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Version 2.0
English
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General Guide to Static and Rapid-Static
GPS System 500
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General Guide to Static and Rapid-Static -2.0.0en
Congratulations on your purchase of a new
System GPS500 from Leica Geosystems.
System GPS500
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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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General Guide to Static and Rapid-Static -2.0.0en
Contents
Contents
Introduction ................................................... 6
Overall planning for a GPS survey .............. 7
Baseline length............................................................ 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
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General 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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Overall 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.
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Overall 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 constellation, 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 independent 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 postprocessing 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 1 1-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.
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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.
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Overall planning for a GPS survey
General Guide to Static and Rapid-Static-2.0.0en
T emporary 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
ü Plan the campaign carefully
ü Consider the job, number of points, accuracy needed
ü Consider connection to existing control
ü Consider the transformation to local coordinates
ü Consider the best ways to observe and compute
ü For high accuracy, keep baselines as short as possible
ü Use temporary reference stations
- Consider the need for independent checks:
- Occupying points twice in different windows
- Closing traverse loops
ü Measuring independent baselines between points
ü Consider using two reference stations
ü Use good windows
ü Consider observing long lines at night
ü For high-accuracy work, try not to squeeze the
maximum number of points into a window
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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 postprocessing 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.
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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:
ü 4 or more satellites above 15° cut-off angle.
ü GDOP ≤ 8.
Whenever possible:
ü 5 or more satellites.
ü GDOP ≤ 5.
ü Satellites above 20°.
Always:
ü Use sky plot to check for obstructions.
ü Recompute GDOP if a satellite is obstructed.
ü Be 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"
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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 1 1-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, nightobservation 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.
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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.
Met hod
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
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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° cutoff 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.
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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 A vailability
(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 Position 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.
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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 postprocessing (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.
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Field observations
General Guide to Static and Rapid-Static-2.0.0en
Fill out a field sheet
Reference Stations
ü No obstructions above 15° cut-off angle.
ü No reflecting surfaces (multipath).
ü Safe, can leave equipment unattended.
ü No transmitters in vicinity.
ü Reliable power supply.
ü Ample memory capacity.
ü Correct configuration parameters (e.g. recording rate).
ü Check antenna height and offset.
ü Does not have to be a known point.
ü It 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
ü 15° cut-off angle.
ü Obstructions should not block signals.
ü No reflecting surfaces (multipath).
ü No transmitters in vicinity.
ü Fully-charged battery.
ü Sufficient memory capacity.
ü Correct configuration parameters (e.g. data-recording
rate).
ü Check antenna height and offset.
ü Observe in good windows.
ü Watch the GDOP ≤ 8.
ü Use Stop and Go Indicator as a guide.
ü Fill 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 dataprocessing stage.
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Field observations
General Guide to Static and Rapid-Static-2.0.0en
Fill out a field sheet, continued
Practical Hints
ü T ribrachs: check the bubble and optical plummet.
ü Level and center the tribrach and tripod correctly.
ü Check the height reading and antenna offset.
ü An error in height affects the entire solution!
ü Use a radio to maintain contact between reference and
rover.
ü Consider orienting the antennas for the most precise
work.
Field Sheet
Point Id.: Date:
Receiver Serial No.: Operator:
Memory card No.:
Type of set up:
Height reading:
Time started tracking:
Time stopped tracking:
Number of epochs:
Number of satellites:
GDOP:
Navigation position: Lat. Long. Height
Notes:
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Importing the data to SKI-Pro
General Guide to Static and Rapid-Static-2.0.0en
Importing the data to SKI-Pro
Checking and editing during
data transfer
Data can be transferred to SKI-Pro
directly via a PC-card slot, or via a
card reader , from the controller
(System 300) or receiver (System
500), or from a disk with backed-up
raw data. During data transfer, the
operator has the opportunity to check
and edit certain data. It is particularly
advisable to check the following:
• Point identification: Check spelling,
upper and lower case letters,
spaces etc.
