Vehicle Swath
2D Swath graphics allows you to easily calculate and graphically display swath for a vehicle.
Use the following steps as a guideline for displaying vehicle swath:
- Select the Swath Type using the drop-down list on the Swath page.
- Enter the appropriate value in the text box next to the chosen Swath Type.
- Select additional options for defining the display of swath using the graphics Options.
Swath Type
Select the type of swath to be displayed in the 2D Graphics window. The vehicle swath displays field-of-view swaths from nadir for a selected ground elevation angle or for a half-angle relative to nadir or a surface distance. The swath is always centered about the ground track and graphical displays of the vehicle swath can be enhanced by cross-hatching, which occurs at the vehicle data sample points. Swath types are outlined in the following sections.
Ground Elevation
The Ground Elevation swath consists of points on the ground having the specified elevation angle. For any given time, there are two points on the swath - a left point and a right point - both lying in the plane perpendicular to the vehicle's ground track at the vehicle's current detic subpoint location.
Vehicle Half Angle
The Vehicle Half Angle swath consists of points on the ground having the specified vehicle half angle or located at the horizon (i.e., at points where the ground elevation is zero). For any given time, there are two points on the swath - a left point and a right point - both lying in the plane perpendicular to the vehicle's ground track at the vehicle's current detic subpoint location.
Swath Half Width
Specifies the swath limits as a distance away from the subsatellite point as measured along the surface of the central body.
Ground Elevation Envelope
Using the text box on the Swath page, indicate the vehicle elevation angle as measured from the ground to the vehicle. For example, a 0 degree elevation angle would be the full field of view from the vehicle's perspective. The easiest way to visualize this is from a ground-based perspective. For a 1 degree elevation angle, an observer standing on the swath limit looking toward the vehicle would see the vehicle as it moves above the horizon. For a 45 degree angle, an observer would see the vehicle "halfway up" in the sky; and for a 90 degree angle, an observer would see the vehicle directly overhead.
At any given time, there are two points on the swath - a left point and a right point - whose rate of change of ground elevation is zero. The curve connecting all the left points and the curve connecting all the right points represent the envelope of the elevation contour curve as it moves with time.
There may be times of the vehicle for which no swath points exist, because there are no points on the ground elevation curve (i.e., those points on the ground meeting the specified vehicle elevation angle) whose rate of change of ground elevation angle is zero. Typically, this occurs when the vehicle changes altitude faster than it moves forward. In such cases, the ground elevation curve grows or shrinks rather than sliding forward. No swath graphics are shown during these intervals; however, the ground elevation curve at the beginning of the interval and at the end of the interval will be drawn using a dashed line as an indicator of the extent of the growth/contraction of the curve over this interval.
Vehicle Half-Angle Envelope
The half-angle relative to nadir. For example, for a 5 degree vehicle half-angle, the swath would be 10 degrees in width, centered at nadir.
Normally, at any given time, there are two points on the swath - a left point and right point - whose rate of change of vehicle half angle is zero. However, there may be times when either (i) the half angle extends beyond the horizon or (ii) there on no points where the rate is zero, depending on the vehicle's trajectory and the size of the half angle. In the first case, we define the swath points at that time using a zero ground elevation criterion; for the second case, the entire curve of points having the specified vehicle half angle is either expanding or shrinking so that there is no envelope curve (and thus, technically, no swath points).
There may be times of the vehicle for which no swath points exist, because there are no points on the cone angle curve (i.e., those points on the ground meeting the specified vehicle half angle) whose rate of change of vehicle half angle is zero. Typically, this occurs when the vehicle changes altitude faster than it moves forward. In such cases, the cone angle curve grows or shrinks rather than sliding forward. No swath graphics are shown during these intervals; however, the cone angle curve at the beginning of the interval and at the end of the interval will be drawn using a dashed line as an indicator of the extent of the growth/contraction of the curve over this interval.
Options
Use the Options to define the display of the chosen swath type. Options are outlined in the following table.