C/A Code Differential GPS for a Cat.II ILS Flight Inspection

Maria F. B. Nunes, Isabel Osorio

Abstract:	"ILS flight inspections involve two phases: the first one is related to the verification of the Localizer and Glide Slope signals; the second one is related to its calibration, in case of being necessary. To perform the ILS verification, it is very important to define the reference generating system, due to the accuracy required for this kind of operation. The Portuguese Air Force (PtAF) is, since 1986, the organization responsible for performing all the civilian and military flight inspections of radio navigation aids in Portugal. To perform these kinds of operations, the PTAF uses the Radio Telemetric Theodolite (RTT) as a reference generating system. Nevertheless, due to the inherent restrictions associated with the use of RTT in alignment inspections ILS and signals structure, the PtAF has promoted the development of viability studies concerning the use of the Differential GPS (DGPS) as a reference generating system. In this paper, the results achieved by using the true reference provided by the DGPS are presented [1]. A Horizontal Position Error (HPE) lower than 4.219 meters and a Vertical Position Error (VPE) lower than 6.472 meters, in 95% of the time was achieved. Also, there is a comparison of the accuracy achieved from the alignment and structure of both the Localizer and the Glide Slope signals in a Cat.II ILS verification originated by using the RTT and the DGPS technology. It has been proved that the use of the RTT can generate accuracy errors concerning the alignment and structure of ILS signals. However, the use of the C/A code DGPS technology results can meet the alignment and structure of the ILS Cat.II verification requirements imposed by ICAO. The use of the C/A code DGPS technology can assure azimuth error values lower than 0.0145¼ for Zone 4 and elevation error values lower than 0.006¼, 95% of the time."
Published in:	Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005) June 27 - 29, 2005 Royal Sonesta Hotel Cambridge, MA
Pages:	704 - 711
Cite this article:	Nunes, Maria F. B., Osorio, Isabel, "C/A Code Differential GPS for a Cat.II ILS Flight Inspection," Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005), Cambridge, MA, June 2005, pp. 704-711.
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