COMPASS 2

Objective

The objective of COMPASS-2 is to achieve the following goal:

Development and Construction of a universal experimental technology platform as a Triple-CubeSat to consolidate an international standard.

To accomplish this goal, the satellite BUS-system will be designed for two universal payloads. The design of the satellite subsystems includes the lessons-learned from the COMPASS-1 system with a higher reliability and efficiency. The idea is to build a universal BUS-system for further space applications and small experiments. Two cubes will carry the payload. The interfaces to the Experiments will be defined in a universal way so that the experiments, which are implemented in the payload cubes, get their own power and data interface. The target orbit is estimated to a low-earth orbit with a medium altitude to have a faster disposal after the mission time.

STR – Structure

The structure of COMPASS-2 is based on the requirements of the CubeSat standard. It is the mounting structure for all components of the satellite. The mounting system is the interface for the PCBs of each system, the battery and solar panels as well as for the payload. COMPASS-2 will become a nano satellite with a size of 100mm x 100mm x 340.5mm. The mass may not exceed 4 kg. The structure is made of lightweight aluminum.

The universal design of the COMPASS-2 system also allows to design a smaller version of it in a Double-Cube-Sat version. The dimensions of 100mm x 100mm x 227mm allow a maximum mass of 2.66 kg.

EPS – Electrical Power System

The functions of the power system are to generate and store electric power for usage by the other spacecraft subsystems. The subsystems can have various specific requirements for voltage, frequency, stability, noise limits or others. These characteristics have to be considered on supplying the different subsystems. It is also a part of the power system to set the operating mode to prevent an early end of mission caused by lack of energy. To maintain the long-term reliability of the system, the power system provides protection to other subsystems against malfunction.

Power Source / Solar Cells

As power source, COMPASS-2 has about 30 solar cells on the surface (depending on the final configuration). Three areas will contain eight solar cells and the fourth area will contain six solar cells. The solar cells have an efficiency of 30.5%. Equipped with these solar cells, the satellite will generate an average power output of 5.1 Watt (flying vertically - Nadir) and 3.5 Watt (flying astronomical horizontally) in sun synchronous orbit.

Power Storage / Batteries

For COMPASS-2 the newest battery technology will be used. Two Nanophosphate High Power Lithium Ion Cells will be providing the satellite with power in night or high power demand phases. The satellite will have an alternating charging system to reduce the charging cycles for each battery. Due to the fact that the batteries are not space-proved, one tasks of the power system team is to test the batteries under space conditions.

Power Distribution & Control

One of the tasks for EPS is to control and distribute the available power. The system will set the satellite in three different modes:

Also it is necessary that the ground station can control the different modes and settings.

COM - Communication

The communications system will provide reliable data connection between ground station and the satellite. The main objective is downloading data and uploading commands and firmware. In comparison to COMPASS-1, this mission will have a focus on channel coding and adaptive modulation in order to improve the amount of data transferred during each flyover. This also includes smart software at the ground station for signal processing. On the classic VHF-UHF-Bands we implemented new modulation methods to provide a higher data rate while staying compatible to default ham radio equipment. Besides there will be a S-Band down-link giving a down-link speed of about 500 kbit/s.

Modulation Methods

Due to the system design, the modulation signal can be created by the COM micro controller. This gives the chance to do the entire modulation signal processing inside the software. Just one transmitter is needed for all transmission types. The software defines which one is used at which moment. The base band waveforms are calculated inside the software and fed into a digital-analog converter. Here the digital waveforms are converted into voltages. A well-dimensioned anti-aliasing filter is used to create a high-quality modulation signal. Depending on the actual demands and link quality, modulation is adaptively and automatically changed by handshaking with the operating ground station. As an experimental addition, hierarchic modulation is used to allow two or more different data rates at same time.

As the COMPASS-2 group is expecting support and help from amateur radio enthusiasts all over the world, the transmission modulation must be compatible to standard amateur radio operators receiving equipment. This means non-coherent receiving with channel width of maximum 6 kHz. Thus, all phase concerning modulations are a no-go. The ground station software will include the possibility for channel equalization, paying attention to the non-linear frequency response of amateur radio receivers.

As an experiment hierarchic modulation using FSK will be used to allow different data speeds at the same time using complex channel coding. The higher data rate needs high link quantity to be decoded properly. The lower one is receivable with high noise impact.

As is it planned to use a PC system with a sound-card connected to the radios, all signal decoding and processing can be done inside of the PC software. This makes the system flexible and low cost comparing to hardware decoders like Terminal Node Controllers. The CPU power of a standard home PC is enough to do all the encoding calculations.

