Archive for June, 2009

SVN 49 Navigation Data parameter changes

June 24th, 2009 | Category: General Navigation,Navigation Accuracy

At a telecon held by Air Force Space Command (AFSPC) last Friday, I asked a question regarding which GPS navigation data parameters were being modified to fix the elevation-based, excessive ranging errors that SVN 49 (PRN 01) is producing.  The answer on the telecon was not as detailed as I would have liked, so I followed up with an e-mail asking for the definitive terms that are being adjusted.  I received their reply today; here is the answer:

"Two methods are being evaluated for mitigating the effects of the SVN-49 problem.  This first method involves adjusting the AF0 and Tgd terms in the broadcast NAV message from SVN-49.  The second method involves adjusting the AF0 and Tgd terms as well as the square root of the semi-major axis and the mean motion difference terms in the broadcast NAV message.  The pros and cons of each method are still being assessed.  The satellite is currently being operated using the second mitigation method without the Tgd adjustment."

So, now we know what's being considered and tested.  I don't doubt that they would consider other parameters as well, if they think they can model the fix better, so this may not be the final answer.  I'll keep the Nog updated if I hear anything further.

More detailed analysis can be found on another blog by Tim Springer.

All comments and questions are appreciated, thanks for following!

Happy Nogging...

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

June 19th, 2009 | Category: General Navigation,Navigation Accuracy

Today I attended the Air Force Space Command (AFSPC) media telecon specifically addressing the high User Range Error (URE) problems on the newest GPS satellite.  PRN 1 was launched on March 24, 2009, carrying the new L5 payload.  The L5 payload was turned on and successfully guaranteed it's spot in the spectrum for future L5 payloads on GPS.  But, while L5 worked, L1 and L2 were having problems; problems no one on the ground had seen before.  The URE from PRN 1 was inconsistent with the other IIR vehicles in that family, causing quite a stir.  Notes from today's telecon describe the situation, detail what happened, who's affected and what the resolution is.

Telecon started at 12:00 PM PDT, June 19, 2009

Col. Dave Madden and Col. Dave Buckman answering questions.

Several media representatives asked questions.

Note that I've paraphrased the questions and answers for brevity and clarity

Question: Does the problem affecting this satellite extend to GPS III?

Answer: (Madden) No, this problem is specific to this satellite only.  It turns out that the L5 payload was added to an existing IIR vehicle using the Auxiliary port [I'm assuming it's the RAP functionality on the satellite - the Reserve Auxiliary Payload]. All ground tests were normal and everything seemed fine.  This Auxiliary port is not the same architecture intended for implementing L5 on the GPS III vehicles.  It turns out that by connecting the L5 payload to the Auxiliary port, L1 and L2 energy is reflected and not compensated for.  To correct this, we've effectively moved the antenna phase center of the antenna and adjusted the navigation message.

Question: Is there any risk to the military's use of GPS?

Answer: (Madden) No.  This satellite, even without a fix, is still well within specification.  The [SIS]URE is between 2-4 meters  depending on where you are on Earth, and it's elevation dependant.

Question: Will L5 on SVN49 be turned off when the next L5 payload is turned on?

Answer: (Madden) We'll probably wait until the 2nd L5 payload is on orbit and active before considering turning the SVN 49 L5 payload off.  The problem on SVN 49 is with L1 and L2, not L5.

Question: Is this problem something that needs to be addressed for the upcoming IIR launch in August 2009?

Answer: (Madden) Initially we were concerned, so we performed a root cause analysis to determine the issues.  This analysis lead to the finding about the Auxiliary port.  We then recreated this situation on the ground in Denver and, with more extensive testing, found the same issue that we have on orbit.  This verifies to us that we've found the problem, clearing the next GPS satellite for launch.

Question: How is the fix for this problem modeled?  Is it a constant bias or something else?

Answer: (Madden) The fix effectively moved the antenna phase center for the satellite to 150 meters behind the satellite.

Question: What navigation parameters are being changed to implement this fix?

Answer: (Thomas Powell, Aerospace) The ephemeris phase center value (later determined to be the Tgd value) and the clock offset values are being modified to allow user's receiver to get a correct URE for this satellite.

Closing remarks from Col. Madden covered the Air Force's concern in the tone of the GAO's analysis for the future of GPS.  Col. Madden reiterated that the Air Force has always met GPS performance commitments and that they have a robust plan for continued health of the constellation. Another issue the GAO neglected he continued, was that the Air Force uses power management to increase the lifetime of satellites in certain cases.

See my Nog on the GAO report issue.

Ok, so now we know the scoop - this is a one vehicle hiccup and one that can be corrected, not too bad!

In the next Nog, I get more technical I promise.  The faithful among you have been waiting for the third installment on predicting GPS accuracy - it's next!  I promise! First two Nogs on that topic here and here.

Until then, smooth sailing.

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