Positioning a Facility, Place, or Target
The Position page enables you to specify the location of the facility, place, or target. The location is specified in two parts: the location of a point on the ground and at a height above the ground. The ground location defines the local horizon for visibility computations, while the height above the ground determines how far "down" an observer at the facility, place, or target location can see. For example, a facility that has a height above ground of zero only has visibility to other objects when the elevation angle relative to that facility is greater than zero.
Select a Position Type and enter the appropriate values. The options available will depend on the Position Type you select.
You can also position a facility, place, or target by clicking anywhere in the 2D Graphics window. If you use the mouse in the 2D Graphics window to specify the position of the facility, place, or target, be sure that the Position page is open. You do not need to click OK or Apply after you have positioned the facility, place, or target in the 2D Graphics window. You will need to click OK or Apply if you have made any other changes on the Position properties page.
For more information on setting object properties using the mouse, see Mouse Shortcuts.
Position parameters
Position type
STK provides five ways of positioning the location of the ground for a facility, place, or target:
| Option | Description |
|---|---|
| Latitude | Latitude is measured in degrees from -90.0 degrees to +90.0 degrees. The geodetic latitude of a point is the angle between the normal to the reference ellipsoid and the equatorial plane. |
| Longitude | Longitude is measured in degrees from -360.0 degrees to +360.0 degrees. The longitude of a point is the angle between the projection of the position vector in the equatorial plane and the prime meridian. It is measured as increasing in a counterclockwise sense when viewed from the north pole. |
| Altitude | Altitude is the distance above or below the reference ellipsoid. Altitude is measured along the normal to the surface of the ellipsoid.
The reference ellipsoid refers to a mathematical geometric model of the earth. The earth is an ellipsoid, not a sphere. STK usually uses one of two models: MSL (Mean Sea Level) and WGS84 (World Geodetic System 1984). These models define the size of the ellipsoid. |
| Option | Description |
|---|---|
| Latitude | Latitude is measured in degrees from -90.0 degrees to +90.0 degrees. The spherical latitude is the angle of the position vector above the equatorial plane. |
| Longitude | Longitude is measured in degrees from -360.0 degrees to +360.0 degrees. The longitude of a point is the angle between the projection of the position vector in the equatorial plane and the prime meridian. It is measured as increasing in a counterclockwise sense when viewed from the north pole. |
| Radius | Radius is the distance of the object from the center of the Earth. |
| Option | Description |
|---|---|
| X | The X component of the object's position vector goes from the center of the Earth through 0 degrees latitude at 0 degrees longitude. |
| Y | This is the Y component of the object's position vector. |
| Z | The Z component of the object's position vector goes from the center of the Earth through the North pole. |
| Option | Description |
|---|---|
| Radius | This is the polar radius, where  . |
| Longitude | Longitude is measured in degrees from -360.0 degrees to +360.0 degrees. The longitude of a point is the angle between the projection of the position vector in the equatorial plane and the prime meridian. It is measured as increasing in a counterclockwise sense when viewed from the north pole. |
| Z | This is the Z component of the object's position vector. |
| Option | Description |
|---|---|
| Latitude | Latitude is measured in degrees from -90.0 degrees to +90.0 degrees. The geocentric latitude of a point is the spherical latitude of the sub-point on the surface of the Earth.
The sub-point refers to the intersection of the projection with the ellipsoid. The point may be on terrain or on a vehicle. The point then can be projected down to the surface, and the intersection is the sub-point. |
| Longitude | Longitude is measured in degrees from -360.0 degrees to +360.0 degrees. The longitude of a point is the angle between the projection of the position vector in the equatorial plane and the prime meridian. It is measured as increasing in a counterclockwise sense when viewed from the north pole. |
| Altitude | Altitude is the distance above or below the reference ellipsoid. Altitude is measured along the normal to the surface of the ellipsoid.
The reference ellipsoid refers to a mathematical geometric model of the earth. The earth is an ellipsoid, not a sphere. STK usually uses one of two models: MSL (Mean Sea Level) and WGS84 (World Geodetic System 1984). These models define the size of the ellipsoid. |
Use terrain data
You can either specify the position of the facility, place, or target or set the altitude automatically by selecting Use Terrain Data.
The terrain data files available in this scenario for visualization and analysis are listed on the Terrain properties page.
Altitude reference
You can reference the altitude from either the mean sea level (MSL) or the central body's reference ellipsoid (WGS84).
Height above ground
Use the Height Above Ground parameter to specify the height of the facility, place, or target above its model of the ground. The height is measured along the normal to surface defined by the reference ellipsoid of the central body. The facility, place, or target models the ground as an ellipsoid passing through the ground position specified by the other position values (for example, the Latitude, Longitude, and Altitude values when the Type is set to Geodetic). A facility, place, or target on the ground itself would have a height of 0.0 m.
A facility, place, or target can look downward only when it has a positive height above ground (more than 1 millimeter) and only to the ellipsoid defined by its ground position. For example, you can model a cell phone tower at 10 meters high above the ground in Denver, and have the cell phone tower be able to look downward only 10 meters to the local ground, not the full mile more to the central body surface, as was allowed in previous versions of STK.
When two facilities/places/targets (both with LOS enabled) do access, each will model their local ground, and both ellipsoids will be considered. Thus, if A can see B, then B can see A; if A cannot see B, then B cannot see A.
Local Time offset from GMT
The local time zone and corresponding time offset from GMT that are assigned to the facility, place, or target are computed automatically based on the object's longitude using time zones spanning 15 degrees in longitude. To override the local time zone for the facility, place, or target, turn the Local Time Offset from GMT option on and enter the time offset. For example, to specify a local time offset corresponding to Eastern Standard Time (EST), enter a value of -5.0 hours. If the facility, place, or target is located in a region of EST that practices daylight savings time, the value would be -4.0 hours for epochs occurring during daylight savings time. The offset is constant and doesn't account for daylight savings time rules, which specify when to switch back and forth from standard time to daylight savings time.
Lighting Obstruction Model
Select from four different lighting obstruction models that control how lighting, such as rise/set and solar intensity, is computed for the eclipse body that is the object's central body. The choices are as follows:
| Model | Description |
|---|---|
| Ground Model | This uses the same shape as used for the Line-Of-Sight access constraint. |
| Terrain | Terrain considers the effects of the surrounding terrain on obstruction. |
| AzElMask | This uses the object's azimuth and elevation mask to determine obstruction. |
| Central Body Shape | This uses the object's central body shape to determine obstruction. |
Other bodies included in the lighting computation continue to use the central body shape, which was called "Shape" prior to STK 12.