AO-40 Transponders Back on the Air!

Discussion in 'Amateur Radio News' started by Guest, Jul 20, 2001.

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  1. Guest

    Guest Guest

    From the ARRL...




    NEWINGTON, CT, Jul 19, 2001--AO-40's transponders are back on the air, following an orbital shift that put the Amateur Radio satellite into an orbit that AMSAT says should be good for many years to come. Ground controller Stacey Mills, W4SM, announced late yesterday that the transponders had been switched back on, with 435 MHz and 1.2 GHz uplinks and a downlink in the 2.4 GHz "S band."



    The transponders have been off since late May, when preparations began to shift AO-40's orbit at perigee. That operation was completed earlier this month, and ground controllers have been readjusting the spacecraft's attitude since then.



    Mills said the transponders would operate from orbital positions MA 10 through MA 99. Uplink frequencies (without taking Doppler into account) are 435.495-435.780 MHz and 1269.211-1269.496 MHz, and the downlink passband is 2401.210-2401.495 MHz. The transponders are inverting, so a downward change in uplink frequency results in an upward frequency shift in the downlink.



    "The current ALON/ALAT [spacecraft attitude, now determined to be 325.5/6.2] will give some very low squint angles, especially in the northern hemisphere, with relatively low ranges during this first half of the orbit," Mills said in a posting to the AMSAT bulletin board. "So signals should be excellent!"



    Mills emphasized that earthbound ops should not use any more uplink power than necessary and should never be any stronger than the middle beacon.



    AMSAT Awards Manager Bruce Paige, KK5DO, in Houston, was among the first stations to get on AO-40 after the transponders were reactivated. In addition to some domestic contacts, he and his daughter, Mahana, W5BTS, worked EA8/DJ9PC in the Canary Islands.



    "It sounds awesome," Paige said. "I am transmitting with 25 watts up, and it sounds great!"




    Michael Mims, K4IZN, says he's on AO-40 with a discarded Primestar TV satellite dish and a "bean can" feed horn. His downconverter is a modified Drake 2880 with no preamplifier. "This is going to be a good bird!" he said.




    Although AO-40's attitude still is not optimal at this point, ground controllers had to suspend magnetorquing operations to adjust it after an onboard sensor lost its view of the sun. Without data from the sun sensor, ground controllers cannot be certain of the satellite's attitude.



    The onboard magnetorquing system--which consists of solenoid coils--makes use of Earth's magnetic field to control the spacecraft's spin and orientation. Magnetorquing is most effective when Earth's magnetic field is strongest, so it typically only takes place at perigee--the point nearest to Earth.



    Mills said now that the ground team has "a very good fix" on the spacecraft, they'll do nothing to change its attitude for several weeks, while the solar angle decreases. Once the sensor regains its view of the sun, efforts to adjust the spacecraft's attitude will resume, so that AO-40's antennas are pointing toward Earth.



    Mills said ground controllers will use the interim period to see if they can re-calculate the so-called "mystery effect" that had been impacting AO-40 at perigee under its former orbit. Mills has indicated that the old orbit, while stable, was too close to Earth for comfort and that the satellite might have been suffering "considerable drag" when it was near Earth. The lower perigee also made magnetorquing difficult.



    Earlier this week, ground controllers used an onboard camera to take images to confirm the satellite's attitude and position.



    In his posting, Mills reminded satellite enthusiasts that there's still a lot of work remaining to commission AO-40, and some systems have not yet been tested at all. "In particular, we may have some days of limited or no transponder activity, and even no beacon activity, while the RUDAK team uploads software and checks out some of the RUDAK functions," he said. These events will be announced, he added.



    The RUDAK--the German acronym stands for "Regenerative Transponder for Amateur Radio Communication"--is a digital transponder system that can be programmed to perform a variety of functions.



    For more information on AO-40, visit the AMSAT-DL Web site, or the AMSAT-NA Web site. AMSAT-DL now offers an AO-40 "Quick Status" page that shows which systems are operational, partially failed, being commissioned, failed, not yet tested or still needing additional testing.
     
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