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Initial orbit results from the TUBiX20 platform
Journal Pre-proof Initial Orbit Results from the TUBiX20 Platform Merlin F. Barschke, Clément Jonglez, Philipp Werner, Philip von Keiser, Karsten Gordon, Mario Starke, Marc Lehmann PII:
S0094-5765(19)31360-8
DOI:
https://doi.org/10.1016/j.actaastro.2019.10.034
Reference:
AA 7730
To appear in:
Acta Astronautica
Received Date: 1 March 2019 Revised Date:
11 October 2019
Accepted Date: 15 October 2019
Please cite this article as: F. Merlin Barschke, C. Jonglez, P. Werner, P. von Keiser, K. Gordon, M. Starke, M. Lehmann, Initial Orbit Results from the TUBiX20 Platform, Acta Astronautica, https:// doi.org/10.1016/j.actaastro.2019.10.034. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
Initial Orbit Results from the TUBiX20 Platform Merlin F. Barschkea , Cl´ement Jongleza , Philipp Wernera , Philip von Keisera , Karsten Gordonb , Mario Starkea , Marc Lehmanna a Technische
Universit¨at Berlin, Institute of Aeronautics and Astronautics, Marchstr. 12, 10587 Berlin, Germany [email protected] b Spacegramming Software and Systems Engineering [email protected]
Abstract Technische Universit¨at Berlin has been active in the field of small satellites for more than 25 years and has successfully developed, built and operated sixteen satellites to date. One of the recent developments of the university is TUBiX20, a highly modular microsatellite platform that was designed to support various different mission types. At the moment, three missions of Technische Universit¨at Berlin are based on TUBiX20, namely TechnoSat, TUBIN and QUEEN. Launched in July 2017, TechnoSat is the first mission to demonstrate the TUBiX20 platform in orbit. TechnoSat’s mission is the demonstration of seven technology payloads that include an S-band transmitter and a reaction wheel system. This paper presents initial orbit results of the TechnoSat mission, focusing on the payloads relevant for future TUBiX20 missions. Here, the evolution of the experiments towards increasing complexity from the commissioning phase into manoeuvres that combine the use of several payloads underlines the platform’s versatility. Furthermore, a quick adaptation of payloads like the S-band transmitter and the reaction wheel system into regular operations was possible by exploiting the platform’s modular architecture. TechnoSat achieved all its mission objectives within the nominal mission time and is now primarily used within its extended mission duration to test and demonstrate capabilities for the upcoming missions. Keywords: orbit results, microsatellite, technology demonstration, in-orbit demonstration, satellite platform
1. Introduction In recent years, small satellites proved that a vast variety of mission scenarios from a number of different application areas can be realized based on this satellite class. In this context, platform designs are often implemented that are used in more than one project to reduce the required development effort per mission. As the requirements imposed on a satellite may diverge significantly between missions, platform concepts commonly rely on modular designs in hard- and software. Technische Universit¨at Berlin has a heritage of more than 25 years in the development and operation of small satellites, having launched sixteen satellites to date [1]. As one of the more recent developments, the university has developed TUBiX20 [2], a modular microsatellite platform which is currently the baseline for three missions. The platform mainly targets towards applications in science, Earth observation and technology demonstration. It is based on a configurable number of single redundant computational nodes that communicate on a Preprint submitted to Acta Astronautica
centralized data bus system. All nodes are located in a central avionics compartment and are connected to a backplane through a standardized interface. According to the needs of a certain mission, components may be added, removed or updated by adjusting the nodes to the needs of the project. In this manner, diverging mission requirements can be supported while limiting the excess capabilities of the final design. Furthermore, this approach facilitates standardization in the hard- and software domain, which in turn allows for extensive reuse. As the first mission based on the TUBiX20 platform, TechnoSat is an in-orbit technology demonstration mission that carries seven payloads. Having completed its nominal mission in summer 2018, payload operations are now complemented by testing new software and operational procedures for following missions within an extended mission period. This paper presents selected orbit results gathered during the first eighteen months of TechnoSat’s operations, as well as their impact on subsequent missions. October 17, 2019
2. TUBiX20 Missions
1. 2. 3. 4. 5. 6. 7.
At present, three missions in various project phases implement the TUBiX20 platform at Technische Universit¨at Berlin. These missions are described in more detail in the following. 2.1. The TechnoSat Mission
Star tracker STELLA [5] Space debris detector SOLID [6, 7] Camera payload Commercial laser ranging retro-reflectors [8, 9, 10] S-band transmitter HiSPiCO [11] Four reaction wheels in tetrahedron configuration Fluid dynamic actuator [12, 13, 14]
The payloads impose only moderate requirements on the attitude control system: three axis stabilization with 20 degree accuracy is required for the verification of the star tracker and the S-band transmitter. Additionally, angular velocity measurements with an accuracy better than 0.1 degree per second are required for testing of the fluid dynamic actuator and the reaction wheel system. To ensure that all experiments can be conducted as planned, a minimal average of 13.3 watts of electrical power needs to be generated under worst case conditions. Table 1 gives an overview of the main parameters of the TechnoSat mission as well as the satellite platform configuration implemented for this mission. Components indicated as payload were cleared for use as nominal platform elements after extensive testing.
The primary objective of the TechnoSat mission is the in-orbit demonstration of seven individual technology payloads [3]. The secondary objective is testing and validation of the newly developed TUBiX20 satellite platform for the first time in orbit. TechnoSat was launched in July 2017 and is currently operated within an extended mission duration (this corresponds to ECSS project phase E). Figure 1 shows a rendering of the TechnoSat spacecraft.
Table 1: Parameters of the TechnoSat mission
Figure 1: Rendered image of the TechnoSat spacecraft in operational configuration with camera and S-band antennas facing upwards [4]
Orbit
600 km, SSO
Launch date Design lifetime
14th of July 2017 1 year
Spacecraft mass
20 kg
Spacecraft volume
465 × 465 × 305 mm
Communications
UHF
Data downlink
S band (payload)
Attitude sensors
IC magnetometers MEMS gyroscopes Sun sensors Fiber optic rate sensors
Attitude actuators
Magnetorquers Reaction wheels (payload)
Orbit determination
Laser ranging (payload)
Figure 3 shows a systems overview of the TechnoSat spacecraft. Here, the six grey boxes outline the subsystems of the spacecraft and its payloads, while in the middle of the schematic the central power (red) and data (blue) buses are depicted. The blocks connected to the central bus system that are shown in dark blue are the aforementioned computing nodes that serve as
Figure 2: Rendered image of the TechnoSat spacecraft with several solar panels removed to show the payloads (modified from [3])
The TechnoSat in-orbit demonstration mission features the following payloads that were developed by industry and different research institutions in Germany (cf. Figure 2): 2
Figure 3: TechnoSat systems overview depicting subsystems and payloads (grey boxes), the central data and power bus system, the nodes that connect to these buses (dark blue), the components connected to the nodes (light blue) and power and data interconnections (modified from [15])
For receiving telecommands as well as sending telemetry to the ground, TechnoSat implements a UHF communications (COM) node with four transceiver channels, each connected to its own antenna. Each two of these transceiver channels with perpendicular arranged antennas are operated in hot redundancy, while the other two are connected to the cold redundant microcontroller of the node. The on-board computer (OBC) is responsible for system-level tasks such as execution of time-tagged command lists, telemetry storage and management of satellite modes. These modes govern a variety of configurations for each subsystem and its devices according to the spacecraft’s current task, such as specific attitude control modes (i.e. target pointing with reaction wheels) with additional payload operations (i.e. camera operation) or data downlink. TechnoSat’s attitude determination and control system (ADCS) comprises of two nodes, the ADCS node that controls the magnetorquers, as well as the six sensor boards, and the fiber optic rate sensor (FOR) node, which connects to three high-precision angular rate sensors. An ADCS sensor board that features two Sun sensors, MEMS gyroscopes and magnetic field sensor ICs is attached to each of the six sides of the satellite. The payload data handling (PDH) node of TechnoSat connects to the camera and the S-band transmitter pay-
interfaces for networking external components such as sensors or actuators. Furthermore, these nodes run the software to control the connected components and to fulfill the high-level tasks of the satellite’s subsystems. The green boxes illustrate components that are directly connected to the central power and data buses, but were developed by the experiment providers and thus do not implement the standardized TUBiX20 hard- and software elements. The star tracker holds a special position here, as it requires a hardware interface (depicted in light blue) to translate to the power redundancy management of the platform. Represented in light blue are devices such as sensors, solar panels, or payloads that are connected to nodes. Here, the depicted number indicates the quantity of implemented instances of the corresponding element. The electrical power system (EPS) of TechnoSat comprises the solar panels, batteries, power conditioning units (PCUs) and the EPS node. Here, each set of solar panels, battery, and PCU form one of the two power generation, storage and distribution paths of the spacecraft. Both paths are operated in hot redundancy, i. e. the spacecraft is still operational if one string fails, however, the performance will be reduced. The EPS node monitors and controls its own elements as well as all nodes of the system. Furthermore, it performs redundancy control for the data bus system and the nodes. 3
loads and is thus responsible for storing pictures before they can be sent to the ground. Furthermore, it runs all software required by the platform to interact with the payloads. TechnoSat is operated by Technische Universit¨at Berlin using the university’s own ground segment. The primary ground station featuring a quad stacked Yagi antenna for UHF communications and a three meter parabolic dish antenna for S band data downlink is located in Berlin. Additional UHF ground stations used for TechnoSat operations are located in Norway and Argentina. Figure 4: Rendering of the TUBIN satellite in operational configuration with the camera system facing upwards [18]
2.2. The TUBIN Mission The second mission based on the TUBiX20 platform is TUBIN, whose mission consists in the demonstration of wildfire detection using microbolometers on a 23 kg microsatellite [16, 17]. To this end, TUBIN carries two infrared microbolometers as well as a camera for the visible range of the spectrum. Introducing two star trackers, a GPS receiver, and fluxgate magnetometers, TUBIN features a significant update of the attitude determination and control capacities compared to TechnoSat in order to fulfill the more stringent pointing accuracy requirements involved by the mission [4]. Here, attitude knowledge better than six arcminutes is required, while the demanded three axis attitude control accuracy in nadir pointing is 180 arcminutes with a stability better than 30 arcminutes per second. For conducting payload experiments, a minimum average power generation of 14.8 watts is required under worst case conditions for TUBIN. Figure 4 shows a rendering of the TUBIN spacecraft. One can see the baffles of the two infrared cameras at the top side of the spacecraft with the one of the visible spectrum range camera in between. The TUBIN spacecraft is currently in production (mission phase D according to ECSS) and is scheduled for launch in 2020.
