I.

INTRODUCTION

The Meteosat Third Generation (MTG) program will ensure the continuity and enhancement of meteorological data from geostationary orbit as currently provided by the Meteosat Second Generation (MSG) system. OHB-Munich, as part of the core team consortium of the industrial prime contractor for the space segment Thales Alenia Space (France), is responsible for the Flexible Combined Imager – Telescope Assembly (FCI-TA) as well as the Infrared Sounder (IRS). This paper reports on the analyses of Sun intrusion and the measures taken to reduce the effects of stray light and solar energy input. Optical, thermal, mechanical and structural engineers teamed up to address the extremely challenging performance requirements.

II.

SUN INTRUSION, A PROBLEM OF EARTH OBSERVING INSTRUMENTS

Instruments on satellites observing the Earth or the Earth’s atmosphere are designed to have a high performance for those illumination scenarios. Any additional power introduced into the system can lead to either degradation of the contrast of the image or thermal/thermo-mechanical instability.

The relative movement of Earth and Sun leads to scenarios in which the Sun is in the vicinity or inside the field of view (FOV) of the instrument. These scenarios occur regularly during normal operation of the system. The criticality of those cases is defined by the time of the duration of such a scenario. For the Meteosat Third Generation Mission a three axis stabilized satellite is used and the two axis scanner mechanism to choose the observed position at the Earths surface is also a new concept. Both leads to possible long lasting static scenarios with sun intrusion into the instruments. Fig. 1 shows the path of the Sun on day 56 of the year as seen by Meteosat Third Generation Infrared Sounder (MTG-IRS). The Sun is passing close to the south pole of the Earth and can illuminate the entrance aperture when the FOV is pointed there.

Fig. 1:

Scan law and path of Sun on day 56 of the year as seen by MTG-IRS.

Another case to consider is a loss of the satellite positioning. In this case any relative position of the instrument FOV to the Sun is possible, leading to direct Sun intrusion.

In case of solar intrusion within the instrument aperture Sun light can enter the optical cavity even if the Sun is not in the detector FOV.

In particular solar power that is collected by the instrument pupil may enter into the system and due to the nature of e.g. a telescope it may be focused in a small spot with high power density hitting and prejudicing instrument parts. Measures must be taken to prevent performance degradation or even worse permanent damages. Fig. 2 to Fig. 3 show the light propagation in the region of mirror 2, field stop and mirror 4 of the front telescope of MTG-IRS (mirrors in blue). With higher angles of the Sun versus the pointing of the instrument the focused light moves out of the mirror surfaces (and thus will not reach the detector) and hits mechanical parts. At the field stop the focused beam causes high power density on the illuminated parts (Fig. 2).

Fig. 2:

Nominal path (0.51° half angle) and path of Sun light with 0.8° with respect to the FOV at M2 and field stop of MTG-IRS front telescope optics

Fig. 3:

Path of Sun light with 2.8° with respect to NADIR at M2 and field stop of MTG-IRS front telescope optics.

Even higher angles lead to illumination of mechanical parts close to M1 and M2 causing thermal problems with e.g. MLI shielding or harness of the mirrors. Fig. 3 shows the Sun illumination with 2.8° versus the FOV with a Sun shield in front of M2 to protect the M2 assembly. Also gaps have to be investigated that could cause light entering the cavities behind the mirrors illuminating unshielded mechanical parts.

Fig. 4 shows the Sun spot hitting the MTG IRS front telescope field stop and the resulting power distribution on mechanical parts for Sun illumination at 0.8° with respect to the FOV and the temperature distribution after 180s of Sun illumination within this condition. This temperature profile leads to thermal stresses in the field stop mounting interface (see Fig. 5).

Fig. 4:

Illumination of the MTG-IRS field stop by the Sun at an angle of 0.8° to the FOV and resulting temperature distribution

Fig. 5:

Thermal stresses due to Sun intrusion on field stop.

