Re: time-nuts Digest, Vol 108, Issue 36
On Sat, 06 Jul 2013 13:59:26 -0400, time-nuts-request@febo.com 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.

Joe Gwinn

On 07/09/2013 04:25 AM, Joseph Gwinn wrote:

A bent pipe is more generic, but there are limits to how much you can
alter the output frequency too.

If the payload is long-term contracted already when the bird is in the
planning stage, then it is another issue.

Cheers,
Magnus

On Wed, Jul 10, 2013 at 08:10:45PM +0200, Magnus Danielson wrote:

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@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."

David,

While I can easily see how you can do closed loop correctioin for Dopplar
from the transmission point for a 'bent pipe' repeater, at any other
location that correction would not be valid, because the paths are not
parallel.

-John

=============

On 07/10/2013 11:08 PM, David I. Emery wrote:

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

Hi

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

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?

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

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

Bob

On Wed, Jul 10, 2013 at 02:42:19PM -0700, J. Forster wrote:

Sorry for my poor choice of words.   That is precisely what I

meant by "for an observer on the ground it is necessary to correct for
the satellite orbit induced doppler".    This is true for ANY observer,
since it would seem certain that the closed loop correction actually is
structured and calculated to cause the satellite to radiate a carrier
(and timing modulation on it) equivalent to what an accurate  local GPS
satellite reference clock would generate if one was aboard the hosted
payload rather than on the ground.  Anything else would make no sense
as it is not incumbent on users to try to figure out ground relative
timing for some unknown uplink antenna somewhere.  And offsetting
radiated uplink time and frequency on the ground to make it right on the
satellite at the output of the bent pipe repeater is very feasible and
more or less a no brainer.

But if the satellite radiates what a local GPS package would and

transmits ephmerides defining  its position and motion it could be
included in a GPS solution and could be used for timing and frequency
purposes the same as any other GPS satellite subject to whatever degree
of relative accuracy the bent pipe clock obtains and the degree to which
the ephemerides in the format transmitted allow an accurate position to
be determined.

And from what I have read it seems very likely the WAAS birds 

meet these criteria..

--
Dave Emery N1PRE/AE, die@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."

Hi

If the WAAS  birds are run in a fashion that gives a true GPS payload performance, why not assign them a SN 32 or below and use them?

If the WAAS birds are not in the "right numbers", why bother to set them up and spend the bucks to make them behave like a nav sat? What's the payoff?

Bob

On Jul 10, 2013, at 7:40 PM, "David I. Emery" die@dieconsulting.com wrote:

On 07/11/2013 01:45 AM, Bob Camp wrote:

In the old days (receiver channels are sparse resource):
If you devote a receiver channel to receive it, let it contribute to
position while it provides the core corrections.

In todays world:
Channels and GPS birds are many, WAAS only contribute to precision and
validation.

This assuming relatively normal commodity receivers.

The fancy receivers (double-frequency, full-blown carrier-phase
pseudo-ranges) had little extra use of the WAAS, except possibly
somewhat quicker lock-in if not being fed from a national reference grid.

Cheers,
Magnus

On Wed, Jul 10, 2013 at 07:45:39PM -0400, Bob Camp wrote:

The patent cited here recently explains... for fixed timing

purposes and basic  anti jam a simple directional antenna pointed at the
WAAS bird allows rejection of many interferers without elaborate and
expensive active steered phased array nulling technology.

And because - given a known fixed ground position - timing and

frequency can work with only one bird visible, this allows
timing/frequency using just the WAAS signal (or signals, they do provide
more than one WAAS frequency).

And potentially if the timing accuracy via the hosted payload is

respectable at least for the needs of many  fixed time/frequency users
this might supply a solution MUCH less resistant to local (nearby)
interferers than the usual more or less hemi pattern GPS antenna would -
as fixed dishes with considerable gain toward the satellite could be
used and in most places they would point well above the horizon and
could be shielded by nearby structures to further reduce jamming
susceptibility from jammers (intentional or unintentional) below or at
the horizon for the site.  For timing/frequency users (certainly an
important subset of the GPS user population) this provides some
protection by antenna pattern that is hard to obtain otherwise (and
users interested in higher precision or redundancy of timing could still
just use another GPS timing system based on normal hemi GPS antennas as
the primary - using the normal SVs - and rely on the dedicated dish
pointed at the WAAS bird only as backup in the event of jamming).

The choice of using different spreading codes from the normal

GPS set for WAAS or using a slightly different one is an overall system
architecture decision... which I guess was made in favor of not tying
up codes for regular SVs for the WAAS birds.  But AFAIK a receiver with
suitable firmware could still extract pseudo ranges and use them.

I guess there is an issue in any frequency translation scheme

with the relationship of carrier and code phase... a homodyne
distortion... due to the random phase of the LO(s)...  but this too can
be predistorted on the ground to come out right and that kept in line
via closed loop tracking of the downlink from a ground site.

I do understand that this insight into a potential further use

of WAAS beyond its use as a data channel and propagation beacon seems
to have happened later and not initially.

--
Dave Emery N1PRE/AE, die@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."