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NASA SkyWatchIntroduction | What's New? | FAQ | Help > Variables Tab Help > Processing and Output

Processing and Output

There are 4 parameters that the user can change that will control the applet processing and output characteristics.

Search Time
This value is used to avoid looking for acquisitions that may never occur. Generally, for low earth orbiting spacecraft, you can expect to get several acquisitions each day provided your latitude is less than the inclination of the orbit. For those cases when there may not be an acquisition possible, this value indicates how long NASA SkyWatch will look for an acquisition before giving up and returning an appropriate error message.

Table Step Size
The table step size controls how often data is displayed on the tablular digital output. The default is 20 seconds which is good for most low earth orbiting satellites. However, if more detailed information is required, lowering the step size is possible. Conversely, increasing the step is possible to reduce the number of rows on the table. For long passes - on the order of an hour or more - the user will want to increase the step size to 60 seconds or more to reduce the number of rows on the table. Note that the same step size is used to plot the Sky Track image.

Drag Coefficient
Generally, spacecraft drag computations use 2.0 as the drag coefficient. This is usually held constant with weight and area being updated to reflect true drag characteristics. NASA SkyWatch allows the user to update the drag coefficient if required.

Plot Horizon
The SkyTrack display can draw the horizon to give the user the added visualization that may be needed to view the satellite. However, since the Earth is always rotating, even during the pass, the horizon will not be in the same place at the end of the pass as it is at the beginning of the pass. NASA SkyWatch gives the user the option of specifying when the horizon is to be drawn. The user can select either the beginning, the middle, or the end. Also, if no horizon line is desired, you can turn that off. Ordinarily, when to plot the horizon is not a problem. You may find that one or two points of the sky track may fall above or below the horizon which is not significant. Drawing the horizon becomes a little more challenging for very long duration passes - on the order of an hour or more. Then, the stars will be rotating 15 degrees per hour and the horizon will not line up where you might think it should.

Curator: Kim Dismukes | Responsible NASA Official: Amiko Kauderer | Updated: 09/23/2011
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