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Spectropolarimeter for Planetary Exploration (SPEX) instrument for ... Relevance of measurement types. Aerosol .... These rockets can ... Candidate Launchers [adapted from 20]. ..... set with a ~160 cm accurate orbit and a ~7 ns accurate.
PRELIMINARY DESIGN OF THE DUTCH-CHINESE FAST MICRO-SATELLITE MISSION D.C. Maessen(1), E. Gill(1), C.J.M. Verhoeven(1), G.T. Zheng(2) (1)

Chair of Space Systems Engineering, Department of Earth Observation and Space Systems, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands, Email: [email protected], [email protected], [email protected] (2) Department of Astronautics and Aeronautics, School of Aerospace Engineering, Tsinghua University, 100084, Beijing, China, Email: [email protected]

ABSTRACT

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In 2007, the Tsinghua University, China, and the Delft University of Technology, The Netherlands, have decided to jointly define, develop and operate a space mission which covers scientific objectives, technology demonstration, and educational objectives alike. The FAST (Formation for Atmospheric Science and Technology Demonstration) mission will allow for a synoptic evaluation of global aerosol data and altitude profiles of the cryosphere with two cooperating microsatellites, FAST-D and FAST-T, flying in formation in 2011. The mission’s scientific payload will consist of two spectropolarimeters and two altimeters. The spectropolarimeters will be used for the detection and characterisation of aerosols in the Earth’s atmosphere with one instrument on each spacecraft. This will improve the data output and temporal resolution. To determine cryospheric height profiles, a laser altimeter will be flown on FAST-D and a radar altimeter on FAST-T. In this paper we present a general outline and current status of the mission. 1. INTRODUCTION

After signing the Letter of Intent, an investigation into the following four candidate missions was carried out, all involving formation flying: 1. Atmospheric science mission, 2. Interferometric synthetic aperture radar (InSAR) mission, 3. Earth mass balance mission, 4. GNSS science mission. The atmospheric science mission would fly the Dutch Spectropolarimeter for Planetary Exploration (SPEX) instrument for global aerosol characterisation. This research would allow contributing data of scientific relevance to models of the Earth’s climate. SPEX is a small (< 5 kg), mechanism-less instrument which is currently under development by a consortium of leading Dutch space institutes and employs a new aerosol detection principle. FF is in this case beneficial due to a significant increase in data return and temporal resolution. The second mission candidate is a formation of microsatellites, each equipped with a small SAR radar instrument (miniSAR) to create digital elevation models or to determine the velocity of objects on ground. In The Netherlands there is already experience with miniSAR on aircraft due to a joint project between DUT and The Netherlands Organisation of Applied Scientific Research (TNO) [1]. In China, development of two radar altimeters is carried out at the universities of Tsinghua and Xi’an. However, typical problems with SAR instruments are the huge amount of data gathered, high power consumption, and large antennas. Therefore, only a limited use would be possible when applying them on micro-satellites, which limits science return. Another option is to use the miniSARs for altimetry, but then there is little reason to do this in a FF setting when conventional nadir-pointing radar is being used (monostatic). However, interferometric or bistatic radar altimetry with non-nadir looking radar beams, similar to the WITTEX mission [2], is an option. An Earth mass balance mission would be a mission comparable to the Gravity Recovery and Climate Experiment (GRACE) mission, which studies temporal

In May 2007, a Letter of Intent was signed between the Tsinghua University, China, and the Delft University of Technology, The Netherlands, to jointly define, develop and operate a space mission with formation flying (FF) micro-satellites in 2011. During a joint workshop in December 2007, an initial payload selection and mission design was carried out. This culminated into a concept for the Formation for Atmospheric Science and Technology Demonstration (FAST) mission which is scheduled to be launched end 2011. As of early 2008, parallel phase A studies are carried out in both Delft and Beijing. The next sections will present several initial candidate missions, the relevance of the selected mission, an overview of the selected mission, a description of the various payloads, expected science products, the technology demonstration topics, as well as the educational goals.