• Make sure that points that have
been observed twice have the
same point identification. Make
sure that different points in the
same project have different point
identifications.
• Height reading: Compare with field
sheets.
Note that some of the above
site-related parameters can
be changed in some
components of SKI-Pro.
However, the af fected
baselines have then to be
recomputed.
Backing up raw data and
projects
After reading in a data set always
make a back-up on either a diskette
or on the hard disk. You can then
erase and reuse the memory card
but you still have the raw data. When
backing up data from several
memory cards, it is advisable to
create a directory for each card.
After importing all the data related to
the project it is often worthwhile
making a backup of the whole
directory where the project is located
before starting to process the data.
Page 23
6X
7X
13X
15X
17X
23
Importing the data to SKI-Pro
General Guide to Static and Rapid-Static-2.0.0en
Deriving initial WGS 84 coordinates for one point
As explained on page 18, the
computation of a baseline requires
that the coordinates of one point are
held fixed. The coordinates of the
other point are computed relative to
the "fixed" point.
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.
Using SKI-Pro it is easy to convert
the grid coordinates of a known point
to geodetic or Cartesian coordinates
on the local ellipsoid. If the
approximate shifts between the local
datum and WGS 84 are known,
WGS 84 coordinates to well within
the required accuracy can be
derived. The local Survey Department or University will usually be able
to provide approximate
transformation parameters.
As explained on page 17, the
reference receiver does not have to
be on a known point. If the reference
receiver was on a new (unknown)
point and a known point was
observed with the roving receiver,
simply compute the first baseline
from the known point (rover) to the
unknown point (reference) in order to
obtain and store the required initial
WGS 84 coordinates for the
reference receiver.
If good initial WGS 84 coordinates for
the reference site are not known or
cannot be derived as explained in the
last two paragraphs, the Single Point
Position computation in SKI-Pro can
be used. When using the Single
Point Position computation always
compute for a site for which there are
several hours of observations. The
effects of Selective Availability should
then average out and the resulting
WGS 84 coordinates should be
correct to within the required 10
meters.
See section
"Need for one known
point in WGS 84"
on page 18 for
further details.
Always keep in mind that poor initial
coordinates for the reference receiver
will affect the baseline computation
and can lead to results outside the
quoted specifications.
Deriving initial WGS 84 coordinates
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Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Data-processing parameters
In the vast majority of cases, the
default settings for data-processing
may be accepted and may never be
altered by the operator. On some
rare occasions the operator may
need to modify one or more of the
data processing parameters. The
most common ones are described
below.
Cut-off angle
It is common practice in GPS
surveying to set a 15° cut-off angle in
the receiver. 15° is also the system
default value in data processing.
Avoid cut-off angles less than 15° if
precise results are to be obtained.
Although you can increase the cut-off
angle you should be cautious when
doing so. If the cut-off angle for data
processing is set higher than in the
receiver some observations will not
be used for the baseline computation
and you may "lose" a satellite. It
could happen that only three
satellites would be used in the
computation instead of four. You
cannot expect a reliable answer with
only three satellites.
It can sometimes be advantageous,
however, to increase the cut-of f angle
to about 20° in case of a disturbed
ionosphere and provided that
sufficient satellites above 20° with
good GDOP have been observed
(use the
Satellite Availability
component in SKI-Pro to check the
GDOP).
You may sometimes find that a
baseline result is outside
specifications even though five
satellites have been observed. If one
of the satellites never rises above
about 20° the observations to this
satellite may be badly affected by the
ionosphere. Raising the cut-off angle
and computing with only four highelevation satellites can sometimes
produce a better result.
Page 25
25
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Ephemeris
SKI-Pro uses the broadcast
ephemeris recorded in the receiver.
This is standard practice throughout
the world for all routine GPS
surveying. For standard GPS survey
work there is little to be gained by
using a precise ephemeris.
Data used for processing
For precise GPS surveying, one will
normally accept the system default
setting
"Automatic"
, which will usually
use
Code
and
Phase
observations.
