Data Hacking

290 Part IV — Playtime
FIGURE 12-1: 100-meter inaccuracy
Those experienced at navigating by map and compass could already achieve this, so GPS was
seen as an “emergency” device handy in bad weather or a device for those who weren’t as competent
at navigating using traditional methods.
However, in May 2000, things changed a lot in the GPS world when former President Clinton
ordered the removal of the degrading Selective Availability error, transforming overnight the
accuracy from a respectable 100 meters to an amazing 10 meters! Figure 12-2 compares this
new accuracy level with the old.
This wasn’t the first time that Selective Availability had been switched off. It is ironic that an error
designed to help deter others from using GPS for military purposes (the fear was that someone
would build a nuclear missile and strap a cheap GPS to the front and thus gain immense precision)
was actually first turned off during a time of war. This was during the Gulf War (1991), and
the error was removed because of a shortage of military GPS units, forcing soldiers to use civilian
units to navigate the deserts.
Even though these days the civilian GPS signal doesn’t contain the deliberate error, the military
(along with certain civilian applications) still uses a different encrypted signal (called P-code) that
is in itself more accurate than the civilian signal (called CA-code) and less prone to jamming and
spoofing (being interfered with by a fake signal).