POLDER

• 1. Introduction

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  POLDER weighs about 30 kilograms, draws 77 watts in imaging mode, and spent its working life staring down at sunlight bouncing off the planet. The name is an acronym: POLarization and Directionality of the Earth's Reflectances. The French space agency CNES built it as a passive optical imaging radiometer and polarimeter. Three times it rode into orbit on someone else's satellite. Each time, the satellite it depended on gave out before the science was finished. So what was a 30-kilogram instrument trying to measure from hundreds of kilometers up? Why build the same device three times? And what does it mean to read the Earth not by the light it makes, but by the light it reflects?

• 2. Reading Reflected Light

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  Solar radiation reflected by Earth's atmosphere was POLDER's entire subject. The instrument never generated its own signal. It watched what the planet sent back toward space and tried to make sense of it. That reflected glow carried information about tropospheric aerosols, the fine particles suspended in the lower atmosphere. It also held clues to sea surface reflectance and to the way land surfaces scatter light in different directions, a property captured by the bidirectional reflectance distribution function. The same measurements fed into the Earth Radiation Budget, the accounting of how much solar energy the planet absorbs versus returns. POLDER's job was to turn a passive shimmer of reflected sunlight into a record of atmosphere, ocean, and ground at once.

• 3. The Push Broom Eye

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  A push broom scanner gave POLDER its way of seeing. Rather than sweeping a single point back and forth, this design reads an entire line at once as the satellite moves forward, like a broom dragged across the surface below. Behind the optics sat a charge-coupled device matrix with a resolution of 242 by 548 pixels. The light passed through a telecentric lens with a focal length of 3.57 millimeters and a focal ratio of 4.6. Its field of view stretched from plus or minus 43 degrees to plus or minus 57 degrees, depending on the tracking method. That wide angle is what let a small instrument take in a broad slice of Earth in every pass.

• 4. Tuned By Wavelength

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  Between 443 and 910 nanometers, POLDER chose what to look at by choosing a color of light. The shorter wavelengths, from 443 to 565 nanometers, typically measured ocean color. The longer wavelengths, from 670 to 910 nanometers, were turned toward vegetation and the water vapor content of the air. The same instrument could shift its attention from sea to plant life simply by reading a different band. For sending its findings home, POLDER transmitted on 465.9875 megahertz at 200 bits per second and received on 401.65 megahertz at 400 bits per second. Its science data flowed at 880 kilobits per second, quantized to 12 bits, the resolution at which each reading was recorded.

• 5. Three Rides, Three Endings

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  On the 17th of August 1996, the first POLDER reached orbit as a passenger instrument aboard ADEOS I. That mission ended on the 30th of June 1997, when communication from the host satellite failed. POLDER 2 followed in December 2002 aboard ADEOS II, only for that effort to stop after ten months when the satellite's solar panel malfunctioned. A third-generation instrument flew on the French PARASOL microsatellite. PARASOL was maneuvered out of the A-train on the 2nd of December 2009 and permanently shut down on the 18th of December 2013, the last of POLDER's three lives in orbit.