Spectra Precision Survey Pro CE v3.60 GPS User Manual - Manual (Page 21)

17
GPS Measurements
This section gives a brief explanation of GPS measurements. First, a
discussion of the basic theory of differential positioning will
familiarize you with different solution types and their expected
precision. Next, step-by-step instructions will describe how to
configure Survey Pro with GPS receivers to perform either Real Time
Kinematic (RTK) GPS or data collection for post processing
differential GPS.
Pseudo-Range Positioning
GPS solutions are computed using pseudo-range positioning: Position
is determined from multiple pseudo-range measurements to different
satellites (or space vehicles SV) at a single measurement epoch. The
position of the GPS receiver antenna is computed by intersecting the
pseudo-ranges from the known SV position in a manner similar to
survey resection. 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, any precise GPS must be performed using
differential positioning.
Differential GPS
Differential GPS (DGPS) positioning involves subtracting a
combination of ranges measured to various satellites from multiple
receivers. 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 specify a starting point.