Sprocket shaped propagation

[gormanst]

Orbital neophyte here,

I am propagating from a PV pair time. Something about my propagation configuration produces a really strange path that looks like this, where the vector must emanate from the Earth’s center:

It’s a very simplistic program producing the ephemeris. The code puts the PV values in a file (output.txt) along with the index. The position vector just got up/down over and over, just like the picture shows.

How can that even happen?

Thank you for your time.

package com.leidos.smac.data.orbit;

import org.hipparchus.geometry.euclidean.threed.Vector3D;
import org.orekit.bodies.*;
import org.orekit.data.*;
import org.orekit.frames.*;
import org.orekit.orbits.*;
import org.orekit.propagation.*;
import org.orekit.propagation.analytical.KeplerianPropagator;
import org.orekit.propagation.analytical.tle.*;
import org.orekit.time.*;
import org.orekit.utils.*;

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Date;

public class Scratch 
{
    public static void main(String[] args)  {
        // Orekit setup data
        File orekitData = new File("src/main/resources/orekit_data");
        DataProvidersManager manager = DataContext.getDefault().getDataProvidersManager();
        manager.addProvider(new DirectoryCrawler(orekitData));
        try {

            AbsoluteDate epoch = new AbsoluteDate(2023, 9, 27, 18, 0, 0.000000, TimeScalesFactory.getUTC());

            double mu =  3.986004415e+14;
            Frame inertialFrame = FramesFactory.getICRF();

            PVCoordinates pvCoordinates = new PVCoordinates(
                    new Vector3D(5.3562909436536320e+06, 1.7374822680742762e+06, 2.9890671950189904e+06),
                    new Vector3D(-1.1435433282106951e+02, 3.9408793506464212e+02, 7.2068475671536287e+00)
            );

            CartesianOrbit initialOrbit = new CartesianOrbit(pvCoordinates, inertialFrame, epoch, mu);

            KeplerianPropagator propagator = new KeplerianPropagator(initialOrbit);

            try (BufferedWriter writer = new BufferedWriter(new FileWriter("output.txt"))) {
                for (int i = 0; i <= 31141; i++) {
                    AbsoluteDate targetDate = epoch.shiftedBy(i);
                    SpacecraftState state = propagator.propagate(targetDate);
                    PVCoordinates pv = state.getPVCoordinates();
                    writer.write(String.format("%.16e  %.16e  %.16e  %.16e  %.16e  %.16e  %.16e%n",
                            (double) i,
                            pv.getPosition().getX(),
                            pv.getPosition().getY(),
                            pv.getPosition().getZ(),
                            pv.getVelocity().getX(),
                            pv.getVelocity().getY(),
                            pv.getVelocity().getZ()));
                }
            } catch (IOException e) {
                System.out.println("Error writing to file: " + e.getMessage());
            }

            System.out.println("Done." );
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
    }
}

[Serrof]

Hi there,

Your orbital speed is way below the km/s mark. Did you forget converting to SI units? That’s the system used by Orekit.

Cheers,
Romain.

[gormanst]

Thank you Serrof,
I think I do have my units correct. I tried multiplying my PV x/y/z values by 1000 which was interesting because the most noticeable change was:

• my position.x value starts at 5.3562909436536320e+09, then constantly decrements until 1.7951806286525593e+09. So it gets rid of the up/down nature it had?
• the velocity vector has negative values for x/y/z almost constantly.
  
  

Things go negative just after the 3rd position is propagated? How did you determine the orbital speed, was it just looking at the velocity vector values?

sg

[luc]

Running your program, I get a proper trajectory (see attached file truncated to the first few thousand lines).
output.txt (431.2 KB).

[gormanst]

Thank you Mr. Maisonobem,
I appreciate you taking the time Sir. My problem must be my inability to interpret my own results. I have an STK (Analytical Graphics’ Satellite Tool Kit ) Ephemeris file that I can draw in my application. It goes out to the geosynchronous belt and back to Earth.

I copied the epoch and initial PV vectors from the STK ephemeris file to propagate it, thinking I would get something very similar to the original STK track. However, it doesn’t look very similar.

Is that bone-headed of me to think I should get similar results from just a PV pair and a time?

Thanks to all for your time.
sg

[luc]

I don’t really know what happens. From the sample output file attached in the previous message, I just used gnuplot to display the positions: using this command line:

gnuplot -e "set terminal pngcairo size 1000,1000 ; set output 'output.png' ; set view 40,20 ; splot 'output.txt' using 2:3:4 with lines"

It gave the following plot:

Just a wild guess: may be you are drawing alternatively position vector and velocity vector instead of the position only?