Leica Geosystems GPS System 500 Configuration guide

Version 2.0
English
50403020
General Guide to Static and Rapid-Static
GPS System 500

2General Guide to Static and Rapid-Static -2.0.0en
Congratulations on your purchase of a new
System GPS500 from Leica Geosystems.
System GPS500

3
General Guide to Static and Rapid-Static -2.0.0en
Introduction
Overall planning for a GPS survey
Mission planning
Observation times and baseline lengths
Field observations
Importing the data to SKI-Pro
Deriving initial WGS 84 coordinates for one point
Data-processing parameters
Baseline selection - Strategy for computation
Interpreting the baseline results
Inspecting the logfile and comparing results
Storing the results
Adjustment, Transformation and output of results
Notes on single-frequency Static and Rapid Static measurements
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4General Guide to Static and Rapid-Static -2.0.0en
Contents
Contents
Introduction ................................................... 6
Overall planning for a GPS survey .............. 7
Baselinelength............................................................ 7
Temporary reference stations for Rapid Static surveys . 8
Check the newly surveyed points ................................ 9
Night versus day observations. Measuring long lines .. 10
Observation schedule -.............................................. 10
best times to observe ................................................ 10
Consider the transformation to local coordinates .........11
Mission planning ......................................... 13
GDOP - Geometric Dilution of Precision..................... 13
Selecting good windows for successful GPS surveying13
Observation times and baseline lengths... 15
Field observations....................................... 17
Reference site........................................................... 17
Need for one known point in WGS 84............................. 18
Observing new points ................................................ 19
Use the Stop and Go Indicator as a guide ...................... 19
Fill out a field sheet.................................................... 20
Importing the data to SKI-Pro .................... 22
Checking and editing during data transfer .................. 22
Backing up raw data and projects .............................. 22
Deriving initial WGS 84 coordinates for
one point ...................................................... 23
Data-processing parameters...................... 24
Cut-off angle ............................................................. 24
Ephemeris ................................................................ 25
Data used for processing........................................... 25
Fix ambiguities up to:................................................. 26
Rms threshold........................................................... 26
Solution type ............................................................ 28
Ionospheric model ..................................................... 28
Use stochastic modelling ........................................... 29
Frequency ................................................................. 29
Tropospheric model................................................... 29
Baseline selection - Strategy for
computation................................................. 30

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General Guide to Static and Rapid-Static -2.0.0en
Interpreting the baseline results............... 32
Baselines up to the limitation value............................. 33
Ambiguities resolved ...................................................... 33
Ambiguities not resolved................................................. 34
Baselines above the limitation value........................... 34
Inspecting the logfile and comparing
results .......................................................... 35
Baselines up to the limitation value............................. 35
Baselines above the limitation value........................... 36
Compare the logfile against the field sheets ............... 36
Compare the results for double fixes.......................... 36
Storing the results....................................... 37
Contents, continued
Contents
Adjustment, Transformation and output of
results .......................................................... 39
Notes on single-frequency static and rapid
static measurements................................... 40

6General Guide to Static and Rapid-Static -2.0.0en
Introduction
Although this guide has been written
specifically for Leica Geosystems
GPS - System 500 and System 300,
much of the information is of a
general nature and applicable to all
GPS surveying. Further information
may be found in the various
guidelines contained in the System
500 or System 300 documentation
material.
Introduction
Surveying with GPS has become
popular due to the advantages of
accuracy, speed, versatility and
economy. The techniques employed
are completely different however,
from those of classical surveying.
Provided that certain basic rules are
followed GPS surveying is relatively
straightforward and will produce good
results. From a practical point of view
it is probably more important to
understand the basic rules for
planning, observing and computing
GPS surveys rather than to have a
detailed theoretical knowledge of the
Global Positioning System.
This guide outlines how to carry out
Static and Rapid Static GPS surveys
and emphasizes those points to
which particular care has to be paid.
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7Overall planning for a GPS survey
General Guide to Static and Rapid-Static-2.0.0en
Overall planning for a GPS survey
Rapid Static surveys feature short
observation times. It is particularly
important for Rapid Static that
ionospheric disturbances are more or
less identical for both sites.
Thus, for all GPS surveying, and for
Rapid Static in particular, it is sound
practice to minimize baseline lengths.
Baseline length
A GPS receiver measures the
incoming phase of the satellite
signals to millimeter precision.
However, as the satellite signals
propagate through space to earth
they pass through and are affected
by the atmosphere. The atmosphere
consists of the ionosphere and the
troposphere. Disturbances in the
atmosphere cause a degradation in
the accuracy of observations.
GPS surveying is a differential
method. A baseline is observed and
computed between two receivers.
When the two receivers observe the
same set of satellites simultaneously,
most of the atmospheric effects
cancel out. The shorter the baseline
the truer this will be, as the more
likely it is that the atmosphere
through which the signals pass to the
two receivers will be identical.

