NGSO budget link

[RomainDell]

Dear Igor,

First of all thank you very much for your development that helped me a lot to quickly calculate link budget.
I found wrong pointing values when using a NGSO satellite Cartesian coordinates and so distance from my antenna were affected also.
So I took the liberty of modifying your function look_angles to make it works properly with this kind of satellite position.

I have already seen the previous Issue addressing this problem but apparently not solving at the end.
Then I performed a check here to see further.
3975f78

Actually the problem is conserving 0 value in the Z plan addresses automatically a satellite on equatorial plan.
The formula is okay for the antenna position and should be the same for the satellite.
(here is a reference to coordinates conversion I use : https://gssc.esa.int/navipedia/index.php/Ellipsoidal_and_Cartesian_Coordinates_Conversion)

So here is what I propose to you as a working fine modification :
(I am sorry I am not used yet to Git usage so I put here my modification if you don't mind.)

First I add radians conversion for sat-lat
sat_lat = radians(sat_lat)
Then just change this :
z_s = 0
to this :
z_s = (N * (1 - e_sq) + sat_alt) * sin(sat_lat)

Then the following steps for pointing calculation remain okay.

Now simulation looks good for all satellite Cartesian coordinates (GSO or NGSO).

Thank you again for this tool !

Kind regards from French Guiana neighbor ;)

[igorauad]

Hi @RomainDell ,

Greetings from your Brazilian neighbor :) I appreciate the correction. The suggested changes make sense. Thanks for that.

I'll merge the changes into the master branch and release a new version on PyPI soon.

By the way, I've been trying to test or document some of the calculations referenced to calculations available in open/public documents like textbooks. If you have a reference like that for NGSO look angles, it would be nice to add it to the docs. In the meantime, c22d9a1 adds a test comparing the Az and El angles obtained with the Skyfield package. They are close to the ones obtained with the updated implementation, but the slant range is still significantly different.

Regards