TT-TDB with SPICE from ephemerides kernels as 'de440t.bsp'

Question

I am a new SPICE user. I would like to ask a question: does anyone know if it is possible with some SPICE function to get the TT-TDB value directly from kernels as 'de440t.bsp', body id '1000000001'?

Answer 1

The data to compute the TDT-TT value is stored in the leap seconds file. The latest version as of now is naif00012.tls.

The description should help in that computation but I must admit never having fully understood it and rely on ESA's definition instead: https://gssc.esa.int/navipedia/index.php/Transformations_between_Time_Systems#TDT_-_TDB.2C_TCB .

Answer 2

I know that the TT-TDB difference can be obtained with the SPICELIB DELTET function: https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/deltet.html

But the goal would be to read it directly from the DE440/LE440 ephemeris, not this way as from 'naif00012.tls: https://gssc.esa.int/navipedia/index.php/Transformations_between_Time_Systems#TDT_-_TDB.2C_TCB

I also know that the TT-TDB difference can be obtained from the JPL Ephemeris Files by looking up the Chebyshev coefficients of triplet 15 and interpolating from ASCII files such as the ftp://ssd.jpl.nasa.gov/pub/eph/planets/ascii/de440t/ascp01950.440t

EPHEMERIS FILES, INTERNAL FORMAT OF THE EPHEMERIS FILES: The 15 triplets give information about the location, order and time-coverage of the chebychev polynomials corresponding to the following 15 items:

   01.- Mercury
   02.- Venus
   03.- Earth-Moon barycenter
   ...
   12.- Earth Nutations in longitude and obliquity (IAU 1980 model)
   13.- Lunar mantle libration
   14.- Lunar mantle angular velocity
   15.- TT-TDB (at geocenter)

What I was looking for is to be able to do the equivalent of the latter directly from the kernel: ftp://ssd.jpl.nasa.gov/pub/eph/planets/bsp/de440t.bsp

with some SPICE routine, using fortran code, for example. some of: https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/index.html

I've tried with SPKPOS, TARGET = '1000000001', but it didn't work.

https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/FORTRAN/spicelib/spkpos.html

What I was looking for was how to obtain the interpolated value of the naf_id = '1000000001' from kernels like 'de440t.bsp', which would be the equivalence to obtain and interpolate from the 15 triplet of the ASCII "ascp01950.440t".