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![Previous (earlier) Story](/images/sty-down.gif "MARS Again Letting US Troops "Phone Home""){kind=small} Previous: MARS Again Letting US Troops "Phone Home" |  Next: FCC Chastises Feuding 75-Meter Occupants |
 AO-40 ground controller Stacey Mills, W4SM. [ARRL Photo] |
NEWINGTON, CT, Nov 29, 2001--Necessary adjustments to AO-40's attitude to compensate for unfavorable sun angles over the next several months will silence the satellite's transponders for a while. Recent reports indicate that AO-40 continues to operate well, providing coverage between many parts of the world.
Command station team member Stacey Mills, W4SM, says, however, that he's puzzled and disappointed by the relatively low numbers of users on AO-40 during the last month or so. "I'm not sure why," he told ARRL this week. "This is prime time, and we won't have such good conditions much longer until the three-month 'bad angle' cycle passes."
A scheduled attitude shift to compensate for the unfavorable sun angle will leave AO-40's antennas pointing away from Earth until next spring and lead to a transponder shutdown period that could start as soon as late December.
The satellite is currently in a long period during which Earth eclipses the sun near perigee--its point closest to Earth. These periods, which began in late August, will rapidly increase in length and continue well into next June. The satellite relies on solar panels for its power.
Mills said that testing and development continue on AO-40's three-axis control system, to account for significant changes in the final orbit, the so-called "mystery effect" and the loss of some sensors. But he said that three-axis control would not be ready in time to avoid the unfavorable solar-angle season, so AO-40 will remain in spin mode, with attitude controlled by onboard magnetorquers. The onboard magnetorquing system--which consists of solenoid coils--makes use of Earth's magnetic field to control the spacecraft's spin and orientation.
"We will stay in spin mode for at least a few more months," Mills said. "Within a few weeks, we will have to change ALAT (AO-40's attitude with respect to Earth) dramatically, probably to about -50 degrees, to allow the sun to pass us by for about three months," he explained. The high "squint angle" (the satellite's antennas' aperture with respect to Earth) will render the S2 transmitter ineffective for transponder use, and the passbands will be shut off temporarily.
Mills estimated that ground controllers may need to start shifting the satellite's attitude starting sometime just before Christmas. He didn't expect a favorable sun angle that would again allow pointing AO-40 directly toward Earth (ALON/ALAT 0/0) until mid-April. "It's possible that we can leave the transponders on during the first part of the move and turn them back on slightly before April 15 as we start back toward 0/0," Mills said, "but you can figure that things will be sub-optimal from about Christmas until April 15."
During the transponder shutdown period, Mills pointed out, telemetry also will be harder to come by. He urged AO-40 telemetry gatherers to be as active as possible during the transponder downtime.
AO-40 Marks First Year in Orbit
 One year ago: |
The next-generation AO-40 satellite marked its first year in orbit during November. The former "Phase 3D" satellite was launched November 16, 2000, aboard an Ariane 5 launcher from the European Spaceport in Kourou, French Guiana.
Transponder operation began May 5, 2001, when the satellite's 435-MHz (U band) and 1.2-GHz (L band) uplinks were connected to the 2.4-GHz (S2 band) downlink.
AO-40 is the largest Amateur Radio satellite ever put into space.
S1 Transmitter Likely Dead
Mills reported earlier this month that ground controllers now assume that AO-40's S1 transmitter is beyond recovery. The S1 transmitter quit abruptly in August, but before going down, it had produced excellent results, with many stations reporting much stronger downlink signals than via the S2 transmitter.
A short program was uploaded to cycle power to S1 transmitter on and off. "The purpose of this test was to determine if the controller circuit to the S1 transmitter--or the S1 transmitter itself--had an intermittent problem relating to power up," Mills said in a posting to the AMSAT Web site. "This program turned the power on and off each second."
Mills said that during the "on" cycle, the routine checked to see if the S1 transmitter was actually drawing any current. But after running the program for approximately 45 minutes--the equivalent of cycling the transmitter on and off 1350 times--the S1 transmitter never drew current.
Mills said that although the S1 transmitter is considered dead, ground control stations will repeat the power-cycling test from time to time "on the remote chance that an intermittent problem has corrected itself." Plans called for performing the same test on the solid-state component of AO-40's X-band transmitter, which also has failed.
Ground controllers also have done additional testing on the 2-meter (V band) transmitter, which had provided the initial beacon for AO-40 following its launch but quit after the nearly catastrophic December 2000 incident that silenced AO-40 for about two weeks and apparently damaged or destroyed some onboard systems.
The AO-40 team has determined that the 2-meter transmitter draws power and warms up, but no output signal has been detected. "Additional testing will be done, listening with EME class stations," Mills said. He conceded, however, that while such experiments may provide evidence about what's wrong with the V-band transmitter, "ultimately it also appears lost."
The current
AO-40 transponder operating schedule and more information are available via the
AMSAT Web site.
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