and the Ferdinand-Braun-Institut, Leibniz-Institut f¨ur H¨ochstfrequenztechnik. For the QUEEN mission, a three axis attitude pointing error below 60 arcminutes is required. The QUEEN payload, which will be operated continuously, has an average power consumption of 25 watts.
3. TechnoSat Mission Operations and Results TechnoSat was launched into a 600 km Sunsynchronous orbit on the 14th of July 2017. Figure 5 shows the integration of TechnoSat on the Fregat upper stage one week before launch.
2.3. The QUEEN Mission QUEEN is a 35 kg quantum technology mission with the objective to demonstrate and test a rubidium two photon vapor cell frequency reference in orbit [19, 20]. Furthermore, it will carry an X-band transceiver, an optical data downlink terminal, and a camera system as secondary payloads. The QUEEN mission, which is currently in definition phase (phase B according to ECSS), is a cooperation project between HumboldtUniversit¨at zu Berlin, Technische Universit¨at Berlin,
Figure 5: Integration of the TechnoSat spacecraft on the Fregat upper stage one week before launch (image credit: Roscosmos)
After the first signals of the spacecraft were received during the first pass over the ground station in Berlin, the launch and early orbit phase (LEOP) and subsequently the commissioning of the platform were conducted until September 2017. This included a software 4
update to optimize the performance of the attitude determination and control system based on data gathered in orbit [4]. Payload commissioning was performed partly in parallel to the platform commissioning activities between August and September 2017. All six payloads that are actively operated (the laser retro-reflectors are entirely passive) were powered successfully and first payload telemetry was gathered. Subsequently, regular payload operations started while experiments to assess the performance of the platform were conducted at the same time. All payloads have been successfully operated in numerous experiments and the data was distributed to the payload developers for evaluation. First orbit results of selected TechnoSat payloads were presented in [10, 14, 21]. Three of the TechnoSat payloads, namely the S-band transmitter, the reaction wheels and the retro-reflectors are implemented as regular platform components within subsequent missions such as TUBIN, after being tested in orbit on TechnoSat. A more detailed description of how results of the TechnoSat mission were considered for the TUBIN design can be found in [4]. Table 2 gives an overview over the operations that were conducted with TechnoSat during the first eighteen months of the mission. Here, a software upload refers to the update of a single node.
maximum power income in Sun pointing was selected as reference value for assessing the power generation and storage performance of TechnoSat. This value was determined to be above 35 watts during the design of TechnoSat. Here, an experiment was conducted that involved minimizing the power income for one Sun period by pointing the side of the satellite without solar cells towards the Sun. In the following Sun period, the attitude was changed to maximize the power income. Figure 6 shows the voltage and the current of the nine solar panels of one of the two solar generation paths of the satellite in the upper plot, as well as voltage and current of the two battery packs in the lower one. The current generated during the first Sun period can be attributed to the Earth’s albedo and the overall development of the albedo induced current can be explained by the orbit geometry. As the satellite performs Sun pointing the irradiation angle of the Earth’s albedo is constantly changing and the generated power remains below 30 percent of the power generated from sunlight that would be expected when pointing the solar panels towards nadir. Furthermore, a significant influence of the Earth’s surface as seen from the satellite, like clouds, water or land can be observed. Nevertheless, it can be seen that the amount of electrical power that is generated by Earth albedo is, under certain circumstances, sufficient to completely supply the satellite with power in nominal operations. For Sun pointing, the peak power that is delivered by the solar panels is more than 35 watts, which agrees well with the design values. After approximately one quarter of the Sun period, one can observe the charge regulator’s transition to constant voltage mode, as would be expected after reaching the design value of 16.8 volts.
Table 2: TechnoSat operations as of January 2019
Orbits Ground stations used
8,187 3
Passes with active operations
2,409
Pictures taken Telemetry downloaded [MiB] Payload data downloaded [MiB] Software uploads performed
17,000 200 1,300
3.2. Using TechnoSat Payloads to evaluate Future TUBiX20 Configurations
16
Although it was originally only planned for a later stage of the mission, experiments that included more than one payload could already be carried out in early October 2017, just three months after launch. As several TechnoSat payloads such as the reaction wheel system, the laser ranging retro-reflectors and the S-band transmitter are used as nominal platform components in upcoming missions such as TUBIN, these experiments allow to evaluate the performance of such platform configurations. In a first such experiment, the reaction wheel system was used to control the attitude of the satellite, while the camera took several pictures of the Earth’s surface. In
3.1. Results of the Satellite Platform To provide redundancy and to maintain flexibility regarding the operation of the payloads, the power system of TechnoSat is designed to include significant margins. As a result, TechnoSat’s performance in power generation and storage are similar to those foreseen for TUBIN. Therefore, TechnoSat’s capabilities in electric power generation and the charging of the spacecraft’s batteries were to be tested in orbit to confirm the design considerations for the power system of TUBIN. As the minimal average power income in a worstcase scenario is difficult to demonstrate in orbit, the 5
solar path A voltage solar panels path A currents
Dec 19, 2018
UTC time
Figure 6: Voltage and current of the nine panels that form one of the two solar power generation paths shown in the upper plot and the same values for the two batteries of the spacecraft depicted in the lower plot (modified from [22])
platform with the aid of selected payloads. Some of the most significant updates, as well as their relevance for future TUBiX20 missions are described in the following.
November, the S-band transmitter was added to this scenario, so that larger amounts of pictures could be sent to the ground than what would have been possible via the nominal UHF transmission path. A collection of color corrected pictures captured by TechnoSat’s camera payload at various places of Earth is shown in Figure 7.
