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In a discussion about some users not recovering as rapidly from Hughes storm-related outages, the following was posted at BroadbandReports.com and I asked for permission to repost here because others might find it interesting: ---------------------- Geosynchronous satellites are not truly "fixed". They're racing along at a speed to match that of a wobbling earth below them. It's this wobble on the earth's axis that - from below - makes it look like the satellite is doing a figure 8 inside a little box. Military and commercial earth station have auto-tracking capabilities that usually make this apparent movement a non-issue. But for those of us with fixed dishes, this "Center Of Box" phenomenon must be taken into account.
When you use your keyboard to input antenna pointing data, the Direcway software generates a "MetersToSatellite" value; simple Pythagorean product derived from the locations of your house, the satellite, and the equator. "MTS" is then used to calculate "EigthMicroSecondsToSatellite". These values are then used in the ranging algorithm used to obtain sync with network timing. It's daisy chain calculating, so the GIGO factor is critical.
The effects of incorrect pointing angles - particularly those affecting transmitter isolation - have already been discussed. For the sake of argument, let's assume you have optimized your antenna pointing angles, and used precise Lat/Lon coordinates to input your geoloco. That very straight Pythagorean line you drew, is actually pointing at a "moving" target. This is where understanding COB comes into play. If you're imprecisely pointed at a "Corner of Box" - and/or range your transmitter when the satellite is OTHER than at COB - subsequent network timing sync will be tenuous, and difficult to reacquire.
Twice a day, the wobble of the earth puts the satellite smack in the middle of its box. THIS is the ideal time to adjust the antenna for optimum transmitter isolation, but from an installer's standpoint - totally impractical. Next best thing is for the customer force-range at Center of Box time. As this is a known phenomenon, each satellite provider provides COB predictions: http://www.satmex.com/english/clientes/centros_caja.php http://www.ses-americom.com/cgi-bin/cob4.pl?AMC-3 http://www.panamsat.com/global_network/center_of_box.asp Determine your OWN center of box time, and force-range accordingly. From that point, the other end of your Pythagorean line will stay much closer to the actual location of your moving target. The resultant network timing sync pulse will necessarily be stronger, and easier to reacquire after disruptions. ------------------------ I'm on G-11, which, according to that Panamsat link, doesn't wobble much. In looking at the links, I see that G4R will get a positioning maneuver on Thursday, so that could throw people off a bit. The optimum pointing times for it now are at 8:36 am and pm, EDT. The SatMex5 tables appear hard to follow, and end two days ago in any case.
quote:Originally posted by Sean Franklin: Don, I think that a glossary definition of COB pointed to this topic (or with the text reproduced there) would be warranted.
Perhaps. I wonder how many non-geeks are able to figure out GIGO?
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What does this mean? "Determine your OWN center of box time," does this depend on my lat/lon? And how does the time of closest approach fit in, as it seems to be the same as COB. Do we need that value at all? John
quote:Originally posted by John Watson: What does this mean? "Determine your OWN center of box time," John
I think that meant to look at your own satellite.
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Originally posted by John Watson: What does this mean? "Determine your OWN center of box time," does this depend on my lat/lon?
It does not depend on Lat/Lon, as I read it. I agree with Blue Roads that he means for the satellite you are on.
And how does the time of closest approach fit in, as it seems to be the same as COB. Do we need that value at all?
I'm not sure COB and closest approach have to be the same thing. If the wobble is parallel to the earth's surface, COB would be the same as closest approach. If the wobble includes any perpendicular component, the COB would be farther away than the closest approach. The very fact that it is referred to as a Box rather than a square leads me to think the wobble is 3-dimensional.
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