GPS Hidden Messages - Computerphile

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Encryption makes precise GPS jamming harder when attackers cannot predict the signal they must disrupt.
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On May 26, every GPS satellite broadcast repeating 101010 instead of random data, then resumed faster random data at midnight May 27.
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GPS contains an unexplained text field labeled “At the discretion of operating command,” hidden from normal consumer receivers.
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GPS forbids the 101010 pattern, yet similar invalid sentinel patterns have crashed GPS receivers before.
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Around the end of May 2011, GPS message rotation accelerated from one every 3.7 days to every 1.8 days.
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GPS satellites generally transmit the same unexplained message simultaneously, then switch to a new one over a short period.
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GPS messages were nearly as random as expected, yet some were strikingly predictable and others unusually surprising sentinel messages.
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GPS operators publish daily system-status announcements but never documented the unusual message patterns.
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Near-duplicate messages are surprising even under perfect encryption, pointing to transmission mistakes, emergencies, or an unexpected system state.
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The exact satellite, frequency, and message-channel mechanism for distributing GPS keys over the air remains unknown.
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GPS military keys once required physical loading through specialized USB devices, making distribution labor-intensive.
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GPS keys expire after one week, so historical observations cannot reveal usable current keys, even with the messages recorded.
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GPS can broadcast one current key encrypted separately for each coalition member, revoking one group's access without changing everyone else's key.
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GPS signals are extraordinarily weak, comparable at transmission to a dim light bulb and nearly negligible at Earth.
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Over-the-air GPS key distribution would replace physical loading with radio delivery, but its security depends on an undisclosed mechanism.
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Military GPS authentication uses encrypted signals and restricted keys, making spoofing much harder because attackers cannot predict signal contents.
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Encrypted military GPS signals make precise jamming harder because attackers cannot easily predict what signal to disrupt.
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Military key management must serve authorized users, block unauthorized access, and repeatedly replace keys as alliances and personnel change.
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Long-lived key hierarchies encrypt short-lived GPS authentication keys, giving military and allied groups different access rights.
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Military GPS encryption blocks spoofing by keeping signal encryption keys known only to authorized users.
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Randomness is not something you can eyeball. Statistical tests and compression estimate information content and entropy.
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In leave-one-out PPMD analysis, messages that expand the compressed corpus contain more information and are less predictable.

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