GPS Units
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Joined: Feb 1999
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The error was called Selective Availability or SA. During the Gulf war, they turned off SA so they could use "off the shelf" gps units. I guess the military version GPS were in short supply. SA was turned back on soon after the war was over. It was great while it lasted.
SA was put into the system so unfriendlies wouldn't be able to use civilian units to get accurate positions. Only military grade units were able to access non-SA signals.
SA became a moot security tool when Differential GPS came on line. With DGPS, you had land based stations sending out a correction signal to DGPS capable units that negated SA.
We've had gps on our boats when the first units became available. That's when a 5 channel Micrologic cost almost $4,000 and got plotters when they cost around $2,000. Then autopilots came online that could interface with the nav system through the NMEA 0183 interface. Those were running around $3,000+ not including installation. Drive units could cost another $1,500+ depending on the steering system on the boat. Slap on a $4,000 radar unit and you were good to go! Oh, I forgot to mention the $300 compass with the NMEA data output that was used by the whole system. I guess you already know that gps only tells you the direction you're moving in, not the direction you're pointed and if you're stopped, it don't tell you anything but your position.
Prior to GPS, we had Loran. Loran, at the time was more accurate if you had the Loran stations near you. It wasn't so accurate in my area and the signal was blocked in a lot of the areas we boated in. In some areas of the country, Loran could get you within a footstep of a position.
At the time of the introduction of maritime gps and associated navigational equipment, it was a bit ironic to see boats running around with navigation systems worth almost as much as the boat!
The advent of gps was kind of neat because if you coupled the data you got from that with the data you got from an accurate water speed indicator, you could figure out how fast the current was running and in which general direction. It also came in handy when you were slow trolling because. Until then, you were never sure if you were actually making any headway if you were going against the current. Sometimes when you're 100 miles from shore, there aren't any landmarks to use to guage your progress or lack of.
Damned, what was the question again? Oh yeah. Is SA off? I'm 99% sure that it's still off. I don't think it will ever go on again.
SA was put into the system so unfriendlies wouldn't be able to use civilian units to get accurate positions. Only military grade units were able to access non-SA signals.
SA became a moot security tool when Differential GPS came on line. With DGPS, you had land based stations sending out a correction signal to DGPS capable units that negated SA.
We've had gps on our boats when the first units became available. That's when a 5 channel Micrologic cost almost $4,000 and got plotters when they cost around $2,000. Then autopilots came online that could interface with the nav system through the NMEA 0183 interface. Those were running around $3,000+ not including installation. Drive units could cost another $1,500+ depending on the steering system on the boat. Slap on a $4,000 radar unit and you were good to go! Oh, I forgot to mention the $300 compass with the NMEA data output that was used by the whole system. I guess you already know that gps only tells you the direction you're moving in, not the direction you're pointed and if you're stopped, it don't tell you anything but your position.
Prior to GPS, we had Loran. Loran, at the time was more accurate if you had the Loran stations near you. It wasn't so accurate in my area and the signal was blocked in a lot of the areas we boated in. In some areas of the country, Loran could get you within a footstep of a position.
At the time of the introduction of maritime gps and associated navigational equipment, it was a bit ironic to see boats running around with navigation systems worth almost as much as the boat!
The advent of gps was kind of neat because if you coupled the data you got from that with the data you got from an accurate water speed indicator, you could figure out how fast the current was running and in which general direction. It also came in handy when you were slow trolling because. Until then, you were never sure if you were actually making any headway if you were going against the current. Sometimes when you're 100 miles from shore, there aren't any landmarks to use to guage your progress or lack of.
Damned, what was the question again? Oh yeah. Is SA off? I'm 99% sure that it's still off. I don't think it will ever go on again.
Last edited by Dennis; Oct 21, 2001 at 06:57 PM.