The TechnoSat mission was designed such that all data produced during experimentation can be downloaded via UHF. However, this required operating the satellite at night and during the weekend and yet the amount of data per experiment was limited. Therefore, the ability to downlink telemetry data via the S-band transmitter was implemented by means of a software update, as this was not yet foreseen when the spacecraft was launched. This significantly expanded the capabilities for experimentation and allowed to test this feature in regular operations with regard to the TUBIN mission that will rely on telemetry downlink via S band due to the comparatively larger amount of data being produced. Furthermore, the picture storage capabilities were extended from five to 1,023 pictures. This significantly enhanced the capabilities of assessing the satellite’s attitude determination and control performance using the camera (cf. Section 3.2.1), and allowed to perform longer imaging campaigns to capture larger regions of Earth’s surface similar to what is foreseen for TUBIN. This in turn made it possible to test the corresponding on-board software and operational procedures. Another prerequisite for continuous imaging was to reduce the time to capture and store a picture, which required 23 seconds upon the launch of the spacecraft. This was reduced to 6.6 seconds, in order to be able to record overlapping pictures in nadir pointing. Exploiting the aforementioned new capabilities, entire orbits can be mapped continuously resulting in more than 1,000 connected
Figure 7: Collection of six color corrected pictures recorded by the TechnoSat spacecraft from orbit (modified from [21])
After extensive testing, the S-band transmitter and the reaction wheel system were cleared for use as regular platform components on TechnoSat in January 2018. This was followed by a major software update in May 2018, significantly increasing the performance of the 6
pictures. Moreover, the additional picture data allowed for further evaluation of the S band performance. The attitude control performance with reaction wheels can generally be improved by avoiding rotational rates near zero for the wheels. For reaction wheel systems featuring four wheels in tetrahedron configuration this can be achieved by exchanging the stored angular momentum between the wheels. Therefore, angular momentum management capabilities of the reaction wheels were added to avoid zero-crossings and thus improve the pointing accuracy of the spacecraft. A streaming mode was implemented allowing for real time downlink of pictures from the camera to the ground without on-board storage. This was used for visual confirmation of the target pointing performance towards the ground station in operated passes.
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should be avoided as well, since a change of the rotational direction leads to a leap in the rotational speed due to the friction characteristics. For a configuration of three reaction wheels, the only cause for the accumulation of angular momentum over time are disturbance torques. Here, magnetorquers are generally used for momentum dumping. For an over-determined system of four wheels, numerical errors can lead to a sum of all four reaction wheels’ angular momentum which is zero, while the angular momentum of the individual wheels is unequal to zero [23]. This in turn means that the angular momentum may be exchanged between the four reaction wheels while keeping their sum constant without creating an additional torque. The TUBiX20 ADCS combines this momentum exchange with the desaturation to achieve continuous momentum management of the reaction wheels, keeping their rotational rate at a constant but configurable value to avoid saturation and zero-crossings. For the specific reaction wheels used for the TechnoSat mission, this is especially beneficial since their internal speed sensor is more accurate for lower rotational rates but unable to detect any rotation near zero. Figures 10 to 11 show the satellite’s angular momentum management and reaction wheels’ desaturation feature. In this experiment, the target angular momentum of 4 mN m s (around 400 rpm) is approached for all four reaction wheels (cf. Figure 10). As Figure 8 shows, the attitude control performance is not affected by the angular momentum management. However, the momentum management is only available while all four wheels are functional. In case of a failure of one reaction wheel, the remaining wheels are still desaturated but zero-crossings are not prevented which may lead to a performance degradation.
60
15:42
15:42
Figure 9: Satellite’s angular rates in the coordinate frame of the camera payload
3.2.1. Attitude Determination and Control The introduction of the reaction wheel system into the platform’s ADCS control loop allowed to evaluate the attitude control performance of TechnoSat with regard to upcoming missions such as TUBIN and QUEEN. Figures 8 to 11 show the performance of the ADCS subsystem during an experiment performed in nadir pointing mode. Figure 8 shows that the target attitude is reached within a few degrees in less than five minutes.
15:40 Mar 9, 2019
X Y Z
−0.5
15:52
UTC time
Figure 8: Angular error of TechnoSat in a nadir pointing experiment conducted in March 2019
In steady-state, the satellite’s angular rate equals the orbital rate of around 0.06 deg /s (cf. Figure 9). The TUBiX20 platform is able to perform nadir pointing with an offset. In this experiment, the camera was pointing in the nadir direction, therefore the satellite’s angular rates in Figure 9 are given in the coordinate frame of the camera payload. When using reaction wheels for attitude control, keeping their rotational rates low is generally advantageous to avoid saturation and reduce mechanical abrasion [23]. On the other hand, rotational rates near zero
Figure 11 shows the norm of the satellite’s angular 7
6
Earth’s surface and resampled based on a priori satellite position and attitude data retrieved from telemetry.
] s. m N m [ m u t n e m o m r al u g n A
4 2
Raw pictures
Reaction wheel 0 Reaction wheel 1 Reaction wheel 2 Reaction wheel 3
0
Knowledge error
−2
Time, position, attitude
Georeferenced picture based on onboard knowledge
−4
Sensor model
(assumed perfect, without mounting error)
Reference satellite imagery from Sentinel-2
Commanded attitude
Ideal picture coordinates
(from telemetry)
−6 15:40 Mar 9, 2019
15:42
15:44
15:46
15:48
15:50
15:52
UTC time
Figure 10: Reaction wheel angular momentum while acquiring nadir pointing within the ADCS experiment
Because the satellite’s knowledge of its time, attitude and position is imperfect, the estimated geographical coordinates of the picture differ from where the actual picture is located. This is where the second stage is applied by registering the picture onto reference imagery from Sentinel-2 satellites, as shown on Figure 13 below. For this purpose, a set of distinguishable features are manually selected on both pictures, and their positions are compared. This step provides the picture’s true geographical coordinates. Finally, the pictures’ coordinates resulting from both stages are compared, and the deformation vectors provide a measure of the attitude knowledge error.
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Magnetorquers' torque Wheels' resulting momentum in satellite frame Satellite's momentum
Performance error
Figure 12: Pointing performance analysis in nadir mode using image registration (modified from [22])
momentum due to the rotation and the reaction wheels’ angular momentum. Since the magnetorquers are also used during the whole experiment, it can be seen that from 15:41, the satellite’s momentum remains nearly constant while the wheels’ momentum decreases. 10
Actual picture coordinates
(registered onto reference)
Sentinel-2 cloudless mosaic fit rrected to Picture co reference Sentinel-2
15:52
UTC time
Figure 11: Satellite angular momentum management and magnetorquer torque during the experiment
To validate the pointing performance of TechnoSat, a series of experiments was conducted during which pictures of the Earth’s surface were taken using the satellite’s camera and then georeferenced. The pictures were processed by means of a two-stage correction similar to [24] as shown in Figure 12. Although this method is generally used for sensor calibration and misalignment estimation, in this case, the attitude error is so dominant that the sensor calibration and misalignment was neglected. In the TUBIN satellite, the attitude determination will be much more accurate due to the star trackers, and this method will be applied to calibrate the camera assembly. In the first stage, each raw picture is projected on the
board sed on on Picture ba ude knowledge & attit n io sit po
Figure 13: Registration of a TechnoSat picture onto Sentinel-2 reference (modified from [22])
On the other hand, the coordinates of the ideal picture were computed based on the attitude that was commanded to the satellite. Such a picture would have been taken if all of the following parameters were perfect: on-board clock, orbit knowledge, attitude knowledge, attitude control. Comparing this ideal case with the ac8
Figure 14 shows the cities of Tokyo and Dubai taken from this series. The pictures furthermore demonstrate the cameras ability for capturing scenes with very low brightness.
tual picture coordinates allows to compute the absolute performance error as defined in the ECSS standard [25]. The results for two series of pictures taken in December 2018 and January 2019 are shown in Table 3 below. The errors in attitude knowledge can mainly be attributed to the limited performance of the used set of sensors. However, it is planned to enhance the attitude knowledge for TechnoSat by improving the calibration and filtering of the given sensors. Another source of error is the time uncertainty due to drifting of the onboard clock and the resolution of the pictures’ timestamps, which will also be addressed by a future update of the satellite’s software. Table 3: Pointing accuracy statistics, measured vs. required [22]
Metric
Mean
Std
Required
Performance error
2.0 deg
1.2 deg
≤ 20 deg
Knowledge error
1.7 deg
1.0 deg
N/A
Figure 14: Processed pictures of Tokyo and Dubai taken by TechnoSat during eclipse
Although involving manual selection of the reference features on pictures, which also limits the number of measurement points, these experiments were able to produce the first absolute pointing performance metrics of the TUBiX20 platform, and the results largely exceed the original pointing performance requirements for TechnoSat. While the attitude determination capabilities will have to be improved for the TUBIN mission by the introduction of star trackers into the sensor suite, the attitude control performance is already well in line with the requirements of future missions such as TUBIN and QUEEN. For the next series of experiments, a similar method will be applied on pictures of the Moon where the envisaged and the actual position of the Moon on the pictures will be compared using image recognition.