"Code only"
can be used for the rapid
calculation of baselines when high
accuracy is not required, for instance
in exploration or offshore work. If only
code observations are evaluated the
accuracy cannot be better than about
0.3m in position.
For the precise measurement of
baselines it should make little
difference whether one processes
code and phase measurements
together or
"Phase only"
. The results
should be more or less identical.
For long lines above about 100 km,
code observations can assist a highaccuracy solution provided that the
ephemerides are sufficiently good.
If code measurements are corrupted
for some reason, one can process
baselines using
"Phase only"
.
For processing kinematic data,
"Automatic"
has to be used for
precise results.
"Code only"
can be
used if high accuracy is not required.
Page 26
26
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Fix ambiguities up to:
With this parameter you can
determine how SKI-Pro should
compute baselines. The system
default value is
20 km
.
For baselines up to this limitation
value, L1 and L2 measurements are
introduced as individual observations
into the least-squares adjustment.
The Lambda search developed by
Prof. Teunissen and his co-workers
at the TU Delft is used as an efficient
approach to find possible candidate
sets of integer ambiguities. The
statistical decision criteria used has
been published previously together
with a different search algorithm, the
Fast Ambiguity Resolution Approach
(FARA) by Dr. E. Frei and is now
called FARA statistics.
For baselines above this limitation
value, a so-called L3 solution is
performed. The L3 observable is a
linear combination of the L1 and L2
measurements. The advantage of the
L3 solution is that it eliminates the
influence of the ionosphere.
However, it also destroys the integer
nature of the ambiguities, therefore
no ambiguity resolution can be
carried out. This is not important,
however, as successful ambiguity
resolution over long distances is in
any case hardly feasible.
Rms threshold
The Rms threshold is used to
minimize the possibility of unreliable
baseline results.
During the computation of a baseline,
the least-squares adjustment
computes the root mean square
(rms) of a single-difference phase
observation (i.e. the rms of unit
weight). This value is compared with
the Rms threshold.
For most GPS surveying applications
one will usually accept the system
default "
Automatic
". This will
automatically select an appropriate
rms parameter depending on the
duration of your occupation.
The rms of a single-difference phase
observation is largely dependent on
the baseline length, observation time,
and ionospheric disturbance.
Ionospheric disturbance is less at
night.
Page 27
27
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Rms threshold, continued
The following table provides a very approximate guide to the rms of a
single difference that a user could expect:
If the rms of a single-difference observation exceeds the rms threshold,
the baseline solution with fixed ambiguities will be rejected and only the
float solution will be presented (ambiguities not resolved).
Note, that the advanced parameter "Use stochastic modelling"
(see page 29) will additionally reduce the rms values of a single
difference.
For Rapid Static observations with up to 10 minutes of measurement
time, one should be cautious about increasing the rms threshold because
an unreasonably high rms value could lead to a weak solution being
accepted.
For longer observation times - let us
say about 30 minutes or more - the
rms threshold can be set higher
without undue risk.
Note that the rms threshold
applies only to baselines up
to the limitation value (see page 26).
For baselines above the limitation
value ambiguity resolution is not
attempted.
Distance Day Observation Night Observation
≤ 10 min > 10 min ≤ 10 min > 10 min
Up to 5 km
< 10 mm < 10 mm < 10 mm < 10 mm
5 to 10 km
< 15 mm < 25 mm < 10 mm < 15 mm
10 to 20 km
< 15 mm < 40 mm < 10 mm < 15 mm
Page 28
28
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Solution type
The solution type parameter applies
to all baseline up to which
ambiguities are attempted to be fixed
(see page 26). If solution type
"Stan-
dard"
is chosen, SKI-Pro will attempt
to fix ambiguities and apply
ionospheric corrections as defined in
the parameter
"Ionospheric model"
.
If solution type "Iono free fixed" is
chosen then the baseline
computation is done in two steps.
First ambiguities are attempted to be
fixed, then in the second step an
ionospheric free solution is calculated
using fixed L1 and L2 ambiguities.
