giving the students an unique chance to experience a real engineering project with real engineering problems. Currently the satellite is still under development and will be delivered for P-POD and launch vehicle integration in January 2002 and then finally launched in April 2003.
The Mission
The main purpose for the AAU CubeSat project is for the involved students to achieve a great deal of knowledge about desiging and constructing Space worthy technology, but the "scientific" mission of the AAU CubeSat is to take pictures of the surface of the Earth and particularly of Denmark by using the on-board camera.
The images recorded by the satellite will later be transmitted to the ground station, located at Aalborg University, from where they will be distributed over the Internet and made accessible for the general public.
Several success criterias have been defined. A basic success criteria in this case is, to develop and build a satellite which will be able to survive the launch and the hazardous environment in its orbit. Another success criteria is establishment of a communication link with the ground station informing about the status of the satelitte.
Finally to point the on board camera towards a specific target on the ground, to take an image and to send this data down to the ground station, represents the final success criteria.
To summarize the above the missions sucess criterias are the following:
- That the involved students have achieved some useful knowledge of space technology.
- That communication is establised with the satellite and housekeeping information is retrieved.
- Take and download any picture.
- Test ACS performance.
- Take pictures of certain locations on earth.
- Take pictures of celestrial objects and experiment with the various subsystems.
The Satellite Structure
The AAU satellite is a CubeSat, thus its measurements have to fulfill the requirements set up by Stanford University and California Polytechnic institute, which originally developed the Cube-Satellite concept.
The size of the satellite has to be 10 x 10 x 10cm, while its mass is limited to below 1kg.
To archive this, light materials are used for the structure of the satellite. Its design will be based on a frame of aluminum with sides made of carbon fibres.
High requirements have been set regarding the structure of the satellite and its integrity, as it has to withstand high temperature variations (+80 and -40 C), vibrations and shocks, radiation, and the vacuum in space.
However, the most vital task when designing the structure will be to keep the weight limited and to be able to fit all necessary subsystems into the structure. Below is the stucture with the different subsystems shown:
The Electronic Systems
The electrical subsystems of the satellite will be controlled from a central on board computer (OBC) based on a C161PI micro controller from Infenion.
It features a 16-bit CPU providing 16 Megabytes of Linear Address Space. The OBC uses three types of memory. A RAM module (4 MB) for picture data, dynamically allocated memory, and buffers. A ROM module of 512kB for initial flightsoftware and 256kB of FlashRom used to upload new software after the satellite is deployed in orbit. The communication between the OBC and the other electrical subsystems are carried out by means of an I2C bus, which connects the Power Supply Unit, the Attitude Determination and Control and The Camera. The Communication Unit is connected directly to the OBC though a parallel interface. The connection between the subsystems can be seen on the figure below:
The Power Supply
The power supply will rely on batteries and solar panels, which will be placed on the surfaces of 5 out of the 6 sides of the satellite.
The solar panels are triple-junction cells from EMCORE. They measure 68.96 x 39.55mm and will be placed in pairs on the satellites sides.
The four on board batteries are developed by DANIONICS and will provide energy when the satellite is eclipsed or whenever the power use exeeds the power input
The available power on board the satellite will depend on the amount of daily sunlight on the solar panels to re-charge the batteries as well as on the environmental effects in the orbit.
The Communication System
In order to receive commands from the ground station as well as for transmitting the on board status and the images recorded during flight, an on board communication unit is used. In this case the on board communication unit and antenna is purchased from OSSS and will communicate with ground by using radio amateur frequencies. The antennas attached to the satellite will be deployed after the satelitte is released from the launch vehicle
To be able to communicate with the AAU-Cubesat from ground a tracking antenna will be used, which follows the satellite's motion over the sky.
The Payload
The main payload for taking the images of Denmark is the on board camera.
It is in this case a digital CMOS camera chip based on a Kodak 1.3 megapixel kac1310, which has been provided by the Danish company DEVITECH. It will take images of the ground with a field of view of about 150 x 115 km and a resolution of 1280 x 1024 pixels with a color depth of 24bit.
The lens in front of the CMOS camera chip is handmade with a diameter of 2cm at a length of 5cm.
Attitude Determination and Control
To be able to take images of the earth, an attitude determination and control system is required. It will point the camera towards the correct target for imaging and for communication. Furthermore it will point the camera away from the sun and point three of the satellites sides with the solar panels towards the sun to ensure a maximum power input.
To control the satellites attitude in orbit three coils are used, which are mounted on three of the satellites sides perpendicular on each other. These will generate magnetic fields which interact with the earths magnetic field, and hereby change the satellites attitude.
To determine the satellites attitude two types of sensors are used. A magnetometer, build up with components from HONEYWELL, to provide information on the direction of the earths magnetic field, and sun sensors. These sun sensors are basically planar photo diodes placed on each side of the satellite to measure the intensity of the incoming sunlight.
Launch and Orbit
The CubeSat will be launched into space together with other CubeSats inside a so called P-POD-deployer. It will be placed on top of the launch vehicle as a secondary payload.
The launch vehicle itself will a Russian Rockot rocket; a former ICBM.
The P-Pod will deploy its load of CubeSats including the AAU CubeSat, in an sun synchronous Low Earth Orbit (LEO) at an altitude of approximately 900km.
Here it will detumble to get a controlled attitude and to perform its task of taking images. The lifetime of the satellite has been assumed to be one year, after which it sooner or later will degrade due to radiation and other environmental effects. The picture below shows the CubeSat in the P-POD:
