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
View of chapters
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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

5
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.
6

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.

8
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.

10
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.

11 Overall planning for a GPS survey
General Guide to Static and Rapid-Static-2.0.0en
Consider the transformation to local coordinates
The common points should be
spread evenly throughout the project
area. For a correct computation of all
transformation parameters (shifts,
rotations, scale), at least three - but
preferably four or more - points have
to be used.
Read the Guidelines to Datum/Map in
the SKI-Pro Documentation for
details on transformation using
Datum/ Map.
System 500 and System 300 provide
accurate relative positions of points
that are observed in a GPS network
and linked in post-processing. The
coordinates are based on the WGS
84 datum.
For most projects it will be necessary
to transform the WGS 84 coordinates
obtained from GPS survey into local
grid coordinates, i.e. into grid
coordinates on the local projection
based on the local ellipsoid.
In order to be able to compute this
transformation, known points with
local coordinates have to be included
in the GPS network. These common
points, with WGS 84 and local
coordinates, are used to determine
the transformation parameters and to
check the consistency of the local
system.

12
Overall planning for a GPS survey General Guide to Static and Rapid-Static-2.0.0en
Temporary Reference Stations
In terms of productivity and accuracy, it is usually
preferable to measure short baselines from several
temporary reference stations rather than trying to
measure long baselines from just one central point.
R-Temporary Reference Site
Example:
Establish 6 temporary reference stations using Static or
Rapid Static.
Check network of temporary reference stations using
double fixes or independent baselines.
Fix new points from temporary reference stations using
Rapid-Static radial baselines.
Consider the need to check critical points.
Consider the transformation to local coordinates, continued
Overall Planning
üPlan the campaign carefully
üConsider the job, number of points, accuracy needed
üConsider connection to existing control
üConsider the transformation to local coordinates
üConsider the best ways to observe and compute
üFor high accuracy, keep baselines as short as possible
üUse temporary reference stations
- Consider the need for independent checks:
- Occupying points twice in different windows
- Closing traverse loops
üMeasuring independent baselines between points
üConsider using two reference stations
üUse good windows
üConsider observing long lines at night
üFor high-accuracy work, try not to squeeze the
maximum number of points into a window

13 Mission planning
General Guide to Static and Rapid-Static-2.0.0en
Mission planning
Selecting good windows for
successful GPS surveying
Poor windows should only be used to
bridge between two or more good
windows when observing for long
periods of time, e.g. at reference
stations and for long lines.
If there are obstructions near a point,
use the sky plot to find out if the
signals from a satellite could be
blocked. This could cause the GDOP
to deteriorate. Check the GDOP by
clicking the satellite "off" in the
Survey Design component. A careful
reconnaissance of such sites is well
worthwhile.
GDOP - Geometric Dilution of
Precision
The GDOP value helps you to judge
the geometry of the satellite
constellation. A low GDOP indicates
good geometry. A high GDOP tells
you that the satellite constellation is
poor. The better (lower) the GDOP
the more likely it is that you will
achieve good results.
Poor satellite geometry can be
compared with the "danger circle" in
a classical resection. If the geometry
is poor, the solution in post-
processing will be weak.
For Rapid Static you should observe
when the GDOP is less than or equal
to 8. A GDOP of 5 or lower is ideal.
For successful, high-accuracy GPS
surveying it is advisable to take the
observations in good windows.
Provided that you know the latitude
and longitude to about 1°, the
satellite summary, GDOP, elevation,
and sky-plot panels in the Survey
Design component of SKI-Pro will
help you to select good windows in
which to observe.
You should take particular care when
selecting windows for Rapid Static
observations.
A suitable observation window for
Rapid Static must have four or more
satellites, with GDOP ≤8, above a
cut-off angle of 15° at both the
reference and roving receiver.

