Sub-setting from a single DE file. about jpl_eph HOT 2 CLOSED

Hi Peter,

Interesting. I've always run with multiple files (and there is an ascp2050.421 available). Looking at the code, though, I think I can see why running with only one file.

"Normally", DE421 would be provided as two files, ascp1900.421 and ascp2050.421. In that case, the determine_year_range() function would (eventually) find both files and set *year_start = 1900, *year_end = 2050, and *year_step = 150. And similarly with any other multi-file case.

With just one file, *year_step is left at the default value of five years, and both *year_start and *year_end are left at 1900. So the program figures it has data covering 1900 to 1905 and thinks : well, I don't have anything that covers your desired time span.

I see two possible fixes :

(1) Make sure both files are available, even though only one is going to get used. Requires no changes to the code.

(2) Add code to say that if only one file is found, *year_step = 1000; That way, if only one file is found, the program says : I don't really know what time span each ASCII file covers, so let's set it to be really long. Worst that'll happen is that we'll read in the entire file and realize that none of it covers the desired time span.

I've implemented the latter and pushed it to GitHub. Shortly after doing so, I saw that I'd made it more complicated than it needed to be; all that was really required was

    if( 1 == n_found)       /* year step wasn't actually determined */
       *year_step = 10000;

I'll probably fix that, but it'll work Just Fine as it is.

BTW, a note of thanks for your work on ASCOM. I am now out of the desktop planetarium business (my Guide 9.1 software is getting increasingly obsolete). But when I was in it, the ability to ignore the bazillion different telescope control protocols saved me a lot of trouble. I had already added code to control various Meade, Celestron, and other telescopes... just enough to make me perfectly happy to delegate further telescopes to ASCOM.

from jpl_eph.

Hi Bill,

Many thanks, your fix works well here and will save anyone else heading down this path! I'm glad ASCOM was of use, you are exactly the target audience we are trying to help.

Best wishes, Peter

from jpl_eph.