# The Empty Field That Wasn't: GPS, OTAD and Two Decades of Encrypted Broadcasts **URL:** \ **Category:** Developing\ **Tags:** gps, gnss, pnt, military, encryption\ **Created:** [June 21, 2026, 12:35pm UTC](https://forum.landoffset.com/t/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/409 "2026-06-21T12:35:39Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![system](https://forum.landoffset.com/uploads/default/original/1X/461c3fb699fff9afe9dbc43189d999b5fa0580cb.png) [@system](https://forum.landoffset.com/u/system)\ **Post date:** [June 21, 2026, 12:35pm UTC](https://forum.landoffset.com/t/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/409/1 "2026-06-21T12:35:40Z")
# The Empty Field That Wasn’t: GPS, OTAD and Two Decades of Encrypted Broadcasts Research published in the May/June 2026 issue of _Inside GNSS_ by Steven J. Murdoch, a Professor of Security Engineering at University College London, details a comprehensive analysis of the GPS L1 C/A Subframe 4, Page 17 special message field ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). The investigation examined 12.16 million observations collected between 2007 and early 2026 from the GFZ Potsdam navigation-bit archive ([GFZ Data Services, n.d.](https://dataservices.gfz-potsdam.de/panmetaworks/showshort.php?id=c22fe9d3-0397-11ee-95b8-f851ad6d1e4b)), revealing that the 176-bit field, which is broadcast every 12.5 minutes by all operational GPS satellites, carries encrypted data consistent with the U.S. military’s Over-the-Air Distribution (OTAD) rekeying network ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). ## Data Analysis and Cryptographic Evidence The GPS Interface Specification (IS-GPS-200) reserves Subframe 4, Page 17 for “special messages with the specific contents at the discretion of the Operating Command” ([U.S. Coast Guard Navigation Center, 2022](https://www.navcen.uscg.gov/sites/default/files/pdf/gps/IS-GPS-200N.pdf)). However, the data analysis found no evidence of conventional text. A chi-squared test on the 45-symbol alphabet yielded a z-score of 1.84, indicating the distribution is statistically indistinguishable from random data ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). Furthermore, an order-8 PPM-D compression model confirmed high marginal entropy, characteristic of encrypted content rather than plaintext or uncompressed data ([Murdoch, 2026b](https://github.com/sjmurdoch/gps-special-messages/blob/main/CLAIMS.md)). Despite the high entropy, structural anomalies were identified. A Prediction by Partial Matching (PPM-D) model identified pairs and groups of unique messages sharing identical substrings at identical byte positions across different broadcast dates, such as a 10-character string shared between two messages on October 8, 2014 ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). These shared substrings are consistent with protocol metadata, such as key identifiers or routing headers, leaking through the cipher ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). Additionally, the research identified three “sentinel” messages with a Shannon entropy of zero, consisting of 22 identical bytes (all-spaces, all-NUL, and all-¬) ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). The all-¬ (0xAA) pattern, which corresponds to the alternating binary sequence 10101010, frequently correlated with periods when satellites lacked operational payloads, such as during commissioning or decommissioning ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). ## Operational Timelines and OTAD Integration The analysis delineated three distinct behavioral regimes in the broadcast history: 1. **Pre-Operational Era (2007–2011):** Payloads rotated approximately every 3.7 days with high sentinel fractions ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). 2. **Operational Era (2011–2022):** Following a synchronized fleet-wide transition to the all-¬ sentinel on May 26, 2011, the rotation rate increased to approximately 1.8 days, with a median message duration of 23 hours ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). This daily cadence aligns with tactical cryptographic key lifetimes and the activation of the OTAD network, as described in a 2015 Space and Missile Systems Center briefing ([Tyley, 2015](https://web.archive.org/web/20250608164932/https://www.gps.gov/multimedia/presentations/2015/04/partnership/tyley.pdf)). OTAD enables the remote delivery of “next black keys” to Selective Availability Anti-Spoofing Module (SAASM) equipped military receivers, replacing manual key-fill procedures ([Ferreira, 2026](https://www.404media.co/the-u-s-military-quietly-turned-gps-into-a-global-numbers-station-evidence-suggests/)). 