Re: The NOG - AFSPC Media Telecon for IIR-20M (svn 49, prn 01) problem

"Mike Jr" <n00spam@xxxxxxxxxxx> wrote in message
news:3af701a4-1be5-4f2c-9919-52973b216fa2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have been thinking (always a dangerous proposition). Consider the
case of two GPS users, one seeing SVN49 at an elevation of 20 degrees
and the second at an elevation of 90 degrees. Hey, it's a big
planet. The user at 20 degrees will see little or no additional error
in his/hers pseudo range measurement. The second user at 90 degrees
will see a huge error.

Both users see the same signal. Can someone explain to me how you
adjust the signal so that both users see a "correct" signal? IT
CANNOT BE DONE.

Let us assume for the sake of argument that you can monkey with the
signal so that both user 1 and user 2 see a pseudo range with a four
to eight meter error. The current constellation average (sans
receiver error) is under a meter, probably close to 3/4 meter. Why on
earth would you want to introduce a bird with this kind of error into
the constellation?

I was going to hold off commenting, as I don't know the specifics, and as
clearly some good experts didn't anticipate this problem it can be dangerous
to guess.

1) It is a corporate fed helical phased array transmitter. What that means
in slightly less jargon is there is a series of waveguides (corporate fed)
that feed the elements of the antenna. Each phased array element is a loaded
helix (what gives you RHCP). Both the waveguides and the helices themselves
have frequency-dependent group and phase delay characteristics. Tuning them
up is somewhat of an art as opposed to pure science. What appears to have
happened is that the change in impedence coming from having the L5 payload
attached (instead of a terminator) messed up the input into the feed
network, causing substantial VSWR going into the network (they're lucky they
didn't get compaction in there). That means the phases going into arms of
the feed network got out of whack in a frequency-dependent way. You know,
you spec everything to be 50 Ohms or whatever, but sometimes things go wrong
even if they meet the spec to within tolerance. Re the art comment...I'm
sure no one has a validated EM-sim of this beast; it's just too knarly.

2) I'm sure they're wishing they had a far field antenna cut on this puppy;
I imagine it's pretty ugly (hell, the nominal ones are pretty ugly; this
will be double ugly). I'd be curious to take some high-gain traces of the
signal across the sky and see if there's extra scatter in the received C/N0
values, like what you get with mutlipath but in this case even at high
angles (and why you'd want a dish to block out the multipath). I've got the
right antenna and feeds available, but I am not an academic so I can't just
spend company or Government money satisfying my curiosity.

3) Net/net, what that means is the group delay varies as a function of
elevation (off axis angle from the satellite antenna maps into elevation
angle of the satellite as seen by the receiver). That's why the code/carrier
measurements were messed up. It's essentially *not* the same signal...it
comes from a different combination of the signals emanating from the 12 or
so elements. Since several meters is multiple wavelengths, that's why I
figure the antenna pattern is likely awful. They must have tuned some things
to get the phase behavior good enough to get any useful pattern though; I
don't know if they can perform per-element group delay measurements though
(likely very tricky).

4) Apparently, you can get most of the effect canceled by treating the
aperture as having a point source phase center 150 meters behind the bird
(Madden's comment). The quick fix would be "fine, just change Root-A".
Unfortunately, that screws up the mean motion calculation, and the
adjustment to delta-n required might exceed the ephemeris range.
Fortunately, to first order, changing the clock bias and/or Tgd can give you
pretty much the same result. I think part of the reason they're holding off
marking the SV healthy is they want to be sure this patch holds over time.

5) Regarding not using this bird (Mike's comment). The URE was already down
to 2.4 meters or so. According to Don Jewell's transcription of the same
telecon, the Wing/2SOPS said they're confident they'll get the URE down to a
meter or better by October.

Lastly, a more detailed technical explanation will likely end up in an
Aerospace report sometime. Whether a paper gets presented to ION or similar
depends on whether it can get past export control. Remember, this is not a
NASA science experiment. Pretty much everything the Air Force touches is
presumed to be a munition by ITAR rules. Presenting a technical paper is an
export of military-related technical data, and has to be reviewed carefully
before release. I'm sure Col Madden, Mr. Powell, and others in the telecon
had to go over what they were going to say with the Program Protection group
at the Wing (Art Fernandez and a few others I know) before they went public.


.

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