[amsat-bb] Re: Langrangian points

[Tony Langdon]

At 09:09 AM 1/14/2007, G0MRF at aol.com wrote:

>At a first 'pass' it would appear that maintaining a satellite at the  L1 and
>L2 points would require some station keeping, hence fuel. They would give
>excellent coverage from 56,000km / 200,000km  respectively.  Tracking the one
>held between the sun and earth would  give really simple tracking. Start by
>pointing at the sun and then move around a  little to peak the signals. - Of
>course the disadvantage is that instead of  cold sky you would have 
>all the Sun
>noise.  The L2 point is only visible at  night...so may be a bit 
>'antisocial' for
>practical use. - I wonder how the  eclipse works out there?

Umm, there seems to be a lot of confusion here.  Firstly, there are 2 
sets of Lagrange points - those of the Earth - Moon system and those 
of the Earth - Sun system.  You seem to be mixing the two.

First, the Earth - Sun system.  L1 is around 1 million miles away, 4x 
the distance of the Moon, so in addition to Sun noise, you have the 
extra 12dB path loss to the satellite.

L2 is on the night side, as you stated.  You still have the million 
mile distance to contend with.

L3 is on the far side of the Sun - 'nuff said ;)

L4 and L5 are at the +/- 60 degree points along Earth's orbit, with 
extreme practical considerations due to distance from Earth.

Now on to the Earth - Moon system.

L1 is between Earth and the Moon, but much closer to the moon end 
(due to the lesser gravity of the Moon).  There would be some 
advantages of putting a satellite there (slightly less distance, full 
time availability of solar power, except during brief eclipses, no 
wild temperature swings), but these would be offset by the 
requirement of fuel for station keeping (L1 is unstable).

L2 is on the far side of the Moon - again, not going to work too 
well.  L1 and L2 would be good points for satellites to service the 
Moon though. :)

L3 is on the far side of the Earth from the Moon.  Comms 
considerations as per a lunar outpost apply, and station keeping as 
per L1 applies.

L4 and L5, being stable would be potential places to park a 
satellite, though some fuel would still be required for attitude 
control.  Distance would be same as for a station on the Moon.

73 de VK3JED
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