US Global Positioning System Research Articles

Precise GPS Positioning by Applying Ionospheric Corrections from an Active Control Network

In this article, initial results are presented of a method to improve fast carrier phase ambiguity resolution over longer baselines (with lengths up to about 200 km). The ionospheric delays in the global positioning system (GPS) data of these long baselines mainly hamper successful integer ambiguity resolution, a prerequisite to obtain precise positions within very short observation time spans. A way to correct the data for significant ionospheric effects is to have a GPS user operate within an active or permanently operating network use ionospheric estimates from this network. A simple way to do so is to interpolate these ionospheric estimates based on the expected spatial behaviour of the ionospheric delays. In this article such a technique is demonstrated for the Dutch Active Control Network (AGRS.NL). One hour of data is used from 4 of the 5 reference stations to obtain very precise ionospheric corrections after fixing of the integer ambiguities within this network. This is no problem because of the relatively long observation time span and known positions of the stations of the AGRS.NL. Next these interpolated corrections are used to correct the GPS data from the fifth station for its ionospheric effects. Initial conclusions about the performance of this technique are drawn in terms of improvement of integer ambiguity resolution for this baseline. © 1999 John Wiley & Sons, Inc.

Vulnerability of the GPS Signal to Jamming

The Global Positioning System (GPS) and Wide Area Augmentation System (WAAS) provide lower power signals that are susceptible to interference. The potential exists for a person or organization to jam GPS or WAAS signals, causing a loss of service over a large area. Accidental interence by extraneous radio transmissions on the GPS frequency also can cause loss of service. Jamming has become a serious issue for GPS, and the U. S. military is making a significant effort to mitigate jamming effects. While jamming is a major concern of the military, interference is of a no lesser concern to the civilian sector. In fact, with varying emphasis both jamming and interference are concerns of the entire GPS user community. This article explores jamming and interference threats to the GPS. We provide a general overview of the GPS signal structure, discuss jamming effects on the GPS, and consider mitigating options. © 1999 John Wiley & Sons, Inc.

Land vehicle use of satellite navigation systems in Japan

Abstract In terms of land users of the US Global Positioning System (GPS), the Japanese market is the most mature in the world. As such, it provides the ‘blueprint’ for how mass markets in other developed countries will respond to GPS and, in the future, to the second generation Global Navigation Satellite System (GNSS-2). The information which follows draws on a recent report prepared by Logica for the UK government, and highlights the growth of the in-car navigation market in Japan and the various technologies being used.

The DoD: stewards of a global information resource, the Navstar global positioning system

In 1973, the US Department of Defense (DoD) began development of the Navstar global positioning system (GPS) and embarked on a journey that would take radio navigation and positioning to what were then unimagined levels of performance. From its inception, GPS was viewed as a revolutionary technology that would enhance the positioning capability of US and allied military forces throughout the world. The DoD has continued to upgrade and improve all of the components of the system to keep pace with technology advances and the requirements of an ever-expanding user community. The GPS user community includes an increasing number of civil, scientific, and commercial applications, ranging from precision scanning to pinpointing disruptions in electric power distribution networks. However, this dual military and civil aspect of GPS has posed significant challenges for the US government and DoD policy makers. Since the inception of GPS, the DoD has been confronted with the need to balance a wide range of different and sometimes competing national security, civil, foreign policy, commercial and scientific interests. The challenge has been to exploit the full civil utility of the system without jeopoardizing national security interests in the process. This challenge will become even more formidable for military leaders as US and allied forces become increasingly reliant on GPS for all types of military operations, and as the applications of the worldwide civil user community continue to expand.

The Navstar Global Positioning System: A Global Utility?

