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02-03-2011, 03:08 PM
cube satellite.doc (Size: 4.71 MB / Downloads: 126)
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What is a cubesat???
type of miniaturized satellite for space research
usually has a volume of exactly one liter (10 cm cube),
weighs no more than one kilogram,
typically uses commercial off-the-shelf electronics components.
List of CubeSats:
The COMPASS-1 CubeSat was designed and built by students from the Aachen University of Applied Sciences in Aachen, Germany.
More than four years were needed to realize this 1 kilogram Picosatellite from scratch into a space-ready flight model.
The launch took place in April 2008 from the Indian space port Sriharikota.
The mission received (and still receives) tremendous support by the radio amateur community, which helped to collect a large amount of data and images from the satellite.
The primary objectives of the compass1 are:
to develop a CubeSat (pico satellite) platform suitable for various scientific and non-scientific missions.
to perform an earth observation mission, using an optical sensor as payload.
to use a Ka-band antenna for technology demonstration.
to use new technologies and methods, in particular micro & nano technology and to keep the costs at a low level.
technology demonstration and validation for space application of COTS products;
communication link for pico satellite;
attitude control system for pico satellite.
The control mode is necessary for the spacecraft to fulfill the mission requirements on the spacecrafts attitude, i.e. the pointing accuracy. Therefore it has to secure a stabilization of the satellite shortly after its deployment from the P-POD. This phase is called detumbling
here we will understand how the spacecraft will operate when it is in orbit and functions as it is supposed to.
For the operator the above mode is valid also. In addition the operator is authorized to send commands concerning the housekeeping and control of the spacecraft and furthermore advanced imaging commands
If for any reason, crucial components of the spacecraft do not work in the expected way, and will therefore hinder the spacecraft to go into the regular operation mode the satellite will react with sending emergency signals. The system switches to the emergency mode and sends beacons periodically. In case that the problem resolves, the system switches back from emergency mode to regular mode.
Launch and deployer
The PSLV-C9 was the launch vehicle. The launch took place in April 2008 from the Indian space port Sriharikota.
CubeSats are typically launched and deployed from a mechanism called a Poly-PicoSatellite Orbital Deployer (P-POD), also developed and built by Cal Poly. P-PODs are mounted to a launch vehicle
carry CubeSats into orbit and deploy them once the proper signal is received from the launch vehicle.
Since CubeSats are all 10x10 cm (regardless of length) they can all be launched and deployed using a common deployment system.
For the design of the satellite we have to concentrate on the requirements from the P-POD supplier. The general guidelines for a CubeSat are:
CubeSats must not present any danger to neighboring CubeSats in the P-POD or to primary payloads,
All satellites must be powered off during integration and launch to prevent any electrical or RF interference with the launch vehicle and primary payloads
CubeSats must use designated space materials approved by NASA,to prevent contamination of other CubeSats and primary payloads during integration, testing, and launch.
Sunlight and Eclipse Times
In space the definition of day and night is completely different to what we are used to on earth. Times of direct sunlight and times of total darkness follow each other in very short periods.
The sensor (a camera) will capture images on request and the OBC will store them in the memory .
The mass will also be very little .
The sensor sits on the front side of the cube. The front side is maintained in a nadir fixed position.
The dish will also be in use only for a short time compared to the mission lifetime.
However, while in use it will probably consume a high amount of power. It has to be assured by the power subsystem that those requirements are met.
The principle operation of the Ka-band transmission is to continuously send the payload information at a given time and duration.
The data from the camera is an array of bytes, in total 640x480 bytes for an VGA image.
To decrease data transfer rate the camera chip provides a solution, that is it can output QVGA resolution image. This mode decreases pixel rate one half.
The resolution default value is 320x240 and can be programmable.
The digital video port also offer RGB Raw Data 16 Bit/8 Bit format.
Attitude Determination and Control
The goal of the Attitude Determination and Control System (ADCS) is to stabilize the spacecraft against all attitude disturbing influences resulting from the environment in the earth orbit in order to point the payload towards a predetermined point on the earth’s surface
The attitude acquisition will be performed by a combination of sun sensors and magnetometer.
The control of the attitude will be done using a magnetorquer which will be supported by the gravity gradient generated by radio-wave dipole antennas pointed towards the earth
The acquisition of the current position will be done by an on-board GPS receiver.
By using a simple lightweight GPS receiver the satellite will autonomously be informed about position and attitude without any further intervention from the ground station
. A GPS antenna will be mounted into one of the side faces of the cube.
ADCS operation mode
A communication with the spacecraft takes place over two types of links.
The uplink carries commands from a ground station to the spacecraft.
These commands are redirected to the OBC and then either processed immediately or stored and executed at a specified time.
The downlink carries data, which consists of two different types of information.
One are the data generated by the payload and those are in this case images.
The other data are information about the spacecraft’s vital characteristics, so-called housekeeping data.
