#makewithmaxim CUBESAT Engineering Model

Introduction

Sending a satellite into space is pretty difficult, not to mention expensive, so instead of sending a bunch of satellites into space to tinker with, why not design a single, highly versatile satellite.One rationale for miniaturizing satellites is to reduce the cost: heavier satellites require larger rockets with greater thrust that also has greater cost to finance. In contrast, smaller and lighter satellites require smaller and cheaper launch vehicles and can sometimes be launched in multiples. They can also be launched 'piggyback', using excess capacity on larger launch vehicles. Miniaturized satellites allow for cheaper designs as well as ease of mass production, although few satellites of any size other than 'communications constellations', where dozens of satellites are used to cover the globe, have been mass-produced in practice.

Another major reason for developing small satellites is the opportunity to enable missions that a larger satellite could not accomplish, such as:
● Constellations for low data rate communications
● Using formations to gather data from multiple points
● In-orbit inspection of larger satellites
● University-related research

Space research is important because it challenges us to solve problems and find solutions which can translate to everyday life here on Earth. The products of space research and space technology are all around us today. From the ballpoint pen, all the way to GPS, special composite materials, special surgical equipment and satellite communication.For a while, only government and military had access to space. However, over the past decade there has been a rapid increase in commercial and public access to space. Private companies can take risks that the government and military cannot, which leads to even bolder and newer technologies being developed. So we are presenting a Nano satellite designed using MAX32630FTHR board.

Nano satellite is designed in 1U CubeSat form-factor so that existing deployment mechanisms can be used to inject satellite to space. Satellite systems essentially consists of space segment and ground segment. Space segment contains satellite systems along with associated payloads. Ground segment consists of RF links and command & Telemetry monitoring section for satellite health monitoring and Data collection from payload onboard satellites. Given below is the detailed block level description each sub systems of Nano satellite project.

1.Nano satellite Subsystems

 1.1 Power Subsystem
 1.2 Guidance & Attitude control Subsystem
 1.3 Onboard Computer Subsystem
 1.4 Telemetry & Tele command Subsystem
 1.5 Sensors, Payload & RF Beacon Subsystem

1.1 POWER SUBSYSTEM

Power Subsystem handles all power requirements of the Nano satellite. The main source of power is the solar panels attached to the satellite. Solar photovoltaic cells convert solar energy into electrical energy. The output of single cell is 2V, 100mA. The cells are connected in series parallel arrangement to obtain raw bus voltage of about 6V, 500mA. The PV array output is fed to a DC-DC Converter for obtaining stabilized DC output. The stabilized DC output is used for charging Li-ion based Battery pack & charging is supervised by a circuit based on CC/CV linear charger TP4056. The output from battery is given to a DC-DC boost Converter to obtain a raw bus voltage of 12V.A dc-dc buck converter is used for obtaining 7.5V for Vin input of MAXFTHR board. An ACS712 based sensor is used for measuring main current. Four 12V SPDT (Single Pole Double Throw) relays along with drivers are used for controlling power to the three reaction wheels and antenna deployment heater mechanism.

1.2 GUIDANCE & ATTITUDE CONTROL SUBSYSTEM

Guidance & Attitude control Subsystem maintains the orientation of Nano satellite stable with respect to earth using an array of sensors and actuators in closed loop fashion. The sensor used in IMU (Inertial Measurement Unit) is MPU9150. The IMU is a 9-DOF (Degree of Freedom) sensor with tri-axial accelerometer, gyroscope and magnetometer. The outputs are angular acceleration, linear acceleration and magnetic field intensity along three axes Yaw, Pitch & Roll. The IMU Output in I2C protocol is interfaced to I2C bus of MAXFTHR board using SCL & SDA lines. The IMU requires 3.3V and is attached rigidly to the body of satellite using vibration damping material so that error and offset due to ambient noises is minimal.

