CASPER C Kernel Viewer
In addition, a tour kernel covering the time period of the observation must also be loaded.
To load a C Kernel, click on the Load C Kernel button at the top left of the C Kernel Viewer window. When the Pickfile Widget appears, select the kernel with the 990624_ prefix. Once it is selected, both the "Current C Kernel" and "C Kernel Summary" status widgets will be updated with information pertaining to the selected kernel. As a check, the start time (UTC) in the "C Kernel Summary" widget should be 1999 JUN 24 13:30:05.314.
The next step is to select a plot start time. For this example, click on the "Plot Start Time (UTC)" input widget, enter the time 1999 June 24 20:20, and press return. As with the Session Data Entry widget, the input time will automatically be updated to UTC format.
After pressing the return key, the "Number of Footprints:" input widget should be highlighted. Enter 15 as the number of footprints to be taken for this observation.
After pressing the return key, the "Footprint Time Interval" input widget should be highlighted. Enter 1m for one minute, and press enter. The number will automatically be updated to read 60 seconds.
Next, the target body and observer need to be specified. To do these, go to the "Select Target Body for Segment Plot:" list, and scroll down until you can select Venus. Make sure that the "Observer for Segment Plot:" is CASSINI by selecting it from the list.
Finally, click on the Make Target Plot button at the top center of the window. This will bring up the Target tool and plot the observation in the drawing widget. This observation will appear in the Target tool window just as a designed CASPER module appears. You can get timing and geometry information using the Utilities->Module/Pointing Data menu option on the Target tool.
Pointing data from a C kernel can be plotted in the Target tool and the Skymap tool just like a CASPER module.When a user has added a module to the Target plot (see Example CASPER Session), it is possible to save the module as a C Kernel. To save the module as a C Kernel click on Sequence Design menu item, then C Kernel Tools, and lastly Save C Kernel.... The Pickfile Widget will appear for the user to input a file name. The user will also be prompted to give the new C Kernel a name to be placed in the header. The observation can then be examined more thoroughly from the C Kernel Viewer widget. Observations created in CASPER can be passed to other CASPER users as C kernels and viewed using the C Kernel Viewer. (It is better to pass observations between CASPER users as saved CASPER mosaics using the Sequence Design->Mosaic Tools->Save Mosaic option in the Target or Skymap tools, but those cannot be used by any programs other than CASPER.) When viewing an observation in CASPER from C kernel data, whether the observation was created by CASPER or is reconstructed spacecraft attitude data, it is important to remember that C kernel data does not include any information about the intent of the observation or its target; it only includes pure spacecraft attitude information.
When a module is made in the Target tool, it is possible to misconstrue that because the pointings are evenly spaced spatially, they are evenly spaced temporally. Below, twe take you through an example that demostrates that the pointings are not evenly spaced temporally and how C Kernel Viewer allows you to deal with that.
First, double check that the appropriate tour file is loaded. It should have SOI_T45.bsp as the suffix. This is especially important if the user is continuing from the previous section.
Next, make a 4x4 GENMOSCD_IVP module beginning at 2005 June 07 00:00. Use the background grid to draw a box that is roughly 3 degrees by 3 degrees. Also, make sure that all FOV instruments are turned off except Galileo SSI camera of the GLL group in the FOV Selection Widget. When finished, a plot that looks similar to this should appear in the Target window.
Before saving the module as a C Kernel, time information needs to be taken from the plot. From the Utilities menu item, select Module/Pointing Data to open the Pointing Information Widget. Scroll until the Pointing Index is 15, and write down the UTC time on scratch paper. The time should be about 13 minutes 0 seconds after the first poiting. Divide the number by 16 to get the time interval between each poiting. It should be about 0.81 minutes. (Remember to close the Pointing Information Widget.)
Now, save the module as a C Kernel as described above. Name the C Kernel time_interval.bck, and enter your last name in the header.
Now, we will want to look at the module from the C Kernel Viewer tool. Click on the C Kernel icon at the top of the Target tool to open the C Kernel Viewer Widget. Select the Load C Kernel button at the top of the widget to open the Pickfile Widget. Select time_interval.bck, which will most likely be in the user's /casper_save/ckernels/ directory. As a check to see that the correct C Kernel has been loaded, the start time should be about 2005 June 7 00:00, and the number of records should be 16.
