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Session B4, Paper #1 Incremental Benefits of Dual Frequency SBAS WAAS enhances the GPS standard positioning service by meeting the stringent integrity, accuracy, availability and continuity requirements of commercial aviation. The system provides en-route through non-precision approach (LNAV), Lateral NAVigation / Vertical NAVigation (LNAV/VNAV), and Localizer Performance with Vertical guidance (LPV) approach capabilities. But WAAS is more than a navigation system for pilots. Most any commercially available GPS receiver bought today is WAAS enabled allowing everyone from hikers and bikers to surveyors, farmers and rescue workers to enjoy the benefits of improved accuracy and integrity in day to day activities. WAAS provides real time corrections and integrity bounds to single frequency L1 users for ionospheric delay as well as corrections to the orbit and clock computed from the satellite ephemeris. The single biggest uncertainty term in the error bounds stems from the estimate of ionospheric delay. Future satellites will provide additional civil frequencies which can be used to correct for the effects of ionospheric delay on the pseudorange measurements. Using two frequencies, users around the world will gain the ability to improve accuracy and correct for ionospheric errors without the need for augmentation which provides opportunity for SBAS to provide improved service. The addition of a second civil signal on block IIRM satellites can improve accuracy by mitigating the effects of ionospheric delay. The benefit of L2C is obstructed by an unfortunate hindrance for WAAS. Aviation users are unable to utilize the L2C signal because it does not lie in an ARNS protected radio frequency band. Unlike L2C, the L5 signal, which will be launched on block IIF satellites and GPS III satellites, lies in a protected ARNS band opening the door to the development of dual frequency L1/L5 aviation receivers which correct for ionospheric delay. The dual frequency version of the satellite based augmentation system will be reverse compatible with legacy (L1 only) user equipment, take far better advantage of the dual frequency L1/L5 GEO satellite ranging capabilities, operate without a loss of availability during solar storms and have improved continuity and availability due to lower uncertainty bounds on the ionospheric delay. Dual frequency SBAS will also drastically improve the performance of systems built in the equatorial region where ionospheric disturbances are observed on a regular basis. Unfortunately, the launch schedule for a full constellation of L5 capable satellites may push out past 2018 forcing aviation users to wait a long time to take advantage of the benefits of these new satellites. To facilitate the seamless cutover from a single frequency system to a dual frequency system, the FAA is considering replacing the existing dual frequency L1/L2 semi codeless receivers in the WAAS ground system with receivers capable of processing L1, L2 semi codeless, L2C and L5 signals. Supposing that such receivers are in place and available in the 2014 timeframe this paper examines the benefits of providing dual frequency corrections with only a partial constellation of L5 capable satellites. Availability and continuity benefits are examined for constellations with 6, 12, 18 and 24 L5 capable satellites. The analysis also examines the benefits of dual frequency corrections for GEO satellites in the event of a GPS satellite outage. Benefits always come at a cost and in the case of providing dual frequency corrections to L1/L5 users the cost is complexity. With a full constellation of L5 capable satellites, the upgrade to dual frequency is relatively simple. With only a partial constellation, the system will have to be enhanced to utilize three frequencies (L1 from all satellites, L2 from older satellites, and L5 from new satellites). For dual frequency operations, new standards will be required to describe the use of the new correction messages. New algorithms will be required in the ground system to process the new signals (and combinations of the new signals) which the various types of user equipment can use. Special consideration will need to be given to ensuring the integrity of the position solution for users which mix single frequency and dual frequency satellites in the position solution. This paper will trade off these costs with the overall advantages of utilizing dual frequency satellites as they are launched. [Return to Program] Session B4, Paper #2 Ephemeris Type A Fault Analysis and Mitigation for LAAS The Local Area Augmentation System (LAAS) has been developed by the FAA to enable precision approach and landing operations using the Global Positioning System (GPS). Each LAAS installation provides services through a LAAS Ground Facility (LGF) which is located at the airport it serves. By monitoring the GPS signals, measurements, and navigation messages, the LGF is able to exclude unhealthy satellites and broadcast real-time range-correction messages for healthy satellites to users via a VHF data link. Airborne users apply these corrections to remove errors that are common between the LGF and the aircraft. The LGF is also responsible for warning the aircraft of