Radiofax

Radiofacsimile, radiofax or HF fax is an analogue mode for transmitting grayscale images via high frequency (HF) radio waves. It was the predecessor to slow-scan television (SSTV). It was the primary method of sending photographs from remote sites (especially islands) from the 1930s to the early 1970s. It is still in limited use for transmitting weather charts and information to ships at sea.

History

Richard H. Ranger, an electrical engineer working at Radio Corporation of America (RCA), invented a method for sending photographs through radio transmissions. He called his system the wireless photoradiogram, in contrast to the fifty-year-old telefacsimile devices which used first telegraphic wires, and then later was adapted to use the newer telephone wires. On 29 November 1924, Ranger's system was used to send a photograph from New York City to London. It was an image of President Calvin Coolidge and was the first transoceanic radio transmission of a photograph. Also that year, AT&T engineer Herbert E. Ives transmitted the first color photograph. Charles J. Young, son of the RCA founder Owen D. Young, and Ernst Alexanderson, developed a radio facsimile system for General Electric. On 12 August 1931 this system successfully transmitted a copy of the Union-Star newspaper of Schenectady, New York to the transatlantic liners America and Minnekahda. It took 15 minutes to copy a single page measuring 8+1⁄2 by 9 inches (220 by 230 mm). Beginning in the late 1930s, the Finch Facsimile system was used to transmit a "radio newspaper" to private homes via commercial AM radio stations and ordinary radio receivers equipped with Finch's printer, which used thermal paper. Sensing a new and potentially golden opportunity, competitors soon entered the field, but the printer and special paper were expensive luxuries, AM radio transmission was very slow and vulnerable to static, and the newspaper was too small. After more than ten years of repeated attempts by Finch and others to establish such a service as a viable business, the public, apparently quite content with its cheaper and much more substantial home-delivered daily newspapers, and with conventional spoken radio bulletins to provide any "hot" news, still showed only a passing curiosity about the new medium. During World War II thousands of photographs were transmitted from Europe, and from the Pacific Islands, to the United States. The major news agencies (AP, UPI, Reuters), maintained their own transoceanic radio facsimile transmitters as close to the action as they could. The iconic flag raising on Iwo Jima was printed in hundreds of American newspapers within a day of being taken, because it was transmitted from Guam to New York City by wireless radiofacsimile, a distance of 12,781 km (7,942 mi). By the late 1940s, radiofax receivers were sufficiently miniaturized to be fitted beneath the dashboard of Western Union's "Telecar" telegram delivery vehicles. In the 1960s, the United States Army transmitted the first photograph via satellite facsimile to Puerto Rico from the Deal Test Site using the Courier satellite.

Weatherfax A decade after the introduction of radiofax National Weather Service (NWS) began transmitting weather maps using the radiofax technology. The NWS named this new service weatherfax (portmanteau word from the words "weather facsimile") The cover of the regular NOAA publication on frequencies and schedules states "Worldwide Marine Radiofacsimile Broadcast Schedules". Facsimile machines were used in the 1950s to transmit weather charts across the United States via land-lines first and then internationally via HF radio. Radio transmission of weather charts provides an enormous amount of flexibility to marine and aviation users for they now have the latest weather information and forecasts at their fingertips to use in the planning of voyages. Radiofax relies on facsimile technology where printed information is scanned line by line and encoded into an electrical signal which can then be transmitted via physical line or radio waves to remote locations. Since the amount of information transmitted per unit time is directly proportional to the bandwidth available, then the speed at which a weather chart can be transmitted will vary depending on the quality of the media used for transmission. Today radiofax data is available via FTP downloads from sites in the Internet such as the ones hosted by the National Oceanic and Atmospheric Administration (NOAA). Radiofax transmissions are also broadcast by NOAA from multiple sites in the country at regular daily schedules. Radio weatherfax transmissions are particularly useful to shipping, where there are limited facilities for accessing the Internet. The term weatherfax was coined after the technology that allows the transmission and reception of weather charts (surface analysis, forecasts, and others) from a transmission site (usually the meteorological office) to a remote site (where the actual users are).

Newspaper fax

Radiofax may also be used to transmit pages of newspapers. Stations like JJC use this way of transmitting news by using radio facsimile technology.

Transmission details Radiofax is transmitted in single sideband which is a refinement of amplitude modulation. The signal shifts up or down a given amount to designate white or black pixels. A deviation less than that for a white or black pixel is taken to be a shade of grey. With correct tuning (1.9 kHz below the assigned frequency for USB, above for LSB), the signal shares some characteristics with SSTV, with black at 1.5 kHz and peak white at 2.3 kHz. Usually, 120 lines per minute (LPM) are sent (For monochrome fax, possible values are: 60, 90, 100, 120, 180, 240. For colour fax, LPM can be: 120, 240). A value known as the index of cooperation (IOC) must also be known to decode a radio fax transmission - this governs the image resolution, and derives from early radio fax machines which used drum readers, and is the product of the total line length and the number of lines per unit length (known sometimes as the factor of cooperation), divided by π. Usually the IOC is 576.

Automatic Picture Transmission format (APT)

APT format permits unattended monitoring of services. It is employed by most terrestrial weather facsimile stations as well as geostationary weather satellites.

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