EntrySat: A 3U cubesat to study the atmospheric reentry environment

The 4S Symposium 2014 – R. F. Garcia

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ENTRYSAT: A 3U CUBESAT TO STUDY THE RE-ENTRY ATMOSP HERIC ENVIRONMENT

Raphael F. Garcia (1), Jérémie Chaix (2), David Mimoun (3), Marc Alomar (4),

EntrySat Student Team

(1) ISAE - Institut Supérieur de l’Aéronautique et de l’Espace, 10 avenue Edouard Belin, 31055 Toulouse, France, [email protected]

(2) ISAE, 31055 Toulouse, France, [email protected] (3) ISAE, 31055 Toulouse, France, [email protected]

(4) ISAE, 31055 Toulouse, France, [email protected]

ABSTRACT The EntrySat is a 3U CubeSat designed to study the uncontrolled atmospheric re-entry. The project, developed by ISAE in collaboration with ONERA, is funded by CNES and is intended to be launched in January 2016, in the context of the QB50 network. The scientific goal is to relate the kinematics of the satellite with the aero-thermodynamic environment during re-entry. In particular, in-flight data will be compared with the computations of MUSIC/FAST, a new 6-degree of freedom code developed by ONERA to predict the trajectory of space debris. According to these requirements, the satellite will measure the temperature, pressure, heat flux, and drag force during re-entry, as well as the trajectory and attitude of the satellite. One of the major technological challenges is the retrieval of data during the re-entry phase, which will be based on the Iridium satellite network. The system design is based on the use of commercial COTS components, and is mostly developed by students from ISAE. As such, the EntrySat has an important educational value in the formation of young engineers.

1 INTRODUCTION

The Low Earth Orbit is getting more and more crowded with space debris. Objects at LEO experience a natural decay process due to the atmospheric drag, which leads to their re-entry into the atmosphere. In fact, this natural process is used as an end-of-life deorbiting strategy for LEO satellites. France has regulated this phase with the Space Operations Act (LSO), whose compliancy is mandatory from December 2010 for any space operation conducted on French territory. This regulation dictates the obligation to de-orbit components of any space system. However, the uncontrolled atmospheric re-entry can have hazardous effects on ground. Most of these objects are monitored by ground-based systems, and computational models are used to estimate their trajectories. However, the precise re-entry point is difficult to predict because of the dependence on many uncertain parameters, such as the attitude of the object, the heat transfer coefficients, the atmospheric density, and the solar activity. The EntrySat is a 3U CubeSat designed to study the atmospheric re-entry. The size of the satellite, 34x10x10 cm3, is similar to the dimensions of secondary debris produced during the breakup of a spacecraft. The satellite will collect data about the temperature, heat flux, pressure, drag force, and attitude during re-entry. All data will be transmitted before its destruction using the Iridium communication system. This mission is intended to be part of the QB50 project, an international network of fifty double and triple CubeSats with the objective of studying the lower thermosphere


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(90 - 320 km). The network will consist of forty 2U CubeSats carrying the same set of sensors, to provide multi-point measurements of the atmosphere, and ten In-Orbit Demonstration CubeSats, which carry their own payload for technological demonstration purposes. At this time, the launch of the constellation is scheduled for January 2016. Valuable data has been obtained in the context of controlled re-entries. A good example is the ATV-1 observation campaign, carried out by ESA and NASA. At the end of its life, the ATV Jules Verne was led to a controlled atmospheric re-entry which was observed by two aircraft, equipped with imaging and spectroscopic instruments [1]. In the United States, the Center for Orbital and Reentry Debris Studies (CORDS) has developed the Reentry Breakup Recorder (REBR) [2]. The REBR is an autonomous data collection system which can be attached to a launcher upper stage or a satellite. When the spacecraft re-enters the atmosphere, following break up, the REBR wakes up and starts collecting data, which is transmitted to the Iridium network before reaching ground. At this time, the REBR has been successfully used in three missions [3], the HTV-2, HTV-3, and ATV-3, all of them transfer vehicles that supply the ISS. In the segment of nanosatellites, another CubeSat of the QB50 constellation is designed to study the atmospheric re-entry, the 3U QARMAN, developed by the von Karman Institute [4]. As opposed to the EntrySat, this satellite contains a thermal protective unit and a drag augmentation system, to increase the stability and the lifetime of the CubeSat. However, the uncontrolled re-entry of space debris has not been studied so far. The high risk of this kind of missions, as well as the short orbital lifetime, are some of the main reasons that prevent space agencies from investing in such projects. In this context, the CubeSat platform is an ideal test bed. The high educational impact, added to the low cost of the project, makes it possible to justify high risk missions, even in case of failure.