any potential integrity threats that cannot easily be resolved by excluding unhealthy satellites. One source of potential errors is the broadcast ephemeris message, which users decode and use to compute GPS satellite positions. In LAAS, potential GPS ephemeris faults are categorized into two types, A and B, based upon whether or not the fault is associated with a satellite maneuver. This work focuses on aviation threats caused by type A faults. To detect and mitigate these threats, we investigate two LGF monitors based on comparing expected ranges and range rates with those measured by the LGF. The effectiveness of these monitors is analyzed and verified in this paper. [Return to Program] Session B4, Paper #3 Enabling the LAAS Airport Surface Movement The Local Area Augmentation System (LAAS) can be used for both precision approach and Differentially Corrected Positioning Service (DCPS) applications. Through its support of DCPS, the LAAS Ground Facility (LGF) is required to meet the integrity requirements of all other operations that could use the LAAS VHF Data Broadcast (VDB). One of our previous works [1] demonstrates that the existing DCPS integrity requirements cannot be met by CAT I LAAS without changes to both the definition of DCPS integrity [2,3] and the airborne receiver requirements [4]. Another of our previous works [7] identifies the changes that are required and recommends specific sets of alternatives. One of its conclusions is that some future applications of LAAS that planned to use DCPS, such as airport surface movement, cannot be supported by DCPS with the CAT I LAAS architecture. It suggests one important further change to the LAAS avionics requirements. The current LAAS MOPS forbids use of the LAAS Position/Velocity/Timing (PVT) outputs if DCPS is not enabled by the LGF [4]. As our previous paper [8] points out, even if DCPS is enabled, it will not support all applications that can make use of the PVT outputs. Therefore, the PVT outputs should be "de-linked" from DCPS so that they can be used independently. PVT applications that cannot be supported by DCPS should be defined as separate applications of LAAS in the same manner as precision approach. We believe that, if airport surface movement is defined as a separate operation, it will be supported by the existing LGF geometry screening that mitigates the ionosphere-anomaly threat for CAT I precision approach. Confirming this hypothesis requires a more-intensive study of the requirements on airport surface movement and is the subject of this paper. The simulation procedure used to analyze the LAAS requirements on airport surface movement has two parts. The first part is to obtain horizontal position error (HPE) and horizontal protection level (HPL) and has been expanded from the methodology in [1,7]. One day of geometries with five-minute updates from 10 major U.S. airports (including Memphis) is used to generate all in view, all N 1, all N 2, ., down to all 4-satellite subset geometries. The maximum supported distance from LGF to user, defined as Dmax and included in the VDB [4], is nominally 45 kilometers, although shorter maximum separations are also evaluated. The simulation is performed at the several LGF-to-user separations within 10 kilometers. Worst-case GPS range errors from the ionosphere anomaly threat model for the Conterminous U.S. (CONUS) [6] are applied to all individual satellites in all allowed subset geometries. The computed HPE and HPL are obtained, and the largest HPE and corresponding HPL are stored for each subset geometry. The second part of the simulation is the existing LGF geometry screening described in [8]. Each geometry whose HPE is not bounded by its HPL is selected. For a certain Horizontal Alert Limit (HAL), a geometry whose HPL is greater than a HAL is screened out. Then, the standard deviation of vertical ionosphere gradient (sigma_vig) is inflated until every HPL of selected geometries is greater than a HAL so that all remained geometries of selected ones can be also removed. Unlike DCPS [7], both case of the worst-case ionosphere impact to any pair of satellites (as done for precision approach) and to individual satellites are considered for airport surface movement. The investigation will be conducted to see whether the existing LGF geometry screening alone can support airport surface movement or not. If it can't support, the results in this paper include various combinations of additional aircraft geometry screening proposed in [7] to lower to lower the Maximum Unprotected Error (MUE) to a usable level while maintaining useful availability. In addition, weighted RAIM using the method in [5] has been evaluated to obtain additional (although not dramatic) reduction of the screening HAL. Airborne ionosphere-gradient monitoring using a variation of the LGF code-carrier divergence technique will also be evaluated. Table and Figures with results for the many alternative tested will be shown in the paper. References: [1] Y. S. Park, S. Pullen, and P. Enge, "Mitigation of Anomalous Ionosphere Threat to Enhance Utility of LAAS Differentially Corrected Positioning Service (DCPS)," Proceedings of IEEE/ION PLANS 2008, Monterey, CA, May 6-8, 2008. [2] Specification: Category I Local Area Augmentation System Ground