Concept Development for the Thermal Management of the Russian American Observational Satellites (ramos)

For the past several years, cryogenically cooled sensors have become an increasingly popular method of observation and study for both space-based and ground-based operations. Accordingly, various cooling techniques have been developed to accommodate this group of sensors. Because of rising performance standards and escalating cost limitations, cryocoolers have become an impressive cooling technique to consider. This report focuses on the use of a mechanical cryocooler in conjunction with the Russian American Observational Satellites (RAMOS), a future pair of earth-imaging satellites which will fly infrared radiometers. The RAMOS program consists of mapping the earth's surface in stereo using two co-orbital satellites. The American Observational Satellite (AOS) will utilize an infrared radiometer with the telescope focal plane assembly (FPA) operating at approximately 60 K. The FPA will be cooled using a multiple cryocooler configuration. The use of multiple coolers introduces redundancy into the cooling system-a redundancy which has been absent from many previously flown satellites. In addition, the cooling system will incorporate various other new technologies, such as thermal disconnects, a thermal storage unit, lowresistance flexible thermal links, etc., to meet the overall system objectives and requirements. Thermal storage units are discussed as a means of eliminating cryocooler self-induced vibration and passively controlling FP A temperatures. Incorporating thermal switches and thermal storage units into a cooling system design can alleviate the concerns of cryocooler vibration and parasitic heat loads. An understanding of these concepts and configurations will assist in the design of similar optical instruments for both spacebased and ground-based exploration campaigns. Introduction and Background During the early 1990's, the United States government began exploring the idea of joint space research and exploration with Russian scientists and engineers. The Russian American Observational Satellites (RAMOS) program is a direct result of this venture. RAMOS is a space research program that involves "an innovative measurement technique, simultaneous stereo-optical imaging (that) addresses the twin concerns of target detection and background suppression. The simultaneous stereo measurement technique also offers the potential for significant improvement in environmental monitoring." Pseudo-stereo imaging is a process that has long been used to· map terrain and other such stationary objects from space. This process, however, is usually performed by a single satellite taking pictures of a fixed target at various times during its orbit. The stereoimaging technique employed by RAMOS has the potential to map not only stationary objects, but highspeed moving targets with images obtained simultaneously from two different orbital positions. The AOS satellite will consist of an infrared radiometer operating at approximately 60 K over a mission lifetime of 2 years. This paper examines various cooling options as well as several cryogenic technologies for use with the RAMOS sensor. As a result of this examination, a conceptual thermal management scheme is presented. Objectives and Requirements The objectives and preliminary requirements for the RAMOS sensor are listed in Table 1. Table 1. Preliminary sensor thermal requirements for RAMOS. Objectives • Determine the instrument thermal requirements and constraints • Develop alternative thermal management options that meet requirements • Perform trade-off studies • Formulate an initial thermal management conceptual design • Devise a passive thermal disconnect mechanism pending the selection of a cryocooler system. • Develop "Vibration Free" Environment • Ensure thermal system compatibility • Refme and optimize the design • Obtain, build, and test certain critical components of the design solution • Finalize the design Instrument Requirement Lifetime Total Mass Limit Approximate System Envelope Maximum Heat Loads (FPA) LaunchPad On-Orbit Maximum Heat Loads (Telescope) LaunchPad On-Orbit Approximate Telescope Temp System Thermal Link Temperature Maximum Input Power Maximum Environmental Temps LaunchPad On-Orbit Spectrometer Temperature Reqs. Optical Bench Temperature Focal Plane Array Temperatures FPA Thermal Drift Requirement Maximum Sensor Operation Period System Readiness Level Flight Proven Maximum Vibration Levels Sensor Operating Sensor Dormant Specification > 2 year <50 kg 36x44x48 em