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|Description:||A description of AO-7 that originally appeared in the AMSAT Newsletter of September 1974 and is reprinted by permission.|
|AO-7's Resurrection:||Jan King, W3GEY, shares his thoughts and feelings on AO-7's Resurrection|
|Timeline:||A Timeline of Events in the life of AO-7|
AMSAT-OSCAR 7 contains two basic experimental repeater packages, redundant command systems, two experimental telemetry systems, and a store-and-forward message storage unit. The spacecraft in solar powered, weighs 65 pounds, and has a three-year anticipated lifetime. It contains beacons on 29.50, 145.98, 435.10 and 2304.1 MHz.
Two types of communications repeaters are aboard the spacecraft, only one of which operates at a time. The first repeater is a higher power, two-watt version of the one-watt two-to-ten motor linear repeater that flow on the OSCAR 6 mission. This unit receives uplink signals between 145.85 and 145.95 MHz, and retransmits them between 29.4 mid 29.5 MHz an the downlink. A 200 milliwatt telemetry beacon provides telemetry data on 29.502 MHz.* Approximately -100 dBm is required at the repeater input terminals for an output of 1 watt. This corresponds to an eirp from the ground of 90 watts for a distance to the satellite of 2,000 miles and a polarization mismatch of 3 dB.
The second repeater, constructed by AMSAT Deutschland e.V., AMSAT's affiliate in Marbach, West Germany, is a 40-kHz* bandwidth linear repeater. It employs an 8-watt PEP power amplifier using the envelope elimination and restoration technique to maintain linear operation over a wide dynamic range with high efficiency. This repeater has an uplink from 432.125 to 432.175 MHz, and a downlink from 145.925 to 145.975 MHz. Since the uplink band in shared with the radiolocation service, an experimental pulse suppression circuit is incorporated in the repeater to reduce the effects of wideband pulsed radar interference in the uplink. Developmental versions of this repeater have flown in high-altitude balloon experiments in Germany, and aircraft flight tests of the repeater prototype unit. A 200 milliwatt telemetry beacon on 145.975' provides telemetry data. Approximately So W.*eirp is required to produce 3 watts of repeater output at a range of 2,000 miles assuming.& polarization mismatch of 3 db.
The two repeaters are operated alternately by means of a timer arrangement, but repeater selection and output power control can also be accomplished by ground command. Each of the repeaters includes a keyed telemetry beacon at the upper edge of the downlink passband to provide housekeeping data and to provide a frequency and amplitude reference marker to assist the amateur in antenna pointing, Doppler frequency compensation, and setting uplink power level. The cross-band 146-to-29.5 and 432-to-146 KNO design of the two repeaters will permit the amateur to monitor his own downlink signal easily, and consequently, he can adjust his power and frequency to continually compensate for changing path loss, repeater loading and Doppler shift.
Redundant command decoders of a design similar to the unit proven highly successful in OSCAR 6 will be flown. The decoder has provisions for 35 separate functions, and is designed to provide a reliable means of controlling the emissions of the repeaters, beacons and other experiments aboard the spacecraft.
AMSAT-OSCAR 7 contains two experimental telemetry systems designed
for use with simple ground terminal equipment. The first system,
developed by the WIA-Project Australis group in Australia, telemeters
60 parameters in 850-Hz shift, 60 WPM five-level Baudot teletype code
to permit printout on standard teletype equipment in a format readily
convertible for direct processing by small digital computer. The second
system telemeters 24 parameters as numbers in standard Morse code and
can be received with pencil and paper. This system was used on OSCAR 6
and proved highly successful as a reliable means of obtaining real-time
telemetry data. An experimental Morse code message storage unit,
Codestore, capable of storing and repeatedly retransmitting 18-word
More& code messages loaded by ground stations in also aboard
AMSAT-OSCAR 7. This unit was first flown on OSCAR 6. The teletype
telemetry encoder amplitude-modulates telemetry beacons on 29.50 MHz
(200 mw), 145.98 MHz (200 mw) and frequency-shift keys the beacon on
435.10 MHz (300-400 mw), as selected by ground command. The Morse code
telemetry encoder and Codestore message storage unit directly key these
beacons as selected by ground command.
"I am certain what has happened (and I know why): The battery did fail short. Virtually all of the cells failed in a shorted mode eventually. This shorted condition placed a shunt across the solar arrays and prevented current from going to the satellite loads (i.e. the transponders, in particular). Some time before G3IOR reported hearing the spacecraft again the short on one of the cells (1 out of 10) failed a second time. This time, it went from short to "open." When it went open this released the shunt that was pulling the array voltage down and allowed the current to pass to the satellite loads.
Since there is no battery at all now (because one cell went open),
when AO-7 goes into eclipse with each orbit, the satellite has no power
to operate and shuts down in the dark. The AO-7 satellite system
predates microprocessors so, in those days we used a logic system to
control the operation of the satellite system. Some of the features of
that logic system (called the Experiment Control Logic or ECL) seem to
be working and some not.
One feature that doesn't seem to work correctly without the battery is a thing called the initial condition generator (ICG). This circuit is supposed to put the satellite, if I recall correctly, into Mode D with one of the beacons going after separation from the launch vehicle. When the satellite comes out of eclipse without a battery, it should think it is being released from the launcher again. Instead, the satellite is randomly reset as it goes into eclipse and comes up in a random mode as has been observed. We seem to have one of two command decoders working and the morse code telemetry system is working very nicely.
The telemetry is very interesting and it was totally scarry to decode the first block of that stuff last year. It was like seeing a ghost for me. You have no idea how emotionally this effected me. Seems strange but, I lived with that thing in the same way someone else would live with a child. I was 27 years old then and my first child was born the same year as the launch. I'm 56 now. So, to have it "tell me" what it was doing after being asleep for all those years was very strange indeed.
It appears that the 24 hour clock that would cycle the transponders
between modes A and B is working but, it gets reset every time the
satellite goes into eclipse. We could control the mode reasonably well
if we could catch the satellite as it comes out of eclipse each time
and command it to the desired mode.
There is another OLD problem though, which works against keeping it
under control. We always had great difficulty commanding the satellite
while it is in Mode B. The receiver is being interfered with by the
transmitter in the transponder. This was never a problem when we had a
battery as the 24 hour timer always brought it back to Mode A after 1
day and we had control again. So, unless you have mega-power at the
command station you can't do much with it while it is in Mode B or C. I
also don't know if it is being regularly commanded. I doubt it. If you
can get people interested in controlling it, then we could make
arrangements to show them how to command it.
So, it probably will be mostly a random satellite, however, when it is in Mode A we could send quite a few interesting commands that would allow users to investigate various equipment on board. One thing that hasn't been played with is an on-board data system called Code Store. It was an early fore-runner to packet radio. I don't think anyone knows if that still works but, we do have a box in the AMSAT Museum that can upload data to that experiment.
People should realize that the solar arrays are old and they don't
put out more than a few watts now. That's the only power available to
run the transponders. So, if they uplink too much power it will just
cause the transmitter to sort of "cave in." The voltage on the
satellite bus begins to sag badly when a heavy demand is placed on the
transponder (by a large uplinking signal) since there is nothing there
to regulate it and then the oscillators in the receiver and transmitter
chain start changing frequency. People call this "FMing" and that is
what is going on.
AO-7 was a major chapter of my life! This whole experience of having it come back is pretty strange stuff to all of aerospace, not just amateur radio.
- Jan King, W3GEY, AO-7 Project Manager
ALL AO-7 INFORMATIONS ARE FROM AMSAT WEB