Mode S Transponder GPS Spoofing Detection via Time-of-Day Validation
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Solution Overview
Problem
Current technologies fail to effectively detect and report GPS signal denial, jamming, or spoofing, which poses a significant threat to both military and civilian aircraft navigation, as they do not provide a long-term solution for continuous countermeasures against hostile attacks on GPS systems.
Innovation Solution
Modifying Mode S transponders to send a platform Time of Day (TOD) message within a Mode S message, allowing a SSR/MSSR/IFF receiving system to detect GPS spoofing by validating TOD and position information from aircraft, using an updated ADS-B squitter message and storing TOD in a spare Binary Data Storage register, enabling passive and active systems to access and validate TOD across the surveillance volume, and triggering alarms for pilots and end users.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If GPS satellites transmit signals for global navigation, then position and time data can be acquired worldwide, but the system becomes vulnerable to denial, jamming, and spoofing attacks
Solution Approach 1:
The patent introduces an intermediary validation system consisting of ground-based SSR/MSSR/IFF radar systems and airborne transponders that act as mediators between GPS satellites and end users. These intermediaries independently verify GPS signal authenticity by comparing TOD messages from multiple satellites with radar-measured positions, thereby detecting spoofing attempts without requiring changes to the GPS satellite system itself.
Solution Approach 2:
The system implements feedback mechanisms where ground and airborne systems continuously monitor GPS signal consistency across multiple satellites, validate TOD messages against independent radar position measurements, and provide alerts when discrepancies indicating spoofing are detected. This closed-loop feedback enables real-time detection and response to GPS denial attacks.
2Device complexity
If GPS receivers rely solely on satellite signals, then the system remains simple and cost-effective, but it cannot detect spoofing or denial of service attacks
Solution Approach 1:
The patent divides the GPS verification function into separate segments: ground-based SSR/MSSR/IFF radar systems perform initial GPS signal validation by comparing TOD messages with radar-measured aircraft positions, while airborne transponders assist in the verification process. This segmentation allows anti-spoofing capabilities to be distributed across multiple independent systems rather than requiring every GPS receiver to be complex and expensive.
Solution Approach 2:
The system leverages existing multi-functional SSR/MSSR/IFF radar infrastructure originally designed for air traffic control and surveillance, enabling these systems to also perform GPS spoofing detection. This universal approach allows anti-spoofing validation to be provided by existing radar systems without requiring dedicated anti-spoofing hardware for each GPS user.
3Measurement precision
If GPS satellites transmit multiple signals including L1 and L2 carriers, then positioning accuracy is improved, but the system remains vulnerable to selective signal jamming and spoofing
Solution Approach 1:
The system performs preliminary validation of GPS signals before they are used for navigation decisions. Ground-based radar systems verify the authenticity of GPS TOD messages and position data from multiple satellites in advance, detecting potential spoofing or jamming attempts before they can compromise navigation accuracy. This preliminary action protects against jamming vulnerability while maintaining the benefits of multiple GPS signal carriers.
Data Source
AI summary
A system and method of identifying and responding to a GPS denial of service includes: configuring a mode S transponder for transmitting a GPS time-of-day message as a downlink format message using a BDS register, and configuring an aircraft surveillance system for receiving one or more GPS time-of-day messages transmitted as a downlink format message. The surveillance system compares the received time-of-day message(s) from the aircraft to a comparison time of day, and validates reception of authentic GPS signals by the aircraft when the received time-of-day message is within a threshold amount of the comparison time of day. The comparison time of day may be the GPS time of day of one of a plurality of aircraft in the surveillance volume or may be the GPS time of day determined by the aircraft surveillance system. An indicator on the transponder indicates counterfeit GPS signals, permitting mitigation od induced navigation error.