Instrument Section
This section lists the instruments whose measurements are included in the USMN network solution. Instruments can have different weights applied. So, for example, if one instrument station was set up in an unstable location, i.e. near a HVAC blower, up high on a large tripod, on a separate concrete pad, etc. The influence of that particular station on the entire network could be reduced by reducing the instrument’s weight relative to the other stations.
Each instrument has a checkbox indicating whether that instrument is allowed to move during the calculation. A USMN bundle (or any bundle, or that matter) is a relative solution. That is, the solution can be reached relative to any instrument. As a result, all instruments may be checked (meaning all instruments will be perturbed to find the solution) or one may be unchecked (so that the solution is reached by perturbing all instruments EXCEPT the unchecked instrument). In both cases, the same end result is reached for the network itself, although the networks will end up in different places in space (and the uncertainties will be relative to different baselines). Regardless, al- lowing all instruments to move will typically take longer to solve, but the resulting point uncertainties will be more uniform and representative of a network in which no one instrument is more important than another. Although the network solution is the same, the calcu- lated uncertainties for the USMN Composite points will be different, because they are relative to different baselines. However, the relative uncertainties (differences between uncertainties) between each pair of points will remain the same.
More than one instrument may be fixed (unchecked) during a solution, although this is typically only done in rare cases and is generally not recommended, as the end result is a solution that is not an ideal spatial representation of an instrument network.
An individual instrument in this list may be double-clicked to access specific instrument properties. This instrument Proper- ties dialog provides access to the following settings:
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Weight Factor. The overall weight applied to the instrument. (weights can also be edited directly in the USMN dialog).
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Moving. This sets the check box determining if a particular instrument is considered to moved as part of the network.
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Degree of Freedom. This set of 7 controls allows restriction of motion of a particular moving instrument as part of the network. Only checked fields are allowed to move. (note: XYZ component restriction are relative to the working frame, for instrument frame reference align the instrument model with the working frame prior to running the USMN).
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Component Uncertainty. Provides direct access to the instruments Uncertainty Variables.
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Point Computation Error Function. Error can be computed using the instruments uncertainty variables. If you choose to do so weights an be applied independently to each variable. Alternatively error can be computed with respect to the XYZ of the working frame.
In addition to individual instrument configurations there is also and option to choose between two Instrument Solution Reference Frame set- tings.
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Instrument Frame. In most cases, this is the best choice. This allows each instrument to be rotated independently about their own standing axis to find the ideal network solution and is the setting SA has always offered.
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Working Frame. This option provides a means to align instru- ments relative to each other in any orientation and hold that orientation independent of their own standing axis.