Satellite-based navigation systems are one of the fastest growing sectors of the space industry. The Navstar Global Positioning System (GPS) is a fully operational US military satellite-based navigation system. New applications, both civilian and military, are continually being developed for GPS and increasing numbers of users worldwide are becoming dependent upon this technology. The airline industry is but one segment of the rapidly growing GPS user base. New technologies evolving from GPS are quickly advancing the usefulness of GPS, which presents the need for international and national policy. The users of GPS technology, such as the airline industry, are having considerable influence over the development of GPS policy as a result of civil users exercising increasing control over the system. The use of GPS for navigation by the airline industry presents the unique challenge of integrating air and space law. This paper will discuss pertinent legal and policy issues that will affect the development of GPS policy such as liability, sovereignty, and GPS availability. According to some authorities, as GPS becomes an international utility it may present the requirement for an international organization, like the International Civil Aviation Organization (ICAO) or another consortium to establish the policy. The International Telecommunication Satellite Organization (INTELSAT) may serve as a model for a navigational consortium. This paper examines the various national and international organizations that may serve as models to develop GPS policy in the future.

Cosine Functions of GPS Carrier Phases for Parameter Estimation

Ambiguity function methods are based on the magnitude of the sum of unit vectors (phasors) defined by differences between observed and calculated global positioning system (GPS) phase data, thereby eliminating integer ambiguity parameters. In a novel approach using only cosines of the carrier phases, least squares techniques are implemented to facilitate convergence from approximate positions to a true position. With dual-frequency phase measurements, a simultaneous estimation of residual ionospheric path delays is proven feasible. Accuracy analyses show centimeter-level root mean square errors for both static and kinematic satellite positionings. Although the approach is not affected by cycle slips, its limitation on position approximation deserves more attention. Together with L1/L2 phase measurements from wide-area tracking or monitoring stations, a potential application for single-frequency GPS users is briefly described.

Relationships between GPS-signal propagation errors and EISCAT observations

Abstract. When travelling through the ionosphere the signals of space-based radio navigation systems such as the Global Positioning System (GPS) are subject to modifications in amplitude, phase and polarization. In particular, phase changes due to refraction lead to propagation errors of up to 50 m for single-frequency GPS users. If both the L1 and the L2 frequencies transmitted by the GPS satellites are measured, first-order range error contributions of the ionosphere can be determined and removed by difference methods. The ionospheric contribution is proportional to the total electron content (TEC) along the ray path between satellite and receiver. Using about ten European GPS receiving stations of the International GPS Service for Geodynamics (IGS), the TEC over Europe is estimated within the geographic ranges -20°≤ λ ≤40°E and 32.5°≤ Φ ≤70°N in longitude and latitude, respectively. The derived TEC maps over Europe contribute to the study of horizontal coupling and transport proces- ses during significant ionospheric events. Due to their comprehensive information about the high-latitude ionosphere, EISCAT observations may help to study the influence of ionospheric phenomena upon propagation errors in GPS navigation systems. Since there are still some accuracy limiting problems to be solved in TEC determination using GPS, data comparison of TEC with vertical electron density profiles derived from EISCAT observations is valuable to enhance the accuracy of propagation-error estimations. This is evident both for absolute TEC calibration as well as for the conversion of ray-path-related observations to vertical TEC. The combination of EISCAT data and GPS-derived TEC data enables a better understanding of large-scale ionospheric processes.

The strain field in northwestern Greece and the Ionian Islands: results inferred from GPS measurements

Recent crustal movements detected by the analysis of repeated satellite geodetic measurements reflect the ongoing geodynamic processes in the Alpine-Mediterranean area. Superimposed on the large-scale counterclockwise rotation of the African plate, complex dynamic processes are affecting the lithospheric fragments between the African and Eurasian plates. Key features to better understand the driving forces and associated seismic activity in the Africa/Eurasia collision zone are the Calabrian and Hellenic arcs. In this paper geodynamic investigations along the West Hellenic arc are discussed. They are based on two epochs (1989 and 1993) of satellite geodetic measurements carried out using the US Global Positioning System (GPS). The results are presented in terms of relative displacements and strain rates. Within the time span of 4 years southwestern Greece has moved to the southwest relative to southeastern Italy by an average of 120 mm, increasing from 80 mm at Lefkada, in the center of the Ionian Islands, to 160 mm at the Peloponnesus. The maximum strain rate is 0.18 μstrain/a located in the vicinity of Lefkada, where anomalously high earthquake activity is observed. The data provide strong evidence for dextral strike-slip motion on the order of 25 mm/a along the Kephalonia Fault Zone (KFZ). The deformation field of the KFZ is interpreted as a transition zone between the kinematics of the Apulian platform and the West Hellenic fold and thrust belts.

