Scientific American - USA (2019-12)

42 Scientific American, December 2019

Satellites

broadcast

data to

receivers

Ground stations

provide orbital

information and

time corrections

to satellites

Circle at

intersection

of two spheres

GPS

signal

Signal from

a jammer

Signal from a spoofer sends ship off course

or prompts crew to make an unneeded correction

Signal from

a spoofer

GPS

signal

Signal from ground-based backup system

GPS: Dependable but Vulnerable

The electric grid, stock markets, banks, airliners and cell-phone networks all

depend on satellite-based GPS for timing their intricate operations. It is not

difficult for hackers to break in and throw off the timing, with potentially harmful

consequences. Many countries have a ground-based network for backup during

an attack, although the U.S. does not.

Satellite

Constellation

The U.S. Air Force

maintains 31 Navstar

satellites that orbit Earth

twice a day and transmit

radio waves to GPS

receivers worldwide.

HOW GPS WORKS

GPS satellites send synchronized signals that specify their position and

time at any moment. A GPS device (person in red circle) receives signals

from at least four satellites and compares the differences in their arrival

times to pinpoint its position. Codes align the receiver’s clock with

atomic clocks on the satellites, giving the exact time.

Accurate Position and Time

The trilateration of four satellite

feeds gives a receiver’s location

and time. A signal from the

first satellite places a receiver

somewhere on a sphere. A sig­

nal from the second satellite

reduces the location to a circle

along the intersection of two

spheres (left). The third signal

defines two points on that

circle, and the fourth signal

determines one point and what

time it is there.

Illustration by Ben Gilliland

© 2019 Scientific American