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CANDIDATE MISSIONS

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variations in the Earth’s gravity field, using low-cost micro-satellites. Studies performed at TU Delft [3, 4] indicate that with a smart relative orbit design and by using very accurate Global Navigation Satellite System (GNSS) receivers on multiple satellites it is possible to do accurate gravity field measurements without accelerometers onboard the spacecraft. The downside is that even with state-of-the-art GNSS-receivers one would need many satellites in order to reach the desired accuracy. Finally, a dedicated GNSS science mission has been studied. Such a mission could perform bistatic GNSSreflectrometry, i.e. altimetry using GNSS signals that are reflected by the Earth’s surface, and/or atmospheric limb sounding using GNSS signals. A trade-off performed between the four candidate missions clearly puts the atmospheric science mission way ahead of the other options, mainly in the fields of science output and mission feasibility. 3. RELEVANCE OF THE ATMOSPHERIC SCIENCE MISSION The following subsections will discuss the importance of performing the earlier mentioned atmospheric science mission. 3.1. Aerosol effects on climate and health Presently, climate change and atmospheric pollution are important political themes. Far-reaching decisions are based on climate change predictions and global atmospheric pollution measurements. Aerosols, small (≤ 100 µm) solid or liquid particles in the atmosphere, contribute to both issues. The effects of aerosols on climate and pollution are fourfold since they [5]: 1. emit, absorb, and scatter radiation (also known as radiative forcing), 2. act as cloud condensation nuclei (resulting in an indirect influence on radiative forcing), 3. catalyse chemical reactions, 4. affect health (smog, fine dust). Thus, aerosols play a significant and complex role in the Earth’s climate. Yet, in a recent publication [6] it is stated that “current uncertainties in the total solar irradiance and aerosol forcings are so large that they preclude meaningful climate model evaluation”. Clearly, current knowledge about aerosols is still lacking (cf. Fig. 1) even though it is acknowledged that a thorough understanding is vital for proper climate modelling.

Figure 1. Estimated climate forcings between 1850 and 2000 [7]. Note the relatively large uncertainties for tropospheric aerosols. 3.2. Relevance of measurement types Aerosol contributions to climate change can be properly modelled when important aerosol characteristics such as composition, size, shape, and number density are better known on a global scale. Studies performed at the University of Amsterdam [8] show that characterisation of aerosols can be done far more accurate by means of polarisation measurement of reflected light instead of the more common flux measurements, as is shown in Fig. 2.

Figure 2. Flux [%] and degree of linear polarisation [-] for measured and simulated particles as function of the scattering angle [deg]. Note that while the flux curves match quite well, the polarisation curves are very different, indicating a wrongly simulated shape [5]. Yet, current aerosol characterisation from space is mainly performed by measuring flux using traditional trace gas instruments like SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY) [9], OMI (Ozone Monitoring Instrument) [10], OSIRIS (Optical Spectrograph and InfraRed Imager System) [11], MODIS (Moderate Resolution Imaging Spectroradiometer) [12], and MISR (Multiangle Imaging SpectroRadiometer) [13]. 3.3. Flown polarization measurement instruments The POLarization and Directionality of the Earth's Reflectances (POLDER) instrument [14] has a mass of 32 kg [15] and was the first instrument that detects aerosols from space by measuring the polarisation of reflected light. This is done at wavelengths of 440 nm, 670 nm, and 865 nm using a special filter wheel

4S Symposium, Rhodes, Greece, 26-30 May, 2008

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(Fig. 3). Downside of this filter wheel is that measurements at different wavelengths and polarisation states are not performed at the same time, resulting in non-negligible errors.

Figure 3. Working principle of POLDER [15]. NASA’s upcoming Glory mission, to be launched in December 2008, will measure both the flux and polarisation of incoming light at nine specific wavelengths (410, 443, 555, 670, 865, 910, 1370, 1610, and 2200 nm) using the APS (Aerosol Polarimetry Sensor) instrument [6]. However, the use of three telescopes and multiple detectors on this mission introduces extra polarization measurement errors and results in a large instrument with a mass of 58 kg [16]. Furthermore, APS and POLDER will not provide any information about aerosols at other wavelengths than those mentioned earlier. Thus, there is still the need for an instrument that is able to measure both flux and polarisation of aerosols in a broad wavelength range. 3.4. The case for FAST SPEX is a small, innovative instrument currently under development that can fulfil the need expressed in the previous subsection by enabling global detection and characterisation of both the flux and the polarisation of natural and anthropogenic aerosols in a broad wavelength range (400-800 nm). To achieve this, it requires only a single detector and no moving parts [17]. Its accuracy is expected to be comparable to or even better than that of the APS instrument. The instrument is compact enough to fit on a small micro-satellite, allowing low-cost and fast access to space. In the light of the foregoing, TU Delft and the Tsinghua University have decided to jointly define, develop and operate the FAST mission using two formation flying micro-satellites to characterise aerosols in the Earth’s atmosphere by employing the SPEX instrument. 4. MISSION OVERVIEW The FAST mission has three equally important top level goals, namely: 1. Characterize atmospheric aerosols and monitor the variation of height profiles in the