The advantage of this approach is
that any ionospheric disturbance is
eliminated while fixed ambiguities are
used; it is recommended to choose
this solution type for all baselines
between 5 km and 20 km, in
particular if daylight observations
have been taken.
Ionospheric model
This parameter is only used for
baselines up to the limitation value
(see page 26,
"Fix ambiguity up to"
),
that is for baselines for which SKIPro will try to resolve ambiguities.
The default parameter is
"
Automatic
", which will automatically
select the best possible choice. If
sufficient observation time is
available on the reference, this will
be the "
Computed model
". In any
other case the "Klobuchar model"
will be taken provided that almanac
data is available. Typically there is
no need to change the default.
A
"Computed model"
may be used
instead of the standard model. This
is computed using differences in the
L1 and L2 signal as received on the
ground at the Sensor.
The advantage of using this model is
that it is calculated according to
conditions prevalent at the time and
position of measurement. At least 45
minutes of data is required for a
Computed model
to be used.
The Standard model is based on an
empirical ionospheric behaviour and
is a function of the hour angle of the
sun. When the Standard model is
chosen corrections are applied to all
phase observations. The corrections
depend on the hour angle of the sun
at the time of measurement and the
elevation of the satellites.
For long lines above the limitation
value (see page 26), the ionospheric
effects are eliminated by evaluating a
linear combination of L1 and L2
measurements, the so-called L3
observable. Ambiguity resolution is
not attempted.
Page 29
29
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Use stochastic modelling
Using this option may support
ambiguity resolution on medium and
longer lines when you suspect the
ionosphere to be quite active.
You should, however, be careful with
shorter baselines since bad data e.g. data influenced by multipath or
obstructions- may be misinterpreted
as being influenced by ionospheric
noise.
This is why by default this setting is
only used for baselines longer than
10 km.
Note that in order to ensure
reliable results this option
will not be used for the processing of
kinematic data.
Frequency
SKI-Pro will automatically select to
process whatever data is available.
Thus there is little point in processing
with anything but "Automatic".
Short observation times with Rapid
Static are only possible with dualfrequency observations. Long lines
can only be processed successfully
using L1 and L2 data.
Selecting "
Iono free float
" makes
SKI-Pro compute an L3 solution even
if the baseline length remains under
the limit to fix ambiguities (see p.26).
Remember, that for an L3 solution
the observation time has to be long
enough.
Tropospheric model
It will not make much difference to
the end result as to whether you
select the
Hopfield
or
Saastamoinen
model, but you should never work
with "
No troposphere
". You cannot
expect to achieve good results if no
tropospheric model is used.
Page 30
30
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Baseline selection - Strategy for computation
Baseline selection - Strategy for computation
Before starting data processing one
should consider carefully how best to
compute the network. Points to be
considered include:
• Obtaining good initial WGS 84
coordinates for one point.
• Connections to existing control.
• Computing the coordinates of
temporary reference stations.
• Rapid static measurements from
temporary reference stations.
• Long lines.
• Short lines.
If more than one temporary-reference
station has been used, this "network"
of temporary-reference stations
should be computed first. This may
also involve the connection to
existing control points. Select and
compute line by line, inspect the
results, and store the coordinates of
temporary reference stations if the
baseline computations are in order.
It is highly advisable to check the
coordinates for each temporaryreference station using double fixes
or other means, as all radial roving
points depend on temporaryreference stations.
Once the "network" of temporaryreference stations has been
computed, all remaining baselines i.e. the radial baselines from the
temporary-reference stations to
roving-receiver points - can be
computed.
If baselines of greatly differing
lengths have to be computed, it can
be worthwhile making two or more
baseline selections and computation
runs. In this way you can select and
compute batches of baselines which
fall into the same category of
parameter sets.
Try to avoid mixing baselines of
totally different lengths in the same
computation run. And avoid mixing
short-observation
"Rapid-Static"
baselines with long-observation
"Static"
baselines.
