COMPASS-2 will become the second student satellite from the FH Aachen, Germany. Like the first satellite project, COMPASS-2 will be developed based on the CubeSat standard. It will be developed as a Triple-CubeSat, which means a total weight of 4 kg and dimensions of 100x100x340.5mm.
The students of the FH-Aachen will develop the BUS system with the necessary subsystems for power generation, power distribution, data handling, communication and data / power interface for the payload. There will be two experimental boxes placed around the BUS-Cube that can easily be replaced using a standardized connection system that will be developed by the students of the FH-Aachen.
The mission 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. All subsystems are designed based on COMPASS-1 including the lessons learned knowledge to develop an improved satellite system with a higher reliability and efficiency. The boxes in the outer cubes will be defined on a universal way so that every experiment, which is implemented in the box with special requirements, gets its own power and data interface. The idea is an universal BUS-system for further space applications for small experiments. The outer cubes will carry the payload and can be connected to each other for a combined experiment. The target orbit is estimated to a low-earth sun-synchronous orbit with a medium altitude to have a faster disposal after the 6 month mission time.

The Structure builds the base frame of the satellite and has to absorb all forces acting on it.

Main Tasks:
The structure of COMPASS-2 is the mounting system for all components of the system like PCBs, the batteries, solar panels and the payload. The structure further is subjected by the requirements for CubeSat standards and has to be strong enough to sustain maximum loading during the launch.


Figure 1:
Main Structure of COMPASS-2Dimension and Mass Requirements:
COMPASS-2 is a nano-satellite with the dimensions of 100 x 100 x 340mm. The mass may not exceed 4 kg and the center of mass have to be within 2 cm of its geometric center.

Structural Requirements:
The structure of the CubeSat must be strong enough to survive maximum loading defined in the testing requirements and cumulative loading of all required tests and launch. The CubeSat structure must be compatible with the P-POD. The use of Aluminium 7075 or 6061is intended for the main structure. Further COMPASS-2 shall meet all other requirements noted in the CubeSat standards.

Picosatellite Deployer
A unique feature of the CubeSat program is the use of a standard deployment system. Through the Poly Picosatellite Orbital Deployer (P-POD) standardization the mission costs and development time are reduced. CubeSats remain stacked inside the P-POD and constrained by a set of hard anodized teflon-impregnated rails until the scheduled orbit is reached.

The Electrical Power System has to generate and store electric power for usage by the other spacecraft subsystems.

Main Tasks:
Next to the power generation, storage and distribution a main task is to guarantee the long-term reliability of the satellite, the power system must provide protection against malfunctions.


Figure 1: Solar CellPower Source (Solar Cells):
For the power source, COMPASS-2 has 30 solar cells on the surface. Three of the long sides will contain eight solar cells and the fourth long side will contain six solar cells. The solar cells will have a 30.5% efficiency. With this solar cells, the satellite will give an average power output of 4.8 Watt in vertical flying and 3.9 Watt in horizontal flying.


Figure 2:
LiFePo BatteryPower Storage (Batteries):
For the COMPASS-2 satellite the newest battery technology will be used, Lithium Ferrum Polymer (LiFePo). The satellite will have a redundant charging system with two batteries to reduce the charging cycles for each battery. Due to the fact, that the batteries aren´t space proofed one of the workspace of the EPS system is to testing the batteries for their work in space. The STR will develop a batterie case, if necessary.

Power Distribution & Control:
One of the biggest workspace for the EPS system is to control and distribute the available power. For this work field good programming skills are needed. The system will set the satellite in three different modes.

• Nominal Mode
• Power Save Mode
• Emergency Mode

Also it is necessary that the groundstation can control the different modes and the settings. The three modes need to be tested to.

The Communication will provide a reliable connection between ground station and the satellite for downloading data and uploading commands and firmware.

