I've written two introductory articles on how trunked radio works. You can read them here and here.

Frequency and site information for P-25 systems are available by clicking here.

• Uniden is selling the BCD996T, a base or mobile scanner capable of tracking nearly all APCO-25 systems, as well as the common analog trunked systems. It has a unique capability of accepting external GPS position information and selecting appropriate frequencies to scan. More here.

• Uniden is selling the BCD396T, a compact handheld scanner capable of tracking nearly all APCO-25 systems, as well as analog trunked systems. More here.

• Radio Shack is selling the PRO-96, a GRE-built scanner capable of monitoring 3600-baud and 9600-baud trunked APCO-25 systems. More here.

• Uniden is selling upgraded digital scanners, the BC296D handheld and the BC796D base/mobile scanners. Each is capable of monitoring APCO-25 systems, including those with 9600 baud control channels. More here.

• Uniden continues to sell the BC250D handheld and the BC785D base/mobile scanners, each of which are capable of monitoring (some) APCO-25 systems through the use of a plug-in BCi25D decoder card. More here.

• AOR is selling the ARD25, an APCO-25 decoder that uses the 10.7 MHz IF output of a separate receiver. More here.

• Propagation environment information from a variety of sources can be found here.
• NOAA maintains the Space Environment Center.
• Information about the Sun-Earth environment at SpaceWeather.com.
• Ham Radio magazine published a brief article on calculating line-of-sight distance. You can see a scan of that article by clicking here.

• Real RTTY signals may once again be on the air, originating from California (read about the station here).

  From an announcement Dated October 31, 2006:
  

  KSM, the coast station of the Maritime Radio Historical Society (MRHS), has been granted two RTTY frequencies by the FCC.

  The frequencies, known officially as narrow band direct printing (NBDP) channels, are:

  8.4330 Mc
  12.6310 Mc

  Authorized power is 5kW.

  The MRHS will use these channels for broadcasting weather, press and other information to the maritime community using RTTY (Baudot) and SITOR FEC modes.

  It will take a while for the MRHS Transmitter Department to bring transmitters on line for these frequencies. But now that we have FCC approval we can begin work on this new project. An announcement will be made when we are ready to begin transmission on these frequencies.

  VY 73,

  Richard Dillman
  Chief Operator, KSM

  You can read more about the Maritime Radio Historical Society here.

• Information regarding the permit required for scanning from a vehicle in Michigan is available here.

  If you're interested in where the towers are located in Michigan's statewide APCO 25 system, click here.

• Some notes on the origin of "73" and other numeric codes can be found here.

• Does anyone know where I can order a McElroy Chart of Codes and Signals as pictured here?

• You can read about maritime Automatic Identification System (AIS) equipment and signals here.

• Back in 1970 the magazine Popular Electronics published this article about detecting changes in local gravity using a CW receiver.

• Information about building a quadrifilar helicoidal antenna can be found here.
• Does anyone know when Motorola built a portable AM/FM radio with a model number of X70E? Some photos of the radio can be seen here.

• Rich Carlson, N9JIG, reviews three scanning magazines (Popular Communications, Monitoring Times, and Scanning USA) on his website here.