3.2.3. Imaging another Satellite Starting at 12:14:27 UTC on 16th of July 2018, a close encounter with the six-unit CubeSat CORVUSBC 1 (later renamed to Landmapper-BC 1) was predicted for TechnoSat. The range between the satellites was expected to decrease down to 236 meters. Between 3.50 km and 17 km, the conditions were suitable for taking pictures of CORVUS-BC 1. Figure 15 shows enhanced sections of three pictures of a larger set in which the satellite can be recognized.
3.2.2. Imaging in Eclipse During the Sun period the attitude determination of TechnoSat is based on the QUEST algorithm [26] which exploits measurements of the Sun direction and the magnetic field vector. The result is then filtered with the aid of readings from the fiber optic rate sensor system. When in eclipse, no data from the Sun sensors is available. Therefore, the attitude determination relies solely on the angular rate measurements from the fiber optic rate sensors. Because these sensors have a low bias and noise, the attitude estimation only drifts slightly during the nearly 35 minutes of eclipse. To demonstrate the spacecraft’s attitude control capabilities in these conditions, several cities were captured at night.
Figure 15: Enhanced sections of three pictures from the set taken of CORVUS BC-1 by TechnoSat at a distance between 3.5 and 17 km
3.3. Operational Scenarios for Future Missions In June and July 2018, a large experimental campaign was conducted imitating the operational scenario and the data volume expected for the TUBIN mission to validate both the platform’s and ground segment’s processing capabilities. The campaign included a total of 32 passes over Australia and Indonesia and on each of those passes TechnoSat took a series of 140 overlapping pictures 9
while pointing the camera towards nadir. Imaging was performed in groups of four consecutive days with two imaging passes per day, followed by a pause of six days while waiting for the next overlapping set of pictures and downloading the retrieved data. Overall, a total of 4,480 pictures (206.9 MiB) were recorded in this experiment. In a similar experiment during the summer of 2018 the eastern Mediterranean Sea was targeted and 28 swaths with a total of 1,120 pictures were taken. A fraction of the captured terrain is shown in Figure 16 that comprises approximately 170 pictures.
Figure 17: Total lunar eclipse and Mars captured by TechnoSat on the 27th of July 2018
management, autonomously configuring components as needed for a specific task. 4. Conclusions and Future Works TechnoSat is the first spacecraft that is based on the TUBiX20 platform of Technische Universit¨at Berlin. It was launched in July 2017, and is successfully conducting payload operations since then. Within the conducted experiments, the platform demonstrated key performance parameters that will be further exploited in the follow-up missions TUBIN and QUEEN. This includes an average overall pointing accuracy of two degrees, the generation of well over 35 watts of electric power and downlink data rates of more than 1 Mbit/s. As an Earth observation mission carrying infrared and visible cameras, TUBIN will need to perform complex observation experiments that include pointing the payloads towards multiple different targets for capturing pictures and calibration frames and downloading large datasets. While the attitude controller could already demonstrate sufficient performance within the TechnoSat mission, the attitude determination needs to be improved for TUBIN. This will be realized by including star trackers into the sensor suite [4]. Due to the payload’s constant power demand of 25 watts, the power generation capabilities of the platform need to be increased significantly for the QUEEN mission [20]. This will be achieved by introducing a primary solar panel that is constantly oriented towards the Sun for power generation. In the future, TechnoSat will continue experimentation within its extended mission duration. Furthermore,
Figure 16: Composite image comprising approximately 170 pictures that shows parts of Egypt and Israel captured by TechnoSat
In July 2018, TechnoSat took a total of nearly 200 pictures of the total lunar eclipse and Mars (cf. Figure 17) while the project team was watching the event from below, celebrating the successful first year of TechnoSat in orbit. This experiment demonstrated the spacecraft’s ability to track a celestial object. Although being very successful, the imaging campaigns showed that the satellite operations would greatly benefit from increased autonomy of the spacecraft. This includes enhanced failure detection, isolation and recovery (FDIR) capabilities, as well as an improved mode management. Currently, TechnoSat enters safe mode upon anomalies such as detected irregularities in the collected measurement data. This shall be refined in a future software update with more elaborate error handling capabilities. Furthermore, for conducting complex tasks such as imaging, a relatively high number of telecommands are currently required. This shall be reduced by introducing a more advanced mode 10
the satellite will be used as a testbed for software features developed for the TUBIN mission. In this context, failure detection, isolation and recovery (FDIR) capabilities as well as the spacecraft’s mode management are especially of interest for testing.
[12]
[13]
Acknowledgements
[14]
The development of the TUBiX20 platform is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) through the German Aerospace Center (DLR) on the basis of a decision of the German Bundestag within the TechnoSat, the TUBIN and the QUEEN mission (Grant No. 50RM1219, 50RM1102, 50WM1754 and 50RU1801).
[15]
[16]
[17]
References [1] M. F. Barschke, K. Brieß, U. Renner, Twenty-five years of satellite development at Technische Universit¨at Berlin, in: Proceedings of the Small Satellites Systems and Services Symposium, Valletta, Malta, 2016. [2] M. F. Barschke, K. Gordon, A generic systems architecture for a single failure tolerant nanosatellite platform, in: Proceedings of the 65th International Astronautical Congress, Toronto, Canada, 2014. [3] M. F. Barschke, K. Gordon, M. Lehmann, K. Brieß, The TechnoSat mission for on-orbit technology demonstration, in: Proceedings of the 65th German Aerospace Congress, Braunschweig, Germany, 2016. [4] K. Gordon, M. F. Barschke, P. Werner, Upgrading TUBiX20 bringing TechnoSat flight experience into the TUBIN mission, in: Proceedings of the Small Satellites Systems and Services Symposium, Sorrento, Italy, 2018. [5] O. Balagurin, H. Kayal, H. Wojtkowiak, Validation and qualification of a CMOS based miniature star tracker for small satellites, in: Proceedings of the Small Satellites Systems and Services Symposium, Portoroˇz, Slovenia, 2012. [6] W. Bauer, O. Romberg, R. Putzar, Experimental verification of an innovative debris detector, Acta Astronautica 117 (2015) 49– 54. [7] W. Bauer, O. Romberg, M. F. Barschke, Space environment characterisation by applying an innovative debris detector, in: Proceedings of the Advanced Maui Optical and Space Surveillance Technologies, Maui, Hawaii, 2015. [8] G. Kirchner, L. Grunwaldt, R. Neubert, F. Koidl, M. F. Barschke, Z. Yoon, H. Fiedler, Laser ranging to nano-satellites in LEO orbits: plans, issues, in: Proceedings of the 18th International Workshop on Laser Ranging, Fujiyoshida, Japan, 2013. [9] L. Grunwaldt, R. Neubert, M. F. Barschke, Optical tests of a large number of small COTS cubes, in: Proceedings of the 20th International Workshop on Laser Ranging, Potsdam, Germany, 2016. [10] P. Wang, H. Almer, G. Kirchner, F. Koidl, M. Steindorfer, M. F. Barschke, P. Werner, M. Starke, khz slr application on the attitude analysis of technosat, in: Proceedings of the 21st International Workshop on Laser Ranging, Canberra, Australia, 2018. [11] R. Alavi, K. Brieß, H. Podolski, J. Riesselmann, A. Weiland, W. Frese, In space verification of the pico-satellite S band transmitter HISPICO on a sounding rocket, in: Proceedings of the
