Satellite-based Positioning Research Articles

The Global Positioning System (GPS) and its use in Geophysical Exploration

The Global Positioning System (GPS) is a satellite-based positioning and navigation system that offers 24-hour, worldwide, three-dimensional positioning. Single point GPS positioning yields coordinates accurate to +/-100m, which are adequate for large-scale, low-accuracy applications. To achieve higher accuracy positioning, relative or differential GPS (D.GPS) is employed, which uses two or more GPS receivers simultaneously. Most geophysical exploration methods must be positioned using this approach, especially those that require subsequent data reduction or processing. The currently available GPS methodologies, their accuracy and suitability for positioning during geophysical exploration, are summarised. Finally, in order to integrate GPS-positioned data with existing data, coordinate transformations are necessary.

A GNSS-Based Time Division Multiple Access Data Link

This article describes a concept for the broadcast of satellite-based position data. This data has the potential for use for air traffic control applications such as situational awareness or collision avoidance. The concept, called Global Navigation Satellite System (GNSS) time synchronized time division multiple access, uses a method of allocating time slots for broadcast of position data by aircraft on an absolute satellite-derived time line. An on-board unit called the GNSS transponder utilizes a satellite receiver, a data link transmitter and receiver, and a processor. Included in the article are descriptions of the GNSS transponder, message formats, and possible extensions of the concept for operational use. © 1994 John Wiley & Sons, Inc.

A Note on the use of the Global Positioning System (GPS) for the Identification of Marine Radar Contacts

The Global Positioning System (GPS) is a cheap, accurate (±100 m), satellite-based position fixing system. These qualities suggest that it might be used in the identification of marine radar targets. The first application of such a system on any scale will probably be as part of a Vessel Traffic Service (VTS) and here GPS would seem to be ideally suited. Vessels entering the VTS control zone would need to be fitted with GPS interfaced with either a dedicated VHF receiver or a ‘guard’ channel on their normal receiver A VTS radar operator needing to identify an echo would work out the echo's position from the radar range and bearing and transmit that position as the interrogating signal. Only the vessel in the control zone with that position (derived from its GPS interface) would respond with its identity. There would thus be no possibility of interference between responses or garbling that can be a problem in interrogation systems based on secondary radar. Even if the VTS had several radars, these could easily be synchronized to prevent simultaneous interrogations. However, outside VTS zones there is a clear need for a universal identification system that would allow any ship to identify any other. It would be unfortunate and wasteful if a system developed for VTS was found to be unsuitable for this general application. This note will therefore discuss the adequacy of GPS as the basis of a universal marine identification system.

Geodesy from ASTROLABE to GPS-A Navigator's View

Since the 17th century, it has been known that the earth's shape is approximately elliposidal. Spherical approximations of the earth's shape, with the exception of meridional parts (meridian distances), were accurate enough for most navigation until the early 1960s. With the advent of a satellite-based Global Positioning System (GPS), navigational accuracy improved to the point where Geodesy, the science concerned with the size and shape of the earth, became very important. This paper addresses the history of geodetic observation or surveying, discusses the size and shape of the earth and the refinements resulting from greater precision of measurement, and describes world geodetic systems compared to older datums. Application of the many facets of Geodesy to navigation, such as the significance of geoidal height and mean sea level, and transformation from WGS-72 to local datums, are also discussed.

A Survey of Satellite-Based Systems for Navigation, Position Surveillance, Traffic Control, and Collision Avoidance

During the past several years there has been increasing interest, attention and commitment directed toward exploiting the capabilities and potential of satellite-based position determination systems. The utility of satellite systems has been extended from surface navigation and geodetic applications into air and space, and to other functional areas such as position surveillance for air traffic control, precise time and time transfer, international maritime and aeronautical position location and reporting services, and collision avoidance. This paper provides a summary of the satellite system concepts, orbital deployments and measurement techniques on which the accomplishment of the various applications is based. The systems and system concepts discussed include: Transit, the Navy Navigation Satellite System; the expanded Transit and Transit improvement program concepts; the Two-In-View configuration; the Defense Navigation Satellite System, including the System 621B and the Timation system concepts; the NASA Position Location and Communication Equipment (PLACE) experiment; the Maritime Satellite program of the Department of Commerce's Maritime Administration; the DOT/FAA Aeronautical Satellite Program; the Location, Identification by Transmission (LIT) and Satellite ATC and Navigation (SATAN) systems; the DOT's Advanced Air Traffic Management System concepts, and the FAA's recently developed ASTRO-DABS concept.