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AO-7 info from Jan



Folks,
Following are two messages from Jan King regarding the signal Pat heard
the other night on 145.975. It clearly was OSCAR-7. Jan's messages
pretty much speak for themselves. Pretty amazing story.
Jim
jim@coloradosatellite.com


My God, I can't believe what Pat was hearing. It has to be AMSAT-OSCAR-7
according to the frequency. AO-6 had a 70 cm beacon, which failed fairly
quickly after launch, and a 2m up/10m down transponder (the original Mode A
transponder). AO-8 had another Mode A transponder and the first Mode J
transponder built by the Japanese. That was, of course, backwards
from Mode B or 2m up/70 cm down. But, AO-7 had both a Mode A and a MODE B
transponder. Mode B had a downlink on 2m. So, of those three satellites,
AO-7 is the only one that had a downlink on 2m. Let me go out to the garage
and check the frequency.
--------- Time Passed Here.---------
Well, the garage files aren't what they once were. Most of the original
files were there but, the Karl Meizer - Mode B file is missing! Damn. Also
missing are the log books, which are historically valuable. The logs aren't
lost. I packed them away with my office stuff somewhere in my garage boxes
which second as a warehouse - so I wouldn't lose them. :-( But, I knew I
had lots of stuff that would tell the frequency plan. The first thing I
found as I was looking was an old ARRL booklet called, "Getting to Know
OSCAR from the Ground Up." I seem to have been a co-author. Hmmm. Don't
even remember it. The transponder had an uplink at 432.125 MHz to 432.175
MHz. The passband was inverting and a little less than 50 kHz wide. The
downlink passband was from 145.925 to 145.975 MHz. THE BEACON WAS AT
145.975 MHz. If I can find the log books I can tell you how far off the
nominal frequency the beacon was as measured back in November 1974 just
before launch. So Pat was hearing AO-7, 24 years after it died! Whew!!
Here's probably what's happening. That thing has a good set of arrays and
the first BCR (battery charge regulator) we ever flew. It's the first
spacecraft we ever had that was capable of overcharging the battery. When
the battery failed the cells began to fail short. One cell after another
failed and the voltage measured on telemetry began to drop. So, the cells
were clearly failing SHORT. Now, after all these years, what happens if any
one of the cells loses the short and becomes open? Then, the entire power
bus becomes unclamped from ground and the spacecraft loads begin to again be
powered but, this time only from the arrays. Now you have a daytime only
satellite but, each time the sun rises at the spacecraft you have a random
generator that either turns on Mode A or Mode B or whatever it wants. So,
occasionally that 70cm/2m transponder transmitter and beacon must least
work. From what you have told me (and without going back and decoding the
old telemetry equations) I can tell you that the following things work in
that spacecraft: The arrays, the BCR, the ISR (instrumentation switching
regulator), the Mode B transmitter and beacon injection circuitry, the Morse
Code telemetry encoder, and the voltage reference circuitry. The latter I
know is working because the last telemetry value is 651. The "6" is just the
row number of the telemetry value but the 51 means that the 1/2 volt
reference is measuring 0.51 volts. I know that telemetry equation by heart
since it was used as the calibration value for the rest of the telemetry
system. So the telemetry has a fair chance of being decoded and making some
sense!!! How about that, man?
Jim that's all amazing for someone who was as close to that thing as I was.
You must remember, that spacecraft was built in my house (in a basement
laboratory) in Lanham, Maryland. Werner and Karl were putting the finishing
touches on that transponder when Ian, my son was being born in the upstairs
bedroom. That afternoon Donna and I went to the hospital to have the baby
while Karl and Werner continued final debugging! So, it doesn't get much
more personal than that.
As the man said, "It's most remarkable." You can post this to the AMSAT-bb
if you want.
73's,
Jan W3GEY
AMSAT-OSCAR-7 Project Manager :-)




Well Jim,
G3IOR's telemetry frame is interesting. Apparently he did hear the AO-7 Mode
B beacon tonight.
I got out my December 1974 and looked up the telemetry equations for the
Morse Code Telemetry Encoder and what I found is in the attached
spreadsheet.
I'm blown away. Most of this stuff makes pretty good sense. In particular,
the temperatures make sense and I would have guessed that they would be the
most sold IF the reference voltage held (which it did). Interpreting some of
this for those who may not understand or don't remember, the telemetry says
the spacecraft was in Mode B; all the other beacons and Mode A were off. It
is possible that the thing had just turned on because the old 24 hour timer
just reset it to Mode B. The damn thing may think it is still on an every
other day cycle. The power output of the transponder is 1.16 watts which
may mean it is transmitting white noise plus beacon power. That seems about
right, but a little low as I recall. The instrumentation switching regulator
is in the middle of it's normal range and seems to be working fine. The
internal temperatures are around 15 deg. C; the external temperatures are
around 5 C and the transponder PA temp, which should be the warmest - IS -
it's 35.1 deg. C. The array current value is bust. I think maybe it always
was. Need to look for some old telemetry to confirm that. The array
current calibrations looks off. The array currents are in the normal range
but all four show current. This can't be. Only two at a time should show
current. Without a battery on line, this is entirely possible. The big
find is that the battery voltage telemetry shows a voltage of 13.9 volts.
Normal is 13.6 to 15.1 volts. So that would suggest the battery was normal
BUT, the 1/2 battery voltage is measuring only 5.8 volts. That can't be.
This imbalance probably means that the 5.8 volts is the correct value for
the lower half of the battery (which is a low value for that half, if the
cells were normal - they are probably not) and there is a break somewhere in
the upper 1/2 of the battery string. My guess is the indicated voltage is
really what the BCR is putting out with only the spacecraft load as a real
load and the battery string has an effective break (or a pretty high
resistance) somewhere in the upper half.
So, this old war horse of a spacecraft seems to have come back from the dead
if only for a few moments. And it is telling us, that even in a 1460 km
high orbit a cheap spacecraft built by a bunch of hams, without very many
high rel parts and without designing for a radiation dose like this, can
last for 27+ years in space as far as a majority of it's electronics is
concerned. Even the damn precision reference voltage regulator is still in
calibration! Pitty Pat did not recognize his old friend when he saw him
again.
Well Jim, you made my day!
73's,
Jan

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