At 03:37 PM 28/02/2009, Darwin Teague wrote: >I did some predictions of Iran's Omid satellite and noticed >something that I thought was odd. >It's closest approach is one minute later for 4 consecutive nights. >Is this just a coincidence, > a common occurence or is it in some special orbit that allows it > to do that? > >3-22 19:55 132-26 >3-23 19:56 135-57 >3-24 19:57 319-63 >3-25 19:58 323-31 > The orbit height is deceasing through air drag and hence the period is decreasing gradually. TLEs show the revoulotions/day relative to the satellite perigee, which unless the inclination is close to 63.6 or its complement 116.4 is a moving target. This is technically called the anomalistic period. The nodal period or the time it takes between equator crossing is the one that describes best the time between closest approaches. The nodal period of OMID is close to 90 minutes at the moment so its close approaches are nearly at same time of day. In OMID;s case its accidental. Because the shape of the Earth causes the node of the orbit to precess, the passes seen on different days are further to the west unless the inclation is 90 degrees. Tony Beresford ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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