Ionospheric Threat Parameterization for Local Area Global-Positioning-System-Based Aircraft Landing Systems Seebany Datta-Barua * San Jose State University, San Jose, California 95192 Jiyun Lee † Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea and Sam Pullen, ‡ Ming Luo, § Alexandru Ene, ¶ Di Qiu, ¶ Godwin Zhang, ** and Per Enge †† Stanford University, Stanford, California 94305 DOI: 10.2514/1.46719 Observations of extreme spatial rates of change of ionospheric electron content and the characterization strategy for mitigation applied by the U.S. 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 L1 frequency) 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. Nomenclature B = satellite clock bias, m b = receiver clock bias, m D = ionospheric threat model maximum ionospheric delay, m f 1 = 1575.42 MHz, GPS L1 frequency f 2 = 1227.60 MHz, GPS L2 frequency g = spatial rate of change of ionospheric delay, also known as slope or gradient, mm=km I = slant ionospheric delay along signal ray-path at a given frequency (in this paper, L1), m K = 40:3m 3 s 2 N = integer ambiguity in number of cycles in carrier-phase measurement, dimensionless N e = electron number density, m 3 r = true range between GPS satellite and receiver, m rx = receiver location sv = GPS satellite location T = slant tropospheric error, m t = time v = ground speed of ionospheric storm or wave, front in the ionospheric threat model, m=s w = ionospheric threat model spatial width of linearly varying region, km x = receiver position " = receiver noise and multipath, m  = GPS carrier signal wavelength, m  = GPS pseudorange measurement, m  vig = nominal standard deviation of ionospheric error broadcast to local area augmentation system users, mm=km  gd = GPS satellite group delay, or interfrequency bias, m  = GPS carrier-phase measurement, m i, j = subscript GPS receiver index k = superscript GPS satellite index I. Introduction T HE U.S. Federal Aviation Administration (FAA) is developing ground-based augmentation of the GPS, known as the local area augmentation system (LAAS), to provide differential GPS (DGPS) corrections to single-frequency users within tens of kilometers of a single airport [1]. The key element of this system is the LAAS ground facility (LGF), which is a DGPS reference station equipped with four GPS receivers (whose antennas are sited at known locations at the airport) and a vhf data broadcast link for sending corrections to users. A simple illustration of this configuration is shown in Fig. 1 (this figure will be addressed again in Sec. III to illustrate an ionospheric threat/user/LGF/satellite configuration). LAAS improves user accuracy by eliminating errors common to the user and reference stations and also provides a guaranteed level of integrity. Integrity is an assurance provided to the user that bounds the difference between the unknown true position and the LAAS-derived position estimate with an extremely high degree of confidence. Anomalies or system failures that would violate this bound must be followed by a timely warning that either establishes a new bound or indicates that no guarantee can be made [2]. These bounds are computed by users in the position domain based on information broadcast by the LGF and are known as the lateral and vertical protection levels (VPL). Typically, the closer a maneuver brings an aircraft to a runway (i.e., the ground), the more stringent the require- ments on integrity become. In particular, for aircraft approach and landing procedures, the vertical alert limits that the VPL must fall within are more stringent due to the presence of nearby obstacles. The current version of LAAS is designed to meet the demands in accuracy, availability, and integrity needed for category 1 precision approaches. To do this, the system monitors many known error Received 13 August 2009; revision received 29 April 2010; accepted for publication 29 April 2010. Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/10 and $10.00 in correspondence with the CCC. * Assistant Professor, Department of Aviation and Technology, One Washington Square; Seebany.Datta-Barua@sjsu.edu. † Assistant Professor, Department of Aerospace Engineering, 335 Gwahangno, Yuseong-gu (Corresponding Author). ‡ Senior Research Engineer, Department of Aeronautics and Astronautics, Durand Building, Room 250. § Research Engineer, Department of Aeronautics and Astronautics, Durand Building, Room 250. ¶ Research Assistant, Department of Aeronautics and Astronautics, Durand Building, Room 250. ** Research and Development Engineer, Department of Aeronautics and Astronautics, Durand Building, Room 250. †† Professor, Department of Aeronautics and Astronautics, Durand Building, Room 250. Member AIAA. JOURNAL OF AIRCRAFT Vol. 47, No. 4, July–August 2010 1141