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.

The scare level regarding the General Accounting Office report on the risk of future GPS failures is rising precipitously.  Let me be one of the first to say - hold on, there's no reason to panic, or sell your Garmin (GRMN) stock.  Many consumers have purchased the now ubiquitous GPS handhelds that tell you where to go.  Providing accurate positioning maps and voice response, they are a tempting buy (but not for me yet, somehow...).  Most folks even regard their device as the GPS, not the system that provides the location signals to their device.  So, what's the truth behind the failure cry and how bad is it really?

The General Accounting Office report, available here: GAO GPS Report, states there is increased risk of future GPS coverage failures because of acquisition problems - basically the next generation of GPS satellites; the Block IIF satellites, are behind schedule.  Also, there are several GPS satellites that are "single-string" meaning they have lost redundancy on one or more components.  This means that if the current component fails, the satellite may not be able to perform its navigation mission.  The GAO report is reporting on increased risk, it is not reporting on GPS failure.  The conclusion in their report is essentially, let's keep a close eye on it - by recommending the appointing of a single GPS oversight authority.

Let's talk specifics - what if the risks the GAO reports were actually to occur?  What if 6 or more satellites were to fail, with no additional satellites being launched and no GPS satellites being moved in orbit to counter poor coverage?  How bad would it get - really?

With that problem statement, I made the following conservative assumptions in order to analyze the problem:

• A GPS user has a 12 channel receiver (able to track 12 GPS satellites at once)
• A GPS receiver won't use any GPS satellites within 5 degrees elevation from their horizon.
• The GPS Receiver will have a combined error of 2 meters (Signal-In-Space and receiver noise, multipath, etc)

Let's look at today's GPS coverage:

This picture shows the maximum navigation error, over 1 day, for the world.  The legend is:

where I've used 10 meters as the max because it's about the width of a typical neighborhood street.

So, everywhere in the US for example, the maximum error you'll see during the day is under 6 meters - roughly half the width of the street.

What about the dark areas?  How bad is the accuracy in those areas, and more importantly, how long is it bad?

The plot below shows that in the dark area in Canada, over the entire day, only a small amount of time is spent with the larger navigation error - roughly 10 minutes.  Even then, the error is only about a street width and a half.

Ok, now on to the fretful stuff.  The GAO is reporting that, because of acquisition issues, GPS accuracy may begin to suffer starting as early as 2010.  Let's look at the situation where GPS starts to lose 1, 2, 3 and more satellites and see how bad our accuracy suffers as a result.

This video shows, in each frame, one additional GPS satellite removed.  There are a total of 9 frames, corresponding to 9 satellites removed.  To decide which satellites to remove, I used data that shows which satellites are most likely to fail based on their loss of redundancy.  I did not use any reliability numbers for these satellites, simply the state of their on-orbit hardware as of March 2009.  The most likely to fail satellites are taken out first, and so on.

The video starts to show some scary colors as we begin to remove large numbers of satellites, but remember - this is the maximum error you will see over the day.  The video points out that instead of localized larger navigation errors like we have today, many more people experience these large errors - but again, for only a short time.  Here's a plot of the worst case scenario along the Eastern seaboard, where 9 GPS satellites have failed, none have been launched, and no movement of GPS satellites has taken place to optimize the coverage.

Throughout the entire day, the accuracy never exceeds 22 meters (about two street widths) and averages roughly 4 meters (less than half a street width).

To counter the scary picture the video paints, I created the plot below to show the average navigation error for the world over one day, with 9 GPS satellites missing.

This result shows that we will still have sufficient GPS coverage for most navigation needs even if the worst was to happen.  For those users in more constrained environments (like canyons, urban or natural) or that have more stringent navigation requirements than knowing which road they are on, there will be additional effects.  It is unlikely that any of this will happen however, given the Air Force's track record for management of the GPS constellation.

So, keep you GPS unit, which ever kind you have, and don't over react when we hear more stories about how GPS will fail - we're nowhere near that result.

Smooth sailing, with an eye toward the sky...

