[time-nuts] Speaking of Costas loops (WAAS)

[David I. Emery]

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.

	What is not clear from anything I have read so far is whether
the UPLINK of the modulated WAAS signal is somewhere in the normal
(usually 6 GHz for C band satellites) uplink frequency band (probably
off one end or the other of the frequency range used).   Seems rather
likely that the ability to reuse the UPLINK common RF hardware
(reflector, feeds, filters, plumbing, maybe transponder front ends and
preamps) would make this a very natural design.

	It also seems clear that doppler and bent pipe conversion 
oscillator correction is done closed loop by having the ground station
that generates the uplinked WAAS signal monitor the downlink from the
bird.    Obviously correcting for the uplink doppler is a matter of
computation from knowing the bird's orbit orbit precisely, something
that would certainly be aided by constantly monitoring the range to the
bird from that WAAS uplink ground station and maybe another couple (for
ionospheric corrections).   Apparently the newer stuff uses two L band
frequencies to improve this (correct for plasma delay).   And the WAAS
signal of course allows continuous measurement of range accurately.

	Correcting for a generally stable but slowly aging conversion
oscillator should be pretty straightforward as well, and presumably such
a closed loop system could hold the downlink frequency to rather tight
tolerances given a reasonably predictable stable oscillator on the bird.
The 240 ms up and back delay does make the loop a bit more complex, but
the bandwidth is very low I would think since the major perturbation is
probably thermal (satellite going into eclipse once a day at certain
times, and changes in sun angle over a day).

	For an observer on the ground it is of course necessary to
correct for the satellite orbit induced doppler... which can be  up to a
couple of hundred Hz or more at 6 GHz - especially with inclined orbit
birds such as the INMARSATs.   The downlink carrier, while more stable
in frequency than GPS bird downlinks is hardly a highly accurate
frequency reference on its own.   But knowing the geo bird ephemeris
(which is broadcast on the WAAS) should allow  single signal time and
frequency solution for an observer at an accurately known location - by
correcting for bird movement.

	How good the closed loops are relative to the precision clocks
on GPS satellites is an interesting question, there seems to be no
obvious design need to reach that level of stability... but it does not
seem impossible to get pretty close.   And much of what has been
achieved here seems related to a cost/power trade off in the hosted
payload in regards to its reference oscillator.


-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."