GPS Spoofing

A malicious actor transmits a counterfeit GPS signal that is slightly stronger than the authentic satellite signal. The target GPS-disciplined oscillator (GPSDO) locks onto this false signal, calculating an incorrect time offset. This introduces a progressive timing error into the PMU's sampling clock, directly corrupting the phase angle measurement of the synchrophasor. The attack exploits the unencrypted, unauthenticated nature of civilian GPS signals.

The primary consequence is a false phase angle shift in the reported measurements. A timing error of just 1 microsecond translates to a phase angle error of 0.018 degrees for a 50 Hz system. A coordinated attack can simulate a non-existent power swing or mask a real instability event. This corrupts the input to critical WAMPAC applications:

• Linear State Estimation (LSE): Produces an incorrect system state.
• Oscillation Detection: Triggers false alarms or misses growing oscillations.
• Angle Difference Monitoring: Shows a fake stress pattern on a transmission corridor.

Unlike GPS jamming, which is a denial-of-service attack causing a complete loss of signal, spoofing is a deception attack. The receiver continues to operate but with a manipulated time reference. This makes spoofing far more dangerous for grid operations, as the data appears valid and no alarm is immediately triggered. A sophisticated attacker can execute a 'carry-off' attack, gradually diverging the time solution from the true GPS time to avoid detection by basic time-quality checks.

Defense-in-depth is required to counter spoofing. Key strategies include:

• Multi-Constellation Receivers: Using Galileo, GLONASS, and BeiDou alongside GPS to cross-validate the time solution.
• Precision Time Protocol (PTP) Backup: Distributing a trusted time reference over a terrestrial fiber network as a secondary source.
• IMU-Aided Tracking: Using an inertial measurement unit to detect the physically impossible jumps in position or time implied by a spoofed signal.
• Absolute Power Monitoring: Detecting the anomalous increase in received signal power typical of a spoofing attack.

A targeted spoofing attack can inject a synthetic forced oscillation into the grid's monitoring system. By subtly varying the timing error over time, an attacker can make a perfectly stable bus appear to oscillate at a specific frequency. This false oscillation can then be detected by a System Integrity Protection Scheme (SIPS), potentially triggering an unnecessary and destabilizing corrective action, such as tripping a major transmission line or shedding load.

GPS spoofing directly degrades the Total Vector Error (TVE) of a PMU. The IEEE C37.118 standard mandates a maximum TVE of 1% for compliance. A timing error introduced by spoofing creates a pure phase angle error, which is a direct component of the TVE calculation. An undetected spoofing attack renders a compliant PMU non-compliant, violating the data quality requirements for mission-critical wide-area monitoring and protection schemes.