AO-40 Attitude Control System Appears Functional

NEWINGTON, CT, Aug 17, 2001--The commissioning of the AO-40 satellite took another giant step forward this week as ground controllers reported success in testing the spacecraft's attitude control system--the momentum wheels. AO-40 controllers hope to use the momentum--or "reaction"--wheel attitude control system to aim its antennas and, eventually, its solar panels. The testing paves the way for possible deployment of the solar array and better signals on the ground.

"We can say with some caution that we have a working three-axis control system!!!" enthused AMSAT-DL President and AO-40 team member Peter Guelzow, DB2OS, in a posting to the AMSAT bulletin board. Until now, AO-40's attitude has been under "spin control."

Ground controllers have been using the onboard magnetorquing system--which consists of solenoid coils and makes use of Earth's magnetic field--to control AO-40's spin and orientation. Magnetorquing is most effective when Earth's magnetic field is strongest, however, so it typically takes place only at perigee--when the satellite is closest to the Earth. The three-axis control offered by the momentum wheels will mean much more positive attitude control from the ground anywhere in the satellite's orbit.

After preliminary tests to energize the momentum wheels earlier this week, ground controllers Stacey Mills, W4SM, and James Miller, G3RUH, ran them up to more than 100 RPM Aug 16 during orbit 365 and left them at this speed for about a half-hour. Telemetry before and after the spin-up, once the solar sensors re-locked, showed that the system appeared to be working properly. The momentum wheels are designed to run at a nominal operating speed of 1000 RPM.

"More tests will be done over the next few weeks, before we will transfer the spacecraft from spin stabilization into three-axis stabilization," Guelzow said.

The momentum wheel system in AO-40 is an experimental version developed by Phase 3D Project Leader Karl Meinzer, DJ4ZC, of the University of Marburg, who came up with a system using frictionless magnetic bearings with two active controlled axes and one passive axis. Working with Meinzer were Michael Scharfe, DK7UX, and Ralf Zimmermann, DL1FDT. AO-40's momentum wheel experience could result in technology to improve the reliability of three-axis-controlled spacecraft in the future.

Correct operation of the momentum wheels is considered essential to solar panel deployment. Earlier this year, Guelzow had said he would "be surprised" if the momentum wheels work okay after what he called "the possible destruction in the inner core of the spacecraft" resulting from an onboard malfunction last December involving the 400-newton propulsion system. He and ground controllers have said that the three-axis control system will be tested extensively before any decision is made to deploy the solar panels on AO-40.

AMSAT reported earlier this week that the 2.4 GHz "S1" transmitter aboard AO-40 suddently went silent August 13. There is no further word on efforts to recover the transmitter, which had been providing an improved AO-40 signal compared to the "S2" transponder, which continues to operate normally.

For more information on AO-40, visit the AMSAT-DL Web site, or the AMSAT-NA Web site.

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