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cryosphere and correlate data for improved science return. 2. Demonstrate autonomous formation flying using various communication architectures with distributed propulsion systems and MEMS technology. 3. Teach cutting-edge technology, broaden international view of students and boost skills through exchange of students and staff members. Around these goals, a highly interesting and challenging mission has been defined. The following subsections will present several aspects of the mission. Later sections will discuss the above mentioned goals in depth. 4.1. Orbital geometry The space segment of the mission consists out of two micro-satellites flying in formation: one developed in Delft, named FAST-D, and one developed in Beijing, named FAST-T. Both spacecraft will carry the SPEX instrument as well as an altimeter. The baseline orbit for the two satellites is a 650 km high sun-synchronous orbit (SSO). The orientation of the orbital plane with respect to the sun will be a few hours before or after 12:00 hr. Reason for this is that the sun has to be well away from zenith for optimum science, as explained in subsection 5.1. The space segment will have an open architecture, meaning that complementary satellites from other nations or institutions can join the formation or train in the future. 4.2. Spacecraft design FAST-D FAST-D will carry two scientific payloads, namely SPEX and a miniature laser altimeter called SILAT (Stereo Imaging Laser Altimeter). The combined mass of the two payloads is less than 13 kg. The total mass of FAST-D is expected to be 40 kg, with peak power requirement of 45 W. FAST-D will be equipped with a radiofrequency (RF) inter-satellite link that is capable of transmitting data and measuring the distance to FAST-T. A GNSS receiver will provide absolute orbital position data. FAST-T The Chinese spacecraft will be based on the Tsinghua-2 spacecraft which is currently under development. Its scientific payloads are SPEX and a radar altimeter. The combined mass of the two payloads is less than 25 kg and the total spacecraft mass is expected to be 80 kg. The power requirement is estimated at 80W, which calls for a deployable solar array. There is the possibility for one additional payload to increase environmental data return. In that case, the radar altimeter will be replaced by SILAT to reduce mass.

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Similar to FAST-D, FAST-T will be equipped with a RF sensor and a GNSS receiver. 4.3. Ground segment, launch, and operations The mission will employ at least two ground stations, one in Delft and the other one in Beijing. Up- and downlink frequencies to the satellites will be in S-band. An important capability which is required of the ground stations is the ability to track two satellites simultaneously, which requires adaptation of the current ground stations in Delft and Beijing. The ground stations will be automated as much as possible to reduce operations costs. No decision has been taken yet on the location of the mission control centre (MCC). Just as for the ground stations, the MCC will be automated as much as possible to reduce operations costs. A Chinese launcher is preferred for this mission. Candidates are the LM-2C (CZ-2C), the LM-2D (CZ2D), and LM-4B (CZ-4B) launchers. These rockets can be launched from the Taiyuan Satellite Launch Center (TSLC) in the province of Shanxi in Northern China (37.5°N latitude [18]) or from the Jiuquan Satellite Launch Center (JSLC) in the province of Gansu in Northwestern China (40.6°N latitude [19]). The launch scenario is that of a single launch from TLSC. Concerning the launch configuration, it is noted that calibration and testing during the early operations phase of the satellites will benefit greatly from stacking the two satellites. It is furthermore noted that the launch will not necessarily be a piggyback launch, which is common for micro-satellites. There is presently a development underway of a smart upper stage for small satellites.

