Hi All, Well, it took me about 4 hours last night, but I finally reduced all the Superbird A observational data from February 25 to December 10th, 2002. Observers were: Frikkie van Zyl, Ron Lee, Ed Cannon, Don Gardner, Leo Barhorst, Bjorn Gimle, Mike McCants and Tom Wagner. I have not computed an updated spin axis yet -- will do so later tonite. One comment about the non-"Main Event" flashes that have been observed by Don Gardner and perhaps others -- this behavior has been seen in years past, and when the surface normals are plotted it is quite clear that it cannot be a simple reflection from a different surface on Superbird A. (The surface normals do not lie on a curve that is concentric with that of the main flashes.) What this means is that the extra flashes result from a double reflection -- probably a combination of a solar wing and the body of the satellite. And this is a good segue into a related topic -- the source of the main flashes themselves. We (or at least I) have assumed all along that these bright glints could only be coming from the very large solar wings on Superbird A. And due to the the fact that the flash brightness is more or less symmetric about the phase shift, and that solar panels have very different optical properties on their front and back sides, the two solar wings on Superbird must be oriented in nearly anti-parallel directions. (From an operations standpoint, this seemed like a reasonable emergency orientation for the solar wings: it ensures that one wing is always sunlit when the satellite is not in eclipse.) Due to a slight misalignment between the two wings, the flashes for one wing start and end a minute or two before the other. This neatly explains what is observed from the ground: flashes starting with a 22.6-second period, transitioning to an 11.3-second period (alternating flashes from each wing), and ending with a 22.6-second period. It also explains why the satellite is spinning up -- differential torque from the arrays due to the unsymmetrical arrangement. There's just one problem: this isn't what's happening! A few months ago while attending a space mission analysis class, I had an opportunity to talk with some other attendees involved in comsat operations. I asked them if it was standard practice to point solar wings in opposite directions when pointing and control for a satellite is temporarily lost. They said this is NEVER done. Solar wings are the primary source of torque on a satellite, and thus symmetry must be maintained. Due to the very different reflective properties of the front and back sides of solar wings, they must point in the same direction to maintain a torque balance. So what does this tell us about the flashes we see from Superbird A? Simply that they cannot be coming from the solar wings. Specular glints can only be produced by the front side of the arrays. So what we've been seeing all along are actually glints from the satellite body! At first this might not seem possible for a satellite at a range of some 38000 km or more. But when you do the math, it's not at all unreasonable. Compare to Iridium for instance. Typical range for a bright Iridium flare is from 900 to 1000 km. Call it 1000 to simplify things, and assume a visual magnitude of -8. Now if you kept everything the same, and just moved an Iridium satellite to a range of 38000 km, the brightness would fall by a factor of 38 squared (1444) which is 7.9 visual magnitudes. That drops your glint magnitude to roughly 0. The flashes from Superbird can be as bright as about +2, though usually +3 is more typical. +2 is a factor of 6.3 dimmer, so you see it really takes very little surface area on a specular surface to produce a +2 or +3 glint from GEO. As it turns out, the sides of the FS-1300 Bus built by Loral Space Systems have a decent specular component based on the images I've seen and are sufficiently large to produce glints of +2. These are the two sides from which the solar wings extend. The other sides do not look favorable for producing glints. All that said, the mystery is: when *DO* the solar wing glints occur? I suppose it's possible that the wings are oriented with their surface normals close to parallel with the spin axis (i.e. very small cone angle). And this cone angle may produce reflections that only rarely intersect the earth (if at all). For curiosity's sake, I decided to take a look at the impact of changing the coning angle on the predicted flash time for central Texas. Using my old axis and precession rate (probably good to within 10 minutes) here's what my software computes for December 12th UT: 89.95 deg (main glint): 03:50 UT 89 deg: 03:57 88 deg: 04:05 85 deg: 04:29 80 deg: 05:08 75 deg: 05:48 70 deg: 06:27 60 deg: 07:46 50 deg: 09:06 40 deg: 10:27 30 deg: 11:51 The good news right now for North America is that a wide range of coning angles can be investigated during our long winter nights. I should point out that this only addresses one half of the coning solution -- cone angles can be greater than 90 degrees (to cover the anti-pole case). These of course will occur earlier than the 03:50 time indicated above. The east coast is in a better position to evaluate the early solutions due to their earlier sunset. By having different observers sign up to keep "glint watch" for times corresponding to different coning angles, it wouldn't take a handful of observers too many nights to cover all the angles. Looks clear here tonite, so I may take a little time to cross off a range of angles. Cheers, 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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