NAVSTAR Global Positioning System Research Articles

Centimeter‐level baseline estimation with GPS interferometry

Abstract The NAVSTAR Global Positioning System can provide geodetic point positioning at the 5 m to 10 m level with a single day of observations using either the Doppler or pseudorange modes. This requires the more precise P code, though it is not clear whether the civilian user will have access to it. An alternate approach is relative positioning with respect to a global network of monitoring stations (or any convenient geodetic datum) using interferometric phase observations of GPS satellites. The Macrometer™2 Model V‐1000 Interferometric Surveyor system, now commercially available, provides a portable relative positioning package that has been shown to yield first‐order baseline determinations on a routine basis. Under real field conditions, the baseline vectors between pairs of points have been estimated to an accuracy of 1:200,000 to 1:1,000,000 in all three components for baselines of one kilometer to several thousand kilometers in length. This is available without access to the GPS codes and requir...

The Use of Pseudo-Satellites for Improving GPS Performance

The NAVSTAR Global Positioning System has proven itself to be extremely accurate under tests of normal operations conducted at the Yuma proving ground. During the same tests of several years ago, a special technique called Differential GPS was also evaluated and shown to have even more startling accuracies. The basic concept of Differential GPS is to have a local receiving station continuously calibrate the remaining bias in the GPS signal and transmit it to the local users. This allows aircraft, ships or land vehicles to refine their estimate of position to the last meter or two. One technique for transmitting such a correction is to use a GPS-like transmitter itself. The exact form of the signal used may be quite significant to operations and accuracy enhancement. The “pseudo-lite” or GPS ground transmitter also provides powerful geometric leverage for the total navigation solution. A pseudo-lite will be called a PL in this paper. This paper will address the geometric problems encountered by the current GPS satellite constellation. Simulation results will show the value of PLs to deal with both poor satellite geometry and potential satellite outages (in the event of failure). Quantitative examples of the performance improvements are included. This paper will also outline potential design alternatives for the PLs.

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 US Federal Radio-navigation Plan

The United States is in the process of determining what the future mix of radio-navigation should be for its users. Air, marine and land navigation requirements are being closely re-examined and the ability of future or existing systems to meet these requirements is being investigated. Events such as the destruction of Korean Airline Flight 007 have focused new attention on the Navstar Global Positioning System (GPS), both by the President and Congress. Behind the selection of a future systems mix is a planning process that has been under way for some time. The first Federal Radio-navigation Plan, published in 1980, listed two key decision points in the planning process; a joint preliminary recommendation to be made by the Departments of Defense (DOD) and Transportation (DOT) in 1983 and a national level decision in 1986 (Fig. 1). A national decision will not be made without an opportunity for user comment and international consultations.

Navigation satellites: their future potential

The United States Department of Defense is developing a new generation of navigation satellites known as the Navstar Global Positioning System or GPS. When the full system of 18 satellites is deployed in the late 1980s, highly accurate information on position, velocity, and time will be available continuously to users anywhere in the world. This capability has already been demonstrated by the existing constellation of five prototype satellites. The original impetus for Navstar was the need for highly accurate positioning information by military aircraft, ships, and ground units. But Navstar also has potential for a variety of civilian uses, which include precision navigation, surveying and accurate time transfer. Moreover, the projected sharp decline in the cost of GPS user equipment will make the system available to a wide class of users.

A Comparison of Multichannel, Sequential and Multiplex GPS Receivers for Air Navigation

As the NAVSTAR Global Positioning System (GPS) begins moving beyond the initial development stage, potential air navigation users are going to have a considerable range of choices for set designs. This paper describes three of the main receiver architecture approaches: multichannel, sequential, and multiplex (MUX). These major options are compared according to functional capabilities, effects of dynamics on navigation accuracy, and relative cost. Oscillator g-loading effects are addressed. The most appropriate choice for a given military or civil air navigation application likely involves a judicious combination of the various basic set architectures. A general familiarity with NAVSTAR/GPS is assumed.

