Hi Paul, Ed and Björn, Paul wrote: > ... more specifically on 09sep I had: > at 53 flashes 20.8047 > at 102 flashes 20.8042 > at 181 flashes 20.8040 > at 205 flashes 20.8041 > to me this shows that I maxed out somewhere between > 53 and 102 meaning the overall uncertainty is somewhere > between 0.05 and 0.1 s, and PPAS seems happy with 0.1 s > & 0.001 s resolution which I seem to be getting as early > as 100 flashes. We're actually in a regime with these GEO flashers where it's possible to achieve accuracies higher than PPAS is probably interested in. However, as you've discovered you can start to do some interesting things when measurement accuracy is high -- in particular, study the impact of spacecraft eclipses on spin rate. Paul mentioned this: > ...but also wondering if the biggest shift occurs during > shadow or immediately after... eg it entered shadow at > 20.815 s and emerged closer to 20.804 s rather than > a constant drift (spin up) over 24 hours?? Since the sun is the driver for pretty much all dead GEO spin-ups, significant changes can be expected during shadow time. In other words, while the acceleration (or deceleration) may look fairly linear when analyzed at low temporal resolution (days), at high resolution (hours) you can see direct evidence of the sun being the driving force. The radiometer effect is a complex one involving optics, spacecraft materials properties (absorption, reflection, emission), geometry, and overall spacecraft structure. When the spacecraft transitions to darkness, infrared emission effects dominate -- high emissivity surfaces (e.g. the "blacker" parts of the spacecraft) emit more IR photons than the reflective parts. If these parts are not distributed in a symmetric fashion, then a net torque will result -- one which can either accelerate or decelerate any already existing spin. Evidently in the case of Cosmos 2282, thermal emission effects are contributors to the spin up. When the satellite is sunlit, you still have thermal emission effects, but you also have visible photon absorption and reflection to add to the equation. (A reflecting photon imparts twice as much force as one that is absorbed or emitted.) Since the solar arrays are furthest from the axis of rotation (and usually represent the majority of the surface area), photon interactions with them will be the dominant source of torque on the spacecraft. It would help to know more information about the solar arrays on Cosmos 2282 (front- and back-side reflectivity/emissivity), but more importantly any non-symmetric features. A symmetric solar array arrangement should not produce any torque since the contributions of each array would be cancelled by the other (I'm assuming there are two). If there are two arrays and they are mirror images of each other (same size, same reflectivity and emissivity, same distance from the axis of rotation, and parallel to one another), then you're left with asymmetries in the spacecraft itself as being the source of torque. My suggestion is to get split times every half-hour -- especially around the times of shadow entry and exit. This will probably show a stepping stair effect on the satellite spin-rate, with much of the increase occurring during eclipse. As we approach autumnal equinox, the satellite is going to be spending more and more time in eclipse each night, so we're in the midst of the greatest acceleration at this time of year (and again in March). Best, Rob ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www.satellite.eu.org/seesat/seesatindex.html
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