Changes between Version 14 and Version 15 of UserApp/Proba-V


Ignore:
Timestamp:
Dec 6, 2011, 1:29:37 PM (8 years ago)
Author:
Iliyankatsarski
Comment:

Legend:

Unmodified
Added
Removed
Modified
  • UserApp/Proba-V

    v14 v15  
    128128
    129129The power distribution and conditioning part of ADPMS supplies an unregulated bus, with each equipment having its internal DC/DC converter. The power conditioning system is designed around a Li-ion battery.
     130= Overview of PROBA-V subsystems =
    130131
    131132
     
    176177AOCS (Attitude and Orbit Control Subsystem) provides three-axis attitude control including high accuracy pointing and maneuvering in different spacecraft attitude modes. The AOCS SW is an extension of the one of PROBA-2, including the following algorithms required by the on-board autonomous mission and payload management: 16)
    177178
    178 - Prediction of land/sea transitions using a land sea mask to reduce the amount of data generated
    179 
    180 - Optimization of attitude in Sun Bathing mode to enhance incoming power while avoiding star tracker blinding
    181 
    182 - Momentum dumping without zero wheel speed crossings during imaging
    183 
    184 - Estimations of remaining spacecraft magnetic dipole to reduce pointing error
    185 
    186 - Autonomous avoidance of star tracker Earth/Sun blinding
    187 
    188 - Inertial mode with fixed scanning rate for moon calibration.
     179 * Prediction of land/sea transitions using a land sea mask to reduce the amount of data generated
     180
     181 * Optimization of attitude in Sun Bathing mode to enhance incoming power while avoiding star tracker blinding
     182
     183 * Momentum dumping without zero wheel speed crossings during imaging
     184
     185 * Estimations of remaining spacecraft magnetic dipole to reduce pointing error
     186
     187 * Autonomous avoidance of star tracker Earth/Sun blinding
     188
     189 * Inertial mode with fixed scanning rate for moon calibration.
    189190
    190191The AOCS hardware selection for PROBA-V consists of a high accuracy double star tracker head, a set of reaction wheels, magnetotorquers, magnetometers and a GPS receiver.
    191 
    192 The main AOCS modes are: Safe, Geodetic, Sun Bathing and Inertial mode.
    193 
    194 - The satellite Safe mode is used to detumble the spacecraft after separation from the launcher and it will be used to recover from spacecraft anomalies.
    195 
    196 - The Geodetic mode is used during nominal observation of the Earth’s vegetation. In this mode the payload is pointed towards the geodetic normal to the Earth’s surface. An extra steering compensation, yaw-steering, is added in this mode, to minimize the image distortion caused by the rotation of the Earth. This yaw-steering maneuver ensures that the spectral imagers are oriented such that the lines of pixels are perpendicular to the ground-trace at each moment. In this mode the star trackers and the GPS receiver are used as sensors and the reaction wheels as actuators.
    197 
    198 - On each orbit, the spacecraft enters the Sun Bathing mode from -56º latitude until entry of eclipse. This is to enhance the incoming power.
    199 
    200 - The Inertial mode coupled with an inertial scanning of the Moon at a fixed rate is used for monthly radiometric full moon instrument calibration purposes. The pointing towards the moon takes 2.5 min, 9 min for scanning the moon and 2.5 min to return to nominal observation mode. It is sufficient to have the moon in the FOV of the SI (Spectral Imager) for a number of pixels.
     192== The main AOCS modes are: Safe, Geodetic, Sun Bathing and Inertial mode. ==
     193
     194
     195 * The satellite Safe mode is used to detumble the spacecraft after separation from the launcher and it will be used to recover from spacecraft anomalies.
     196
     197 * The Geodetic mode is used during nominal observation of the Earth’s vegetation. In this mode the payload is pointed towards the geodetic normal to the Earth’s surface. An extra steering compensation, yaw-steering, is added in this mode, to minimize the image distortion caused by the rotation of the Earth. This yaw-steering maneuver ensures that the spectral imagers are oriented such that the lines of pixels are perpendicular to the ground-trace at each moment. In this mode the star trackers and the GPS receiver are used as sensors and the reaction wheels as actuators.
     198
     199 * On each orbit, the spacecraft enters the Sun Bathing mode from -56º latitude until entry of eclipse. This is to enhance the incoming power.
     200
     201 * The Inertial mode coupled with an inertial scanning of the Moon at a fixed rate is used for monthly radiometric full moon instrument calibration purposes. The pointing towards the moon takes 2.5 min, 9 min for scanning the moon and 2.5 min to return to nominal observation mode. It is sufficient to have the moon in the FOV of the SI (Spectral Imager) for a number of pixels.
    201202
    202203Beyond the technology demonstration through the PROBA program, it is also noted that the AOCS software technology developed in the course of this program is now the baseline of the AOCS of a major operational mission of the GMES (Global Monitoring for Environment and Security) program: Sentinel-3. NGC Aerospace Ltd (NGC) of Sherbrooke, (Québec), Canada was responsible for the design, implementation and validation of the autonomous GNC (Guidance, Navigation and Control) algorithms implemented as part of the AOCS software of PROBA-1 and PROBA-2. NGC has the same responsibilities for the PROBA-V mission (Ref. 16).
    203204
    204205EPS (Electric Power Subsystem): The PVA (Photo-Voltaic Array) uses GaAs triple junction cells with an of efficiency of 28%. To obtain the operating voltage of 31.5 V, 18 cells are included in each string in series with a blocking diode. The PVA consists of a total of 25 solar strings taken into account the loss of one string on the most contributing PVA panel. The average solar string power under EOL conditions (summer solstice and T = 40°C) yields 12.8 W. The maximal incoming power at EOL during an orbit is 144 W. The energy budget for PROBA-V is derived for a bus power consumption of 140 W assuming a worst case day in the summer and while not taken into account the effect of albedo. A worst case power budget analysis indicated a maximum capacity discharge of 1.66 Ah. Use of a Li-ion battery. The battery cells provide a capacity of 1.5 Ah per string. The PROBA-V battery is sized to 12 Ah taking into account capacity fading and loss of a string.
     206== Launch ==
     207
    205208
    206209Launch: A launch of the PROBA-V spacecraft as a secondary payload is planned for Q4 2012. The primary launcher is currently assumed to be Vega with the VESTA adapter.
     
