Preliminary Evaluation of Passive Thermal Control for the Soil Moisture Active and Passive (SMAP) Radiometer

NASA’s Earth observing Soil Moisture Active & Passive (SMAP) Mission is scheduled to launch in November 2014 into a 685 km near-polar, sun synchronous orbit. As one of the four first-tier missions recommended by the National Research Council's Committee on Earth Science and Applications from Space, SMAP will provide comprehensive global mapping measurements of soil moisture and freeze/thaw state in order to enhance understanding of the processes that link the water, energy, and carbon cycles. The primary objectives of SMAP are to improve worldwide weather and flood forecasting, enhance climate prediction, and refine drought and agriculture monitoring during its 3 year mission. The SMAP instrument architecture incorporates an L-band radar and an L-band radiometer which share a common feed horn and parabolic mesh reflector. The instrument rotates about the nadir axis at approximately 14 rpm, thereby providing a conically scanning wide swath antenna beam that is capable of achieving global coverage within 3 days. In order to make the necessary precise surface emission measurements from space, the electronics and hardware associated with the radiometer in particular must meet very tight short term (instantaneous and orbital) and long term (monthly and mission) thermal stabilities. Short term orbital stabilities, for example, must not exceed 0.6 o C/orbit, while longer term mission drift must not exceed 15 o C. Maintaining these tight thermal stabilities is quite challenging because the sensitive electronics are located on a fast spinning platform that can either be in full sunlight or eclipse, thus exposing them to a highly transient environment. In the interest of providing a low cost solution to proper thermal management of the instrument, a passive design approach was first implemented early in the design cycle. A Thermal Desktop model was created in order to help evaluate the passive design and assist the project with deciding if a more advanced active control scheme would be required in order to meet the tight stabilities with sufficient margin. This paper will discuss the preliminary thermal model predictions and summarize what thermal stabilities can be realistically achieved through passive means on a fast spinning platform exposed to both direct sunlight and eclipse in low Earth orbit. The SMAP mission has not been formally approved by NASA. The decision to proceed with the mission will not occur until the completion of the National Environmental Policy Act (NEPA) process. Material in this document related to SMAP is for information purposes only.