Session F1: AVIATION APPLICATIONS
Paper #1

GNSS PERFORMANCES LIMITATION OVER AIRPORTS AT HIGH LATITUDE AND IN GEO-SIGNALS OVERLAPPING ZONE:
F. Costa, G. Nobile, A. Parente, C. Carnebianca, IDS - Ingegneria Dei Sistemi S.p.A., Italy

As well known, the Initial Operational Capability (IOC) of the GNSS elements under implementation (WAAS, EGNOS, MSAS) shall rely on a reduced set of geostationary satellites with respect to that required by a full operational system. Moreover, the Geo-signals interoperability under assessment, may not provide a seamless solution as expected. In this scenario, Aeronautical users in terminal phases of flight during high bank angle manoeuvres, may experience service performance limitation, due to the following conditions :
* complete loss of the Geo-signals at high latitude Airports or,
* need to switch to a Geo-signal pertaining to another Regional Area

The objective of this paper is to analyse and quantify from the Users's point of view the GNSS operational limitations over selected Airports at high latitude and in Geo-signals overlapping zones , and, at the same time, verify the flexibility of the Satnav Flight Procedures to overcome or mitigate potential constraints.

The study cases are based on flight trajectories, simulating approaches and missed approaches covering extreme conditions as concerns bank angle and orographic environment, carried out with SAPET (Satnav Performance Evaluation Tool) a product developed for Flight Procedure Design and Validation.

The results show that :
* for high latitude Airports, loss of Geo-signals ( antenna mask angle dependent ) is experienced during high bank manoeuvres and it can be translated into Aircraft bank and heading angles restriction
* for the overlapping zones, need to switch between interoperable Geo-signals, in case of lack of full interoperability, may be considered as complete loss of Geo-signals leading to (even though less critical) Aircraft heading angles restriction as well.
* Inclusion of the Aircraft optical mask mitigates the above restrictions, due to the improvement of satellites visibility below the local horizon.

In conclusion, this paper characterizes the initial GNSS performances and limitations from User's operational view point during Terminal Phase of Flight in areas where the Geo-signal availability is most critical, such as high latitude sites and interoperable Geo-signals overlapping zones. Restrictions to aircraft bank angle and heading, along mitigation effects ,when the optical mask of the aircraft is considered, are quantified, providing useful contribution to Satnav Flight Procedure implementation process.
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Session F1: AVIATION APPLICATIONS
Paper #2

SENSITIVITY OF WAAS LNAV/VNAV COVERAGE WITH THE NEW INTEGRITY MONITOR DESIGN:
R. Fries, S. Alexander, C. Griffith, E. Altshuler, S. Peck, Raytheon Company

Raytheon is developing the Wide Area Augmentation System (WAAS) under contract with the Federal Aviation Administration (FAA). Raytheon is currently in the process of finalizing the algorithms to be used in redesigned Integrity Data Monitor (IDM). The new IDM was devised to be provably safe, proving a probability of Hazardously Misleading Information (HMI) being presented to the user of less than 10-7 per approach. This approach is defined to be Lateral Navigation/Vertical Navigation (LNAV/VNAV); an approach procedure that combines a basic vertical navigation capability with conventional non-precision approach lateral guidance. LNAV/VNAV capability requires that the Vertical Protection Limit (VPL) be less than 50m.

Raytheon is currently in the process of Algorithm Validation of the new IDM. The redesigned IDM has included a number of new algorithms. Algorithm Validation includes: alarm analysis/mitigation, parameter tuning, performance analysis and timing of the algorithms in the IDM. Alarm mitigation and parameter tuning are chosen to maximize system availability and continuity. As algorithm validation progresses, WAAS LNAV/VNAV Availability has been shown to be highly sensitive some of the features of these new algorithms plus existing ones. These areas include: ionospheric threat models, Ionosphere Pierce Point (IPP) Coverage, radius size, tuned Parameters such as false alarm rate and levels of HMI protection, and general alarming. This paper will give a brief overview of each one of these areas and explore its effect on WAAS LNAV/VNAV Availability.
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Session F1: AVIATION APPLICATIONS
Paper #3

