Keep Watching the Ice
Meet the satellites bringing data to the discussion of global warming
- By Ben Iannotta
- Air & Space magazine, September 2006
(Page 4 of 7)
NASA had planned to run ICESat’s three lasers in sequence until they burned out—ideally after at least five years of continuous measurements. But ICESat managers were shocked when one of the lasers fizzled after just 36 days. NASA rushed to assemble a review panel headed by veteran NASA engineer Bob Kichak, an expert in spacecraft power systems, to determine what went wrong and whether anything could be done to save the remaining two lasers.
Inside each ICESat laser, dozens of half-inch-long light-emitting diodes project light onto a metallic crystal. The electrons in the crystal become so energized that they release pulses of laser light. The diodes are linked to a power supply by numerous hair-like gold wires. NASA officials disassembled a spare unit purchased from the manufacturer, Spectra Laser Diodes of San Jose, California, and examined the diodes under high magnification. Kichak’s board concluded that “excessive” amounts of metallic indium solder, used to hold the diode assembly together, had most likely contacted the gold wires and eroded them, causing a catastrophic loss of power. The most likely cause for the presence of the extra solder was a manufacturing error.
Based on Kichak’s recommendations, NASA ordered a complete overhaul of ICESat’s operating plan. To conserve the lasers, operators can turn them on only three times a year for 33-day-long “campaigns.” ICESat’s cooling system would slow any chemical erosion. Engineers eventually decided to run the lasers at a temperature of 14 degrees Fahrenheit, says Zwally.
The decision dimmed the laser’s visible, green light, so ICESat’s secondary measurements of aerosols and clouds can be gathered only at night. The ice and land elevation readings aren’t compromised because they require only the lasers’ infrared channel, Zwally says .
NASA officials breathed cautious sighs of relief as the second and third lasers were turned on and ICESat gathered data successfully. The second lasted about 100 days before officials decided to turn on the third.
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