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Deselecting Unavailability

The ten year anniversary of the death of Selective Availability

Glen Gibbons
Here's to the anniversary of the end of an idea whose time came and, thankfully, has gone.

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Only a decade ago, but a world away: 2000.

The last year of the old century that everyone thought was the first of the new.

When flying was still a delight, rather than a worrisome bother.

When the expected — a global Y2K bug–bitten IT meltdown — didn’t happen, and the much-anticipated but still-unexpected did: the United States turned off GPS selective availability.

On May 2, NASA Administrator Dan Goldin flew to Scotland, pre-empting the Department of Transportation officials scheduled at the GNSS 2000 conference opening session, to tell the world (but particularly Europe) that SA was going away.

Selective availability — or SA — was the practice of dithering the time-of-transmit component of the navigation message in satellite signals so as to degrade the accuracy of unaided GPS positioning: no worse than 100 meters 95 percent of the time. Which was a much less reliable and useful performance standard than GNSS users had come to desire.

I was at the meeting in Edinburgh and remember three things quite clearly: the frost on the steep cobbled streets of the city during my morning runs, the aged single-malt whiskey samples at the evening reception, and Goldin’s imperious and irascible behavior in delivering his message.

This was still in the era of U.S. GNSS hegemony. GLONASS was down and seemingly out. Galileo was a name, not a program. And the first launch of a Chinese satnav/communications system called Beidou wouldn’t happen until later that year.

Meanwhile, the United States appeared to view GNSS as a competitive, zero-sum game. The timing of Goldin’s appearance at the Scotland conference was hardly coincidental. Europe was on the brink of making a substantial financial investment in developing Galileo, and U.S. officials saw turning SA off as a way to demonstrate the altruistic stewardship of GPS as a global resource that needed no alternatives.

As we know, Europe ultimately went ahead with the Galileo program, and Russia has since rebuilt and modernized its GNSS system. But the consequences of the 2000 decision — probably immeasurable at that moment — are still rippling through the world today as the number of GPS receivers in use soars toward the billions.

“The result [of turning off SA] has been an immediate improvement in the position accuracy of the GPS Standard Positioning Service,” Goldin told his Edinburgh audience.

“How much improvement? Yesterday, the GPS receiver in your automobile would have been able to tell you what block you were on. Today, it can tell you whether you are in front of your house or not.”

The NASA administrator didn’t even mention and perhaps couldn’t conceive of what became the real drivers of GPS consumer sales: cell phones and portable navigation devices.

But obviously others could.

The day after Goldin’s announcement, an Oregonian named Dave Ulmer went out and buried what he called a “GPS stash” in rural Clackamas County, posted the coordinates on an Internet bulletin board, and challenged other GPS users to find it. Today, there are more than a million geocaches worldwide and three to four million zealous geocachers — including my sister, nephew, and brother-in-law — scouring the countryside and planting their own latter-day stashes.

More crucially, tens of millions of mobile phones now have the ability to automatically report the location of emergency 911 callers with pinpoint accuracy.

SA is, indeed, finally dead. The GPS III generation of satellites now being developed by Lockheed Martin won’t even have the capability any more.

So, here’s to the anniversary of the end of an idea whose time came and, thankfully, has gone.

Copyright © 2018 Gibbons Media & Research LLC, all rights reserved.

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