North-facing satellites??

ard126k

Active Member
I'm sure this is a really stupid question but I've been thinking about the trouble some folks have where they don't have line-of sight to the South.

Why aren't there satellites over the North Pole? Couldn't they be geostationary there as well?

I'll understand if this isn't worth answering.

Alex.
 

fernandez

Distinguished Member
Most communication satellites orbit at or near the equator. In the northern hemisphere this is obviously to the south. In the southern hemisphere it is to the north, so the dishes would point north
 

pedro2000uk

Distinguished Member
I'm sure this is a really stupid question but I've been thinking about the trouble some folks have where they don't have line-of sight to the South.

Why aren't there satellites over the North Pole? Couldn't they be geostationary there as well?

I'll understand if this isn't worth answering.

Alex.

There is only one place that can support geostationary (gso free ride) orbits for satellites and that's over the equator at about 22000 miles on the Clarke Belt - due to the rotation of the Earth & it's gravity ... ........................
 
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grahamlthompson

In memoriam
There is only one place that can support geostationary (free ride) orbits for satellites and that's over the equator at about 22000 miles on the Clarke Belt - due to the rotation of the Earth & it's gravity ... it doesn't work anywhere else. Any closer (above the equator) & they fall to Earth- any further out & they shoot off into space and anywhere else requires power to stay in orbit. GSO satellites only need a bit of power to stay in position.

There are satellites to the north... & those that orbit N S and low orbit stuff etc.. but not DTH TV stuff.

Small inacuracy. There are many stable orbital heights, the whole point about the 22000 mls (22,236mls) is that the orbital speed to maintain that height exactly matches the earths rotation in that 1 orbit takes 1436 minutes. No power is required to maintain an orbit at any altitude provided it's above the atmosphere, this applies to any orbital plane. Power is required simply to maintain an accurate position due to small external pertubations (even Sunlight exerts a small force on the satellite). A short burn to either speed up or slow down a satellite results in a slightly higher or lower altitude so the satellite either moves forward in respect of the Earth or backwards until an equal and opposite burn restores the geosynchronous requirement of 3.07KM/sec. This is how they can be moved to a different orbital location using a minimal amount of the stored propellant. A satellite at the same velocity in a pole to pole orbit will still orbit in 1 sidereal day but of course the point on the Earths surface directly below it will change as the Earth rotates.
 

pedro2000uk

Distinguished Member
Small inacuracy. There .........


that's generous Graham - I wrote it and I can only blame a long day & too toooo much coffee.. (and no googling & wikying btw... 22263mls hmmm).. I think I know what I sent to may- I mean meant to say.. but most of that was [edit].... :laugh:
 

Tarzion

Well-known Member
The Aussies and the Kiwis point their dish towards the north rather than the south and maybe a few other countries which are on the other side of the Equator
 

grahamlthompson

In memoriam
The Aussies and the Kiwis point their dish towards the north rather than the south and maybe a few other countries which are on the other side of the Equator

Any country South of the Equator has to point a dish north (if close to the equator of course it will only be a tiny bit), the satellites are exactly above the equator, that's the only place where they will appear stationary from the Earth. A dish exactly on the equator will point East or West only unless it happens to be directly below the orbital location. For example a dish at 0 deg latitude (ie on the equator) and 28.2deg E longitude would point straight up to align on Astra 2.

If you are interested you can locate the bit of Africa directly under Astra 2 by using Google Earth and typing in the co-ordinates. No prizes for finding the nearest place to the exact spot
 
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Alex5

Active Member
I'm sure this is a really stupid question but I've been thinking about the trouble some folks have where they don't have line-of sight to the South.

Why aren't there satellites over the North Pole? Couldn't they be geostationary there as well?

I'll understand if this isn't worth answering.

Alex.

Well, the Earth rotates around it's own axis making a turn in 24 hours. Obviously, with the Earth rotating, everything on it's surface like a satellite dish rotates as well.

Any object put on the Earth's orbit like a broadcast satellite rotates around the Earth.

So, we have 2 moving objects: a dish and a satellite. We need to have both two moving in a synchronized way so that dish will always point to the satellite.

The only way to achieve it is to make the satellite orbiting around the Equator with a speed that matches the speed of Earth's rotation - this is done by putting it on a certain elevation. The satellite will then appear stationary to the dish, and this only and one orbit is called the Geostationary orbit.

Systems where no dishes pointing to the satellite are used to pick up signal allow other orbits to be used - example is the well known GPS.
 

ard126k

Active Member
Thanks to Graham for a nice clear explanation for non-astrophysicists like me! I just had this picture of a satellite floating above the pole, forgot they have to keep moving to stay up!
Alex.
 

grahamlthompson

In memoriam
Thanks to Graham for a nice clear explanation for non-astrophysicists like me! I just had this picture of a satellite floating above the pole, forgot they have to keep moving to stay up!
Alex.

Newtons laws apply equally to Sputnik 1 and the Moon both Earth orbiting satelittes. Same principle applies to the Earth orbiting the Sun :smashin:

Real thanks should go to the late and great Arthur C Clarke who first twigged if you stuck up a satellite at the right velocity directly along the equatorial plane in the same direction as the Earths rotation that it would stay directly over the same spot. Any astronomer will tell you that the Earth takes slightly less than 24hrs for 1 full rotation, they have to use a time system based on the real Earths rotation in order to locate and track celestial objects (sidereal time).
 
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Chris Muriel

Distinguished Member
It's also interesting to look at the elevation of satellite dishes in equatorial areas. They look weird, especially prime focus dishes - pointing close to vertical.

Chris Muriel, San Jose Airport , but in 24 hours time.....
 

Tight Git

Distinguished Member
It's also interesting to look at the elevation of satellite dishes in equatorial areas. They look weird, especially prime focus dishes - pointing close to vertical.
That's only for satellites in the arc immediately above.

For those satellites close to the horizon, the dishes in equatorial regions would look just like our Sky dishes!

Of course, a movable dish on the equator would be an "up and over" jobbie! :laugh:
 

Alan Mac

Well-known Member
Well, the Earth rotates around it's own axis making a turn in 24 hours.

The Earth’s rotation time (and consequently the geostationary orbit time) is 23 hours 56 minutes and 4 seconds (approximately!!).


Alan
 

bob1

Well-known Member

grahamlthompson

In memoriam
Easy answer then just move house:).

Agreed unlike in Blighty where it's chilly up north, in OZ it's chilly down South :D
 

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