Old Akro, the backup for a GPS is another GPS. There is lots of interest in keeping the satellites functioning by lots of users, many not even anything to do with aviation.
Your example on the ground isn't a good one. There aren't multiple reflections from nearby buildings when you are flying... I hope.
Glider pilots have been using data loggers for the last 20 years for contest scoring. These use simple commercial grade GPS receivers and usually don't have a to[ of aircraft mounted antenna. I'm not aware of any widespread failures or unhappiness. Most gliders carry two units.
If you are worried, have a second GPS. If worried about the GPS satellite constellation be assured that you can easily get combined GPS/Glonass receivers and even now combined GPS/Glonass/Beidou/Galileo/QZSS/WAAS receivers and if you want precision navigation Differential GPS. Ask any broad acre farmer about that. The commercial chips that do this sell for under $20. You'll soon have about 100 satellites from several independent constellations to do your navigation with.
I only fly day VFR but I have at least 4 GPS units on board. A panel mounted one, two iPads running AvPlan, a phone running Avplan and there are a couple of neat apps that simulate VOR and NDB locations as well as a ILS. These all use GPS data on your phone.