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Launch Pad Return

Date: Sat, 22 Jun 2002 16:00:08 -0600
To: amsat-bb@AMSAT.Org, bod@AMSAT.Org
From: Jim White <jim@coloradosatellite.com>
Subject: [amsat-bb] 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. Jimjim@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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