Re: Hirzon angles??
With GPS, they knew from the start that four satellites in view would be required, three if the receiver had a good atomic clock. The system preceding Navstar-GPS was Transit; That one, with three satellites, was fine to provide daily position updates to your handy battleship or nuclear sub. The rest could be - and still can be - handled with inertial navigation.
I did get a navigation fix back when only four Galileo sats were usable. I also just managed to build a receiver and get a fix before Loran was crippled. There are a few ways to build a navigation system, but if it isn't really expensive, it's not politically hot, so forget about those solutions.