1.

INTRODUCTION

EarthCARE is a joint European-Japanese mission addressing the need for a better understanding of the interactions between cloud, radiative and aerosol processes that pay a role in climate regulation.

The EarthCARE mission aims to improve the representation and understanding of the Earth’s radiative balance in climate and numerical weather forecast models by acquiring vertical profiles of clouds and aerosols, as well as the radiances at the top of the atmosphere.

The EartCARE mission will fly four instruments, being:

Of these the MSI is the only instrument with a swath, thus providing context information.

During the phase A study period there have been two parallel teams studying concepts for the Earthcare satellite.

Within both teams, the MSI instrument is split into two sections:

Due to mass and size restraints, the TIR instruments use microbolometer detectors as the prime sensing elements.

During the phase A study for which an extension is still running, two teams (one lead by Astrium, the other by Alcatel) have performed a preliminary design for both the mission and the instruments.

TNO Science and Industry (formerly TNO TPD) has taken part in this study as the prime contractor for the Multispectral Imager instrument as part of the Astrium team.

2.

MICROBOLOMETER SPECTROMETER MIBS

Within the Astrium team, TNO has designed a novel concept for the Thermal infrared channels Fig. 2. depicts the result of the phase A study as presented in the final report.

Fig. 1.

Earthcare artists impression

Fig. 2.

Phase A MSI instrument

Fig. 3.

Mechanical setup MIBS

Fig. 4.

Slit assembly central part

Fig. 5.

Slit assembly folding mirror

Fig. 6.

Camera housing

Fig. 7.

Collimator mirror blank

Fig. 8.

Pointing mirror blank

Fig. 9.

Main body

The smaller section contains the VNS channels, and the larger section is the so called microbolometer spectrometer.

A number of years ago, this concept couldn’t give a high enough NETD in order to be feasible for monitoring of low temperature targets.

However, continuous performance improvements in obtainable noise levels for microbolometers (that are still going on) have led to the availability of bolometer detectors that could fulfill and even exceed the requirements of MSI while using the spectrometer concept.

Because these improvements are mainly driven by military applications within the USA, ITAR restrictions will be a main problem in obtaining these detectors, but due to the fact that microbolometers are also finding their way in things like automotive safety it is very likely that improved detectors will be available within the foreseeable future from sources outside of the USA.

Although spectrometers have a radiometric disadvantage as compared to the more conventional beamsplitter operated systems (which is disputable for instruments with more than 2 channels) the main advantage is the higher spectral purity and the larger number of channels that can be provided at the same or marginally increased costs.(some extra signal processing electronics might be needed for the instrument control unit, and at satellite level more data will need to be handled.)

Although extensive modeling has been performed, it was decided to build a breadboard to show the feasibility of the concept.

The breadboard will closely resemble the final instrument, apart from the facts that it will not be light weighted and will operate at room temperature.

The main assemblies (slit assembly and camera assembly) as well as the mirrors are all highly similar to the foreseen flight configuration.

The breadboard therefore, can easily be converted into a flight capable instrument through exchange of the pointing mirror assembly, the cold blackbody and the detector. (if necessary the optical bench can be light weighted also)

Currently the parts to be used for the breadboard have been ordered and are under production.

As can be seen in the following pictures, some are already nearing completion.

It is the intention of TNO to assemble the breadboard before the end of the year and do some first order characterization measurements.

The aim of this project (which is supported by the Dutch NIVR) is to show that the final instrument can meet the requirements as posed on the EarthCARE MSI instrument once the foreseen detectors are available.