The EarthCARE satellite mission objective is the observation of clouds and aerosols from low Earth orbit. The payload will include active remote sensing instruments being the W-band Cloud Profiling Radar (CPR) and the ATLID LIDAR. These are supported by the passive instruments Broadband Radiometer (BBR) and the Multispectral Imager (MSI) providing the radiometric and spatial context of the ground scene being probed. The MSI will form Earth images over a swath width of 150 km; it will image the Earth atmosphere in 7 spectral bands. The MSI instrument consists of two parts: the Visible, Near infrared and Short wave infrared (VNS) unit, and the Thermal InfraRed (TIR) unit. Subject of this paper is the VNS unit.<p> </p>In the VNS optical unit, the ground scene is imaged in four spectral bands onto four linear detectors via separate optical channels. Driving requirements for the VNS instrument performance are the spectral sensitivity including out-of-band rejection, the MTF, co-registration and the inter-channel radiometric accuracy. The radiometric accuracy performance of the VNS is supported by in-orbit calibration, in which direct solar radiation is fed into the instrument via a set of quasi volume diffusers.<p> </p>The compact optical concept with challenging stability requirements together with the strict thermal constraints have led to a sophisticated opto-mechanical design.<p> </p>This paper, being the second of a sequence of two on the Multispectral Imager describes the VNS instrument concept chosen to fulfil the performance requirements within the resource and accommodation constraints.
The Tropospheric Monitoring Instrument TROPOMI is ready for system level verification. All sub-units have been integrated and tested and final integration at Dutch Space in Leiden has been completed. The instrument will be subjected to a testing and calibration program and is expected to be ready for delivery to the spacecraft early 2015. Using TROPOMI measurements, scientists will be able to improve and continue the study of the Earth’s atmosphere and to monitor air quality, on both global and local scale.
METIS is the 'Mid-infrared ELT Imager and Spectrograph' for the European Extremely Large Telescope. This E-ELT
instrument will cover the thermal/mid-infrared wavelength range from 3 to 14 μm and will require cryogenic cooling of
detectors and optics. We present a vibration-free cooling technology for this instrument based on sorption coolers
developed at the University of Twente in collaboration with Dutch Space. In the baseline design, the instrument has four
temperature levels: N-band: detector at 8 K and optics at 25 K; L/M-band: detector at 40K and optics at 77 K. The latter
temperature is established by a liquid nitrogen supply with adequate cooling power. The cooling powers required at the
lower three levels are 0.4 W, 1.1 W, and 1.4 W, respectively. The cryogenic cooling technology that we propose uses a
compressor based on the cyclic adsorption and desorption of a working gas on a sorber material such as activated carbon.
Under desorption, a high pressure can be established. When expanding the high-pressure fluid over a flow restriction,
cooling is obtained. The big advantage of this cooling technology is that, apart from passive valves, it contains no
moving parts and, therefore, generates no vibrations. This, obviously, is highly attractive in sensitive, high-performance
optical systems. A further advantage is the high temperature stability down to the mK level. In a Dutch national research
program we aim to develop a cooler demonstrator for METIS. In the paper we will describe our cooler technology and
discuss the developments towards the METIS cooler demonstrator.
Passive cooling has shown to be a very dependable cryogenic cooling method for space missions. Several missions
employ passive radiators to cool down their delicate sensor systems for many years, without consuming power, without
exporting vibrations or producing electromagnetic interference. So for a number of applications, passive cooling is a
good choice. At lower temperatures, the passive coolers run into limitations that prohibit accommodation on a spacecraft.
The approach to this issue has been to find a technology able to supplement passive cooling for lower temperatures,
which maintains as much as possible of the advantages of passive coolers.
Sorption cooling employs a closed cycle Joule-Thomson expansion process to achieve the cooling effect. Sorption cells
perform the compression phase in this cycle. At a low temperature and pressure, these cells adsorb the working fluid. At
a higher temperature they desorb the fluid and thus produce a high-pressure flow to the restriction in the cold stage. The
sorption process selected for this application is of the physical type, which is completely reversible. It does not suffer
from degradation as is the case with chemical sorption of e.g. hydrogen in metal hydrides. Sorption coolers include no
moving parts except for some check valves, they export neither mechanical vibrations nor electromagnetic interference,
and are potentially very dependable due to their simplicity. The required cooling temperature determines the type of
working fluid to be applied. Sorption coolers can be used in conjunction with passive cooling for heat rejection at
This paper starts with a brief discussion on applications of passive coolers in different types of orbits and the limitations
on passive cooling at low cooling temperatures.
Next, the working principle of sorption cooling is summarized. The DARWIN mission is chosen as an example
application of sorption and passive cooling and special attention is paid to the reduction of the radiator area needed by
the sorption cooler.
By examining the performance of alternative working fluids suitable for different cooling temperatures, the application
field of this type of sorption cooling is currently expanded.