Thanks for the info per the ngs.noaa site. I thought I'd been all over that site in the last 2 or 3 years. Guess I missed that info, or didn't realize what at was at the time.

You know I've been questioning on the main board for some time now about solar influences on GPS work. All I want to know is when to suspect solar problems as the culprit in suspicious vectors. I'll keep plugging till I'm satisfied.

As for interpreting the info on the ngs.noaa site, let me ask for your input. The way I see it, the total electron content increases during the daylight hours, most noticeably during the summer months. And, due to more direct sunlight, even more so in the lower lattitudes. If this is the case, and high TEC is detrimental to GPS signals, then one would conclude this a good reason to incorporate night sessions for longer lines. True?

As for geomagnetic storms, the same ngs.noaa site seemed to indicate that in summer time a geomag storm tended to lower the TEC, yet in the winter months the same would tend to increase the TEC.

As I see this, at my latitude of around 32°30, my day time sessions are more affected by TEC than if I were at say around 44° N lat. True? Any way to actually determine with certainty that "less than desireable residuals" could be due to TEC, or for that matter geomag storms?

I don't have many problem sessions, but when I do, like any other problem, I want to find the source.

Thanks for your input

J.D. in Northeast Texas

I am not Mr. Geodesist, but I have battled with the solar problem for the last 3 years with Topcon Turbo SII units. I have seen a connection with high tec values and failed baselines. My software will reflect good sigma values, but the rms values will be sky high. Loop closure precision will be low even with low sigma values. After a failed session I could look back at the tec values for that day and they would be high. You are right that tec values start out low in the morning, but climb as the day progresses. With our units on most days I could get reasonable sessions up until about 11:00 AM local time. Some days I would get no good data. Sessions at night would have to take place late (probably about 1:00 am) to allow the tec to decay to an acceptable level.

I am no expert on the ionosphere, but here is what I know about effects on GPS vector accuracy. The short answers to your questions are:

(1) The lower the TEC values the better your GPS vectors will be, so night sessions will give you better results, the longer the baseline the greater will be the accuracy gained.

(2) In general a magnetic storm will always tend to raise the TEC, since the number of electrons is primarily the result of the amount of incoming ultraviolet radiation from the sun, which increases with a magnetic storm.

(3) The ionospheric effects on GPS signals tend to be the greatest at 10 to 20 degrees latitude and decrease toward mid latitudes.

Here is some elaberation. In the case of single frequency GPS receivers models are used to estimate the the ionosphereic effect on GPS observations. These models are far from perfect. The error in a GPS solution due to the effects of the ionosphere depends, not on the accuracy of the modeled value of the total electron content (TEC) at each station individually, but on the accuracy obtained from the model for the difference in TEC between the two stations at the ends of a GPS vector. The closer together the two stations are the more nearly a GPS signal from a satellite follows the same ionospheric path to the two stations and the smaller the difference in the computed error for the two stations is likely to be.

As the two stations at the ends of a GPS vector become further apart, the larger, percentage wise, the error in the difference between corrections is likely to be. And as the percentagee error becomes larger it becomes increasingly important that the corrections themselves be as small as possible. The TEC of the ionosphere usually has values lying in the range 10^16 to 10^18 electrons for a one meter square column. For a TEC of 10^16 the ionospheric correction is about 15 cm; for a TEC of 10^18 the correction is about 15 m. With this very large difference in the potential error due to the ionosphere as the TEC increases, the use of night observations with lower TEC values can result in very large improvements in results for long vectors.

As for the TEC, can fluctions be predicted based on solar weather predictions? I suppose not entirely as the TEC is affected more by direct sunlight/heating. I was under the impression from the data on ngs.noaa that geomagnetic storms in summer would actually tend to lower the TEC, and have the opposite effect in winter. I may have misread.
Another question you may have some idea of concern solar wind. It would seem to me that high solar winds from coronal mass ejections would tend to cause a wave action or undulation (I just like to use that word - sound sexy) in the ionosphere, thus causing more unpredictability in the iono estimate.

One thing I've noticed in the last couple of months in watching the graphs on the solar conditions has to do with the x-ray flux. The x-ray flux spikes at the moment of a CME, the larger-more earth directed the CME, the larger the spike. This seems to be a very good predictor of events to come within 48 hours or so. Does this sound correct?

Thanks for your help.

J.D.

Thanks for sharing your experiences. This is what started the whole AFPD thing a few weeks (months?) ago. I just get stuck on a subject and won't let go until I'm satisfied.

Understanding potential errors in gps due to solar influences are just as practical as understanding atmospheric effects on edm. It's all part of knowing your tools, isn't it.

Thanks

J.D.

I have never had a problem with solar disturbances, even though I use GPS every day for static observations.

I always carry a 6 inch diameter solar filter, the kind that is designed to fit over the end of a telescope. I watch the sunspots move across the sun.

One day earlier this year, we had an extraordinary flare of solar ionic activity, which caused Aurora Borialis effects as far south as Mexico. That same day, I gathered 5 static shots with perfect solutions.

If you want something to worry about, worry about trees. Trees have an enormous effect on the solutions to GPS positions. The biggest improvement that can be made to GPS is to equip them with frequencies that are less muddled by trees. You see, the smart bombs that run with GPS guidance are over the treetops, and so GPS was designed without regard to these troublemakers.

You can observe the ionosphere in a state of helplessness, or you can put a better system in orbit.

Maybe 0.01 percent of the commercial use of GPS happens within 50 feet of the ground, I think.

Nearly Normal wrote:

I have never had a problem with solar disturbances, even though I use GPS every day for static observations.

Would you, by any chance, be using dual-frequency receivers?