Abshire says that even with initial drawbacks, the lasers have performed better than expected. Satellite-based instruments often have Achilles’ heels, and in the case of ICESat, the weakness is a slight laser-pointing error as ICESat crosses in and out of Earth’s shadow. To measure the effect, experts from the Scripps Institution of Oceanography in La Jolla, California, drove SUVs over a salt flat in the mountains of Bolivia using GPS receivers to survey the terrain. ICESat passes within range of this salt flat every two weeks, and during those passes, controllers program the satellite to turn sideways so that its laser bounces off the salt flat. By comparing those readings to the ones from the GPS receivers, scientists know that ICESat is operating with an absolute accuracy as fine as seven centimeters (2.7 inches), better than the original target of 10 centimeters, Abshire says. When mathematical models are applied to subtract the known aiming errors, the instrument has proven to make repeatable measurements with a margin of error as low as three
The new ICESat operating plan has made the job of steering and controlling the spacecraft more complicated for the staff and students at the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder. LASP officials had assumed they would settle in to a pattern of routine monitoring from the lab’s cozy mission operations center. But during a teleconference in 2003, NASA managers outlined the new operating scenario to the LASP staff. “When we went to that telecon we went, ‘Whoa! They’re going to correct a manufacturing defect with an operational workaround,’ which
wasn’t unheard of,” says veteran satellite controller Jack Faber, an operations engineer who also teaches satellite control at the University of Colorado. “The bottom line was LASP is going to have to do a lot of work.”
The satellite is steered not by rockets but by electric-powered reaction wheels, which spin at differing rates to create a predictable torque that changes the direction in which the satellite points.
Since ICESat studies the changes in glaciers, volcanoes, and river basins that might be off its orbital track, LASP controllers must be ready to turn it to face targets of opportunity. Now that ICESat is run intermittently, the controllers have to work harder during the limited time. Controllers must upload aiming commands approximately once a week instead of once a month, as was the original plan.
ICESat’s laser campaigns are intense affairs in which controllers closely monitor the temperature of the laser subsystems for signs of trouble. Each time the satellite comes into range of a ground station, controllers have about 12 minutes to send commands and download the all-important temperature readings.
On one sunny afternoon at LASP, ICESat has risen above the horizon north of Alaska and into range of the Svalbard ground station in the Norwegian Arctic.
Faber and Chris Bunch, a 22-year-old mechanical engineering student, are ready to make contact with the craft, and Bunch is twirling a pen nervously in one hand.
Faber’s graying beard and jeans may look typically Boulder, but he’s anything but casual about running the operations center. He once famously booted the LASP director out of the control center for being distracting during the critical moments after a satellite launch.