Local Area Augmentation System Research Articles

Integration of the ground-based augmentation system in continuous descent operations

Predictability is a necessity in today's commercial flight operations if delays are to be minimized. Possible mitigations, such as the implementation of continuous descent arrivals (CDAs), may help generate the required procedural predictability to curtail these delays. However, on the basis of its individual avionics suite, each aircraft follows a different barometric vertical descent path; furthermore, the aircraft may be given additional level-off segments by air traffic control to compensate for possible deviations in the vertical profile. Therefore, aircraft flying the current descent profiles are too unpredictable and thus the procedures cannot be implemented in complex airspace structures or when traffic is moderate to high. This study introduces a mitigation to this problem by taking the global positioning system (GPS) one step further by integrating the GPS-based ground-based augmentation system (GBAS) into the new optimized continuous decent arrival procedure, subsequently, allowing for the introduction of a geometrical vertical guidance, namely, the GBASCDA, which will make the aircraft flying the descent profile predictable. With the increase in availability of GBAS aircraft equipage, specifically the GPS landing system (GLS), and the approaching amendment of the Appendix E of the Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment–RTCA DO-253D, this idea of the integration of the GBAS on the arrival/descent segment as a viable solution may be finally coming within reach.

Estimation of overbound on ionospheric spatial decorrelation over low‐latitude region for ground‐based augmentation systems

In ground-based augmentation systems (GBAS), such as local area augmentation system (LAAS) of USA, variation of ionospheric delay between LAAS ground facility and the user leads to residual ionospheric error. Hence, the users need to compute the bounds on residual ionospheric error with the help of a parameter, σvertical_ionospheric_gradient (σvig). As σvig is a function of geographic region, the estimated value of σvig for mid-latitude regions (4 mm/km) is not appropriate for low-latitude regions where ionosphere is highly varying and is characterised by large spatial gradients. In this study, gradients are computed with processed total electron content data from 16 stations of global positioning system aided geo augmented navigation network and using the time-step method. The distribution of gradients is compared with one-sigma Gaussian distribution to obtain ‘inflation factor’. σvig overbound is computed for several days data comprising of quiet, moderate and storm conditions of ionosphere using the corresponding inflation factors. It is found that overbound values of 4.5 and 14.4 mm/km are required to bind the gradients under quiet and active conditions. These findings are useful for designing a safe and reliable GBAS for low-latitude stations.

INVESTIGATION OF THE EFFECT OF IONOSPHERIC GRADIENTS ON GPS SIGNALS IN THE CONTEXT OF LAAS

Local Area Augmentation System (LAAS) is expected to enable the pilots to guide the aircraft more precisely and safely into busy airports even in poor visibility conditions. The anomalous low and equatorial latitude Ionosphere is severe threat to the LAAS system. To characterize the anomalous ionospheric gradients, the performance of an ionospheric threat model is evaluated. In our investigation, in contrast to the reported work available in the open literature, smoothed code phase measurements are used in the threat model to obtain precise ionospheric time delay. The three key parameters of the threat model gradient slope (mm/km), width (km) and front speed (m/s) are used in the analysis. Further, geometry screening using Maximum Ionosphere Induced Error in Vertical (MIEV) as a key parameter is carried out to identify the stationary gradients and its impact on system performance for CAT-I operations. A maximum ionospheric gradient of 355.74mm/km over a distance of approximately 75km is reported at mid latitudes. Whereas, in our flndings at low/equatorial latitudes even within a distance of approximately 4km a maximum gradient of 460mm/km is observed, which is comparatively very high. Our results show that, there is necessity to enhance upper bound for the ionospheric gradients threat space over low latitudes.

