Updated: Friday, April 9 11:00am US Pacific. I added more specific information regarding signing up for Space Weather Prediction Center email alerts. See below.
It’s time to touch on the solar activity subject again, as there was an event earlier this week and rumors began to fly. The mainstream press jumped on a story back in January when the first solar flare of Solar Cycle 24 occurred. Of course, journalists were writing about worst-case scenarios in the event of extreme solar events that could cause power grids to fail, GPS to stop working, etc.
While that is true, it’s a real stretch and the typical “sky is falling” reporting. In reality, the solar flare back in January had no effect on GPS operations. In fact, it would take an event 10-20 times stronger than last January’s to begin to notice any effect on GPS operations. Earlier this week (Monday 0800 GMT), the first geomagnetic storm of Solar Cycle 24 occurred.
Geomagnetic storms are the ones that will give GPS users problems, although this one didn’t because it was relatively minor. The last geomagnetic storm strong enough to noticeably affect GPS users occurred in December 2006. During such an event, it might interrupt your GPS receiver for 10-15 minutes. Most users would not notice or they might attribute it to a local system malfunction. By the time they investigate and reset the system, the event would have passed and the user is back in operation. It would be barely noticeable, if at all.
According to Joe Kunches of the NOAA Space Weather Prediction Center, a geomagnetic storm is a global event (as opposed to a regional event) that is caused by a highly energized solar wind that is fast and embedded with a strong magnetic field. In the following chart, you can see how this week’s event illustrates this.
Source: NOAA Space Weather Prediction Center
In the above chart, the top panel illustrates how the magnetic field becomes much more turbulent starting at 0700 GMT. The fourth panel on the chart denotes the solar wind speed, which ramped up to approximately 2,000,000 mph (3,218,688 kph) at its peak.
Extreme geomagnetic storms = Dynamic TEC = GPS interruptions
There needs to be very turbulent solar wind that disturbs the Earth’s geomagnetic field in order for GPS operations to be affected. For those of you who are familiar with the Total Electron Count (TEC), a dynamic TEC density in the ionosphere is what really messes up GPS operations. If the TEC is stable, the ionospheric models work fine and we get really good GPS performance like we’ve seen in the past few years in between solar cycles.
GPS L1 users are affected most by a dynamic TEC density in the ionosphere. These are users of WAAS, DGPS, and commercial L1 correction services like OmniSTAR VBS (not their XP or HP service). During the extreme geomagnetic event in October 2003, published simulations (Yousuf, Skone, Coster, University of Calgary, ION NTM 2005) that illustrated the WAAS maximum horizontal error (95th percentile) blew out to 25 meters while single baseline DGPS maximum horizontal error (95th percentile) blew out to 18 meters. This extreme event lasted for several days.
This doesn’t mean you’re going to have major problems in the future if you are using WAAS (or another SBAS) or DGPS, but just that high-performance GPS L1 receivers are the most susceptible to extreme solar events. In the case of the December 2006 event, SBAS and DGPS users might have experienced 10-15 minutes of unusual behavior depending on their locations. According to Kunches, high latitude geographic regions (60+ degrees latitude) and the region within 10 degrees of the geomagnetic equator (as opposed to the geographic equator) are affected the most by geomagnetic storms.
GPS L1/L2 receivers are less susceptible to extreme solar events because they can actively model the affects of the ionosphere, but they are not immune. Extreme events such as in October 2003 can cause a loss of phase lock, especially on L2 with GPS receivers that utilize codeless/semicodeless techniques, which are virtually all of the dual-frequency GPS receivers on the market today. The L2 signal-to-noise (SNR) ratio on L2 is quite a bit lower due to the codeless/semicodeless technique so it is more susceptible.
GPS L1/L2 receivers using L2C will be less affected (assuming a sufficient number of GPS satellites are broadcasting L2C) due to a stronger SNR.
Not the time to panic
The reason I wrote this article is to share what I’ve learned about the effects of solar storms on GPS operations from speaking with a number of different scientists. This isn’t meant to be a warning of impending doom for GPS users or anything or that sort. Extreme events typically occur near the solar peak and then again during the decline of the cycle. The peak is estimated to occur around May 2013, so the typical extreme events affecting GPS would likely occur in 2013, 2014, and 2015. It’s too early to start worrying much about it now.
However, as Solar Cycle 24 ramps up, we’ll see more and more geomagnetic storm activity. If you’re a high-performance GPS user (meter or sub-meter level GPS L1 and GPS L1/L2), I think it’s a good idea to monitor space weather now. Fortunately, the NOAA Space Weather Prediction Center (where Kunches works) provides a service that will notify you of unusual space weather by e-mail. You can sign up to receive e-mail alerts at http://www.swpc.noaa.gov
Following are detailed instructions for signing up for alerts:
-Goto the Space Weather Prediction Center website.
-Click on Email products (under the Support Services menu on the left)
-Create an account if you don’t have one already (it’s free).
-Click on Subscribe
You don’t want to subscribe to everything. Here are the ones specific for GPS operations:
-Advisories/Space Weather Bulletin
-Geomagnetic Storm Products/(sign up for both Alerts and Warnings for K6, K7, K8, K9 events.
-For high latitude (55 degrees and higher) users, also sign up for Alerts and Warnings for K4 and K5 events.
Following are some good reference links regarding the Solar Cycle and TEC:
GPS World article in January 2010 (scroll to end of article)
GPS World article in October 2009 (follow-up to other October 2009 article)
GPS World article in October 2009
GPS World article in May 2003
Latest NOAA prediction on Solar Cycle 24
Solar Cycle 24 page
Real-time TEC plot from the Jet Propulsion Lab
Wikipedia description of the Ionosphere
Wikipedia description of the Total Electron Content (TEC)
RF ID (Radio frequency Identification) in Survey Monuments
If you haven’t been followi
ng my Geospatial Solutions Weekly newsletter (sign up here for free), you might want to sign up and read the article I wrote on how RF ID is going to be a technology very much used by surveyors in the future. You can read the article by clicking here.
Webinar later this month (April 22, 10 a.m. Pacific time, 6 p.m. GMT): GPS, GLONASS, and SBAS Constellation Updates
There’s been a lot of infrastructure changes with GPS, GLONASS, and SBAS in the past six months. We’ve already got several hundred people registered for this webinar. It’s going to be a good one. Here are some of the questions I’ve received already and will be addressing:
- When and where will the new FAA WAAS GPS Satellite cover?
- Will the accuracy of hand-held units be increased with these latest changes?
- What developments will make GPS & GLONASS work better together? In terms of RTK accuracy.
There have been some questions as to whether you can receive continuing education credit (PDH, CEUs, etc.) by attending the webinar. Please e-mail me directly with these requests and I will do my best to accomodate.
See you next time.
Follow me on Twitter at http://twitter.com/GPSGIS_Eric
Prince’s death on April 21 highlights a fatal flaw in the United States’ antiquated 911 emergency system. When you call from cell phone, 911 doesn’t automatically know where you are. 911 often can’t determine the location of an emergency, even when the call for help comes from a GPS-equipped smartphone. Often the 911 operator can only zero in the nearest cell tower, which can be several miles away or in the next county.
