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The Interchangeability Problem: Signals, Coordinate Frames, and Time--Rita M. Lollock, Thomas D. Powell, and Thomas A. Stansell
Pages 75-82

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From page 75...
... To achieve satisfactory navigation results the receiver had to compensate for differences in system time, coordinate frame, signal frequencies, spreading codes, message formats, and separate filter delays within the receiver. Nevertheless, this and subsequent commercial receivers were able to demonstrate that two rather dissimilar systems could be used together to achieve better results than with either system alone, especially in difficult reception environments, even though the systems were far from "interchangeable." In recent years new systems have been proposed and are being developed, including Compass, Galileo, and QZSS (Quasi-Zenith Satellite System)
From page 76...
... The internationally accepted definition of interoperability is the "ability of open global and regional satellite navigation and timing services to be used together to provide better capabilities at the user level than would be achieved by relying solely on one service or signal." As a result of international cooperation, new and modernized GNSS signals will have characteristics that substantially improve interoperability as compared with the original GPS and GLONASS signals. Some have suggested that the goal should be not only interoperability but also interchangeability, meaning that there should be no discernible differences between the signals from different systems.
From page 77...
... INTEROPERABILITY PROGRESS AND STATUS As a result of international bilateral and multilateral meetings, the advantages of GNSS interoperability have been recognized and major strides made toward achieving the goal. A particularly important achievement is that right hand circular polarization, code division multiple access (CDMA)
From page 78...
... GPS L1C employs a time multiplexed binary offset carrier (BOC) or TMBOC waveform, whereas the Galileo E1 OS employs a composite BOC (CBOC)
From page 79...
... This allows use of common analog receiver components for each of these signals. All other signal differences, such as the spreading codes, data rates, message structures, etc., are handled by digital electronics and software.
From page 80...
... With identical center frequencies and signal spectra, reference stations can precisely measure the time offset between satellites of dif ferent systems. Although providers could use this information to drive their own system clocks to agree with any one of the other systems, it would be preferable to establish an international GNSS time standard against which each system could measure its time and drive any difference toward zero.
From page 81...
... When signals are used for safety of life applications, it is particularly important for providers not only to define the signals and the expected performance standards but also to rapidly issue service notifications as is done by GPS Notice Advisories to Navstar Users (NANUs)
From page 82...
... 82 GLOBAL NAVIGATION SATELLITE SYSTEMS FIGURE 4 Better user accuracy is achievable via better position dilution of precision Lollocketal_Fig4.eps (PDOP) due to multi-GNSS interoperability.


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