Comparison of NAVSTAR satellite L band ionospheric calibrations with Faraday rotation measurements

Using the Faraday rotation technique, the Deep Space Network of the Jet Propulsion Laboratory (JPL) monitors ionospheric total electron content (TEC) at its three complexes at Goldstone, California, Canberra, Australia, and Madrid, Spain, to correct data used in the orbit determination process for ionospheric effects. With the number of earth satellites suitable for Faraday rotation measurements steadily decreasing, JPL is investigating other ways to determine ionospheric TEC. One promising method has been developed by MacDoran and Spitzmesser. This method exploits the coherent P code modulation of the two L band signals (1575.42 and 1227.60 MHz) transmitted by satellites of the NAVSTAR Global Positioning System (GPS), but does not require knowledge of the code. It has been named SLIC, for satellite L band ionospheric calibration. With the full complement of 18 GPS satellites in operation in 1987, continuous ionospheric sampling in at least four directions will be feasible with SLIC. In this paper, TEC data measured at Goldstone using SLIC are compared with the results of mapping Goldstone Faraday TEC to the five GPS satellite lines of sight. The results of the two methods are in reasonable agreement.

Principles and Present Status

The Navstar Global Positioning System (GPS) is a satellite-based passive radio-navigation system under development by the US Department of Defense (DOD) and due for operational deployment in the late 1980s. Each satellite carries an atomic frequency standard and transmits L-band spread-spectrum signals whose carrier frequencies and code epochs are synchronized with the satellite clocks. A suitably equipped user may measure the arrival time and frequency of the signals from at least four satellites to obtain accurate three-dimensional position and velocity information and, as a by-product, an accurate ‘system’ time.

Experimental Results of Using the GPS for Landsat 4 Onboard Navigation

The first use of the Navstar Global Positioning System (GPS) by a spaceborne user occurred with the launch of NASA's Landsat 4 on July 16, 1982. One of the experimental packages onboard Landsat 4 is the Global Positioning System Package (GPSPAC). A brief description of the GPSPAC is presented and the operational history of the GPSPAC experiment onboard Landsat 4 is outlined. The responsibility for the control of the GPSPAC experiment and for the validation of its results resides at the Goddard Space Flight Center (GSFC). The role of the Naval Surface Weapons Center (NSWC) primarily is to aid GSFC in achieving the objectives of the GPSPAC experiment. Results of NSWC's evaluation of the GPSPAC performance are presented.

Navstar Global Positioning System

The NAVSTAR Global Positioning System (GPS) Program began a decade ago when the United States Air Force, Army, Marine Corps, Navy, and the Defense Mapping Agency (DMA) combined their technical resources to develop a highly‐accurate, space‐based navigation system. The GPS concept is the outgrowth of extensive studies, analysis, tests, and operational demonstrations which validated system performance and feasibility. The GPS will provide extremely accurate three‐dimensional position, velocity, and timing information to an unlimited number of suitably equipped users and do so on a continuous basis. Currently the GPS program is in the Full‐Scale Engineering Development (FSED) Phase. In this phase, the operational effectiveness of the GPS concept for both military and civilian users will be verified.

Applications of Time Transfer Using NAVSTAR GPS

The Navstar Global Positioning System (GPS) allows extremely accurate and global determination of time, as well as position and velocity, and it does it economically. Without even using the precise GPS codes and dual frequency measurements, the transfer of time with GPS can be accomplished to better than 100 nanoseconds. In fact, with a simultaneous common view approach, relative timing can be accomplished to better than 10 nanoseconds. Although a widespread need for this type of accuracy may not be apparent, one would be surprised at the number of applications for that accuracy surface once that its availability has been established. Probably more important than the accuracy capability is the ease by which time transfer can be accomplished with GPS, both technically and logistically. GPS time transfer systems are more portable than precise clocks primarily because batteries are not required to keep the clock running. Time with GPS can be reinitialized at any time. Economically, because of the technology involved, the current cost of precise clocks (Cesium) is on a steady rise, while GPS time transfer systems will become less expensive as time passes, enhancing the advantage over the use of precise clocks. Currently, GPS is not an operational system. For time transfer it is, but not continuously, available (only available on the order of 16 to 20 hours a day). However, as the number of satellites increases, the time transfer will become continuous long before GPS is declared fully operational. It is that time that GPS time transfer systems could replace many of the precise clocks in existence. This paper will discuss how GPS time transfer satisfies the time keeping requirements of various timing applications, enhances the time keeping capabilities of those applications and reduces the expense of time keeping in those applications. The performance of GPS time transfer in its different modes of usage will also be discussed.