2 MISSION OBJECTIVES

2.1 Science Research

The scientific objectives of the mission, which have been defined in collaboration with ONERA, are summarized in table 1. The payload has been chosen to relate the kinematics of the CubeSat with the aero-thermodynamic environment during atmospheric re-entry. This data will be compared with the predictions of a new model developed by ONERA, MUSIC/FAST [5]. This model couples the aero-thermodynamic loads with the dynamics of the object, taking into account ablation and fragmentation phenomena. So far, the code has been tested and calibrated only for large spacecraft. The EntrySat mission will be used to increase the accuracy of the model for small debris.

Table 1. Scientific Objectives

Scientific Objectives Measurements Sensors

Investigate the kinematics of

uncontrolled space debris

Position GPS

Velocity Attitude IMU, Magnetometer

Study the variation of aerodynamic

pressure during re-entry

Absolute Pressure Absolute Pressure Sensor

Drag Force Analyse the satellite integrity

during re-entry

Internal, External Temperature Type-K Thermocouples Heat Flux Heat Flux Sensor


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Figure 1. Concept of Operations.

2.2 Technological Demonstration

The EntrySat is part of the ten In-Orbit Demonstration CubeSats of the QB50 project. As opposed to the standard CubeSats of the constellation, their objective is to explore new technological concepts in the field of nanosatellites. The main challenge that the EntrySat has to overcome is the retrieval of data during the re-entry phase. Since the satellite will disintegrate during re-entry, data has to be recovered in-flight. This situation presents various problems. At an altitude of about 200 km, the time window over a ground station will be of just a few minutes, which seriously compromises the capability of downloading data and commanding the satellite using traditional communication systems, such as UHF/VHF. Furthermore, the re-entry of the CubeSat will occur in a short period of time over an undetermined area. Even if the ground segment consists of a network of ground stations, it cannot be guaranteed that any of them will be under visibility. To overcome this problem, the use of a satellite relay network came out as the only reliable solution. Several systems in LEO/MEO orbit were considered, such as the Globalstar, the Orbcomm and the Iridium networks. After the analysis of requirements for coverage, power consumption, mass, and availability, the Iridium network was considered as the best option for the CubeSat platform. This system has been successfully used in similar missions, such as the REBR and the PhoneSat Bell, from NASA. However, the use of Iridium modems in space is not widespread, and it needs special authorization. For example, the TechEdSat CubeSat had to deactivate the Iridium modem because they could not obtain the FCC license on time. At this moment, the legal aspects concerning the use of Iridium in the EntrySat mission are being considered.

2.3 Education

A key objective of the EntrySat project is to educate young engineers with a real, hands-on project. The design and implementation of the satellite is carried out by about thirty students from ISAE, under mentorship and supervision of faculty. For most of the students, this is the first contact with a


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Figure 2. Satellite Layout.

complex engineering project. The short life-cycle of the project enables them to participate in several phases of the development, giving an overall picture of how space engineering projects are conducted. This project is an invaluable opportunity that complements their academic formation at ISAE.

