Seeking DE406 file formats

I am looking for a clear explanation of the Unix binary file format for
the NASA JPL DE406 long-range ephemeris. Specifically, I need to know
the byte order (big-endian or little-endian), and for each field I need
the data type, length in bytes, and meaning.

The best documentation I have found so far is vague and refers to
"words", which vary from system to system. Looking at the C source
code for test programs doesn't help much because I don't know how much
space is used for an int, 2 bytes or 4 bytes. This varies with
compilers and switches used during compilation.

I tried reading two 8-byte double precision values from the beginning
ofone of the files; according to the documentation i have I should have
gotten the starting and ending Julian day number for the file, but
instead I got 5.03839446078370E+223 and 6.55996434388144E+232, so
something is clearly wrong.

Any help would be appreciated.

--- Brian

wrote:

I am looking for a clear explanation of the Unix binary file format for
the NASA JPL DE406 long-range ephemeris. Specifically, I need to know
the byte order (big-endian or little-endian), and for each field I need
the data type, length in bytes, and meaning.


The data types you'll find in the unix.406 file are character arrays
(of various lengths), integers (4 byte) and double precision reals
(8 bytes).

For example, beginning with byte A5D_hex (counting the first byte
as 0), we have in that file the byte sequence

41 23 15 A1 00 00 00 00 41 45 7D C8
40 00 00 00 40 50 00 00 00 00 00 00
00 00 00 9C 41 A1 D5 5D 5D 61 CA C1


which breaks down as follows:

A5D..A64: d. prec. 41 23 15 A1 00 00 00 00 = 625360.5 decimal, starting JD of ephemeris
A65..A6C: d. prec. 41 45 7D C8 40 00 00 00 = 2816912.5 decimal, ending JD of ephemeris
A6D..A74: d. prec 40 50 00 00 00 00 00 00 = 64.0 decimal, block length in days
A75..A78: integer 00 00 00 9C = 156 decimal, number of constants in header
A79..A80: d. prec. 41 A1 D5 5D 5D 61 CA C1 = 149597870.69 decimal, astronomical unit

....
I tried reading two 8-byte double precision values from the beginning
ofone of the files; according to the documentation i have I should have
gotten the starting and ending Julian day number for the file, but
instead I got 5.03839446078370E+223 and 6.55996434388144E+232, so
something is clearly wrong.

I'm not sure about partial files, but the full unix.406 is divided
into records of 5824 bytes each, with the ephemeris data starting
in the third record:

2D80..2D87: d.prec. 41 23 15 A1 00 00 00 00 = 625360.5 decimal, starting JD of this data block
2D88..2D8F: d.prec. 41 23 16 21 00 00 00 00 = 625424.5 decimal, ending JD of this data block
2D90..2D97: d.prec. C1 89 DF 47 C6 72 0A 81 = -54257912.806 decimal, first Chebyshev coefficient


For the type and meaning of each number, please refer to the TESTEPH.F
source code provided with the ephemeris data. It explicitly demonstrates
how to read everything.


Bye,
Thomas

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Thomas Schmidt e-mail: