I.

INTRODUCTION

METimage is a cross-purpose medium resolution, multi-spectral optical imaging instrument dedicated for operational meteorology, oceanography, and climate applications. It is implemented as passive imaging spectro-radiometer, capable of measuring thermal radiance emitted by the Earth and solar backscattered radiation in a broad spectral range. The instrument is mounted on MetOp-SG satellite A.

We present the METimage instrument design and discuss the achievable instrument performance.

II.

MISSION CONTEXT

The METimage instrument is designed to serve the VIS/IR Imaging Mission (VII) of the EUMETSAT Polar System – Second Generation (EPS-SG) [1]. EPS-SG shall provide global observations which cover a broad spectral range (from UV to mid infrared), are related to different spatial coverage (global and regional), and are characterized by a variety of different time scales, in order to continue and enhance the services offered by the EPS system. EPS-SG is Europe’s contribution to the Joint Polar System (JPS), which is composed of EUMETSAT’s EPS-SG satellites, NOAA’s JPSS satellites and shared ground systems and services. By global coverage and a variety of passive and active sensors at low Earth orbits a significant improvement of the numerical weather prediction is targeted.

The MetOp-SG satellites – two series of satellites, with three units in each series – constitute the space segment of EPS-SG. The Satellite A series focuses on optical instruments and atmospheric sounders, while the Satellite B series focuses on microwave instruments. The three satellites in each series provide a total nominal lifetime of 21 years. The METimage instrument is embarked on the Satellite A nadir panel, see Fig. 1.

Fig. 1

METimage onboard MetOp-SG Satellite A. (Courtesy Airbus Defence & Space SAS)

III.

INSTRUMENT & OBSERVATION CONCEPT

METimage is a passive imaging spectro-radiometer, capable of measuring thermal radiance emitted by the Earth and solar backscattered radiation in 20 spectral bands from 443 to 13.345nm. The instrument onboard MetOps-SG satellites A is placed in a Sun-synchronous polar orbit with an average altitude of 830km. Full Earth coverage is achieved within the 14 orbits of 1 day.

METimage achieves global coverage with 500m square pixels by continuous scanning orthogonal to the flight direction. Due to the scan motion, the image moves sequentially over the detector channels. By proper timing of the sampling, a certain pixel in the image is measured sequentially by different spectral channels, see Fig. 2.

Fig. 2

METimage scanning principle, illustrating the sequential measurement of a pixel p within 1 scan over time t.

The METimage instrument is designed to cover a large across track swath of ~2670km, corresponding to ±53deg scan angle, with a constant spatial sampling angle across the swath and a spatial resolution of 500m at nadir. It is implemented as in-beam scanner with static telescope and synchronous field de-rotation. The scanner is rotating continuously clockwise about the telescope axis. The scan period of 1.729s is defined by the satellite orbit and the spatial resolution. Field de-rotation, realized by a set of planar mirror reflections, ensures a regular imaging geometry over the full scan range.

The scanning principle also allows for regular views to dedicated calibration sources without interrupting the scientific observation, see Fig. 3. A two point calibration scheme is implemented, i.e. each signal channel can be re-calibrated with two different calibration sources at different signal levels. A dark signal level (deep space view DV) is provided for offset correction while the bright sources (solar calibration device SCAD, thermal calibration device TCAD) provide the gain calibration. The calibration sources are placed in front of the scan mirror to allow for the calibration of the complete optical and electrical detection chain.

Fig. 3

METimage observation principle, illustrating the Earth view as well as the calibration views.

The telescope is designed to offer an instantaneous field of view (IFOV) of 1.6deg. This supports the scanning of 24 ground pixel of 500m resolution in flight direction and provides 10 slots for spectral channels or TDI stages for each of the 3 focal planes (spectral bands VNIR, SMWIR and LVWIR). The spectral band separation between the focal planes is accommodated by 2 dichroic beam splitters, the channel separation within each band is implemented by spectral bandpass filters.

The instrument key parameters are given in Tab. 1.

Tab. 1

Key instrument parameters.

IV.

