Sub-systems

Detector:

The purpose of the detector sub-team is, unsurprisingly, to ensure that the X-ray detector carried on EXACT functions properly. This component is an essential part of EXACT because it enables the CubeSat to detect the X-ray spectra that will be analyzed for the two missions. Our work as a part of this sub-team is to calibrate the detector and work with the other sub-teams to ensure that the structure, power, and data handling systems account for the detector properly. 

To be a bit more specific, there are a few hardware components that make up the detector itself. The detector that will be on EXACT is actually a combination of two different types of detectors. One type, called a solid state detector, contains a semiconductive material that emits an electrical signal when an X-ray hits it. This detector is used as a supplementary detector for lower energy ranges. The other type is a scintillator detector that contains scintillation crystals that convert incident X-rays into flashes of light, which are then picked up by a component called a SiPM (Silicon Photomultiplier) that converts that flash of light into an electrical signal. This is the primary type of detector on EXACT because scintillation at this energy range requires little power and provides precise results. These detectors in tandem make up the detector on EXACT that will convert X-ray photons into electrical signals sent as time-stamped data packets to the flight computer.

X-123 Detector

EPS:

The Electrical Power Systems(EPS) team is responsible for designing the hardware that goes into the satellite as well as monitoring the power budget for the system. We work with the other sub-teams to design a system that best fits their needs in addition to making sure that the satellite remains balanced in terms of power during the mission.Tasks that EPS performs are designing printed circuit boards(PCBs), simulating circuits to evaluate their power consumption, soldering PCBs, and testing the boards by checking voltages and functionality. Additionally, EPS writes code for microcontrollers that are on the designed PCBs and ADCS  and write scripts in MATLAB to estimate power requirements during different stages of flight.

The hardware that’s involved in our subsystem are the PCBs that we design. These boards all connect vertically in the satellite to form the stack. The boards within the stack are the: Solar Array PCB, CDH PCB, Avionics PCB, Control PCB, and the Deployment PCB. These boards all have different purposes but are equally important to the functionality of the satellite. Additionally, we use a maximum power point tracker that will deliver optimal operating voltage to the system, a power distribution unit that delivers different amounts of power to different components/ boards simultaneously, and a battery that will power the satellite.

EPS block diagram

ADCS:

The Attitude Determination and Control System (ADCS) is vital to the mission success of the satellite. In order to obtain usable data for the science mission objectives, the satellite’s position and orientation in space must be established. This is known as Attitude Determination, one of the primary duties of the ADCS. The other main role is the Control System, in which the satellite is pointed(controlled) towards a desired orientation. The orientation of the satellite is determined by taking measurements from multiple sensors. These include a GPS, magnetometer, and IMU. A GPS determines position of the satellite, while magnetometer measures the magnetic field and IMU measures the angular velocity of the satellite. This data is then passed through an Extended Kalman Filter and the attitude is calculated. The control system uses magnetorquers to turn the satellite into the position we want. Our team is currently using a MATLAB simulation environment to test and improve our system, and is developing plans to perform hardware testing in a laboratory setting.

 

ADCS Block Diagram

Structures:

Structures is responsible for designing and manufacturing the walls of the satellite and internal mechanical supports. Apart from holding the internal components like electrical boards and the detector together, having an external shell that protects the internal components and detector is crucial as well to prevent unwanted materials like screws from leaving or entering the satellite for the safety of spacecrafts and its crews. Structures mostly works with metals and sometimes polymers to form the satellite. Our work is crucial to ensure all components fit together within the satellite and assemble them in the right steps for maximum space efficiency and structural integrity. We perform several tests such as vibration to ensure the satellite does not fall apart during launch, and work closely with EPS and Thermal to assemble and run other tests for a fully functional satellite.

CAD model of EXACT outer-housing

Thermal:

The Thermal Subteam runs thermal and structural simulations to model the conditions that the satellite will encounter during various phases of the mission. ANSYS, a finite element analysis software, is used to simulate the launch and orbit of the satellite. By modifying various parameters, thermal and structural performance during the entire mission. These simulations highlight specific areas of concern and from there, methods of thermal management will be investigated. In addition to ANSYS, Matlab is also used to run 1+ node simulations. 

The Thermal Subteam aims to support all other subteams by providing thermal and structural analysis for various mission and design parameters. We also provide validation and verification for finalized designs.

Thermal ANSYS Simulation

Comms:

The Communications Subteam, or COMMS, is solely responsible for communication between the satellite and the ground. Our team is crucial in receiving information on how the satellite is doing while in orbit. We are responsible for developing the Link Budget, a document that accounts for the gains and losses of the exchanges of signals between the ground station and the satellite. This budget then helps us to verify that we will be able with the satellite.Communication happens over two radios, both communicating to separate computers in order to package, transmit, and receive the data. The communication protocols are developed in Python and C/C++. We have a partnership with the Aerospace Corporation to test their radio hardware. Our team performs tests to check the performance of the radios as well as in tandem with CDH to verify the code functionality.

ADV Radio and Patch Antenna

CDH:

The CDH (Command and Data Handling) team is responsible for designing and implementing a robust and redundant flight computer that accomplishes the satellite mission and is resilient to hardware and software errors happening onboard. The flight software is installed on two microprocessors, which constitute our flight computer board. During run-time, the software runs on one microprocessor, while having the other shut down. Such implementation of cold redundancy allows the satellite to recover from extreme situations, such as a single-bit failure on the running microprocessor. Additionally, the flight computer board is directly connected to hardware installed on the satellite, such as science instruments, a radio module and various sensors and actuators. This design allows the flight software to interface directly with hardware through serial protocols, like SPI and UART. 

Under default procedure, the flight computer collects data from science instruments, while it maintains its orientation and communication with ground stations through ADCS (Attitude Determination & Control System) and COMMS (Communication System). In special circumstances, such as power shortage, the flight computer is designed to react accordingly, for instance, turning off noncritical hardware and charging the battery. 

The quality and performance of a flight computer is critical to the success of a satellite mission. For this reason, we perform rigorous testing, like documented unit and integration testing on written software modules, to ensure that its behaviors meet the mission’s expectations.

CDH Default Mode Example