Jamming of Drones

• Jamming injects high-power noise into the summing junction of the receiver
• $R_x$ is the receiver of the drone
• $T_x$ is the transmitter of the user
• In a control system, jamming injects high-power noise into the summing junction of the receiver

Signal-to-Noise Ratio

• The drone receiver needs a certain level of SNR, such as 10 dB, to demodulate the pilot's command
• The jammer blasts that same frequency with enough power that the SNR drops below the threshold, such as $\text{SNR}<0$
• The "Pilot Command" signal becomes unintelligible noise
• The receiver output to the Flight Controller goes to "Null" or "Hold"

Drone's Reaction

Hover and Wait

IF (Link_Lost) THEN (Hold Position for X seconds)

Return to Home, or RTH

IF (Link_Lost > 3 seconds) THEN (Climb to Safe Altitude -> Fly to GPS_Home_Coordinates)

• If we only jam the control link, the drone still has GPS to fly back home
• We need to jam both the control link and GPS, more specifically GNSS L1/L2, which forces the drone to fall back to mode C

Land Now, or Drift

IF (Link_Lost AND GPS_Lost) THEN (Descend Slowly) or (Disarm Motors)

• Commercial Drones, such as DJI: Slowly descend to the ground, or auto-land, to save the airframe
• FPV or Homemade Drones: Often configured to "Drop," or cut the motors immediately, to prevent flyaways. The drone falls like a brick
• Drift: Without GPS, the drone enters "ATTI Mode," or Attitude Mode. It keeps itself level with the IMU, or accelerometer and gyro, but drifts with the wind because it cannot hold a position

Frequency Hopping Spread Spectrum

FHSS is a dynamic system where the carrier frequency changes rapidly, such as 100 times per second, based on a pseudorandom sequence known only to the Transmitter, or Drone, and Receiver, or Pilot.

When jamming, the drone changes frequency. For example, at $t=0$, it transmits on 2.401 GHz, but it is jammed. At $t=1$, it transmits on 2.450 GHz.

Vanilla Jammer

• Blasts noise throughout the entire 2.4 GHz band
• Stops the drone but also kills Wi-Fi in the building

Intelligent Jammer

• Acts like a state observer
• Listens to the drone's signal and identifies specific protocols
• Instead of jamming all frequencies, identifies the next state, or frequency, or jams the specific Synchronization Packets, which are the chain of frequencies it can transmit on
• If you jam the synchronization packets, the drone and controller lose their "shared clock"
• They desynchronize