- GPS satellites have been broadcasting encrypted messages (likely U.S. military OTAD rekeying) in the Subframe 4, Page 17 field since at least 2007, using 176 bits of the navigation message.
- The content is high-entropy and statistically indistinguishable from random, not text, as confirmed by frequency analysis and compression models.
- Three distinct operational regimes are identified: pre-OTAD (2007–2011) with ~2.5-day payload rotation, operational OTAD (2011–2022) with daily rotation, and post-2022 era slowing to ~2.3–4.2 days.
- Coordinated fleet-wide events in January and May 2011 mark the system's operational activation, while the all-¬ sentinel (0xAA) ended fleet-wide in July 2020.
- In December 2023, TEXT-prefixed messages appeared across the constellation, indicating a migration to a new format, possibly linked to OCX or GPS III.
- The article calls for GNSS engineers to monitor this field, as it has implications for signal authentication, operational transparency, and receiver firmware updates.