Author:
Dmitri
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Date Posted: 09:01:27 10/30/14 Thu
My research indicates that Selective Availability was actually turned off in May of 2000 (the end of Slick Willy Clinton's administration). My own experimentation with it (for wilderness canoeing and hiking) before and after that time period, showed it had settled down a lot during accuracy checks with my own little hand-held unit (not for automobile navigation). I could paddle my canoe away from a beach and then follow a track back and take out on my footprints made when leaving the beach. Before that, I could be as much as a quarter mile off or more (several hundred yards away), though it was often much less than that extreme.
Dmitri
>>I remember back in 2002 I was in New Jersey for the
>>first time. It was after dark and I was going from the
>>Newark airport to Berkley Heights. I had printed maps
>>before I left so I was covered. I tried the Hertz
>>Neverlost system just for grins. It showed that I was
>>driving in the river not on the highway. Although I
>>did use the system succesfully over a number of years
>>from that point on I called it Everlost!
>>
>>Kirby
>
>
>You were driving in the river due to a feature called
>"selective availability" or just "SA".
>
>"Selective availability" was an intentional
>degradation of public GPS signal accuracy supposedly
>included in the design for national security reasons.
>
>When switched on, SA allowed the system controllers to
>offset the displayed location by a selectable amount
>in a selected direction, usually either east or west.
>
>My first personal experience in using GPS was in a
>Hertz rental in 1999 or 2000, in the San Francisco Bay
>area. My passenger said that we were "driving in the
>weeds" that paralleled the highways.
>
>SA was switched off in September 2007. A friend of
>mine told me that he was driving on highway 15 between
>Los Angeles and Las Vegas at the moment when it was
>actually switched off and the icon representing his
>car on his Magellan receiver screen "swept" from 500
>feet east in the weeds to about 1000 feet west into
>the desert, then "jumped" to the exact lane he was in
>and stayed there.
>
>More info:
>
>href="http://www.cs.cmu.edu/~sensing-sensors/readings/G
>PS_History-MR614.appb.pdf">http://www.cs.cmu.edu/~sensi
>ng-sensors/readings/GPS_History-MR614.appb.pdf
>
>A bit of history:
>My first exposure to what is now called GPS was at an
>amateur radio club meeting in the mid 1980s, where the
>speaker was one of the engineers that developed the
>system (and who was also a licensed ham). The
>presenter mentioned that there was a poster on the
>wall of their R&D lab showing the Q-developed locator
>screen in center console of James Bond's Aston Martin
>(as shown in the 1964 movie Goldfinger).
>
>A number of photos were shown in the ham radio club
>presentation. One was of the first usable vehicular
>receiver - all it displayed was latitude and longitude
>on a set of numeric displays.
>
>Another of the more interesting photos was of a mobile
>GPS mapping display receiver demonstration vehicle - a
>large van with three racks of equipment containing a
>CRT-based graphics display terminal, it's supporting
>hardware, a couple of rack-mount disk drives, a
>minicomputer system to do the GPS location
>calculations, and an attached external floating point
>processor to help it do that. The third rack held
>nothing but satellite radio receivers.
>
>There was a satellite receiving antenna mounted on an
>attached trailer - and under the antenna was mounted a
>15kw 240v generator to power the equipment racks, the
>disk drives plus the three RV air conditioners
>(installed in the roof of the van).
>
>The fragility of the disk drives of the day precluded
>the unit from being used while in motion - any
>vertical movement, including rough asphalt or even bad
>tire vibration would crash the disk heads into the
>spinning platters.
>
>From the time the van came to a stop to the time the
>operator could generate a location was about 15-20
>minutes - assuming the generator actually started, and
>there were no computer or satellite receiver glitches.
>
>Mike
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