|BEESAT-2 - Berlin Experimental and Educational Satellite
|Dipl.-Ing. Sebastian Trowitzsch
|Federal Ministry of Economics and Technology
|50 RM 1006
BEESAT-2 is a pico satellite mission hosted by the Chair of Space Technology of Professor Dr.-Ing. Klaus Brieß. The satellite was built according to the CubeSat design specification by staff members of the Chair of Space Technology with active participation of students. Major components of the satellite bus had already been designed during the precursor mission BEESAT-1. These components form the space proven technological basis of the satellite BEESAT-2.
The primary scientific objective of the mission is the technical verification of an innovative attitude control system for picosatellites under space conditions. Miniaturized reaction wheels are used as actuators. Those reaction wheels were already verified successfully on orbit during the research project "Microwheels II".
The primary mission objective is the technical verification of miniaturized reaction wheels for attitude stabilization of a picosatellite. A small camera for Earth observation is used as an exemplary payload. The secondary mission objective is the successful integration of mission operations into lecture courses of the Chair of Space Technology.
|Passing the acceptance test campaign
|Three months of mission operations
|Proven functionality of the attitude control loop, verified by Sensors
|Successful transmission of one picture
|Satellite pointing verified by Camera
The picosatellite mission BEESAT-2 is part of the ongoing research project 'Microwheels III' of the Chair of Space Technology. The project 'Microwheels III' is funded by the German Aerospace Center (DLR) with funds from Federal Ministry of Economics and Technology (BMWi) on the basis of a decision by the German Bundestag (Grant No.: 50RM1006).
|Dipl.-Ing. Sebastian Trowitzsch
|Dipl.-Ing. Frank Baumann (ehem.)
|Michael Jetzschmann (ehem.)
|Johannes Lieb (ehem.)
|Pascal Thabaut (ehem.)
The major design driver for the satellite architecture is the requirement for an implementation of a failure tolerant design. Most of the used components are commercially available, mainly automotive grade. Failure tolerance of the system was achieved by the use of redundant subsystems that either run in parallel, referred to as 'hot-redundant', or lie dormant, referred to as 'cold-redundant'. Radiation induced effects on components are counteracted by prepending a securing circuitry that completely disconnect the components from power supply in case of a malfunction. Possible permanent damage due to excessive heat during latch ups can thus be prevented. The block diagram shows how the individual subsystems are interconnected by a redundant communication bus. The controller area network bus (CAN 2.0B) is used for subsystem communication. The CAN-bus works cold-redundant. Redundancy is supervised by the power control unit (PCU).
The cubical satellite is equipped with triple-junction gallium-arsenide (GaAs) solar cells on all sides. These solar cells provide an efficiency of 26.8%. The solar generator consists of parallel strings of two cells in series. Each string is protected by a Schottky bypass-diode. The solar generator powers the redundant battery charge regulators. The satellite is equipped with two physically separated lithium-polymer batteries that are sealed with epoxy resin and stowed in an aluminum compartment to protect them from the vacuum of space. Each battery is designed for a nominal lifetime of one year, providing a nominal voltage of 7.4V and a capacity of 1250mAh.
The chosen design of two separated, hot redundant power supply chains is able to tolerate outages of one system without putting satellite operations at risk.
A power control and distribution unit (PCU) supervises battery charging and power consumption of 20 individually switchable components on board the satellite. Those components are powered by a regulated power bus of either 5V or 3.3V.
The PCU is protected by security measures embedded into the operating software and by external securing circuitry. Radiation induced effects can thus be handled effectively.
The on-board computer of BEESAT-2 is identical to the flight hardware of BEESAT-1 that operated successfully for more than three years in a sun-synchronous orbit at an altitude of 720km since September 2009.
The on-board computer has been implemented cold-redundant and consists of a 32 bit ARM7 micro controller running at 60 MHz, 2MB SRAM data memory and 16MB of Flash-ROM program memory. Additionally, 4MB of Flash-ROM are used for storing telemetry collected while orbiting Earth that is transmitted during ground station contact.
The on-board computer supports the acquisition of 52 separate analogue channels.
The on-board computer assembly is additionally equipped with components of the communication subsystem and sensors for attitude determination, namely one hot-redundant three-axis magnetometer and three single-axis MEMS gyroscopes.
The attitude control system of BEESAT-2 comprises of all the necessary components for complete attitude stabilization and control in three axes. The control loop is cycling at 2 Hz.
