So you are saying that the system IS always encrypted? If it's not encrypted then it's perfectly legal to listen to, even if it uses a strange and proprietary encoding.
The question is whethe the voice packets are always encrypted. If they are, then it's a no go. If they're not, then it does not matter if the radio is programed to receive a given channel or not. If it's not encrypted then you might be able to grab the signal and pipe it into the radio's decoder circuitry. It's just "packet sniffing." You just make it blindly decode whatever voice stream is fed to it, but that's not going to work if it's actually encrypted.
It's so secure even the people you want to talk to can't receive it half the time!
While I am NOT a proponent of Opensky systems at all for general public-safety communications, I do understand the design architecture and why they've done some of the things they've done. It's a great data platform, it's robust, stabile, and secure; just ask FedEx, they've been using Opensky technology for sometime now.
The entire data stream is encrypted with varying levels of AES; depending upon what the individual contract and municipality requested in the original bid or subsequent modifications thereafter. As for government communications using Opensky; forget it... These guys are using specialized formats of encryption that are not available to the general public-safety market; especially those overseas countries like Iraq and Afghanistan... The systems aboard the naval vessels and various bases throughout the U.S. soil are also tightening up their encryption algorithms with the implementation of these new Opensky systems.
The voice paths are generally encrypted from site to site, mobile to mobile, portable to portable. Some agencies, like Oakland County, Michigan have taken this a step further and encrypted the traffic on their wired network backbone between the dispatch consoles and the remote stations. The encryption choices with this platform are becoming endless.
Again, Opensky in my opinion is much better for mobile data than anything else; even your comment about users not being able to hear each other is in-fact somewhat justified. Early on, when it was M/A-COM and they were trying to get their four talk paths per channel feature to work; there were MANY lost communications. Since Harris has taken over; Opensky has been overhauled and is now a fully functional four talk path secure digital trunking system that offers excellent voice and data options to various agencies...
Welcome to the 21st century of communications; yes it sucks, but remember, Harris isn't the only bad guy here, Motorola's new Project-25 Phase-II systems are being pushed by sales personnel to include 100% encrypted features for all users... There's allot of wasteful spending of Homeland Security money going on in government these days... Is there a need for some of these things, sure for tactical operations and surveillance, but do the trash trucks really need to have encryption? You're a taxpayer, you be the judge...
Anyway, good luck trying to decode Opensky; I'll be watching this thread with the utmost curiosity into your results.