I have been playing with OP25 on my Linux system, monitoring my local P25 systems.  I have found a link that explains the download and installation of OP25 and I have provided the link on my OP25 page.

OP25 is a not-for-profit project to bring together folks that are interested in implementing APCO P25 using a software-defined radio. Our goal is to build a software-defined analyzer for APCO P25 signals that is available under the GNU Public License (GPL).

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APCO Project 25 is the digital communications standard used by many police and emergency services throughout the world. Most notably the US, Canada and Australia deploy systems based on P25. Compared to existing analogue systems P25 offers improved spectrum use, coverage and flexibility. Provision is made to ensure the confidentiality of traffic, to allow the use of trunking and the provision of data in addition to voice services.