Specification for Orbit

1. Overview

1. functions

The Orbit class calculates the position and velocity of satellites with the following propagation mode.
- KEPLER : Kepler orbit propagation without disturbances and thruster maneuver
- SGP4 : SGP4 propagation using TLE without thruster maneuver
- RK4 : RK4 propagation with disturbances and thruster maneuver
- ENCKE : Encke's orbit propagation with disturbances and thruster maneuver
- RELATIVE : Relative dynamics (for formation flying simulation)
The KEPLER mode is the simplest and general for all users.
The SGP4 mode is useful for LEO satellites users without orbit control.
When users need to analyze the orbit disturbance force effect or the orbit control algorithm, the users should choose RK4 or ENCKE mode.
For multiple satellite operation, RELATIVE mode is useful.`

2. files

src/dynamics/orbit/orbit.hpp, cpp
- Definition of Orbit base class
src/dynamics/orbit/initialize_orbit.hpp, .cpp
- Make an instance of orbit class.

3. How to use

Make an instance of the orbit class in Initialize function in dynamics.cpp
Select propagate_mode in the spacecraft's ini file(eg. sample_spacecraft.ini).
Select initialize_mode in the spacecraft's ini file(eg. sample_spacecraft.ini).
- This setting works only for RK4, KEPLER, and ENCKE mode.
Set the orbit information in the ini file
- You can see the details in the documents for each orbit propagation mode.
The definition of the coordinate system is decided at [CELESTIAL_INFORMATION] section in sample_simulation_base.ini.

2. Explanation of Algorithm

In the Orbit base class provides the following common functions for every propagator.

1. `UpdateByAttitude` function

The UpdateByAttitude function simply converts the velocity vector of the spacecraft (in ECI coordinate system) to body coordinate system notation using the argument: quaternion_i2b.

2. `SetAcceleration_i_m_s2`, `AddAcceleration_i_m_s2`, `AddForce_i_N`, `AddForce_b_N` function

These functions provides the interface to include orbital disturbance or control forces.

2. `TransformEcefToGeodetic` function

The TransformEcefToGeodetic function calculates latitude[rad], longitude[rad], and altitude[m]. Currently, we use the Geodetic class to calculate the information.

3. `TransformEciToEcef` function

The TransformEciToEcef function can convert the position and the velocity of the satellite from the ECI frame to the ECEF frame, which considers the earth's rotation.

3. Results of verifications

The verification results are described in the document of each propagation method.

4. References

NA