Real-time navigation using the global positioning system

This paper presents the results of an investigation of the application of the Global Positioning System (GPS) to real-time integrated missile navigation. We present quantifiable measures of navigation accuracy as a function of GPS user segment parameters. These user segment parameters include antenna phase response accuracy, single versus dual frequency, and Kalman filter structure and size. We also formulate some new phase-locked loop (PLL) filter designs for application in GPS receivers, and demonstrate their superiority over more conventional filters. >

Sole means navigation in US Navy aircraft

The current edition of the US Federal Radionavigation Plan, issued in 1984, presents a consolidated federal plane on the management of those radionavigation systems which are used by both the civilian and military sectors. It states the US Dept. of Defense (DoD) goal to phase out the use of TACAN, VOR/DME, OMEGA, LORAN C, and TRANSIT in military platforms and for Global Positioning System (GPS) to become the standard radionavigation system for DoD. This would eliminate all the current sole-means air navigation systems (TACAN and VOR/DME) aboard military aircraft. Instrument Flight Rule (IFR) operations within controlled airspace requires an operating sole-means air navigation system to be aboard the aircraft. The authors investigate the requirements for GPS certification as a sole means air navigation system in the US National Airspace System (NAS); discuss the implication for GPS user equipment (UE) hardware and software; describe the actual UE implementation; and discuss approaches for UE integration with flight instruments on US Navy aircraft.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

GPS Satellite-to-User Range Accuracies: A Calibration Experiment

From 20 October to 17 November 1986, three Contracting and three Government institutions took part in a cooperative evaluation of the accuracy of the Global Positioning System (GPS). In effect, acting as independent GPS users employing CIA-code single-frequency or P-code dual-frequency receivers, each organization computed satellite-to-user range prediction errors. Each institutional user computed statistical summaries of these User Range Errors (URE). The root-mean-square (rms) of the URE’s across all satellites, and over all 30-days of the test, as reported by each user ranged from a high of 4.8 meters to a low of 3.1 meters. The C/A-code users experienced the largest rms URE. The GPS is required to provide 6 meter rms URE. For the same test period, the GPS Operational Control System (OCS) computed URE performance measures of 2.0 meters (rms) and 2.5 meters (rms). This suggests that these GPS performance measures might be improved. Throughout the 30 day test, users reported that the daily rms URE were approximately steady-state. However, just prior to the test, the rms URE did appear correlated over several days with rms values near 6 meters. This URE degradation appeared to improve as NAVSTARS 3,6, and 10 were less and less eclipsed by the Earth shadow.

Civil Aviation Application of Navstar

Collins Government Avionics Division (CGAD) of Rockwell International became involved with the Navstar Global Positioning System (GPS) in 1974. Since that time, the division has accomplished several key milestones in the development of GPS user equipment including the first transatlantic flight of an aircraft using GPS signals.

The Integrated Inertial Navigation System AN/ASN-132

The US Army has a requirement for a highly accurate inertial/TACAN hybrid navigation system. This paper describes the navigation system architecture developed to meet this requirement, the specific hardware selected and the development status. At the beginning of the Integrated Inertial Navigation System (IINS) development program it was anticipated that significant technological advances would take place in its life cycle. In addition, mission requirements were expected to change as the system evolved. Particular attention was therefore given to the IINS system architecture, specifically the partitioning of hardware and software functions as well as the means of data transmittal between the subsystem elements. The paper describes in detail the design trade-offs made as well as hardware selections. The system is configured using several different manufacturers' equipment making the data interface and its configuration control of great importance. The above considerations lead to the conclusion that the appropriate data bus interface standard would be MIL-STD-1553. This interface standard not only facilitates configuration control during system development, but will allow rapid integration with future aircraft systems that will use it as a primary means of data transmittal. The paper describes in detail the signal interface between the subsystem element, onboard mission equipment and flight instruments. The most significant change anticipated in the course of the IINS life cycle is the substitution of Global Positioning System User Equipment for the external update function currently provided by TACAN. Another area where technology insertion may be deemed appropriate is the substitution of a strapdown inertial navigation unit (INU) employing ring laser gyros (RLG) for the currently employed conventional gimballed INU. The INU employed in the IINS is built to a form, fit and function specification, which will allow substitution of a RLG INU when such a unit is qualified and in production. The system hardware and software is structured so that these changes can be accomplished with a minimum of effort. The paper also describes the federated processing structure which allows this flexibility with particular attention to the inertial/TACAN update algorithm and data bus control functions. The paper concludes with a brief summary of the development status of the IINS, currently in full scale engineering development.

