Applications Of Hf Radio Communication

HF is probably the most economic means of low data rate transmission over long distances (e.g., > 200 mi). It might be argued that meteor burst communication is yet more economic in some circumstances. Performance makes this difference. As we will show in Chapter 13, meteor burst transmission links have the disadvantage of waiting time between short data packets; HF does not.

Traditionally, since the 1930s, HF has been the mainstay of ship-shore-ship communication. Satellite communication offered by INMARSAT [International Marine Satellite (consortium)] certainly provides more reliable ser-

Radio System Design for Telecommunications, Third Edition By Roger L. Freeman Copyright © 2007 John Wiley & Sons, Inc.

vice, but HF continues to be the only long-distance communication means for many vessels. We will not argue rationale one way or the other.

Ship - shore HF communication service includes the following:

• CW or continuous wave, which connotes keying a RF carrier on and off, forming the dots and dashes of the international Morse code

• Selective calling teleprinter service (ITU-R M.493-11, Ref. 2) using frequency shift keying (FSK) with a wide frequency shift

• Advanced automatic digital message service using ALE (automatic link establishment), with near real-time sounding, forward error correction

• Single sideband (SSB) voice telephony, often using improved Lincom-pex; such HF connectivities frequently access the PSTN (public switched telecommunication network)

• Simplex teleprinter service with narrow and wide shift FSK as used by many of the world's navies such as the U.S. Navy's Fox broadcast, merchant marine broadcast (MERCAST), and Hydro broadcasts (Refs. 3 and 4)

HF is widely employed for propaganda broadcast such as the U.S. Voice of America and BBC, in Cuba, Russia, and many, many other countries. It is also used for religious broadcasting such as the large HCJB facility in Quito, Ecuador. Many of these modern HF broadcast installations are very large and expensive and boast of megawatts of EIRP (effective isotropic radiated power).

The armed forces of the world make wide use of HF as a primary means of connectivity, or as an essential backup capability. In this case it can be used for voice, vocoded secure voice, record traffic, data, and facsimile. It can even be used for freeze-frame video.

HF also can provide very inexpensive point-to-point teleprinter/data service: simplex, half-duplex, full-duplex; four-channel time division multiplex (TDM); and 16-channel narrowband FDM using voice frequency carrier telegraph (VFCT) techniques.

Weather maps to ships and other entities are broadcast on HF by facsimile transmission using narrowband frequency modulation. It is also used for the transmission of time signals such as WWV on 5, 10, 15, and 20 MHz from Ft. Collins, Colorado (U.S.A.). There are numerous other HF time emitters around the world. Of course, these are eclipsed by surface and satellite systems such as Loran C (100 kHz), OMEGA (10 kHz), and GPS and GOES satellite (L-band) systems.

HF has many disadvantages. Some of these are:

• Low information rate. The maximum bandwidth by radio regulation is four 3-kHz independent sideband voice channels in a quasi-frequency division configuration with low data rates (e.g., about 2400 bps per 3-kHz voice channel).

• Degraded link time availability when compared to satellite, fiberoptic, coaxial cable, wire pair, troposcatter, and line-of-sight (LOS) microwave communication. HF link time availabilities vary from 80% up to better than 95% for some new spread spectrum wideband adaptive systems.

• Impairments include dispersion in both the time and frequency domains. Fading is endemic on skywave links. Atmospheric, galactic, and man-made interference noise are among the primary causes of low availability besides the basic propagation phenomenon itself.

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