[amsat-bb] Re: can RF change the orientation of a satellite?

Daniel Schultz n8fgv at usa.net
Sun Oct 24 18:15:04 PDT 2010

It makes sense. Satellites are subject to solar radiation pressure from
absorption of sunlight, they are also subject to radiation pressure from
emission of RF radiation. The wavelength of the radiation does not enter into
this calculation so the pressure from radio frequency radiation is the same as
that of visible light radiation.

Radiation pressure is the electromagnetic power flux density divided by the
speed of light. If a 100 watt transponder emits RF in a narrow beam from a 1
square meter antenna, the radiation pressure on the satellite will be 100
watts / (1 square meter * 3E8 meters/sec) = 0.33 micropascals. The force on a
1 square meter antenna will thus be 0.33 micronewtons. This is not the same as
ion propulsion because no physical mass is expelled from the satellite.

For amateur satellites with omnidirectional antennas, the RF is emitted in all
directions and the force cancels out. Intelsat birds emit RF in tightly
focused beams so there will be a net force on the satellite, and if the
antenna is off axis from the satellite's center of gravity this force will
exert a torque which could cause the momentum wheels to spin up.

The uplink RF is minuscule (microwatts per square meter or less) but the lack
of uplink signals causes the linear transponders to have zero RF output (other
than noise), thus causing reduced off axis pressure on the satellite.

Dan Schultz N8FGV

>I couldn't help but notice that on this website
>http://www.intelsat.com/resources/galaxy-15/faqs.asp about "zombiesat"
>Galaxy 15, Intelsat figures that the lack of RF on thr transponder package
>since their loss of control earlier this year, will actually make it take
>longer to saturate it's momentum-wheel orientation stablizer system.  This,
>compared to the manufacturer's original estimate of achieving the saturation
>condition beuing several months sooner having assumed normal full-load of RF
>on the transponders.

>My question is: How is this possible?  Does the RF put acceleration-forces
>on the transmitting antenna of the satellite similar to ion propulsion, or
>does the uplink RF push on the receiving antenna?  Is there some other
>mechanism that can electronically alter the forces acting on the body of the
>satellite based on the amount of RF power?

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