Overview of communication system
A TNC (Terminal Node Control) is a microprocessor,
previously also introduced as micro control unit (MCU),
which has the following functions for downlink:
- read the data to be transmitted and store them if necessary;
- pack the data into protocol format;
- send data to modem.
The other way around it works in the following manner:
- receive commands from modem;
- unpack commands;
- redirect commands to OBC.
Command and Data Handling
The command and data handling subsystem provides the main bus for the data exchange between all other subsystems.
All data exchanged through the bus is in binary format made up by zeros and ones.
The system manages three digital data streams, each critical to the spacecraft and each with distinctive characteristics. Those are:
- data from the payload
- housekeeping data
Hardware is everything physical, e.g. the
PCB for the main bus and the electrical
The main bus assures the dataflow within the spacecraft. Every subsystem is connected to the bus.
I²C bus is used because of its suitable data rate and high flexibility.
The controllers of all subsystems have an inbuilt I²C bus hardware module, making it easier to write the software.
It controls the spacecrafts operations in normal mode.
The OBC is the main data handling system.
It handles the exchange of data between payload, memory and communication.
Every command received from ground will be decoded by the OBC and stored in the memory.
Because of its relatively high energy consumption, the OBC will be shut down in case of entering the power saving emergency mode.
To prevent the OBC to hang up, an external WDT, watchdog timer, will also be mounted on the PCB. It forces the MCU to restart and reboot, so it can go on doing its work.
The language used for programming will be C/C++ whereas some routines, which need to be optimized for speed, might be written in Assembler code.
The spacecraft can only perform as long as it has power
Functional breakdown of power subsystems
This system has to store the required power for eclipse times and needs to be functional over the entire mission lifetime.
Power Distribution, Regulation & Control
The input from the five with solar cells covered sides needs to be converted to a stable voltage level.
The charge controller manages the charging of the batteries and supplies the power. When the kill switch is open, current flows through the power lines to the power board controller and the communications system controller always.
All elements in the spacecraft have an influence on the thermal housekeeping by either emitting or absorbing energy or both respectively.
Since the CubeSat is very limited in space and mass we cannot expect to have the components insulated from each other.
Here the solution lies in the useful configuration of the elements in order to protect vulnerable components with stringent temperature boundaries.
Thermal Control Components
The following components and devices are
used extensively in thermal control
subsystems of common satellites.
Structure Elements and Interfaces
The mission operations really start when
the spacecraft is in orbit.
This section deals with three main issues concerning the mission operations aspect.
Those are the communication architecture, the ground station and the user interface.
Bit Error Rate
The ground station is the counterpart to the satellites communications system
Images of the Earth
The small camera on the bottom side of the satellite can be commanded from ground to take color pictures of the earth. Several images have been downloaded
The resolution is 640x480 pixels in 8bit resolution.
The GPS receiver has already been activated several times. It initializes and operates without malfunction but fails to establish contact to any GPS satellites. The problem is traced back to an improper integration of the patch antenna at the front panel
Three-Axis Attitude Control
The implementation of an attitude control system is highly complex, partly due to the fact that its proper operation cannot be fully verified on earth. It is therefore disappointing to report that due to a software problem (bug) in the attitude determination software for reading and processing the analog sun sensors measurements correctly, the on-board process does not achieve a useful estimate of the satellite's actual attitude
Up to this very date, the satellite is still doing well in space and shows no sign of significant degradation.
The power system is constantly supplying energy to the on-boards electronics
produces enough excessive power to charge the batteries for the eclipse times.
The communication system is also working just fine.
It is remarkable that the COMPASS-1 spacecraft comprises of a total of nine (9) microcontrollers (one on EPS, COM, ADCS, and CDHS, and one on each sun sensor), all of whom apparently work very well in space.
No single-point failure or malfunction appeared that resulted in a significant degradation of the mission operation.
This is particularly a great success, as most of the systems are engineered with COTS components, which were designed for use in terrestrial applications, most dominantly in industrial applications. The fact that the satellite up to present still operates nominally, verifies that the spacecraft is well suitable for low earth orbit missions
Other Applications Of CubeSats:
Earth remote sensing
StudSat is a student satellite conceptualized and currently being designed by undergraduate students across India. It is a picosatellite and first of its kind in India.
Mission type-Remote sensing technology
Launch date-May 2010
Launch site-Satish dhawan space centre
Regime-Sun synchronous circular orbit
‘The satellite itself is not much bigger than a
cup of tea, though the involved project and implimentation
aspects (from project and implimentation management to
system engineering and qualification
testing) resemble very much any other
satellite mission, however on a lower scale.
Through the combination of bright, novel
ideas of young engineers-to-be and the use
of commercial off-the-self components, the
realization of cheap and fast responsive
pico- and nanosatellite missions become
not only feasible, but also worthwhile!’