aerosol characteristics at a certain location over several hours. The separation of the two spacecraft in mode B is a trade off between propellant usage, manoeuvre time and required temporal separation. 4.4. Timeline Currently, the project is in phase A, which will end in October 2008. Phase B is planned to last one year, which leaves two years for the phases C and D in order to be able to launch the satellites end of 2011. 5. PAYLOAD DESCRIPTION The three payloads that will fly on the FAST-D and FAST-T satellites are presented in the next subsections. 5.1. SPEX The Spectropolarimeter for Planetary Exploration or SPEX, is currently under development by a consortium of Dutch companies and institutions consisting of Dutch Space, TNO, the Netherlands Institute for Space Research (SRON), the Netherlands Foundation for Research in Astronomy (ASTRON), and the University of Utrecht. It is primarily intended for planetary exploration, but can easily be adapted to study aerosols in the Earth’s atmosphere. Since the instrument is intended to be used in particular in an orbit around Mars, it has to have a low mass (goal is < 5 kg), low power consumption, and high reliability. This has resulted in the choice not to put any mechanisms like scanning mirrors in the instrument. Downside of this choice is that the swath width becomes fairly limited, as shown in Tab. 1. Table 1. SPEX key characteristics [adapted from 17]. Wavelength range Spectral resolution Spectral sampling Spatial resolution Viewing directions FOV per viewing direction Mass (excl. electronics and thermal hardware) Dimensions spectropolarimeter subsystem Power requirement Temperature requirement Operational constraints

Figure 4. Candidate Launchers [adapted from 20]. The mission will consist out of two modes, lasting one year each: in mode A, the satellites will fly in formation with an along-track separation of ~560 ± 10 km for optimal SPEX science data return; in mode B, the separation between the satellites will be increased substantially (train) in order to measure changes in

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Data rate for 650 km SSO orbit Data storage

400-800 nm < 2 nm 2 detector pixels 10 km @ nadir from 300 km 0°, ±18°, ±36°, ±54°, limb forward and backward view 7°x1.7° (cross track x along track) 2 kg (target) 130 x 130 x 60 mm3 CMOS detector: < 0.5 W Close to room temperature Measurements only at sunlit side of the planet of interest. Pointing knowledge and stability of platform smaller than 360″. 12.4 Gbit/day (no compression) To be handled by the satellite

The current instrument baseline specification for SPEX is a shoebox size module containing 9 fields of view (FOVs, of which 2 are limb looking) imaging the planet’s radiance for different phase angles. Full linear

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polarimetry is performed through encoding the input polarization in a sinusoidal modulation of the spectrum. The spectral dependence of the degree and angle of linear polarization are retrieved from a single intensity measurement [21]. The reason that SPEX has seven planet-looking viewing directions (0°, ±18°, ±36°, ±54°) is that flux and polarisation measurement at a certain location needs to be performed from multiple angles in order to do proper aerosol characterisation. This is also shown in Fig. 2 where flux and polarisation are plotted as function of the scattering angle. This angle is depicted in Fig. 5 and represents the angle with which sunlight is reflected by, in this case, aerosols in the atmosphere. Because of this relation between polarisation and scattering angle, it is preferred to have the sun relatively close to the horizon as seen from the satellite (i.e. large phase angle). This will result in the largest spread in observed scattering angle and therefore the best set of measuring points needed to determine the aerosol properties.

Figure 5. Definition of the scattering angle. 5.2. SILAT The Stereo Imaging Laser Altimeter (SILAT) is currently under development at cosine Research B.V. in Leiden, The Netherlands. Initially, it was intended to fly on ESA’s Mercury mission (BepiColombo). The current design is made for a possible Europa mission and can easily be adapted for an Earth observation mission. These modifications are expected to result in a decrease in mass as compared to the value stated in Tab. 2. SILAT is an example of a highly integrated payload where three different instruments are combined to create a new instrument with superior characteristics than all three instruments separately. It consists firstly out of a miniature laser altimeter (LAT) that employs the principle of photon counting to measure height variations. Secondly, there is a high resolution camera (HRC) that can make detailed colour pictures of the planet surface. Thirdly, there is a stereoscopic forward looking camera (SCAM, 27° off nadir). Combined in one instrument, this allows the creation of very accurate three dimensional colour maps of the planet surface.