Facility. Washington, D.C., Federal Aviation Administration, FAA-E-2937A, April 17, 2002. [3] Minimum Aviation System Performance Standards for the Local Area Augmentation System (LAAS), Washington, D.C., RTCA SC-159, WG-4, DO-245A, December 9, 2004. [4] Minimum Operational Performance Standards for GPS Local Area Augmentation System Airborne Equipment. Washington, D.C., RTCA SC-159, WG-4, DO-253C, December 16, 2008. [5] T. Walter and P. Enge, "Weighted RAIM for Precision Approach," Proceedings of ION GPS-95, Palm Springs, CA, September 12-15, 1995. [6] S. Pullen, Y.S. Park, and P. Enge, "Impact and Mitigation of Ionospheric Anomalies on Ground Based Augmentation of GNSS," Radio Science, Vol. 44, 2009. [7] Y.S. Park, S. Pullen, and P. Enge, "Enabling the LAAS Differentially Corrected Positioning Service (DCPS): Design and Requirements Alternatives," Proceedings of ION GNSS 2009, Savannah, GA, September 22-25, 2009. [8] J. Lee, M. Luo, S. Pullen, Y.S. Park, M. Brenner, and P. Enge, "Position-Domain Geometry Screening to Maximize LAAS Availability in the Presence of Ionosphere Anomalies," Proceedings of ION GNSS 2006, Fort Worth, TX, September 26-29, 2006, pp. 393-408. [Return to Program] Session B4, Paper #4 Testbed for Dual-Constellation GBAS Concepts Satellite based Navigation Systems (Global Navigation Satellite System GNSS) will become a major element in the navigation infrastructure of the future. In addition to classical en-route and terminal navigation, where GNSS is increasingly used, approach and landing procedures are being developed and implemented based on GNSS. To meet the requirements of integrity, accuracy, continuity and availability for precision approach and landing operations augmentation systems are needed. Currently there are two augmentation systems for these periods of flight available: Ground Based Augmentation Systems (GBAS) and Space Based Augmentation Systems (SBAS). At the Research Airport of Braunschweig a Galileo Test Environment called "aviationGATE" is currently under development by the Institute of Flight Guidance, using a total of 9 ground located transmitters to cover an area of up to 5.500 square-kilometres. Each of these ground transmitters is able to send three navigation signals in both frequency bands (I/NAV on E1b and E5b, F/NAV on E5a) or freely defined contents. First components are in a test stage. Based on the different experiences with GBAS data gathering campaigns in the past this Test Bed will be further developed and optimized. An initial concept has already been designed and will be discussed in the paper which includes among others the implementation of an aircraft positioning module and an aviationGATE correction message uplink module. With the coverage area including the airport as well as the already implemented GPS approaches this test bed will serve as an ideal environment for tests of GBAS CAT II/III dual constellation concepts. This paper will describe the "aviationGATE" at the research airport of Braunschweig with its capabilities and its planned expansions towards a test bed for GBAS multi constellation research. The possibilities and limitations of such a test environment for early GBAS GAST-D prototype equipment and system concepts will be presented. [Return to Program] Session B4, Paper #5 Multi-Layer Modeling and Simulation of the Effects of Ionospheric Scintillation on Service Availability of the GPS Augmentation Systems This paper describes a multi-layer technique using modeling and simulation to assess the impact of ionospheric scintillation on satellite navigation systems. It is well known that GPS receivers in low- and high-latitude regions may suffer from rapid amplitude and phase fluctuations and signal scatter known as scintillation, causing loss of lock and cycle slips to dual as well as single frequency users. Scintillation poses a challenge to the GPS augmentation service providers (SBAS and GBAS) to provide a high probability of navigational service availability in affected regions. Over the recent decades, many studies and data analyses have been conducted to understand effect of scintillation which affects users of satellite based navigation systems. Some of the more significant data has been translated into modeling and simulation tools to help characterize scintillation effects and the world-wide scintillation environment. The objective of our modeling and simulation process is to combine hardware-in-the-loop assessments of scintillation effects on GPS sensors with probabilistic models to predict the overall system impact with respect to time and location. The technique involves the use of three steps. The first provides a high fidelity assessment of the performance of the GPS sensors to determine the levels of tolerable scintillation. The second step uses the output of the first step as thresholds to determine the probability of disruption for the desired scintillation environment. Lastly, the overall system impact is assessed using the probabilities of disruption determined in the second step with a system availability model which reflects the target system's architecture and algorithms. To perform the three steps, a combination of three simulation tools was used for characterization, data generation and analysis. For the first