OBSERVATIONS FROM SCOTLAND

Uosat2 shares a lot of similarities with Oscar7. In particular, both birds use a hardwired telemetry system which switches the outputs from the various telemetry channels to a SINGLE A/D converter before being transmitted as 'frames' of data.

The frames take 4.84 seconds each to transmit and are 'framed' in a rigid manner with a header block containing the satellite ID and the internal clock time value.

UOSAT-2           0903306024319

000000010001020002030003040004050005060006070007080008090009
100001110000120003130002140005150004160007170006180009190008
20000221000322000023000124000625000726000427000528000A29000B
30000331000232000133000034000735000636000537000438000B39000A
40000441000542000643000744000045000146000247000348000C49000D
50000551000452000753000654000155000056000357000258000D59000C
60800E615FC1620105633305644402651E0C66600667000168000E69000F

I downloaded and analysed data for many years from U0SAT2.
The frame shown is interesting, but useless, and I'll explain why.

The UoSAT2 telemetry system is hard-wired in CMOS and uses a single A/D converter which is sequentially switched between channels using hardwired logic. This is very similar in concept to the system used on AO7 which does the same thing in the absence of useful data… it still polls the telemetry channels creating a data ‘frame’ filled with… err.. umm… nothing!

Uosat2 is no different.

The 'lights are on, but there's no-one home'. That is to say that the counter-multiplexer circuits are still operating perfectly, but the A/D sampling circuitry is delivering nothing to them. Why? Well the A/D requires a reference voltage, to which sampled data are compared. In the absence of the reference, the process doesn’t work. Just like AO7 with it's GOOD 449/450/451 data values...representing adequate voltage to the precision reference .

The A/D's in both AO7 and UoSAT2 seems to require a higher bus voltage than the CMOS counters, shift registers etc do. Actually, CMOS is quite forgiving of voltage variations, and as has been adequately demonstrated, it has very good rad-hardness properties (albeit helped with generous radiation screening in place)

If you look at the telemetry frame shown, you'll see the channel numbers but no data content. This is extremely similar to the behaviour of Oscar7 when there is insufficient power available, which is almost always!

This of also of course means that the battery has failed o/c, as Ni-CD’s almost NEVER do. They normally fail short. Lucky us!

It also should be mentioned that both craft are very old and cosmic radiation will have darkened the cover-slide glasses protecting the solar cells, so there is likely in both birds to be a very limited supply of power.

So in many ways, UoSAT2 has suffered the same fate as Oscar7... and for the same reasons IMHO. There *may* be differences in the subtle ways that they failed, but essentially BOTH have batteries that are NOT short circuited… but they may be dragging the bus voltages down… acting as a resistor, rather than a storage battery, AND the A/D's have insufficient voltage to their precision voltage references to operate properly. Still, at least UoSAT2 sends nothing instead of drivel :-)

One thing is clear….. first generation CMOS has proved to be exceptionally resilient in a high radiation environment. Other technologies have not fared so well. We should learn from the experience with these old ladies of space.
There are practical lessons there to be learned and acted upon.

I’m not a gambler, but I would bet that without the batteries on board, both UoSAT2 and Oscar7 could still work reasonably well… in full sunlight. UoSAT7 has indeed worked fairly well in the past, when in full sunlight.

This of course, raises a new raft of questions re batteries on spacecraft. perhaps we’d fare better without them. Seriously!

In a different vein, I had hoped for great things from DELFI C3’s linear transponder battery-less system…. we could have learned a great deal from it, but alas it was never to be. What a wasted opportunity IMHO.

Have a good Christmas folks.

73 de Al.
GM1SXX

For the Eagle eyed die-hard satellite fans, I also posted most of this on Mike DK3WN's site. Thanks Mike!

P.S. I should have mentioned that row 7 of the data frame, the last line, with numbers beginning with 6 are displaying data. This is the 'Status' data which consists of only binary flags and which doesn't make use of the D/A converter. They are simple on/off flags indicating the status of various systems. Although they are generated by the onboard multiplexers, they don't need the A/D to be operational to function. I thought I'd better explain why some of the telemetry may make sense, but I can't vouch for that :-)