About Designated Objects

When PERCY encounters a body number in a command, it normally expects to find data for that body in one of its ephemeris files. However, it recognizes 99 special body numbers---1001 through 1099---as referring to bodies which cannot be found in any ephemeris file.

Normally, these bodies are undefined: attempting to use an undefined body in any way---for example, as one of the bodies in a FIND command---results in an error.

The DESIGNATE command allows you to spontaneously create a new body---called a designated object--by supplying a set of coordinates in any of a number of Target Reference Frames recognized by PERCY. Once a designated object has been created, it can be used in any command, just as an ephemeris body would be. (You may remove a designated object at any time with the UNDESIGNATE command.)

Most of the frames are defined in terms of the locations of the target body, the Earth, the Sun, and the observing body. However, the magnetic systems (MAGNETOCENTRIC and TORUS) have some additional optional terms that can be used to rotate and displace the reference system from the normal planetocentric coordinate frame attached to the ephemeris object.

In fact, there are only two significant ways in which a designated object differs from an ephemeris body.

1.

2.

Creating a designated object

Each DESIGNATE command begins by specifying the ID code of the object for which a definition is being supplied.

   DESIGNATE \body(1001:1099)

Every designated object must be defined in terms of a Target Reference Frame (TRF), which must be centered at an ephemeris body. The center and the frame must be identified next.

   CENTER \body
 
        REFERENCE   (1:1){ CARTOGRAPHIC
                         | EARTH MERIDIAN
                         | GREAT CIRCLE
                         | MAGNETIC
                         | NORTH POSITION ANGLE
                         | PLANETOCENTRIC
                         | PLANETOGRAPHIC
                         | RECTANGULAR
                         | SOLAR MERIDIAN
                         | SOLAR POSITION ANGLE
                         | STELLAR
                         | TORUS }

Once the center and the frame have been identified, the coordinates of the object within the frame must be specified. The coordinates depend on the frame being used. However, all coordinates can be defined as either constant or linear expressions. For example,

   LATITUDE 44.35 DEGREES

specifies that the latitude of the object (in whatever reference frame) is fixed at 44.35 degrees, while

   LATITUDE 44.35 DEGREES 0.003 DEGREES/SECOND

specifies that the latitude is 44.35 degrees at some reference epoch, and increases thereafter at the rate of 0.003 degrees per second.

Whenever at least one of the coordinates is defined as a linear expression, a reference epoch for the expression must be supplied. (Otherwise, the expression cannot be evaluated.)

   REFERENCE EPOCH \epoch

Regardless of whether the coordinates are constant or linear, you may limit the interval during which the definition is valid.

   (0:2){ FROM \epoch
        | TO   \epoch }

If limits are not supplied the object is assumed to be defined for all epochs.

About Designated Objects

Creating a designated object

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