step, the Cornell Scintillation Simulator was used by permission of Cornell University to inject simulated scintillation effects, in a hardware-in-the-loop fashion, into the GPS receiver. The scintillation simulated by the Cornell tool is based on extensive libraries of past scintillation data including equatorial data. Measures of scintillation in amplitude and phase are quantified by S4 index and spima(phi)? phase variations respectively. User Action Files based on desired S4 index and time correlation parameters are thus loaded into the Spirent 7700 GPS Simulator which is connected to the GPS receiver under test. Data collected from the receiver is then analyzed to determine the quantity and duration of loss of lock as well as cycle slip counts and other anomalies. Assessment of these statistics provides tolerable levels of scintillation for the receiver under test which can be parameterized based on S4 and sigma(phi). The next tool is the Wideband Model (WBMOD) which was used by permission from AFRL to predict the probability of scintillation effects in terms of S4 and sigma(phi). The estimates are based on climatological models for global distribution and behavior of ionospheric plasma-density irregularities causing scintillation and are parameterized based on solar activity (sun spot number), level of ionospheric disturbance (Kp), day of year and other parameters to define the world-wide scintillation environment. The scintillation characterized by the WBMOD tool is also based on extensive libraries of past scintillation data including equatorial data. The outputs are given in terms of probability of the threshold levels of S4 or sigma(phi) along the lines of site for the desired scenario. The availability model reads the probabilities of disruption per line of sight and assesses the system performance for the architecture and algorithms of the system of interest. A dual frequency differential GPS system utilizing floating point ambiguity resolution was studied; however given sufficient fidelity any desired system design could be applied. The combined result is a very controlled, repeatable, hardware-in-the-loop test which can evaluate the GPS receiver performance as well as assessment of the overall system under the given climate conditions to provide a characterization. The results are obtained on the basis of high fidelity prior data which can save considerable time, resources and difficulty over performing live data collection under scintillation conditions. [Return to Program] Session B4, Paper #6 A New GBAS NSE Model for CAT II/III Autoland Simulations Since many years, civil aviation has identified GNSS as an attractive mean to provide navigation services for every phase of flight due to its wide coverage area. However, to do so, GNSS has to meet requirements in terms of accuracy, integrity, availability and continuity. To achieve this performance, augmentation systems have been developed to correct the GPS L1 C/A measurements and to monitor the quality of the received Signal-In-Space (SIS). Different solutions exist depending on where and how the augmentation is implemented. We can distinguish between ABAS, SBAS and GBAS. In particular GBAS (Ground Based Augmentation System) allows guarantying a very high level of performance in a given coverage area around an airport. GBAS is composed of a ground station able to compute differential pseudorange corrections and to monitor the quality of the SIS. This station includes several receivers making pseudorange measurements used to compute corrections and integrity information which are sent through a RF data link to the surrounding aircrafts. Using this information, the user receiver is able to correct its measurements but also to exclude some satellites and to compute protection levels which are an evaluation of the confidence that the user can have in the final position solution. Currently, GBAS is foreseen as an important source of innovation for Civil Aviation since it may reach civil aviation requirements down to CAT II/III minima unlike other GNSS augmentation systems. Indeed, GBAS has already been certified for CAT I precision approaches, and it is anticipated that GBAS can provide further performance to meet CAT II/III requirements. GBAS could thus be an alternative to ILS for precision approaches. This possibility is actively investigated and ICAO and Industry standardization bodies are currently deriving requirements for GBAS CAT II/III. The development of these new requirements follows a new concept using GBAS in an innovative fashion. Indeed, this new concept named "GAST D" results from a performance based approach taking credit of aircraft capabilities to allow the use of GBAS technology to reach CAT II/III minima, instead of putting all the constraints on the Signal in Space. In this new approach, there will be a transfer of responsibility from the ground station to the aircraft, unlike ILS. New requirements on the Signal-In-Space and on the airborne side will impact the noise and the errors affecting the outputs of the fault-free on-board receiver which is the interface between GBAS SIS and the autopilot guidance laws. Autoland demonstrations for CAT II/III certification require numerous simulations to assess statistically the aircraft