Understanding Different Orbit Propagators

Systems Tool Kit (STK) has different orbit propagators to model the motion of satellites. This article provides an overview of the various propagators available in STK, their specific use cases, and examples of how they can be applied.

Types of Orbit Propagators in STK

1. Analytic Propagators

Analytic propagators approximate the motion of a satellite and are ideal for scenarios where maintaining an orbit is necessary without modeling the maintenance maneuvers.

• TwoBody (Keplerian Motion): This propagator considers only the gravitational force from the Earth, which is modeled as a point mass. It's useful for simple and quick simulations.
• J2 and J4 Perturbation (First-Order): This propagator accounts for secular variations in the orbit due to the Earth's oblateness. It provides a more accurate model compared to the TwoBody propagator by considering the Earth's shape.
• Note: These are available in STK Pro

2. External Propagators

STK supports several external propagators, which can be used when propagation was already performed in another tool or previously. Instead of recalculating the position, the External Propagators use the given ephemeris and interpolate between the points. These propagators are integrated into STK to extend its capabilities.

• STK External Ephemeris: Allows the use of external ephemeris data. This data can be generated by STK, generated by ODTK, generated by COMSPOC or generated by external tools. COMSPOC provides their High-Definition Ephemeris (HIDEph) data in STK external format as a default file format. 
• SPICE: Used for planetary science and astronomy applications, integrating data from NASA's SPICE toolkit.
• GPS (SP3) & SP3: Useful for GPS satellite data and precise positioning applications.
• Real-Time: Enables real-time simulation and tracking. Using RealTime Data  
• NASA Ephemeris from Horizons System: Modeling Comets and Asteroids using Nasa Ephemeris data 
• Note: These are available in STK Pro

3. Common Propagator 

This propagator is available and works well with the Standard Public Database from Space-Track.org. This propagator is useful for knowing an approximate location of a satellite based off of tracking by the Space Force. 

• Simplified General Perturbations (SGP4) propagator is a standard US Space Force (USSF) propagator that uses GP (General Perturbation) data. Typically, the data is formatted in two-line mean element (TLE) format, though you can use other formats. SGP4 considers secular and periodic variations due to Earth oblateness, solar and lunar gravitational effects, gravitational resonance effects, and orbital decay using a drag model. 
• Note: This is available in STK Pro

4. Special Perturbations (SP) Propagators 

These propagators use numerical integration of force models to generate high-fidelity ephemeris. They are suitable for detailed and accurate simulations. These are the highest quality and most accurate propagators available for STK. 

• HPOP (High Precision Orbit Propagator): A high-fidelity propagator that considers various perturbative forces such as gravitational forces from celestial bodies, atmospheric drag, solar radiation pressure, and more.
• Astrogator: Designed for mission planning and analysis, this propagator provides detailed modeling capabilities for complex space missions. The HPOP propagator an propagator included in Astrogator along with other more specific propagators like Cislunar, and Lunar HPOP. 
• Long-term Orbit Predictor (LOP) provides an accurate prediction of the motion of a satellite’s orbit over many months or years. This is for predicting the decay of a satellite after years of orbital perturbations. 
• Note: These propagators are available only in STK Premium Space

Choosing the Right Propagator

The choice of propagator in STK depends on the specific needs of your simulation. Each propagator has its own set of parameters and data requirements. Here are some factors to consider:

• For low-fidelity, fast simulations, analytic propagators like TwoBody or J2 are suitable. STK Pro is required. 
• For high-fidelity needs, HPOP or Astrogator would be more appropriate. STK Premium Space is required.
• If your simulation involves external data or real-time tracking, external propagators or real-time propagators should be used. These are available in STK Pro. 
• If you are planning any maneuvers, Astrogator is Required, which is available in STK Premium Space.
• If you require any perturbations like drag, solar radiation pressure, or detailed gravity models, Astrogator or HPOP is required, which is available in STK Premium Space.
• If you require interplanetary or outside of Earth’s Orbit, AND you don’t have the external ephemeris for these missions, Astrogator is require for the mission planning, which is available in STK Premium Space.  

Conclusion

STK offers a diverse range of orbit propagators to cater to different simulation requirements, from simple analytic models to complex high-fidelity integrations. Understanding these options allows you to select the most appropriate propagator for your specific simulation needs, ensuring accurate and efficient modeling of satellite orbits.

If you need additional clarification on using these propagators, feel free to reach out for more detailed guidance.

If you have any further questions or need assistance, we are here to help! Our dedicated Tec-Support team is ready to provide prompt and personalized assistance tailored to your needs. Please don't hesitate to reach out by submitting a ticket by going to https://lsas-tec.freshdesk.com/support/tickets/new. We look forward to assisting you and ensuring a positive experience.  

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LSAS Tec-Support Team