24 Hr Orbits

[Zatnikitelman]

I was wondering, since the Earth makes 1 full rotation in 24 hrs that means that a satellite appears in the same location at the same time every day right? So if I have an inclined orbit with a period of 24 hrs, and a periapsis that is initially over let's say Atlanta, will the Periapsis always remain over Atlanta? Or will this move around the planet? I know it will wobble atleast a little due to non spherical gravity, moon, sun etc.
Thanks,
Zat

 

[BlueDragon8144]

true, because after one orbit, the earth's orientation is the same as it was one orbit ago, since both rotated at the same speed. Normally, it would shift over time, but that's because of the Earth's rotation, which takes longer to rotate than for the spacecraft to orbit.

 

[tomek]

Well, the periapsis doesn't really stay over Atlanta (it stays stationary with respect to center of the Earth, while Atlanta moves with Earth rotation). But every time you are at the periapsis, you'll be over Atlanta.

 

[BlueDragon8144]

well, yeah, thats what I meant, but since you can't really visually see where the periapsis is over till you are there, I was kinda refering to when the spacecraft is at the periapsis and not where it is at all times, because frankly, it really doesn't matter where it is till you are at it.

 

[tomek]

I'm pointing that out to avoid confusion, not just to be a smartass (though that too of course). It's just that someone without clear understanding of this stuff might read "periapsis always stays over Atlanta" and get a picture of an orbit rotating along with the planet.

 

[Andy44]

I was wondering, since the Earth makes 1 full rotation in 24 hrs that means that a satellite appears in the same location at the same time every day right?

No, only if you are in a "synchronous orbit", which has a period which is a multiple of 24 hrs.

So if I have an inclined orbit with a period of 24 hrs, and a periapsis that is initially over let's say Atlanta, will the Periapsis always remain over Atlanta? Or will this move around the planet? I know it will wobble atleast a little due to non spherical gravity, moon, sun etc.
Thanks,
Zat

The answer, in this case, is almost yes, since 24 hours is obviously a multiple of itself.

I say almost, because the periapsis will only remain stationary in 3-space if you have disabled non-spherical Earth effects modeling. If non-shperical effects are being modeled, the argument of periapsis and the right ascension of the ascending node will both rotate with time. I don't have them in front of me, but there are simple equations to approximately calculate both of these.

 

[agentgonzo]

The Earth rotates 360° about its axis every approx 23h56'. This is know as a Sidereal Day. 24 hours is how long it takes to rotate 360° *plus* the extra rotation needed because the Earth has moved a bit further around its orbit of the Sun. So you'd need an orbit of 23h56' for your satellite

See http://en.wikipedia.org/wiki/Sidereal_day

 

[Mission_CDR]

wouldn't need an inclination of 0 degrees to stay over one spot?

 

[jarmonik]

I was wondering, since the Earth makes 1 full rotation in 24 hrs that means that a satellite appears in the same location at the same time every day right? So if I have an inclined orbit with a period of 24 hrs, and a periapsis that is initially over let's say Atlanta, will the Periapsis always remain over Atlanta? Or will this move around the planet? I know it will wobble atleast a little due to non spherical gravity, moon, sun etc.
Thanks,
Zat

Nope, the periapis wouldn't stay over Atlanta due to rotation of orbit apsides. The periapis could easily drift about 1deg per day.

The drift can be calculated with an equation (8) from Doc/Technotes/gravity.pdf

 

[tomek]

wouldn't need an inclination of 0 degrees to stay over one spot?

That's geostationary, a special case of geosynchronous. It has 0 inclination, 0 eccentricity and ~24 hours orbital period. In such orbit, a spacecraft always stays above the same spot.

If orbit is geosynchronous, but not geostationary, then a spacecraft doesn't stay above one spot, but every time it flies through a specific point on such orbit, it is above the same spot on Earth.

 

[Zatnikitelman]

Thanks guys! I was dreaming of a weather satellite constellation that could keep tabs on my area continuously and so thought of maybe 6 or 8 sats in the same orbit with period=86,400s, but wasn't sure if that would work.
Thanks!