capability to autoland when the autopilot is receiving deviations from an ILS receiver for instance. It is thus necessary to identify precisely the GBAS GLS behavior to perform autoland simulation, in line with applicable regulations for CAT II/III operations. As a consequence, there is a need for a model depicting the behavior of the outputs of the GBAS system for CAT II/III performance validation. A model has been proposed in the past but the lack of information on the methods used to validate it makes it necessary to investigate further this subject for a better understanding and to provide an advanced theoretical knowledge of GBAS. The aim of this paper is to present a GBAS L1 C/A model reproducing with sufficient fidelity the behavior of the system for CAT II/III autoland simulations. Early results concerning the study of a state of the art GBAS NSE model have already been published in ION GNSS 2009 proceedings. This previous study focused on the nominal GBAS NSE model. Its advantages and weaknesses have been identified and updates have been proposed. This new paper describes additional features of the model which are the step function generator, the limit case generator and the fault mode generator. Moreover, it defines the final whole architecture of the proposed updated GBAS NSE model. The first section is an overview of the architecture of the model which can be divided into three different modules: the nominal NSE generator, the step function generator and the limit case generator. The proposed implementation supports GAST-C but also GAST-D requirements such as geometry screening and 30 seconds code-carrier smoothing and can therefore be used for CAT-I and CAT-II/III autoland simulations. The nominal NSE generator is briefly described in the second section. It allows generating nominal GBAS NSE by assuming that it can be modeled as three independent sequences filtered by a second order filter accounting for tracking loops and code carrier smoothing filter effects and scaled by sigma factors drawn from experimental distributions. Third section is a description of the step function generator which represents the impact of satellite constellation changes on the GBAS NSE. The probabilities of step events and their statistics have been evaluated through simulations. Step magnitudes are then obtained in the model by generating samples from Gaussian distributions scaled by sigmas drawn from the observed distribution of the standard deviation of steps magnitude. The last section is a review of the limit case generator which produces extreme errors for limit case autoland simulations. [Return to Program] Session B4, Alternate #1 A Comparison of the Improvement of Positioning Accuracy in the Edge of the EGNOS Territory - In Coordinate Domain and Pseudorange Domain 1 Introduction EGNOS (European Geostationary Navigation Overlay Service) entered its operational phase in October 2009 and its primary service is now available to all users equipped with EGNOS-compatible receivers. EGNOS is actually almost a standard functionality for lots of commercially available satellite navigation receivers nowadays. A crucial question now topical is how EGNOS works in the edge of its service area, where the availability of the system is decreased. At the border of the EGNOS territory, the availability of EGNOS SIS is degraded because of 1. low elevation angle (for example: in Finland) ; 2. missing corrections (especially ionospheric correction); 3. and the obstacle in vicinity where the visibilities to EGNOS GEO satellites are poor. To address this shortcoming, lots of substitutionary technologies for Non-GEO transmissions of EGNOS SIS have been investigated and tested, such as EGNOS TRAN, pseudolite transmission and ESA's SISNeT. 2. Motive Both Finland and Malta are located on the border of the EGNOS service area, the performance is degraded comparing to that in central Europe: some of the EGNOS provided measurement corrections are missing, especially when considering ionospheric corrections. The EGNOS performance is degraded due to the shortcoming of the missing corrections. A low-end receiver typical for mass-market products is chosen for assessment in this paper, and the evaluated EGNOS performance in the low-end receiver gives a higher reference value for industrial designers. The low-end GPS receivers provide the end user with the NMEA messages only, with which it is impossible to apply all corrections into the pseudorange domain. This study compares the EGNOS real-time performance in position coordinate domain with that of pseudorange domain. For some time-critical applications, the few minutes it takes to initialize the EGNOS correction may be too critical to the solution and ruin the user experience. Because of poor visibilities of EGNOS GEO satellite, the initialization may take even longer in some cases. Meanwhile, the flexibility of the ESA's SISNeT solution offers an alternative for time-critical applications by shortening the EGNOS correction initialization time to one minute. 