From the maker Harrison Sand:
"In this guide I will explain how to build an inexpensive SDR (software defined radio) that can be used to receive police, fire, taxi, and other digital radio transmissions. I will do my best to explain the steps in detail, and provide further resources to help along the way.
For the radio enthusiasts, this setup is capable of following trunked, digital voice radio transmissions with a single receiver.
Read the full howto here: https://sites.google.com/site/policescannerhowto/"