Archive for May, 2009
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:
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...No comments
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.5 comments