3 CONCEPT of OPERATIONS

The concept of operations is summarized in figure 1. At this moment, the launch date of the QB50 constellation is set for January 2016. The satellites will be launched by an Ukrainian Cyclone-4 from Alcantara, Brazil, to an orbit of altitude 380 km and inclination 98 degrees. After successful ejection from the deployment rack, the EntrySat will wait 30 minutes before deploying the UHF/VHF antennas. Once the link with the ground station has been established, the satellite will enter a commissioning phase that will last approximately one week. During this time, the payload instruments will be tested, as well as the communication link with Iridium. During the orbital phase, the satellite will perform measurements at a rate of 300 s. The goal is to have the EntrySat in an optimal condition to begin the re-entry phase. The transition to the final phase will be autonomous, triggered by an altitude threshold to be determined. It will be possible also to order the transition by telecommand, as well as to cancel it, to recover from unexpected transitions. The re-entry phase will be the core part of the mission. During this phase, which will last a few days, the payload sensors will retrieve data at a rate of 1 s, up to the destruction of the satellite. All data will be retrieved using the Iridium network.

4 MISSION DESIGN

4.1 System Requirements

The design of the EntrySat is the result of a number of requirements and factors, including

1. Comply with the scientific requirements specified by ONERA. 2. Meet the requirements set by the QB50 project. This includes the fulfilment of the CubeSat

Design Specification set by CalPoly, as well as constraints regarding the QB 50 deployment system, the StackPack.

3. Make the project feasible for undergraduate students. Most of the work is done by


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inexperienced students, under a limited budget. This fact requires keeping the design as simple as possible.

As a result of the last factor, it has been decided to use Commercial Off-The-Shelf (COTS) components that comply with the CubeSat standard. This strategy reduces the development effort, saves time, and increases the probability of success of the mission. For example, the On-Board Computer (OBC) and the Electrical Power Subsystem (EPS) are both provided by GomSpace, and the 3U Structure, the Magnetorquer and the UHF/VHF Transceiver are provided by ISIS. The other devices are COTS components coming from ground applications, but that comply with industrial environmental requirements. The only component that will be integrally developed for this mission is the Sensor Board, which is the interface between the payload sensors and the OBC.

4.2 System Design

The EntrySat is based on a 3U CubeSat platform, of dimensions 34x10x10 cm3 and a maximum weight of 3 kg. The system layout is shown in figure 2. The configuration diagram in figure 3 shows the subsystems and the main interconnections. The system has a centralized architecture, where the OBC is the master of the I2C bus. This protocol is implemented in most of the CubeSat COTS components, which are interconnected via the PC104 stack connector. All the subsystems operate as slaves, responding only to requests from the bus master.

4.3 Payload

In order to comply with the scientific requirements described in section 2.1, the EntrySat incorporates five absolute pressure sensors and six heat flux sensors. The absolute pressure sensors are mounted on the side faces and on the front face of the CubeSat. The heat flux sensors are mounted on all the six faces. The mounting technique is under study, and the final choice will depend on the results of the vibration tests. All the sensors are interfaced with the OBC by means of the Sensor Board. This component, a joint design of ISAE and EREMS, incorporates three Analog-to-Digital Converters (ADC) with an I2C interface. The board, which is controlled by the OBC, will also filter high frequency noise coming from the sensors, and will provide independent switching control of the pressure and thermal segment. The payload is completed with a GPS and an Inertial Measurement Unit (IMU). The GPS will provide position and velocity data, necessary to determine the trajectory of the satellite during re-entry. In addition, it will synchronize the on-board clock with real-time UTC data. The attitude of the satellite will be determined by the IMU. This data will be used only during the re-entry phase, not for the attitude control algorithm.

4.4 ADCS

The attitude control requirements of the QB50 mission are not very demanding. The payload doesn’t need accurate positioning, but controlled attitude is required for the correct operation of the satellite. The main functions of the ADCS will be to reduce the tip-off rates after ejection, to position the UHF/VHF antennas, and to orient the satellite in an optimal position for solar energy harvesting. The satellite will have an active control system based on a magnetorquer, and the attitude will be estimated from the measurements of a magnetometer embedded in the OBC.


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Figure 3. Configuration Diagram.

At this moment, one of the main challenges is to ensure the communication link during re-entry. In this context, tumbling should be avoided, since the angular rate could seriously compromise the link with the Iridium constellation. To increase the stability of the satellite, the center of gravity will be displaced 2 cm forward. Other passive stabilization systems are currently under study.