Geodetic theory

Spurred by a tremendous increase in new and highly accurate data and by prospects for further improvements, theoretical studies in geodesy have concentrated on devising new methods, or adapting extradisciplinary ones, to process the data efficiently and accurately. Satellite altimetry, the NAVSTAR Global Positioning System, satellite Doppler, satellite‐to‐satellite tracking, satellite laser, VLBI, satellite interferometry, and inertial navigation advancements are all contributing to provide the geodetic, and indeed the geophysical, community with a wealth of new data. Least‐squares collocation has achieved prominence through numerous applications, but for efficiency it must compete with other methods, such as those founded in spectral theory. Also, the improved measurement accuracy has mandated the development of more realistic geodetic models where former approximations can no longer be tolerated.

Multiconfiguration Kalman filter design for high-performance GPS navigation

This paper describes the design, implementation, and performance of a real-time multiconfiguration Kalman filter for high-performance Navstar global positioning system (GPS) navigation. The design provides extreme flexibility in order to operate with a wide variety of host sensors. It configures automatically (four filter configurations) based upon the host vehicle requirements and sensor availability, in order to process GPS measurements and provide the best estimate of the navigation states. Two new techniques, namely an unaided dead-reckoning Kalman filter implementation and an automatic inertial platform tilt estimation control scheme, are developed to improve the navigation accuracy, especially for high-dynamics applications. Performance results are presented to demonstrate the advantages of these techniques.

NAVSTAR: Global positioning system—Ten years later

With the award of one of the first multiyear procurements to be authorized by the Congress, the Department of Defense celebrates the tenth anniversary of a major Joint-Service / Agency program that promises to revolutionize the art and science of navigation. This contract, totaling 1.17 billion dollars, will result in delivery by the end of this decade of 28 satellites that are to become the heart of the NAVSTAR Global Positioning System (GPS). This paper traces the history of this program from its inception, identifying and discussing the major technologies that made it all possible. In so doing, the concept of operations of the entire system, including the satellites, their associated ground-based control network, and the family of user equipment being developed and tested to meet the broad scope of Department of Defence positioning and navigation requirements is introduced.

Navigation systems performance versus civil aviation requirements

This paper presents comparative assessments of navigation systems in terms of their ability to satisfy current and projected civil aviation requirements. Emphasis is placed on U.S. operations. The navigation systems considered as candidates, either separately or in combination, for meeting post-1995 requirements are Loran-C, Omega/ VLF, GPS, VOR, DME, Doppler, and INS. Each navigation system is characterized by a brief description of the facilities used and the techniques employed in providing navigation capability. The technical performance capability of each navigation system is compared with both existing and projected system-use accuracy requirements. Of the two error components that determine system-use accuracy, flight technical error (FTE) and equipment error, it is shown that FTE is the limiting factor in the improvement of system-use accuracy. Each system is also evaluated to identify the limitations imposed by signal-coverage considerations. The paper concludes with the recommendation to maintain a navigation system mix of VOR, DME, Loran-C, Omega/VLF, and INS as an effective means of satisfying both current and post-1995 civil aviation requirements. Doppler is not included, because of its poor accuracy. It is considered premature at this time to make any commitments regarding civil implementation of the proposed NAVSTAR Global Positioning System (GPS).

The Application of NAVSTAR Differential GPS in the Civilian Community*

The NAVSTAR Global Positioning System (GPS), currently being developed for the Department of Defense, is a space based navigation system that will provide the user with precise position, velocity and time information on a 24-hour basis and in all weather conditions at any point on the globe. The potential unauthorized use of this source of very precise navigation information has prompted the DOD to intentionally contaminate the signals and to provide only authorized users with the necessary information to recover the original signals. The high probability of the implementation of these techniques, known as Selective Availability, indicated the need for a variation of the baseline GPS, so as to allow for an assured, uninterrupted level of accuracy, if the system was to be used effectively in the civilian sector. Differential GPS provides such a capability. This paper presents the current thinking in the implementation of Selective Availability and its impact on the civilian use of GPS. Further, different possible implementations of differential GPS are presented and their performance compared with the baseline system, with or without the use of Selective Availability. Finally, the support in the civilian community for the development of differential GPS and the potential market size are assessed.

