One of the most significant changes in the scanning hobby since the introduction of the original trunk-tracking radios is taking place right now as new digital-capable scanners hit the market. Uniden is the first to sell scanners that have the ability to monitor public safety and other radio systems that use the APCO Project 25 (P-25) suite of standards. The standards grew out of work by the Association of Public-Safety Communications Officers and have since become a popular choice for new police, fire, and other agency radio systems. Background on Project 25 can be found in my March 2001 Tracking the Trunks column.

With the introduction of these new P-25 scanners it will become important to know where to look for digital systems. The following list contains identification and frequency information for many known P-25 systems. Listings are sorted by state, and then by the organization name within the state. Each organization will have one or more transmitter locations. Each location will transmit on one or more frequencies.

Remember that Project 25 is a set of standards, some mandatory and many optional. Most system designers have chosen to use only a few of the available standards. Here are some brief notes about the differences that you may encounter when scanning.

All P-25 systems use the same protocols over the air, called the Common Air Interface (CAI). An APCO-25 compatible scanner will be able to understand a system that uses this standard. It will not understand AEGIS or other digital systems that do not use the P-25 CAI.

Voice traffic may be digital only, or a mix of analog and digital. Digital voice may be encoded using one of two methods: either IMBE (Improved Multi-Band Excitation) or VSELP (Vector Sum Excited Linear Predictive). IMBE is the Project 25 standard, which you should be able to hear (if it is not encrypted). VSELP is a Motorola proprietary scheme and is not supported by any consumer scanner. Cleveland is an example of a city that uses P-25 CAI and digital voice, but because they use VSELP voice coding the voice traffic cannot be heard.

Regardless of the encoding method, digital voice may either be encrypted or unencrypted. (Remember that encoding and encryption are not the same thing!) There are different types of encryption, but for the scanner listener it doesn't matter since the result is unintelligible audio.

Frequencies in a P-25 system may either be part of a trunking group or may be conventional (non-trunked). If they are trunked, the control channel for the trunking group may run at either 3600 baud or 9600 baud. The 3600-baud control channel is used in systems with older radios that cannot handle the higher speed. The 9600-baud channels will be found in newer systems that are completely digital. Preliminary reports indicate that the new Uniden scanners will not be able to trunk-track 9600-baud systems, so users will have to scan those as they would a conventional system.

One final note: there are many more systems out there than we have room to print. This list, along with updates and additions, will be maintained on my website at http://www.signalharbor.com, so check there for the latest information. If you have details about a particular system, or have found one that isn't on the list, please send me the details via electronic mail to dan@signalharbor.com or write to me in care of this magazine.