[Spice_discussion] kernel data

[Yukio Yamamoto]

Dear Ed,

I recommend dumping ephemeris data in ASCII.
You can find out the program ‘spkdiff' in the following URL:

http://naif.jpl.nasa.gov/naif/utilities.html

You can dump the Earth’s ephemeris like this:

$ spkdiff -b1 399 -s 3600 de432s.bsp


If you want to implement SPK reader in Python although it is challenging,
I’ll show you some hints to implement.

To understand the SPK format, you should know DAF format in advance.

http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/daf.html

DAF format provides a mechanism to store double precision array, and
SPK format is a DAF based format to keep ephemeris.

Also, SPK supports several types to express ephemeris. The detail is in the following URL:

http://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/req/spk.html

The target file: de432s.bsp is SPK Data Type 2, and it keeps the ephemeris data
in Chebyshev polynomial coefficients,  therefore, you have to calculate a position
and a velocity from these coefficients.

To support your understanding, the utility program ‘spy’ is also available.
The following is a command example to dump SPK file:

$ spy -cl -start lsk_cmd DUMP DATA SPK de432s.bsp

where lsk_cmd is a text file including only one line:
LOAD naif0011.tls;

and the file 'naif0011.tls' must be in the same directory.
Using the program ‘spy', you can understand 'DE-0432LE-0432’ is a Segment ID in DAF format.

Best regards,
Yukio


Yukio YAMAMOTO
yukio at planeta.sci.isas.jaxa.jp/yamamoto.yukio at jaxa.jp
Japan Aerospace Exploration Agency (JAXA)
Institute of Space and Astronautical Science (ISAS)
3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, JAPAN
TEL: +81-50-3362-2612 / FAX: +81-42-759-8768