A novel LAAS pseudo-range error over-bound method based on improved pseudo-range error distribution model

The high level of safety demand of civil aviation requests local area augmentation system (LAAS) extremely high navigation integrity performance. A new LAAS pseudo-range error over-bound method is proposed in this paper to improve the integrity of LAAS. Firstly, a more practical pseudo-range error distribution model is established. Then, by calculating the relationship between the statistical uncertainty of the model parameter and the integrity risk, a new method is proposed to calculate the pseudo-range error over-bound model. This method can effectively reduce the inflation factor and the resulting conservativeness of the over-bound model. Comparative experiments show that the method proposed in this paper performs better and satisfies the requirements of real applications.

Mitigation of Code-carrier Divergence Effect using an Optimized Filter for LAAS Applications

Local Area Augmentation System (LAAS) is an augmentation to GPS, for providing aircraft precision approach and landing. LAAS relies on Hatch Alter for code smoothing but its Altered range is affected by code-carrier divergence occurring during ionospheric storms. Substituting traditional Hatch filter by optimized filter reduces this effect to a considerable extent. The objective of this paper is to estimate code-carrier divergence and mitigate it using the optimized filter. The performance of both Hatch and optimized filters is analyzed for different ionosphere conditions such as the geomagnetic storm of October 2003 over a low latitude station. Investigations showed that the optimized filter outperforms Hatch Filter for disturbed day data, whereas for quiet day data, it does not provide any additional advantage. The results obtained are agreeing with the results published elsewhere and are encouraging.

High-Integrity GPS/INS Integrated Navigation with Error Detection and Application to LAAS

A dynamic state realization for tightly coupling Global Positioning System (GPS) measurements with an Inertial Navigation System (INS) is described. The realization, based on the direct fusion of GPS and INS systems through Kalman filter state dynamics, explicitly accounts for temporal and spatial decorrelation of GPS measurement errors (such as tropospheric, ionospheric, and multipath errors) through state augmentation, thereby ensuring Kalman filter integrity under fault-free error conditions. Predicted system performance for a Local Area Augmentation System (LAAS) aircraft precision approach application is evaluated using covariance analysis and validated with flight data.Built-in fault detection mechanisms based on the Kalman filter innovations are also evaluated to help provide integrity under certain fault conditions. It is shown that an algorithm based on the integral of Kalman filter innovations outperforms other existing GPS fault detection methods in the detection of slowly developing ranging errors, such as those caused by ionospheric and tropospheric anomalies.

Multiple reference consistency check for LAAS: a novel position domain approach

Since the traditional Maximum Likelihood-based range domain multiple reference consistency check (MRCC) has limitations in satisfying the integrity requirement of CAT II/III for civil aviation, a Kalman filter-based position domain method has been developed for fault detection and exclusion in the Local Area Augmentation System MRCC process. The position domain method developed in this paper seeks to address the limitations of range domain-based MRCC by focusing not only on improving the performance of the fault detection but also on the integrity risk requirement for MRCC. In addition, the issue of the stability of the Kalman filter in relation to the position domain approach is considered. GPS range corrections from multiple reference receivers are fused by the adaptive Kalman filter at the master station for detecting and excluding the single reference receiver' failure. The performance of the developed Kalman filter-based MRCC has been compared with the traditional method using experimental data. The results reveal that the vertical protection level is slightly better in the traditional method compared with the developed Kalman filter-based approach under the fault-free case. However, the availability can be improved to over 97% in the proposed method relative to the traditional method under the single-fault case. Furthermore, the fault-tolerant positioning result with an accuracy improvement of more than 32% can be achieved even if different fault types are considered under the single-fault case. In particular, the algorithm can be a candidate option as an augmentable complement for the traditional MRCC and can be implemented in a master station element of the LAAS integrity monitoring architecture.

Design of the pulse duty cycle of the pseudolite signal in local area augmentation systems

The short-pulse TDMA signal is an excellent solution to the near-far problem in the pseudolite augmentation systems, and the pulse duty cycle is an important element in the solution. This paper aims at designing the duty cycle of the pseudolite pulse signals with a theoretical method. At first, the scope of the duty cycle is derived theoretically with the object of ensuring the reception of both the pseudolite signals and the satellite signals, and the expression for the theoretical upper limit is given. Then based on the proposed expression, the duty cycles of Galileo pseudolite pulse signals are designed. The theoretical upper limit can guide the design of the short-pulse TDMA pseudolite signals.

