The image on the left displays position scatter after correcting for ionosphere derived from L1 and L2 pseudoranges. The image in the center displays positional scatter after applying the GPS broadcast ionosphere model corrections (ICD-GPS-200C) to the L1 pseudoranges. The image on the right displays position scatter when no ionosphere correction is applied to the L1 pseudoranges.
 

Date	Dual Freq. Measured Ionosphere correction 95%	Single Freq. Broadcast Ionosphere correction 95%	Single Freq. No Ionosphere correction 95%
May 03	6.18 m	6.99 m	8.04 m
May 04	6.52 m	6.99 m	7.27 m
May 05	11.40 m *	6.68 m	7.28 m
May 06	6.78 m	6.76 m	6.73 m
May 07	6.37 m	6.47 m	6.20 m
May 08	6.31 m	6.30 m	6.45 m
May 09	6.50 m	6.63 m	7.54 m
May 10	6.39 m	6.88 m	7.30 m
May 11	6.59 m	6.27 m	6.74 m
May 12	6.08 m	5.99 m	6.49 m
May 13	-- data set incomplete --
May 14	6.75 m	7.80 m	9.44 m
May 15	6.94 m	7.12 m	8.89 m
May 16	6.40 m	7.36 m	9.10 m
May 17	6.70 m	8.18 m	8.82 m
May 18	6.34 m	7.28 m	8.76 m
May 19	13.47 m *	6.07 m	7.19 m
May 20	6.30 m	5.55 m	6.92 m
May 21	6.22 m	6.85 m	8.43 m
May 22	6.41 m	6.08 m	7.63 m
May 23	6.36 m	6.58 m	8.40 m
May 24	6.39 m	7.70 m	7.91 m
May 25	6.69 m	6.75 m	7.46 m
May 26	6.90 m	5.63 m	6.52 m
May 27	7.61 m	7.44 m	7.32 m
May 28	6.74 m	6.54 m	6.64 m
May 29	6.29 m	6.23 m	6.36 m
May 30	6.25 m	7.38 m	7.76 m
May 31	7.22 m	7.41 m	7.67 m
* Note: May 05 & May 19 data have L2 disturbance.

The table at left summarizes the accuracies using different ionosphere corrections. Click on any of the results for a full-size image of the underlying 24-hour data set. The numbers in the table represent the radii of circles bounding 95% of the computed points. 

Now that SA has been removed, the impact of ionospheric error on positional scatter is clearly evident. As seen in the figures, ionospheric effects cause the long streams extending from the center of the distributions. The best ionospheric corrections are typically, although not always, obtained through the dual frequency correction. The dual frequency results produce a more evenly clustered distribution of data.

Patterns are clearly evident in the table. Dual frequency results tend to provide a uniform level of accuracy. (There is no explanation for the 9.31 meter dual frequency result of May 5, 2000.) The single frequency, no-model results show the greatest variations. Of particular interest is the large, no-model increase on and after May 14. The single frequency, broadcast ion model scatters tend to fall between the other results.

In the interest of completeness, it should be pointed out that dual frequency ionosphere correction represents a measurement of the ionosphere, and is subject to the multipath error on both the L1 and on the L2 pseudoranges. As seen on May 7, this effect may have lead to a result with a larger 95% scatter, when compared to the single frequency, no-model results. Even so, the most robust results are obtained through the dual frequency ionosphere correction.

For additional information:

Dr. Richard Snay
National Geodetic Survey, NOAA
Richard Snay

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