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. 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?
Hi, 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!
73 Auke ----- Original Message ----- From: "Daniel Schultz" n8fgv@usa.net To: amsat-bb@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. 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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Auke de Jong, VE6PWN -
Daniel Schultz