Page 31
31
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Baseline selection - Strategy for computation
Data Import and Computation
Check and edit during data transfer:
ü Point identification
ü Height reading and antenna offset
ü WGS 84 coordinates of initial point
ü Back up raw data and project
Baseline selection - Strategy for computation, continued
Consider the following carefully:
• How best to compute the network
• The need for good WGS 84 coordinates for one point
• Connection to existing control
• The need to transform to local coordinates
• Computation of network of temporary reference
stations
• Computation of new points from temporary reference
stations
• Long lines
• Short lines
• Data-processing parameters
Page 32
32
Data-processing pameters
General Guide to Static and Rapid-Static-2.0.0en
Interpreting the baseline results
When interpreting the results, one
has to distinguish between baselines
up to the limitation value (
"Fix
ambiguities up to"
) and baselines
above this value (see page 26).
For baselines up to the limitation
value, ambiguity resolution using the
Lambda search and the FARA
statistics is always attempted.
Interpreting the baseline results
For baselines above the limitation
value, a so-called L3 solution (linear
combination of L1 and L2
measurements) is performed. This
eliminates the ionospheric effects but
destroys the integer nature of the
ambiguities. Thus ambiguity
resolution is not carried out.
Page 33
33
Interpreting the baseline results
General Guide to Static and Rapid-Static-2.0.0en
Baselines up to the limitation value
These will usually be the "true
values".
However , one should also be aware
that very severe ionospheric
disturbances can cause systematic
biases in the phase observations. In
this case, although the results of the
least-squares adjustment will be
statistically correct, they could be
biased away from the true values.
The statistical methods implemented
in F ARA are based on very restrictive
criteria in order to try to ensure the
highest probability of a reliable result.
When the ambiguities are resolved,
you know that SKI-Pro has found a
"most probable" solution with an rms
value that is significantly lower than
for any other possible ambiguity set.
If the guidelines for baseline lengths,
observation windows, number of
satellites, GDOP, and observation
times are followed (combined
perhaps with your own experience),
the results of baselines for which the
ambiguities are resolved should be
within the system specifications.
Nevertheless, as explained above, it
is simply impossible to eliminate
completely the possibility of the
occasional biased result.
Ambiguities resolved
For baselines up to 20 km (system
default for
"Fix ambiguities up to"
),
ambiguity resolution should always
be successful if good results are to
be achieved.
For baselines up to the limitation
value, SKI-Pro searches for all
possible combinations of ambiguities
and evaluates the rms of a singledifference observation for each set of
ambiguities. It then compares the two
solutions with the lowest rms values.
If there is a significant difference
between the two rms values, the
ambiguity set yielding the lowest rms
value is considered as the correct
one. This decision is based on
statistical methods.
The reader will realize, of course, that
a least-squares adjustment can only
provide the "most probable" values.
Page 34
34
Interpreting the baseline results
General Guide to Static and Rapid-Static-2.0.0en
Note that for baselines up
to 20 km it should normally
be possible to resolve the
ambiguities provided that
sufficient observations have
been taken (see page 15
for a guide to baseline
lengths and observation
times). If the ambiguities
are not resolved check the
rms values in the logfile
(see next page).
Baselines above the
limitation value
Ambiguities not resolved
For baselines above the limitation
value (system default = 20 km), SKIPro eliminates the ionospheric effects
but does not attempt to resolve
ambiguities.
Thus the result will always show
"Ambiguities not resolved"
(Ambiguity
status = no).
Note that there is usually no
benefit in trying to resolve
ambiguities for lines over
20km.
As already explained, ambiguity
resolution should always be
successful for baselines up to 20 km
if good results are to be obtained.
If insufficient observations were
taken or the satellite constellation
was poor , SKI-Pro will not be able to
resolve the ambiguities. If the
ambiguities are not resolved it is
most unlikely that the system
specifications will be achieved.
