Batteryless satellite operations. August 2006 Allan Copland GM1SXX
While engaged in email exchanges with William Leijanaar PE1RAH (designer/builder of the Dutch satellite transponder aboard HAMSAT/VUSAT) we both seem to have come up with an idea that while not exactly new, is perhaps now of its time. Satellites often fail because of bad batteries. Ni-Cd batteries generally fail short-circuit, thereby ending the life of the mission. BCR's have helped with some satellites, but they only help... satellites with or without BCR's will eventually suffer failed or seriously degraded batteries. It's inevitable. Batteries are 'wet' chemical technology. They are prone to electrolyte loss, dendrites and a multitude of other undesirable degrading effects. Sometimes, just sometimes, the batteries may NOT fail short circuit, at least after being shorted for some time, chemical changes may conspire to cause an open circuit, as in the case of Oscar 7.
Oscar7 was not the first satellite to come alive after a long period with shorted batteries, the Russian RS-1 bird did likewise but failed again fairly soon after. Oscar 7 has remained available in sunlight, thanks to it bucking the trend of Ni-Cd batteries going short circuit. In doing so, it has continued to provide excellent service to radio amateurs around the globe when in sunlight. It has also demonstrated that batteries are a double edged sword. In the case of AO7, the loss of the battery has saved its mission.
Some radio amateurs have now realised that satellites without batteries are not such a bad idea. After all, with no battery, there's less to go wrong. Not having a battery onboard may be seen by some as a catastrophe while for others, it barely matters, or matters not at all.
For a simple amateur communications satellite with only a single transponder, it would entirely feasible to have battery-less daylight-only operation. Such operation could even add to the challenge of satellite working. It could also dramatically enhance the life expectancy of any satellite built in this way. While discarding the battery, one might also discard any on-board computer, telemetry, command and BCR system, because strictly speaking, they are not required in a simple transponder only comsat. Granted, some sort of tracking SMPS power system might allow more of the power from the panels to be utilised but if you ditch the battery, there's less of a reason to include a BCR. It only adds to the complexity and the ways in which a bird can fail. Even on a battery-less satellite, a simple CW beacon would be very desirable as with all linear transponder satellites. The most simple of PIC or other micropower CPU's and a simple two transistor oscillator/buffer could easily do that job with a power drain in the low tens of milliwatts.
If the rules of the game change, and there's a possibility that they might, it would be possible to add a simple system to disable the transponder via a command link should that be necessary. It's interesting to note that many early satellites were built to operate with no means of ever disabling them.
Oscar7 has shown us the way forward. It's ancient, but it works, and it works mainly because its batteries are U/S! True, AO7's power output is very limited because of degradation of the glass cover-slides that protect the solar cells, *but* Oscar7 is in a rather high circular orbit that exposes it to a lot of radiation dose (over 100K RADS to date) whereas most LEO birds are in rather lower, and therefore more benign orbits. The fact that it is well utilised shows that there are operators for whom the 'easy option' is not their preferred choice. With a battery-less LEO bird in a lower orbit, say 650-1000Kms, the signal would undoubtedly be considerably stronger than Oscar7 while having a decent footprint providing interesting possibilities for the satellite enthusiast.
William, PE1RAH has built quite a number of transponders. One was flown successfully on VUSAT/HAMSAT thus proving that he has the know-how to build viable space hardware. His suggestion to sandwich a transponder between a pair of solar arrays might not be ideal because the transponder would drop out with edge-on, orientations. A cuboid or equilateral triangular structure with basic passive magnetic stabilisation might be better but I do believe that the idea merits serious investigation.
Were one to ditch the standard cubesat launcher and instead use a squashed six-inch parallelogram shaped spring-assisted structure instead, equilateral triangular shaped battery-less satellites could be packed into the launcher system in pairs thus saving space and increasing the number of payloads carried. Increasing the panel size from 4 to 6 inches square would make a very noticeable difference to the available power (see my other CUBESAT postings) while not greatly increasing the size of the spring-loaded 'launcher' .
Below is reproduced a copy of an email sent to me recently by William PE1RAH.
73 Al.
GM1SXX
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Hi Al,
About your question about the transponder power.
When I purely look at the RF power (eg of HAMSAT) the 1W is peak power.
This means when the PA is full open, but in most times it sends only the noise floor, some 20dB down. So it sends about 10mW of noise what is quite small.
The main battery eaters are the bias currents. With some smart designing they can be lowered a lot, but many other problems will occur, like compression points will go down and overloading of stages will occur more early. This makes the dynamic range smaller, and protections are needed like AGC and smaller RX filters (to get ride of to much out of band signals) etc...
With a battery, power can be stored over a long time because duty cycle is low.
However, when the batteries get bad, the transponders life is also ended.
What is needed is just a transponder and a large enough solar panel.
With a thin sized transponder in can be sandwiched between two solar panels and there it goes.
A beacon can easy be added in a transponder, as an easy and small oscillator on the IF frequency can be used for CW, just by on/off switching. A good uController is the Texas Instruments MSP430 family, they use only few micro-amps !!! I even did some tests by let it run on DC power made from the RF of my handheld at 3meter distance :o) Same as the RFID system...
These uController devices are also used in those cubesat kits...
Just for your idea: The beacon on my transponder makes about 30mW !!!
For a LEO you don't need a lot of RF power. However, how larger the bandwidth, the more stations that can be on, the more total power is wished...
The more bandwidth gives also a larger amount of noise that is on the downlink, what makes also more total RF power. For a low battery power transponder it is better to have a smaller bandwidth.
But it is like a car, there are many kinds of cars, it just depends on what car you need.
You have high speed cars, powerfull cars to carry loads, cars that are used once a week, or cars that are used by many people...
For me as designer it is difficult to know what kind to make as it all depends on many parameters,
73 de PE1RAH William