It's an odd story, since until pretty recently most North American police radio was plaintext to begin with.
First you need to make darn triple check extra sure that when you deploy it, you won't change it. It is a one-shot switch and whoever gets to your site is stuck with the configuration for days, weeeks, months. And you cannot tell them "my bad, try again".
Then if you have a sensible setup, you would redirect immediately to HTTPS anyway.
Sure, it protects you from some marginal risks (such as you not setting your cookies to secure mode) but then you have other problems and HSTS will bite you when you prod the security settings without a good plan.
its an interesting domain but hard to get solid info on unless you are working on these types of projects or for some MoD somewhere. most info out there on the net is about old tech.
as far as i know preshared keys are common. hard to rekey ofc in case of compromise so likely they have some tricks up their sleeves to make sure if for instance a unit is overtaken by enemy not all coms are compromised by this key in the device. (guesswork here ofc..) dont think much of this stuff uses priv/pub keys and https or vpn like auth schemes etc.
To put it very simply, radios communicate with a central Key Management Facility (KMF) using a special key (UKEK, Unique Key Encryption Key) to securely transport the new key material. There's more to it than that, of course, but these features are heavily used by the feds and also by larger state and local systems -- because manually re-keying each radio is a huge pain.
Allow me to speculate massively. Hiss sounds more like weak signal acquisition. Perhaps in this case, Mitnick was interfering but not defeating encryption.
If there is a clear pattern to it, then that’s either unencrypted framing, or bad encryption. (Think 90’s cable TV ‘scrambling’).