The image on the left displays position scatter after applying the GPS broadcast ionosphere model corrections (ICD-GPS-200C) to the L1 pseudoranges. The image in the center displays position scatter after applying an additional correction for seasonal troposphere delay to the L1 pseudoranges. The image on the right displays position scatter when IGS rapid orbits and clocks are substituted for the broadcast orbit and clock data.
 

Date	Single Freq. Broadcast Ionosphere correction 95%	Single Freq. Seasonal Troposphere correction 95%	Single Freq. Rapid Orbit & Clock 95%
May 03	6.99 m	4.49 m	3.78 m
May 04	6.99 m	4.59 m	2.82 m
May 05	6.68 m	4.45 m	2.82 m
May 06	6.76 m	4.27 m	3.21 m
May 07	6.47 m	3.38 m	3.19 m
May 08	6.30 m	4.17 m	2.63 m
May 09	6.63 m	4.56 m	3.66 m
May 10	6.88 m	4.63 m	3.96 m
May 11	6.27 m	5.14 m	3.23 m
May 12	5.99 m	4.12 m	3.29 m
May 13	-- data set incomplete --
May 14	7.80 m	6.87 m	4.77 m
May 15	7.12 m	4.40 m	4.03 m
May 16	7.36 m	7.21 m	5.54 m
May 17	8.18 m	6.65 m	5.42 m
May 18	7.28 m	5.97 m	4.89 m
May 19	6.07 m	4.05 m	3.66 m
May 20	5.55 m	3.23 m	3.04 m
May 21	6.85 m	5.83 m	4.42 m
May 22	6.08 m	5.07 m	3.56 m
May 23	6.58 m	4.75 m	3.04 m
May 24	7.70 m	7.38 m	7.18 m
May 25	6.75 m	5.32 m	3.74 m
May 26	5.63 m	4.45 m	2.88 m
May 27	7.44 m	5.16 m	2.51 m
May 28	6.54 m	3.90 m	2.58 m
May 29	6.23 m	4.22 m	2.68 m
May 30	7.38 m	5.45 m	5.42 m
May 31	7.41 m	6.02 m	2.63 m

The table at left summarizes the accuracies when progressively including the different corrections described above. 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 in meters bounding 95% of the computed points.

These results indicate the accuracy that is obtainable from single frequency L1 pseudorange data. In particular, the second column reflects the accuracy currently obtainable without “outside” data, such as precise GPS clocks or surface barometric pressure. The third column shows the improvement if precise orbit and clock data could be used in place of the broadcast information, or if broadcast orbits and clocks could be upgraded. The third column indicates the possibilities if precise GPS orbit and clock data are quickly gathered in a global network, computed and predicted over short time intervals, and made available through the Internet.

The data, and figures, show considerable variation, and, as a general rule, do not perform as well as the dual frequency results. Note that there are occasions where the single frequency data, supplemented by the broadcast ionosphere model, show less 95% scatter than the comparable dual frequency data. Dual frequency data represent a measurement of the ionosphere, and are subject to the multipath error on both the L1 and on the L2 pseudoranges. 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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