I would like to know if it is possible to program orbit and signals of GNSS satellites in GNSS Skydel simulators, including LEO orbits (to simulate LEO GNSS PNT), and if yes, which Skydel simulator allows this.
All constellation orbits can be manually programmed per constellation per SV and will be overridden if custom RINEX is uploaded. Only GPS signals can be manually programmed:
Supporting documentation can be found at http://manuals.spectracom.com/skydel/index.html#settings-gps-orbits
These settings are not limited to specific hardware platforms or license features. They come standard with the constellations that have been enabled.
After programming an orbit, will a user need to update the orbital parameters of the almanac and ephemeris to match the new orbit?
By default, when changing the information in the Orbits settings, it affects both the calculation of the satellite position and the navigation message. In other words, there is nothing special to do to have the ephemeris and almanac updated accordingly. With the introduction of data sets in version 21.11, it might not be the case. For example, starting with 21.11, you can have different data sets for ephemeris, almanac and the orbit calculation. Take a look at this post for more details.