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Preparation of Coverage Maps

While the 1989 study used an advanced computer mapping program developed by and unique to NTIA’s Institute for Telecommunications Sciences (ITS), several software programs are now available from commercial vendors with similar capabilities.  The maps prepared for this study used commercially available signal propagation mapping software.    

The FM coverage maps and population estimates were prepared by NTIA/PTFP staff using the Probe II signal propagation software developed by V-Soft Communications of Cedar Falls, IA.  The software used for this project relied on FCC transmitter and antenna information accessed directly from the FCC license database via the Internet (www.fcc.gov) on October 1, 2003.   In a few cases where negative antenna heights are listed on the FCC database, staff corrected to zero height in order to utilize the software. The FM mapping took place during October, November and December 2003, and AM mapping was done in March 2004.  The map range for the study was 300 kilometers from each transmission site.  The software contains topographical data from the U.S. Geological Survey and performs calculations using the Longley-Rice propagation model, which takes into account the terrain as well as other technical specifications.  Finally, the software automatically performs population calculations based on 2000 U.S. Census data.

Two FM contours are drawn on the maps.  The primary service area (shown in yellow) is determined to have a minimum signal strength of 70 dBu.  The 70 dBu level is sometimes referred to as “City Grade.”  The secondary contour (shown in red) has a minimum of 60 dBu.  The 60 dBu contour was used to determine the population considered to receive service.  The 60 dBu level is recognized as the area in which a reliable signal can be received using an ordinary radio receiver and antenna.  It would not be uncommon, however, for signals to be received beyond the contour lines. 

The V-Soft Communications' Probe II software was also used by NPR's Engineering Division to develop the maps showing AM coverage.  The primary service contour (indicated by a green line) is 66 dBu (2 mV/m) and the secondary contour (indicated by a red line) is 54 dBu (.5 mV/m). NTIA provided NPR with information on AM stations and parameters similar to our research on FM stations, and we wish to acknowledge the role of NPR's Jan Andrews and Mike Starling in creating the AM maps.

As in the 1989 study, with the exception of Alaska, only the FM coverage statistics were used to calculate the number of persons and percentage covered for each state.   The reasons for not including AM coverage in the general population statistics are explained in the discussion regarding AM Coverage in the Findings section later in this report.

Only full-power stations and boosters have call signs printed on the maps.   In order to create a more readable map, the translators are represented only by their transmitter locations without call letters.  Likewise, the names of counties and other geographical information are not shown.   The addition of county lines and grid coordinates was thought to be helpful in locating the specific areas lacking coverage.  State maps may not show all stations in adjoining states.

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