MOVE-II (Munich Orbital Verification Experiment II) is a nanosatellite (CubeSat) under development at TU Munich (TUM) as a follow-on to the First-MOVE mission, with launch currently expected in late 2017/early 2018. About 60 Bachelor and Master students are currently working on the satellite mostly in their free time, with one PhD student as full-time project manager. MOVE-II is a cooperation between the Institute of Astronautics (Lehrstuhl für Raumfahrttechnik, LRT) and the student group WARR, which has more than 50 years of experience with spaceflight and rocketry projects.
Our mission is funded by the German Aerospace Center DLR (FKZ 50RM 1509) as an educational project, and we are developing a satellite capable of supporting a scientific payload with challenging requirements. The goal of our mission is to test and verify the so-called satellite bus, meaning all parts of the satellite required to run the payload. On MOVE-II, this includes communications, on-board data handling, the attitude control system, the power supply system, the structure and the thermal control system.
The structure of the satellite is probably the most obvious and shape giving part, holding all electrical boards, side panels, solar cells and other components. Its main function is to keep all parts in their defined position and enable a suitable operation condition.
Especially during launch of the satellite the structure is exposed to huge stress due to high acceleration, vibration and shocks. It must be designed to withstand these loads while having a low mass. Furthermore the design should enable easy and fast integration for optimized testing.
Since MOVE-II will have 4 deployable flap panels covered with solar cells a HDRM (hold down and release) mechanism has been developed. This mechanism uses shape memory alloy parts for activation and deployment. The mechanism holds the panels in its folded position and releases it after a certain time. A lot of tests will be done to ensure a high reliability since the deployment of the solar panels is very critical. No solar panels, no power, no mission success.
Electrical Power System (EPS)
The electrical power system has the responsibility to get power into the satellite, store it and distribute it to the subsystems while monitoring the current flow. The EPS team designs the power architecture as well as most of the concept of operations to satisfy our mission goals while considering the energetic restrictions (such as power shortage).
MOVE -II will have foldable solar panels that will be deployed once we are in orbit. From that we route the power through our custom array architecture to maximize the harnessed power into the EPS board, which we will most likely buy from a company called ClydeSpace, who manufacture dedicated CubeSat hardware. There will be a lithium-polymer battery attached to store the power until it is needed in one of the subsystems. Each subsystem is connected via a monitored switch that detects overcurrent or overvoltage and shuts down the system in case of a failure. These shorts sometimes occur on satellites due to radiation damage.
Command and Data Handling (CDH)
The command and data handling module is where all information from the various subsystems flows together. It is responsible for the safe storage of this data in space and works closely together with the communication system to execute commands coming from the ground station and to send back the collected data.
Opposed to other CubeSats being build, our satellite will not utilize multiple microcontrollers to achieve this task, but one custom designed processor implemented on an FPGA (a chip with programmable logic gates). This has the advantage that we only have to create and maintain software for one processor.
To ensure that this central part of our satellite will remain functional independent of radiation in space, precautions like radiation safe filesystems (FTRFS), error correction and debug facilities have to be integrated into this system. A lot of research is being done on these technologies within our CDH team.
The communications subsystems (COM) provide wireless data links to the satellite. MOVE-II has two transceivers that provide data links with low and high data rates.
The low data rate transceiver operates in the UHF and VHF amateur radio bands. Its purpose is to provide a basic low power link, which is used for telemetry, tracking and control (TT&C). These bands are used because of the comparatively low free-space losses, which allows us to use low-gain omnidirectional antennas. This way, the system can provide a very robust radio link regardless of the orientation of the satellite.
The high data rate transceiver operates in the lower S-Band, the same frequency range that was used in the Apollo missions. This transceiver can be used to up and download larger amounts of data. This can be used to transfer new software to the CDH subsystem or to a payload. The S-Band transceiver consumes significantly more power than the UHF/VHF transceiver, but the radio link is much more power efficient. This is because antennas with significantly higher gain can be used both on ground and on the satellite.