After the last Nog, someone pointed me to the following picture, published by the GPS Operations Center from this site: http://gps.afspc.af.mil/gpsoc/performance_reports.aspx, (pick the Position Errors By Satellite value in the drop down box)

This shows that PRN 1, on April 30, 2009, had a very large User Range Error (URE), larger than any other satellite in the entire GPS constellation. If you've been following the Nog for awhile, you know that a URE this big can lead to large navigation errors.  This is certainly the reason PRN 1 is not healthy yet.  URE's are calculated using ephemeris and clock errors.  Ephemeris errors are rarely a problem, so it's likely that this satellite has a clock problem.

SVN 49, PRN 1 was launched Mar 24 and if previous experience is any guide, it should have been set healthy quite awhile ago.  The standard time frame for this activity is a month, but we're over that mark now.  This satellite is also broadcasting the new L5 signal.

The GPS Operations Center says that the Air Force is continuing testing on that satellite and will be releasing a statement regarding this issue.

So what are the possible issues?  Caution, speculation ahead!

The satellite is on-slot, it's been in the almanac since late March.  It started broadcasting L5 on April 10, but still remains unhealthy for navigation using L1-L2 C/A-P(Y).  The time between when it's on-orbit and it becomes healthy is usually spent characterizing the on-board clock - the heart of the navigation and timing function of the satellite.  Once the clock has "settled down" (a technical term), it can be set healthy for navigation.  So, one could speculate that there is a problem with one of the clocks (there are three Rubidium atomic clocks on-board each GPS IIR satellite), or possibly an issue with the addition of the L5 signal.  If you have a receiver that tracks through the unhealthy setting on the satellite, you could watch the how the signal changes over time and make some conclusions.  If you do, I'd love to hear from you!

Until we get an official word from the Air Force, we'll just have to be content with our 30 healthy satellites.

Learned at the National Space Symposium this week that the new GPS satellite got great orbit pre-positioning, prior to it's 5 AKM burns to reach its designated MEO slot (no, that's not Middle Earth Online for you LOTR fans, it's this).  This means its on station sooner than planned and the CPU, MDU and Rubidium clock are all on and warming up.  L1/L2 should be broadcasting healthy soon and L5 will happen shortly thereafter.  L5 will be dataless - but the first L5 broadcast will satisfy the ITU requirements for frequency slot allocation.  It's a grand time.

Just a quick note for all of you keeping score, today (Mar 26, 2009) at 2031 UTC, the US Air Force decommissioned SVN 35 (PRN 5) per NANU 2009023.  It was on it's last clock, and in fact had be turned off before.  It was brought back to keep the constellation coverage up.  With the launch of the new satellite, and PRN 5 's declining behavior (See GPS Satellite Performance), it has now been shut off for good.  PRN 5 is now available for reuse by a future GPS satellite.

At 0434 EDT this morning a Delta II launch vehicle carried the next GPS satellite into space.  The flight lasted 68 minutes, according to United Launch Alliance.  From this point, the satellite will maneuver into it's designated slot.  See the previous Nog for further details.

InsideGNSS and Patrick AFB are reporting that the latest Block IIR satellite (IIR-20) is ready for launch.  The launch is scheduled for March 24, 2009 between 0434 and 0449 EDT.  The GPS Operations Center reports that IIR-20 also known as SVN 49, will take PRN 1 and be placed in slot B2.  This slot is currently occupied by PRN 30, so this new IIR is probably a replacement.

SVN 49 has an L5 payload aboard and is intended to secure the L5 spectrum GPS is planning to use for civil signals.  According to InsideGNSS:

Meanwhile, the U.S. Air Force is in a race against the clock to get the new L5 signal on the air by August 26, 2009, in order to meet an International Telecommunications Union (ITU) deadline for securing a preferential L5 frequency allocation for GPS operations.
Problems have pushed the GPS program much closer to the deadline that expected.

The first and probably only opportunity to meet the deadline: a modernized GPS Block IIR-M satellite — IIR-20(M) — with an experimental L5 signal demonstration payload scheduled for a March 24 launch.

But if there are problems with the launch or the vehicle - ?:

“Originally, the U.S. planned to meet the deadline with the first IIF satellite,” said the [GPS Wing] spokesman. “The IIR-20 demo payload was developed as the back-up plan.”