HIGH SPEED LORAN-C DATA COMMUNICATIONS - FLIGHT TEST RESULTS:
B. Peterson, K. Dykstra, Peterson Integrated Geopositioning, LLC; P. Swaszek, University of Rhode Island; J. Boyer, United States Coast Guard LORAN Support Unit; M. Narins, Federal Aviation Administration

This paper reports on the results of flight tests in an ongoing project to develop methods for transmitting GPS Wide Area Augmentation System (WAAS) data over LORAN. The project is being funded by the Federal Aviation Administration under their program to determine what potential benefits LORAN-C might provide to the National Airspace System. The full 250-bit WAAS message of 250 bits will be transmitted in one second. This information bandwidth is achieved via 16-ary Intrapulse Frequency Modulation (IFM). Two types of forward error correction are being evaluated; the standard rate convolutional encoding as in the WAAS signal specification but with interleaving, and Reed-Solomon block encoding.

Two series of flight tests are scheduled. The first will be in May and June and will be done using the LORAN Support Unit's test transmitter at Wildwood, NJ. The second series will use the Tok, Alaska. The FAA Technical Center's Convair aircraft at flying at speeds of up to 250 knots will be used in the testing. The results of both series of tests will be presented.
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Session F1: AVIATION APPLICATIONS
Paper #4

VISUALIZING AIRBORNE GPS RECEIVER PERFORMANCE VIA SIMULATION 3-DIMENSIONAL VISUALIZATION AND BLANKING TOOL (3D-VBT):
M. Chaloupka, Control Systems Research Inc.; D. McDowell, WR-ALC/LKNE

BACKGROUND
Engineers at the Warner Robins GPS Integrated Support Facility (ISF) are tasked to maintain fielded GPS User Equipment (UE). This effort entails normal software upgrades and verification through GPS simulation. Occasionally, UE receiver performance anomalies are reported from field users for investigation. It is the role of the ISF to accurately stimulate the failure mode making it observable for subsequent corrective action. Through use of GPS RF simulators and receiver hardware-in-the-loop (HWIL) testing, ISF engineers attempt to reconstruct the events by constructing simulation scenarios. Prior to the receipt of the 3-Dimensional Visualization and Blanking Tool (3D-VBT), simulation engineers relied on non real-time detailed trajectory analysis to determine instances when satellite or jammer line of sight visibility to the GPS antenna was obstructed by aircraft fuselage, terrain or other features. Then, simualtor scenario files are modified to disable specific satellite or jammer RF power based on time intervals to simulate periods of blockage. This time consuming process did indeed result in the receiver reselecting constellations and thus injecting noise into the tracking loops. However, with the 3D-VBT system, satellite or jammer visibility is automatically performed during execution.

DESCRIPTION
This ariticle describes ISF staff experience with a new PC-based 3-dimentional visualizaion and blanking tool (3D-VBT) to analyze GPS receiver performance. This simulator enhancement enables engineering anlysis of field reported anomalies to focus on antenna, fuselage, and satellite constellation interactions in both clear and jamming enviroments. Development of the 3D-VBT was aided by Small Business Innovation Research (SBIR) contracts awarded to Control Research Systems. This new tool allows the ISF staff to automatically reproduce satellite and jammer obscuration effects in real-time and quickly analyze receiver performance against simulator truth data.
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Session F1: AVIATION APPLICATIONS
Paper #5

GPS/IRS HYBRIDIZATION: FAULT DETECTION AND ISOLATION OF MORE THAN ONE FAILURES:
A.C. Escher, C. Macabiau, ENAC, France; N. Martin, Thales Avionics

Measurements made by a GPS receiver on-board an aircraft are nominally affected by ionospheric error, multi-path, noise, jamming, and they may be subject to satellite failure. Yet they provide good long term accuracy. As inertial sensors do not depend on external signals, they are not sensitive to these effects, but position obtained from inertial measurements is affected by slow low frequency drift.

