Showing papers in "Annual of Navigation in 1994"

Federated Kalman filter simulation results

Abstract: This paper describes federated filter applications to integrated, fault-tolerant navigation systems, with emphasis on real-time implementation issues and numerical simulation results. The federated filter is a near-optimal estimator for decentralized, multi-sensor data fusion. Its decentralized estimation archi- tecture is based on theoretically sound information-sharing principles. A federated filter consists of one or more sensor- dedicated local filters, generally operating in parallel, plus a master combining filter. The master filter periodically com- bines (fuses) the local filter solutions to form the best total solution. Fusion generally occurs at a reduced rate, relative to the local measurement rates. The method can provide sig- nificant improvements in data throughput, fault tolerance, and system modularity. Numerical simulation results are pre- sented for an example multi-sensor navigation system. These results demonstrate federated filter performance characteristics in terms of estimation accuracy, fault-tolerance, and computation speed. This work was supported by the Defense Small Business Innovation

Signal Characteristics of GPS User Antennas

Abstract: The performance of the GPS user antenna has a great effect on the accuracy and precision of the data that is obtained This effect is often ignored or misunderstood This paper presents anechoic chamber measurements of the most important performance characteristics of eight models and configurations of GPS user antennas, and explores the effect of these characteristics on the data collected

Required navigation performance (rnp) for precision approach and landing with gnss application

Abstract: A methodology to determine the required navigation performance (RNP) for aircraft precision approach and landing under instrument meteorological conditions (IMC) is described. The RNP in turn defines an aircraft containment surface about the nominal flight path, called a tunnel, which specifies the allowed approach and landing flight path limits. If the aircraft and its navigation system satisfy the RNP, then the aircraft will successfully traverse the tunnel, terminating in a safe landing. The tunnel is defined by four RNP parameters: accuracy, integrity, continuity, and availability.

Isolation of GPS Multipath and Receiver Tracking Errors

Abstract: GPS receiver dynamic tracking error is an often misunderstood concept. Flight test results and field experiences have shown dynamics-induced errors that can be related directly to the receiver architecture. To evaluate the effects of multipath in airborne receivers, it is necessary to isolate multipath from receiver tracking errors. Separation of multipath and receiver tracking errors can be difficult, particularly for a highly dynamic receiver platform. This results from the fact that the two error sources can be similar in magnitude and frequency. Historically, receiver dynamic tracking error has often been ignored or overlooked when analyzing multipath data. This paper describes a controlled experiment that allows for the isolation of multipath and receiver tracking errors. The theory behind the dynamic tracking error phenomenon is described, along with the isolation technique. Data collection and analysis results are presented to confirm the viability of the technique.

GPS receiver RF interference monitoring, mitigation, and analysis techniques

Abstract: There have been numerous papers published on GPS integrity monitoring from the perspective of detecting and mitigating the effects of space segment failures. Another important aspect of GPS signal integrity that has received less attention is the monitoring and mitigation of radio frequency (RF) interference, which can result in degraded accuracy performance or complete loss of receiver tracking. This paper describes RF interference monitoring and mitigation techniques that have been developed for military GPS receiver applications where intentional jamming must be anticipated. RF interference analysis techniques are also presented. These techniques should be considered if RF interference becomes a source of GPS signal integrity problems for civil avionics applications.

Comparison of real-time ionospheric algorithms for a gps wide-area augmentation system (waas)

Abstract: The Federal Aviation Administration (FAA) Satellite Program Office is developing a GPS Wide-Area Augmentation System (WAAS) to support a precision approach capability down to or near the lowest Category I (CAT I) decision height (DH) of 200 ft. In one of the candidate architectures under development, a vector of corrections is sent to the user via geostationary communications satellites (e.g., Inmarsat). This correction vector includes components for ionospheric, clock, and ephemeris corrections. The purpose of this paper is to evaluate the performance of the grid-based algorithms and other real-time ionospheric algorithms that could be implemented at the ground ionospheric reference stations, as well as at the airborne receiver. Results show that all of the ionospheric algorithms used in this paper (grid-based, least-squares, and spherical harmonics) provide roughly equivalent performance. Based on an extensive data collection program, the error in estimating ionospheric delay is derived. An analysis of WAAS accuracy performance is also presented.

Electrostatically force balanced silicon accelerometer

Abstract: An accelerometer is fabricated by forming a proofmass (294, 295, 544) and at least one associated hinge (296, 522) in a silicon substrate (12, 112) by ion implantation and the formation of anoxide support layer (40, 42, 44) below the proofmass, subsequently integrally bonding two complementary proofmass (294, 295) and substrate structures (112) together, and then removing the oxide support layer (272) to leave the proofmass (294, 295) supported by the hinge (296) within the body of silicon material. The proofmass (300) may be electrically connected to a lead (174) extending through an etched recess (178) in one of the substrates; and the proofmass (294, 295, 300) may be electrically isolated or separated from the substrates (112) by an oxide layer (272) and by a change in conductivity type of the semiconductor material where the hinge (296) is structurally mounted to the substrates.