If GPS cannot secure the L5 spectrum before August 26, 2009, ITU regulations state that GPS risks losing unconditional use of that band - instead providing priority usage of that spectrum to which ever GNSS system begins broadcasting on it first - so hopes are high on this launch and success of this vehicle.

Block IIF problems

The Block IIF program has suffered a recent setback - a power anomaly affecting all signals on the L2 frequency discovered during testing.  According to InsideGNSS:

Discovery of a power anomaly in signal generator of the first GPS Block IIF space vehicle (SV) has thrown a new wrinkle into the long-delayed follow-on generation of spacecraft.

In the words of a GPS Wing spokesman at the Space and Missile Systems Center (SMC), Los Angeles Air Force Base, California, “In reviewing test data from the final phase of SV1 thermal vacuum test, [government and Boeing mission assurance teams] identified a new concern that a component in the L2 transmitter may not have sufficient design margin to operate at its highest required power throughout the satellite lifetime.”

“Boeing has identified multiple options for addressing the concern and is working parallel solutions to deliver redesigned transmitters this summer,” said the GPS Wing spokesman.

The launch of the first IIF satellite was expected in October of this year, but that has now been moved to "late 2009" with a second launch not to be scheduled for at least 6 months afterward.

I was a little reluctant to open a Twitter account, not because I didn't think the tech was cool, but could I possibly have that much to say each day?  In such short sentences?  Well, I figured out that on a daily basis I may not have much to say, but GPS does.  I wanted to provide some useful information to GPS followers, something that could be said in a few words.

To that end, I created an account on Twitter with a user name GPSToday.  This account I figured, could send 'tweets' to followers about GPS events, like accuracy statistics, satellite outages, etc.  But this type of information would take a lot of my time to create and update on a regular basis.  Ahhh, but wait, the AGI Navigation component can be coded in any way, shape or form.  I could use this to create a program that automatically did what I needed and produce the results automatically.

The first application: GPS Accuracy Stats over the globe each day.  Whether you're aware or not, GPS accuracy varies each day - due to satellite outages, GPS signal quality and many other factors.  Getting a quick glance of GPS accuracy and status on Twitter can keep you informed with no work on your part.  So what's available?

Here's a picture of a sample GPSToday Daily Accuracy tweet:

I use the AGI Navigation Accuracy Library, Dynamic Geometry Library and the Spatial Analysis Library to calculate the global position error, at 5 degree grid increments and 60 sec time steps.  I then find the Maximum, Mean and Minimum statistics over the globe for the day.  Once I have this information, I construct a string that states what you see in the picture above and use Twitterizer post the tweet.  I can't believe how easy this was to do.

On the machine I use to calculate the global accuracy, I used Windows Scheduler to set up the run every day at midnight. When it completes, The code will send me an e-mail that it finished and update the GPSToday status with the message above.  Also, if there were any satellite outages, a tweet with that info would be posted as well.

Computing global accuracy is easy using the Spatial Library component.  A peek at the documentation here, then heading to the Programmer's Guide, Overview, Coverage section, show lots of examples of how to compute coverage.  Down towards the bottom are some navigation examples also.  The coverage algorithm first calculates access over the grid (not at specific times, but based on the assets and constraints you assigned to the grid).   Once Access is calculated, you can evaluate a Figure of Merit (FOM), such as Navigation Accuracy, on that calculated Access at given time steps.  Also built in are statistical functions to allow statistical calculations over the entire grid and time, or just across time at a specified grid point.  Nice.

The best part of all this is that the access and FOM calculations are multi-threaded and core aware - the library will take advantage of all the cores on your machine simply by setting the following:

CoverageDefinitionOnCentralBody m_CoverageDefinition;

m_CoverageDefinition.MultithreadCoverage = true;

So, with the components, a little time and the help of a couple of tools, getting a new requirement coded and out the door happened in very little time.

If you don't have a twitter account, consider getting one, if just to follow how well GPS is doing everyday.  Follow this Twitter user: GPSToday.  Oh, You can find me at TedDriver too.

Happy tweeting!