MODERN TRENDS AND TECHNIQUES IN HF RADIO COMMUNICATION
In times of increased necessity for international crisis management, secure and protected Radio communications especially at sea are more important than ever before. Mobile Naval forces require reliable, jam resistant and secure voice and data connections. Standardized communication media, such as email, internet via radio, has long been part of today’s naval communication and is now indispensable.
Even today, entire frequency spectrum of VLF, LF, HF, VHF, and UHF, as well as UHF, SHF and soon also EHF SATCOM is used to set up communication with units and forces at sea. Which of these transmission media is selected depends mainly on the availability of the information channel in terms of coverage range, transmission speed and, last but not the least, required data rate. However, the decisive factor is, and will continue to be, the geographical distance to the receiving station. We can distinguish between three such distances as line of sight (LOS), extended line of sight (ELOS), up to approximately 300 NM and beyond line of sight (BLOS) for coverage beyond 300 NM. Two viable options available for communication beyond line of sight and worldwide are following:
• High Frequency Communication (2 to 30 MHz)
• Satellite Communication
The key benefit of HF Radio is that it provides communication over very long distances (worldwide with a suitable aerial). It achieves this with "sky wave" communication, where the HF signal is reflected off the ionosphere (illustrated above) for long distance communication, or using Near Vertical Incidence Skywave (NVIS) for shorter distances. This benefit comes with a number of drawbacks: • Slow speed. Rates vary from 75 to 12,800 bits per second, with 1,200 bits per second typical. This is insufficient to support many desirable applications. • Cannot send and receive at the same time.
• The equipment (radios, modems, batteries, and antenna) is large and heavy. A small system with a whip antenna will weigh several kilograms. • Transmission can suffer from noise and errors.
Transmissions at radio frequencies higher than HF (VHF, UHF, EHF, SHF) overcome all of the drawbacks of HF, and can provide high bandwidth data communications enabling key communication capabilities. The main restriction of all of these frequencies is that they are limited to 'line of sight'. For some technologies, distance more than line of sight can be achieved by use of a slightly curved communications path. This gives a little extra distance, but distance is still a key constraint. SATELLITE
Satellite communication is becoming the preferred choice for long distance communication, offering relatively high bandwidth. However, there are a number of drawbacks: • Area coverage of a given satellite or satellite system is often constrained, and may not provide what is needed. • Satellites are expensive.
• Military satellite terminals are quite a bit larger and heavier than HF radios. Some commercial terminals are smaller • Satellite will not work in all terrain - for example it is not suitable in jungle. • National control may be an issue for countries that cannot afford their own satellite systems, and need to rely on commercial or foreign systems. • Satellite ground stations are subject to jamming and other threats. • The satellite is vulnerable single point of failure. China's January 2007 'demonstration' of it's anti-satellite capabilities is clear evidence of their vulnerability. Satellites are also potentially vulnerable to attack by laser or EMP (electromagnetic pulse). • Many countries are not capable of maintenance, fault identification and rectification of satellite space segments Keeping in mind the aforesaid for the military in many nations, if not in all nations, HF is a...
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