Re: Trouble aboard latest Global Positioning System satellite

On Jun 20, 4:59 am, Mike Jr <n00s...@xxxxxxxxxxx> wrote:
On Jun 19, 11:01 pm, Sam Wormley <sworml...@xxxxxxxxx> wrote:



Mike Jr wrote:
On Jun 17, 11:47 pm, "Wolfgang S. Rupprecht" <wolfgang.rupprecht
+gnus200...@xxxxxxxxx> wrote:
The Air Force has identified the cause of signal anomalies for PRN01
and will attempt to work around the problem.  If they can do that
PRN01 might be set usable this fall.
http://www.insidegnss.com/node/1573
While I'm really happy that the military provides the GPS for our use,
the announcements they issue read like a bad joke about military
intelligence.  I've read about a dozen reports now that "a signal
anomaly has been detected and they think they can correct it", but the
reports all stop there.  If they really know what is going wrong why
isn't there a proper explanation in that press release?

-wolfgang
--
Wolfgang S. Rupprecht              Android 1.5 (Cupcake) and Fedora-11

What you have is the data.  Follow the link to be found here, see page
35.
http://groups.google.com/group/sci.geo.satellite-nav/browse_thread/th....

Now think.  They bolted on an L5 transmitter but it feeds a single
phased array antenna, the same phased array antenna feed by the L1
transmitter.  The IIRM birds were not designed with an extra connector
ready made for a new transmitter.  I am guessing that the interface to
the phased array antenna had to be changed and changed in a hurry.
Somebody at ITT (I am guessing) made a judgment about swapping in a
new interface that did have a new connector.  Clearly that new
interface didn't work as expected.

The wing has limited funds.  I am guessing that a decision was made to
not spend the money needed to do near and far field testing.   What
kind of technical advice the wing got and what they did with it is
unknown.

You lay down your money and you take your chances.

--Mike Jr

   Pretty good Mike Jr!

The SVN-49 Story: What Went Wrong, How It Got Found — and Fixed
   http://sidt.gpsworld.com/gpssidt/content/printContentPopup.jsp?id=605179

Jun 19, 2009
By: Don Jewell

During a very reassuring teleconference today with Colonel David Madden (GPSW/CC) and
Colonel David Buckman (AFSPC - GPS Command Lead), we learned the true story of exactly
what happened to SVN-49, aka IIR-20(M), launched March 24, and why it has not been set to
a healthy status. This teleconference should put an end to all the speculation concerning
SVN-49 and its future status. In sum, there is nothing wrong with the L1, L2, or L5 signal
transmitters, and they will not have to undergo expensive re-testing.

According to Colonel Madden, when the previous attempts to orbit a GPS vehicle with the L5
payload all failed due to delays in various programs, the decision was made to place a
demonstration-only L5 payload on SVN-49 to accomplish three goals:
1) Place an L5 signal in orbit to prove that the L5 signal, which has a different signal
structure, would not interfere with the legacy signals being broadcast from the satellite.
2) To allow the L5 user equipment developers to see the L5 signal from space so that they
could proceed in their development of L5 receivers.
3) reserve the L5 spectrum, according to ITU specifications, for the US to use at a later
date.

While initially it appeared to most observers that the first test was not met, in that it
was originally thought that the L5 signal was interfering with the legacy GPS signals,
this turns out not to be the case. Colonel Madden explained that when the decision was
made to place the L5 signal payload on the SVN-49 vehicle, the decision was also made to
integrate with the satellite through an auxiliary payload port already built into the
satellite bus that was not being used on this particular mission. While the installation
and checkout appears to have proceeded normally, there was not really an opportunity to
test the satellite signals from a distance without a significant delay in the program and
launch schedule, and the drop-dead date for the ITU signal requirements was rapidly
approaching.

Therefore, the system tests were limited, and since the additional L5 payload was
primarily made-up of a space-certified but modified L2 transmitter package left over from
the IIRM line, no hardware problems were expected. After several weeks of testing SVN-49
performance on orbit the GPS Wing, Lockheed Martin (LMCO) and the 2nd Space Operations
Squadron (2SOPS), have now determined that the observed signal anomaly is not with the L5
payload or the L5 signal itself. Indeed the anomaly is such that once the demo L5 payload
is turned off, the signal errors will still exist, and the navigation parameters fix will
continue to be implemented for the useful life of the satellite which could be as many as
16 years.

Colonel Madden explained that after a full-scale root-cause analysis was run at the LMCO
facility in Denver, Colorado, the real problem was determined to be the way the L5 payload
is integrated through the auxiliary payload port that causes reflected energy from the L1
and L2 signals to be reflected back into the broadcast antenna. This reflected signal
causes a phase shift between the L1 and L2 signals, which affects how they are being
formed and transmitted. The physical manifestation of the problem for users is that the
resultant phase-center bias makes the satellite appear to be up to 150 meters closer to
the Earth than is actually the case. This of course disrupts the ephemeris calculations of
the satellite and makes the clock appear to also be in error.

