What's Wrong With CUBESATs 7th August 2006 Allan Copland GM1SXX
CUBESATs provide Universities with a cheap method of launching small payloads. They do nothing at all for the average amateur satellite fan wanting a bird with transponders.
When I say small, I mean SMALL. The specification for a CUBESAT demands that each satellite structure be no larger than a 100*100*100 millimetre cube. That's very small indeed. The size is a compromise between flying as many satellites as possible in a small space and being able to provide enough payload space and power for the onboard experiments.
Off the shelf CUBESAT development system.
That small square box in the photo is the CUBESAT spaceframe.
The choice of a four inch cube is extremely limiting as I hope to demonstrate and severely limits what such a satellite can do.
With regard to communications between the satellite and its groundstation CUBESATs normally only carry a receiver for command decoding etc and an FM transmitter. Because the power available to a CUBESAT is extremely limited, transmissions are normally made in short bursts.
It should be becoming very obvious to the reader that the small amount of power available effectively cripples such a satellite if some sort of transponder operation is desirable.
To date, no CUBESATs have ever carried a transponder. Such a transponder would not only have to be more efficient than any ever flown before, they would have to use an extremely small (and extremely efficient) final amplifier stage.
So, how much power can the solar panels on a CUBESAT generate?
That to my knowledge has never been published but we can make a few 'back-of-the-envelope' calculations that will give us some sort of take on the problems.
The solar irradiance constant is approximately 1380watts per square metre.
Given a 100% effective solar collector, this is the amount of power that a solar panel of one meter square in earth orbit would collect when pointed directly at the sun.
This equates to 0.138watts per square Centimetres of surface area.
Solar arrays, even the best ones, are very inefficient and typical values for efficiency are in the 20-25% range, so lets aim high for our model and choose 25%
25% of 0.138 = 0.0345 watts (per square centimetre).
We now have a ballpark figure for the power per unit area .... (0.0345W/square Cm)
CUBESATS are 10Cms square but not all the space is available for solar panels so lets chop off 20% to give our available surface area.
(10*10)*0.8 = 80 square Cms of surface area per panel.
Multiply 80 by 0.0345 and we get 2.76
Thats watts! Our puny little 10Cms square panel covered 80% by 25% efficient cells (no spaces or connecting strips!) can only deliver 2.76Watts of power when facing directly at the sun.
This model is an oversimplification, because with real satellites you almost never have a single panel oriented this way. The illumination is constantly changing, but I deliberately want to keep things simple.
Lets scale those dimensions, active areas and power outputs for other sizes of panels.
panel size active area power generated
================================
100mm 80 2.76watts
120mm 115.2 3.9744watts
150mm 180 6.21watts
180mm 259.2 8.9424W
200mm 320 11.04W
You should be able to see at a glance what's wrong with four inch CUBESATs.
In a single word 'POWER'... or the sheer lack of it.
Be aware that this is a much simplified model only describing a single panel faced directly sunward. In the real world, things are more complex because a real satellite rotates and the illumination is variable. The model does however show the best case scenario for a single panel under optimum illumination.
Add to that the fact that current CUBESAT antennae need to fit inside that 100MM square 'envelope' for launch and you can see even more reasons why they are useless as a COMSAT platform.
CUBESATs are too small to have any real use for amateur satellite use (IE transponders) but larger CUBESATs are a promising format for low power LEO linear transponders.
A seven inch cube (18Cms square) or an eight inch cube (20Cms square) would elevate CUBESATS into a different league.
They could then become true 'amateur satellites' given that they carried a minimum of power guzzling equipment and only a single efficient transponder.
The space and mass saved onboard in going down this road could perhaps be used to carry larger batteries or alternatively, a large'ish lightweight satellite could be constructed if launch mass is the most important factor.
Amateur COMSATs don't need to be overly complex. Reducing the onboard systems to the minimum required to support a single transponder could pay dividends elsewhere. As an example, HAMSAT/HM-1 is an outstanding success and its a pretty simple satellite. It doesn't carry much dead weight. A simple command receiver, CMOS based command decoder, a BCR and a simple telemetry beacon in addition to the transponder could do a perfectly good job. I'm with William of Occam!
"'Tis vain to do with much what can be done with a little." (Lord Occam 1640)
Perhaps the future of amateur satellites (not educational ones) could be LARGE CUBESATs that do something useful for radio amateurs... transpond.
Small Cubesats ... Pah!
73 Allan Copland