INSTRUMENT DESIGN

The orbit and observational parameters together with the detector parameters lead to a TMA design for the telescope. The pupil stop is between the scanner mirror and M1, a field stop is placed on the intermediate image after M2 to limit the out-of-field stray light. The rotation of the scan mirror is compensated by a beam de-rotator following the telescope. The de-rotator is electrically synchronized with the scanner and rotates at half of the scanner frequency. It is implemented with as 5 mirror system to minimize polarization effects. A set of dichroic beam splitters separates the beam into 3 spectral bands and folding mirrors direct the beams to the VNIR detector and the IR intermediate images. At these intermediate images the spectral channels are spatially separated by filter stripes, and re-imaged onto the IR detectors to cope with the detector geometry (magnification of 0.135). Inside this re-imaging optics a cold pupil stop reduces slightly the aperture to remove the direct light coming from the (warm) entrance pupil stop. The overall METimage optical design is depicted in Fig. 4.

Fig. 4

METimage optical design.

The instrument consists of the optical head as well as of the electronic units located inside the spacecraft’s payload equipment bay. The optical is constituted of four major parts (see Fig. 5):

Fig. 5

METimage optical head (structural parts transparent, MLI not shown).

The instrument structure is made of Aluminum/CFRP sandwich panels and is actively thermal controlled. The Earth view baffle is defined by the instrument’s scan range and by the deep space view at -66.6deg.

The thermal calibration device (TCAD) is a black plate with well-known emissivity and high accuracy temperature control (sensors with 1mK accuracy over lifetime). The solar calibration device (SCAD) is implemented as a reflective diffusor made of Spectralon. To cope with aging effects, the diffusor is illuminated only once in several days. In addition a reference diffusor is available to determine the degradation over time. Details on instrument calibration are given in [2].

The scan mirror and the de-rotator optical assembly are mounted on respective mechanisms, which are properly aligned and actively controlled to ensure the electrical synchronization. The telescope optics is implemented on a separate structure to allow for proper alignment.

The cryogenic subsystem is implemented on a separate structure to allow for a stand-alone integration and verification but also to ease the integration into the optical head structure, see Fig. 6.

Fig. 6

Cryogenic subsystem.

The cryostat design with 2 inner shields ensures a detector temperature of 60K and temperatures of up to ~80K for the re-imaging optics. Two windows allow the optical paths to enter the cryostat. The refractive re-imaging optics for each IR band is mounted inside a Titanium structure which interfaces the cryostat housing with GFRP struts to minimize the thermal loads. The IR detectors are coupled to the pulse tube cryo-coolers via highly conductive thermal links. The cryo-cooler compressors are linked via heat pipes to the radiator.

The beam splitters and fold mirrors as well as the VNIR filter assembly and FPA are implemented on a separate bench. This bench is mounted on the cryostat to make feasible the alignment of the re-imaging optics to the beam splitters and to improve the spatial co-registration between the VNIR channels and the IR channels.

The proximity electronics are used for detector output signal pre-amplification. They are mounted as close as possible to the detectors and are interfacing the front end electronics (FEE), which is located in the external electric assembly. The FEE performs the analog to digital conversion and data serialization.

The data acquired from the Earth view and the calibration views is provided via the FEE to the METimage central electronics (MCE). The MCE controls the entire instrument, including thermal control, mechanism synchronization, cryo-cooler operation, or data handling. It performs SNR bit trimming to reduce the overall data volume. The MCE is located inside the payload equipment bay and is connected to the optical head via a harness of ~3m length. The TM/TC interface to the spacecraft is via SpaceWire.

VII.

INSTRUMENT PERFORMANCE

The key performance parameters of the METimage instrument are summarized in Tab. 2 [3]. The instrument is designed to fulfill these requirements in an optimum way. Verification of the requirements is discussed in [2].

Tab. 2

Key performance parameters.

A particular driver for the instrument performance is the on-ground storage of the second and third instrument flight model. The instrument is stored onboard the spacecraft and subject to yearly maintenance activities. A critical parameter is the particulate and molecular contamination during the storage period. The second and third flight models have to be re-calibrated prior to launch to ensure a proper instrument characterization.

VIII.

CONCLUSIONS & OUTLOOK

The METimage instrument is in the detailed design phase. The instrument concept has been established, the main subsystem contractors have been selected, and the detailed design of the major subsystems is ongoing. The major challenges are the stringent requirements on the optics in terms of performance, straylight and alignment as well as the required calibration and verification effort.

The next milestones are the CDR as well as the delivery of the EFM and the STM. The launch of the first flight model onboard MetOp-SG satellite A1 is planned for 2021, the 2 further flight models follow in 7 years intervals.