Instrument Mounts

The DIRSIG4 platform model has a nominal heading in the +Y direction, so across-track scanning is along the X-axis (or about the Y-axis) and along-track scanning is along the Y-axis (or about the in X-axis). Not this old holds true if the attachment between the mount and the platform does not introduce a rotation of the mounts coordinate frame (which only happens when the user deliberately manipulates the mount attachment). For more information about the coordinate conventions consult the Coordinates Systems document and the Affine Transform document.

In DIRSIG4, mounts were limited to platform-relative rotation only. In the DIRSIG5 BasicPlatform plugin the ability to model platform-relative translation was added for a select set of data-driven mounts:

Each mount has the option to include jitter, which will add deviations to the nominal pointing of the current mount. This feature allows the user to incorporate unprogrammed motion into the platform-relative pointing. This could be used to emulate shaking in a camera ball on an aircraft or flex in mounts used to bolt instruments to the platform. Like the jitter support in other parts of the DIRSIG model, the user can specify temporally uncorrelated and correlated jitter on a per-axis basis.

Most of the dynamic mounts have a "clock" associated with them. This clock has the same interface as those associated with components like a focal plane, which allows the clock to be setup as either an independent clock or one slaved to the master clock on the platform. This clock defines the frequency that a dynamic mount will repeat the scan pattern. For example, a sinusoidal "whisk" pattern with a clock set to 30 Hz will repeat the full sweep (across and back to the starting position) 30 times per second.

The Mount Editor in the graphical user interface allows the user to visualize a dynamic mount by plotting the along-track and across-track pointing angles as a function of time. The Animate panel at the bottom of the Mount Editor allows the user to control the visualization parameters.

The images below show different points in time in the animation of a "Lemniscate" scan. The Fraction Displayed was increased to 0.90 so that more of the scan pattern could be visualized during each step.

The "static" mount points the attached instrument(s) in a fixed direction with respect to the platform. The user defines this pointing direction via a set of constant right-handed X, Y and Z axis Euler rotation angles. The user must supply the order that the axis rotations are perform in (see the affine transform document for more information on Euler rotation operations). For example, the "rotation order" of "xyz" indicates to first apply the x rotation, then the y rotation, then the z rotation.

But default, the model configures a static mount with these pointing angles all set to zero, which results in the instrument pointing straight down out of the aircraft or satellite bus.

The "lemniscate scan" mount provides the user with a way to produce a "figure 8" style scan pattern. The user has control over the the along-track and across-track angular ranges.

The "line scan" mount provides the user with a constant angular velocity scan in the across-track direction. The scan direction follows a right-hand

The "tabulated" mount allows the user to model an arbitrary, periodic scanning mount by inputting XYZ Euler rotation angles as a function of "scan fraction". The "scan fraction" ranges from 0 to 1, and is a way to convey where the mount is in a scan independent of time or the scan rate. A scan fraction of 0 is the start of the scan and a scan fraction of 1 is the end of a scan. As a result, a fraction of 0.5 is the middle of the scan. For a continuous repeating scan, then the state at 0 and 1 should be the same. The user only needs to supply known time/rotation pairs and the model will linearly interpolate rotations for intermediate times.

The user has two choices for how the angle vs. scan fraction data is supplied.

The reason that this mount supports either internal or external data is to better support different scenarios. A fixed scan pattern that never changes from collection to collection can be stored as "internal" data. If the pointing is specific to a specific collection, then it might make more sense to store that data "external" to the platform file in order to avoid having multiple copies of platform files that simply vary in the command data driving the pointing.

In either case, the user must provided the axis rotation order. The choice of axis rotation order has a large impact on the resulting orientation (see the Affine Transform document for more information on the impact of rotation order).

The "whisk scan" mount scans the attached instrument(s) in the across-track direction. The DIRSIG4 platform models have a nominal heading in the +Y direction, so across-track scanning is an Y axis rotation. At the present time, the X and Z rotations are constant and 0 valued. The "whisk scan" mount is a dynamic mount, so the description includes a scan rate that defines the rate at which the mount sweeps and entire scan. The across-track scan covers user-defined angular range using a sinusoidal function.

The "tracking mount" will always point at a designated piece of scene geometry. It can handle moving platforms, moving target geometry, and offset instrument mount attachments. To specify what piece of scene geometry is tracked by the mount, the user must provide the name of a unique instance inside an GLIST file.

This mount is very similar to the Tabulated mount, except that the primary variable is time rather than scan fraction. This makes it easier to configure a scenario like a camera ball that rotates to specific locations at specific times.

Starting with DIRSIG 4.2.3, instrument mounts could be controlled through a scripting language. This provides additional flexibility when the built-in dynamic mount options prove insufficient and is best utilized when the desired scan pattern can be easily characterized by an analytical calculation. The supported scripting language is ECMA-262, which is an international standard scripting language. The ECMA-262 standard description can be found at the following external link.