The image on the left displays height scatter after applying the GPS broadcast ionosphere model corrections (ICD-GPS-200C) to the L1 pseudoranges. The image in the center displays height scatter after applying an additional correction for seasonal troposphere delay to the L1 pseudoranges. The image on the right displays height 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	19.33 m	6.38 m	5.26 m
May 04	19.89 m	8.23 m	4.82 m
May 05	21.94 m	6.43 m	3.83 m
May 06	21.69 m	7.10 m	4.71 m
May 07	22.45 m	6.03 m	5.54 m
May 08	24.20 m	7.47 m	7.19 m
May 09	21.96 m	6.24 m	5.93 m
May 10	22.86 m	6.21 m	5.89 m
May 11	21.07 m	7.66 m	4.24 m
May 12	19.04 m	8.00 m	6.53 m
May 13	-- data set incomplete --
May 14	17.68 m	8.65 m	5.82 m
May 15	19.56 m	6.94 m	4.75 m
May 16	20.34 m	6.40 m	5.17 m
May 17	20.99 m	11.75 m	6.63 m
May 18	20.11 m	7.36 m	4.53 m
May 19	21.33 m	6.80 m	5.23 m
May 20	20.66 m	5.88 m	4.22 m
May 21	19.81 m	7.13 m	4.69 m
May 22	21.09 m	9.27 m	4.32 m
May 23	20.93 m	6.53 m	4.19 m
May 24	21.20 m	9.78 m	8.08 m
May 25	19.29 m	8.77 m	6.42 m
May 26	17.81 m	8.65 m	6.64 m
May 27	20.83 m	5.72 m	4.71 m
May 28	21.69 m	6.88 m	4.94 m
May 29	21.27 m	5.31 m	4.06 m
May 30	20.48 m	6.40 m	4.27 m
May 31	21.77 m	7.89 m	3.48 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 +/- limit in meters bounding 95% of the computed heights.

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. Note that profound improvements in height accuracy are possible with application of a general troposphere model. 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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