possibility of water ice and characterize the effects of prolonged exposure to the lunar radiation environment.
In addition to its exploration mission, LRO will return scientific data that will help NASA better understand the moon's topography and composition.
"Current lunar maps are not as accurate as we'll need to return people safely to the moon," Ronald Zellar, team lead for the LRO laser ranging system, said in a statement. "In order to make an accurate map, first you need to know where you are. Knowing the precise range to LRO is necessary for its instruments to produce much more accurate maps, with errors reduced to the size of humans or rovers."
Engineers use a telescope at the ground station on the Goddard campus to direct laser pulses toward LRO. The range to LRO is calculated by measuring how long it took the laser to reach the spacecraft. The laser ranging to LRO is one way, meaning that the laser is directed at LRO, which records the time of arrival and sends the data back to ground stations on Earth by its radio telemetry link.
According to NASA, this is the first time repeated, one-way tracking has been used for spacecraft ranging. Typical satellite laser ranging, used for spacecraft in low-Earth orbit, is two-way, meaning the laser is simply reflected off the spacecraft and the time of flight recorded when it returns to the ground.
There are, however, challenges, including interference, precise time measurement, and the age-old problems of hitting a moving target and plain, old bad weather.
The laser pulses from Earth are carefully timed to avoid interfering with the instrument's operation. Since the instrument sends laser pulses 28 times per second to the lunar surface, the laser ranging pulses are sent at the same rate but shifted in time to avoid interference.
"It's like shooting at a spinning coin from a mile away and being able to hit it on the edge as it spins," said Gregory Neumann, a geophysicist at NASA Goddard.
As for accurate time measurement, the range to LRO is calculated by measuring how long it took the laser to reach the spacecraft, meaning any variations in the time measurements will produce variations in the range estimates. LRO has a timing system that uses a crystal oscillator-the heart of which is a vibrating crystal-to measure time precisely. The oscillator is accurate to one part in a trillion over an hour.
However, the rate at which the crystal vibrates changes with temperature, so the crystal is housed in a small oven that must be carefully controlled to maintain a stable temperature.
Then there's the difficulty of hitting a moving target. LRO is constantly moving in its orbit, and Goddard scientists must fire the laser pulses at a point in front of the spacecraft to compensate for the spacecraft's motion while the pulse is in-flight toward the moon.
And there is the weather. The laser can't penetrate thick cloud cover, so laser ranging is not available in those situations. "We're ranging to LRO whenever the moon is visible, 24 hours a day, seven days a week," said Jan McGarry of NASA Goddard, ground system lead for laser ranging.