Determination of reliability parameters of HPL and VPL technical safety in the procedure of a non-precision landing approach NPA GNSS with using GPS and GLONASS navigation systems in air transport

Published: 30 June 2018 The paper presents the results of determining the HPL and VPL safety parameters used to evaluate the reliability of aircraft positioning in air transport. The HPL and VPL security level parameters were determined using GPS and GLONASS systems for the NPA GNSS nonprecision landing approach. The work also compares the HPL and VPL values with the technical standards published by ICAO.

The paper presents the results of determining the HPL and VPL safety parameters used to evaluate the reliability of aircraft positioning in air transport. The HPL and VPL security level parameters were determined using GPS and GLONASS systems for the NPA GNSS nonprecision landing approach. The work also compares the HPL and VPL values with the technical standards published by ICAO.

Introduction
One of the fundamental tasks and requirements for the implementation of the GNSS satellite air transport technology is to increase the safety of air operations. The level of technical safety is particularly critical within the aircraft approach in the final phase of the flight. Within the implementation of GNSS satellite technology in air transport, there are 3 basic aircraft positioning systems that determine the type of landing aircraft approach [Jafernik et al. 2016 The NPA approach is quite commonly used in the RNAV area navigation concept, and is also implemented for GPS, GLONASS or GALILEO navigation systems. The position of the aircraft in the NPA GNSS (GPS or GLONASS or GALILEO) approach is defined only and solely in the horizontal plane, but there are no recommendations for navigation in the vertical plane [Fellner et al. 2016]. It should be emphasized that the position of the aircraft is determined by means of the navigation device and the Flight Management System (FMS) integrated with the GNSS receiver. The GNSS on-board receiver must have the Receiver Autonomous Integrity Monitoring (RAIM) function, which enables continuous monitoring and the autonomy of indications of navigation parameters. The technical safety factors in the NPA GNSS approach are determined by the positioning reliability coefficients: HPL (Horizontal Protection Level) and VPL (Vertical Protection Level) [Kazmierczak et al. 2014]. It must be noted that certification of HPL and VPL parameters is defined and implemented by the International Civil Aviation Organization (ICAO) [Krzykowska et al. 2014].
The HPL and VPL technical security parameters allow determining the area of the actual and reliable position of the aircraft in the airspace. This information is of paramount importance for 'Search and Rescue' service in aviation rescue using satellite and navigation systems (e.g. COSPAS-SARSAT) [Wetoszka 2016]. It should be emphasized that the installation of verification system of the HPL and VPL safety parameters for onboard equipment on rescue aircrafts allows, among others, for: -development of a unified security, interoperability and cooperation system for rescue and search operations carried out by aircraft crews, -implementation of the flight crew teaching and training system with new instrumentation and onboard software equipped with the GNSS satellite technology elements, -safe access to the crash site by the rescue team, -avoiding collisions with other aircrafts heading for the disaster region, -determining the cruising altitude for the aircraft rescue column, -defining a safety zone for horizontal navigation conducted by rescue aircrafts, -narrowing the potential area of search for catastrophe victims, -logistical support of the crash site, -enabling air navigation at the hard-to-reach crash site.
The primary purpose of this research paper is to determine the numerical values for the HPL and VPL safety parameters using the GNSS sensor in the air transport area. The HPL and VPL parameters were determined using the GPS and GLONASS observation as part of the NPA GNSS non-precision landing approach. The research experiment was carried out using the Topcon HiperPro mobile receiver mounted on the Cessna 172 board, which performed a test flight over the military airport in Dęblin on 1 st June 2010. The satellite data from a geodetic receiver that was not coupled to the FMS onboard computer and did not have any official boarding certification were used for the experiment purposes. Such a use of the geographic receiver aimed to check its suitability for determining the HPL and VPL reliability parameters in air navigation. The article compares the values of HPL and VPL parameters with technical standards published by ICAO (Chicago Convention of 1944, Annex 10 'Aeronautical Telecommunications' Volume I 'Radio Navigation Aids'). The whole work is divided into: introduction, 3 theoretical chapters, conclusions and research literature (bibliography).

