Spectra Precision Survey Pro v4.6 Recon and Nomad User Manual - Manual (Page 233)

GPS Overview
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GPS Measurements
GPS coordinates are computed using pseudo-range positioning.
Pseudo-range positioning involves intersecting the ranges from the
known SV position in a manner similar to a distance – distance
intersection problem. Four SVs are required to determine three
position dimensions and time. Position dimensions are computed by
the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ)
coordinates.
A pseudo-range solution will be one of two types: autonomous, or
differential. A single GPS receiver can compute an autonomous
position from ranges to four or more SV. This single receiver position
is extremely coarse. One can expect errors in the order of 100-m on a
bad day. For this reason, precise GPS measurements must be
performed using differential positioning.
Differential GPS
Differential GPS (DGPS) positioning involves subtracting a
combination of ranges measured to various satellites from two or
more receivers measuring the same satellites simultaneously. When
the signals are subtracted, the major error sources cancel each other
out. However, because you are computing a difference in ranges, the
DGPS measurement solves for a coordinate difference and not a
coordinate. To compute a coordinate using a coordinate difference,
you must first specify a starting point. In RTK survey, this starting
point is the reference position the base receiver broadcasts over the
data link. In a post processing survey, this starting point is specified
by the user in the processing software.
Differential Solutions: Types and Quality
A differential GPS solution can be calculated using either the GPS
code data, or the GPS carrier phase wave. A carrier phase solution
can be calculated from either float ambiguities or fixed ambiguities.
An explanation of these terms, and the expected precision from the
different solution types is explained below.