Sam Pullen

Wide area augmentation of the Global Positioning System

The Wide Area Augmentation System (WAAS) is being deployed by the Federal Aviation Administration (FAA) to augment the Global Positioning System (GPS). The WAAS will aid GPS with the following three services. First, it will broadcast spread-spectrum ranging signals from communication satellites. The airborne WAAS receiver will add these new ranging signals to the GPS constellation of measurements. By so doing, the augmented position fix will be less sensitive to the failure of individual system components, thus improving time availability and continuity of service. Second, the WAAS will use a nationwide ground network to monitor the health of all satellites over our airspace and flag situations which threaten flight safety. This data will be modulated on to the WAAS ranging signals and broadcast to the users, thereby guaranteeing the integrity of the airborne position fix. Third, the WAAS will use the ground network to develop corrections for the errors which currently limit the accuracy of unaugmented GPS. This data will also be included on the WAAS broadcast and will improve position accuracy from approximately 100 m to 8 m. When complete, the augmented system will provide an accurate position fix from satellites to an unlimited number of aircraft across the nation. It will be the primary navigation system for aircraft in oceanic routes, enroute over our domestic airspace, in crowded metropolitan airspaces, and on airport approach.

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

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 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.

Paired overbounding and application to GPS augmentation

The relationship between range-domain and position-domain errors remains an open issue for GPS augmentation programs, such as the Federal Aviation Administrations Local Area Augmentation System (LAAS). This paper introduces a theorem that guarantees a conservative error bound (overbound) in the position domain given similarly conservative overbounds for broadcast pseudorange statistics. This paired overbound theorem requires that a cumulative distribution function (CDF) be constructed to bound both sides of the range-domain error distribution. The paired overbound theorem holds for arbitrary error distributions, even those that are non-zero mean, asymmetric or multimodal. Two applications of the paired overbound theorem to GPS augmentation are also discussed. First, the theorem is employed to construct an inflation factor for a non-zero mean Gaussian distribution; in the context of a simulation of worst-case satellite geometries for 10 locations in the United States and Europe, the required inflation factor for broadcast sigma is only 1.18, even for biases as large as 10 cm for each satellite. Second, the theorem is applied to bound a bimodal multipath model tightly; the result shaves more than 40% off the previously established inflation factor derived through a more overly conservative analysis.

Assessment of Ionosphere Spatial Decorrelation for Global Positioning System-Based Aircraft Landing Systems

Ground-based augmentations of the global positioning system demand guaranteed integrity to support aircraft precision approach and landing navigation. To quantitatively evaluate navigation integrity, an aircraft computes vertical and lateral protection levels as position-error bounds using the standard deviation of ionosphere spatial decorrelation. Thus, it is necessary to estimate typical ionospheric gradients for nominal days and to determine an appropriate upper bound to sufficiently cover the differential error due to the ionosphere spatial decorrelation. Both station-pair and time-step methods are used to assess the standard deviation of vertical (or zenith) ionospheric gradients (σ vig ). The station-pair method compares the simultaneous zenith delays from two different reference stations to a single satellite and observes the difference in delay across the known ionosphere pierce point separation. Because these ionosphere pierce point separations limit the observability of the station-pair method, the time-step method is also used to better understand ionospheric gradients at short distance scales (10-40 km). The time-step method compares the ionospheric delay of a single line of sight at one epoch with the delay for the same line of sight at another epoch a short time (a few to tens of minutes) later. This method has the advantage of removing interfrequency bias calibration errors on different satellites and receivers while possibly introducing an estimation error due to temporal ionospheric gradients. The results of this study demonstrate that typical values of σ vig are on the order of 1-3 mm/km for nonstormy ionospheric conditions. As a result, σ vig of 4 mm/km is conservative enough to bound ionosphere spatial decorrelation for nominal days and still leave enough margin for more active days and for non-Gaussian tail behavior.

Ionospheric Threat Mitigation by Geometry Screening in Ground-Based Augmentation Systems

Large spatial variations in ionospheric delay of Global Navigation Satellite System signals observed during severe ionospheric storms pose potential threats to the integrity of the Ground-Based Augmentation System, which supports aircraft precision approaches and landing. Range-domain monitoring within the Ground-Based Augmentation System ground facility cannot completely eliminate all possible ionospheric threats, because ionospheric gradients are not observable to the ground monitor if they impact the satellite-to-ground lines of sight with the worst-possible geometry and velocity. This paper proposes an algorithm called position-domain geometry screening to remove potentially hazardous satellite geometries under worst-case ionospheric conditions. This is done by inflating one ormore integrity parameters broadcast by the ground facility. Hence, the integrity of the system can be guaranteedwithout anymodification of existing avionics. This paper develops an algorithm that allows the ground station to conservatively estimate the worst-case ionospheric errors for Ground-Based Augmentation System users. The results of this algorithm determine which potential aircraft satellite geometries are safe and which are unsafe, and inflation of the broadcast vig parameter is used to make all unsafe geometries unusable for the Ground-Based Augmentation System. Although the elimination of unsafe geometries reduces system availability, this paper shows that acceptable availability for category I precision approaches is attainable at Memphis International Airport and Newark Liberty International Airport while guaranteeing system integrity under anomalous ionospheric gradients.