Both transceivers are custom products manufactured by the MultiMediaStudio Dipl. Ing. Rolf-Dieter Klein in cooperation with students from the COM team.
Furthermore, the COM subsystem also provides the ground section of the radio link. Members of the team build the rotatable antenna rack for the UHF/VHF antennas, the radio receiver software and the satellite control software.
Attitude Determination and Control (ADCS)
The main objective of the Attitude Determination and Control System (ADCS) is to stabilize and to align the satellite towards a reference orientation. The ADCS requires two main types of hardware components, sensors and actuators. Three orthogonally mounted magnetic coils, referred to as magentorquers, are used as actuators for active attitude control. The interaction of the magnetic moment of the coils with the Earth’s magnetic field produce a (very small, but sufficient) control torque. The main task of active attitude control of MOVE-II is twofold. At first the satellite has to dampen the very high angular velocity rates after ejecting from a CubeSat deployer at the beginning of the mission. Only after this so-called detumbling the next mission steps can be initialized. The second task is stabilizing and maintaining the desired attitude during normal operation against environmental perturbations in order to point the S-Band antenna towards the receiving antenna on Earth’s surface.
To enable control, the actual attitude of the CubeSat has to be determined and therefore sensor data has to be evaluated by algorithms to estimate attitude. Sun sensors and magnetometers are used as reference sensors, gyroscopes as inertial sensors. The ADCS concept has to be verified in simulations and all hardware components must be tested in a special test facility, which simulates space environment.
The thermal design of MOVE-II has to keep all sensitive components within acceptable temperature margins at all times. Apart from an electric battery heater, the design has to rely purely on passive methods.
Heat exchange in space works differently than on Earth, as air convection is entirely suppressed. This means that heat can only be transferred through thermal radiation and internal conduction. Solar radiation and earth albedo radiation will heat up our satellite, while it radiates heat towards the space background. Our main methods to control internal temperatures are an appropriate positioning of heat-dissipating internal components, and the use of multi-layer insulation (MLI).
During the satellite’s design process, numerical thermal simulations will be created. In order to test and refine these simulations, thermal tests of different subsystems will be carried out and correlated with our numerical models. The TUM chair of astronautics runs a thermal vacuum chamber which can be used for those tests. Apart from verifying thermal designs, thermal vacuum experiments can prove the general functionality of MOVE-II systems under vacuum conditions.
Assembly, Integration & Testing (AIT)
In order to make sure that MOVE-II will work properly in space, we’ll have to prove all its functionalities prior to the launch. We’ll also have to meet requirements provided by our stakeholders. And last but not least, we’ll have to ensure, that we’ll be able to integrate MOVE-II subsystems without damaging them. For that reason we already built a 3-D mockup model (see picture) of our satellite and are also currently working on a so-called integration procedure, in which all the necessary steps for a successful integration are documented.
Starting later this year, a major part of our work will be to prepare, to monitor and to document all testing activities. This includes functional testing as well as the classic environmental tests for space hardware (e.g. thermal vacuum tests, vibrational & acceleration tests). Thereby, we have to document every single test we execute. As we are not able to perform all tests by ourselves, we will do both in-house testing as well as testing in professional facilities around Munich.
Last but not least our team develops ground support equipment that helps us handling the satellite without damaging it and we also handle the flight hardware and the cleanroom at the Institute of Astronautics.
The operations subsystem is responsible for observing and controlling MOVE-II from our ground station at the LRT.
This starts in the Mission Control Center where all the operators will be located during the mission and includes for example the design and implementation of the user interface, the server backend and the definition of interfaces with all the different subsystems. Here data will be visualized to enable the operators to react on certain information. Furthermore every subsystem can change settings on the satellite from here, or analyze the downloaded data.
The operations subsystem will make use of all the functionality that has been implemented on the satellite itself and is the main interface between the ground station and the launched MOVE-II.