Orbit question

"KFH" kirjoitti viestissä
...
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

(KFH) wrote in message ...
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 is something called a sun-synchronous orbit. You only get one
pass over your target area per day, but it happens at the same time
each day. You end up with a pretty elliptical orbit at around 97
degrees inclination. A completely-LEO orbit (mostly circular) with a
continuous grounnd track is only possible with an orbit directly over
the equator. Otherwise, the rotation of the earth and nodal
regression will throw you off.