High Fidelity Antenna Model Validation Results of a GNSS Multipath Limiting Antenna

The local area augmentation system (LAAS) is a proposed satellite-based landing system by the Federal Aviation Administration (FAA) to serve as a navigation aid for commercial aircraft during the approach and landing phase of flight. The LAAS utilizes a special ground-based antenna array called the multipath limiting antenna (MLA) to provide differentially corrected global navigation satellite system (GNSS) data to oncoming aircraft as well as to reduce the largest error contributor to pseudo-range estimates; the ground multipath. The performance of the MLA is characterized in terms of its desired-to-undesired (D/U) gain ratio, phase center variation (PCV) and group delay (GD). A high fidelity antenna model (HFAM) of the MLA is presented that estimates the D/U, PCV, and GD as a function of elevation angle. The model consists of two main components: (1) a microwave feed distribution circuit model created using a circuit simulator, and (2) a radiating structure model developed using a full-wave method of moments (MoM) field solver. The performance of the model was assessed against antenna range measurements. The results of the HFAM show good correlation for the D/U and PCV while a significant bias was observed on the results for the GD. The dependence of the GD on the radiating element's impedance, tuning process, and Doppler frequency shift are demonstrated via simulations.

Algorithm and Flight Test Results to Exchange Code Noise and Multipath for Biases in Dual Frequency Differential GPS for Precision Approach

A modified Code Noise and Multipath (CNMP) algorithm is presented for dual-frequency differential GPS precision approach and landing. Pseudorange noise and multipath are exchanged for reducible biases in both reference station and aircraft ranging measurements. The second frequency is used only to correct the code-minus-carrier observable. Corrected pseudorange measurements are combined using a single-frequency carrier phase position domain smoothing (CPDS) algorithm. Flight test vertical and horizontal navigation system error is reduced from 0.80 m to 0.32 m and from 0.41 m to 0.33 m, respectively, as compared to Local Area Augmentation System single-frequency processing. For six included 150-s precision approaches, 95% error drops from 0.56 m to 0.26 m vertically and 0.28 m to 0.14 m horizontally. Composite protection levels (PLs) using CNMP-derived quantities are much smaller than modified LAAS PLs. Mean PL values are reduced from 5.90 m to 2.60 m vertically and from 3.24 m to 1.38 m horizontally.

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.

Effects of Pseudolite Positioning on DOP in LAAS

In this paper, effects on DOP (Dilution of Precision) due to augmentation of Global Positioning System (GPS) with pseudolites are investigated. For this purpose, a typical Local Area Augmentation System (LAAS) scenario is consi-dered by placing pseudolites in various positions. It is found that only properly located pseudolites can improve the DOP. DOP values with two pseudolites located on either side of the run way are found to be the best. Geometric DOP (max) was found to be nearly 4 due to only GPS and came down to approximately 2 due to augmentation with two pseudolites. Implementation aspects of Bayes and Kalman filters while estimating DOP values are also examined.

A WIDE BAND ANTENNA FOR MULTI-CONSTELLATION GNSS AND AUGMENTATION SYSTEMS

Local Area Augmentation System (LAAS) based on multi- constellation GNSS can provide improved accuracy, availability and integrity needed to support all weather category II and III precision approach landing of aircraft. In order to receive satellite signals of GNSS, an antenna working over wide frequency band and high phase center stability is preferred. Commonly used antennas like crossed dipoles, patch etc. are inherently narrow band. This paper describes the design and development of half-cardioid shaped dual arm, wide band printed circuit antenna. The antenna has low VSWR of < 3 : 1, a stable phase center and good right hand circularly polarized radiation patterns covering full L-band frequencies. The simulated and measured results compare well. This compact antenna can also be used on ground, ship and airborne platforms to receive signals from multiple GNSS satellites above the horizon.