3. **Modern Era (2022–Present):** A coordinated change in May 2022 reduced the rotation rate to 4.3 days, slowing further to 6.8 days by early 2026 ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). This shift occurred without public Notice Advisory to Navstar Users (NANU) documentation and coincided with broader GPS modernization efforts, including M-Code deployment and OCX ground segment developments ([Clark, 2026](https://arstechnica.com/space/2026/03/after-16-years-and-8-billion-the-militarys-new-gps-software-still-doesnt-work/)). ## Recent Protocol Formatting Changes In December 2023, the first message containing a recognizable string—the four-byte prefix “TEXT”—was broadcast on PRN 8 ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). This format subsequently expanded in a staged deployment, reaching a 10-PRN distribution event in March 2024, and later stabilizing into daily broadcasts migrating across specific satellites (PRN 1, PRN 21, PRN 20) throughout 2024 and 2025 ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). While the prefix aligns with the IS-GPS-200 specification for text payloads, the trailing 18 bytes remain high-entropy ciphertext ([Murdoch, 2026a](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/)). The dataset, source code, and verification scripts supporting this research are archived on Zenodo ([Murdoch, 2026c](https://zenodo.org/records/20500661)). ## References Clark, S. (2026, March 30). After 16 years and $8 billion, the military’s new GPS software still doesn’t work. _Ars Technica_. [After 16 years and $8 billion, the military's new GPS software still doesn't work - Ars Technica](https://arstechnica.com/space/2026/03/after-16-years-and-8-billion-the-militarys-new-gps-software-still-doesnt-work/) Ferreira, B. (2026, June 5). The U.S. Military Quietly Turned GPS Into a Global ‘Numbers Station,’ Evidence Suggests. _404 Media_. [The U.S. Military Quietly Turned GPS Into a Global ‘Numbers Station,’ Evidence Suggests](https://www.404media.co/the-u-s-military-quietly-turned-gps-into-a-global-numbers-station-evidence-suggests/) GFZ Data Services. (n.d.). _GPS C/A Navigation Message Data Bits_. GFZ Helmholtz Centre Potsdam. [GPS C/A Navigation Message Data Bits](https://dataservices.gfz-potsdam.de/panmetaworks/showshort.php?id=c22fe9d3-0397-11ee-95b8-f851ad6d1e4b) Murdoch, S. J. (2026a, June 9). The Empty Field That Wasn’t: GPS, OTAD and Two Decades of Encrypted Broadcasts. _Inside GNSS_. [The Empty Field that Wasn't: GPS, OTAD and Two Decades of Encrypted Broadcasts - Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design](https://insidegnss.com/the-empty-field-that-wasnt-gps-otad-and-two-decades-of-encrypted-broadcasts/) Murdoch, S. J. (2026b, June 2). _Article claims and verifiers_ [Source code]. GitHub. [gps-special-messages/CLAIMS.md at main · sjmurdoch/gps-special-messages · GitHub](https://github.com/sjmurdoch/gps-special-messages/blob/main/CLAIMS.md) Murdoch, S. J. (2026c, June 2). _The Empty Field That Wasn’t: GPS, OTAD and Two Decades of Encrypted Broadcasts (pipeline, analysis, figures, and verifiers for the GPS L1 C/A Subframe 4 Page 17 corpus, 2007–2026)_ [Software and data]. Zenodo. [The Empty Field That Wasn't: GPS, OTAD and Two Decades of Encrypted Broadcasts (pipeline, analysis, figures, and verifiers for the GPS L1 C/A Subframe 4 Page 17 corpus, 2007–2026) | Zenodo](https://doi.org/10.5281/zenodo.20500661) Tyley, S. (2015, April 29). _Over-the-Air Distribution (OTAD) Update_ [Briefing]. 2015 GPS Partnership Conference. Space and Missile Systems Center. [Wayback Machine](https://web.archive.org/web/20250608164932/https://www.gps.gov/multimedia/presentations/2015/04/partnership/tyley.pdf) U.S. Coast Guard Navigation Center. (2022, August 1). _Global Positioning System Directorate Systems Engineering & Integration Interface Specification IS-GPS-200, Revision N_. [https://www.navcen.uscg.gov/sites/default/files/pdf/gps/IS-GPS-200N.pdf](https://www.navcen.uscg.gov/sites/default/files/pdf/gps/IS-GPS-200N.pdf) _AI generated_