On 07/10/2013 11:08 PM, David I. Emery wrote:
On Wed, Jul 10, 2013 at 08:10:45PM +0200, Magnus Danielson wrote:
On 07/09/2013 04:25 AM, Joseph Gwinn wrote:
Yes, of course, but I don't think I explained very well. The issue was
more economic than technical.
There isn't much spare space, weight, or power in the birds, technology
moves rapidly, and the satellite companies don't want to have expensive
satellites that no longer generate rental income because something
became obsolete. So they ruthlessly simplify. A bent pipe will handle
any possible band-limited modulation, no matter if currently known or
not, and so is the safest solution.
Now WAAS may have become important enough to command dedicated
hardware, but that came later, to the degree it came at all.
A bent pipe is more generic, but there are limits to how much you can
alter the output frequency too.
It seems completely inconceivable to me that either the antenna
system (particularly feeds) or transponder RF hardware on any commercial
Ku or C or Ka or X band satellite could possibly be frequency agile
enough to tune to 1575.42 MHz unless it was purpose designed to radiate
on that frequency from the start.
So any hosted WAAS payload is completely application specific.
I was thinking along the same lines, but I have too little experience in
RF design for birds. There are several potential other uses for L-band
transmission if tweaking a little up or down from L1 is feasible,
otherwise it's pretty application specific.
WAAS links primarily provides an information channel, so it doesn't have
to be very accurate. However, as you devote a channel to it, you might
as well use it to produce pseudo-ranges, but it seems like they didn't
care too much on the carrier-phase part compared to the code-phase, but
10 years back not many receivers used the code phase for nav at all, but
carrier smoothed code should at least be common now, so for those it may
not fully meet the needs. The added precision for the other channels
compensate thought.
Cheers,
Magnus
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