Therefore, the broadcast navigation message parameters have been modified to reflect that
the SVN-49 satellite phase center location is corrected by 150 meters and the relative
clock offset has been corrected, meaning that the satellite position will appear to be
nominal for receivers on the Earth. This means that the satellite which currently is
broadcasting a position accuracy in the 6-meter range, depending on the latitude, when set
to healthy should be broadcasting a position that is within the normal 2-4 meter accuracy
range, with hopes that it might be closer to the current constellation average of .75
meters, the best that it has ever been since the inception of the GPS program.

Since we currently have 30 operating GPS satellites in orbit with another three residual
satellites that can be reactivated in about 10 days to two weeks, there is no need to rush
SVN-49 into service, so a little more time testing and adjusting the correction parameters
is the prudent way to go. Currently the Wing expects the 2 SOPS at Schriever to set the
satellite to healthy status sometime in the September-October 2009 timeframe. This is
around the same time that IIRM-21 should be set healthy as well, if all goes according to
plan.

For those of you paying attention to launch schedules, this should have been about the
same time that the first IIF satellite would have been launched, but it looks like that
will now not take place until the January-February timeframe in 2010. This fits well with
my earlier predictions that we would not see a IIF launch in 2009. But as I am sure we
would all agree, it is better to get it right than just launch on schedule and have a
failure. Besides, according to Colonel Madden, launch-vehicle availability is really the
culprit for the delayed IIF launch; but I am sure Boeing appreciates he extra time.
The news comes at a welcome time for all the rest of us as well, alleviating some of the
current concerns about the GPS modernization program.

This also allows everyone, including LMCO and Boeing, to breathe a sigh of relief because
it means there is nothing wrong with the L1, L2 or L5 signal transmitters and they will
not have to undergo expensive re-testing.

My hat’s off to the folks at LMCO, the GPS Wing, and the operators at Schriever for
keeping at this task until they uncovered the real problem, and for continuing to provide
the best PNT system in the world today.

I didn't mean to be sexist ( see correction below).

Interesting.  So they did have a spare connector just one not designed
exactly for the purpose that it was put to. The wing needed to
preserve the L5 frequency which they accomplished.  The price was
higher than they wanted.

I wonder where you make the fix?  In the Kalman Filter?  I hope the
wing has some *really* smart people around to help with that.

Alfred Sloan could run GM in the 1920's by just looking at the
financial numbers because he had people in the various divisions
looking at how to make cars.  Problem is, every one wanted Sloan's job
so everybody began looking only at the financial numbers.  Look how
well that turned out.

GPS *is* rocket science.

--Mike Jr

.

Relevant Pages

  • Re: Trouble aboard latest Global Positioning System satellite
    ... Colonel David Buckman (AFSPC - GPS Command Lead), we learned the true story of exactly ... demonstration-only L5 payload on SVN-49 to accomplish three goals: ... would not interfere with the legacy signals being broadcast from the satellite. ...
    (sci.geo.satellite-nav)
  • Re: Trouble aboard latest Global Positioning System satellite
    ... During a very reassuring teleconference today with Colonel David Madden (GPSW/CC) and Colonel David Buckman, we learned the true story of exactly what happened to SVN-49, aka IIR-20, launched March 24, and why it has not been set to a healthy status. ... According to Colonel Madden, when the previous attempts to orbit a GPS vehicle with the L5 payload all failed due to delays in various programs, the decision was made to place a demonstration-only L5 payload on SVN-49 to accomplish three goals: ... Place an L5 signal in orbit to prove that the L5 signal, which has a different signal structure, would not interfere with the legacy signals being broadcast from the satellite. ...
    (sci.geo.satellite-nav)
  • Re: And yet an even more radical proposal.
    ... Is this different for parabolic antennas? ... You need receivers for these frequencies. ... >receivers even above the satellite TV approach. ... >> local signals can be suppressed. ...
    (sci.physics)
  • Re: And yet an even more radical proposal.
    ... Is this different for parabolic antennas? ... You need receivers for these frequencies. ... >receivers even above the satellite TV approach. ... >> local signals can be suppressed. ...
    (sci.astro)
  • Re: Could DirecTV satellite dishes be used for the Square Kilometer Array - and a more radical propo
    ... - Bandwidth is restricted to the data rate of getting signals from the dishes to the central place. ... > SetiLeague site lists sellers of specially made preamps for SETI ... > This site states the rate of growth of satellite TV subscribers is at ... > This method of keeping the receiving antennas fixed while detection ...
    (sci.physics)