OPUS Projects
User Instructions
and Technical Guide
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National Geodetic Survey
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Although the defaults can be changed at any time, changes made after processing may not be
reflected in the already completed processing results requiring one to reprocess some or all of the
session solutions and network adjustments.
Figure 1.201 - Data processing
defaults
Each of these processing defaults is
examined individually over the the next
few sections.
User Instructions and Technical Guide
OPUS Projects
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National Geodetic Survey
1.3.5.2.4.1
Output Reference Frame
Coordinates in the current ITRF/IGS are always included in the processing reports, but the coordinates in
an additional reference (user selected) will be reported if possible. The default for this additional
Output Reference Frame is selected using a pull-down menu (Figure 1.201). Note that the default in
newly created projects is ‘LET OPUS CHOOSE’ and causes the latest NAD 83 realization for the regions
covered by the North American Datum of 1983, to be reported. Your project’s
specifications or location
may require the use of a different reference frame so other menu items are provided for prior
realizations of NAD 83, and current and legacy realizations of ITRF, IGS, and WGS84 global reference
frames. It is recommended that you use the default when your project is in the United States or its
territories.
1.3.5.2.4.2
Output Geoid Model
A pull-down menu is also used for the
Output Geoid Model (Figure 1.201). Note that, here too, the
default in newly created projects is ‘LET OPUS CHOOSE’ causing the latest NGS hybrid geoid model, to be
used if possible, but other choices are available including some gravimetric geoids. It
is recommended
that you use the default when your project is in the United States or its territories.
A point of clarity is worthwhile here:
The conversion from ellipsoid heights to orthometric heights is
done as a purely algebraic removal of the geoid undulation from the ellipsoid height. Those users
familiar with “Height Modernization” should not confuse this conversion with an actual least squares
adjustment performed in the orthometric height domain.
1.3.5.2.4.3
GNSS
Currently the only choice in the
GNSS pull-down menu (Figure 1.201) is
G (GPS-only), but the capability
to use other GNSS in the data processing may be added in the future. Note that you may upload data
files (RINEX) containing GLONASS data and the NGS may archive it, but the GLONASS data is currently
stripped out automatically when processed by OPUS.
1.3.5.2.4.4
Tropo Model
OP provides limited user control over how the effects of water vapor on the satellite signals as they
traverse the tropospheric
portion of the atmosphere, commonly called the tropo corrections, are
parameterized (or modeled) in order to remove them (Figure 1.16). One strategy fits line segments,
representing the tropo corrections, to the data within specified time spans with the requirement that
the endpoints of the different segments connect. When plotted, this can create a continuous line, but
one composed of linear pieces
(Figure 1.21). OP uses the
jargon
piecewise linear (PWL).
This
is a realistic way to
represent a time-dependent
effect like this, but it can be a
poor choice if the data are
sparse or have gaps. For
sparse data, a simpler but less
realistic alternate strategy is
offered. With this alternate
strategy, only a step offset,
rather than an offset and slope
as
in the PWL strategy, is fit to
the data in each time segment.
When plotted, these corrections collectively look something like stair steps, thus OP calls this the
step-
offset (SO) method which is currently the default preference setting. The general rule-of-thumb is use
the program default
SO method if you have clear weather (high pressure) over your entire project and
few mask obstructions. If not, then use the
PWL method.
Figure 1.21 - PWL and SO modeling methods
SO model
PWL model
OPUS Projects
User Instructions and Technical Guide
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1.3.5.2.4.5
Tropo Interval(s)
The data intervals, mentioned in the preceding section, can also be specified by users (Figure 1.201).
For the
SO and
PWL methods the default interval is 1800 seconds (30 minutes). These values should
not be changed unless the user has a specific reason.
1.3.5.2.4.6
Elevation Cutoff (deg)
The default
Elevation Cutoff mask, in degrees above the horizon, can be specified (Figure 1.201). Data
from satellites below the specified mask will be ignored. The default for new projects is a conservative
but common value of 15 degrees. By eliminating low elevation satellites, some of the problematic
effects of the troposphere and ionospheric delays may be avoided. Pre-planning the GNSS observations
on each project mark by preparing a 'mark obstruction diagram' will produce a cleaner
data set for each
observation. Trees, buildings, hilly topography, etc. reduce satellite visibility, and may cause cycle slips
because of temporary losses of signal from a satellite. Most survey grade receivers allow the elevation
cutoff to be specified for an individual observation. A common strategy it to set the
receiver’s elevation
cutoff mask at a lower value, such as 10 degrees, then apply a higher elevation cutoff mask for individual
session processing. You will be given the opportunity to adjust the Elevation Cutoff mask seperately for
each session you process which is a good idea for sessions containing marks with
impared visibility
masks.