3. System Configuration and Implementation Following the motivation, two PDA based user terminal EGNOS application programs are under test to evaluate the performance of EGNOS. The first one is upgraded from the original SISNeT receiver developed by the Finnish Geodetic Institute under ESA contract, which applies the EGNOS augmentations in position coordinate domain. The second approach considered is the user terminal for EGNOS-pseudolite (PL) based approach which applies the corrections in the pseudorange domain. Considering that effect of Kalman filter may complicate the conclusions, it is necessary to disable the Kalman filter option both on receiver side and user terminal side. Typically, a low-end GPS receiver provides the end user with NMEA messages only. In this paper, the EGNOS corrections are estimated in position coordinate domain with a NMEA output of the Fastrax iTrax03 based receiver. As a counterpart, the pseudorange information from the same receiver is also obtained via a binary "iTalk" protocol, and the position is calculated based on the pseudorange domain with the pseudorange corrections simultaneously. Both the position coordinate and the pseudorange domain solutions are based on a PDA real-timely. 4. Experiments and results The GPS antenna is placed on a known position and connected to a Fastrax GPS receiver. Two serial ports of the receiver are connected to two user terminals: one contains the NMEA stream to the SISNeT receiver and the other contains the binary "iTalk" stream to the user terminal of the EGNOS-PL implementation. The SISNeT receiver fetches the EGNOS stream via an internet connection and the user terminal of the EGNOS-PL approach receives the EGNOS SIS from the GEO satellite. Series of experiments were carried out both in Finland and Malta to evaluate the EGNOS real-time performance with a low-end GPS receiver (Fastrax 03 series). The augmentation corrections included in the EGNOS message are applied to compute the final user coordinates both in position coordinate domain and pseudorange domain, side by side. The first test case was carried out to evaluate the EGNOS performance in Finland. The raw data were collected on a surveyed site on the roof of an office building of the Finnish Geodetic Institute for 1 hour. The coordinates of the antenna of the receiver were determined with an RTK (Real Time Kinematic) solution. During the test, the SISNeT receiver fetched the EGNOS stream from the SISNeT server via a fixed-line internet connection. The second case was carried out to evaluate the EGNOS performance in Malta. The only difference to the first case was that the SISNeT receiver fetched the EGNOS stream from the SISNeT server via a GPRS wireless internet connection. 5. Conclusion From the primary analysis, the EGNOS corrections improve the positioning accuracy by applying the corrections to the position coordinate domain and the pseudorange domain. It is concluded that the EGNOS corrections improve the positioning accuracy a bit in the low-end receiver. There are two reasons for this result 1. The noise of the embedded ARM chip in the low-end receiver overwhelms the system noise and this is the error that EGNOS can not correct. 2. Since Finland and Malta are located in the edge of the EGNOS service area, due to the limited layout of the monitoring stations, some of the GPS satellites in view always get no ionospheric correction at all, which decreases the geometry of the constellation comparing with GPS standalone solution, since then these no-correction signals are not used in the solution. ESA's SISNeT service can shorten the initialization time effectively, which can be a substitute augmentation service for any time-critical EGNOS application. [Return to Program] Session B4, Alternate #2 An Examination of Practical Means to Authenticate Navigation Data Channels for Aviation The integrity of navigation system outputs is a paramount concern for safety of life applications, such as aircraft landing. Aviation systems such as spaced based augmentation system (SBAS) and ground based augmentation system (GBAS) provide information to assure the performance of Global Navigation Satellite Systems (GNSS) and to bound inaccuracies in the GNSS derived position. However, these systems are not designed to provide assurance against GNSS spoofing or spoofing of the data contained in its own broadcasts. The former problem can be mitigated by many means such as directional antennas or authentication signals. This paper focuses on the later. It discusses the need for such authentication and details means to achieve important elements to enable robust authentication. Specifically, it develops key components necessary to enable feasible and reasonable implementation of data authentication that is compatible with the currently field augmentation systems. Finally, it uses both SBAS and GBAS as case studies and provides a data authentication design that is compatible with and reasonable for these systems. GNSS augmentation systems primarily provide information to aid GNSS integrity. Fundamental to these systems is information integrity. Part of information integrity is the ability to authenticate the source of the data. Currently, this assurance is not generally built into these systems. Traditional data authentication techniques can be used to provide this assurance. However, augmentation systems have requirements that differ from the channels for which these techniques were designed. In particular, the data is more time sensitive