Changes between Version 8 and Version 9 of UserApp/Proba-V

Dec 4, 2011, 4:13:05 PM (8 years ago)


  • UserApp/Proba-V

    v8 v9  
    2120Bearing a different designation from its predecessors, Proba-V is an operational as well as experimental mission, designed to serve an existing user community.
    217216Data compression: The massive amount of data produced by the instrument is beyond the capabilities of the bandwidth available on board of a small satellite. Data are reduced by using a lossless data compression algorithm implemented in a specific electronics. The data compression ratio obtained using standard CCSDS compression algorithms (CCSDS 133.0 B-1) is shown in Table 2.
    222218Compression ratio
    242237Table 2: Overview of compression rates
    256251|+'''Figure 5: Photo of the X-band transmitter (image credit: CNES, ESA)'''
    258 |[[File:ProbaV_AutoE.jpeg]]
    259 |}
     253|[wiki:File:ProbaV_AutoE.jpeg File:ProbaV AutoE.jpeg]
     255= Sensor complement: (VGT-P) =
     258The PROBA-Vegetation payload is a multispectral spectrometer with 4 spectral bands and with a very large swath of 2285 km to guarantee daily coverage above 35 latitude. The payload consists of 3 identical SI (Spectral Imagers), each with a very compact TMA telescope. Each TMA, having a FOV of 34º, contains 4 spectral bands: 3 bands in the visible range and one band in the SWIR spectral range. 19) 20)
     260VGT-P is restricted to imaging land and dedicated calibration zones. On-board the spacecraft there is for each spectral imager a land sea mask that is provided by the PI (Principal Investigator). The land sea mask removes the pixels that contain only sea and it dictates when each SI should be in imaging mode.
     262OIP (Optronic Instruments & Products, Belgium) is the industrial prime contractor for the payload and is responsible for the design and development of the PROBA-V instrument and AMOS (Belgium) is responsible for the manufacturing and alignment of the telescope. The major payload challenge lies in the fact that the wide-swath imaging instrument has to fit into a small satellite with limited resources. The TMAs and the SWIR FPA have to be developed for the VGT-P since no COTS products are available
     264Each SI (Spectral Imager) contains one telescope, a beam splitter to separate the VNIR from the SWIR spectral bands, spectral bandpass filters to select the spectral bands, and the VNIR and SWIR focal plane arrays. The spectral bands will be realized by spectral bandpass filters centered on 460, 658, 834 and 1610 nm, with bandwidths of respectively 42, 82, 121 and 80 nm. The filters will be applied on the detector windows.
     266The optical axis of the central telescope will point to nadir and the two outer telescopes will point 34º from nadir. Together the three TMAs will cover a complete FOV of 102º. The optical system is telecentric, and the aperture is located at the position of the second (spherical) mirror.
     268{| align="center"
     269|+'''Figure 6: Conceptual accommodation of the VGT-P inside the PROBA-V spacecraft (image credit: OIP, ESA)'''
     271|[wiki:File:ProbaV_AutoD.jpeg File:ProbaV AutoD.jpeg]
     274Figure 6 shows the payload mounted on the PROBA-V platform. Given the reduced size of the platform, a H-shape structure, the only practical location of the payload is on the anti-velocity panel. This accommodation, with respect to a solution with the payload in the middle of the structure, has the advantage of a very simple assembly and clean mechanical interface. The drawback is a larger temperature gradient due to the close vicinity of the payload to the solar panel.
     276{| align="center"
     277|+'''Figure 7: Block diagram of the VGT-P (image credit: OIP)'''
     279|[wiki:File:ProbaV_AutoC.jpeg File:ProbaV AutoC.jpeg]
     282Legend to Figure 7:
     284 *  ROE (Read Out Electronics)
     286 *  PSU (Power Supply Unit)
     288 *  DHU (Data Handling Unit)
     290 *  PEU (Peripheral Electronics Unit)
     292 *  MLI (Multi-Layered Insulation)
     294TMA telescope development: VGT-P makes use of a set of three such telescopes, identical to each other. The purpose of the related ESA GSTP (General Support Technology Program) development is to demonstrate the feasibility of one item of the set with respect to its required optical quality, and to secure the instrument development. The entire telescope is an athermal design made of the same aluminium material. The mirrors quality is achieved by SPDT and the alignment rely on the very precise matching of the mirrors with the mounting structure.
     296Taking into account the mission constraints and objectives, including the innovative features of the instrument, a full-aluminum design was selected. This choice allows taking benefit from the recent developments in ultra-precision milling and turning techniques, as well as in optical aluminum production. Furthermore, this leads to a homothetic telescope behavior. The optical performance requirement of the telescope with regard to MTF (Modulation Transfer Function) is given in Table 4.
