Two Body, J2, and J4 Special Options

For the Two Body, J2, and J4 propagators, click Special Options... on the Orbit properties page to open the Special Options window, where you can specify the propagation frame of the satellite. For the J2 and J4 propagators, you can also use this window to specify an ellipse option.

For Propagation Frame, select from Inertial, TrueOfDate, and TrueOfOrbitEpoch. The initial conditions will be converted to the selected frame during the computation of the orbit. The analytic orbit equations will be applied to the orbit initial conditions with the results being interpreted in the propagation frame. For example, a selection of the TrueOfDate frame with the TwoBody propagator will result in an orbit whose orbital elements are constant when reported in the TrueOfDate frame. This capability is useful for the construction of prototypical orbits where a constant relationship between the plane of the orbit and the equator of the central body is desired.

The Ellipse Options are defined in the following table.

Option	Description
Osculating	Representative of the particular point on a satellite trajectory, but the actual trajectory differs from its two-body counterpart at other points in time. If ON, the relationship among the Semimajor Axis length, nodal Mean Motion and Period of the satellite varies during the satellite's trajectory. These variations, including the effects of Earth oblateness, are caused by the propagators. The default option is to use two-body (Osculating) relationships among various sets of the classical elements entered on the Orbit page.
Secularly Precessing	Remains constant in shape and size but changes orientation in a continuous, consistent manner. If ON, the conversions between Semimajor Axis, nodal Period and Mean Motion consider the precession of the orbit. In this way, you can accurately specify nodal periods in the J2 and J4 propagators to assist in the design of orbits with strict period requirements or repeating ground tracks. The Secularly Precessing option can be useful if you want to design an orbit with a specific nodal period needed to produce a repeating ground track. In this case, select Secularly Precessing and set the Period on the Orbit page. STK then determines the correct semimajor axis based on the specified Period and other initial conditions.

Option

Description

Osculating

Representative of the particular point on a satellite trajectory, but the actual trajectory differs from its two-body counterpart at other points in time. If ON, the relationship among the Semimajor Axis length, nodal Mean Motion and Period of the satellite varies during the satellite's trajectory. These variations, including the effects of Earth oblateness, are caused by the propagators.
The default option is to use two-body (Osculating) relationships among various sets of the classical elements entered on the Orbit page.

Secularly Precessing

Remains constant in shape and size but changes orientation in a continuous, consistent manner. If ON, the conversions between Semimajor Axis, nodal Period and Mean Motion consider the precession of the orbit. In this way, you can accurately specify nodal periods in the J2 and J4 propagators to assist in the design of orbits with strict period requirements or repeating ground tracks.

The Secularly Precessing option can be useful if you want to design an orbit with a specific nodal period needed to produce a repeating ground track. In this case, select Secularly Precessing and set the Period on the Orbit page. STK then determines the correct semimajor axis based on the specified Period and other initial conditions.