Sigma Overbounding using a Position Domain Method for the Local Area Augmentaion of GPS

The local area augmentation system (LAAS) is a differential GPS navigation system being developed to support aircraft precision approach and landing navigation with guaranteed integrity and availability. While the system promises to support Category I operations, significant technical challenges are encountered in supporting Category II and III operations. The primary concern has been the need to guarantee compliance with stringent requirements for navigation availability. This paper describes how a position domain method (PDM) may be used to improve system availability by reducing the inflation factor for standard deviations of pseudo-range correction errors. Used in combination with the current range domain method (RDM), a 30% reduction in the inflation factor is achieved with the same safety standard. LAAS prototype testing verifies the utility of the PDM to enhance Category II/III user availability.

Impact and mitigation of ionospheric anomalies on ground‐based augmentation of GNSS

This paper describes the impact that extreme ionospheric spatial gradients occurring during severe ionospheric storms have on Global Navigation Satellite System Ground Based Augmentation Systems and how the U.S. Local Area Augmentation System (LAAS) mitigates the integrity risk due to these events. Gradients in slant ionospheric delay of as large as 425 mm/km over baselines of 40–100 km have been observed in the United States during ionospheric storms since April 2000 by both the U.S. Wide Area Augmentation System and the network of Continuously Operating Reference Stations receivers. Because ionospheric gradients affecting a LAAS site may not be observed by the LAAS Ground Facility (LGF) before users are affected, a simulation‐based method has been developed to determine, in near real time, which potential LAAS user geometries would be unacceptably threatened by a hypothetical worst‐case ionosphere gradient. Geometries of this type are made unavailable to LAAS users by having the LGF inflate the broadcast ionospheric‐gradient sigma, which increases the Vertical Protection Level of the unacceptable geometries so that they exceed the allowed Vertical Alert Limit; thus ensuring user safety.

Cascade Dual Frequency Smoothing for Local Area Augmentation System

Both temporal and spatial gradients of ionosphere errors have adverse effects upon local area augmentation system (LAAS). This article proposes a cascade dual frequency smoothing (CDFS) method to overcome this problem. One dual frequency smoothing filter is used to secure a precise estimate of ionosphere error, which is then used to modify ionosphere error in the code pseudorange measurement before it is put into another dual frequency smoothing (DFS) filter to attenuate noise. Thus, the ionosphere error is thoroughly removed from the smoothing process and excessive noise induced by GPS L2 code measurement is suppressed. Effectiveness of CDFS and accuracy of CDFS LAAS is analyzed based on data collected from LAAS test bed in Communication, Navigation, Surveilliance/Air Traffic Management (CNS/ATM) Labs, Civil Aviation Administration of China (CAAC). The analysis demonstrates that CDFS method can eliminate residual errors in the smoothed pseudorange raised by the ionosphere temporal gradient and differential correction errors caused by the ionosphere spatial gradient.

Formulation of a time-varying maximum allowable error for ground-based augmentation systems

Ground-based augmentation systems (GBAS), such as the federal aviation administration's local area augmentation system (LAAS), protect GPS users against signal anomalies by monitoring for rare satellite faults. This paper introduces a new, time-varying MERR (maximum-allowable error in range) formulation that enables proof of integrity for ground-based fault monitoring. The time-varying MERR supersedes earlier MERR methods which relied on a static integrity test. The utility of these earlier methods was limited in that they neglected GBAS time-to-alert (TTA) requirements and restricted the choice of the monitor filter, requiring its impulse response to match that of the user ranging-error filter. By contrast, the time-varying MERR explicitly incorporates TTA and places no restrictions on monitor filter design. These attributes are critical for correctly evaluating system integrity and for crediting the integrity benefits of GBAS systems with aggressive filter designs and process timing.