14
Mission planning General Guide to Static and Rapid-Static-2.0.0en
Selecting good windows for successful GPS surveying, continued
Selecting Good Windows
Window for Rapid Static:
ü4 or more satellites above 15° cut-off angle.
üGDOP ≤8.
Whenever possible:
ü5 or more satellites.
üGDOP ≤5.
üSatellites above 20°.
Always:
üUse sky plot to check for obstructions.
üRecompute GDOP if a satellite is obstructed.
üBe wary if 2 out of 4 or 5 satellites are low (<20°).
Example:
Good window - GDOP low and stable Poor window - GDOP high
Avoid observing during this "spike"

15 Observation times and baseline lengths
General Guide to Static and Rapid-Static-2.0.0en
Observation times and baseline lengths
Unless one is extremely restrictive, it is impossible to quote observation times
that can be fully guaranteed. The following table provides a guide. It is based
on tests in mid-latitudes under the current levels of ionospheric disturbance
with a dual frequency Sensor.
Ionospheric activity is currently increasing to a high level in an 11-year cycle.
As the activity increases it can be expected that observation times have to be
increased or baseline lengths reduced. Ionospheric activity is also a function
of position on the earth's surface. The influence is usually less in mid latitudes
than in polar and equatorial regions.
Note that signals from low-elevation satellites are more affected by
atmospheric disturbance than those from high satellites. For Rapid
Static observations, it can be worth increasing the observation times if
two out of four or five satellites are low ( say < 20°).
The observation time required for an
accurate result in post-processing
depends on several factors: baseline
length, number of satellites, satellite
geometry (GDOP), ionosphere.
As you will only take Rapid Static
observations when there are four or
more satellites with GDOP < 8, the
required observation time is mainly a
function of the baseline length and
ionospheric disturbance.
Ionospheric disturbance varies with
time and position on the earth's
surface. As ionospheric disturbance
is much lower at night, night-
observation times for Rapid Static
can often be halved, or the baseline
range doubled. Thus it can be
advantageous to measure baselines
from about 20km to 30 km at night.

16
Observation times and baseline lengths General Guide to Static and Rapid-Static-2.0.0en
Observation times and baseline lengths, continued
Times and Baseline Lengths
Observation time depends upon:
• Baseline length
• Number of satellites
• Satellite geometry (GDOP)
• Ionosphere
Ionospheric disturbance varies with time, day/night, month, year, position
on earth's surface.
The table provides an approximate guide to baseline lengths and observation
times for mid latitudes under the current levels of ionospheric activity when
using a dual frequency Sensor.
Obs.
Method No. sats. GDOP ≤8 Baseline Length Approximate time observation
By day By night
Rapid
Static 4 or more
4 or more
5 or more
Up to 5 km
5 to 10 km
10 to 15 km
5 to 10 mins
10 to 20 mins
Over 20 mins
5 mins
5 to 10 mins
5 to 20 mins
Static 4 or more
4 or more 15 to 30 km Over 30 km 1 to 2 hours 2 to 3 hours 1 hour 2 hours

17 Field observations
General Guide to Static and Rapid-Static-2.0.0en
Note that the reference
receiver does not have to be
set up on a known point. It is
far better to establish
temporary reference stations
at sites that fulfill the
requirements listed above
than to set up the reference
receiver on known points that
are not suitable for GPS
observations.
For computing the transformation
from WGS 84 to the local system,
known points with local coordinates
have to be included in the GPS
network. These points do not have to
be used as reference stations. They
can be measured with the roving
receiver.
Field observations
Reference site
GPS surveying is a differential
technique with baselines being
"observed" and computed from the
reference to the rover. As many
baselines will often be measured
from the same reference station, the
choice and reliability of reference
stations are of particular importance.
Sites for reference stations should be
chosen for their suitability for GPS
observations. A good site should
have the following characteristics:
•No obstructions above the 15° cut-
off angle.
•No reflecting surfaces that could
cause multipath.
•Safe, away from traffic and
passers-by. Possible to leave the
receiver unattended.
•No powerful transmitters (radio,
TV antennas, etc.) in the vicinity.
The results for all roving points will
depend on the performance of the
reference receiver! Thus the
reference receiver must operate
reliably:
•Power supply must be ensured.
Use a fully-charged battery.
Consider connecting two batteries.
When possible, consider a
transformer connected to the
mains.
•Check that there is ample capacity
left in the memory device for
storing all observations.
•Double-check the antenna height
and offset.
•Make sure that the mission
parameters (observation type,
recording rate etc.) are correctly
set and match those of the roving
receiver.

