That was a very interesting article but I'm concerned it did not report an accurate picture of the situation. The author seems to confuse GPS and AIS.
The loss of GPS position was likely due to interference
with the satellite
signals, or jamming, which could be intentional or inadvertent. GPS signals are microwave frequency, while AIS uses two channels in the VHF marine
spectrum. To lose data from both devices at the same time suggests the presence of a very strong electrical
signal that overwhelms all radio reception
. The article didn't mention the failure of bridge-to-bridge or bridge-to-shore VHF communications
, though, so perhaps what was meant by "its AIS transponder had failed" was the AIS was not reporting GPS positions from any nearby ships. That would be expected in an area where GPS receivers
experienced a blackout.
This sentence concerns me as well: "it and its neighbor had also been spoofed—their true position and speed replaced by false coordinates broadcast from the ground." That isn't the way GPS works. Coordinates are not broadcast--either from satellites or from the ground. GPS receivers
calculate their position from the time it takes signals from four satellites to reach them, a trick they accomplish by trying various time corrections until one yields a single-point fix. Retransmitting satellite
signals with a slight delay and at sufficiently higher power to make the GPS receivers accept the delayed signals would cause all affected receivers to report erroneous, but different positions. That doesn't explain the circles, though.
The article's attempted explanation of the GPS system is incorrect. Three satellites would be sufficient to fix the receiver's position if the receiver's clock were perfectly synchronized with the clocks aboard the satellites. The fourth satellite signal allows the receiver to adjust its internal clock until it yields a good fix. By assuming the receiver is at mean sea level time correction and a fix can be obtained with just three satellites.
This doesn't make sense to me: "While GPS satellites broadcast several different signals intended for both military and civilian use, AIS relies on just one of them." AIS transmits a GPS position and additional data about a vessel over marine VHF frequencies. There's nothing special about a position reported by an AIS transponder. It depends on how good the GPS receiver is which it uses for position information.