Detecting RFI Through Integrity Monitoring at a DGPS Reference Station

Because GPS is a radio navigation system which has a very low power level, it is vulnerable to RFI. Excessive RFI could cause receiver performance degradation, such as degradation of position accuracy, loss of lock and increased acquisition time. After GPS modernization plans introduce dual-frequency civil signals to mitigate ionospheric errors, RFI will remain as one of the dominant threats for differential GPS navigation systems. Examples of safety-critical civil aviation and military missions threatened by RFI include the Local Area Augmentation System (LAAS) and the Joint Precision Approach and Landing System (JPALS). This paper focuses on RFI mitigation through integrity monitoring for a DGPS system like LAAS or JPALS. The mitigation strategy consists of two parts. First, the paper develops a new RFI detection method, using a raw divergence statistic. Second, the paper investigates strategies for maintaining integrity in the case that RFI is detected.To validate the utility of the divergence-based RFI monitor, this paper takes an experimental approach. The experiments assess the performance of the divergence metric and compare it to existing alternatives for RFI detection, such as metrics for Automatic Gain Control (AGC) and carrier-to-noise ratio (C/N0). Generating a monitoring threshold for these statistics proves challenging, because the threshold depends both on the type of RFI threat (e.g. continuous wave, narrow band, wideband, pulsed) and on environmental conditions, such as temperature. As experiments illustrate, the divergence statistic resolves these limitations, as divergence directly estimates ranging source error, independent of the type of RFI threat or the environmental conditions.

Performance and Availability Analysis of a Simple Local Airport Position Domain Monitor for WAAS

: The FAA has investigated the feasibility of a Local Airport Monitor (LAM) using the Wide Area Augmentation System (WAAS) with Local Area Augmentation System (LAAS) avionics for Category I (CAT I) precision approaches. This paper presents a simple LAM concept using position domain monitoring on the ground. A position solution using WAAS corrections with locally measured pseudoranges is compared to the surveyed position of the LAM reference antenna. If this position difference exceeds a threshold for any set of satellites the LAAS avionics might select, WAAS cannot be used for CAT I service. Availability of this LAM concept over CONUS is predicted to be 0.995 for a vertical alert limit (VAL) of 10 m with the optimal 24 GPS constellation. Operation of the method on real WAAS corrections and local pseudorange measurements gives 100 percent availability and no monitor false alarms with a current satellite constellation and a quiet ionosphere.

Sigma-mean monitoring for the local area augmentation of GPS

The local area augmentation system (LAAS) is a ground-based differential GPS system being developed to support aircraft precision approach and landing navigation with guaranteed integrity. To quantitatively appraise navigation integrity, an aircraft computes vertical and lateral protection levels using the standard deviation of pseudo-range correction errors, /spl sigma//sub pr/spl I.bar/gnd/, broadcast by the LAAS ground facility (LGF). Thus, one significant integrity risk is that the true standard deviation (sigma) of the pseudo-range correction error distribution may grow to exceed the broadcast correction error sigma or that the true mean of the correction error distribution becomes excessive during LAAS operation. This event may occur due to unexpected anomalies of GPS measurements. To insure that the true error distribution is bounded by a zero-mean Gaussian distribution with the broadcast sigma value, real-time sigma and mean monitoring is necessary. Both direct estimation and cumulative sum (CUSUM) methods are useful to detect violations with acceptable residual integrity risk. For sigma monitoring, the estimation method more rapidly detects small violations of /spl sigma//sub pr/spl I.bar/gnd/ but the fast initial response (FIR) CUSUM variant more promptly detects significant violations that would pose a larger threat to user integrity. For the purposes of mean monitoring, the FIR CUSUM variant is superior to the estimation method in detecting any mean violations. The results demonstrate that real-time protection is achievable against all sizes of sigma/mean failures that can threaten navigation integrity.