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Re: future of satellite business (was Re: More about the Ariane failure)

> Then there's Ka band.  FCC issued a bunch of licenses a few years ago for 
> Ka band satellite systems (20/30 GHz).  Many of the proposed systems were 
> proposed only to get one of the licenses, and then see what would 
> happen.  There were people who were serious, such as Teledesic, but it is 
> difficult to discuss Teledesic while keeping a straight face.  The original 
> system involved a constellation of over 800 LEO satellites.  This would 
> have offered high speed internet access to everyone in the world via 
> satellite.  The large number of satellites meant that there would always be 
> one nearly overhead, providing a short range (good link budget) and low 
> latency.  From a communication point of view, this made great sense.  Only 
> problem is it would have required an impossible amount of money.

The other scary thing their approach was based on was high speed
optical inter-satellite links.  This is quite a risky bit in the 
whole system design, as getting the links to work is probably
somewhat challanging, not to mention the routing protocols and
algorithms to keep track of a constantly changing topology.

One of their competitors (who also essentially flamed-out) had a less
risky design with no inter-satellite links, but with sets of steerable
antennas on the (LEO) spacecraft.  There were then somewhat simpler
algorithms to schedule each spacecraft's transponders and antenna so
that it would illuminate a particular "cell" on the ground.  Between
all the spacecraft, the idea is that at least one would always be
available to provide coverage within the cell.  This presumed a
groundstation in or near each cell on the ground, with backhaul
done on the ground rather than in the space segment between the
continuously changing LEO satellite toplology.

It's also worth keeping in mind that the space-based data services
seem to really be though of a geosync birds (with the couple of
hundred millisecond propoagation delay per hop) and various LEO
systems that push much lower latencies.  You see fewer and fewer
applications in the telecom space using satellite links because
of the latency.  Clearly for broadcast sort of applications, it's
no big deal and you get the replication in air "for free" 


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