Integrity monitoring for precision approach using kinematic gps and a ground-based pseudolite

Abstract: Although DGPS is inherently robust to space-vehicle-related ranging errors, failures in the differential station or airborne receiver can be a threat to continuous precise positioning. The stringent required navigation performance (RNP) for Category III precision landing, therefore, establishes the need for an active integrity verification system. Carrier tracking of ground-based pseudolites can provide comprehensive integrity. The large geometry change that occurs during overflight of a ground-based pseudolite and the great precision of carrier-phase measurements can provide the leverage for effective autonomous integrity monitoring. The measurement residual statistic can be used to detect a wide range of conceivable fault scenarios occurring during the pseudolite overflight, including cycle slips, intentional tampering, and airborne database errors. The potential for autonomous integrity verification for Category III approach is demonstrated through analysis, simulation, and flight tests performed in a single-engine aircraft.

GPS System Integration and Field Approaches in Precision Farming

Abstract: Two phases of a precision farming project are described. Phase I consists of the collection of data from four test fields across Alberta using differential GPS (DGPS I combined with crop yield monitors and electromagnetic f EM) ground conductivity meters for measuring salinity. A geographic information system (GIS) is used to analyze and combine various layers of information obtained from each field in order to analyze yield variation. Positions better than 50 cm horizontally and 1 m vertically are obtained in DGPS mode using a robust carrier-phase smoothing of the code approach. The accuracy of the DGPS positions is verified independently through a crossover point analysis and a comparison with an ambiguity resolution on the fly t OTF) solution. DGPS positions are used to generate maps, which indicate that fields are not homogeneous in crop yield when they have been treated wlthout consideration for variability of soil, salinity, topography, or field history. Phase II, which consists of the application of variable-rate fertilizer based on the information gathered in Phase I, is summarized.

Real-time on-the-fly kinematic gps system results

Abstract: The development of a real-time positioning system using GPS carrier phase was formally initiated by the U. S. Army Corps of Engineers in 1988. The end goal of this effort was to have ready by 1993 an operational prototype capable of subdecimeter positions in three dimensions in real time. This goal has been met. Testing of this prototype began in March 1993. The first demonstration was in August 1993, with the first public demonstration following in October. The system typically resolves integer ambiguities on-the-fly (OTF) in 15 s or less, and requires no static initialization. It is robust and reliable to use for precisely positioning any mobile platform out to approximately 20 km. Testing has been performed under varying operating conditions to evaluate the limits of OTF ambiguity resolution for precisely positioning moving platforms. This paper summarizes the results of those tests. Early real-time tests performed have shown accuracies of l-3 cm in all three dimensions. Design of the final real-time system and its integration with hydrographic survey platforms are also discussed.

GPS Orbit Determination for the Extreme Ultraviolet Explorer

Abstract: A single-frequency GPS receiver was launched with the Extreme Ultraviolet Explorer (EUVE) mission in the summer of 1992. The low altitude of the EUVE and the placement of dual antennas directly on the spacecraft body provide an opportunity to investigate error sources and the level of accuracy obtainable for future satellite missions. GPS data from several periods have been processed with GIPSY-OASIS II software. Data from EUVE and from a global network of GPS receivers were combined to eliminate the effects of selective availability. Methods for removing ionosphere delay error from the single-frequency data and orbit solutions using the reduced dynamic technique were investigated. Solutions were assessed with internal consistency tests and through comparison with the GTDS orbit solutions produced at the Goddard Space Flight Center. Orbit precision and accuracy with GPS are at the 1 m level.

Real-time flight testing using integrity beacons for GPS category III precision landing

Abstract: A new system based on GPS Integrity Beacons (low-power pseudolite marker beacon transmitters situated underneath the approach path) has been developed to provide the highest possible performance (including accuracy, integrity, and availability) for Category III precision landing using the Global Navigation Satellite System (GNSS). The landing system based on Integrity Beacons defines the Kinematic GNSS Landing System (KGLS). Two new developments are reported: first, real-time, centimeter-level kinematic satellite positioning using the integrity beacon is evaluated in flight testing, and second, a new paradigm for real-time differential satellite positioning is introduced that eliminates the traditional data link. A special type of Integrity Beacon is the Omni Marker, which rebroadcasts each received satellite signal coherently using a new PRN code. An aircraft receiving both the direct signal and the rebroadcast is capable of differential ranging without a digital data link. The latency associated with traditional differential positioning is completely eliminated.