Because the two systems are complementary, GPS/IRS hybridization is a good candidate to fulfill demanding civil aviation requirements. When the integrity of the GPS measurements is ensured these measurements may be used to calibrate inertial position and improve accuracy. This can be done in a tightly coupled manner through a Kalman filter.
Calibrated IRS can ensure coasting while maintaining good short term accuracy and helps detect large GPS failures.

But, the hybridized system must also be able to detect slowly growing errors on GPS measurements that may affect inertial calibration. These slowly growing errors may affect one or several GPS measurements, depending whether they are due to a satellite failure or jamming for example.

Fault detection and exclusion capacity of Receiver Autonomous Integrity Monitoring, is limited for GPS, as it is designed for one failure only, and doesn't provide enough availability for NPA. Therefore other solutions have been proposed, among these the extrapolation method or the averaging solution separation method.

The study reported in this paper presents a method for fault detection and isolation using information from a tightly coupled GPS/IRS system when more than one failures occur at a time during operations ranging from en-route to CAT I, with the objective to satisfy the availability requirement.

The paper starts with a review of civil aviation requirements and a brief definition of detection algorithm parameters in the case where multiple failures occur at a time. This section also contains an assessment of signal in space degradation modes such as satellite failure and jamming.

A model of a tightly coupled GPS/IRS system is presented. Then, the results of the detection and isolation satellite failures based on the former method are described.

The whole system is now composed of a complete Kalman reinforced by a concurrent method in order to exclude failed measurements from several satellites. The proposed method provides alarms and exclusion levels.

The simulated performance of the algorithm when submitted to several failures is then presented: false alarm rate, horizontal and vertical protection levels, time to alert. This performance is compared with respect to civil aviation requirements.
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Session F1: AVIATION APPLICATIONS
Paper #6

THE USE OF RAW GPS FOR VERTICAL NAVIGATION:
J.D.A. Graham, NAV CANADA

Several organizations concerned with flight safety have recommended that vertical navigation be provided wherever possible to reduce CFIT accidents. The safety benefits associated with a stabilized approach are well established.

Barometric vertical navigation (Baro VNAV) has been available to pilots of FMS-equipped aircraft for some time. However, the operators of aircraft using panel-mount GPS receivers have not benefited from this technology.

A low-cost baro serializer was installed on a Cessna 172 and used to provide vertical guidance on the intermediate and final approach segments of standalone GPS approaches. Initial testing indicated that the concept was viable, and that a 95% navigation sensor error (NSE) of 13 metres was achieved.

It was thought that raw GPS might be used to provide an independent integrity check of the barometric altitude, and that the performance of such a system would be improved in the absence of selective availability. The results of flight testing indicated that raw GPS consistently outperformed the baro sensor for accuracy, stability, and noise. This led to the proposal to use the GPS signal in lieu of baro for vertical guidance.

Static GPS altitude data were collected at a single site which yielded a 95% accuracy of 7.2 metres. It is noted that while these results are rather limited, and do not consider the effects of ionospheric anomalies or satellite malfunctions, they are probably indicative of the nominal accuracy that might be expected.

The paper assesses three methods of providing vertical integrity. The first, and easiest to implement, considers the VNAV guidance to be advisory only, leaving the pilot to ensure that stepdown altitudes are respected, using the altimeter as the primary vertical reference. The other two employ barometric altitude as an independent check of the GPS-derived altitude. In all cases, lateral integrity is provided by RAIM.

The paper also considers several operational issues associated with this application of GPS, and describes the test program conducted to address those issues.

