Scientific American - USA (2019-12)

December 2019, ScientificAmerican.com 43

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

Jamming

GPS radio transmissions have very low power when they

reach Earth. A hacker near a receiver can drown out the

broadcast by blasting meaningless noise at the same

frequency, making it hard for the receiver to stay con­

nected to the signal or to lock onto it in the first place.

Spoofing

Each GPS satellite sends a unique code that identifies it. A hacker near a receiver can

pick up the incoming codes, then retransmit them, slowly increasing their power until

the receiver switches to the hacker as the originating source. The hacker can then send

new radio signals that misdirect the receiver or fool human operators into thinking

they are off course, which they might mistakenly try to correct.

SOURCE: NATIONAL MARITIME PNT OFFICE (

U.S. transmitter map

)

TWO WAYS TO HACK IN

Interfering with GPS timing can lead to electric­grid blackouts, stock market crashes and airliners that lose guidance during landing. Hackers can overpower

(jam) or mimic (spoof) the radio waves GPS satellites transmit, giving receivers false information.

BACKUP SYSTEM TO COUNTER ATTACKS

Many countries have a backup network that some receivers can switch to if satellite broadcasts are hacked or lost. Master stations and antennas on the ground

emit strong, low-frequency radio waves that are very difficult to jam or spoof. A receiver picks up the signals from several pairs of transmitters to determine its

location and time, though with less accuracy than with GPS. To be effective, a network should cover a country or region; one possible U.S. configuration is shown.

© 2019 Scientific American