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

Auke de Jong, VE6PWN sparkycivic at shaw.ca
Sun Oct 24 21:26:52 PDT 2010

Thanks for the informative reply!

So it follows that this class of satellite with a maximum RF power output 
of, say 3000W with all transponders fully loaded, that would add up to 
significant forces!  I'm also fairly certain that the primary transmitting 
reflector hangs from the side of these satellites, opposite the receiving 
reflector, so this is making sense.  Until now, it was the last factor I 
would have expected for prolonging the saga of Zombiesat!

Maybe they could  send up some carriers at the passband edges of each 
transponder while in-between orbital slots just to hasten the progress of 
the saturation of the momentum-wheels?(just kidding)   I'll be reading with 
great interest, any news of how Galaxy 15 behaves after it has finally run 
it's batteries down to cutoff, and if it can show what the fault actually 
was, upon possible resumption of telemetry.
There could be a lesson in this for Amateur satellite designers and 
builders, where fault-tolerance is definitely JUST AS important as 
fault-avoidance - we always like our hardware to be useful to the last 
drop(AO-7).  Actually, Oscar 7 has now spent the bulk of her wakeful time in 
a FAULTY state, while still very useful... legendary!

----- Original Message ----- 
From: "Daniel Schultz" <n8fgv at usa.net>
To: <amsat-bb at amsat.org>
Sent: Sunday, October 24, 2010 7:15 PM
Subject: [amsat-bb] Re: can RF change the orientation of a satellite?

> 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. 
>>compared to the manufacturer's original estimate of achieving the 
>>condition beuing several months sooner having assumed normal full-load of 
>>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 
>>satellite based on the amount of RF power?
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