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.