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60th International Astronautical Congress, Daejeon, South Korea, 2009. D. Noack, K. Brieß, Laboratory investigation of a fluid-dynamic actuator designed for CubeSats, Acta Astronautica 96 (2014) 78–82. D. Noack, J. Ludwig, P. Werner, M. F. Barschke, K. Brieß, FDAA6 – a fluid-dynamic attitude control system for TechnoSat, in: Proceedings of the Nano-Satellite Symposium, MatsuyamaEhime, Japan, 2017. D. Noack, M. F. Barschke, P. Werner, K. Brieß, FDA in space first in-orbit results of a fluid dynamic attitude control system, in: Proceedings of the Small Satellites Systems and Services Symposium, Sorrento, Italy, 2018. M. F. Barschke, K. Gordon, Enabling flexible payload management through modularity, in: Proceedings of the 65th International Astronautical Congress, Jerusalem, Israel, 2015. M. F. Barschke, J. Bartholom¨aus, K. Gordon, M. Lehmann, K. Brieß, The TUBIN mission for wildfire detection using nanosatellites, CEAS Space Journal 9 (2017) 183–194. J. Bartholom¨aus, M. F. Barschke, M. Lehmann, The TUBIN mission within the context of present and future satellite-based fire detection systems, in: Proceedings of the 12th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, 2019. J. Bartholom¨aus, M. F. Barschke, M. Lehmann, The TUBIN mission within the context of present and future satellite-based fire detection systems, in: Proceedings of the 12th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, 2019. M. F. Barschke, A. Dinkelaker, J. Bartholom¨aus, P. Werner, H. Christopher, M. Krutzik, Optical quantum technology in space using small satellites, in: Proceedings of the 68th International Astronautical Congress, Adelaide, Australia, 2017. A. N. Dinkelaker, A. Kaparthy, S. Reher, A. Bawamia, C. K¨urbis, R. Smol, H. Christopher, A. Wicht, P. Werner, J. Bartholom¨aus, S. Rotter, R. J¨ordens, M. F. Barschke, M. Krutzik, Optical quantum technologies for compact rubidium vapor-cell frequency standards in space using small satellites, Journal of the British Interplanetary Society 72 (2019) 74– 82. M. F. Barschke, P. Werner, K. Gordon, M. Lehmann, W. Frese, D. Noack, L. Grunwaldt, G. Kirchner, P. Wang, B. Schlepp, Initial results from the TechnoSat in-orbit demonstration mission, in: Proceedings of the 31st AIAA/USU Conference on Small Satellites, Logan, USA, 2018. M. F. Barschke, J. Bartholom¨aus, J. M. H. Crespo, K. Gordon, C. Jonglez, P. von Keiser, D. K¨oltzsch, J. Leglise, M. Lehmann, C. Meumann, S. Reinert, S. Rotter, M. Starke, P. Werner, L. Zander, TechnoSat - results from the first 18 months of operation, in: Proceedings of the 12th IAA Symposium on Small Satellites for Earth Observation, Berlin, Germany, 2019. Z. Yoon, T. Terzibaschian, C. Raschke, O. Maibaum, Fault tolerant attitude control system of OOV-TET satellite - design and test results, in: Proceedings of the Small Satellites Systems and Services Symposium, 2010. R. A. Schowengerdt, Remote sensing: models and methods for image processing, Elsevier, 2006. ECSS, Control Performance Standard ECSS-E-ST-60-10C, ESA-ESTEC Requirements & Standards Division (2008). M. D. Shuster, S. D. Oh, Three-axis attitude determination from vector observations, Journalof Guidance and Control 4 (1981) 70–77.
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We introduce a highly modular satellite platform architecture The platform’s first mission is a 20 kg technology demonstration satellite Initial orbit results are presented to showcase the performance of the platform Thousands of images are taken to demonstrate attitude pointing and operations
S0094-5765(19)31360-8
DOI:
https://doi.org/10.1016/j.actaastro.2019.10.034
Reference:
AA 7730
To appear in:
Acta Astronautica
Received Date: 1 March 2019 Revised Date:
11 October 2019
Accepted Date: 15 October 2019
Please cite this article as: F. Merlin Barschke, C. Jonglez, P. Werner, P. von Keiser, K. Gordon, M. Starke, M. Lehmann, Initial Orbit Results from the TUBiX20 Platform, Acta Astronautica, https:// doi.org/10.1016/j.actaastro.2019.10.034. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 IAA. Published by Elsevier Ltd. All rights reserved.
Initial Orbit Results from the TUBiX20 Platform Merlin F. Barschkea , Cl´ement Jongleza , Philipp Wernera , Philip von Keisera , Karsten Gordonb , Mario Starkea , Marc Lehmanna a Technische
Universit¨at Berlin, Institute of Aeronautics and Astronautics, Marchstr. 12, 10587 Berlin, Germany [email protected] b Spacegramming Software and Systems Engineering [email protected]
Abstract Technische Universit¨at Berlin has been active in the field of small satellites for more than 25 years and has successfully developed, built and operated sixteen satellites to date. One of the recent developments of the university is TUBiX20, a highly modular microsatellite platform that was designed to support various different mission types. At the moment, three missions of Technische Universit¨at Berlin are based on TUBiX20, namely TechnoSat, TUBIN and QUEEN. Launched in July 2017, TechnoSat is the first mission to demonstrate the TUBiX20 platform in orbit. TechnoSat’s mission is the demonstration of seven technology payloads that include an S-band transmitter and a reaction wheel system. This paper presents initial orbit results of the TechnoSat mission, focusing on the payloads relevant for future TUBiX20 missions. Here, the evolution of the experiments towards increasing complexity from the commissioning phase into manoeuvres that combine the use of several payloads underlines the platform’s versatility. Furthermore, a quick adaptation of payloads like the S-band transmitter and the reaction wheel system into regular operations was possible by exploiting the platform’s modular architecture. TechnoSat achieved all its mission objectives within the nominal mission time and is now primarily used within its extended mission duration to test and demonstrate capabilities for the upcoming missions. Keywords: orbit results, microsatellite, technology demonstration, in-orbit demonstration, satellite platform
1. Introduction In recent years, small satellites proved that a vast variety of mission scenarios from a number of different application areas can be realized based on this satellite class. In this context, platform designs are often implemented that are used in more than one project to reduce the required development effort per mission. As the requirements imposed on a satellite may diverge significantly between missions, platform concepts commonly rely on modular designs in hard- and software. Technische Universit¨at Berlin has a heritage of more than 25 years in the development and operation of small satellites, having launched sixteen satellites to date [1]. As one of the more recent developments, the university has developed TUBiX20 [2], a modular microsatellite platform which is currently the baseline for three missions. The platform mainly targets towards applications in science, Earth observation and technology demonstration. It is based on a configurable number of single redundant computational nodes that communicate on a Preprint submitted to Acta Astronautica
centralized data bus system. All nodes are located in a central avionics compartment and are connected to a backplane through a standardized interface. According to the needs of a certain mission, components may be added, removed or updated by adjusting the nodes to the needs of the project. In this manner, diverging mission requirements can be supported while limiting the excess capabilities of the final design. Furthermore, this approach facilitates standardization in the hard- and software domain, which in turn allows for extensive reuse. As the first mission based on the TUBiX20 platform, TechnoSat is an in-orbit technology demonstration mission that carries seven payloads. Having completed its nominal mission in summer 2018, payload operations are now complemented by testing new software and operational procedures for following missions within an extended mission period. This paper presents selected orbit results gathered during the first eighteen months of TechnoSat’s operations, as well as their impact on subsequent missions. October 17, 2019
2. TUBiX20 Missions
1. 2. 3. 4. 5. 6. 7.
At present, three missions in various project phases implement the TUBiX20 platform at Technische Universit¨at Berlin. These missions are described in more detail in the following. 2.1. The TechnoSat Mission
Star tracker STELLA [5] Space debris detector SOLID [6, 7] Camera payload Commercial laser ranging retro-reflectors [8, 9, 10] S-band transmitter HiSPiCO [11] Four reaction wheels in tetrahedron configuration Fluid dynamic actuator [12, 13, 14]
The payloads impose only moderate requirements on the attitude control system: three axis stabilization with 20 degree accuracy is required for the verification of the star tracker and the S-band transmitter. Additionally, angular velocity measurements with an accuracy better than 0.1 degree per second are required for testing of the fluid dynamic actuator and the reaction wheel system. To ensure that all experiments can be conducted as planned, a minimal average of 13.3 watts of electrical power needs to be generated under worst case conditions. Table 1 gives an overview of the main parameters of the TechnoSat mission as well as the satellite platform configuration implemented for this mission. Components indicated as payload were cleared for use as nominal platform elements after extensive testing.
