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

Bob Camp lists at rtty.us
Wed Jul 10 18:12:21 EDT 2013


On Jul 10, 2013, at 5:08 PM, David I. Emery <die at dieconsulting.com> 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.
> 	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.    

Clear from what documentation? I have not seen anything that says the WAAS is any better than the doppler spec. Uncorrected doppler is still *way* below the level on the nav sats. Why correct it?

> 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.

That's only the first layer, you still need atmospheric correction for a low angle bird along with a few other things.

> 	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.

I still don't see how it will be as good as a normal GPSDO, let alone better.


> -- 
>  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."
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