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The development of the instrument is well underway and a laboratory test setup of the laser altimeter exists. The next development step is to test it on an aircraft.

Figure 6. Mechanical layout of SILAT. Not shown here are thermal and straylight baffles, the laser source and its power supply [22]. Table 2. SILAT key characteristics [adapted from 23]. Current values follow from an instrument design for a Europa mission and can change if a dedicated Earth observation instrument is made. Overall Total mass [kg] 7.77 Dimensions [mm 3 ] 287x327x327 Power consumption [W] 11.92 Temperature range [K] 263-303 Platform pointing accuracy 24 (TBC) required [arcsec] Platform pointing stability 0.17 (TBC) required [arcsec/ms] Data rate for 650 km SSO 30.7 (no compression, orbit [Mbit/s] 100% duty cycle) LAT Power consumption [W] 10.34 Vertical resolution [m] 0.15 Repetition frequency [Hz] 10000 Wavelength [nm] 532 Emitter FOV [rad] 0.000050 Receiver FOV [rad] 0.000150 Detector type Single Photon Avalanche Diode HRC Power consumption [W] 0.5 FOV [deg] 3.7 IFOV [deg] 0.0018 (0.031 mrad) Sensor type Active Pixel Sensor (APS) Filter configuration [nm] 404, 559, 671 Shutter time [ms] 2.5 SCAM Power consumption [W] 1.076 Pointing direction [degrees 27 off nadir] FOV [deg] 4.7 (TBC) IFOV [deg] 0.0045 (0.079 mrad) Sensor type APS Filter configuration [nm] 559 Shutter time [ms] 6.3

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5.3. Radar altimeter At the time of writing, no decision has been taken whether taking a Chinese radar altimeter from Tsinghua University or from Xi’an University of Technology. Both candidate radar altimeters are still under development. The expected mass for both devices is roughly 20 kg. Important to note is that, if an extra environment sensor is added, the altimeter may be changed to an altimeter with less mass, which might be SILAT. 6. SCIENCE OUTPUT Due to the presence of several payloads on multiple satellites, various science products can be obtained. The next subsections will discuss these. 6.1. Science output from SPEX As explained earlier, the SPEX instrument is dedicated to characterise aerosols in the atmosphere. The instrument has seven fixed planet-looking FOVs. For proper aerosol characterisation, the aerosols need to be detected from as many different angles as possible, in this case seven. However, as the Earth rotates underneath the satellites’ orbit, there will be many cases in which a certain location is seen by only a few FOVs, resulting in an incomplete characterisation of the aerosols at the location. Adding a second SPEX instrument ~560 km behind the first one (mode A of the mission) alleviates this problem and thus will significantly increase the SPEX data return. Therefore, by having two satellites cooperating in this manner, the amount of useful data gathered is more than doubled.

employ a Chinese radar altimeter, while FAST-D will employ the miniature Dutch laser altimeter SILAT. This opens up several unique opportunities: 1. Cross-calibration and comparison of two fundamentally different types of altimeters. 2. The combination of a laser and a radar altimeter offers the interesting potential of directly measuring snow thickness on ice since radar waves will travel through the snow and be reflected by the ice while the light waves will be reflected by the snow. 3. Using a laser and a radar altimeter, clouds can be detected since radar waves are unobstructed by clouds while light is reflected. 6.3. Combined altimeter and SPEX science Next to separate SPEX and altimeter science, the data gathered can also be combined to obtain further useful information: 1. Correlation of altimeter data with aerosol measurements made with SPEX provides the opportunity to directly study the influence of aerosols on another important contributor to the Earth’s climate, namely ice and snow. 2. The combination of SPEX, SILAT, and the radar altimeter allows for combined aerosol and cloud retrieval, which is highly relevant science because aerosols influence cloud formation to a large extent. A similar strategy will also be applied with the A-train formation where the combination of Glory’s APS, the CALIPSO lidar, and the CloudSat radar will provide similar data [6]. 7. TECHNOLOGY DEMONSTRATION As stated earlier, technology demonstration is a major mission goal. So far, six technology demonstration topics have been identified. 7.1. Autonomous FF with micro-satellites

