REC LPFM Advisory Letter #20 - LPFM applications near large bodies of water
LPFM applicants located within 16 kilometers (10 miles) of a large body of water, such as the Atlantic or Pacific Ocean should be aware that the effective radiated power (ERP) that is automatically assigned by the FCC's Licensing Management System (LMS) may be significantly lower than what the application may be entitled to.
For most applications filed in LMS for LPFM services, the FCC uses the National Elevation Dataset 1 arc second (NED1) terrain data. In Alaska, a similar dataset is used at 3 arc seconds (NED3).
HAAT is calculated by taking the elevations from 50 points along a specific heading/radial between 2 and 10 miles (3~16 km) and then averaging them and comparing them to the radiation center elevation at the transmitter site. This process is repeated seven more times at equally spaced radials by 45 degrees (e.g. 0, 45, 90, 135, etc.). The result of each of those 8 radials is then averaged and that average is the HAAT.
REC has noticed that when calculating the HAAT using the NED1 dataset at locations near the ocean, that extremely high HAATs along certain radials towards the ocean will be registered among certain radials resulting in an overall HAAT that is much higher than expected. This issue is pronounced most for LPFM proposals in Hawaii. This issue also impacts the shape of contour maps provided by REC Networks tools such as FCCdata.org and other services that use the FCC's Contours API.
REC has researched the issue and we have concluded that the issue is related to oceanic topography.
Other FCC terrain data sources such as the original FCC30 and the GLOBE terrain data sets will consider oceanic areas as an elevation of zero. However, the NED1 dataset will consider the actual depth of the ocean floor at a particular point and consider it a negative elevation below sea level. We have discovered when HAAT is measured along a particular radial that exhibits these very high HAAT calculations, if the distance along the radial is reduced (for example from between 2 and 10 miles to 2 and 6 miles), the HAAT will be greatly lowered and thus resulting in a higher HAAT. Because of the topograhy off the coast of Hawaii where there are significant cliffs in the ocean floor, the issue is most pronounced there.
FCC Audio Division staff has been notified of REC's theory on this matter and they state that they will do their own measurements and not depend on the LMS default HAAT.
To obtain the proper HAAT for a location, tools need to be able to support the FCC30 and/or GLOBE data sets. This includes the FCC's HAAT Calculator website, V-Soft and ComStudy 2. Applicants using one of those methods should include an exhibit that includes either a screen shot of the calculator website or data from a third party tool to demonstrate the correct HAAT and thus, ERP. The REC HAAT Calculator supports NED1, NED3 and GLOBE terrain data but does not support FCC30.
Locations that are within 10 miles of the ocean and do not have any other terrain past 10 miles but within the 34 dBu interfering contour along the radial in question can reduce the area determined by limting the average elevation along the radial to only the land area from 2 miles from the transmitter site to the shoreline. In cases where the shoreline is less than 2 miles away, the entire radial can be removed completely from consideration and the average will be calculated based only on the remaining radials. For more information on how to use this method, see §73.313(d)(2) of the FCC Rules.
Applicants handled by REC Networks full service who are in these areas will be provided with HAAT calculations that apply the §73.313(d)(2) rules if using the rule will result in a higher ERP.
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This LPFM Advisory Letter was originally published on December 13, 2023.