In the "Plot Start Time:" field, input 2005 June 7, and press enter. Set the "Number of Footprints" to 15. And, in the "Footprint Time Interval:" field, input the time interval that was calculated earlier (approximately 0.81m). Update the "Observer" to CASSINI SIM. Finally, click on the Make Target Plot button.
Once completed, the Target tool should display a picture that does not look like the original observation, but, rather,
Fortunately, the C Kernel Viewer tool will still allow the user to view the module as it was originally created. To do this, click on the Load C Kernel icon in the Target widget to bring up the C Kernel Viewer widget. Highlight the "Plot Start Time" input widget and press the return key since the correct input time (2005 JUN 07 00:00) should already be displayed. PLEASE NOTE: if this is not done, CASPER will begin plotting at the end of the kernel file. CASPER must be "retold" what the start time is. The "Number of Footprints" should already read 15, and it does not need to be re-entered. Highlight the "Footprint Time Interval" input widget and enter 0. Finally, click on the Make Target Plot button.
Once completed, the Target tool should display a picture that looks like the original observation minus one pointing since we have asked it to do only 15 footprints,
The CASPER C kernel viewer allows you to extract detailed geometric information for a planned or previous observation for which C kernel data exist. This is the most straightforward way to determine where an instrument is pointed during the course of an extended observation. For detailed information at a single point in time, the CASPER Physical Model Interface is also useful.
The C Kernel Viewer offers two options for viewing detailed pointing profiles: create an IDL data file or interactively plot the data within CASPER. Because IDL is designed for quick and easy visualization and manipulation of just this type of data, the recommended route is to save the pointing information in an IDL data file and then work with that data in a separate IDL session independent of CASPER. The plotting tool is useful for quick looks at the data prior to saving and before exiting CASPER.
Pointing information is generated after you enter the following information:
and then click on either the Save Pointing File or Plot Pointing buttons (instead of the Make Target Plot) button. This is the same information you need to enter when plotting the C kernel data in the Target or Skymap tools with the Make Target Plot button. For the Save Pointing File and Plot Pointing options, you can enter a very large number of footprints without encountering any memory problems. These tools are very useful for studying the detailed pointing profile at high time resolution and/or over a long period of time.
When you click on the Save Pointing File button CASPER will compute the following information for every footprint requested.
Because this may be a very large number of footprints (tens of thousands), the operation may take a minute or two. A status message at the bottom of the C Kernel Viewer informs you of progress. When the data is computed a file name requestor will prompt you for a file name to save the data to. Pointing files are given a .pnt file name suffix by CASPER. The default location is in your casper_save/output/ directory. All angles are in degrees.
To use the saved pointing information, open an IDL session independent of CASPER. CASPER need not be running (and it may be easier not to have it running) when working with the pointing file. At the IDL prompt enter the command:
restore,'filename.pnt'
where filename is the name you gave the file. You must of course be in the correct directory for the file or specify the full file path. A brief summary of the data is provided in the variable readme which you can see with:
IDL> print,readme
The data is stored in an array of structures called pointing_save. The various tag names of the structure can be seen with:
IDL> help,pointing_save,/struc
The number of elements in the pointing_save array is equal to the number of footprints you specified when you created the file with the C Kernel Viewer. To plot the pointing profile in inertial space, for example, do:
IDL> plot,pointing_save.ra,pointing_save.dec
To plot phase angle as a function of time, do:
IDL> plot,pointing_save.scet,pointing_save.phase
Use IDL's features to manipulate, sort, filter, and view the data in whatever ways suit your particular needs.
The C Kernel Viewer provides a simple plotting tool which allows you to plot data in this fashion from within CASPER. After specifying your C kernel data parameters as above, click on the Plot Pointing button to open a new plotting tool. By associating the Y (and X, optionally) axes with a data vector (such as phase angle) you can immediately plot the data within CASPER using this plotting tool. The tool also provides the ability to adjust the plot parameters including linestyle, plot symbol, axis and main titles, plotting range, and it allows plotting to the PostScript device as well as to the screen.