Figure 7. FAST-D and FAST-T in mode A, resulting in overlapping SPEX FOVs [24]. In mode B of the mission, the separation between the satellites will be several hours. This is interesting for atmospheric scientists, as it allows them to study changes in aerosol characteristics on the same day over many locations. 6.2. Science output from altimeters The primary objective of the altimeters on the two spacecraft is to monitor seasonal changes in height profiles in the cryosphere. However, the altimeters on the two spacecraft will not be similar: FAST-T will

4S Symposium, Rhodes, Greece, 26-30 May, 2008

Although already demonstrated, autonomous FF is still a field under development. Therefore, the FF aspect of the FAST mission is considered to be technology demonstration in itself. In the field of FF, FAST will demonstrate: •

The first micro-satellite formation addressing science



Increased science data return through FF



Flexible adaptation of relative geometry to maximize data return (FF) or resolve temporal atmospheric changes (train)



Inherent robustness due to risk mitigation



Scalability for further satellites



An “open space segment” architecture



The implementation of a true sensor web

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7.2. Propellant optimised distributed propulsion When two satellites are flying in formation in a low Earth orbit (LEO), they will slowly drift apart. This is mainly due to differential air drag and a differential J2effect. For FF, it is vital that the satellites remain within a specified distance with respect to each other. Therefore, the inter-satellite distance needs to be adjusted periodically using thrusters. Since there are two satellites, there is the opportunity for redundancy by installing propulsion systems on both satellites: if one fails, the other one can still perform formation keeping manoeuvres. By having a direct inter-satellite link, it is possible to implement an autonomous decision process in the space segment (based on predefined parameters) which propulsion system to fire and when to do this in order to minimize propellant usage. 7.3. Distributed, fault tolerant, and out-of-core computing Two satellites with an inter-satellite link can be regarded as a distributed dual-core processor. Provided that there is enough bandwidth available on the intersatellite link, this fact can be exploited to enhance the computing power as compared to a single processor. Combined with the technique of out-of-core (OOC) computing, large problems such as precise orbit determination (POD) can be solved on-board. In very general terms, OOC allows one to divide large problems (matrices) into many small problems that can each be handled individually by the on-board computer (OBC) and then be put back together to arrive at a solved large problem. Next to POD, the technique can amongst others also be used for precise attitude determination (PAD), GNSS radio occultation bending angle pre-processing, and higher data compression ratios. The above techniques can be taken one step further by incorporating fault detection algorithms to counteract errors caused by radiation effects such as single event upsets (SEUs). By employing this software technique, less radiation hardness is required from the OBC’s hardware components, allowing them to be cheaper. 7.4. Multiple inter-satellite communication paths In the first part of the mission (mode A), there is a direct line of sight (LOS) between the satellites. Therefore, the nominal communication path is a direct inter-satellite link. The direct link also allows for relative ranging between the satellites using the same signal. In the second part of the mission (mode B), the intersatellite distance is so large that there is no LOS, thus inter-satellite communication has to be routed. To do this, there are two options: using ground stations-in-theloop to relay the data (‘standard’ solution) or using a commercial satellite communication network such as Orbcomm, Iridium or Globalstar. To the knowledge of the authors, this second option has never been tried

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before, but is in principle possible. This is deduced from experiments performed by the German company OHBSystem AG in Bremen using their RUBIN experiment testbed. With it, they have successfully demonstrated satellite-to-ground and ground-to-satellite communication by sending e-mail messages via the Orbcomm constellation [25]. RUBIN-8-AIS (AIS stands for Automatic Identification System), launched April 28, 2008, will also experiment with communication via the Iridium constellation [26]. Thus, near real-time satelliteto-satellite communications using the Orbcomm constellation as a relay is possible. In addition, the OHB experiments show that communication satellites can certainly be used to send telemetry data to the ground.

