GPS Subframe 4 Page 17 Carries Military Ciphertext for Two Decades (benthamsgaze.org)

• Analysis of 12.16 million GPS observations (2007-2026) reveals encrypted military key distribution via Subframe 4 Page 17.
• Payloads are statistically random but contain structural patterns like placeholders and repeated strings.
• System supports U.S. OTAD network for secure key delivery to military receivers.
• Fleet-wide changes in 2011, 2022, and 2023 indicate operational developments.

"Researchers at UCL analyzed 19 years of GPS L1 C/A data and found that a special field (Subframe 4, Page 17) has been broadcasting encrypted military key distribution messages (OTAD) in plain sight. The payloads are statistically indistinguishable from random noise, but identifiable patterns like placeholders and repeated strings reveal the underlying protocol. The system supports the U.S. Over-the-Air Distribution network for secure military GPS receivers. The analysis is published in Inside GNSS and open-source code is available."