If the ambiguities are not resolved in
Rapid Static (short observation
times) it is difficult to give an
indication of accuracy . However, as a
rough guide, one could multiply the
sigma values for each estimated
coordinate by 10 in order to obtain an
approximate estimate of the accuracy
of the baseline computation.
Page 35
35
Inspecting the logfile and comparing results
General Guide to Static and Rapid-Static-2.0.0en
Inspecting the logfile and comparing results
As explained in section
"Rms
threshold"
(see page 26), if the rms
float exceeds the rms threshold, the
baseline solution with fixed
ambiguities will be rejected and only
the float solution will be presented
(ambiguities not resolved). Thus if
ambiguities are resolved the rms float
and rms fix have to be lower than the
rms threshold.
The table on page 27 provides an
approximate guide to the rms values
(float and fix) that can be expected.
If the rms threshold is lower than the
rms float or rms fix one can consider
manually increasing the rms
threshold value. However, as
explained on page 27, one should
exercise a certain amount of caution
when doing this for Rapid Static
observations with up to 10 minutes of
measurement time.
The reason is that this could allow
unreasonably high rms float and fix
values and could therefore lead to a
weak solution being accepted.
Manually widening the rms threshold
value for successful baseline
computation requires a certain
amount of experience and
judgement.
If baselines of greatly differing
lengths have to be computed, it is
advisable to make two or more
computation runs. In this way you
can select and compute batches of
baselines which fall into the same
category of processing parameter
sets.
Baselines up to the limitation value
For baselines up to the limitation
value, ambiguity resolution using the
Lambda search and the FARA
statistics is always attempted.
When you look at the logfile, you will
find a summary of the FARA
statistics at the end of each baseline
output. You should check the
following:
• Number of satellites: there should
always be at least four.
• The rms float: this is the rms value
before fixing ambiguities.
• The rms fix: this is the rms value
after fixing ambiguities. The rms
fix will usually be slightly higher
than the rms float.
Page 36
36
Inspecting the logfile and comparing results
General Guide to Static and Rapid-Static-2.0.0en
Compare the results for
double fixes
Baselines above the
limitation value
Compare the logfile against
the field sheets
For baselines above the limitation
value (system default = 20 km), SKIPro eliminates the ionospheric effects
but does not attempt to resolve
ambiguities.
When inspecting the logfile check the
following:
• The number of satellites observed.
• The rms of unit weight
The rms of unit weight should be less
than about 20 mm for lines of about
20 km to 50 km. For lines over 50 km
the rms of unit weight will usually be
higher due to the minor inaccuracies
in the broadcast ephemeris.
If the results are not as good as you
would expect, it can be well
worthwhile comparing the information
in the logfile with that in the field
sheets. Check if the number of
satellites used in the baseline
computation is the same as that
noted in the field sheets. Remember
to check the reference station as well
as the rover. If the number of the
satellites is not the same, the GDOP
values could be higher than you
expected. Check the actual GDOP
for the satellites used in the
computation using the Satellite
Availability component of SKI-Pro.
If a point was observed twice in
different windows or two reference
receivers were operating
simultaneously , you should compare
the resulting coordinates.
Page 37
37
Storing the results
General Guide to Static and Rapid-Static-2.0.0en
Storing the results
After inspecting the summary of
results and the logfile, store the
results that meet your accuracy
requirements.
The coordinates are averaged
(weighted mean) if more than one
solution for a point is stored. For
instance if you store the coordinates
for point A from one baseline solution
and then you compute and store the
coordinates for point A again from
another baseline solution, the stored
coordinates will be updated to the
weighted mean values from the two
solutions. The weighted mean is
taken provided the coordinates agree
in both height and position to within
the
"Limits for Automatic Coordinate
Averaging"
set in SKI-Pro (default =
0.075m).
It follows that you should exercise a
certain amount of care when storing
points that have been fixed in more
than one baseline computation.
Compare the results before storing.
Page 38
38
Storing the results
General Guide to Static and Rapid-Static-2.0.0en
Storing the results, continued
• Baselines above the limitation value
(default = 20 km):
L3 solution, ambiguity resolution not attempted.
