Extending Required Navigation Performance to IncludeTime Based Operations and the Vertical Dimension

Kurzfassung

New Air Traffic Management (ATM) concepts aim at enabling an increase in air traffic while at the same time maintaining the same or a better level of safety. Key enablers for these new ATM concepts are evolving technologies which are successively included in aircraft guidance and control functions. Some components of theses evolving ideas were already introduced in the ATM environment– such as Required Navigation Performance (RNP) - or are in the process of being researched and tested such as Required Time of Arrival (RTA) or advanced RNP. Moreover, with the evolution of air to ground datalink communications, full four-dimensional trajectories could be negotiated between air traffic control and the aircraft and flown in free route airspace [0]. While research into datalink communications, the trajectory management process and conflict resolutiosn are actively being pursued by the ATM research community, so far no requirements for a continuous four dimensional airborne navigation performance have been investigated or specified. Four-dimensional navigation requirements would describe the minimum capability of the aircraft to adhere to the trajectory that was assigned by or negotiated with ATC in the cross-track, along track, vertical and temporal dimension. As such, they would define a 4D RNP concept evolving from the current cross-track RNP specifications [1]. Presently, RNP is a designator for an area navigation system for use within a performance based navigation concept. RNP also includes continuous monitoring of navigation performance and alerting of the pilot in case of failure [1]. Only lateral (or cross-track) RNP accuracy is indicated by a number following the letters RNP, (i.e. RNP 0.3 for 0.3 nm accuracy). In this case, accuracy relates to the Total System Error (TSE) which is a combination of the Flight Technical Error (FTE) and the Navigation Sensor Error (NSE), and designates the 95% uncertainty bounds. Vertical guidance during an RNP operation is accomplished using barometrical vertical navigation (Baro VNav). Limiting factors for vertical navigation are the barometric uncertainty of the altimeter and the human factor in determining the pressure value for the pilot to set in the airplane and no monitoring or position error estimation exists for such Baro VNav systems. In the work presented here, we derive requirements for the missing dimensions vertical, along track and time from high level ICAO prerequisites and presently applicable separation standards. In addition, we introduce new algorithms based on augmented GNSS that continuously monitor the system performance in those additional dimensions. A four dimensional RNP concept needs to be seen as an extension of the current RNP definitions and must include a required navigation performance for the vertical and the along track dimension as well as for time. The along-track or longitudinal component is tightly connected to the time component by means of speed and the RTA at each given waypoint. Thus, from a top-down point of view, it makes sense to define requirements jointly for the along-track / longitudinal component and time through the concept of required time of arrival and speed control. Vertical Required Navigation Performance (vRNP) could be specified as a separate component, since it is largely independent of the other dimensions. Vertical performance requirements are already formulated to some degree for legacy systems. Attachment A to Volume 2 of [1] describes the accuracy requirements of the altimeter system for Baro VNav approach operations. The ICAO documentation on Reduced Vertical Separation Minimums (RVSM) [2] specifies the integrity requirements for altitude in a high density enroute traffic environment. For the vertical RNP, we merge the existing requirements from both documents. Assuming a zero mean Gaussian distribution for the altitude error, a system with a standard deviation of 5.05 m can fulfill this requirement. The limiting factor is the accuracy required by [1], whilst [2] would allow a tail heavy distribution. It is notable that RVSM requires about the same error probability for a vertical error exceeding 90m which the PBN manual requires for an error exceeding 15m during instrument approach. In order to complete the requirement for vRNP, we suggest a monitoring and alerting function which warns the pilot in the case of malfunctions of the barometric altimeter. The monitor performance must also comply with the previously defined error curve. We show that an algorithm using a GNSS solution augmented by a regional augmentation system such as WAAS or EGNOS can be used for monitoring the barometric altitude. The suggested system is capable of monitoring altitude deviations in level flight and up to a certain descent or climb path angle. In the case of longitudinal and time navigation performance no such prerequisites as for vertical RNP exist. Same track separations considerations are largely based on collision risk models that are specifically tailored to a target airspace. Here, for along track RNP and the desired target level of safety given by ICAO, we define requirements for arrival at any point on a trajectory at a given time. Required arrival time accuracy at waypoints is be closely linked to a minimum along track separation requirement as well as separation requirements at merging points of trajectories. We found that the required along track accuracy depends largely on the number of aircraft on a specific route and their respective speed. Results show that, for example, in order to reach a target level of safety of 5x10^-9 with 10 aircraft per hour on a given route crossing a given point, an along track accuracy of 2875 m is needed. With a speed of 400 knots this is equivalent to a required temporal accuracy of 13.5 s assuming no uncertainty on the speed. Along –track position and velocity monitoring is already accomplished by the algorithm for receiver autonomous integrity monitoring (RAIM). The onboard clock needs to be synchronized to a common time base, preferably UTC as it is already used as common reference time for aviation. Since satellite navigation systems are time based, the navigation solution already incorporates clock synchronization with the GNSS time base accurately up to a few milliseconds. Therefore, if the flight management computer is synchronized with GPS time, a precise time reference is always assured, as long as a navigation solution is available. Regional augmentation systems can support the clock accuracy by detecting common system biases in the satellite navigation system that would otherwise map to the clock correction. For redundancy of the clock monitoring, we recommend that two independent systems are used to derive separate clock solutions. One form of cross checking can be a comparison of the UTC time derived from GPS with the UTC time derived from EGNOS or Galileo.