Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 40

GPS technology has developed into a new multidomain (land, marine, aero-
space), multipurpose (in-transit navigation, precise positioning, precise timing,
satellite orbit determination, disaster monitoring, resource survey, construction,
municipal planning, marine development, traffic control, etc.), multimodel (geode-
sic type, timing-based, hand-held, integrated, vehicle-borne, ship-borne, airborne,
satellite-borne, missile-borne, etc.), high-tech international industry. GPS has been
widely applied in aerospace, fishing, guided tours, and agricultural production. It is
said that “the applications of GPS are only restricted by human imagination.”
GPS Measurement and Positioning Methods
GPS positioning methods are diverse. Users can use different positioning methods
that are appropriate for their different purposes. GPS positioning methods can be
classified according to different criteria as follows (see e.g., Liu et al. 1996):
According to Observed Values Adopted by Positioning
Pseudo-Range Positioning. The observed values adopted in the pseudo-range
positioning are GPS pseudo-ranges, which can be a C/A code pseudo-range or a
P code pseudo-range. This positioning method has the advantages of simple data
processing, a low demand for positioning conditions, no integer ambiguity, and an
easier realization of real-time location. The disadvantage is the low accuracy of
observed values. The accuracy of the C/A code pseudo-range observations is
generally 3 m and that of the P code pseudo-range observations is about 30 cm,
which results in low accuracy of positioning results.
Carrier Phase Positioning. The observations adopted in the carrier phase
positioning are GPS carrier phase observations; namely, L 1 carrier, L 2 carrier, or
a linear combination of these. The advantage of carrier phase positioning is the high
accuracy of observations, with a tolerance of better than 2 mm; however, it is
complicated in data processing and has integer ambiguity.
According to Modes of Positioning
Absolute Positioning. Absolute positioning, also known as precise point position-
ing (PPP), is a positioning model in which one receiver is used. The absolute
coordinates of the receiver antenna are determined in this mode. The mode is
simple in operation, so it can be used in stand-alone operation. It is generally
used for navigation and other applications of low accuracy.
Relative Positioning. Relative positioning, also known as differential position-
ing, employs more than two receivers to observe simultaneously in order to
determine the mutual relationship between positions of the receiver antennae.