GPS World, March 2016

0 5 10 15 20 2 1 0 1 2 FCB 0 5 10 15 20 6 5 4 3 2 Satellite widelane relative clock error nanoseconds DC 0 5 10 15 20 Time hours 2 1 0 1 2 IRC to the DC satellite carrier phase clock The satellite clock corrections presented were not differenced with respect to a reference satellite to illustrate their differences in an absolute nature If the clocks are differenced in a relative nature they are equivalent The data gaps in the FCB products are expected because of the regional nature of the products Unlike the DC and IRC products the FCB pseudorange clocks illustrate different trends such as those between hours 3 and 4 The noise illustrated in the IRC clock can be removed either by filtering or by differencing with respect to another satellite clock In Figure 4 we present the relative satellite clock error for the transformed FCB upper subplot and IRC lower subplot products For the original DC product middle subplot a simple moving average filter was applied with a bin size of five minutes to reduce the noise and illustrate the underlying equipment delay The relative satellite clock error represents the difference between the pseudorange and carrier phase clocks The distinct differences of the products are easily visible such as the filtering present within FCB and IRC products in contrast to the DC The underlying relative satellite clock error is also significantly different in contrast to the DC product such that FCB and IRC have an average relative satellite clock error of 0041 0101 nanoseconds and 0645 0005 nanoseconds respectively whereas the DC has an average of 8465 1546 nanoseconds Figure 5 shows the relative satellite widelane clock error for the transformed FCB upper subplot and IRC lower subplot products For the original DC product middle subplot a simple moving average filter was applied with a bin size of five minutes to reduce the noise and illustrate 54 GPS WORLD WWW GPSWORLD COM MARCH 2016 the underlying equipment delay The relative satellite clock error represents the difference between the widelane clocks and phase clocks Similar to the relative satellite clock error the differences in the transformed relative satellite widelane clock error are noticeable As expected the transformed FCB has a constant widelane estimate of 024 nanoseconds whereas the transformed IRC and DC have an average widelane estimate of 00589 0002 and 36704 034 nanoseconds respectively PERFORMANCE OF TRANSFORMED PRODUCTS One of the metrics we can use to examine the performance of the transformed products is the quality of the solution in the position domain The solutions were examined with respect to the time for convergence to a pre defined threshold and position stability We used five stations from the Scripps Orbit and Permanent Array Center SOPAC network for days 23 to 30 of 2015 These five stations were selected because of the regional nature of FCB products provided by SOPAC We show the results for site Brand Basin BRAN on day of year 30 of 2015 as it reflects the performance of the whole dataset processed In FIGURES 6 to 8 we show the varying convergence periods at the site BRAN on day of year 30 for the float and fixed solutions using the different PPP AR products where fixed means the ambiguity resolved solution and float the unresolved solution Figure 6 uses the decoupled clock products and the fixed solution performs as expected After a few minutes the solution attains the correct ambiguity candidate and a fixed state is maintained The performance of the fixed solution using the IRC products is depicted in Figure 7 Initial convergence is similar to the DC products in the northing and easting components where a fixed state is attained after a few epochs In the up component the solution quality deteriorates after 30 minutes What is also easily visible is the solution sensitivity to changes in the satellite geometry As the number of satellites changes the fixed ambiguities change causing datum shifts in the user solution Similar trends were also observed when the transformed FCB products were used with the results presented in Figure 8 The solution deterioration is most evident in the easting component as the incorrect integer candidate is selected CHALLENGES OF INTEROPERABILITY Interoperability of the various PPP AR products is a challenging task because of the different qualities of the publicly available products limited literature documenting the conventions adopted within the network solution of the providers and unclear definitions of the corrections FIGURE 5 Transformed FCB and IRC products to code phase relative widelane clock correction on day of year 28 of 2015 for PRN 10 DC was included for comparison Linear trend has been removed