Figure 8. RUBIN-2 communications architecture [25]. 7.5. MEMS For the FAST satellites, the intention is to demonstrate newly developed Dutch and Chinese micro electromechanical systems (MEMS) where appropriate. For FAST-D, hardware developed in the Dutch MicroNed program [27] can be used, such as a micro-propulsion system, a radio transceiver, and a micro sun sensor. Furthermore, TU Delft and Tsinghua University both feature MEMS research centres: DIMES (Delft Institute of Microsystems and Nanoelectronics) in Delft and IMETU (Institute of Microelectronics of Tsinghua University) in Beijing. There already exist good relations between these institutions and since both universities collaborate in the FAST mission, there is a potential for collaboration between these research centres on this topic. 7.6. Real-time GPS ephemerides updates GPS ephemerides broadcasted by the GPS satellites are currently updated every two hours. This results in a relatively large position error for a satellite using a GPS receiver for navigation. In principle, it is possible to reduce this error by uploading real-time GPS ephemerides data sets provided by the IGS (International GNSS Service). These can be used to correct the ephemerides received from the GPS satellites. The ultra-rapid IGS data set is released four times each day and contains 48 hours of predicted GPS satellite orbits. The first 24 hours are computed from

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observations and the second 24 hours are predicted orbits [28]. The GPS satellite orbit accuracy in this data set is ~10 cm, while the GPS satellite clock accuracy is ~5 ns [29]. Compared to the ephemerides broadcasted by the GPS satellites themselves, this is a significant improvement as the satellites themselves provide a data set with a ~160 cm accurate orbit and a ~7 ns accurate clock [29]. For the FAST mission, the intention is to use the IGS data set to obtain very accurate GPS-based navigation results onboard the spacecraft. 8. EDUCATION Since the FAST mission is an academic endeavour, education is an important mission goal. Thus, students will be involved in design activities of various mission aspects. This will not only benefit the students themselves, but also future employers since they can hire employees with experience from a real space project. For FAST, three educational goals have been set: 1. Boost of skills through the exchange of staff and students (internship, MSc, PhD), 2. Teach cutting edge technology using the current mission as an example, 3. Broaden the international view of students. By exposing students and staff to a different culture and a different way of working, they are confronted with a different way of approaching problems than what they have been taught. This will benefit them in future projects. Furthermore, exchanging staff and students will strengthen the bonds between the two universities. Next to being taught the ‘basics’ of space engineering, students in the Bachelor and Master program will be taught cutting edge space technology using the FAST mission as an example. This will prepare them better for their future careers as they are aware of present day technology. Involving students in a bi-lateral project will broaden their international view by making them aware of other than the well-known centres of excellence for space systems engineering. 9. CONCLUSIONS In this paper, we have presented the international atmospheric science mission FAST. For FAST, TU Delft and the Tsinghua University cooperate to gather aerosol and cryosphere data using two micro-satellites flying in formation in 2011. Both satellites will carry a small spectropolarimeter for aerosol characterisation and an altimeter for cryosphere height variation determination. One satellite will carry a laser altimeter while the other will carry a radar altimeter. Currently, the mass of the satellites is expected to be 40 kg and 80 kg respectively.

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The design of this first true micro-satellite formation and train in space results in a significant increase in aerosol data return and various synergetic payload data combinations. It is an open and flexible test bed for the demonstration of emerging technologies and will serve as an excellent educational project due to its international character and the use of cutting edge technologies. Currently, parallel phase A studies are being performed in Delft and Beijing. These are scheduled to be finished on October 2008. Launch of the satellites is scheduled for end 2011. 10. ACKNOWLEDGEMENT The authors would like to acknowledge the help of Daphne Stam of SRON and Yongliang Ma of Tsinghua University for providing valuable information about conducting atmospheric science. Jian Yang of Tsinghua University is thanked for information about radar altimeters. Also appreciated is the detailed information on SPEX and SILAT provided by Erik Laan of TNO and Scott Moon of cosine Research respectively. 11. REFERENCES 1. Steeghs P., Halsema E. van, Hoogeboom P., MiniSAR: A Miniature, Lightweight, Low Cost, Scalable SAR System, in Proceedings of CEOS SAR Calibration/Validation Workshop ‘01, Tokyo, Japan, 2-5 April, 2001, pp. 125-128. 2. Raney R.K., Porter D.L., Fountain G.H., WITTEX: A constellation of three small satellites that meet aggressive requirements for radar altimetry, Acta Astronautica, Vol. 52, Issues 9-12, May-June 2003, pp. 777-783. 3. Encarnacao J., Ditmar P., Liu X., Analysis of Satellite Formations in the Context of Gravity Field Retrieval, in Proceedings of the 3rd International Symposium on Formation Flying, Missions and Technologies, ESA/ESTEC, Noordwijk, The Netherlands, April 23-25, 2008. 4. Ditmar P., et al., On a feasibility of modeling temporal gravity field variations from orbits of non-dedicated satellites, in Proceedings of IUGG XXIV, Perugia, Italy, July 2-13, 2007. 5. Stam D.M., Atmospheric Research with Microsats, Presentation, Delft, The Netherlands, February 12, 2008. 6. Mishchenko M.I., et al., Accurate Monitoring of Terrestrial Aerosols and Total Solar Irradiance: Introducing the Glory Mission, Bulletin of the American Meteorological Society, Vol. 88, Issue 5, pp. 677-691, May 2007. 7. Hansen J., et al., Global warming in the twenty-first century: An alternative scenario, Proceedings of the National Academy of Sciences of the United

