A. VNS Driving Requirements and Constraints

The VNS shall provide co-registered images of four spectral channels. The required spectral bands are detailed in TABLE I. The other main requirements are a swath width of 150km, ground sampling distance of 500m, MTF larger than 0.25, polarization sensitivity below 2.5% and a radiometric accuracy better than 10% absolute and 1% relative between the channels.

TABLE I.

VNS SPECTRAL BANDS

The requirements are coupled with the constraints of a large range for the spacecraft orbit altitude and a demanding temperature environment.

The combination of demanding requirements and constraints result in a challenging opto-mechanical design with strict constraints on temperature stability, mechanical alignment and mechanical stability [3].

B. VNS Design

The VNS consist of the following three assemblies, which are directly mounted on the MSI Optical Bench:

Fig. II-1

3D CAD model of the VNS

The VNS has two separate apertures (Fig. II-2), one 4.85mm diameter aperture for the VIS, NIR and SWIR-1 channels, and a second 10.47mm diameter aperture for the SWIR-2 channel. The larger aperture for the SWIR-2 channel compensates for the weaker light levels coming from the earth. The light collected through the VIS/NIR/SWIR1 aperture is separated in different spectral regions using a set of dichroics.

Fig. II-2:

VNS camera optical configuration

The scene of the four channels is imaged on linear detector arrays using refractive telescopes. Inside of the telescopes a spectral band filter ensures the required wavelength range. The focal length (22.2mm) of the telescopes is based on the orbit height, ground sampling distance requirement and the 25μm detector pixel pitch.

The four detector arrays are developed and manufactured by Xenics (Leuven, Belgium). The detector packages consist of a thick film hybrid circuit with a centered 512 pixel linear photodiode array and two readout integrated circuits (corresponding to odd/even photosites). The VIS and NIR channel detectors employ silicon photodiodes, while the SWIR channels employ InGaAs photodiodes.

The in-orbit operating temperature of the VIS/NIR and SWIR-1 detectors are 300K, while the operational temperature of SWIR-2 is 235K. The reduced SWIR-2 temperature is implemented to achieve the required signal to noise levels and radiometric accuracy. In order to reach its operating temperature the SWIR-2 detector is thermally isolated from the VNS camera and thermally coupled to the VNS SWIR-2 radiator using a flex link and cold finger (see Fig. II-3). The thermal isolation of the SWIR-2 detector is achieved by the implementation of titanium flexures in the camera design.

Fig. II-3

SWIR-2 camera assembly

The radiator panel, with a surface of 0.175m², is designed to reach temperature well below the target. An active thermal control circuit stabilizes the detector temperature at 235K.

During the in-flight operations the VNS will be regularly calibrated. Dark measurements will be performed for offset corrections during eclipse of the orbit; sun calibration will be performed for response calibration during passes over the South Pole region. During the sun calibration the sun light will illuminate a pair of Quasi Volume Diffusers (QVD). The QVDs are mounted on a rotating carousel (see Fig. II-4), which is used to switch between dark, sun and earth viewing modes.

Fig. II-4

VNS Calibration mechanism

The rotating carousel is driven by a Phytron VSS-43 motor and is guided by ESTL procured bearings.

C. VNS Calibration Mechanism Life Test

The first physical model built of the VNS is the Life Test Model (LTM), see Fig. II-5. The LTM consists of the VNS Calibration Mechanism Assembly (CMA) and is used to perform an accelerated life test on the Mechanism.

Fig. II-5:

VNS life test model during vibration test at CSL

The test program of the VNS LTM consisted of:

The VNS LTM successfully passed all these tests maintaining the required performance and is therefore considered qualified for the MSI program.

D. VNS Engineering Confidence Model

The second physical model built is the Engineering Confidence Model (ECM), see Fig. II-6. The purpose of the ECM is to validate the overall instrument concept prior to proceeding with the Proto-Flight Model (PFM). The VNS ECM is mechanically and electrically representative. Two of the four optical channels (VIS and SWIR1) and the Calibration Mechanism are fully functional.

Fig. II-6

VNS ECM (Sun Baffle not assembled)

During the assembly of the VNS ECM, the assembly and alignment procedures have been checked and refined. Using the refined alignment procedures, the scene can be imaged at the detectors within a position accuracy of a few micrometers. These accuracies result in a VNS instrument pointing and channels co-registration well within the requirements.

After full assembly the VNS ECM has been subjected to a qualification level vibration campaign. The pre and post vibration performance checks show that the pointing and coregistration shifts are well within the requirements. This validates the mechanical integrity and stability of the VNS concept.

At the moment of writing this paper the full performance test campaign on the ECM are being carried out. The performance tests already completed show:

All these performance parameters are within their respective requirement.

Before the end of the third quarter of 2012 all performance test and a dry-run calibration will be concluded on the VNS ECM.

A. TIR Requirements and Design

The TIR provides the other 3 spectral bands of the MSI instrument. Its optical design has been reported before [4]. To recap, the spectral bands are shown in TABLE II.

