An air traffic control system, for use by a controller controlling multiple aircraft, comprising a processor, an input device and a display device, further comprising: trajectory prediction means for calculating a trajectory for each aircraft, for inputting aircraft detected position data, and for recalculating the trajectories based on said position data, and conflict detection means for detecting, based on the trajectories, future circumstances under which pairs of aircraft violate predetermined proximity tests, and for causing a display on the display device indicating said circumstances, further comprising means for inputting instruction data corresponding to instructions issued by the controller to an aircraft, and in which the proximity indication means is arranged to use a first proximity test and a second, more restrictive, proximity test; and in which the system is arranged to display a symbol representing pairs of aircraft which violate the second test in a first display mode, and those which violate the first set but not the second set in a second display mode, and in which the system is arranged, on input of a said instruction by a controller in relation to a pair of aircraft, to change the display mode of the symbol for said pair from the second mode to a third mode indicating that no further action is necessary.

Air Traffic Control

Extreme space weather: impacts on engineered systems and infrastructure

Observations of extreme spatial rates of change of ionospheric electron content and the characterization strategy for mitigation applied by the US local area augmentation system are shown During extreme ionospheric activity, the gradient suffered by a global navigation satellite system user a few kilometers away from a ground reference station may reach as high as 425 mm of delay (at the GPS L1frequency) per km of user separation The method of data analysis that produced these results is described, and a threat space that parameterizes these possible threats to user integrity is defined Certain configurations of user, reference station, global navigation satellite system satellite, and ionospheric storm-enhanced density may inhibit detection of the anomalous ionosphere by the reference station

Ionospheric Threat Parameterization for Local Area Global-Positioning-System-Based Aircraft Landing Systems

In the 2020 time frame, the Global Positioning System (GPS) will be fully modernized, and other satellite navigation systems will be operational. With an additional layer of fault detection, these systems will provide vertical guidance worldwide. This capability will be born of three important technologies. First and foremost, avionics will receive signals on two frequencies: L1/E1 and L5/E5a. This frequency diversity will do much to obviate the impact of ionospheric storms that troubles aviation use of GPS today. Secondly, a multiplicity of data broadcasts will be available to convey integrity information from the ground to the airborne users. These will include the navigation satellites themselves, geostationary satellites, and possibly terrestrial transmitters. However, the most important change will be the most subtle. The fault monitoring burden will be split between the aircraft and the supporting ground systems in a new way relative to the fault-detection techniques used in 2008. This new integrity allocation and the associated architectures are the subject of this paper.

Worldwide Vertical Guidance of Aircraft Based on Modernized GPS and New Integrity Augmentations

Carrier-phase ranging measurements from Global Positioning System (GPS) and low-Earth-orbiting Iridium telecommunication satellites are integrated in a precision navigation system named iGPS. The basic goal of the system is to enhance GPS positioning and timing performance, especially under jamming. In addition, large satellite geometry variations generated by fast-moving Iridium spacecraft enable rapid estimation of floating cycle ambiguities. Augmentation of GPS with Iridium satellites also guarantees signal redundancy, which enables Receiver Autonomous Integrity Monitoring (RAIM). In this work, parametric models are developed for iGPS measurement error sources and for wide-area corrections from an assumed network of ground reference stations. A fixed-interval positioning and cycle ambiguity estimation algorithm is derived and a residual-based carrier-phase RAIM detection method is investigated for integrity against single-satellite step and ramp-type faults of all magnitudes and start-times. Predicted overall performance is quantified for various ground, space, and user segment configurations.

Analysis of Iridium-Augmented GPS for Floating Carrier Phase Positioning

The Wide Area Augmentation System (WAAS) will
provide real-time differential GPS corrections and integrity
information for aircraft navigation use. The most
stringent application of this system will be precision
approach, where the system guides the aircraft to within a
few hundred feet of the ground. Precision approach
operations require the use of differential ionospheric
corrections. WAAS must incorporate information from
reference stations to create a correction map of the
ionosphere. More importantly, this map must contain
confidence bounds describing the integrity of the
corrections. The confidence bounds must be large enough
to describe the error in the correction, but tight enough to
allow the operation to proceed. The difficulty in
generating these corrections is that the reference station
measurements are not co-located with the aviation user
measurements. For an undisturbed ionosphere over the
Conterminous United States (CONUS), this is not a
problem as the ionosphere is nominally well behaved.
However, a concern is that irregularities in the ionosphere
will decrease the correlation between the ionosphere
observed by the reference stations and that seen by the
user. Therefore, it is essential to detect when such
irregularities may be present and adjust the confidence
bounds accordingly.
The approach outlined in this paper conservatively bounds
the ionospheric errors even for the worst observed
ionospheric conditions to date, using data sets taken from
the operational receivers in the WAAS reference station
network. As we progress through the current solar cycle
and gather more data on the behavior of the ionosphere,
many of our pessimistic assumptions will be relaxed.
This will result in higher availability while maintaining
full integrity.

Robust Detection of Ionospheric Irregularities

The approach outlined in this paper conservatively bounds the ionospheric errors even for the worst observed ionospheric conditions to date, using data sets taken from the operational receivers in the WAAS reference station network.

Robust detection of ionospheric irregularities

Stochastic models of aircraft flight paths and a method for deriving such models from recorded air traffic data. Each stochastic model involves identifying the flight plan for one or more aircraft; identifying important parameters from each flight plan, such as aircraft type, cruise altitude, and airspeed; optionally identifying flight plan amendments for each flight; representing each route of flight as a series of navigational fixes; representing at least one aircraft flight parameter probabilistically; modeling realistic differences in at least one dimension between each planned route of flight and the flight path as it might actually be flown; and communicating the modeled deviations or simulated flight paths to the user. At least one aircraft flight parameter is represented as a random variable with a particular statistical distribution, such as a normal (Gaussian), Laplacian, or logistic distribution; or with a more complex algorithm containing one or more random elements. The modeled flight parameters may be any of lateral position, longitudinal position, climb altitude, descent altitude, climb airspeed, descent airspeed, cruise airspeed, cruise altitude transition, or response time to a flight plan amendment.

System and method for stochastic aircraft flight-path modeling

When first commissioned, the Wide Area Augmentation 
System (WAAS) will support enroute navigation, 
terminal area navigation, nonprecision approaches and 
Lateral Navigation/Vertical Navigation (LNAV/VNAV) 
approaches. An analysis was made to examine the 
feasibility of an additional service that is closer to 
Category (CAT) I Precision Approach called GNSS 
Landing System (GLS) service. This paper is focused on 
one part of this analysis, which dealt with possible 
enhancements to the ionospheric integrity function, i.e., 
the function that generates Grid Ionospheric Vertical 
Errors (GIVEs). Four design options corresponding to 
two system architectures and two GIVE algorithms will 
be briefly discussed. The initial performance results 
obtained for these four options will be presented.

Roland Lejeune

Papers

Robust Detection of Ionospheric Irregularities

Robust detection of ionospheric irregularities

System and method for stochastic aircraft flight-path modeling

Trade Study of Improvements to the WAAS Ionospheric Integrity Function for GNSS Landing System (GLS)

Performance of a Tomographic Approach to SBAS Ionospheric Estimation in the Equatorial Region