Ed Haywood
En-Route
A reminder on how GPS works. A network of satellites broadcast time-stamped signals. The receiver uses elapsed time of the signal from transmission to receipt to calculate the distance from each satellite. Distance from one satellite results in a sphere of possible locations. Intersection of two spheres results in a circle. Three spheres results in 2 points. Four spheres (or 3 plus the surface of the earth in a pinch) results in one point. Thus the minimum satellites required for accurate position is four. All calculations are performed by the receiver.
The system is designed so that at least 7 satellites are visible from anywhere on earth. In most of the US, somewhere between 8 and 12 satellites are visible.
Every additional visible satellite over the minimum of four provides confirmation and error checking capability. If you are receiving 10 satellites and one or more of the computed spheres fails to coincide with the consensus of the rest, that input is easily discarded.
This illustrates why spoofing is so technically challenging. To successfully spoof a receiver, the spoofer would have to mimic the signal of at least four separate satellites to provide an alternate consensus to fool the receiver. The transmitters must be line of sight from the receiver. For an aircraft at 30K AGL, the maximum LOS is approximately 211 miles.
Can Iran set up transmitters to mess with flights in or close to their airspace? Sure. But spoofing US airspace would be so operationally challenging as to be nearly impossible except in the context of a superpower war.
The system is designed so that at least 7 satellites are visible from anywhere on earth. In most of the US, somewhere between 8 and 12 satellites are visible.
Every additional visible satellite over the minimum of four provides confirmation and error checking capability. If you are receiving 10 satellites and one or more of the computed spheres fails to coincide with the consensus of the rest, that input is easily discarded.
This illustrates why spoofing is so technically challenging. To successfully spoof a receiver, the spoofer would have to mimic the signal of at least four separate satellites to provide an alternate consensus to fool the receiver. The transmitters must be line of sight from the receiver. For an aircraft at 30K AGL, the maximum LOS is approximately 211 miles.
Can Iran set up transmitters to mess with flights in or close to their airspace? Sure. But spoofing US airspace would be so operationally challenging as to be nearly impossible except in the context of a superpower war.
