Kantronics KPC-4 - Page 39

WEFAX Mode
General
Facsimile reception (WEFAX) is available with your TNC since the weather facsimile
broadcasts are using an 800 Hz FSK shift which is compatible with the Kantronics
1200 baud modem. In order to receive WEFAX, you must have a program for your
computer which will receive 8 bit data from the TNC and format it properly for display
on your computer screen, or your printer. Kantronics has available three terminal
programs which meet these requirements — MAXFAX for the Commodore 64 and 128
computers, SuperFax II for the PC, and WeFaxWorks for the Apple Macintosh
computer.
Background: HF WEFAX Transmissions and Resolution
Many of the weather charts broadcast on HF as WEFAX are hand drawn synoptic
maps, i.e. a summary of weather conditions, showing such things as atmospheric
pressure and surface temperature over wide areas. Many of these maps show whole
continents or oceans. These maps and charts are often produced by a particular
weather service at their own HF radio station site. Two predominant stations are
NAM, the Norfolk US Navy Fleet Weather Service station, and CFH, the Canadian
Forces Halifax station.
Full size charts are eighteen (18) inches wide and vary in length from 10 to 18 inches.
In past years, many charts were generated on a rotating drum at 60 scan lines per
minute, but modern day charts are digitized (placed in computer storage) at the rate of
120 scan lines per minute on a flat-bed scanner. The scanner digitizes with a resolution
of 96 lines or pixels per inch. So, the maximum resolution of a hand-drawn synoptic
chart, when digitized, is nearly 1800 by 1800 pixels!
In addition to the temperature and pressure charts, redigitized satellite photos are
sometimes transmitted over HF. The sources of most.of these satellite weather photos
are the National Oceanic and Atmospheric Administration (NOAA) operated weather
satellites.
All WEFAX charts and pictures received on HF come from an HF ground station,
not from any of the satellites. As mentioned above, many stations generate their
own weather charts. Weather photos coming from the satellites are received first
by a ground station, reformatted, and then relayed by telephone line to the HF
transmission site for dissemination.
When the WEFAX maps and charts are transmitted on HF, some of the resolution is
lost.
This
occurs
because
the
FSK
modulation
scheme
used
by
all
manufacturers
of
HF
WEFAX
transmission
equipment
will
not
support
the
baud
rate
needed
for
full
horizontal
resolution.
The
FSK
format
has
been
kept,
however,
to
make
today's
equipment
compatible
with
that
produced
earlier,
and
received-map
resolution
is
acceptable.
© Copyright 1989, 1990, Kantronics, Inc. All Rights Reserved. A
Duplication of this manual or the firmware without Aug. 13, 1990 E Version 3.0
permission of Kantronics, Inc. is prohibited. ) Operations Manual