Spectra Precision ProMark 800 Reference Manual - Manual (Page 79)

67
Precise Surveying - Field Applications & Concepts
Accuracy will also depend on the RTK correction data
received from the base:
• The further the distance between the surveyed point and
the base, i.e. the longer the baseline length, the higher the
theoretical uncertainty affecting the position result.
• The lower the reception level of the received RTK
correction data, the less the data involved in the
processing and the higher the measurement uncertainty
affecting the position result.
Accuracy will also depend on whether the survey is run in real
time (RTK) or post-processing. In post-processing, because
the system processes all the collected raw data, accuracies
are better than in real time, provided the observation times
are long enough.
Expression of
Accuracy
For all Spectra Precision precise surveying systems, the
expression of the global accuracy on position is the sum of a
constant term and a variable term, as expressed in the
equation below.
Where:
• X is the constant term, in centimeters, indicating the
global uncertainty on position measurements (an rms
value, see Accuracy Measures on page 68). X qualifies the
intrinsic quality of the receiver and its algorithms under
nominal receptions conditions (i.e. open sky, a minimum
number of satellites is received and good GDOP). It may
be different for the vertical and horizontal components of
position.
• Y is the variable term, expressed in parts per million (ppm)
of baseline length. For example, if Y=1 and the baseline
length in your survey is about 8 km, then Y brings about
an additional, and nominal, 8 millimeters uncertainty on
all positions. The value of Y also reflects the quality of the
receiver and the algorithms used. Like X, Y may be
different for the vertical and horizontal components of
position.
For your information, usual figures of accuracy for fixed RTK
positions are given in the table below for nominal reception
conditions (open sky, good GDOP, 5 to 7 satellites received).
But remember these values are specific to each model.
Accuracy Xcm Yppm + =