Orbital radius of Geo-stationary satellite Could you please tell, Why all the geo-stationary satellites are to be dropped at same height from earth? Why can't it be closer or away from its regular orbit(ie, 35,000 km)? If all satellites are dropped in the same orbit, then will not those collide one another?
 A: Geostationary satellites are less likely to collide since they are all moving in the same direction.  But they aren't all perfectly positioned and so do drift - even 'perfectly' positioned ones do drift in a figure of 8 around their intended point. 
But if there ever is a collision, or an explosion, the debris is going to stay there essentially for ever - unlike stuff in LEO which will re-enter the atmosphere. And you don't have an alternative geostationary to choose from. If there is ever a large cloud of debris in the GSO slot for N. America then you are all going to have to switch to cable.
On the other hand GSO is become less important as more and more comms switches to fibre.
A: About the collision question; By definition, a geostationary satellite has a frequency of rotation equal to  earth's frequency of rotation. That means it's a specific angular velocity. If two satellites orbit with the same angular velocity they will always maintain the same distance.
About the radius for geostationary satellites; The velocity of the satellite is a function of the radius. So in order to have a specific period you need a specific radius.
From Newton's second law, we have that the centripetal acceleration of the satellite is equal to the gravitational force $m\vec{a_c}=\frac{GMm}{r^2}\hat{r}$. 
$|\vec{a_c}|=\frac{GM}{r^2}|\hat{r}|$
$\omega^2 r=\frac{GM}{r^2}$
$r=(\frac{GM}{\omega ^2})^{1/3}$
If you put the frequency equivalent to a geostationary orbit into this equation, you'll have the appropriate radius.
