NASA Precision Laser ICESat-2 Measures Heights of Rough Arctic Surface

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LOMPOC, Calif., Oct. 22, 2019 — Four months after launch, the lidar laser aboard NASA’s Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2), in conjunction with the Airborne Topographic Mapper (ATM) and Land Vegetation and Ice Sensor (LVIS) lasers aboard NASA’s Operation IceBridge airborne campaign, succeeded in their mission to take identical measurements of ice sheet elevation and sea ice thickness in the Arctic, in addition to measurements of land topography, vegetation characteristics, and clouds.

ICESat-2 carries a single instrument, the Advanced Topographic Laser Altimeter System, or ATLAS. Like GLAS, the altimeter on the first ICESat mission, ATLAS measures the travel times of laser pulses to calculate the distance between the spacecraft and Earth’s surface.

 A refrozen lead in the Arctic Ocean sea ice. Courtesy of NASA/Jeremy Harbeck.

According to NASA, the laser light is at 532 nm, a bright green on the visible spectrum. The lasers fire at 10,000 pulses per second, 250 times faster than GLAS, which sent 40 pulses per second. With the increased pulse rate, ATLAS can take measurements every 2.3 ft along the satellite’s ground path.

About 20 trillion photons leave ATLAS through its box structure with each pulse, with only a dozen returning to the satellite’s telescope. To catch the remaining photons, ATLAS is equipped with a beryllium telescope, 2.6 ft in diameter. ATLAS engineers then designed an additional tool known as the Laser Reference System to accurately calibrate the satellite’s alignment with the returning photons.

To prevent the detectors from being swamped by captured sunlight, filters are designed to only allow light measuring exactly 532 nm. The travel data for each photon is then communicated to the electronics and communication system on ICESat-2’s spacecraft and sent to the ground. Computer programs can then calculate travel time and satellite position and tell researchers the distance the photon traveled, giving an accurate measurement for how high the ice surface is above open water, a measurement known as freeboard.

The first launch of ICESat-2, which cost NASA an estimated $96.6 million, took place Sept. 15, 2018, from Vandenberg Air Force Base Space Launch Complex 2 aboard a Delta II 7420-10C. With four flights’ worth of data comparisons traveling more than 600 miles (1000 km), the two data sets of arctic ice measurements conducted from air and space were deemed a near perfect match, a correlation of 0.95 of a possible 1.0.

“If you look at the height profiles from ICESat-2 and IceBridge, you can tell that they’re almost the same,” said Ron Kwok, a sea ice scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and lead author of the new study. “It’s a good pitch for the quality of the ICESat-2 data.”

When Kwok and his colleagues compared freeboard measurements from ICESat-2 and Operation IceBridge's ATM instrument for the April flights, they were within 0.8 to 1.6 in. (2 to 4 cm) of each other.

IceBridge mission scientist John Sonntag had to calculate where and when ICESat-2 was going to be over a specific spot in the Arctic Ocean, and how to navigate the plane to be there at the same time, all while timing for ideal weather conditions of low winds and clear skies.

“He was able to nail it,” Kwok said. “He was able to get to that part of the Arctic so there’s almost zero time lag between ICESat-2 and the airplane.”

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Published: October 2019
A quantum of electromagnetic energy of a single mode; i.e., a single wavelength, direction and polarization. As a unit of energy, each photon equals hn, h being Planck's constant and n, the frequency of the propagating electromagnetic wave. The momentum of the photon in the direction of propagation is hn/c, c being the speed of light.
Research & TechnologyLasersNASAICESat-2photonsCaliforniaGoddard Space Flight Centerberylliumphoton

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