How Does the High-Precision Orbit Propagator (HPOP) Work?

This articles provides a high-level overview of how the High-Precision Orbit Propagator (HPOP) works. If you are interested in a more detailed explanation of the different components of HPOP, you can read about them on the STK help pages.

Inputs

Initial State Vector – Typically position and velocity
Force Model – User-configurated list of forces acting on the satellite

Notes: For real satellites, the initial state vector is likely coming from an orbit determination system. For best accuracy, you should specify the same force model for orbit prediction in HPOP as you use in the orbit determination process. HPOP uses a force model to compute the accelerations on the spacecraft that result from factors such as central body gravity, third-body gravity, atmospheric drag, solar radiation pressure, thrust, etc.

Numerical Integration

Numerical Integration Method – General mathematical technique for numerically integrating solutions to ordinary differential equations (ODE)
Equations of Motion – Formulation of ODE in specific variables; typically expressed in terms of Cartesian position and velocity

Notes: HPOP applies the numerical integration method to the equations of motion using the configured force model. An evaluation of the force model computes the total acceleration of the spacecraft given the time, position, and velocity. HPOP uses the acceleration in the equations of motion to compute the rate of change of the elements of the orbit state, most commonly position and velocity. The numerical integration algorithm uses a series of force model evaluations to approximate the motion of the spacecraft over a single integration step. Some numerical integration methods contain automatic error control that first attempts to estimate the maximum size of the integration error over the step and then compares that maximum to a user-supplied acceptable tolerance. If the estimated maximum error is higher than the tolerance, the integration process reduces the step size and tries again. Alternatively, you can specify a fixed step size, which results in the process ignoring integration error.

Outputs

Propagated State Vectors – Typically position and velocity for each integration time step

If the process has not reached the specified end time, a step’s output state vector becomes the initial state for the next step. If a step's state vector is at the specified end time, then the process finishes, and the set of all propagated state vectors along with their associated epochs becomes an ephemeris.

Notes: There are many details that can affect the computation of the orbit. There are two significant measures of accuracy during orbit propagation: The accuracy with which the force model is integrated, and the accuracy of the force model itself. For instance, it is possible to numerically integrate a two-body force model with extreme accuracy when measured against the analytical solution of the problem. Unfortunately, the two-body model is not an accurate model of the forces on the spacecraft.