SEDSAT-2 Communications Design Notes
From SEDSWiki
Contents |
Research/Background reading
Details on comms system on other cubesats
A list of current cubesat projects
University of Leicester Cubesat project
Expected communication windows
Orbitron was used to predict the visibility of the CUTE-1 satellite (as an example of a cubesat in a LEO orbit). The following are the communication windows above 10 degrees from the horizon, in a 24 hour period between the 2nd March and 3rd March.
SEDSAT-2 Communications Design Notes 20080302
The Subsystem
MCU
MCU Development, the Java MCU simulator, and MCU design/development tasks
Terminal Node Controller
Possible commercial TNCs
- http://www.paccomm.com/spirit.html
- http://www.paccomm.com/tiny2mk2.html
- http://www.paccomm.com/handi.html
- http://www.paccomm.com/pico.html
- http://www.timewave.com/products/pk96.html
- http://www.tnc-x.com/
- http://www.elcom.gr/sv2agw/hardware.htm
- http://www.wimo.com/cgi-bin/verteiler.pl?url=packet-radio-tnc_e.html
Modulation
An overview of modulation schemes and benefits and disadvantages of each. SEDSAT-2 Communications Design Notes 20071125
An introduction to channel modelling SEDSAT-2 Communications Design Notes 20080227
PSK
FSK
Simulation of a simple FSK transmitter, and a diagram of a simple FSK receiver. SEDSAT-2 Communications Design Notes 20080229
Frequency
420 Mhz
GHz communications feasibility
SEDSAT-2 Communications Design Notes 20071124
Protocol
AX.25
FX.25
Custom protocol
Full-duplex communication over a half-duplex channel
Channel Modelling
Antenna
Background/Theory
Dipole
Grid Oscillator
- Design
- Due to the complicated electromagnetic field interactions between the active devices, metal grating, and dielectric substrate, modeling of the grid can be very difficult. To address these problems, edge effects are ignored by assuming that the grid is infinite in extent, as shown below
- This infinite-array approximation provides planes of symmetry that allows characterization of the entire grid with the unit cell.
- The unit cell can be separated into two parts: a passive part (consisting of the substrate, metal grid, mirror, and free space), and an active part (consisting of the transistor). The passive part is characterized
- using full-wave electromagnetic techniques which results in a two-port scattering-parameter network. The active part is characterized with the transistor scattering parameters.
- Physical Specifications and Integration
- An important aspect of the design is achieving certain physical specifications required from the overall CubeSat perspective. The areas of consideration are: mounting schematics, circuitry layout, and weight. There are several constraints for the mounting of the grid oscillator since the grid requires the placement on the exterior of the cube. The first goal is to minimize the physical size in order to share a face of the cube with a solar panel for maximization of power, as shown below
- Since we are using 6x6 array each with unit cell of 6x6 mm or 12x12mm , which comes to be 36x36 mm or 72x72mm respectively(which gives us enough space to be accommodated with solar panel ).
- The grid can be mounted to a face of the satellite using an adhesive and possibly screws.
- Tentative specification of the antenna
- NxN grid printed on 0.254 mm substrate with refractive index = 10.2
- High electron mobility transistors as active devices.
- 12.7mm thick HiK dielectric material between substrate and mirror
References
http://cubesat.atl.calpoly.edu/media/Documents/Papers/antenna_paper.pdf
Some Questions Regarding Grid Oscillator Antennas
- can this be used as a secondary payload? (so comms can switch between dipole and grid oscillator in order to test this)
- how does this affect the structure of the cubesat (will one face of the cubesat replaced with grid oscillator?)
- will we have any problems meeting the cubesat design specifications if we have a grid oscillator on one side?
- will the grid oscillator sit on top of the aluminium surface of the cubesat?
- if so, structures can't use/buy a pre-made cubesat structure as these are exactly 10cm x 10cm x 10cm. So with a (for example) 4mm thick grid oscillator on one side, we'll be above the spec for the cubesat size. What size must the main cubesat structure be, to be able to fit the grid oscillator on an outside panel whilst still meeting the size requirements?
- what will the grid oscillator be made of?
- as it will be outside the structure, can it survive the large temperature range of space (-100degrees to +125 degrees)?
- can it survive the radiation environment in LEO?
- if the grid oscillator works, then we'll be able to communicate at a higher data rate than if using a dipole. But this means that the communication equipment on the cubesat (and on ground) will have the be designed with the higher data-rate grid-oscillator in mind. How much extra power/space will this require?
- how will this impact the ADCS system - if one side of the cubesat is a camera which is pointing at earth, and another size is our grid oscillator, then when we want to transmit, the satellite will need to be rotated to point the grid oscillator side towards earth. Does ADCS have the capability to do this?
Transceiver
DJ C7T
Mass/Power Budgets
Funding/Sponsorship
Funding/Sponsorship brainstorm task
Other Notes
SEDSIC'07 Notes
Subsystem Notes as of SEDSIC '07
Links
SEDSAT-2 General Meeting
Some questions from Comms for the SEDSAT-2 General Meeting on 2007/12/30. SEDSAT-2_Communications_Design_Notes_20071229