4.5 Communication Subsystem

The communication subsystem will be the only one with some degree of redundancy. Due to the requirements of the re-entry phase, the EntrySat will have two communication systems: a UHF/VHF transceiver and an Iridium modem. The main communication system during the orbital phase will be a UHF downlink / VHF uplink transceiver from ISIS. The satellite will be operated from the ISAE ground station facilities, in Toulouse. The satellite will also emit a continuous wave (CW) beacon, once every thirty seconds. This beacon will consist of a basic set of housekeeping data, the Whole Orbit Data (WOD), which is required for all the satellites that are part of the QB50 constellation. The UHF/VHF transceiver will use four tape antennas in a turnstile configuration. The antennas, made of a shape-memory alloy, are part of the ISIS Deployable Antenna System. The deployment, which is ordered by the OBC, will occur 30 minutes after the ejection from the launcher. During the re-entry phase, the Iridium system will be used to download scientific data. We have chosen the Iridium 9602 modem, which has a small footprint and a reduced mass and power consumption. The modem is interfaced to a patch antenna of very low profile. The data service that will be used is the Iridium Short Burst Data (SBD), which can send messages of a maximum length of 340 bytes. The modem sends messages to the Iridium satellite network, which in turn are sent to


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the Iridium Gateway. From there, the message is transmitted to the end user by e-mail.

4.6 Command and Data Handling

The C&DH subsystem is based on the Nanomind A712D on-board computer, from GomSpace. The board is based on an ARM7TDMI 40 MHz processor, and has 2 MB of RAM, 4MB for code storage, and a microSD socket for up to 2 GB of storage. The main connector is a PC104 bus, which contains the I2C and power lines. The board also includes a magnetometer, which will be used for the ADCS algorithm, and three PWM outputs for driving a magnetorquer. The flight software (FSW) is implemented by the freeRTOS real-time operating system, a pre-emptive, open-source RTOS designed for small embedded applications. The use of an RTOS provides many advantages to the development of the FSW. With an RTOS, code is organized in tasks, improving the modularity of the software. This is a key asset that facilitates team development: tasks can be developed independently, with clean interfaces for intertask communication. Another benefit comes from the fact that tasks can block on events. Polling is avoided, improving the efficiency of the system.

5 CONCLUSIONS

The EntrySat is a 3U CubeSat designed to study the atmospheric re-entry. The project has a launch opportunity in 2016 in the context of the QB50 project, an international network of fifty CubeSats for multi-point measurements in the lower thermosphere, funded by the European Union 7th Framework Programme (FP7). The EntrySat project, developed by ISAE in collaboration with ONERA, is financially supported by CNES. At this point, we have submitted the Critical Design Review (CDR) to the QB50 committee. Depending on the results of this review, the CubeSat will be finally selected for the QB50 constellation. This project is the first CubeSat that has been designed by ISAE. As such, the project is a valuable learning experience for engineering students, who face for the first time the complexity of a space engineering project. Another project that is currently on-going is the 3U CubeSat JumpSat, a qualification satellite to be launched in 2017.

6 REFERENCES

[1] Blasco A., et al. Analysis of the ATV1 Re-Entry Using Near-UV Spectroscopic Data from the ESA/NASA Multi-Instrument Aircraft Observation Campaign, 62nd IAC, 2011. [2] Ailor, W. H. et al. Spacecraft Re-entry Breakup Recorder, United States Patent No. 6,895,314 B2, May 17, 2005. [3] Feistel, A. S., Weaver, M. A. and Ailor, W. H., Comparison of Reentry Breakup Measurements for Three Atmospheric Reentries, 6th IAASS Conference, 2013. [4] Bailet, G. et al. Qubesat for Aerothermodynamic Research and Measurement on AblatioN, 4th International ARA Days, Arcachon, France, 2013. [5] Prevereaud, Y. et al. Predicting the Atmospheric Re-entry of Space Debris through the QB50 EntrySat Mission, Proceedings of the 6th European Conference on Space Debris, 2013.

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EntrySat: A 3U cubesat to study the atmospheric reentry environment