Spec_Sgp4.md

Specification for SGP4 Orbit Propagation

1. Overview

1. functions

• The Sgp4OrbitPropagation class calculates the position and velocity of satellites by using SGP4 method with TLE.

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.
• src/dynamics/orbit/sgp4_orbit_propagation.hpp, .cpp
  • Library of SGP4
  • /src/library/external/sgp4

3. How to use

• Select propagate_mode = SGP4 in the spacecraft's ini file.
• Set the TLE you need like the following example

  tle1=1 25544U 98067A   20076.51604214  .00016717  00000-0  10270-3 0  9005
  tle2=2 25544  51.6412  86.9962 0006063  30.9353 329.2153 15.49228202 17647
  

2. Explanation of Algorithm

1. Propagate function

• The difference between the current Julian day and the original period in TLE in units of [minutes] (elapse_time_min) is calculated, and it is used in the argument of the sgp4 function of the SGP4 calculation execution function. At the same time, the geodetic system definition (whichconst) and the trajectory information structure (satrec) are also required, which are defined at the call of the constructor. The position [m] and velocity [m/s] of the spacecraft are assigned to the member variables sat_position_i_ and sat_velocity_i_ as the output of the sgp4 function. Note that the values, in this case are the values from the ECI coordinate system.

3. Results of verifications

1. Verification of SGP4

1. Overview

• Verify whether the propagation of SGP4 is correctly installed or not.
• By comparing the propagation result of SGP4 in STK simulator and S2E

2. Conditions for the verification

• Conduct verification using the two different initial TLE cases with different time spans.

1. Hodoyoshi orbit : (span:10000 second)

   • TLE

   40299U 14070B   20001.00000000 -.00003285  00000-0 -13738-3 0 00007
   40299 097.3451 081.6192 0014521 069.5674 178.3972 15.23569636286180
   

2. ISS Release orbit (span:10 days)

   • TLE

   99999U   20001.00000000  .00000007  00000-0  93906-7 0 00002
   99999 053.4260 297.1689 0008542 245.4975 274.8981 15.55688139000015
   

3. Results

1. Hodoyoshi orbit : (span:10000 second)

   • Left: STK, Right: S2E
     
     The outputs of the satellite position are almost the same between the two simulators.

2. ISS Release orbit : (span:10000 second)

   • Left: STK, Right: S2E
     
     The outputs of the satellite position are almost the same between the two simulators.

4. References

NA