Temperatures and deformations have to be analysed and, if needed, the instrument geometry has to be adapted to prevent damage to the instrument due to solar power impact. This design effort has to take into account various different and sometimes contradictory requirements of the participating engineering disciplines. The Sun propagation within an optical instrument has to be analysed carefully and the design has to consider the related constraints beginning in the early project phase. In particular in large telescope design the issue increases in complexity, main reasons are:

Large tolerance chains (mechanical and optical)

III.

DESIGN PROCESS

In order to reduce the impact of Sun intrusion to the instrument performance a dedicated design process has been developed at OHB-Munich. The design of the instrument has to consider two main aspects:

Point 1) is considered in:

Point 2) can be performed by a proper design of baffles and local sun shielding (baffling). In fact, the design of the instrument baffling has the double function to protect the instrument thermally from the Sun and to limit the impact of the Sun intrusion on the optical performance. In Fig. 6 the generic baffling design flow is shown.

Dynamic displacements contradict the optical need for closed cavities. Mechanical parts need clearance between each other causing gaps that possibly introduce light into the not shielded regions of the instrument.

The two points above determinate with which angles the Sun light enters the instrument.

Finally the analysis has to consider all the deviations from the nominal design, in particular:

Fig. 6:

Design Flow

Fig. 7:

Sun propagation analysis flow

The deviations above can be considered by the definition of a worst case scenario.

The results of the Sun propagation analysis are used as input for a new thermal and, if needed, structural simulation to verify the thermal, optical and mechanical design including AIT aspects. In addition, a updated redesign of the baffle and shielding structures may follow.

These steps usually have to be repeated until a reasonable compromise for the contradicting restraints of optical, thermal and structural needs can be found. Fig. 8 shows the baffle and shielding components introduced in the MTG-IRS front telescope to reduce Sun intrusion and stray light impact on the instrument.

Fig. 8:

Baffle and shielding components introduced for Sun intrusion and stray light reduction

When preliminary design assumptions, budgets and requirements are violated during the project development (e.g. large displacements, large alignment range, high temperatures) the complete design loop has to be repeated to generate a non-conflicting design baseline. Design modifications have to be implemented and checked in all models to secure the instrument design and performances.

An example of FCI-TA vane thermal analysis is shown in Fig. 9 and Fig. 10.

Fig. 9

Thermal analysis on the baffling vanes; geometrical model of a vane with Sun spot

Fig. 10:

Thermal analysis on the baffling vanes; example of temperature profiles

In the example different designs of the baffling vanes have been traded to identify the best compromise in term of thermal stability vs. mechanical design. Different cases of solar intrusion have been considered. At the end a proper mechanical design has been selected in term of material, geometry, mass, coating, manufacturing process, etc.

IV.

MULTIDISCIPLINARY DATA EXCHANGE

For the analysis loop dedicated tools for the different purposes are applied.

The performance of this chain of simulations interfacing different tools is crucial and time consuming. No direct transfer of results is possible due to different input and output formats of the tools. Interfaces in this simulation chain are:

Fig. 11:

Thermal mapping tool parameters

Fig. 12:

Mapping process flow

With this tool the geometry is first divided into single parts. The model parting must detect/prescribe the part limits. The meshes are also read in. Thermal mapping is done in two steps: First the algorithm defines temperatures for the main structural (FEM) nodes as a function of thermal elements. Then the not covered structural areas are defined based on available temperatures by interpolation (no further use of the thermal model). This creates the temperature maps for the structural simulation and is exported in a NASTRAN format. All parts and the temperature mapping have to be verified visually. The thermal mapping tool allows a faster conversion of thermal simulation data to structural temperature input. The mapping has to be done for every Sun position and, in case of time dependent simulations, for every time step.

V.

