User Instructions
and Technical Guide
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National Geodetic Survey
Figure 1.26 - Example 'MST' network
Figure 1.25 - 'MST'
network design
MST: A 'minimal spanning tree' (c.f., e.g.,
Wang et al. 1977) connects all the project
marks and CORS together seeking the
fewest and shortest possible unique
baselines. All project marks and CORS are
designated as hubs. "Generally speaking, a
shorter baseline leads to a better common
view of the satellites. For a network, an
optimal choice should be that the
summation of the weighted lengths of all
independent baselines should be minimal".
2
For
a network with n stations, there are
[n(n-1)/2] possible baselines, of which only
n−1 can be truly independent baselines.
The task is to find exactly those
(n−1)
possible baselines which are absolutely
independent from each other and are as
short as possible. This is especially useful for ambiguity fixing because there are usually some remaining
baseline length-dependent effects that can be mitigated by this network design strategy. Figure 1.25
shows the baselines selected using the MST Network Design. Because the number of project marks is
small, the minimal spanning tree looks identical to the CORS design in Figure 1.24.
Figure 1.26 shows a larger network using the MST Network Design. In this case, it is obvious that only
the shortest baselines between all points (project marks and CORS) are considered. Solution results
using this network design may be very dependent on which stations are constrained. Successive chains
of unconstrained project marks can lead to poor coordinates because of the propagation of
coordinate
uncertainties through the chain. The MST method may be most appropriate for networks with a few
project marks or with well distributed constrained marks.
2
Xu, G., GPS: Theory, Algorithms and Applications Solutions, 2003, 2007, Springer
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User Instructions and Technical Guide
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Figure 1.27 - 'TRI' network design
Figure 1.28 - Delaunay algorithm
Figure 1.30 - Mark co-location definition
TRI: The triangle network design (Figure
1.27) selects baselines using the Delaunay
triangulation algorithm. Invented by Boris
Delaunay in 1934
(2)
, this algorithm selects
lines connecting points such that no point
falls inside the circumscribing circle of any
triangle, i.e. the circle connecting the
three vertices of the triangle. Other
possible lines connecting points are
ignored. This is represented in the
example shown in Figure 1.28. Delaunay
triangulation maximizes the minimum
angle of all the angles of
the triangles
defined thereby avoiding “skinny
triangles” as much as possible. This design
may also permit the possibility of a station
being several successive stations removed
from a constrained station. Planning the
location of constrained CORS stations
within the overall network design is very important. Note the example in figure 1.27 shows that all
project marks and CORS are automatically selected hubs. Note that this method is the only choice
within OP where baseline independence is no longer a limiting factor in the number of baselines being
formed. This method also incorporates some trivial (dependent) baselines and the
mark confidence
statistics (uncertainties) have been compensated for the dependent baseline occurance.
General Limitations: For the
USER,
CORS, and
MST
network design there is currently a programmed limit of 99
project marks plus (including) CORS maximum per session
(this limit may change in the future). For the TRI network
design there is no size limit at this time. However, there are
also practical size limits dictated by the fact that OP is a
Web-based tool (cloud program). Experience indicates this
practical size limit is about 50 project marks per session.
More information about network design and specific survey
types may be described more fully in Section 3 (OPUS
Projects Technical Guide) and appendices of this document.
1.3.5.2.6
Project Mark Co-location Definition
The
Project Mark Co-location Definition
gives project managers some control over
how data files are associated together
with a mark ID. Here again, this definition
will be disabled (grayed out) after session
processing has begun and cannot be
changed. Two strategies are available.
Mark ID: RINEX and other
proprietary data file types use a mark
identifier string as the first four characters of the file name. If you are confident that this will be the
User Instructions and Technical Guide
OPUS Projects
40 |
P a g e
NOAA | National Geodetic Survey
Figure 1.31 - Save project preferences
case for all of your project’s data files, select '
Mark ID', then OP will group similarly named data files
together and name the project marks by the four character mark identifiers from the GNSS data file
names.
Position: On the other hand, if project mark identifiers are not the first four characters of the GNSS
data file names, the '
Position' selection will cause OP to group data files by their OPUS solution
coordinates. Data files whose OPUS solution positions fall within the
Maximum Position Difference
specified (1 m is the new project default and is shown in Figure 1.30) will be grouped together. OP will
attempt to name each project mark uniquely. If all the data
files associated with a mark, and only the
data files associated with that mark use the same first four characters in their file names, the project
mark will be given that four character name. If not, a name will be automatically generated starting
with “a001”. If “a001” is already in use, the numeric portion will be incremented until a unique name is
created.
The most common error users make is to not properly name the marks by editing the first 4
characters of the file name (RINEX).
1.3.5.2.7
Saving Project Preferences
Once you’ve reviewed all of the preferences and made the necessary changes, click the
Save Changes
and Close button at the top of the web page to apply these preferences to your project.
1.3.5.3
Project List
The
Project List button sends an email to the project manager listing all her or his projects.
1.3.5.4
Design
A placeholder for future enhancements. Currently this control is disabled (grayed out).
1.3.5.5
Serfil
A placeholder for future enhancements. Currently this control is disabled (grayed out).
1.3.5.6
Solutions button
The Solutions button displays a pop-up window with controls to rename or delete session solutions or
network adjustments. The standard pop-up window controls appear at the top of the window (Figure
1.32) with the
Help and
Refresh buttons
to the top-left, the
Apply Changes and Close button at the top-
center, and the
Close button to the top-right. Rules and guidelines for solution names are listed next.
This window identifies the lists of solutions and network adjustments that have been completed. Each
item in these lists includes
Rename and
Confirm fields which are used to change those names, and a
Delete checkbox. Changes are validated before they are applied. If errors are found, the changes will
not be applied and a message describing the problems appears. If any session solution included in a
network adjustment is deleted, the dependent network adjustment will be deleted also.