The primary objective of the TechnoSat mission is the in-orbit demonstration of seven individual technology payloads [3]. The secondary objective is testing and validation of the newly developed TUBiX20 satellite platform for the first time in orbit. TechnoSat was launched in July 2017 and is currently operated within an extended mission duration (this corresponds to ECSS project phase E). Figure 1 shows a rendering of the TechnoSat spacecraft.
Table 1: Parameters of the TechnoSat mission
Figure 1: Rendered image of the TechnoSat spacecraft in operational configuration with camera and S-band antennas facing upwards [4]
Orbit
600 km, SSO
Launch date Design lifetime
14th of July 2017 1 year
Spacecraft mass
20 kg
Spacecraft volume
465 × 465 × 305 mm
Communications
UHF
Data downlink
S band (payload)
Attitude sensors
IC magnetometers MEMS gyroscopes Sun sensors Fiber optic rate sensors
Attitude actuators
Magnetorquers Reaction wheels (payload)
Orbit determination
Laser ranging (payload)
Figure 3 shows a systems overview of the TechnoSat spacecraft. Here, the six grey boxes outline the subsystems of the spacecraft and its payloads, while in the middle of the schematic the central power (red) and data (blue) buses are depicted. The blocks connected to the central bus system that are shown in dark blue are the aforementioned computing nodes that serve as
Figure 2: Rendered image of the TechnoSat spacecraft with several solar panels removed to show the payloads (modified from [3])
The TechnoSat in-orbit demonstration mission features the following payloads that were developed by industry and different research institutions in Germany (cf. Figure 2): 2
Figure 3: TechnoSat systems overview depicting subsystems and payloads (grey boxes), the central data and power bus system, the nodes that connect to these buses (dark blue), the components connected to the nodes (light blue) and power and data interconnections (modified from [15])
For receiving telecommands as well as sending telemetry to the ground, TechnoSat implements a UHF communications (COM) node with four transceiver channels, each connected to its own antenna. Each two of these transceiver channels with perpendicular arranged antennas are operated in hot redundancy, while the other two are connected to the cold redundant microcontroller of the node. The on-board computer (OBC) is responsible for system-level tasks such as execution of time-tagged command lists, telemetry storage and management of satellite modes. These modes govern a variety of configurations for each subsystem and its devices according to the spacecraft’s current task, such as specific attitude control modes (i.e. target pointing with reaction wheels) with additional payload operations (i.e. camera operation) or data downlink. TechnoSat’s attitude determination and control system (ADCS) comprises of two nodes, the ADCS node that controls the magnetorquers, as well as the six sensor boards, and the fiber optic rate sensor (FOR) node, which connects to three high-precision angular rate sensors. An ADCS sensor board that features two Sun sensors, MEMS gyroscopes and magnetic field sensor ICs is attached to each of the six sides of the satellite. The payload data handling (PDH) node of TechnoSat connects to the camera and the S-band transmitter pay-
interfaces for networking external components such as sensors or actuators. Furthermore, these nodes run the software to control the connected components and to fulfill the high-level tasks of the satellite’s subsystems. The green boxes illustrate components that are directly connected to the central power and data buses, but were developed by the experiment providers and thus do not implement the standardized TUBiX20 hard- and software elements. The star tracker holds a special position here, as it requires a hardware interface (depicted in light blue) to translate to the power redundancy management of the platform. Represented in light blue are devices such as sensors, solar panels, or payloads that are connected to nodes. Here, the depicted number indicates the quantity of implemented instances of the corresponding element. The electrical power system (EPS) of TechnoSat comprises the solar panels, batteries, power conditioning units (PCUs) and the EPS node. Here, each set of solar panels, battery, and PCU form one of the two power generation, storage and distribution paths of the spacecraft. Both paths are operated in hot redundancy, i. e. the spacecraft is still operational if one string fails, however, the performance will be reduced. The EPS node monitors and controls its own elements as well as all nodes of the system. Furthermore, it performs redundancy control for the data bus system and the nodes. 3
loads and is thus responsible for storing pictures before they can be sent to the ground. Furthermore, it runs all software required by the platform to interact with the payloads. TechnoSat is operated by Technische Universit¨at Berlin using the university’s own ground segment. The primary ground station featuring a quad stacked Yagi antenna for UHF communications and a three meter parabolic dish antenna for S band data downlink is located in Berlin. Additional UHF ground stations used for TechnoSat operations are located in Norway and Argentina. Figure 4: Rendering of the TUBIN satellite in operational configuration with the camera system facing upwards [18]
2.2. The TUBIN Mission The second mission based on the TUBiX20 platform is TUBIN, whose mission consists in the demonstration of wildfire detection using microbolometers on a 23 kg microsatellite [16, 17]. To this end, TUBIN carries two infrared microbolometers as well as a camera for the visible range of the spectrum. Introducing two star trackers, a GPS receiver, and fluxgate magnetometers, TUBIN features a significant update of the attitude determination and control capacities compared to TechnoSat in order to fulfill the more stringent pointing accuracy requirements involved by the mission [4]. Here, attitude knowledge better than six arcminutes is required, while the demanded three axis attitude control accuracy in nadir pointing is 180 arcminutes with a stability better than 30 arcminutes per second. For conducting payload experiments, a minimum average power generation of 14.8 watts is required under worst case conditions for TUBIN. Figure 4 shows a rendering of the TUBIN spacecraft. One can see the baffles of the two infrared cameras at the top side of the spacecraft with the one of the visible spectrum range camera in between. The TUBIN spacecraft is currently in production (mission phase D according to ECSS) and is scheduled for launch in 2020.
and the Ferdinand-Braun-Institut, Leibniz-Institut f¨ur H¨ochstfrequenztechnik. For the QUEEN mission, a three axis attitude pointing error below 60 arcminutes is required. The QUEEN payload, which will be operated continuously, has an average power consumption of 25 watts.
3. TechnoSat Mission Operations and Results TechnoSat was launched into a 600 km Sunsynchronous orbit on the 14th of July 2017. Figure 5 shows the integration of TechnoSat on the Fregat upper stage one week before launch.
2.3. The QUEEN Mission QUEEN is a 35 kg quantum technology mission with the objective to demonstrate and test a rubidium two photon vapor cell frequency reference in orbit [19, 20]. Furthermore, it will carry an X-band transceiver, an optical data downlink terminal, and a camera system as secondary payloads. The QUEEN mission, which is currently in definition phase (phase B according to ECSS), is a cooperation project between HumboldtUniversit¨at zu Berlin, Technische Universit¨at Berlin,
Figure 5: Integration of the TechnoSat spacecraft on the Fregat upper stage one week before launch (image credit: Roscosmos)
After the first signals of the spacecraft were received during the first pass over the ground station in Berlin, the launch and early orbit phase (LEOP) and subsequently the commissioning of the platform were conducted until September 2017. This included a software 4
update to optimize the performance of the attitude determination and control system based on data gathered in orbit [4]. Payload commissioning was performed partly in parallel to the platform commissioning activities between August and September 2017. All six payloads that are actively operated (the laser retro-reflectors are entirely passive) were powered successfully and first payload telemetry was gathered. Subsequently, regular payload operations started while experiments to assess the performance of the platform were conducted at the same time. All payloads have been successfully operated in numerous experiments and the data was distributed to the payload developers for evaluation. First orbit results of selected TechnoSat payloads were presented in [10, 14, 21]. Three of the TechnoSat payloads, namely the S-band transmitter, the reaction wheels and the retro-reflectors are implemented as regular platform components within subsequent missions such as TUBIN, after being tested in orbit on TechnoSat. A more detailed description of how results of the TechnoSat mission were considered for the TUBIN design can be found in [4]. Table 2 gives an overview over the operations that were conducted with TechnoSat during the first eighteen months of the mission. Here, a software upload refers to the update of a single node.
maximum power income in Sun pointing was selected as reference value for assessing the power generation and storage performance of TechnoSat. This value was determined to be above 35 watts during the design of TechnoSat. Here, an experiment was conducted that involved minimizing the power income for one Sun period by pointing the side of the satellite without solar cells towards the Sun. In the following Sun period, the attitude was changed to maximize the power income. Figure 6 shows the voltage and the current of the nine solar panels of one of the two solar generation paths of the satellite in the upper plot, as well as voltage and current of the two battery packs in the lower one. The current generated during the first Sun period can be attributed to the Earth’s albedo and the overall development of the albedo induced current can be explained by the orbit geometry. As the satellite performs Sun pointing the irradiation angle of the Earth’s albedo is constantly changing and the generated power remains below 30 percent of the power generated from sunlight that would be expected when pointing the solar panels towards nadir. Furthermore, a significant influence of the Earth’s surface as seen from the satellite, like clouds, water or land can be observed. Nevertheless, it can be seen that the amount of electrical power that is generated by Earth albedo is, under certain circumstances, sufficient to completely supply the satellite with power in nominal operations. For Sun pointing, the peak power that is delivered by the solar panels is more than 35 watts, which agrees well with the design values. After approximately one quarter of the Sun period, one can observe the charge regulator’s transition to constant voltage mode, as would be expected after reaching the design value of 16.8 volts.