Geodetic Positioning Using a Global Positioning System of Satellites

This report analyzes various techniques of using range, Doppler or phase measurements of signals from the NAVSTAR Global Positioning System (GPS) to determine absolute or relative positions of the observer. The first two chapters give an introduction and background, the third chapter considers the relative strengths of the various techniques in the absence of systematic errors, the fourth chapter gives a detailed development and error analysis of dynamic positioning techniques, the fifth chapter discusses interferometric solutions based on phase measurements and the sixth chapter summarizes the study.Several changes have been made in the proposed satellite system and receivers since the publication of this report. These changes require reinterpretation of the results of simulations given in the report. The GPS is now proposed to consist of 18 satellites rather than the 24 satellites assumed in the study. This change degrades the geometric strength of solutions by about 50%. The study assumed observations would be made on satellites sequentially in the dynamic mode or on pairs of satellites in the interferometric mode. All receivers now proposed for use in geodetic applications will make simultaneous or quasi‐simultaneous observations on four or more satellites. This change primarily reduces the time on site required to achieve a given level of accuracy. The receivers now being developed by MacDoran and Counselman for interferometric observations are drastically different from those originally proposed. However, the principal error sources are not significantly affected by the changes in measurement technique. Since Fell gives a detailed break down of the contribution of each error source to the total error and of the variation of the errors with time on site, the results given can be used to evaluate the accuracies currently achievable or those which would be obtained under other assumptions.

DMAHTC Support to National Ocean Survey Loran-C Charting

The selection of Loran-C as the primary radionavigation system for the United States Coastal Confluence Zone (CCZ) requires, by joint agreement, that the Defense Mapping Agency Hydrographic/Topographic Center provide corrections to Loran-C lattices overprinted on National Ocean Survey (NOS) charts. Additional Secondary Factor (ASF) Corrections to warp the lattices are necessary if the system is to meet ¼ nautical mile or better positioning accuracy required by the Department of Transportation (DOT) and specified in the 1977 National Plan for Navigation. The Coast Guard, which has statutory responsibility for the Loran-C Radionavigation System, expects it to be the principal aid to navigation for civil use in the CCZ until the year 2000. For Department of Defense (DoD) purposes the system will be replaced eventually by the NAVSTAR Global Positioning System (GPS). This paper reviews future plans for refinement of the techniques to compute ASF corrections, as well as past and present computational methods. In addition, it examines the status of the Loran-C Radionavigation System and the status of NOS charts which support the system. The U.S. Coast Guard has surveyed critical areas in the CCZ to verify the accuracy of Loran-C NOS chart lattices. An example survey in the offshore area of the west coast is depicted and discussed.

Global Positioning System Sensitivity Experiment

Abstract : This experiment demonstrated the obtainable accuracy for estimating changes in a receiver antenna position using Doppler techniques and the NAVSTAR Global Positioning System (GPS). The antenna was periodically moved to three locations approximately 2 to 3 meters apart. The moves occurred within a one minute Doppler count interval every fifteenth interval until at least six Doppler measurements were made for each location change. These measurements were used to determine the observed change in range from the antenna to the GPS satellite due only to the movement of the antenna. The calculated change in range was computed from the surveyed distance moved and the angular differences in elevation and azimuth between the direction of the move and the direction of the satellite. The observed and calculated changes in range were compared. The three-dimensional antenna position changes were estimated using six observed change in range measurements for each location change. The estimated results were compared with surveyed values and demonstrated subdecimeter accuracy. (Author)

Aerospace: Navstar: The all-purpose satellite: The military sees Navstar as a way of keeping wide-ranging forces on target, and potential civilian users are showing interest too

Discusses the developments and applications of the Navstar system, particularly in the fields of global navigation and position determination. The Navstar Global Positioning System (GPS) is basically a military system but its potential for civilian use includes maritime, aviation and space applications. The function of the Navstar system and its main parts (satellites, ground stations and users' facilities such as ground vehicles, ships or aircrafts), is discussed.