Missile Properties | Satellite Properties

Two-Body, J2 Perturbation & J4 Perturbation Propagators

Two-Body, J2Perturbation and J4Perturbation are analytical propagators that generate ephemeris by evaluating a formula. Two-Body's formula is exact (i.e. the formula generates the known solution for a vehicle moving about a central body considering only the effect of the body viewed as a point mass) but is not an accurate model of a vehicle's actual force environment. J2Perturbation includes the point mass effect as well as the dominant effect of the asymmetry in the gravitational field (i.e. the J2 term in the gravity field, representing North/South hemisphere oblateness); J4 additionally considers the next most important oblateness effects (i.e., the J2^2 and J4 terms in addition to J2). None of these propagators model atmospheric drag, solar radiation pressure or third body gravity; they only account for a few terms of a full gravity field model.

These propagators are often used in early studies (where vehicle data is usually unavailable for producing more accurate ephemeris) to perform trending analysis: J2 Perturbation is often used for short analyses (weeks) and J4Perturbation often for long analyses (months, years). They are particularly useful for modeling "ideal" maintained orbits without having to model the maintenance maneuvers themselves. For satellites, these maintained orbits can be constructed using the Orbit Wizard.

The solutions produced by the J2Perturbation and J4Perturbation propagators are approximate, based upon Keplerian mean elements. In general, forces on a satellite cause the Keplerian mean elements to drift over time (secular changes) and oscillate (usually with small amplitude). In particular, the J2 and J4 terms cause only periodic oscillations to semi-major axis, eccentricity and inclination, while producing drift in argument of perigee, right ascension and mean anomaly. STK's J2Perturbation and J4Perturbation propagators model only the secular drift in the elements (the drift in mean anomaly can best be seen as a change to the period of motion of the satellite). Many of the ideal maintained orbits are designed in a manner to take advantage of the prevailing secular drift caused by J2 to achieve their missions.

After choosing the Two-Body, J2 Perturbation or J4 Perturbation propagator, you must define its parameters:

1. Get a new initial vector with the Initial State Tool (optional).
2. Set the vehicle's Start Time, Stop Time, and Step Size. Use the current scenario time period by turning on Use Scenario Analysis Period.
3. Choose a Coordinate System.
4. Choose a Coordinate Type and enter values for the orbital parameters.
5. Use the Special Options... button to set the propagation frame of the satellite. For the J2 and J4 propagators, you can also specify an ellipse option.