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   From: no-forename.jarvi@icon.fi   
      
   "KFH"  kirjoitti viestissä   
   news:NOSPAM-3105041954370001@66-44-113-194.s448.apx3.lnhdc.md.dialup.rcn.com...   
   > I have been thinking about a concept for a remote sensing satellite where   
   > the satellite in LEO would be in an orbit that always passed over the same   
   > ground track every orbit.  The purpose would be for multiple daily   
   > observations of one specific area.   
   > I have been told that such an orbit is not stable and without using a lot   
   > of fuel it would drift off of the ground track.  Is that true?  Or can   
   > there be a stable orbit of this nature?   
   > Thanks   
   > Fred   
      
      
   There are at least following different solutions:   
   1. An orbit along the equator flies every time over all places on equator.   
   2. An orbit through the poles flies every round over both poles (can see   
   only the poles every time).   
   3  An orbit with flight time 23 hours 56 minutes 4.1 seconds flies every   
   time over all same places.   
   4. Any orbit having flight time 23 hours 56 min. 4.1 sec multiplied by n,   
   where n is an even number 1, 2, 3, etc.   
   5. Any orbit having orbit time 23 h 56min 4.1 s divided by 2, 3, 4, etc.   
   6. Any orbit having orbit time 23 h 56min 4.1 s multiplied or divided by   
   2/3, 3/4, 2/5, 3/5 4/5 etc. However these orbits do not every time fly over   
   the same places.   
      
   The orbit stays in relation to stars. The earth rotates once in a sidereal   
   (astral) day,  23h 56min 4.1s.  So any orbit time that fits exactly with   
   this or its even multitude will make the satellite to pass over the same   
   ground track. every day 3min 5,9s earlier than previous day.   
      
   Antti Järvi   
      
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