High above the Ice

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For centuries, the parts of our planet consisting of frozen water – the cryosphere – were considered hostile and useless. Those who ventured into the Arctic and Antarctic regions did so despite the icy climate. They were not searching for knowledge but for new trade routes and precious furs. Systematic exploration of the polar regions did not start until the late 19th century. It has not been until only recently that scientists have become aware of the importance of these regions for global climate. They discovered that the high reflectivity of ice and snow, as well as their low thermal diffusiveness, are major factors in the complex mechanisms of our atmosphere.

Non-melting snow-covered surfaces typically reflect 80% to 90% of the incoming solar radiation back into the atmosphere. This keeps the average temperature in the polar regions cold and the snow from melting. At the same time, it is much more difficult for temperature waves to penetrate snow and ice than air. Generally speaking, snow cover insulates the ground surface, while sea ice does the same to the underlying ocean water. Both of these facts have a significant effect on the exchange of heat and moisture between the planet’s surface and its atmosphere. Scientists are only just beginning to unravel the complex feedback mechanisms that define our climate. Due to the increase in global warming, they are expanding their efforts.

Scanning from Above

One way to help understand these correlations better is continuous surveillance of the cryosphere. Considering the generally hostile environment of the polar regions, it is better to gather the necessary data from a safe distance – for example, from space. This is the mission of the ICEsat-2 satellite, which NASA launched in September 2018. Its only cargo is the advanced topographic laser altimeter system (ATLAS), a space-based LiDAR instrument that scans the Earth’s surface with six green laser beams at a wavelength of 532 nm. Emitting 10,000 pulses per second, ATLAS can take measurements at a resolution of 0.7 meters along the satellite’s ground path. The receiving telescope aboard contains highly sensitive single photon counting detectors. After all, only a handful of the 20 trillion photons released with each laser pulse hits the surface and returns to the satellite. ICEsat-2 circles the earth in a near-circular, near-polar orbit approximately 496 km above the ground. At a speed of 6.9 kilometers per second, it takes 91 days to complete the orbital path. Every three months it produces a grid of the entire earth, covering not only the cryosphere but topography measurements of land and water surfaces, cities, and forests around the globe.

Perfectly Aligned

Considering the extremely low percentage of photons that return to the satellite, the laser emitter and telescope have to be perfectly aligned to each other, so that none of the photons goes astray. In the ICEsat-2 satellite this is achieved using an integrated laser reference system (LRS). On both the emitting and receiving end, reference beams are redirected to the LRS, which then adjusts the beam steering mechanism (BSM) (see figure). It is crucial to the mission that the reference beams be transmitted with the utmost accuracy. To achieve this goal, the ATLAS system uses patented lateral transfer hollow retroreflectors (LTHR) by PLX. The laser light is guided through an arrangement of first-surface mirrors – a flat mirror on one end and a roof mirror on the other – and redirected at a 180° angle so that both the incoming and outgoing beams are parallel to each other. Unlike standard solid retroreflectors, the optical path runs through the air rather than through solid material. This principle eliminates the effects of material absorption. The mirrors are connected by tubing made from the same material. At PLX, all these parts are fused together in a proprietary process. The resulting monolithic structure features extremely high thermal stability and is insensitive to mechanical influences such as vibrations or shock. With sub-arc second beam deviation and a wavefront error that is better than λ/10, LTHRs are the obvious choice for high-precision space applications.

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Retroreflectors are commonly known from bicycle reflectors (cat’s eyes). They reflect incoming light parallel to the original beam regardless of the angle of incidence. In high-tech applications, such as laser tracking, space-based interferometry, and long-path spectroscopy, the same principle is used for beam guidance at the utmost precision. At LASER COMPONENTS, we offer both solid and hollow retroreflectors by PLX, a specialized manufacturer from New York, USA.

Further product information:
Retroreflectors for High Power Applications

Manufacturer:
PLX Inc.

Contact:

Contact Person:	Rainer Franke
Company:	Laser Components Germany GmbH
Address:	Werner-von-Siemens-Str. 15
ZIP / City:	82140 Olching
Phone:	+49 (0) 8142 2864-39
Fax:	+49 (0) 8142 2864-11
Email:	rainer.franke@lasercomponents.com