TABLE II.

TIR SPECTRAL BANDS

As with the VNS, the system constraints lead to a significant design challenge which the unit’s external appearance (Fig. III-1) belies.

Fig. III-1

TIR (left) CAD (right) ECM

The internal layout of the TIR comprises an f/8 telescope upstream of a dichroic and filter assembly which provides the spectral division into the 3 TIR bands. Downstream of this assembly is an f/1 relay lens, responsible for directing the 3 colour channels onto a 2D α-Si microbolometer detector. This system is read out in Time-Delay-Integration (TDI) mode by the front end electronics (FEE) such that the output is a linear co-added, background subtracted image suitable for the instrument’s pushbroom operation in flight.

Fig. III-2 shows the TIR located on the OBM, with its outer shroud removed. The unit has three target apertures – the primary science Earth (nadir) viewing port, the Cold Space viewing port for dark scene reference and an on-board calibration blackbody aperture providing the bright scene reference. The blackbody is a passive component, thermally tied to the TIR optical bench and will be monitored by high precision platinum resistance thermometers in the PFM. In the ECM, these PRTs are substituted by thermistors.

Fig. III-2

MSI OBM from above: TIR cover, SWIR2 radiator, harnesses omitted omitted

Selection of one of 3 target apertures directed through the optical chain onto the focal plane assembly is performed by the TIR calibration mechanism assembly. The focal plane array (FPA) and relay lens system design have been described in detail [5].

B. TIR Calibration Mechanism Assembly

The TIR requires regular calibration, by pointing the optical beam at an on-board blackbody source and to a cold space viewport. After viewing and recording the reference sources, the optical beam is then aimed back at Earth for science data acquisition. This motion is carried out by reflecting the beam off a rotating calibration mirror. The mirror is a two surface, front coated item. One side of the mirror is plane, and this is used for Earth view and cold space view. The other side of the mirror is concave, providing focusing down on to the TIR blackbody aperture. To perform its function, the mirror has to be flipped through more than 180°. The stability and positional repeatability of the mirror for Earth viewing is very strict, so there is a hard end-stop at that point.

Fig. III-3

TIR CMA ECM design

The TIR CMA ECM is based around a Phytron VSS-42 motor. The shaft assembly and other components are SSTL design. The ECM equipment is used as a qualification model; it diverges from the traditional mechanism life test model in that it is used in the next level assembly, the TIR ECM, and is not intended to be subjected to post-test constructional analysis.

Fig. III-4

TIR CMA ECM and the equipment test campaign per model

At the time of writing, the TIR CMA ECM has withstood a combined xyz random vibration response of 65.7 g_rms at the motor. In the life test, the CMA has experienced 44117 cycles; 40000 cycles were the target to meet the orbit lifetime under qualification conditions.

The TIR CMA ECM has also undergone an exported torque (microvibration) test, illustrated in Fig. III-5. The results are expected to be finalized by September 2012. The measurements and specific analysis approach used show that the CMA exports very low moments (<0.04 Nm) and low forces (<0.12 N) between 1 Hz and 1 kHz.

Fig. III-5

TIR CMA ECM microvibration test

C. TIR Calibration Blackbody (TIR-BB) EQM

As with the TIR CMA ECM, the TIR-BB is qualified at subassembly level. The blackbody is a standard qualification model, with only the temperature sensors differing from the flight model. The primary motivation for qualification is the black coating response within the specific internal geometry afforded by the TIR-BB to environmental testing.

Fig. III-6

TIR CMA (left) design (right) EQM

The test flow is shown in Fig. III-7. The thermal test extrema were -40°C to +70°C.

Fig. III-7

TIR CMA qualification testing campaign

The TIR-BB EQM passed its qualification at equipment level and has been incorporated into the TIR ECM.

D. TIR ECM

The purpose of the TIR ECM is to address the structural and performance risks prior to entering the PFM phase of the instrument programme.

Structurally the major risks arise in integration of key subassemblies of this unit. Specifically, the risk of misalignment of the components within a subassembly, and of subassemblies to each other is significant enough to warrant a high fidelity ECM to rehearse on.

The use of TDI as the readout configuration also demands excellent optical aberration control; again the principal risk here is judged to arise from assembly activities.

Strict control of the background pedestal on the signal is necessary to sustain the required radiometric accuracy over an orbit interval before recalibration. The TIR is designed for thermally stability rather than thermal control responsivity.

With all this in mind, from a structural viewpoint the TIR ECM is equivalent to a prototype.

The ECM serves to prove the assembly and integration processes and trial the bespoke ground support equipment made to enable the functional and performance tests. It also provides a base for the development of user friendly data acquisition and processing software for the future stages of the MSI and EarthCARE programme.

Fig. III-8

TIR ECM internal (left) FPA end (right) from TIR-BB end

At present the TIR ECM is undergoing its performance tests prior to mechanical environment testing. First interface and functional tests have been completed, with the unit passing. The test programme for the TIR ECM is expected to be complete by the last quarter of 2012.