>Plamsat On Board Computer

Plamsat On Board Computer

Palmsat (shown left) is a Pico-satellite being developed for Surrey Satellite Technologies Limited (SSTL), who are the world leaders in the design and building of low-cost small-sized satellites. A Pico-Satellite is a class of satellite weighing less than 1Kg and Palmsat was designed to be around the same size as a coke can. The purpose of this project was to build the on board computer (OBC) that essentially controlled the spacecraft and all the other onboard systems. I designed the OBC using a PIC18 microcontroller at the core. This project was for my final year project at the University of Surrey. I received a first for the project, however a fully working prototype could not be built using the technologies I had available in the lab. However, the designs and research that went into the project should provide a good starting point for other similar projects, and is the reason I have put the project online. The reports/schematics can be downloaded from the bottom of this page.

Introduction:

With launch costs between $20,000-$30,000 a kilo, launching satellites into space can be prohibitive for small organisations, educational institutions and individuals who want to learn about space. Other factors that increase the cost of a mission is the cost of the satellite and its components, that have to be highly reliable because satellites cannot be repaired once in orbit because of the prohibitive costs. SSTL came up with a different approach; its prime objective is to make access to space possible for everyone. To date, SSTL has designed and launched many mini-satellites, and more recently deployed a nano-satellite, weighing less than 5 Kg. All these efforts have pulled down the cost of space missions significantly, yet it is still too expensive for educational institutions and individuals to launch a satellite. Pico-satellite PALMSAT is the next step in the series. A pico-satellite will be very small, and weigh less than a kilo, using cheap, off the shelf components. Palmsat will serve as a foundation for future pico-satellites. It will allow students and individuals to develop their skills and interests in space technologies. It may also be launched with larger satellites to monitor their deployment once in orbit, such as the opening of solar cells. An important philosophy in designing Palmsat is that it uses cheap, miniature off the shelf components. Hence Palmsat will serve as a medium for demonstrating that this philosophy can be used in space, where to date a large expense is put into highly reliable designs and components.

The Palmsat Architecture:

Palmsat is a group project, with each subsystem of the satellite being designed by a different student. The subsystems are described briefly below:

Transmitter: Transmits data from the satellite on UHF band, 9.6K baud.
Receiver: Receives data from the ground station on the VHF band, must be constantly on to receive data at any time.
On Board Computer (OBC): Manages all the subsystems of Palmsat. Transfers and stores data and processes commands received from the groundstation.
Power: This subsystem regulates the power from the solar cells and batteries. It also decides when to charge and discharge the batteries.
Payload: Can contain anything, but for the purposes of the original Palmsat, the payload will be a small, low-resolution camera.

Each subsystem will be designed to fit onto a small circuit board the size of a credit card, which will be slotted side by side inside Palmsat.

The total satellite should weigh less than one kilogram, and the satellite will receive an average solar power of about 2 Ws, so must run on less power than this. It is intended to use the following interfaces between each subsystem:

USART: OBC, RF Subsystem

SPI/I2C: OBC, memory, power controller and payload controller.

The area of this particular project is to design the On Board Computer (OBC). The OBC is the ‘brain’ of the satellite controlling all the other subsystems. It controls the flow of data between the subsystems; records and processes data received from the subsystems, and executes commands received the ground station.

Determination Of OBC Functions:

In order to design the OBC’s hardware and software for this project, the expected functionality of the OBC must be known. These are divided into two groups; essential functions are the functions that must be addressed at the start of the project, and optional functions are functions that may be implemented later in the OBC, but are may not be essential in the final design of Palmsat.

Essential Functions

Optional Functions

Recording the telemetry data of Palmsat for sending to groundstation for analysis.
Encoding/Decoding of data packets to/from groundstation.
Processing of telecommands from groundstation, including time delayed commands.
Subsystem controller, acting as a watchdog on other subsystems, resetting if necessary, and controlling the flow of data between them.
Stores payload and telemetry data for transferring later to groundstation.

Compression of telemetry data.
Self-reprogramming of new software into program memory and boot-loading the new software while in space.

In order to have the essential functions, it should be clear that the OBC would need to have, as well as a microprocessor:

An analogue-to-digital converter, for recording telemetry data on the satellite.
Memory for storing the payload and telemetry data.
Reliable memory for storing the software to run on the OBC.
It was decided it was desirable to have the self re-programming feature on the OBC as well, so the system must be capable of this.
One or more buses for communicating between the devices and subsystems.
Timers for watchdog functions as well as running time delayed commands. This may also require an external oscillator.

Design Constraints:

The following constraints have been imposed on the design of the OBC:

Power (2 W) and weight (1 Kg) limits should not be exceeded.
Design should consist of off the shelf components.
The components should be suitable for the space environment.
In spacecraft design, it is necessary to provide redundancy of components to avoid critical failures in a mission. However, redundancy adds to power consumption, weight and cost. Therefore, the design will use a single string solution, with no redundant components.
The design should be flexible, so that future changes can be made to suit particular missions.

Project Files:

Palmsat_Report.pdf (1.32MB)