     298SPDT (Single Point Diamond Turning): Diamond turning is a process of mechanical machining of precision elements using Computer Numerical Control (CNC) lathes equipped with natural or synthetic diamond-tipped cutting elements. The SPDT process is widely used to manufacture high-quality aspheric optical elements from crystals, metals, acrylic, and other materials. Optical elements produced by the means of diamond turning are used in optical assemblies in telescopes, scientic research instruments and numerous other systems and divices. Diamond turning is specifically useful when cutting materials that feature aspheric shapes such as TMA surfaces.
     301{| border="1"
     302|+ '''Table 4: Performance requirements of MTF'''
     305|Nominal MTF (%)
     306|2? MTF (%)
     307|Max. frequency (lp/mm)
     330{| align="center"
     331|+'''Figure 8: Optical design concept of the TMA (ray tracing diagram), image credit: OIP'''
     333|[wiki:File:ProbaV_AutoB.jpeg File:ProbaV AutoB.jpeg]
     336Baffle design (Ref. 20): The aim of the baffle design is to block the out-of-field light which could enter the instrument and reach the detector, directly or through one or several reflections on the mirrors. This 1st order analysis didn’t consider vanes on the baffles and diffusion on M1 of out-of-field light.
     338The preliminary baffle layout is presented in Figure 9. It comprises 7 baffles: 1 at the entrance aperture of the instrument and 6 placed inside the instrument. An aperture stop is also placed at the level of the secondary mirror.
     340The baffle #1 is placed at the entrance of the instrument. Its role is to limit the out-of-field light that could directly reach the mirrors. The combination of the baffles #1 and #2 stops the direct view of the M3 mirror through the instrument entrance. The length of the upper side of the entrance baffle is defined to stop the light which could directly reach the M3 mirror and that could not be stopped by the lower side of the entrance baffle and by baffle #2. Some out-of-field light can also reach the M2 and M3 mirrors after reflecting on M1. This cannot be totally avoided but the length of the lower side of baffle #1 has been chosen in such a way that this straylight is stopped by the baffle #3 after reflecting on M3. The baffle #3 is placed below the M2 mirror and stops the direct view of the M1 mirror by the VNIR detector. The baffle #4 is a critical location where reflection or diffusion on the M2 structure can occur and bring stray light to the VNIR detector which is very close. Vanes will be placed at this location. The baffles #5 and #6 are placed near the focal planes to isolate the detectors from each other. The baffle #7 avoids a direct view to the SWIR detector from the M1 or M3 mirrors.
     342{| align="center"
     343|+'''Figure 9: Proba-V TMA preliminary baffles layout (image credit: CSL, OIP, ESA/ESTEC)'''
     345|[wiki:File:ProbaV_AutoA.jpeg File:ProbaV AutoA.jpeg]
     348{| align="center"
     349|+'''Figure 10: Illustration of the optical assembly of VGT-P and two star trackers on the optical bench (image credit: OIP)'''
     351|[wiki:File:ProbaV_Auto9.jpeg File:ProbaV Auto9.jpeg]
     354SWIR detector development: This development concerns the large format SWIR focal plane array containing at least 2704 pixels with 25 µm pitch. The solution selected uses the mechanical butting technique with 3 overlapping detectors of 1024 pixels and approximately 80 pixels in the overlap area. In Figure 11 the linear detector arrays are shown in green, while the ROICs (Readout Integrated Circuits) are presented in red. Xenics NV of Leuven, Belgium, is developing the InGaAs SWIR detector array.
     356Several techniques were evaluated to realize the required alignment accuracy of the 3 PDA (Photo Diode Array) subarrays in the FPA. The requested alignment accuracies are:
     358- In plane alignment accuracy, ?X and ?Y = ± 12.5 µm
     360- Out of plane alignment accuracy, ?Z = ± 50.0 µm
     362- Subarray PDA separation = < 1.5 mm.
     364{| align="center"
     365|+'''Figure 11: Schematic view of of the mechanically butted SWIR detector array (image credit: OIP, Xenics)'''
     367|[wiki:File:ProbaV_Auto8.jpeg File:ProbaV Auto8.jpeg]
     370{| align="center"
     371|+'''Figure 12: Drawing of the subarray alignment tools with the 3 PDAs (green) mounted on the mount (image credit: OIP, Xenics)'''
     373|[wiki:File:ProbaV_Auto7.jpeg File:ProbaV Auto7.jpeg]
     376{| align="center"
     377|+'''Figure 13: Photo of the fully assembled FPA in its package (image credit: OIP, Xenics)'''
     379|[wiki:File:ProbaV_Auto6.jpeg File:ProbaV Auto6.jpeg]
     381= Thermal design of the VGT-P instrument:  =
     384One of the major drawbacks of using multiple optical systems in parallel while imaging, is the effect of pointing inaccuracies due to thermo-elastic and mechanical deformations. It is obvious that such pointing errors can easily destroy the quality of the images. For the VGT-P, the stringent geo-location requirements demand the instrument to be thermally stabilized as much as possible to reduce any thermo-elastic disturbances.