Results should meet specifications provided
sufficient observations are taken.
Long lines need long observation times.
• Inspect double fixes, independent baselines etc.
• Store results that meet accuracy requirements.
• Coordinates averaged if more than one result stored.
Interpreting and Storing the Results
• For lines up to 20 km, ambiguity resolution should be
successful if high-accuracy results are to be obtained.
• For long lines over 20 km, the L3 solution without
ambiguity resolution will normally be used.
• Baselines up to the limitation value
(default = 20 km):
Ambiguity resolution always attempted.
Ambiguities resolved (Ambiguity status = yes):
SKI-Pro has found most probable solution.
Results should normally meet specifications.
Ambiguities not resolved (Ambiguity status = no):
Float solution presented.
Result outside specifications, inspect logfile.
Consider increasing the rms threshold and
recomputing.
Page 39
39
General Guide to Static and Rapid-Static-2.0.0en
Adjustment, Transformation and output of results
6
7
13
15
17
22
23
24X
30X
32X
35X
37X
After the observations have been
computed, you may wish to adjust
the results if multiple observations to
points exist. This provides the best
estimates for the position of the
points. See SKI-Pro online help
"Adjustment"
for further details.
The results of the baseline
computations are coordinates in the
WGS 84 system. Using a
"Coordinate System"
in SKI-Pro,
these coordinates can be
transformed into coordinates in any
local datum or grid system.
Adjustment, Transformation and output of results
Page 40
40
General Guide to Static and Rapid-Static-2.0.0en
Adjustment, Transformation and output of results
When measuring with the SR510
(System 500) or SR9400 / SR261
(System 300) there are several
points that should be noted in order
that the measurements are
successful and good results can be
obtained.
Only observation windows with a
minimum of 5 satellites above 15°
and a good GDOP (< 8) should be
used.
The minimum observation time in
Static or Rapid Static should never
be less than 15 minutes.
As a rule of thumb the baseline
observation time should be 5 minutes
per kilometre of the baseline length
with a minimum time of 15 minutes.
Notes on single-frequency static and rapid static measurements
Recommended (minimum)
observation times:
A Rapid Static observation can
usually be considered to be
successful when SKI-Pro can resolve
the ambiguities. Providing an
estimate of the required observation
time is more difficult for single
frequency receivers than for dual
frequency equipment as considerably
less information is available for the
post processing software. Never the
less, the above table should serve as
a guide.
By default, SKI-Pro will not attempt to
resolve ambiguities if less than 9
minutes of (rapid) static, singlefrequency data is available. This is
done in order to avoid unreliable
results. Once the ambiguities are
resolved correctly the length of the
baseline will normally be accurate to
about 5 - 10 mm plus 2 ppm. These
default settings can be changed in
the Data Processing component of
SKI-Pro, but this is not
recommended.
Notes on single-frequency/rapid static measurements
Baseline-length Observation time
1 km 15 min.
2 km 15 min
3 km 15 min
4 km 20 min
5 km 25 min
6 km 30 min
7 km 35 min
8 km 40 min
9 km 45 min
10 km 50 min
> 10 km > 60 min
Page 41
41
General Guide to Static and Rapid-Static-2.0.0en
Adjustment, Transformation and output of results
6
7
13
15
17
22
23
24X
30X
32X
35X
37X
Notes on single-frequency static and rapid static measurements, continued
Notes on single-frequency/rapid static measurements
If the highest possible accuracy
should be achieved it is
recommended to orient the antennas
in a common direction.
On long baselines above 10 km the
accuracy which can be achieved with
single frequency Sensors is inferior
to that which can be achieved with
dual frequency Sensors due to
ionospheric effects which cannot be
eliminated with single frequency data.
Users who have previously worked
with dual frequency equipment
should be aware of this fact.
Page 42
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Ask your local Leica Geosystems agent
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Original text
712168-2.0.0en
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