Access Charges and Related Issues for Civil GPS Users

The Global Positioning System (GPS) will be a giant navigation utility paid for by the U.S. taxpayers. Access charges will permit the operational costs to be borne by those who actually use the system, both foreign and domestic, rather than by the taxpayers. Civil users who pay for a service will not be ignored, as they are now with the Transit system. User charges will permit operational continuity when the DoD no longer needs GPS. An implementation technique based on annual replacement of an LSI chip which decrypts the satellite message is described. A low annual charge is recommended to permit maximum system use and to optimize annual revenues. Counterfeiting is shown to be impractical, except by pirate radio transmissions. The design challenges related to interaction with military requirements, equitable costs for different classes of users, and the transition from one access chip to the next are described. Institutional issues are outlined, including who should develop the user charge technique. It is proposed that higher charges apply to those who purchase foreign-built equipment to promote U.S. employment, balance of payments, and to provide a return on the U.S. taxpayer investment in the system development. Those civil users who need maxium accuracy are frustrated with the Government policy of accuracy denial. This is described, and the probable development of differential GPS techniques to recover accuracy is evaluated. The creation of differential GPS standards is encouraged. Finally, another plea for greater civil user involvement in federal navigation planning is aired.

National aerospace meeting of the Institute of Navigation

The program for this year's aerospace meeting of The Institute of Navigation addressed developments in the evolving Global Positioning System (GPS) of navigation satellites, inertial navigation systems, and other electronic navigation systems and their applications. Also included in the program were a limited number of papers addressing the geodetic use of the GPS system.The Global Positioning System is a constellation of 18 navigation satellites being developed by the Department of Defense to provide instantaneous worldwide navigation. The system will support a multitude of military applications. The first paper by Jacobson reviewed the engineering development of GPS navigation receivers stressing the use of common hardware and software modules. A later paper by Ould described the mechanization of a digital receiver for GPS applications designed for faster acquisition of the spread spectrum satellite transmissions than analog receivers. The paper by Brady discussed the worldwide coverage that is provided by the limited number of satellites that will constitute the GPS constellation through 1983. The capability provided by the satellites presently on orbit would support a variety of experiments at almost any location. Tables of multiple satellite availability are provided for numerous worldwide locations. For civil aviation applications, Vogel addressed the satellite geometry considerations for low cost GPS user equipment, Esposito described the Federal Aviation Administration acceptance tests of a GPS navigation receiver, and Hopkins discussed the design and capability of an integrated GPS strapdown attitude and heading reference system for avionics.

Aircraft Navigation with the Limited Operational Phase of the NAVSTAR Global Positioning System

The navstar Global Positioning System (GPS) will soon be in the limited operational phase. During this phase, a GPS user will not generally be able to determine his position using satellite measurements only. This paper describes the simulation of an aircraft navigation technique which uses the limited operational phase GPS and barometric altimeter measurements. For this research effort, the GPS user was assumed to be a cargo–type aircraft equipped with a single–frequency, sequentially–tracking GPS receiver and a barometric altimeter. The flight profile consisted of a New York to Chicago flight and included takeoff and landing maneuvers. A small amount of wind gusts were simulated. To make the simulation more realistic, random effects were included in the user and satellite clocks, in the atmospheric radio delays, and in the measurement process itself. The tested navigation algorithm is based on the well-known Kalman filter equations. Three different models of aircraft acceleration were evaluated. Results indicate that, for a wide range of filter parameters, navigation errors are generally less than 100 meters. Poor geometry, however, results in kilometer–sized position errors.

GPS User Equipment Error Models

the basis for establishing the fundamental mathematical error models associated with measurements of the Global Positioning System (GPS) signals by various types of user equipments is provided in this paper. The GPS user equipment error models which are described are intended to be utilized for various system performance and mission effectiveness analyses.