While raw GPS will not likely support the level of service that WAAS will accommodate, it represents a means of providing vertical guidance to LNAV minima in regions where WAAS GLS will be unavailable, and should be practical to implement in newer C129A-certified receivers.
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Session F1: AVIATION APPLICATIONS
Paper #7

AVAILABILITY OF GPS/INS INTEGRATION METHODS:
T. Murphy, M. Harris, Boeing Air Traffic Management; M.S. Braasch, Ohio University

This paper presents the results of an availability analysis for several GPS/INS integration methods. The availability of a range of RNP levels for current fielded Airplane Based Augmentation Systems (ABAS) designs, and the Multiple Solutions Separation (MSS) augmentations is presented. This performance is compared with the availability of RNP service using RAIM with fault detection (RAIM FD) and RAIM with fault detection and exclusion (RAIM FDE) augmentations.

The availability analysis was performed using several different assumed GPS satellite constellation configurations (e.g. 24 satellites, 30 satellites etc.). Constellation states operating for 24 hours with up to 4 satellites removed were considered and weighted by the probability of their occurrence. The availability of a range of RNP levels (up to 4 nautical miles) given each augmentation is presented in graphical form for several user locations. Some discussion of the regional variation of the results is included.

The paper shows the relative merits of tightly coupled GPS/INS integration for GPS integrity monitoring in terms of increased availability of specific levels of service. The paper further contrasts this difference to the sensitivity of service availability to other factors such as the number of satellites in the constellation as well as the satellite reliability (i.e. MTBF, MTTR etc.)
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Session F1: AVIATION APPLICATIONS
Paper #8

MONITORING AIR FORCE EJECTION SEAT TESTS USING GPS-BASED POSITION AND VELOCITY:
B.R. Tredway, M.M. Miller, AFIT/ENG

Test and evaluation of the United States Air Force's latest aircraft ejection seat technology requires very accurate position and velocity profiles during each test run to determine the relative positions between the aircraft, ejection seat, manikin and the ground. Current test and evaluation systems, which rely on expensive camera systems to determine the position and velocity profiles, are limited in their reliability and ability to track more than two objects at a time.

This paper presents preliminary design and test results from a new GPS-based system capable of monitoring all major ejection test components (including multiple ejection seat systems) during an entire ejection seat test run. Small, low-power, lightweight GPS receivers, capable of handing high-accelerations, are mounted in the ejection seat and the manikin to maintain track of each component during the ejection seat firing, manikin separation, and landing.

This paper presents the system design and its performance compared to various simulated ejection seat profiles (actual system test results will be provided if an actual ejection seat run is accomplished and analyzed before the conference).
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Session F1: AVIATION APPLICATIONS
Alternate #1

DETECTION OF MULTIPATH INFLUENCE AT DGPS REFERENCE STATIONS:
M. Hoper, H. Hoffmann, T. Boelow, Aerodata Systems GmbH, Germany

The disturbance of GPS measurements caused by multipath influence is known as a major source of error in current GPS applications and limits the availability of a carrier phase solution. Different approaches are already available to minimize this influence. Until one of these approaches leads to a product that is available at a reasonable price, multipath will keep disturbing the user's position.

To enhance the navigation system's integrity, the receiver autonomous integrity monitoring (RAIM) is used. But it can usually identify only one disturbed satellite. The possibility of detecting signals influenced by multipath only by inspecting range and carrier phase measurements of the satellites would further increase the integrity of GPS based navigation systems. Knowledge about the signal's multipath disturbance for each channel would allow identifying and excluding more than one influenced satellite if there are no other constraints, like GDOP.

During the research a method was found that extracts an indication value for the magnitude of multipath influence based on the receiver's raw data. This value can be calculated online for each visible satellite and refers to a previously defined time window. The algorithm mainly uses code phase information and thus is not applicable for carrier phase multipath detection. This method was developed to increase the integrity and availability of a GPS carrier phase solution, which is need for flight inspections purposes.

This paper deals with this detection of multipath influence at DGPS reference stations, which means for stationary applications, and provides an overview about the concept of the detector.
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