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States of America, Vol. 97, No. 18, pp. 9875-9880, August 29, 2000. 8. Volten H., et al., The Amsterdam Light Scattering Database, http://www.astro.uva.nl/scatter/ 9. SCIAMACHY product and validation site, http://www.sciamachy.org/ http://www.knmi.nl/omi/publ10. OMI website, nl/instrument/index.html 11. Llewellyn E.J., et al., The OSIRIS instrument on the Odin spacecraft, Canadian Journal of Physics, Vol. 82, pp. 411-422, 2004. 12. MODIS website, http://modis.gsfc.nasa.gov/about/specifications.ph p 13. MISR website, http://wwwmisr.jpl.nasa.gov/index.html 14. Breon F.-M., et al., Multi-directional and polarization measurements with POLDER: lessons learned, Presentation at the 4th International Workshop on Multiangular Measurements and Models, Sydney, Australia, March 20-24, 2006, http://smsc.cnes.fr/POLDER/A_produits_scie.htm 15. POLDER instrument, Technical Features, CNES, http://smsc.cnes.fr/POLDER/GP_instrument.htm 16. Earth Science Reference Handbook, Glory, pp. 141147, http://eospso.gsfc.nasa.gov/eos_homepage/mission _profiles/docs/Glory.pdf 17. Laan E., SPEX Interface Control Document for the FAST mission, issue 0.2, TNO Science and Industry, The Netherlands, April 18, 2008. 18. Space Today Online, Space Launch Sites Around the World, http://www.spacetoday.org/Rockets/Spaceports/La unchSites.html#Taiyuan 19. China Academy of Launch Vehicle Technology, LM-2C User’s Manual, Issue 1999. 20. Go Taikonauts!, Chinese Launch Vehicle Overview, http://www.geocities.com/CapeCanaveral/launchp ad/1921/launch.htm 21. Laan, E., Specsheet for SPEX - Spectropolarimeter for Planetary Exploration, November 12, 2007. 22. Kraft S., et al., Demonstration of Highly Integrated Payload Architectures and Instrumentation for future planetary Missions, Proceedings of SPIE 5978-23, Remote Sensing Europe, Brugge, 2005. 23. Moon S., Specsheet SILAT, issue 0.2, FAST internal document, Leiden, May 15, 2008. 24. Giuditta N., et al., Assignment from group2 WP290: Interfaces between FAST-D and FAST-T, Microsatellite Engineering course document, Delft University of Technology, Delft, The Netherlands, April 22, 2008.

4S Symposium, Rhodes, Greece, 26-30 May, 2008

25. OHB-System AG, RubinX – Experimental Space Technology Program, http://www.ohbsystem.de/gb/Satellites/Missions/rubinx.html# 26. Kalnins I., Fuchs M., Small- and Nano-Satellites from Bremen, Presentation, February 15, 2008, http://www.lvh.it/lvhPortal/Download.po?did=120 6547942 27. Gill E., et al., MISAT: Designing a Series of Powerful Small Satellites based upon Micro Systems Technology, IAC-07-B4.6.05, 58th International Astronautical Congress, Hyderabad, India, September 24-28, 2007 28. IGS Central Bureau, IGS Product Availability, http://igscb.jpl.nasa.gov/components/prods_cb.htm l 29. IGS Central Bureau, IGS Products, http://igscb.jpl.nasa.gov/components/prods.html

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