Think you’re a real sharpshooter?
Hitting an object about the size of a minivan shouldn’t be a problem, then.
The catch? Your target is 250,000 miles away.
Oh, and it’s moving at 3,600 miles per hour.
That feat may be impossible for a single man, but NASA’s Goddard Space Flight Center manages to achieve it 28 times per second. The target? Its Lunar Reconnaissance Orbiter spacecraft, which orbits the moon.
The target practice is part of the first laser ranging effort to track a spacecraft beyond low-Earth orbit on a daily basis. The goal is to provide distance measurements accurate to 10 centimeters, or about four inches — far more accurate than current microwave stations, which track the LRO within 65 feet of accuracy.
“Current lunar maps are not as accurate as we’ll need to return people safely to the moon,” said Ronald Zellar of NASA Goddard, team lead for the LRO laser ranging system, 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.”
But that’s not all. By improving tracking accuracy of the LRO, we also improve our knowledge of the moon’s orientation and gravity, which helps us understand its interior structure.
Engineers at Goddard use a telescope at the ground station to direct laser pulses toward the spacecraft. The range to the LRO is calculated by measuring how long it took the laser to reach the spacecraft.
The laser ranging to LRO is one-way, meaning the LRO records the time of the laser’s arrival and sends data back to ground stations on Earth by its radio telemetry link
It’s the first time repeated, one-way tracking has been used for spacecraft ranging. The advantage of a one-way system is that the laser system used is less expensive and draws less power.
(Typical satellite laser ranging, used for spacecraft in low-Earth orbit, is two-way, meaning the laser is reflected off the spacecraft and recorded when it completes its return trip.)
But Earth provides its own complications to the process.
LRO’s laser tracking presents unique challenges, however. First, there’s the issue of avoiding interference. The laser pulses from Earth are received by a small telescope on LRO and transferred to the spacecraft’s laser altimeter instrument. The detector on this instrument performs double-duty, detecting both the laser ranging pulses as well as the pulses from its own laser reflected off the lunar surface. The instrument’s laser is used to build three-dimensional (topographic) maps of the lunar landscape and those pulses could hit the detector at the same time as the laser ranging pulses from Earth, confusing the data. So the pulses from Earth have to be 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 Neumann.
But that’s not all. Ensuring accuracy with the timing equipment used to record the laser is paramount, and NASA’s timing system — which uses an oscillator made of a vibrating crystal housed in a controlled-temperature oven — is accurate to one part in a trillion over an hour.
Along with that, NASA must rely on the microwave tracking system to predict where the moving target will be and fire ahead of it to compensate for the movement.
And if there’s heavy cloud cover, the process is stalled until it clears. After all, even the sharpest sharpshooter can’t shoot in the dark.
[via]