The mathematical model of HPL and VPL parameters determination for the NPA GNSS approach
The HPL and VPL parameters allow for the determination of technical safety levels using the GNSS sensor in flight operations. The HPL parameter is referred to as a permissible error of the aircraft position in the horizontal plane for the safety needs of the flight operation being performed. In turn, the VPL safety parameter defines the maximum permissible vertical position error of the aircraft for the purpose of the safety of the air operation [Ciecko et al. 2015]. The HPL and VPL values are determined with a probability of 95%. The values of HPL and VPL parameters can be specified using mathematical formulas as below [Jokinen et al. 2011]: where: kH -coefficient determining navigation in the horizontal plane, kH -6.18 for 'en-route' navigation, kH -6.0 for the precision approach (PA),

kV
-coefficient determining navigation in the vertical plane,

The research experiment and research results
The HPL and VPL technical security parameters for the NPA GNSS non-precision landing approach were determined using GPS and GLONASS observations for aircraft positioning under the method of Single Point Positioning (SPP). The research experiment was conducted for C/A code observations on L1 frequencies in GPS and GLONASS system. The GPS / GLONASS code observations were used in the research test, recorded and stored in the internal memory of the Topcon HiperPro receiver. The Topcon HiperPro dual-frequency receiver was mounted on-board on the Cessna 172 during the airplane test at the Dęblin Airport (see Fig. 1). The test flight was conducted on 1 June 2010 as part of a research project carried out by a team of experts from the Polish Air Force Academy (WSOSP) in Dęblin. The basic aim of the air test was to implement landing approaches using the GNSS satellite technology for the airport in Dęblin and to verify the operation of the experimental GNSS-based on-board system [Cwiklak and Jafernik 2010].
GPS / GLONASS observations in the RINEX 2.11 format were used in the calculations to determine the coordinates of the aircraft (see Fig. 1) and the positioning accuracy in the BLh geodetic system. Numerical calculations were made in the RTKLIB program in the RTKPOST module. In order to obtain a solution for the position of the aircraft, the following initial RTKPOST configuration parameters were set up in the RTKLIB program [Takasu 2013  The obtained accuracy values (mB, mL, mh) of the aircraft positioning were used to determine the HPL and VPL parameters from Equation (1) for the GPS and GLONASS solution. When determining the HPL and VPL values, the following figures were used for the parameters: kH = 6.18 and kV = 5.33. Figure 2    The average value of the difference for the HPL parameter results is -17.601 with the Root Mean Square (RMS) error of 8.5 m, and for the VPL parameter results, respectively, -21,078 m with the RMS error of 7.1 m. The spread of results for the dHPL difference is from -49.463 to -1.337 m, and for the dVPL difference from -60.379 to 7.868 m respectively. The median parameter for the dHPL result set is -16.623 m, while for the dVPL result set -21.016 m, respectively. It should be emphasized that the HPL and VPL reliability results from the GLONASS solution are definitely greater than from the GPS solution.

Discussion
The International Civil Aviation Organization ICAO has introduced a metric framework related to the value of HPL and VPL security levels for landing approach procedures with the use of the GNSS sensor. For the NPA GNSS non-precision approach, the values for HPL and VPL are respectively ]: -556 m for horizontal navigation, -the VPL parameter is not determined for vertical navigation.

Conclusion
This article discusses the results of determining the HPL and VPL security levels for the NPA GNSS non-precision landing approach procedure. The HPL and VPL values based on the GPS and GLONASS solutions were identified during the studies. The navigational data of the geodetic mobile receiver installed on board of the Cessna 172 aircraft was used in the research experiment. Furthermore, the HPL and VPL differences between the GPS and GLONASS solutions were determined. The obtained results of the HPL parameters from the GPS and GLONASS solutions do not exceed the critical limits for horizontal air navigation. For vertical navigation, VPL values are not certified by ICAO and constitute only information material for potential readers.
The obtained results of the satellite positioning reliability parameters were aimed at checking the suitability of the Topcon HiperPro geodetic receiver to determine the HPL and VPL parameters for air transport purposes. The results of the verification of HPL and VPL technical safety parameters presented in this work are also important in search and rescue operations. A uniform safety system based on the GNSS navigation system procedure can be very helpful in air rescue. Assimilation of safety procedures to the HPL and VPL parameters can accelerate the decision-making process in rescue operations and facilitate communication and logistics between rescue teams.