Table 2: TechnoSat operations as of January 2019
Orbits Ground stations used
8,187 3
Passes with active operations
2,409
Pictures taken Telemetry downloaded [MiB] Payload data downloaded [MiB] Software uploads performed
17,000 200 1,300
3.2. Using TechnoSat Payloads to evaluate Future TUBiX20 Configurations
16
Although it was originally only planned for a later stage of the mission, experiments that included more than one payload could already be carried out in early October 2017, just three months after launch. As several TechnoSat payloads such as the reaction wheel system, the laser ranging retro-reflectors and the S-band transmitter are used as nominal platform components in upcoming missions such as TUBIN, these experiments allow to evaluate the performance of such platform configurations. In a first such experiment, the reaction wheel system was used to control the attitude of the satellite, while the camera took several pictures of the Earth’s surface. In
3.1. Results of the Satellite Platform To provide redundancy and to maintain flexibility regarding the operation of the payloads, the power system of TechnoSat is designed to include significant margins. As a result, TechnoSat’s performance in power generation and storage are similar to those foreseen for TUBIN. Therefore, TechnoSat’s capabilities in electric power generation and the charging of the spacecraft’s batteries were to be tested in orbit to confirm the design considerations for the power system of TUBIN. As the minimal average power income in a worstcase scenario is difficult to demonstrate in orbit, the 5
solar path A voltage solar panels path A currents
Dec 19, 2018
UTC time
Figure 6: Voltage and current of the nine panels that form one of the two solar power generation paths shown in the upper plot and the same values for the two batteries of the spacecraft depicted in the lower plot (modified from [22])
platform with the aid of selected payloads. Some of the most significant updates, as well as their relevance for future TUBiX20 missions are described in the following.
November, the S-band transmitter was added to this scenario, so that larger amounts of pictures could be sent to the ground than what would have been possible via the nominal UHF transmission path. A collection of color corrected pictures captured by TechnoSat’s camera payload at various places of Earth is shown in Figure 7.
The TechnoSat mission was designed such that all data produced during experimentation can be downloaded via UHF. However, this required operating the satellite at night and during the weekend and yet the amount of data per experiment was limited. Therefore, the ability to downlink telemetry data via the S-band transmitter was implemented by means of a software update, as this was not yet foreseen when the spacecraft was launched. This significantly expanded the capabilities for experimentation and allowed to test this feature in regular operations with regard to the TUBIN mission that will rely on telemetry downlink via S band due to the comparatively larger amount of data being produced. Furthermore, the picture storage capabilities were extended from five to 1,023 pictures. This significantly enhanced the capabilities of assessing the satellite’s attitude determination and control performance using the camera (cf. Section 3.2.1), and allowed to perform longer imaging campaigns to capture larger regions of Earth’s surface similar to what is foreseen for TUBIN. This in turn made it possible to test the corresponding on-board software and operational procedures. Another prerequisite for continuous imaging was to reduce the time to capture and store a picture, which required 23 seconds upon the launch of the spacecraft. This was reduced to 6.6 seconds, in order to be able to record overlapping pictures in nadir pointing. Exploiting the aforementioned new capabilities, entire orbits can be mapped continuously resulting in more than 1,000 connected
Figure 7: Collection of six color corrected pictures recorded by the TechnoSat spacecraft from orbit (modified from [21])
After extensive testing, the S-band transmitter and the reaction wheel system were cleared for use as regular platform components on TechnoSat in January 2018. This was followed by a major software update in May 2018, significantly increasing the performance of the 6
pictures. Moreover, the additional picture data allowed for further evaluation of the S band performance. The attitude control performance with reaction wheels can generally be improved by avoiding rotational rates near zero for the wheels. For reaction wheel systems featuring four wheels in tetrahedron configuration this can be achieved by exchanging the stored angular momentum between the wheels. Therefore, angular momentum management capabilities of the reaction wheels were added to avoid zero-crossings and thus improve the pointing accuracy of the spacecraft. A streaming mode was implemented allowing for real time downlink of pictures from the camera to the ground without on-board storage. This was used for visual confirmation of the target pointing performance towards the ground station in operated passes.
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should be avoided as well, since a change of the rotational direction leads to a leap in the rotational speed due to the friction characteristics. For a configuration of three reaction wheels, the only cause for the accumulation of angular momentum over time are disturbance torques. Here, magnetorquers are generally used for momentum dumping. For an over-determined system of four wheels, numerical errors can lead to a sum of all four reaction wheels’ angular momentum which is zero, while the angular momentum of the individual wheels is unequal to zero [23]. This in turn means that the angular momentum may be exchanged between the four reaction wheels while keeping their sum constant without creating an additional torque. The TUBiX20 ADCS combines this momentum exchange with the desaturation to achieve continuous momentum management of the reaction wheels, keeping their rotational rate at a constant but configurable value to avoid saturation and zero-crossings. For the specific reaction wheels used for the TechnoSat mission, this is especially beneficial since their internal speed sensor is more accurate for lower rotational rates but unable to detect any rotation near zero. Figures 10 to 11 show the satellite’s angular momentum management and reaction wheels’ desaturation feature. In this experiment, the target angular momentum of 4 mN m s (around 400 rpm) is approached for all four reaction wheels (cf. Figure 10). As Figure 8 shows, the attitude control performance is not affected by the angular momentum management. However, the momentum management is only available while all four wheels are functional. In case of a failure of one reaction wheel, the remaining wheels are still desaturated but zero-crossings are not prevented which may lead to a performance degradation.
60
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Figure 9: Satellite’s angular rates in the coordinate frame of the camera payload
3.2.1. Attitude Determination and Control The introduction of the reaction wheel system into the platform’s ADCS control loop allowed to evaluate the attitude control performance of TechnoSat with regard to upcoming missions such as TUBIN and QUEEN. Figures 8 to 11 show the performance of the ADCS subsystem during an experiment performed in nadir pointing mode. Figure 8 shows that the target attitude is reached within a few degrees in less than five minutes.
15:40 Mar 9, 2019
X Y Z
−0.5
15:52
UTC time
Figure 8: Angular error of TechnoSat in a nadir pointing experiment conducted in March 2019
In steady-state, the satellite’s angular rate equals the orbital rate of around 0.06 deg /s (cf. Figure 9). The TUBiX20 platform is able to perform nadir pointing with an offset. In this experiment, the camera was pointing in the nadir direction, therefore the satellite’s angular rates in Figure 9 are given in the coordinate frame of the camera payload. When using reaction wheels for attitude control, keeping their rotational rates low is generally advantageous to avoid saturation and reduce mechanical abrasion [23]. On the other hand, rotational rates near zero
Figure 11 shows the norm of the satellite’s angular 7
6
Earth’s surface and resampled based on a priori satellite position and attitude data retrieved from telemetry.
] s. m N m [ m u t n e m o m r al u g n A
4 2
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Reaction wheel 0 Reaction wheel 1 Reaction wheel 2 Reaction wheel 3
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Georeferenced picture based on onboard knowledge
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Sensor model
(assumed perfect, without mounting error)
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(from telemetry)
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Figure 10: Reaction wheel angular momentum while acquiring nadir pointing within the ADCS experiment
Because the satellite’s knowledge of its time, attitude and position is imperfect, the estimated geographical coordinates of the picture differ from where the actual picture is located. This is where the second stage is applied by registering the picture onto reference imagery from Sentinel-2 satellites, as shown on Figure 13 below. For this purpose, a set of distinguishable features are manually selected on both pictures, and their positions are compared. This step provides the picture’s true geographical coordinates. Finally, the pictures’ coordinates resulting from both stages are compared, and the deformation vectors provide a measure of the attitude knowledge error.