SUN INTRUSION VERIFICATION

Sun intrusion verification shall be done in a space simulator. An exemplary facility (LSS in ESTEC) can be described as follows: Three parts comprise the thermal vacuum chamber: the main vacuum chamber, where the instrument is placed during the test, the auxiliary chamber, where the mirror and the cryo pumps are located, and the seismic block, installed at the bottom of the main Chamber with the aim to accommodate the interface structure to the instrument. Between the instrument and the seismic block a spin box or other structure to rotate and shift the instrument is installed.

The Sun simulator consists of lamps whose radiation is projected through an optical integrator onto the collimation mirror placed in the auxiliary chamber. Its purpose is to provide a homogeneous parallel light beam with respect to the horizontal plane and the rotation around the vertical axis of the main chamber.

In principle every Sun position can be simulated within these facilities since the instrument can be rotated and tilted with respect to the incoming light. The problem arising here is that these facilities are built for thermal test of the full instrument structure and not to reproduce the beam inside the optical instrument. The divergence of the simulated Sun beam is larger than the divergence of the real Sun (most space simulators have a minimum cone angle around 2°) leading to larger spots inside the instrument during test compared to space. Depending on the position inside the instrument and the type of Sun simulator the spot size can be up to seven times larger inside the IRS instrument than the real solar spot in flight reducing the power density by a factor of 49.

Sun intrusion verification with the existing space simulators is fully representative up to M1 where the light is not focussed. At M2 the spot has roughly double the size (Fig. 13) but (for Sun in the FOV) is still fully on the mirror so thermal simulations could be simulated for the reduced power input and compared to the test.

Fig. 13:

Spot size enlargement at M2 and field stop of MTG-IRS for Sun divergence angle 0.25° (space) (a), and 1.9° (test facility) (b) solar cone angle.

At the field stop the spot size is multiple times as large as the nominal Sun spot (Fig. 13 b). Here the test conditions are too far from the real in-orbit Sun intrusion conditions to predict thermal effects.

To correlate the used models to the Sun intrusion test in a space simulator the models have to be adapted to the larger source. ASAP simulations have to be performed to predict the spot size and power distribution inside the instrument. The ASAP power distribution then has to be mapped to the ESATAN mesh for a simulation of the larger solar source. Thermal predictions can then be established with the adapted model considering the sun simulator behavior which can then be used to assess the test results.

VI.

CONCLUSION AND WAY FORWARD

The solar intrusion in large optical instruments for Earth observation is are an important issue to be understood and considered during instrument design because it may lead to instrument performance degradation or even to permanent corruption of the instrument functions.

The mitigation of the Sun intrusion impact on a large optical instrument is a multidisciplinary task where all the technical disciplines are involved. In the frame of the MTG project OHB-Munich has experienced the definition and implementation of a design process to properly identify and analyse the effects of Sun intrusion. The analysis results have been used to implement mitigation options to secure the instrument design. Due to the multidisciplinary nature of the topic, the design loops within the design flow are time and cost intensive. For that reason OHB-Munich is currently investigating a methodology to improve the efficiency of the design loop reducing duration and cost. At the same time OHB-Munich is targeting the generation of a process which may even produce more accurate and reliable analysis results.

The first step is to reduce the inefficiency of the model exchange. Interfacing between simulation tools is inefficient and error prone. Values have to be inter- and extrapolated to match the meshes of other simulation software. For the interface between ESATAN and NASTRAN a special mapping tool has been developed by OHB-Munich. This tool reduces the time needed for mapping by about a factor of 6. Two options are available for the exchange between optical and thermal simulations:

With applying an extended mapping tool the advantages of single simulations would be preserved but the overall development time would not reduce sufficiently. On the other hand ASAP, ESATAN and NASTRAN are ESA approved simulation tools.

The use of a single tool will reduce the need of importing the CAD geometry into different programs, and changes in geometry will only have to be implemented once. Results of one simulation will be directly used for the next step of the design flow.

OHB-Munich has kicked-off an investigation to consolidate the needs and potential improvements of the analysis chain. This investigation includes also a tool survey