Main Tasks:
The amount of data in mission operation and the access time, the time of a flyover, have to be analyzed. Therefore it is necessary to decide, if there will be data processing to compress the data aboard the satellite or not. In comparison to COMPASS-1, this mission will have a focus on channel coding and adaptive modulation in order improve the amount of data transferred during each flyover. Signal creation by the COM microcontroller gives the chance to do all the 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 (SDR - Software Defined Radio). 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.

HAM Networking:
As the COMPASS-2 group is expecting support and help from ham radio enthusiasts all over the world, the transmission modulation must be compatible to standard ham 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.

Antenna Configuration:

Figure 1: Ejected C-2
Antenna ConfigurationThe 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 downlink will be on the 70cm amateur radio band (437 MHz - referring to the 70cm band plan). On the downlink 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.

The Command and Data Handling System is the data- and telecommand administration system.

Main Tasks:
The CDHS data storage service stores the internally generated data of the satellite, which includes the housekeeping data, payload data, image data, firmware data and the transmitted telecommands from the ground station. In a regular time interval the CDHS requests data from each subsystem and stores this data in the designated data storage devices. The internal generated data (housekeeping data and mission data) have to be transmittable during one pass of the satellite. The incoming telecommands are received by the COM and then forwarded to the CDHS, which then decodes and sends it to the targeted subsystem.

Bus Systems:
For the command system communication the I2C-Bus will be used to serve that purpose. This bus system allows multi-master transmission, which makes the design flexible and easy to integrate. Subsystems of the COMPASS-2 are also interfacing their housekeeping data via multiple SPI buses. The CDHS requests the housekeeping data and mission data via this I2C command bus line. In a time overflow of a request to send data to the CDHS, the subsystems send their data to an I2C interfaced memory module.

Memory Storage:
The memory of the satellite has to be in one way or another classified in priorities, although classifying the data may be done by hardware and/or software. A mixture of both guaranties an optimal operation. Understanding the data leads us to the right design decisions and successful module selection. Three relevant choices have been made:

• EEPROM
• FRAM
• Flash

They are advantageous in different ways. Therefore a mixture of them works ideally.

The Attitude Determination and Control System is responsible for a accurate attitude positioning necessary for the experiments and a good communication.
Further ADCS is responsible for defining all coordinate systems required for each phase of C-2.

Attitude Determination:

Figure 1:
Body FrameThe attitude determination will be realized using Digital Sun Sensors (DSS) combined with a magnetometer and a spin rate sensor. The requirements to the sensor technology are very high, because the active attiude control is just as good as the measurement accuracy.

Attitude Control:
In order to build an attitude control system for an universal CubeSat-BUS it would be desirable to have different options to adjust the system for a certain mission. A general requirement is that an active attitude control should be feasible. An attitude control system has to compensate all disturbance torques. Calculations with Magnet Torquers and Reaction Wheels showed, that an attitude control system might only be functional up from 400km height.

Requirements:
All attitude control maneuvers are performed only in the sun period of the orbit. This is a safety precaution, to minimize the risk of discharging the battery to a critical level. Geometrical dimensions of the coils, mass and power budgets as well as the occurring temperatures are given by the responsible subsystems. If it is necessary the supplied voltage of EPS can be transformed with degradation. CDHS delivers the attitude control unit with clocking frequency and all other in flight information, like the Kepler Elements.

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.


Figure 1:
Thermal analyse of C-1It has been pointed out, that the EPS chooses an accumulator-type which has a wide operating temperature range. At this point the conclusion is a non regulating and only analysing Thermal Control SystemSensors will just measure and validate the previous analysis values and the test chamber values. In face of the low weight of the heatfoils the TCS disclaims on the heatfoils unless the thermal testing will show us difficulties in insulation or not reproducible results due to imprecise forecasts. The use of PCM (Phase Changing Material) does not enter the equation due to the wide range of temperature of the accumulator; nevertheless we will keep it in the back of our minds.

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.