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On 2018-09-18 5:45, Brad Morris wrote:
> I'll correct myself on the government seizure part.  Apparently you can buy
> older, somehow neutered units and the government doesn't come after you
> (anymore).

The great strength of a military GPS receiver is access to encrypted
signals radiated by the satellites. These provide enhanced accuracy and
resistance to jamming and spoofing. But you can't unlock this capability
unless the receiver is loaded with the current cryptographic key.

During my USAF days one of my duties was the preflight loading of keys
for secure voice radios, IFF Mode 4, and GPS. These keys were all
different, and I was always a little stressed making sure they were the
correct ones for the time of the flight. Cipher changes (old replaced by
new) were in terms of UTC, so the offset from local time had to be kept
in mind.

Also, there were different conventions for specifying the time. One
cipher was designated by its expiration time, another by the time it
became valid. That was second nature to the specialists who had been
trained from scratch in this field, but I wasn't one of those and had to
always think it through carefully. The crypto safe contained keys for
several days in advance. It was easy to grab the wrong one.

Ciphers were stored in KYK-13 key transfer devices. A "kick thirteen"
was about the size and shape of a bar of soap and could hold six keys.
To key the GPS, connect the KYK-13 to the GPS receptacle in the cockpit,
set the 1 - 6 selector to the register that contains the correct key,
turn it on, and watch for the flash of the red light. Turn off the
KYK-13, disconnect, and you're done. It takes only a few seconds.

I never had occasion to load a hand held receiver such as a PLGR, but I
suspect the method is almost the same.

http://www.cryptomuseum.com/crypto/usa/kyk13/index.htm

   

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