GPS Hidden Messages - Computerphile

Embedding forbidden sentinel patterns like 101010 can cause receiver failures because some devices encounter unexpected field values that trigger unhandled conditions or bugs.

Over-the-air rekeying is attractive because physically reprogramming thousands of receivers is labor-intensive and keys need frequent rotation due to device turnover and alliance changes, so broadcasting encrypted key material greatly reduces logistical burden if a secure delivery exists.

The military uses encrypted GPS signals because encryption prevents outsiders from predicting or fabricating valid signals, which raises the technical bar for spoofing and precise jamming.

On 26 May 2011 all satellites switched to a 101010 sentinel at midnight, and because the per-message rotation rate changed at the same moment this sentinel likely flagged an over-the-air rekeying or coordinated operational event.

Many operational events encoded in GPS special pages are invisible to ordinary users because consumer receivers ignore pages unrelated to positioning and time, so sentinel patterns or rekeying signals only surface if a device exposes debug fields.

Measuring distance to a satellite alone leaves you on a sphere, so receivers also need the satellites' encoded orbital positions to intersect multiple distance spheres and compute a unique location.

You can quantify how much new information a GPS message adds by leaving it out of a corpus and compressing the rest, because the size increase when that message is reinserted measures its surprisal or entropy.

Some 'at discretion' GPS pages were far more predictable or strikingly surprising because their compressibility and entropy diverge from random behavior, revealing deliberate markers or special operational states inside the stream.