18
Field observations General Guide to Static and Rapid-Static-2.0.0en
Need for one known point in WGS 84
The computation of a baseline in
data processing requires that the
coordinates of one point (reference)
are held fixed. The coordinates of the
other point (rover) are computed
relative to the "fixed" point.
In order to avoid that the results are
influenced by systematic errors, the
coordinates for the "fixed" point have
to be known to within about 20
meters in the WGS 84 coordinate
system. Whenever possible, the
WGS 84 coordinates for the "fixed"
point should be known to within about
10 meters otherwise scale errors of
about 1 to 2 ppm will be introduced.
This means that for any precise GPS
survey the absolute coordinates of
one site in the network have to be
known in WGS 84 to about 10
meters. WGS 84 coordinates for one
site will often be available or can be
easily derived as explained on page
23.
If WGS 84 coordinates for one site
are not known or cannot be derived,
the Single Point Position computation
in SKI-Pro can be used. Remember,
however, that Selective Availability
(SA) may be switched on. The only
way to overcome SA is to observe for
sufficient time for the effects of SA to
be averaged out in the Single Point
Position computation.
The reference receiver will usually
observe for several hours as the
rover moves from point to point. In
such a case, the Single Point Positi-
on for the reference receiver
computed in SKI-Pro should be
relatively free from the effects of SA.
If a Single Point Position is computed
from only a few minutes of
observations, the effects of Selective
Availability will not be averaged out.
The result could be wrong by 100m
or more due to SA.
When computing the Single Point
Position for the starting point of a
network, always compute for a site
for which you have several hours of
observations. The resulting WGS 84
coordinates should then be correct to
within about 10 meters.
The minimum observation for the
computation of a reliable Single Point
Position is probably about 2 to 3
hours with four or more satellites and
good GDOP. The longer the
observation time, the better the
Single Point Position will be.

19 Field observations
General Guide to Static and Rapid-Static-2.0.0en
Observing new points Use the Stop and Go Indicator as a guide
As the Stop and Go Indicator can
only monitor the roving receiver it can
only provide an estimate for the
required measuring time. It should be
used only as a guide.
The operator of the roving receiver
should also pay attention to certain
points. This is particularly important
for Rapid Static surveys with short
measuring times.
• Make sure that the configuration
parameters (e.g. recording rate
etc.) are correctly set and match
those of the reference receiver.
• Check the antenna height and
offset.
• Watch the GDOP when observing
for only a short time at a point.
• For 5 to 10mm + 1 ppm accuracy
with Rapid Static, only take
measurements with GDOP ≤8.
The Stop and Go Indicator on the
sensor provides the roving-receiver
operator with an approximate guide
to measuring times for Rapid Static
observations with four or more
satellites and GDOP less than or
equal to 8. It estimates when
sufficient observations should have
been taken for successful post-
processing (ambiguity resolution) to
be possible.
At the present time estimates are
calculated for two baseline ranges,
0 to 5 km and 5 to 10 km. The
estimates are based approximately
on the current situation for GPS
observations in mid latitudes and
assume that the reference and roving
receiver are tracking the same
satellites.

20
Field observations General Guide to Static and Rapid-Static-2.0.0en
Fill out a field sheet
Reference Stations
üNo obstructions above 15° cut-off angle.
üNo reflecting surfaces (multipath).
üSafe, can leave equipment unattended.
üNo transmitters in vicinity.
üReliable power supply.
üAmple memory capacity.
üCorrect configuration parameters (e.g. recording rate).
üCheck antenna height and offset.
üDoes not have to be a known point.
üIt is better to establish temporary reference stations at
good sites rather than at unsuitable known points.
For precise GPS surveying, WGS 84 coordinates for one
point have to be known to about 10 meters.
Roving Receiver
ü15° cut-off angle.
üObstructions should not block signals.
üNo reflecting surfaces (multipath).
üNo transmitters in vicinity.
üFully-chargedbattery.
üSufficient memory capacity.
üCorrect configuration parameters (e.g. data-recording
rate).
üCheck antenna height and offset.
üObserve in good windows.
üWatch the GDOP ≤8.
üUse Stop and Go Indicator as a guide.
üFill out a field sheet.
As with all survey work, it is well worthwhile filling out a
field sheet for each site when taking GPS observations.
Field sheets facilitate checking and editing at the data-
processing stage.