Help / KnownX MPT / Propagation
Integration, time and frames
How the equations of motion are stepped forward, and how time and frames are defined.
Integrators
| Integrator | Used for |
|---|---|
| Semi-implicit (symplectic) Euler | Interactive scenarios in the workspace |
| Fourth-order Runge-Kutta | Validation and benchmarking |
Both integrators use the same force models, so a force cannot be present in one and missing from the other.
Time acceleration
The time rate can be doubled or halved with = and -. At high rates each frame is divided into smaller steps, and the rate backs off by itself if a step would become too large.
A run at a very high time rate uses a longer physics step. Use the Validation Center, not a fast-forwarded run, when the question is accuracy.
Time
Scenarios can be tied to a UTC epoch. When they are, the Earth’s rotation is set from Greenwich Mean Sidereal Time for that date, so the ground beneath the spacecraft is where it would really be.
Frames
Orbital states are held in an Earth-centred inertial frame. Results compared with SGP4 are expressed relative to the reference orbit as radial, in-track and cross-track components.
Units
Scenarios are defined in SI units: metres, kilograms, seconds, newtons. All calculation is in double precision.