NAV file sourcing

[plutonheaven]

I propose to download exclusively BRDC00IGS_R_<yyyy><doy>0000_01D_MN.rnx files, because they contain:

• all constellations (including GLONASS)
• ionospheric corrections

However, this means that we can only download files after 2016-01-01. Earlier files are available on IGS servers, but with a different format and we would have to check the availability of iono corr and GLONASS.

[plutonheaven]

@jtec, is the availability of NAV files earlier than 2016 required for you?

[jtec]

@plutonheaven I have seen that FTP server going down for a while in the past, is the simplification worth the potential loss in ephemeris availability?

[plutonheaven]

I think we can add more sources, both http and ftp.
The main catch is to focus on BRDC00IGS_R_<yyyy><doy>0000_01D_MN.rnx so that we would be sure to have all constellations.
I'll complete this with more souces of RNX NAV file.

[plutonheaven]

I added back the http download from https://igs.bkg.bund.de/root_ftp/IGS/BRDC/, but the complete NAV file is available only after 2022.

We have 2 FTP and 1 HTTP sources. Let's consider that we are covered at least for a while.