Paired overbounding for nonideal LAAS and WAAS error distributions

A significant challenge in fielding space-based and ground-based augmentation systems (SBAS and GBAS) for GPS involves the validation of navigation integrity, which requires the establishment of error bounds for aircraft position. This paper introduces a new approach to validating position-domain integrity by using two-sided envelopes for each ranging source. This paired-bounding approach allows for error distributions of arbitrary form and thus improves on earlier integrity validation approaches restricted to zero-mean, symmetric, and unimodal distributions

Targeted Parameter Inflation within Ground-Based Augmentation Systems to Minimize Anomalous Ionospheric Impact

Anomalous ionospheric conditions can cause large variations in propagation delays of transionospheric radio waves, such as global navigation satellite system (GNSS) signals. Although very rare, extremely large spatial variations pose potential threats to ground-based augmentation system (GBAS) users. Because GBAS provide safety-of-life services, namely precision approach and landing aircraft guidance, system safety must be guaranteed under these unusual conditions. Position-domain geometry-screening algorithms have been previously developed to mitigate anomalous ionospheric threats. These algorithms prevent aircraft from using potentially unsafe GNSS geometries if anomalous ionospheric conditions are present. The simplest ground-based geometry-screening algorithm inflates the broadcast vig parameter inGBAS to signalwhose geometries should not be used.However, the vig parameter is not satellite-specific, and its inflation affects all satellites in view. Hence, it causes a higher than necessary availability penalty. A new targeted parameter inflation algorithm is proposed that minimizes the availability penalty by inflating the satellite-specific broadcast parameters: prgnd andP values. In this new algorithm, prgnd and P values are inflated by solving optimization problems. The broadcast parameters obtained from this algorithm provide significantly higher availability than optimal vig inflation at Newark Liberty International Airport and Memphis International Airport without compromising system safety. It is also demonstrated that the computational burden of this algorithm is low enough for real-time GBAS operations.

Navigation, Interference Suppression, and Fault Monitoring in the Sea-Based Joint Precision Approach and Landing System

The United States Navy seeks the capability to land manned and unmanned aerial vehicles autonomously on an aircraft carrier using GPS. To deliver this capability, the Navy is developing a navigation system called the Sea-Based Joint Precision Approach and Landing System (JPALS). Because standard GPS is not sufficiently precise to land aircraft on a shortened, constantly moving runway, Sea-Based JPALS leverages dual-frequency, carrier-phase differential GPS navigation. Carrier phase measurements, derived from the sinusoidal waveforms underlying the GPS signal, are very precise but not necessarily accurate unless the user resolves the ambiguity associated with the sinusoids periodicity. Ensuring the validity of ambiguity resolution is the central challenge for the high-integrity, safety-critical JPALS application. Based on a multi-year, multi-institution collaborative study, this paper proposes a navigation and monitoring architecture designed to meet the guidance quality challenge posed by Sea-Based JPALS. In particular, we propose a two-stage navigation algorithm that meets the aggressive integrity-risk requirement for Sea-Based JPALS by first filtering a combination of GPS observables and subsequently exploiting those observables to resolve the carrier ambiguity. Because JPALS-equipped aircraft may encounter jamming, we also discuss interference mitigation technologies, such as inertial fusion and array antennas, which, with appropriate algorithmic modifications, can ensure integrity under Radio Frequency Interference (RFI) conditions. Lastly, we recommend a fault monitoring strategy tailored to the two-stage navigation algorithm. Monitoring will detect and isolate rare anomalies such as ionosphere storms or satellite ephemeris errors which would otherwise corrupt ambiguity resolution and positioning in Sea-Based JPALS.

Strictly Positive Real H Controller Synthesis via Iterative Algorithms for Convex Optimization

The synthesis of strictly positive real H 2 controllers for collocated control of large space structures is addressed. To perform this synthesis, a convex optimization technique has already been developed using linear matrix inequalities. This existing technique is based on two design criteria: strict positive realness and the use of H 2 norm as the criterion for optimization. It adopts a common Lyapunov solution to both of these criteria, which results in undesirable conservatism. To reduce this conservatism, a new synthesis technique based on iterative algorithms that can produce superior, noncommon Lyapunov solutions is proposed. Even if a common Lyapunov solution is infeasible, the proposed technique can yield feasible, strictly positive real H 2 controllers. An illustrated example is included.

Assessment of Nominal Ionosphere Spatial Decorrelation for LAAS

The authors would like to thank Todd Walter (Stanford),Jason Rife (Stanford), Ming Luo (Stanford), Juan Blanch (Stanford), Boris Pervan (IIT), Livio Gratton (IIT), and John Warburton (FAA Technical Center) for their help during this research. We also would like to express special thanks to Attila Komjathy at JPL for providing data and comments. The advice and interest of many other people in the Stanford GPS research group is appreciated, as is funding support from the FAA Satellite Navigation Program Office. The opinions discussed here are those of the authors and do not necessarily represent those of the FAA, the GPS Joint Program Office, or other affiliated agencies.