and the bandwidth is much more limited. Additionally, user and system equipment are designed for decades of service with little to no upgrades. As a result, aviation requires data authentication that is 1) fast, 2) robust to message loss, 3) not resource intensive 4) self contained and 5) robust to future attacks. Traditional data authentication techniques must be adapted to perform to these specifications under limited bandwidth conditions. Meeting these desired qualities seem very difficult given the constrained bandwidth. However, the unique characteristics of augmentation systems and its operational use can also be used to aid the design. These attributes limit the types of attacks that are feasible against the system as well as provide means to cross check information. Important considerations for any data authentication are key distribution and authentication protocol. This paper presents a key distribution protocol that utilizes the operation of the aircraft and air traffic to aid in key verification. The goal is to reduce the infrastructure, bandwidth and networks necessary to distribute and certify keys. The paper also presents several options for data authentication protocols. Protocols based on asymmetric and symmetric key are discussed. In particular, modifications to the Time Efficient Stream Loss-tolerant Authentication (TESLA) protocol are developed to help provide the desired characteristics mentioned previously. The paper uses the current L1 SBAS and GBAS as case studies. The paper will show that a reasonable method can be achieved on the current SBAS using about 20% of bandwidth. Message types for authentication and the queuing strategy is outlined. The method is compatible to current SBAS user equipment in that they will not be adversely affected. [Return to Program] Session B4, Alternate #3 A Software-Based Receiver Sampling Frequency Calibration Technique and its Application in GPS Signal Quality Monitoring We present a novel software-based technique to accurately calibrate a GPS receiver's RF front end ADC sampling frequency. Due to manufacturing and environmental reasons, a GPS receiver's actual and manufacturer specified sampling frequency may differ significantly. Accurate knowledge of the sampling frequency is fundamental for high accuracy and high sensitivity receiver signal processing. Additionally, applications such as pre-correlation signal quality monitoring employing periodic averaging and dithered sampling techniques to enhance the signal-to-noise ratio (SNR) and sampling resolution depend on the accuracy of the sampling frequency [1]. An analysis of the receiver signal processing performance degradation in the presence of sampling frequency error is presented in the paper as a motivation for the project. This performance degradation is much more serious than the one caused by the deviation of sampling/re-sampling rate from the integer multiples of the pseudorandom noise code rate for periodic averaging. Experimental measurements and subsequent software receiver processing results are presented to support the analysis. The method we developed to estimate the accurate sampling rate is a solely software-based technique requiring no additional hardware other than the GPS receiver RF front end output samples. The technique applies a sequence of operations including carrier (plus Doppler) wipe off, navigation data removal, periodic averaging preprocessing, and hierarchical correlation peak search to digital samples of a software receiver RF front end output. It is important to note that there is a prior software-based technique designed for the same purpose [2]. In that method, however, it is necessary to have a priori knowledge of the specific frequency plan of the RF front end circuit and complex receiver output data fitting. The performance of the technique is evaluated using RF simulator signals as well as live GPS signals collected by GPS data acquisition equipments manufactured by several different vendors. Several performance measures are used to support the evaluation including clear Power Spectrum Density (PSD) envelopes, waveforms/eye-diagrams, constellation plots, amplitude probability density histograms, and correlation-related characteristics. The results show that we can calibrate the sampling rate with an accuracy resolution down to 0.125Hz, the reciprocal of the averaging duration, and the pre-correlation SNR can be potentially improved by 39dB using periodic averaging after the calibration. Detailed sampling frequency error impact on pre-correlation signal quality monitoring, the proposed software-based technique to calibrate the sampling frequency using GPS receiver RF front end output, and the technique performance evaluations using RF simulator and live GPS signals from multiple vendors will be presented in the paper. [1] Marco Pini, Dennis Akos. Exploiting GNSS Signal Structure to Enhance Observability. IEEE Transactions on Aerospace and Electronic Systems, October 2007.. [2] Liou, L. L., D. M. Lin, J. B. Tsui, J. Schamus, Y. Morton, "Frequency Characterization of A/D Converter in Software GPS Receivers," Proc. 2005 Joint IEEE Int'l Frequency Control Sym. & Precise Time & Time Interval Sys. & Appl. Meeting, Vancouver, Canada, August 2005. [Return to Program] |