The data after processing by guidance & control algorithm is used to generate the required control signals for the actuators. The actuators used are three reaction wheel along three axes Yaw, Pitch & Roll. The reaction wheels work under principle of conservation of angular momentum. The reaction wheels are attached to a BLDC (Brushless Direct Current) motor of 7200 rpm and BLDC motor is connected to a BLDC drive circuit. Three reaction wheels are placed in orthogonal fashion inside satellite frame. The various control signals used are EN (enable), DIR (Direction) and PWM (Pulse Width Modulation) to control ON/OFF, direction and speed respectively of each reaction wheels.

1.3 ONBOARD COMPUTER SUBSYSTEM

Onboard Computer is the brain of the satellite and is responsible for generating the control signals for actuation based on the outputs of the various sensor onboard the Nano satellite. OBC algorithm is implemented on the MAX32630FTHR board. The system is designed as a fault tolerant embedded system so that it can withstand event such as SEU (single event upset) and SEL (single event latch up).A radiation hardened WDT (Watchdog Timer) is implemented to periodically reset the microcontroller board in case of failure such as latch up by using a counter, clocked by a separate clock signal from a crystal oscillator. All sensors and actuators are interfaced to the Board by using interfaces such as UART, SPI, I2C, PWM, ADC and GPIO. Supervisory program is also executed to monitor for any error in execution of tasks. Onboard Autonomy is implemented to take care of task execution in case of communication blackouts. In case of communication failure or Low power Alarm mode, Radio Transmission is switched off and data is periodically backed up to SD card based storage system in CSV format. Certain signals are routed out from MAX32630FTHR using 74LS244 based buffer.

1.4 TELEMETRY & TELECOMMAND SUBSYSTEM

Telemetry Subsystem is used for remote monitoring of satellite parameters and housekeeping information regarding voltage, current and temperature signatures at various critical points. ADS1218 is an 8 channel 24 bit delta-sigma converter with UART interface and can be connected in daisy chained format for expanding the number of analog monitoring channels for monitoring voltage and temperature at various points. Voltage divider with suitable protection network is used for sampling various voltages. LM35 based temperature sensor is used for temperature monitoring and is connected to MAX32630FTHR ADC input. ACS712, a hall-effect sensor based current monitoring IC is used for monitoring the current flow along raw battery power bus.
Tele command Subsystem is used to manually control subsystems on board the satellite from the ground control station. Upon the reception of tele command frames from ground they are decoded by the MAX32630FTHR board and suitable command are executed by switching the relays ON/OFF. Relays are driven by ULN2803 based 8 channel Darlington driver as commanded by the GPIOs of MAX32630FTHR. Tele command system facilitates the isolation of faulty subsystem in satellite so that rest of the systems can function normally, thereby implementing FDIR (Fault Detection, Isolation and Reconfiguration) scheme. Telemetry & Tele command Subsystem RF link is implemented using XBEE RF modules operating at 2.4GHz. RF Modules must be configured prior to operation to work on point-point link by assigning same PAN-ID and proper DH-DL address. RF module at satellite side is
interfaced to MAX32630FTHR board using the UART interface. For range enhancement high gain patch antenna is connected to external antenna port of XBEE module via 50 ohm co-axial cable.

1.5 SENSORS, PAYLOAD & RF BEACON SUBSYSTEM

Payloads are the instruments on-board the satellite for the purpose of scientific study and analysis. In Nano satellite a low-resolution camera is interfaced with the OBC through a NTSC input.This payload integrates a camera capable of capturing pictures in the visible spectrum at VGA resolutions (640 x 480). This can be used for analysis of outer space. Using a radiation sensor the Nano satellite is capable of counting ionized particles and perform radiation dosimetry. The radiation measured by this payload will change for each point in the orbit trajectory, allowing to have a baseline reference to correlate with the rest of the satellite's measurements. A hardware counter in the OBC'sCPU is used to record the number of ionized particles detected by the device. A four quadrant sun sensor is constructed out of photodiodes and Trans impedance amplifiers for measuring out emissions from sun. A catalytic heater based gas analyzer is used for analyzing the gas concentration. All sensors are interfaced to the OBC through Analog front ends (Sensor AFE). A standalone UHF RF beacon Transmitter working in 433 MHz band is used for echoing the satellite ID and details in a standard CW Morse code format. The transmitter is built around the Radiometrix 433MHx RF module.

