I was never interested in actually recieving let alone decrypting police data. It is unlawful to defeat any encrypted transmission regaurdless of the sender, purpose, or medium IIRC. I initially looked into this stuff becuase I thought it might be possible to apply conventional direction finding techniques to various police (and other, such as WiFi hotspots, cell antennas, etc) RF emissions. The system would be completely passive.
The most obvious and useful purpose of this data would be to place this icon on my gps screen; however for any such system to be useful it must be fairly reliable. Reliability requires continuous tracking which is impossible if police radio transmissions are intermittent rather than continuous.