Frequencies and Antenna Configuration

Referring to the design of COMPASS-1, the design of COMPASS-2 will be very close to the old one, but with improvements and extensions. We will use just one transmit frequency. Due to the arbitrary modulation system, it's just the onboard software that defines the service. The uplink will be in the 2m amateur radio band (145 MHz - referring to the 2m band plan) receiving FM modulated data packets and DTMF commands as a backup. The down-link will be on the 70 cm amateur radio band (437 MHz - referring to the 70 cm band plan). On the down-link data frequency we will be able to send packet data, SSTV still picture images, Morse code and maybe some short voice messages. The Morse code transmission is elementary necessary for receiving the status directly after the deployment out of the launch pod, recovering the satellite and long time health measurement of the satellite. Nearly every radio amateur is familiar to Morse code, so it is possible to get telemetry from every part of the world.

Ground Station

The satellite will be operated by the ground station at the FH Aachen University of Applied Sciences. The radio station contains an ICOM IC-910H, an IC-821H and two PCs. The IC-910H is used for sending on the 2m band with 100 Watts and receiving packet data in the 70 cm band. The bandwidth is 6 kHz. The antenna set consists of four cross-Yagi antennas with a +20dB preamplifier for UHF (70 cm receiving) and two cross-Yagi antennas for VHF (2m sending), both right turning polarized, controlled via an Egis Rotor with full horizontal coverage. The azimuth is able to drive 450° and the elevator can rotate up to 90°. For tracking the satellite and for the antenna rotor control the station uses SATPC32, which also adjusts the radios frequency to compensate the Doppler shift. For a correct tracking and Doppler shift correction it is necessary to update the Kepler elements weekly from Celestrak, a free of charge service for tracking satellites in space. The structure of the software will be modular giving the chance to work and improve on the components separately. Key features are:

In addition to the ground station in Aachen, many private amateur radio stations joined us as backup stations during the mission operation of COMPASS-1. The COMPASS-2 Team also will encourage the stations to participate loading data and decoding Morse code from COMPASS-2.

Main Tasks

The CDHS is the main board of the COMPASS-2 Satellite. It is responsible for managing all general and system data. It is also responsible for forwarding commands from the ground station to the subsystems and when requested sending internally generated data back to earth.

Redundancy

Firmware update: To allow further improvements after the launch of the satellite, it is wise to allow reprogramming the main controllers of the subsystems. Multiple firmware storage modules are implemented in the design. For more security in worst-case scenario, the default firmware, which is stored previously on the satellite, will be recalled and the controllers of the satellite will be programmed as prior to launch.

Data recovery

In case of a total loss of the command an data handling system, routines implemented on the COM system will allow a direct access to the stored data. This will be used to recover the generated data until subsystem loss, allowing access to HK data for failure analysis or even download science data.

Storage Media

Out of many storage medium technologies the CDHS picks out the most suitable technology for the application it is intended to be used for:

System Architecture

Controller Area Network - CAN: The satellite provides a CAN bus system for designated data routes. The following are the groups of data an SPI bus is designated to. Serial Bus

The communication of commands between the COM and the CDHS system will occur through a serial interface. A second bus will be used for data transfer, allowing the access of both, COM and ADCS, subsystems to the storage media.

Main Tasks

The major objective of this subsystem is to obtain an attitude control, which fulfills the requirements of pointing, stability and agility of as many payloads as possible. Due to the size, the weightbudget, the powerbudget and the desire to use of the shelf products the main requirements can just be obtained to a certain level. Parts of the subsystem are and will be manufactured by the team. Due to the fact, that not all of the parts have been finished it is not possible to estimate and simulate the overall performance of this subsystem yet.

Environmental Effects on Attitude Control

To develop an attitude control system it is necessary to be aware of the environmental effects acting on the satellite in a certain orbit. The following environmental effects have to be considered:

These effects interacting with the satellite generate torques. These torques can either be used for passive or active attitude control or be regarded as disturbance torques. The estimation of the maximum acting torques is possible in a conservative analytical way. For more precise estimations and for average values a simulation of the acting torques is necessary. The more precise these values are the better an optimisation of the attitude control system is possible.

Actuators

To realize the generic concept of COMPASS-2 the ADCS team has to develop all actuators, because of the low volume budgets given by the CubeSat standards the actuators have to be miniaturized. The following actuators are developed to get implemented on the COMPASS-2 Triple-CubeSat:

Reaction Wheels

Well designed Reaction wheels can provide a high actuating accuracy which do not depend on environmental effects. There is a development of reaction wheels by the COMPASS-2 team in progress.

Magnetorquers

COMPASS-2 will use air core coils like the previous COMPASS project as magnetorquers for the active attitude control. The reason for choosing magnetorquers as part of the attitude control system is the reliability and the simple design. Air core coils were chosen, because we are able to optimize and manufacture them ourselves. Ferrite core coils are more efficient regarding space and power consumption, but the optimization, calculation and production is much more complicated. So we decided to keep it as simple and cost efficient as possible and use air core coil magnetorquers as our prime active attitude control actuators. There will be three actuators oriented perpendicular to each other. Through this orientation attitude control is possible regardless how the satellite is oriented to the magnetic field lines.