     386Since the PROBA platform is fairly limited in the delivery of power, VGT-P needs to be very efficient in its power use. As a direct consequence, there is no possibility to have an active thermal control system to stabilize the instrument. The thermal design of the instrument must therefore be very carefully assessed and engineered.
     388 *  Thermal isolation: Firstly, as the surrounding satellite panels are heavily fluctuating in temperature during the orbit, it is of the utmost importance to shield the instrument thermally from these platform variations. To reduce the radiative heat loads from the environment, the instrument is completely wrapped in a 12 layer MLI. To reduce the conductive heat loads from the mounting plane, the instrument is mounted by means of titanium quasi isostatic mounting feet. These quasi isostatic mounting feet also play a major role in the transfer of the thermo-mechanical deformations from the underlying platform to the optical bench as they strongly reduce these deformations. Therefore, these titanium flexures as they are called not only serve as a thermal isolation, but also acts as a thermo-elastic isolator.
     390Power reduction: A natural step to reduce the thermo-elastic effects on the instrument is to reduce as much as possible the heat load on the optomechanics. Therefore, all non critical and heavy heat dissipating detector read-out electronics are separated from the optics. The FPAs of the telescope only contain the detector and electronic components which drive the radiometric performances of the instrument. These FPA electronics are connected through a flex rigid to the ROE (Read-Out Electronics) which is thermally and structurally disconnected from the optomechanics. All major heat dissipating components are located in there.
     392Obiously, also the central electronics (DHU and PSU) are separated from the optomechanical imaging system. By doing this, the total power dissipation on the optical bench is only 9W, which is less than ¼ of the total power dissipation of the complete VTG-P instrument.
     394 *  Heat dissipation: To dissipate this heat load, a radiator is needed. Several concepts were proposed and analyzed. The most efficient radiators point towards deep space which would enable us to cool down the complete instrument to very cold temperatures. This had a drawback that additional heaters would have been needed to stabilize the thermal regime of the instrument to normal working temperature. Moreover, as the instrument is always pointing downwards towards Earth, the radiator would have been located on the side of the instrument which naturally induces an asymmetry in the optomechanics. Such asymmetry is not desired in an imaging sensor with stringent pointing requirements. Moreover, heat pipes would have been mandatory to extract as efficient as possible all heat of the detectors towards the radiator which unnecessarily complicated the complete design.
     396From a thermo-elastic point of view, it was highly desirable to respect the symmetry of the instrument as much as possible and to symmetrically extract the heat from the FPA’s on the optical bench. Thus, it was chosen to locate the radiator in front of the instrument and point it towards the earth surface. As the earth is thermally quite stable at a fairly modest temperature and as the payload is always pointed nadir, this is the perfect heat drain for the instrument. The implementation of this concept reduces the complexity dramatically: the radiator, covered with aluminized Teflon, is connected through two thermal straps towards the front of the instrument without the need to install heat pipes.
     398 *  Stability: Stability is the key aspect of thermo-elastic performance. Of course, without the possibility of an active thermal control system, stability is quite difficult to achieve in a thermal environment which is constantly varying over the orbit.
     400To tackle this problem, the first stage was to avoid the randomness in the heat loads on the instrument and to have constant thermal regime along the orbit. As the payload is encircling the Earth with its radiator pointing at nadir, the heat load on the radiator is subjected to a varying regime from sunlit to eclipse and back. From the point of view of efficient power use, the imaging circuits on the instrument are switched off by the satellite if no imaging is needed (over the oceans, over the poles, during eclipse). This would induce different thermal regimes from one orbit to the other, which is not acceptable from pointing point of view. But leaving all electronics switched on during non operation is a no go considering the lack of power. As a compromise, during sunlit conditions and when the imaging electronics is powered off, a heater located on the detector with a heat load equal to the heat load of the detector and FPA is powered. In this way, the heat loads on the optical system remain constant during sunlit. During eclipse, all is switched off. - As a consequence, a constant thermal regime on the optics is established: during 1/3 of the orbit (eclipse) the radiator faces only IR and the instrument is switched off. During the 2/3 of the orbit, the radiator sees IR and albedo and the instrument is switched on.
     402 *  Gradients: The final challenge in the thermal design is to avoid thermal gradients in the instrument as gradients are hard to control and can severely affect the thermo-elastic performance. As already described, the heat extraction has respected the symmetry of the instrument. An unavoidable asymmetry is the location of the Star Trackers as they have their own limitations. The heat load from the FPA and the detectors on the telescopes is normally entering the instrument through the TMAs to the top skin of the optical bench. However, this would heavily distort and bend the optical bench as the top skin will expand more than the bottom. To reduce this effect, thermal straps are designed to extract most of the heat (4/5) from the detectors and the FPA towards the optical bench, the rest is still entering the TMA structure. To reduce the thermal bending, the heat straps are mounted on the side of the optical bench to avoid the bending of the bench.
     404[[ For further info]]