Spacecraft landing on the Moon and Mars have yet to be able to choose their landing sites: they touch down wherever their trajectories take them. But the most scientifically rich terrain also tends to be the most hazardous. Now NASA is developing an optical sensor that will, for the first time, allow spacecraft to identify safe landing locations and navigate toward them.

The technology is a light detection and ranging (LIDAR) system that sends three continuous beams of laser light to the surface. It measures the properties of the light that bounces back to determine the velocity and position of the spacecraft relative to the surface, in three dimensions. "It is much more accurate than any other available, similar technology in terms of determining your coordinates relative to the surface, and it is going to be revolutionary in that area," say Bob Reisse, the project manager for the system at NASA's Langley Research Center, in Hampton, VA.

Traditional LIDAR uses short pulses of laser light and measures the time it takes them to return to the emitter. Instead, the new system measures the Doppler shift--the change in frequency and wavelength--of the return beam. "The beam has to be more or less continuous for long enough to make the measurement," says Reisse. "We also need [the beam] to be stable, and continuous lasers are much more stable than continuous bursts." In addition, whereas traditional LIDAR uses one beam, the new system uses three. "Essentially, it is an entirely different technology," Reisse says. The combination of the Doppler shift measurement and the added beams allows the spacecraft to calculate its velocity down to the order of centimeters per second, and its position to the order of centimeters, at a range of one to two kilometers from the planet's surface, says Reisse.