Individual convergences, even back-to-back, can have significantly different behavior. Because of the variation in individual convergences, convergence performance is normally evaluated using some type of aggregate statistic. Figure 4, for instance, shows the 95th and 68th horizontal convergence percentiles for the same data used in Figure 3. Corresponding convergence times for several horizontal error levels are given in Table 1.
|50 cm||3.6 min|
|20 cm||13.2 min|
|15 cm||19.8 min|
|10 cm||35.6 min|
The data for Figure 4 and Table 1 was collected under near-ideal conditions at NovAtel's Calgary headquarters. Actual customer performance will differ, depending on local observing conditions and natural variations in correction feed quality.
Converged Solution Accuracy
Once converged, variations in PPP position accuracy are largely due to the correction feed quality. The TerraStar correction feed has outstanding quality, yielding PPP positions with excellent stability. Figure 5, for instance, shows the post-convergence variation in horizontal errors for a one week period. The error never exceeds 20 cm. The corresponding cumulative sum percentages in Figure 6 show that the 95% horizontal error is less than 6 cm.
When the signals to the GNSS satellites are interrupted, the unknown starting point of the carrier phase range measurement changes and the corresponding ambiguity must be reset. If this only occurs on a few satellites, then the remaining, uninterrupted signals will be enough to maintain a position such that when tracking is re-established to the lost satellites the ambiguities will converge quickly.
Unfortunately, if tracking is lost to too many satellites, the PPP filter will lose its position, and both it and the ambiguities will have to converge again. Done naively, this would be a full convergence. In NovAtel CORRECT with TerraStar PPP, however, the ionosphere effects are estimated within the filter. This information is then propagated past the signal interruption, and used as additional information to aid the re-convergence. Figure 7 shows how these ionosphere constraints can improve reconvergence following a 10-second complete signal interruption.
Employing satellite-based delivery of data ensures that the corrections are available in remote areas, where other telecommunications infrastructure is not available.
For example, agricultural sites are typically very large. Using RTK-based methods for machine guidance in remote locations might require more than one base station, which adds cost and logistical complexity. NovAtel CORRECT with TerraStar provides decimetre or better accuracy at a more cost-effective price point than more conventional, logistically complex base station solutions.
As well, delivery over L-Band satellite means any user who can see the satellite has access to the corrections (between 70 degrees north and 70 degrees south).
Using satellite-based delivery ensures higher reliability of the service and makes subscribing and using the service easier for end-users.
Perhaps one of the greatest benefits of the NovAtel CORRECT with TerraStar service is subscription flexibility. A range of PPP services are available through NovAtel to suit any customer's business model. Why pay for centimetre positioning if all you need is decimetre accuracy? Why pay for a full year subscription if you only need decimetre accuracy for three months?
With NovAtel® in control of the entire positioning solution, future innovation-including seamless integration with all positioning modes and correction types-is assured.