Gravity Gradient Boom (GGB)

A gravity gradient boom is a passive attitude control actuator. Therefore no power or control loop is needed during operation. Using a gravity gradient boom just two pointings of the satellite with regard to the earth center is possible. Either pointing to the earth center or pointing away from the earth center. So it is a severe limitation of attitude control and just usable in certain missions. There is a development of a gravity gradient boom, but as an add on system only if reasonable for a certain mission.

Aerodynamic drag stabilization

A further passive actuating torque will be created by the aerodynamic drag in the low earth orbit and may lead to a helpful stabilization of the satellite. Aerodynamic drag and the resulting torque cannot be avoided and the team will try to get partial advantage if possible.

Digital multi-image sensors (DMIS)

Digital multi-image sensors (DMIS) are digital camera–based instruments for detecting the center of the sun and calculating the angle between the sun vector and the camera's view vector on the x and y axis. Additionally, the earth's horizon and the rotation rate of the satellite can be determined. Therefore an attitude measurement using the earth's horizon is possible, too. Furthermore the sensors are able to store colored images for later transmission to earth.

3-Axis Magnetometer

The magnetometer is absolutely necessary for running the magnetorquers. Further they are needed as a reference for calculating the attitude matrix with the help of the earth’s magnetic field.

Spin Rate Sensor (Gyro)

The gyros are only for redundancy. These sensors deliver a reference to the calculated tumbling frequency calculated by the DSS and the magnetometer. These sensors determine compared to the DSS and the magnetometer the attitude inertial and not by using a reference. The gyros make use of a micro-electro-mechanical system which determines the actual attitude with the help of the coriolis force produced by a rotating body.

TCS – Thermal Control System

Due to the small amount of dissipating heat and the wide operating temperature range of the (LiFePo4) accumulator (-30 °C till +60 °C), the TCS will be a passive regulated system with the advantage of energy saving while keeping the satellite and its subsystems within the allowable temperature limits. The limits are normally -40 °C till +125 °C for commercial off-the-shelf products, but some parts of the COM-subsystem are more temperature sensitive and should not exceed +85 °C. The problem is here the high energy consumption and the involved high local (on an area of only 9mm²) dissipation.

Resource management

The temperature will not be monitored all the time but at a given period to realize low power consumption. The temperature-information search will be as intelligent as possible. This means, that if the temperature at one point of the CubeSat exceed a certain threshold a few times, the program will automatically collect more information in a shorter period to locate the reason and to ensure better research for future missions.

Main tasks

The energy from the incoming electromagnetic radiation is basically converted into heat. Thus it is the most important base for the thermal budget. Relevant for the TCS is the direct solar radiation, the reflected solar radiation from the earth/moon and the infrared radiation of the earth/moon. Therefore it is very important to know the chronological sequence and the orbital parameters because these parameters will all be considered and processed in our thermal analysis. All influences have to be combined with the temperature range of the electrical parts in the spacecraft. Precisely because we are using commercial off-the-shelf-products, it is indispensable to hold the satellite in an "earth like" environment. Therefore we are using some several insulation techniques like foils, coatings and so on to get the satellite in this marginal temperature range as well as to guarantee a good environment for micro controllers, sensors and the payload.

Methods and techniques

We combine some ways of setting up the temperature:

Payload

In the following, the requirements for one experiment box are listed. Given these “hard facts”, any payload provider will be able to build its payload without any much communication with the COMPASS-2 Team to adapt the payload to the BUS cube:

the overall energy needed for each payload can be scheduled between the customers via FH Aachen.

data transfer protocol:

CAN

I2C

max data rate : amount of data rate can be scheduled between the customers via the FH Aachen

Testing

Testing must be performed to achieve all launch provider requirements as well as any additional testing requirements deemed necessary to ensure the safety of the CubeSats and the P-POD. All flight hardware will undergo qualification and acceptance testing. The P-PODs will be tested in a similar manner to ensure the safety and feasibility before the CubeSats will be integrated. All CubeSats have to pass the following tests:

Vibration Test

Vibration tests serve the simulation of dynamic mechanical loads. This is tested with oscillations in the frequency response range of 1 to 2000 Hz.

The goals of vibration tests are:

For the vibration test, the satellite is housed in a test POD which is mounted to a shaker table. The vibration loads are measured immediately by acceleration sensors, which are mounted on several points on the test POD. To improve the measured values, several sensors are monitoring the vibration behavior. All these values are computed to an average value for further analyses. The vibration load tests are performed through the three main axes.

Solar-Simulation Test

The solar intensity in low-earth orbit is about 1368 W/m². With the sun simulator, the effects from the high solar radiation flux onto the spacecraft are tested. The sun-simulator is using a xenon lamp to simulate the solar radiation. The spectrum of this lamp is - except of a spectral peak, caused by the properties of xenon - very similar to the natural sunlight.

Thermal-Vacuum Test

During this test, the satellite is put into a high-vacuum chamber for several temperature cycles and the thermal behavior of the satellite is tested.

All tests will be executed at the FH Aachen.

Outlook and Project Status

COMPASS-2 will be a good platform for companies and research institutes to bring small experiments and new technologies rapidly into space at low cost.