Performance of category IIIB automatic landings using C/A-code tracking differential GPS

Abstract: An experimental DGPS precision approach and landing system was installed and flight tested on the NASA Langley Transport Systems Research Vehicle (TSRV). The GPS ground reference station and avionics units used 10-channel, narrow-correlator, C/A-code tracking receiver engines. The avionics drove the aircraft flight control system with ILS “look-alike” vertical and horizontal angular deviations derived solely from DGPS C/A-code tracking position and velocity. The avionics did not make use of kinematic carrier-phase tracking with on-the-fly cycle ambiguity resolution techniques. A total of 40 DGPS-guided approaches and landings were performed at the NASA Wallops Flight Facility, 31 of them hands-off, automatic landings. Total system error, measured by laser tracker, met the proposed Category III required navigation performance (RNP) or “tunnel concept” accuracy requirements by substantial margins. Touchdown dispersion for the 31 automatic landings also met Category III RNP requirements with significant margin.

Novel Techniques for Loran‐C Skywave Estimation

Abstract: It is conventionally assumed that Loran-C receivers process only samples taken before the first skywave components arrive, 35-60 u s after the groundwave. This technique has significant limitations when implemented in receivers of finite bandwidth, which increase the rise times of the pulses and substantially reduce the amplitudes of the groundwave signals at the 30 KS point. For this reason, many current receivers take samples later in the pulse and so suffer skywave errors. This paper examines the consequences of this practice. It then proposes a new class of techniques to allow the onset of skywave signals to be detected so that the sampling point can be adjusted accordingly. Novel algorithms for distinguishing skywaves from the groundwave are presented. Their benefits, and the computing resources they require, are assessed. The paper includes theoretical analyses, simulations, and Monte Carlo experiments to demonstrate the performance of the techniques proposed.

Interferometric GPS Flight Reference/Autoland System: Flight Test Results

Abstract: An interferometric GPS flight reference system (IGPS FRS) has been implemented and flight tested on a transport category aircraft. The underlying principle of operation is similar to that used for kinematic surveying, and is also referred to as differential carrier-phase tracking. The objectives of the IGPS FRS include 0.1 m accuracy RMS (each axis); one or more updates per second; UTC time synchronization better than 0.1 ms; real-time, all-weather operation; and repeatable flight paths. The latter requirement calls for full aircraft integration and coupled flight. This paper discusses the IGPS FRS architecture, its principles of operation, the aircraft integration, and the initial static calibration and flight test performance results. These are the first reported results for an interferometric GPS flight reference/autoland system that resolves carrier-phase ambiguities in flight while simultaneously providing high-precision guidance to a transport category aircraft through direct coupling to its flight control system.*

Precision strike concepts exploiting relative gps techniques

Abstract: Many future weapons and weapon carriers are expected to have an avionics suite that includes an integrated INS/GPS set. This relatively inexpensive means of navigation motivates examination of whether very high accuracy (∼ 10 ft circular error probable [CEP]) may be obtainable using only this set of weapon avionics operating in either a relative or differential GPS mode. This paper presents several different weapon system concepts that exploit such a capability to attack both fixed and stationary (but mobile) targets. It also addresses the theory behind each concept that enables the high accuracy and discusses the assumptions relating to each technique. Concepts explained include the use of aircraft equipped with an INS/GPS/syn-thetic aperture radar (SAR) avionics suite to perform a real-time relative targeting function for weapon initialization. The importance of reasonable aircraft maneuvers to enhance observability and speed up the three-dimensional (3-D) targeting fire control solution is also addressed. Simulation results for several realistic scenarios are presented.

What are vessel traffic services, and what can they really do?.

Abstract: Vessel traffic services, referred to as VTS systems, are being used in many ports and waterways worldwide as a means to reduce operational and environmental risk in marine transportation. These objectives are achieved principally by providing information, rather than by following the aviation model of traffic control. Recent tanker accidents that caused major pollution incidents have precipitated public calls for expansion of VTS operations, including direct traffic control. This paper describes VTS operational concepts from the perspective of a former VTS director and provides a model for implementing VTS systems. It also examines the applicability and feasibility of passive and active measures that can be used by shore-based traffic regulators to prevent marine incidents and accidents in varying operational situations.

Future lunar landing navigation schemes, with emphasis on precision landings

Abstract: Preliminary descriptions and analyses of a precision lunar landing using various navigation scenarios are presented. The scenerios assume the Common Lunar Lander (CLL) as a reference mission. Current state-of-the-art or off-the-shelf technology is emphasized to keep mission cost to a minimum—an important driver in the possible funding of future space missions. It is concluded that precision navigation sensor suites are required to meet certain stringent mission navigation requirements.

