[amsat-bb] TROPOSHPERIC DUCTING FORECASTS

[Gunther Meisse]

An interesting story which arrived at our TV station this morning,

TROPOSHPERIC DUCTING FORECASTS
The troposphere is the lowest portion of the earth's atmosphere, extending
from the earth's surface to approximately 4 to 12 miles above (varying by
location and season). Under certain conditions (discussed below) FM radio
and TV signals in the VHF and UHF frequency bands can propagate over great
distances through the troposphere due to a phenomenon called tropospheric
ducting. A Canadian meteorologist and self-avowed radio/TV "DX"
(long-distance) enthusiast, William R. Hepburn, has developed a method for
predicting tropospheric ducting and provides forecasts (available on the
Internet) using this method, which predict radio and TV signal strength
propagation as a result of the ducting phenomenon.

"William Hepburn's Worldwide Tropospheric Ducting Forecasts" are available
online at www.dxinfocentre.com/tropo.html. The principal content on this
Website is the forecast maps. Six-day preview as well as 42-hour preview (in
6 hour increments) maps for virtually every region of the world are provided
(an example map of Eastern North America is shown here). The purpose of
these maps is to display potential duct paths for VHF, UHF and microwave
signals, indicated by the color shading on the maps using the "Hepburn Tropo
Index" (see scale on the left-hand side of the map). This index indicates
the degree of tropospheric bending forecast to occur over a particular area,
which is an indication of the overall tropospheric radio signal strength on
a linear scale from 0 to 10 (with 10 representing an "extremely intense
opening" for propagation by ducting). Also shown on the maps (indicated by
the letter "U") are predicted "unstable signal areas" where weather
conditions could potentially disrupt signal paths and cause unusual and
sometimes rapid variations in signal strengths.
Mr. Hepburn's Web page also provides some background information on ducting,
indicating that vertical boundaries between different types of air masses
(for example, warm air over cooler air) can refract radio signals. When the
vertical boundary becomes especially sharp, the amount of refraction can
become so severe that signal propagation is extended a great distance as
though caught in a duct, thus the reference to tropospheric ducting.
The raw data for the maps (temperature, humidity & pressure) are extracted
from the Meteorological Service of Canada's global weather model. Mr.
Hepburn has written a proprietary algorithm which approximately mirrors the
atmospheric conditions necessary to produce tropospheric ducting (warm dry
air overriding cool moist air). He developed this algorithm from over 30
years of real-time distant radio & TV observations coupled with 25 years
experience as a meteorologist. Using a freeware meteorological program
called GrADS, the data is ingested, the algorithms are applied, and the maps
are produced.
According to Mr. Hepburn, in the U.S., the Gulf States & Florida are most
susceptible to ducting, especially during the spring months when the sea
temperatures are still relatively low. In the Midwest, Great Lakes &
Northeast, ducting is more common in the fall. Ducting is rare west of the
Rockies except along the Pacific Coast & Hawaii.
The ducting forecast Web page is just one of the sites featured on "William
Hepburn's Radio & TV DX Information Centre" Website (www.dxinfocentre.com),
which also includes information on radionavigation, worldwide broadcast
station lists, and a collection of "TV DX photos" showing screen shots of
distant TV signals (as far as 1,000 miles away) received by Mr. Hepburn in
Grimsby, ON, Canada (located about 50 miles NW of Buffalo, NY).