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Magnetorquers' torque Wheels' resulting momentum in satellite frame Satellite's momentum
Performance error
Figure 12: Pointing performance analysis in nadir mode using image registration (modified from [22])
momentum due to the rotation and the reaction wheels’ angular momentum. Since the magnetorquers are also used during the whole experiment, it can be seen that from 15:41, the satellite’s momentum remains nearly constant while the wheels’ momentum decreases. 10
Actual picture coordinates
(registered onto reference)
Sentinel-2 cloudless mosaic fit rrected to Picture co reference Sentinel-2
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Figure 11: Satellite angular momentum management and magnetorquer torque during the experiment
To validate the pointing performance of TechnoSat, a series of experiments was conducted during which pictures of the Earth’s surface were taken using the satellite’s camera and then georeferenced. The pictures were processed by means of a two-stage correction similar to [24] as shown in Figure 12. Although this method is generally used for sensor calibration and misalignment estimation, in this case, the attitude error is so dominant that the sensor calibration and misalignment was neglected. In the TUBIN satellite, the attitude determination will be much more accurate due to the star trackers, and this method will be applied to calibrate the camera assembly. In the first stage, each raw picture is projected on the
board sed on on Picture ba ude knowledge & attit n io sit po
Figure 13: Registration of a TechnoSat picture onto Sentinel-2 reference (modified from [22])
On the other hand, the coordinates of the ideal picture were computed based on the attitude that was commanded to the satellite. Such a picture would have been taken if all of the following parameters were perfect: on-board clock, orbit knowledge, attitude knowledge, attitude control. Comparing this ideal case with the ac8
Figure 14 shows the cities of Tokyo and Dubai taken from this series. The pictures furthermore demonstrate the cameras ability for capturing scenes with very low brightness.
tual picture coordinates allows to compute the absolute performance error as defined in the ECSS standard [25]. The results for two series of pictures taken in December 2018 and January 2019 are shown in Table 3 below. The errors in attitude knowledge can mainly be attributed to the limited performance of the used set of sensors. However, it is planned to enhance the attitude knowledge for TechnoSat by improving the calibration and filtering of the given sensors. Another source of error is the time uncertainty due to drifting of the onboard clock and the resolution of the pictures’ timestamps, which will also be addressed by a future update of the satellite’s software. Table 3: Pointing accuracy statistics, measured vs. required [22]
Metric
Mean
Std
Required
Performance error
2.0 deg
1.2 deg
≤ 20 deg
Knowledge error
1.7 deg
1.0 deg
N/A
Figure 14: Processed pictures of Tokyo and Dubai taken by TechnoSat during eclipse
Although involving manual selection of the reference features on pictures, which also limits the number of measurement points, these experiments were able to produce the first absolute pointing performance metrics of the TUBiX20 platform, and the results largely exceed the original pointing performance requirements for TechnoSat. While the attitude determination capabilities will have to be improved for the TUBIN mission by the introduction of star trackers into the sensor suite, the attitude control performance is already well in line with the requirements of future missions such as TUBIN and QUEEN. For the next series of experiments, a similar method will be applied on pictures of the Moon where the envisaged and the actual position of the Moon on the pictures will be compared using image recognition.
3.2.3. Imaging another Satellite Starting at 12:14:27 UTC on 16th of July 2018, a close encounter with the six-unit CubeSat CORVUSBC 1 (later renamed to Landmapper-BC 1) was predicted for TechnoSat. The range between the satellites was expected to decrease down to 236 meters. Between 3.50 km and 17 km, the conditions were suitable for taking pictures of CORVUS-BC 1. Figure 15 shows enhanced sections of three pictures of a larger set in which the satellite can be recognized.
3.2.2. Imaging in Eclipse During the Sun period the attitude determination of TechnoSat is based on the QUEST algorithm [26] which exploits measurements of the Sun direction and the magnetic field vector. The result is then filtered with the aid of readings from the fiber optic rate sensor system. When in eclipse, no data from the Sun sensors is available. Therefore, the attitude determination relies solely on the angular rate measurements from the fiber optic rate sensors. Because these sensors have a low bias and noise, the attitude estimation only drifts slightly during the nearly 35 minutes of eclipse. To demonstrate the spacecraft’s attitude control capabilities in these conditions, several cities were captured at night.
Figure 15: Enhanced sections of three pictures from the set taken of CORVUS BC-1 by TechnoSat at a distance between 3.5 and 17 km
3.3. Operational Scenarios for Future Missions In June and July 2018, a large experimental campaign was conducted imitating the operational scenario and the data volume expected for the TUBIN mission to validate both the platform’s and ground segment’s processing capabilities. The campaign included a total of 32 passes over Australia and Indonesia and on each of those passes TechnoSat took a series of 140 overlapping pictures 9
while pointing the camera towards nadir. Imaging was performed in groups of four consecutive days with two imaging passes per day, followed by a pause of six days while waiting for the next overlapping set of pictures and downloading the retrieved data. Overall, a total of 4,480 pictures (206.9 MiB) were recorded in this experiment. In a similar experiment during the summer of 2018 the eastern Mediterranean Sea was targeted and 28 swaths with a total of 1,120 pictures were taken. A fraction of the captured terrain is shown in Figure 16 that comprises approximately 170 pictures.
Figure 17: Total lunar eclipse and Mars captured by TechnoSat on the 27th of July 2018
management, autonomously configuring components as needed for a specific task. 4. Conclusions and Future Works TechnoSat is the first spacecraft that is based on the TUBiX20 platform of Technische Universit¨at Berlin. It was launched in July 2017, and is successfully conducting payload operations since then. Within the conducted experiments, the platform demonstrated key performance parameters that will be further exploited in the follow-up missions TUBIN and QUEEN. This includes an average overall pointing accuracy of two degrees, the generation of well over 35 watts of electric power and downlink data rates of more than 1 Mbit/s. As an Earth observation mission carrying infrared and visible cameras, TUBIN will need to perform complex observation experiments that include pointing the payloads towards multiple different targets for capturing pictures and calibration frames and downloading large datasets. While the attitude controller could already demonstrate sufficient performance within the TechnoSat mission, the attitude determination needs to be improved for TUBIN. This will be realized by including star trackers into the sensor suite [4]. Due to the payload’s constant power demand of 25 watts, the power generation capabilities of the platform need to be increased significantly for the QUEEN mission [20]. This will be achieved by introducing a primary solar panel that is constantly oriented towards the Sun for power generation. In the future, TechnoSat will continue experimentation within its extended mission duration. Furthermore,
Figure 16: Composite image comprising approximately 170 pictures that shows parts of Egypt and Israel captured by TechnoSat
In July 2018, TechnoSat took a total of nearly 200 pictures of the total lunar eclipse and Mars (cf. Figure 17) while the project team was watching the event from below, celebrating the successful first year of TechnoSat in orbit. This experiment demonstrated the spacecraft’s ability to track a celestial object. Although being very successful, the imaging campaigns showed that the satellite operations would greatly benefit from increased autonomy of the spacecraft. This includes enhanced failure detection, isolation and recovery (FDIR) capabilities, as well as an improved mode management. Currently, TechnoSat enters safe mode upon anomalies such as detected irregularities in the collected measurement data. This shall be refined in a future software update with more elaborate error handling capabilities. Furthermore, for conducting complex tasks such as imaging, a relatively high number of telecommands are currently required. This shall be reduced by introducing a more advanced mode 10
the satellite will be used as a testbed for software features developed for the TUBIN mission. In this context, failure detection, isolation and recovery (FDIR) capabilities as well as the spacecraft’s mode management are especially of interest for testing.
[12]
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Acknowledgements
[14]
The development of the TUBiX20 platform is funded by the Federal Ministry for Economic Affairs and Energy (BMWi) through the German Aerospace Center (DLR) on the basis of a decision of the German Bundestag within the TechnoSat, the TUBIN and the QUEEN mission (Grant No. 50RM1219, 50RM1102, 50WM1754 and 50RU1801).
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We introduce a highly modular satellite platform architecture The platform’s first mission is a 20 kg technology demonstration satellite Initial orbit results are presented to showcase the performance of the platform Thousands of images are taken to demonstrate attitude pointing and operations