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The data base on here is 100% compiled be listener submission. There are no secret frequencies police use that they hide from the public and they can’t easily change them. All of their frequencies require an FCC license that is publicly available and searchable. The challenge is to find out what frequencies are still in use, what agency is using them and for what purpose. If the police really don’t want you to listen they will encrypt their transmission. No scanner can decode the transmission and it is illegal to so. You will read a lot on these forums about the big “E” that means that the agency went full time encrypted and can no longer be monitored. That is becoming more and more common as agencies upgrade their systems.