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Introduction

PH20 is the ephemeris of Phoebe. This ephemeris of Phoebe is based on a numerical model fitted to the observations from 1898 to 2019.
The dynamical model is using planetary positions from JPL DE438 and Saturnian satellites positions from NOE-6-2018 (Lainey, 2018).
This ephemeris is available for the period from 1874.07.01 to 2050.02.08 and through Chebychev polynomia (ph20.dat) readable with the Fortran subroutines contained in r-ph.f90. This is an update version of PH15 with additional observations from Gomes-Júnior et al. (pers.comm.), and 6 positions from occultations on 2017-2019 (Gomes-Júnior et al., 2020).

Files

• README.txt : Explanation file
• ph20.dat : Chebychev polynomia
• r-ph.f90 : Fortran subroutines used to compute ephemeris
• test.f90 : Fortran program to test the computation

The ephemeris can be obtained directly with SPICE Librairy and the bsp file:

• ph20.bsp (42Mo)

Fortran subroutines

• header: Read the header of ph20.dat to load information such as degree of polynomia or period of validity
  • Out
    • nc : degree of polynomia
    • ni : number of intervals
    • a : first date of validity
    • b : last date of validity
• load: Load the coefficients of Chebychev polynomia
  • In
    • nc : degree of polynomia (given by header)
    • ni : number of intervals (given by header)
  • Out
    • cf : coefficients
    • dat : limit period of intervals
• pos_ph12: Compute the position and velocity for a specific date (in TDB)
  • In
    • a : first date of validity (given by header)
    • b : last date of validity (given by header)
    • nc : degree of polynomia (given by header)
    • ni : number of intervals (given by header)
    • cf : coefficients (given by load)
    • dat : period of intervals (given by load)
    • t : time in Julian Day(TDB) (given by user)
  • Out
    • x : 6-dimension vector position-velocity

Units

Positions and velocities are delivered in au and au/day.

Example

Computation of test program should provide:

DJ =  2450000.500
x  =     -0.0565316104734     0.0683540174366     0.0384508370152
v  =      0.0006274576389     0.0005863229487     0.0001848029282
  
DJ =  2450010.500
x  =     -0.0500077290975     0.0738915529928     0.0401179713132
v  =      0.0006760354170     0.0005207173121     0.0001485799683
  
DJ =  2450020.500
x  =     -0.0430361433293     0.0787599891531     0.0414219409170
v  =      0.0007170404792     0.0004525963860     0.0001122071413
  
DJ =  2450030.500
x  =     -0.0356913416105     0.0829368323360     0.0423623003300
v  =      0.0007507182745     0.0003824785448     0.0000758891585
  
DJ =  2450040.500
x  =     -0.0280454438945     0.0864043934686     0.0429404882505
v  =      0.0007772954037     0.0003108087142     0.0000397995649
  
DJ =  2450050.500
x  =     -0.0201684321859     0.0891491228030     0.0431595536339
v  =      0.0007969718296     0.0002379721470     0.0000040880057
  
DJ =  2450060.500
x  =     -0.0121284456428     0.0911610794445     0.0430239512615
v  =      0.0008099155578     0.0001643073731    -0.0000311130287
  
DJ =  2450070.500
x  =     -0.0039921164298     0.0924335247788     0.0425393992645
v  =      0.0008162594981     0.0000901181616    -0.0000656824203
  
DJ =  2450080.500
x  =      0.0041750697347     0.0929626287928     0.0417127914132
v  =      0.0008160997886     0.0000156842819    -0.0000995053260
  
DJ =  2450090.500
x  =      0.0123083990441     0.0927472817637     0.0405521585330
v  =      0.0008094946852    -0.0000587282769    -0.0001324683699
  
DJ =  2450100.500
x  =      0.0203435508692     0.0917890086296     0.0390666778986
v  =      0.0007964646385    -0.0001328569834    -0.0001644549520
  

Previous versions

• PH12 : original version of 2012
• PH15 : version of 2015
• PH20 : current version (2020)

Reference

Desmars J, Li S.N., Tajeddine R., Peng Q.Y., Tang Z.H.
Phoebe's Orbit from ground-based and space-based observations
A&A, 553, 36D, 2013, 10p.
ads arXiv

Author

J. Desmars, IMCCE/IPSA/OBSPM, josselin.desmars@obspm.fr.

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