Demonstration of GPS Automatic Dependent Surveillance of Aircraft Using Spontaneous Mode S Broadcast Messages

Abstract: A new automatic dependent surveillance (ADS) system concept combining GPS-based positions with Mode S data communications is described. Several potential applications of this concept are presented, with emphasis on surface surveillance at airports. The navigation and data link performance are analyzed. Compact ADS position formats are included. The results of the first tests at Hanscom Field, demonstrating the feasibility of the spontaneous broadcast of ADS positions using Mode S messages, are presented. Test aircraft, vehicles, avionics equipment, and the ground system configuration are described. Avionics standards and GPS interface requirements are discussed. Multipath and airport surface coverage issues are addressed. Further testing in an operational environment is continuing at Logan Airport.

1993 At-Sea Evaluation of ECDIS

Abstract: The electronic chart display and information system (ECDIS) is a navigation tool that superimposes electronic chart, ship's position, and radar information on one display. The purpose of the test described in this paper was to compare baseline performance (i.e., a mariner using traditional techniques of a paper chart, radar/automated radar plotting aid (ARPA), and visual piloting) and performance with ECDIS. Evaluation criteria included safety of navigation, reduction of navigation workload, mariner comfort with chart features and navigation functions, and integration of radar features. Results indicated that when using ECDIS with radar overlay, the average workload was the lowest, leaving more time for collision avoidance. The mariners reported close agreement with the features and functions required by the International Maritime Organization/International Hydrographic Organization (IMO/IHO) Draft Performance Standard for ECDIS. Statistical analyses performed showed navigational method, visibility, and the presence of a radar overlay to be the most significant factors for improving navigational performance.

On estimating map model errors and GPS position errors: Applying more science to the art of navigation

Abstract: To decide whether a desired maneuver can or cannot be undertaken safely, a prudent navigator must be aware of both the current spatial uncertainty of the vehicle's positioning system and the spatial uncertainty of the navigational map model being used to depict the theater of operations. This paper discusses the electronic chart (EC) implications of both GPS vessel positioning errors and the relatively large data modeling errors specific to bathymetric map models (charts). It proposes and demonstrates software solutions that statistically evaluate both of these spatial uncertainties and graphically integrates the two stochastic models within an EC environment. The depicted estimates of bathymetric map model errors were computed using a modified kriging algorithm that interpolates both a gridded seafloor model and a stochastic surface that describes the fidelity of this model. This algorithm is being developed as part of an international effort to define quality standards for digital bathymetric data. The paper also describes how GPS real-time position error estimates were validated experimentally, and how enhancements to the estimation algorithm were devised.

An Operational Test of a Vehicle Emergency Location Service in Colorado

Abstract: This Federal Highway Administration (FHWA)-sponsored operational test will evaluate the use of GPS (location) and cellular phone (two-way communications) to provide assistance to travelers in an emergency dispatch area of more than 12,000 m2 in central Colorado. The test will implement 2,000 in-vehicle units using a low-cost GPS location device. The major objectives of the operational test are to evaluate the impact of the system and response network on emergency response activities and time, and public safety; to evaluate a low-cost system that allows users to request roadside assistance; and to identify the necessary structure, responsibilities, and service levels of a travel assistance center for commercial operation of the system. The operational test Mayday service will first become available to travelers in mid-1995. The system architecture to be implemented and the operational test program are described in this paper.

Availability assessment of alternative category i approach and landing system architectures.

Abstract: This paper presents results of a comparative assessment of the availability of alternative differential GPS (DGPS) precision approach and landing system architectures. The baseline availability for comparison is developed based on the performance of the current Instrument Landing System (ILS). The analysis addresses the number and positioning of geostationary satellites that are needed to achieve the availability requirements and identifies a possible role for pseudolites. Analysis of temporal availability characteristics of DGPS provides additional operational insight.

Analyses Supporting FAA Decisions Made During the Development of Supplemental GPS Avonics Requirements

Abstract: During 1992, members of the FAA (Federal Aviation Administration) Navigation Satellite Operational Implementation Team (SOIT) developed the requirements for Technical Standard Order (TSO) C-129, which describes capabilities of GPS avionics to be used as a supplemental means of navigation for en route through nonprecision approach phases of flight. During the writing of TSO C-129, the SOIT requested that analyses of certain issues be performed, including the effect of certain types of altimeter aiding on receiver autonomous integrity monitoring (RAIM) availability, and the risk of continuing flight in the event of an unpredicted loss of RAIM after an aircraft crosses the final approach fix (FAF) of a nonprecision approach. This paper summarizes technical analyses used in the development of these requirements. The results showed that altimeter aiding significantly improved RAIM availability, and that the risk of continuing an approach in case of an unpredicted loss of RAIM under certain conditions was acceptable. Operational and technical considerations, including the results of these analyses, led the SOIT to require certain capabilities of GPS avionics to be covered under TSO C-129