A high accuracy GPS module is used in Nano satellite for obtaining positional information such as latitude, longitude altitude etc. GPS module with built in patch antenna working at 3.3V is connected to OBC via a UART interface. ISL29023 based ambient light sensor, TMP006 infrared temperature sensor, BMP80 barometric pressure sensor and SHT21 humidity sensor are connected to OBC via I2C link. An active thermal control system is realized using Peltier module. Depending on the internal and external cluster temperature direction of current in Peltier is reversed using a DPDT (Double pole double throw relay) to obtain required heating/cooling effect. A laser module is provided for conducting one way differential Doppler ranging demo and can be activated upon the tele command on/off relay activation.

2. Ground Subsystem

Ground subsystem consists of antennas and receiver subsystem. The telemetry and tele command section consists of XBEE module paired with the one on the Nano satellite. GUI (graphical User Interface) for Tele command uploading and telemetry viewing is implemented in processing based GUI. For listening to the beacon Transmission from Nano satellite, an SDR (Software Defined radio) is configured around RTL-SDR 2832U Dongle. For the beacon transmitter signal Reception, a Qadrifilar helix Antenna and LNA (low noise amplifier) is connected to the SDR antenna input via an SMA connector. Antenna is attached on a servo motor based pan/tilt assembly which is 3-D printed. A Morse code reader android app is used for decoding Morse code beacon signal from Nano satellite.

Nano satellite Firmware

Nano satellite firmware is developed in mbed online IDE. Libraries are used for peripherals such as GPS, I2C sensors etc. Pins for interface and their mode of operation is given in the initialization section of code. All communication interface such as I2C, UART and their parameters are specified in the setup section of code. A round robin scheduler RTOS is used for multitasking application. The Code is well commented and each task and interface is given as a separate function so that debugging process is simplified. Also in each function terminal printing serial messages are given for ease of debugging.

Nano satellite Assembly

Nano satellite engineering model is developed using commercial off the shelf components. Acrylic sheet is used as a platform for various stage integration. Uniform holes grid spaced at 1cm are put on acrylic sheet using laser engraver so that sub-assemblies and PCB can easily be tied or screwed to the sheet. Different Layers are stacked on to each other using M3 nut and bolts. In each stages all connections are terminated using D-connectors for the ease of assembly and debugging or checkout operation. Each stage is checked and verified by checkout systems consisting of GPIO simulators, voltmeter and ammeter to ensure proper working operation. Finally main wire harness for interconnecting each stage is fabricated and integration and testing is done.

Problems Faced

Nano satellite development opened lot of challenges to solve such as problems of noise e and ground loop. To prevent ground lifting due to ground loop formation and crosstalk separate ground for analog, digital and RF is devised and finally all grounds are terminate in a single point. To prevent the problems of relay chatter and switching effects decoupling is provided in each supply entry point. All RF connections are made using 50 ohm RF cable to avoid problem of signal reflections.

Nano satellite Upgrades

Nano satellite can be enhanced by adding more sensors and payloads. Right now only an analog black and white camera is used for imaging purpose, which pose certain limitations on imaging. Only Ethernet based streaming of NTSC camera image is demonstrated. On board image acquisition and sending using ZigBee transceiver firm ware needs to be developed along with picture framing GUI in ground station.

Project Conclusion

Nano satellite makes it possible for everyone to do his or her own space exploration. Space exploration is no longer reserved for the NASA and the billion dollar companies. It is now possible for everyone to be on what has been said to be the final frontier. This is actually the perfect time to take part in this revolution and for everyone to contribute to the space technology and exploration. Nano satellite offers the chance for everyday people to control a satellite for different purposes such as exploration, entertainment and experiments. It takes advantage of the existing technologies and platforms so that more and more people can participate in the space technology. With more and more people participating, the space industry is likely to go on through countless innovations.