My 1st GPS was a Garmin, available at any West Marine. I just remembered, mine was a 2nd generation. It was a model 75...the 1st generation was a model 45.
http://www8.garmin.com/manuals/GPS75_OwnersManual.pdf
It was 1st available in 1992 according to the Owner's Manual above.
It cost ~$400 at that time. It had Lat, Long, distance to way point, man-overbaord capability (reverse course to way point), speed over ground, heading and a few other things. Of course those were mostely just calculator functions. It would track up to 8 satellites at a time.
.........accuracy was more like +/- 90 meters. It was also intigratable with my Radar unit (for display purposes only) and auto-pilot.
Just a few years later far better GPS units were available for ~$100.
SA blocking was only open to the military at that time and to my knowledge was not available at any cost.
The accuracy that you are talking about was available with the old "Sat-Nav" units. My sat-nav cost $1200 in 1987 and that thing sucked up power like crazy (5A if I remember correctly). It would only track 3 satellites at a time and there were areas of the world that I might only get one accurate fix a day. However, when you are only making 100-175 miles a day, 1 fix was OK.
BTW.......a good sextant fix from the deck of a boat was +/- 3 miles. Even 1/4 of a mile would have been amazing in those days. The advent of the early GPS built over confidence in the crew. People got lazy and boats ran aground from not paying attention, mainly.
This isn't a very flattering picture and I was a little off on the date. This pic of our early Sat-Nav was taken May 1986. The Latitude reading was 00.000.00S, as we crossed the Equator, on our way to Samoa from Palmyra. We spent 3 months on Palmyra all by ourselves (Palmyra was a deserted Island 900 miles south of Hawaii. Today, it is inhabited by the Conservatory of the Pacific)..
These days, I wouldn't dare try to board an airplane looking like that.........