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Overall planning for a GPS survey General Guide to Static and Rapid-Static-2.0.0en
Temporary reference stations for Rapid Static surveys
In terms of productivity and accuracy,
it is much more advantageous to
measure short baselines (e.g. 5km)
from several temporary reference
stations rather than trying to measure
long baselines (e.g. 15 km) from one
central point.
As observation time and accuracy
are mainly a function of baseline
length, it is highly recommended that
baseline lengths should be kept to a
minimum.
Depending on the area and number
of points to be surveyed by GPS, you
should consider establishing one or
more temporary reference stations.
Baselines radiating from a temporary
reference station can be several
kilometers in length. Remember,
however, that it is advantageous to
minimize baseline lengths. The table
on page 16 provides a guide to
baseline lengths and observation
times.

9Overall planning for a GPS survey
General Guide to Static and Rapid-Static-2.0.0en
Check the newly surveyed points
Depending on the accuracy required,
the user should be prepared to check
newly surveyed points. This is
particularly important if observation
times have been cut to a minimum
and recommendations regarding
GDOP ignored.
For a completely independent check:
Occupy a point a second time in a
different window. This ensures that
the set-up, the satellite constellat-
ion, and the atmospheric
conditions are different.
Close a traverse loop with a
baseline from the last point to the
starting point.
Measure independent baselines
between points in networks
A partial check can be obtained by
using two reference stations instead
of one. You will then have two fixes
for each point but each will be based
on the same roving-receiver
observations and set-up.
In all types of survey work it is sound
practice to cross check using inde-
pendent measurements. In classical
survey you check for inaccurate or
wrong control points, wrong
instrument orientation, incorrect
instrument and target heights, etc.
You close traverses and level loops,
you fix points twice, you measure
check distances! Depending on the
job and accuracy needed it is well
worthwhile applying the same
principles to GPS surveying.
One should be particularly careful
with Rapid Static with short
observation times. If the observation
time is too short, or the satellite
geometry (GDOP) is poor, or the
ionospheric disturbances are very
severe, it can happen that the post-
processing software will resolve
ambiguities but the results may
exceed the quoted specifications.

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Overall planning for a GPS survey General Guide to Static and Rapid-Static-2.0.0en
Observation schedule -
best times to observe
For baselines up to about 20 km, one
will usually attempt to resolve the
ambiguities using the Rapid Static
algorithm in SKI-Pro post-processing
software.
For baselines over 20 km, it is
usually not advisable to resolve
ambiguities. In this case a different
post-processing algorithm is used in
SKI-Pro. This algorithm eliminates
ionospheric influences to a large
degree but destroys the integer
nature of the ambiguities.
Night versus day observations. Measuring long lines
Generally speaking, the longer the
baseline the longer one has to
observe.
The ionosphere is activated by solar
radiation. Thus ionospheric
disturbance is much more severe by
day than by night. As a result, the
baseline range for night observations
with Rapid Static can be roughly
double that of day observations. Or,
put another way, observation times
for a baseline can often be halved at
night.
At the present time ionospheric
activity is increasing in an 11-year
cycle.
The table on page 16 provides a
guide to baseline lengths and
observation times under the current
ionospheric conditions.
When you inspect the satellite
summary and GDOP plots, you will
usually see several good windows
(see page 14) distributed through a
24 hour period. You should try to
work with Rapid Static during good
windows, and plan your schedule
carefully.
It is impossible to plan GPS
observations to the minute. Rather
than trying to squeeze the maximum
number of points into a window by
cutting observation times to the bare
minimum, it is usually better to
measure one point less and to
observe for a few minutes longer.
Particularly for high-accuracy work, it
pays to be conservative and not to
risk poor results.