The sensor suite contains Sun sensors developed in-house that are mounted onto each of the six surface panels. Each Sun sensor is able to precisely determine the angle of incident sunlight in two axes and delivers a unit vector to the Sun in the body fixed reference frame.
Additionally, two three-axis magnetometers allow measuring the Earth's magnetic field based on the anisotropic effect in magneto-resistive materials. Three single-axis analog MEMS gyroscopes, complemented by one next generation, digital three-axis MEMS gyroscope, measure the current rotation rate of the satellite around its body axes.
In order to obtain an attitude information from these sensors, the on-board computer needs reference vectors. From known orbital parameters and UTC time, the current position of the satellite in orbit can be calculated using the SGP4 algorithm. Besides the position vector from the Earth's center to the satellite, an inertial vector pointing from the satellite towards the Sun is also calculated. To achieve higher accuracy of the Sun vector, even the movement of the Sun around the barycenter of the solar system is taken into account. Knowing the orbit position of the satellite, another reference vector can be calculated by using the current reference model of the Earth's magnetic field (IGRF11).
Both reference vectors, the Sun vector and the magnetic field vector, are combined with measurement values from up to five sensors by a statistical estimation algorithm (QUEST), yielding attitude information of the satellite with respect to the inertial coordinate system. Using QUEST, the rotation matrix obtained shows the lowest error between the calculated and measured vectors.
Regarding the actuators for attitude control, BEESAT-2 is equipped with six magnetic coils implemented into the surface panels as well as three micro reaction wheels mounted along the body axes of the satellite.
The magnetic coils are designed for damping the satellite's rotation by interacting with the Earth's magnetic field. For this purpose, knowledge of the satellite's attitude is not mandatory.
The reaction wheels are used to point the satellite towards a certain target. For this purpose, several closed-control loops are used: a damping mode, to reduce the satellite's rotation to zero, the slew-mode, to perform large angle maneuvers along the shortest available track, and finally, the fine-pointing mode that maintains the orientation of the satellite towards a specific target with high accuracy.
It is possible to point the satellite to either a predefined inertial target (inertial-pointing) or downwards to the center of the Earth (nadir-pointing).
The reaction wheel system has its own electronics for driving the brushless motors. With a 10Hz control loop cycle frequency, either angular momentum (via rotation) of the wheels or resulting torque (via acceleration) can finely be controlled. Both the magnetic coils system as well as the micro wheels system can be operated manually by the ground personnel for testing.
BEESAT-2 communicates with the ground station using the ultra high frequency (UHF) band at 435.9500MHz, which is dedicated to amateur radio satellites. The modulation used is Gaussian minimum shift keying (GMSK) with a modulation index of 0.3. The nominal data rate is 4800 bits per second. Half-duplex communication is used. On demand, the data rate can be increased up to 9600 bits per second. The communication link is secured by forward-error-correction (FEC), checksums (CRC), interleaving and scrambling. Transmitted data is unencrypted.
The communication system is designed fully redundant as well. Each unit consits of an 8 bit micro controller, a modem, a transceiver, and an antenna made of a trimmed spring-steel strip. The antenna is safely wound around the satellite in launch configuration and is released after a programmable time delay once the satellite is deployed from the launch container.
The two monopole antennas are arranged in such a way that in the case of unfavorable orientation of one antenna with respect to the ground station, the adjacent second antenna is just perfectly aligned. Both transceivers are in receive mode at the same time. However, signal transmission is only performed by the transceiver that yields the better communication link, i. e. by the one that is oriented best towards the ground station. Nevertheless, each communication link can be used selectively by the operator.
Thermal design of the satellite is implemented completely passively by carefully choosing optical surface properties of the satellite to fine-tune the radiation balance with the space environment. Thus, an acceptable average temperature as well as a minimal temperature fluctuation of the components between sun and shadow phase during one orbit can be achieved. Together, all subsystems provide 24 temperature measurements that are transmitted to the ground station as part of the telemetry.
The satellite structure is made of aluminum and makes up 25% of the total satellite mass, including all joints, screws and bolts. According to the CubeSat specification, BEESAT-2 incorporates redundant deployment switches that disconnect the electronics from the power supply during launch and ascent. The satellite is activated once it is deployed from the launch container and either one of the two switches is closed.
In launch configuration, two heat-wire mechanisms keep the antennas in place at high tensile force. In order to release each antenna, the heat-wire mechanisms can independently be activated.