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It’s all a question of timing

One snippet that caught my attention this week is NASA's use of laser to track the Lunar Reconnaissance Orbiter (LRO) to within 4 inches – an outstanding feat of accuracy that one geophysicist at NASA Goddard says is like 'shooting at a spinning coin from a mile away and being able to hit it on the edge as it spins.' This is NASA's first laser ranging effort to track a spacecraft beyond low-Earth orbit on a daily basis; for comparison, microwave stations track the LRO to a precision of about 65 feet.

A little background: As you probably know, lasers are regularly used to track Earth-orbiting satellites, but Lunar-orbit laser-ranging – that's about 250,000 miles out into space – requires a different approach. Notably, the LRO is said to be the size of a minivan, but rotates the moon somewhat quicker, at nearly 3,000 miles an hour!

So what's the secret behind this repeated, one-way tracking capability? Well it's all about timing, according to NASA.

Earth-orbiting satellites typically have a reflector that deflects laser light from a ground-source back to a receiver on the ground, but the LRO has an on-board photon detector to pick up the incoming light. This data is transmitted back to NASA via a radio telemetry link. The first element of the timing challenge is to avoid interfering with the LRO's own laser-tracking duties, which involve sending a pulse 28 times per second to the lunar surface in order to accurately chart the moon's topology and composition. Laser pulses from Earth are sent at the same rate but shifted in time.

The range to LRO is calculated by measuring how long it took the laser to reach the spacecraft. The LRO's timing system uses a crystal oscillator, maintained at a stable temperature to ensure its accuracy to one part in a trillion over an hour. That a crystal oscillator is used in this application is perhaps contrary to what you would imagine. In the past an atomic-resonance device would have been the usual recourse for this kind of accuracy. But thanks to issues of size, weight, power, reliability, and even short-term frequency stability, quartz oscillators are in demand for space missions, according to Symmetricom, which has delivered ultrastable oscillators for the LRO.

The LRO's ultimate mission is to accurately map the lunar surface, in advance of a safe return of people to the moon. But with the 40th anniversary of the first moonwalk just past, NASA's goal of returning to the moon by 2020 looks in doubt. It is, therefore, somehow heartening to know that a return to the moon by man is simply a question of timing.

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