At the risk of taking us further off topic….. SA was turned off in May of 2000 by executive order. https://www.gps.gov/systems/gps/modernization/sa/ <https://www.gps.gov/systems/gps/modernization/sa/>
The position quality transition brought on by the shutdown of SA was a milestone event for the marine research community (among many others.) This event was captured at the time by many and is characterized by the amazing graph on the page above - fond memories. Some (many of our tiny community) stayed up and watched the data show the transition in real-time - after stumbling around the world’s oceans for a decade it was AWESOME. I’ve been working with GPS/GNSS receivers (mostly on research ships at sea, but some geodesy) since the early 1980s when our “portable” (two full height relay racks) Y-set had two HP 2100s and a 9-track tape drive see: https://www.ion.org/museum/item_view.cfm?cid=7&scid=9&iid=43 <https://www.ion.org/museum/item_view.cfm?cid=7&scid=9&iid=43> (the picture only has the receiver hardware, it took serious comping in that era to derive real-time solutions and log the data. Now our cellphones and watches do it.) Moving that system from ship to ship set the bar for my Compaq “luggable” later on ;-) A substantial part of the early orbit-correction data collection and analysis that led to differential GPS was led by the civilian surveying and geodesy community trying to figure out how to get high accuracy post-processed solutions during SA. (Not unlike the amateur radio digital experiments that led to packet radio… I still do no not take GPS (and GNSS) for granted. -Dale KB1ZKD --- "Getting out there and doing it; that’s what really counts. At the end of a career, at the end of anything, it’s not about the awards. It’s about the friends and family.” -Daryl Miller, McKinley climbing ranger > On Jan 24, 2018, at 17:41 , Fred Jensen <[email protected]> wrote: > > Yep. The "civilian channel" on GPS was a gift to everyone from the US > Department of Defense, I suppose they could turn it off if they so wish, but > it is so ingrained in today's culture that they won't. Years ago, the > channel was purposely perturbed to limit the horizontal accuracy to something > between +/-100 to 200 meters. It was called "Selective Availability." > > The US Coast Guard [another branch of the US military] promptly installed GPS > receivers in very carefully surveyed key locations, and published the error > between the known position and the GPS reported position. They called it > Differential GPS. If you had a DGPS receiver, and were close enough to the > DGPS receiver, it would use the broadcast error to correct the received > position. In the US at least, they're still broadcasting from various sites > in the 280 – 460 KHz range which you can receive if your K3 has the new > synthesizer [and you have the BPSK decoder and a cheat-sheet for the format]. > Over time, commercial interests also began doing this too. > > Quite awhile back, someone in the guvmint must have realized that SA wasn't > really working all that well at limiting position precision on the civilian > channel and gave up. The DGPS transmissions continue, or at least did in > 2015. So much these days depends on GPS, it's hard to see the "100 ms > channel" going away. > > 73, > > Fred ["Skip"] K6DGW > Sparks NV DM09dn > Washoe County > On 1/24/2018 11:18 AM, Ken Chandler wrote: >> Course, Governments/Military have overall control over civilian use of the >> Birds! or certainly use to have, whether that’s still the case I’m unsure! >> >> >> >> Ken.. G0ORH >> > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Message delivered to [email protected] > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
