The following is from the February 1989 issue of QEX from an article
written by H. Paul Shuch, N6TX, called A Low-Noise Preamp For Weather
Satellite VISSR Reception.

Geostationary weather satellites, from which radio amateurs have long
delighted in recovering earth images, actually provide two distinct
products.  WEFAX, or weather facsimile is the more familiar of the two,
and consists of preprocessed and enhanced frames, taken at visible or
infrared wavelengths, relayed via a frequency modulated microwave
carrier, in slow scan format (four lines per second).  Because of its
relatively narrow bandwidth (30 kHz per channel) and respectable output
power (on the order of 5 watts), the WEFAX signal is commonly received
with little effort using relatively simple equipment.  But the data is
secondhand.  WEFAX images are analog retransmissions of raw satellite
data that has been downlinked to a central datacollection facility for
processing, then returned to the satellite for distribution.  In WEFAX
mode, the satellite is thus serving as a repeater.

In its other operating mode, the familiar WEFAX satellite provides a
wide-band digital signal variously called VISSR for Visible and Infrared
Spin-Scan Radiometer, the image sensor on the satellites, VAS, for VISSR
Atmospheric Sounder, or HRPT for High-Resolution Pictur Transmission,
which highlights the signals advantage over WEFAX.  throughout this
article, I shall use the term VISSR.

The ultimate microwave challenge is reception and display of raw satellite
data prior to processing.  this has until recently been an elusive goal,
because in the digital mode, the satellite is transmitting at greater
than 2 Mbits per second.  Hence, a wide receiver bandwidth (on the order
of 8 MHz) is required.  Thus, though the VISSR transmitter power is on a
par with that of the WEFAX transmission, the modulation sidebands are
spread over a frequency spectrum about 300 times as wide.  The resulting
low spectral density makes it necessary to employ some rather
sophisticated receiving equipment; government stations emply 60 foot
dishes, parametric amplifiers and large mainframe computers for image
processing.