Found via Paulo Amoroso's blog https://journal.paoloamoroso.com/blogroll, but Hannes has mentioned it too.
See https://sattrackcam.blogspot.com/2019/10/the-structure-of-space-orbital-families.html and https://sattrackcam.blogspot.com/2020/09/the-structure-of-space-2-formation-of.html, but other articles are interesting too.

Datenportale der ESA, EUMETSAT, USGS und des DLR. Die hier aufgelisteten Datenportale beinhalten Satellitendaten und keine Prozessierungsumgebung.

Has I2C, UART interfaces and PPS output.

Antenna analysis software NEC2 in C with GUI.
NORAD sdp4/sgp4 satellite orbit modelling software in C and Pascal.

This is an all new two volume text, completely revised Jan 2011,
text covers how the GPS works from basics all the way through the
operation and implementation of the GPS100SC receiver, a custom
single channel receiver. The text is unique in that not only is
theory covered but the complete details of a functional design
are presented. Detailed schematics (down to part level) and all
design CAD files are included. In addition a chapter is devoted
to the Zarlink chip set, an open software source receiver and all
new material on the Turbo Rogue receiver designed by JPL. There
is no other text like this text. If you want to know how gps
works and how a receiver can be implemented this book is for you.

Wiki about satellite navigation, developed under an ESA contract.

Information about cardboard models of satellites (page in german, some of the links point to english sites).

SaVi - satellite constellation visualization. Simulate Iridium, Globalstar, Sirius, GPS, Teledesic and other systems, modify them, or design your own. Uses Tcl/Tk and Unix libraries; 3D viewing option requires Geomview running on X Window. BSD license.