7. Concluding remarks

Figure 5 shows the PL for the along-track and cross-track directions (shown as PLAT and PLCT) and the absolute values of the positioning errors along these directions (denoted as errAT and errCT) using an integrity risk of 10�<sup>4</sup> and 10�<sup>6</sup> as examples. The figure shows that the RTK with correct ambiguity fixing gives positioning errors within a few centimeters. The average absolute value of the AT and CT positioning errors are 0.058 and 0.054 m, respectively. The FDE method detected 15 code observations with severe irregularities, which are attributed to high multipath in this environment. These observations were excluded from further processing. There were a few cases where the ambiguity fixing seemed to be incorrect by one or two cycles, which were not detected by the FDE procedure. However, the PL adapted to these situations and bounded this error as illustrated in the Figure 5. Inspection of the Figure also shows that when using RTK with correct ambiguity fixing, an Alert Limit (AL) can be safely chosen as 1 m. The sub-decimeter positioning accuracy of RTK is bounded by a tight protection level. The positioning errors in the test were always bounded by the PLs, and PLs < ALs for the whole period, with an integrity availability of 100%. The medians of the PL for the AT and CT using different integrity risk (PHMI) values are given in the Table 1. Both table and Figure 5

Figure 5. PLAT and PLCT and positioning errors in the AT and CT directions for the integrated positioning systems using

Integrity risk (PHMI) 10�<sup>3</sup> 10�<sup>4</sup> 10�<sup>5</sup> 10�<sup>6</sup> PLAT 0.176 0.197 0.215 0.232 PLCT 0.148 0.164 0.177 0.188

Table 1. Median PLAT & PLCT in meters for different values of integrity risk (PHMI).

<sup>P</sup>HMI <sup>¼</sup> <sup>10</sup>�<sup>4</sup> (top panel) and <sup>P</sup>HMI <sup>¼</sup> <sup>10</sup>�<sup>6</sup> (bottom panel).

40 Multifunctional Operation and Application of GPS

show that the PLs increase with the decrease of the allowed integrity risk.

In this chapter the concepts of integrity and IM have been introduced, and the main RAIM methods currently in use or under development have been presented. As these methods were developed in the aviation context, their adoption in land applications has been discussed. The positioning methods used in land applications still satisfy the assumptions made by current RAIM algorithms, though great care shall be taken in addressing the larger number of vulnerabilities affecting the positioning system, in particular multipath and the carrier-phase specific vulnerabilities. Some preliminary but promising results of the application of a RAIM algorithm in urban environment were shown. Further research and practical experiments are necessary to strengthen the confidence in the models.
