Surveying with GPS
This chapter is mainly concerned with the practical aspects of GPS surveying and addresses planning, performance, and in situ data processing. Overlapping with other chapters is intentional to provide complete information in a single chapter for readers m
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Introduction
This chapter is mainly concerned with the practical aspects of GPS surveying and addresses planning, performance, and in situ data processing. Overlapping with other chapters is intentional to provide complete information in a single chapter for readers more interested in practical considerations.
7.1.1
Terminology definitions
The enormous interest in GPS is reflected by the numerous papers published today. Unfortunately, a standard terminology is missing, although several authors have attempted to provide a list of terms used. To avoid confusion in the following sections, some of the more important definitions are given here and are used throughout the entire text .
Code mnge versus carrier phase Typically, GPS observables are pseudoranges derived from code or carrier phase measurements. Generally speaking, the accuracy of code ranges is at the meter level, whereas the accuracy of carrier phases is in the millimeter range. The accuracy of code ranges can be improved, however, by the narrow correlator spacing technology or by smoothing techniques. Unlike the carrier phases, the code ranges are virtually unambiguous. This makes code ranges immune from cycle slips (i.e., changes of the phase ambiguities) and to some extent from site obstructions. The determination of the phase ambiguities is a critical issue in GPS surveying. Real-time processing versus postprocessing To qualify as real-time GPS, the position results must be available in the field immediately or while still on the station. The results are denoted as "instantaneous" if the observables of a single epoch are used for the position computation and the processing time is negligible. The original concept of GPS aimed at instantaneous navigation of moving vehicles (i,e., ships, cars, aircraft) by unsmoothed code pseudoranges. A different and less stringent definition is "real-time" which includes computing results with a slight delay. Precisely speaking, these are quasi (or near) real-time results. Today, radio data links allow the combination of measurements from different sites in real time. Postprocessing refers to when data are processed after the fact . B. Hofmann-Wellenhof et al., Global Positioning System © Springer-Verlag/Wien 2001
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7 Surveying with GPS
Point positioning versus relative positioning The coordinates of a single point are determined by point positioning when using a single receiver which measures pseudoranges to (normally four or more) satellites. Instead of "point positioning" the term "single-p oint positioning" or t he term "a bsolut e point positioning" is used. Here, the term "a bsolut e" reflects the opposite of "rela t ive" . Instead of "relative positioning" the term "differential positioning" is often used. Note, however, that the two methods are (at least theoretically) different. Differential positioning is rather an improved single-point positioning technique and is based on applying (predicted) corrections to pseudoranges at an unknown site. The technique provides instantaneous solutions (u sually d
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