    235238
    236239
    237 Table 2: Overview of compression rates
    238 
    239 The selection of an S-band transceiver and the development of an innovative and generic X-band transmitter for small satellites has been initiated in a collaborative program between CNES and ESA and is funded under GSTP-5 (General Support Technology Program-5). The X-band transmitter is a high-performance device optimized for the needs and constraints of small platforms for which small volume, low mass, low power consumption, and low cost cost are important parameters. Moreover, some key features such as modulation (filtered Offset-QSK), coding scheme (convolutional 7 ½), data and clock interfaces (LVDS packet wire serial interface) have been selected in compliance with CCSDS recommendations, but also to ease the interoperability with most of the existing on-board computers and ground station demodulators. 17)
     240Table 2: Overview of compression rates== S-band ==
     241
     242The selection of an S-band transceiver and the development of an innovative and generic X-band transmitter for small satellites has been initiated in a collaborative program between CNES and ESA and is funded under GSTP-5 (General Support Technology Program-5). The X-band transmitter is a high-performance device optimized for the needs and constraints of small platforms for which small volume, low mass, low power consumption, and low cost cost are important parameters. Moreover, some key features such as modulation (filtered Offset-QSK), coding scheme (convolutional 7 ½), data and clock interfaces (LVDS packet wire serial interface) have been selected in compliance with CCSDS recommendations, but also to ease the interoperability with most of the existing on-board computers and ground station demodulators.
     243== X-band ==
     244
    240245
    241246The development of the new X-band transmitter is based almost exclusively on COTS components to achieve at the same time high performances and low recurrent cost. The transmitter also features an innovative functionality with an on-board programmable RF output power from 1-10 W which allows to match finely with the chosen bit rate, and to reduce as much as possible the margins of the link budget and therefore the consumption power. PROBA-V is the first mission to use this newly developed transmitter. The transmitter has a mass of 1 kg, a size of 160 mm x 115 mm x 46 mm, an in-orbit life time of 5 years, and a radiation hardness of 10 krad. Data rates from 10-100 Mbit/s are available. The X-band transmitter was manufactured by TES Electonic Solutions of Bruz, France. 18)
     
    291296
    292297 *  MLI (Multi-Layered Insulation)
     298
     299== TMA telescope development ==
    293300
    294301TMA 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.
     
    356363Several 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:
    357364
    358 - In plane alignment accuracy, ?X and ?Y = ± 12.5 µm
    359 
    360 - Out of plane alignment accuracy, ?Z = ± 50.0 µm
    361 
    362 - Subarray PDA separation = < 1.5 mm.
     365 * In plane alignment accuracy, ?X and ?Y = ± 12.5 µm
     366
     367 * Out of plane alignment accuracy, ?Z = ± 50.0 µm
     368
     369 * Subarray PDA separation = < 1.5 mm.
    363370
    364371{| align="center"
     
    385392
    386393Since 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.
    387 
    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.
     394== Thermal isolation: ==
     395
     396
     397Firstly, 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.
    389398
    390399Power 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.
    391400
    392401Obiously, 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.
    393 
    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.
     402==  Heat dissipation:  ==
     403
     404To 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.
    395405
    396406From 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.
    397 
    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.
     407== Stability: ==
     408Stability 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.
    399409
    400410To 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.
    401 
    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.
     411== Gradients:  ==
     412
     413The 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.
    403414
    404415[http://events.eoportal.org/presentations/7111/10001905.html For further info File:ProbaV Auto6.jpeg]
     
    418429|130 Kg
    419430|160 Kg
     431! Orbit
     432| Altitude between 700 km and 800 km, Sun-synchronous, Inclination 98.298 degrees
     433| Sun-synchronised polar orbit, 820 km, with a 10:30 AM local time at the descending node
     434!Launcher
     435|Rockot
     436|To be decided – designed to be compatible with Vega, Soyuz or Falcon 1E launchers.
    420437|}
    421438