The importance of helicopters in the battlefield scenarios is significantly increasing as the new weaponry and guidance control systems allow use of helicopters in a multiplicity of roles, ranging from anti-tank operations to rapid transportation of men and materials. Missions are often assigned to the helicopters that require flying at close distance from the enemy positions. In these cases, low-altitude flying is necessary for survivability and this type of operation, designated as Nap-Of-Earth (NOE) flying, is a well established helicopter tactic for concealment against enemy fire. In the ordinary modes of operation, radio communications between the helicopter and other platforms in the tactical area are supported by the VHF1 (30 to 88 MHz), VHF2 (108 to 172 MHz)m, and UHF (225 to 400 MHz) frequency bands over line-of-sight (LOS) links. The VHF1 band is used by the helicopter to communicate with ground forces during RECCE missions and to interact with other battlefield units for mission management and briefing. The VHF2 band is dedicated to communications with the Base for ATC purposes, while the UHF band is universally used for Ground-Air-Ground and Air-to-Air links in Air Defense operations. As NOE altitudes for helicopters are typically in the order of a few feet, LOS conditions are seldom achieved due to interposed obstacles (such as vegetation or a hilly landscape) or simply because at NOE altitudes the distance between the communicators may well exceed the LOS range. In this scenario precluding effective use of tactical VHF, helicopter to ground and helicopter to helicopter links can be reliably established over battlefield distances by use of the HF propagation mode known as Near-Vertical - Incidence-Skywave (NVIS), whereby an RF signal with a high angle of incidence (greater than 80 degrees) is reflected by the ionosphere back to earth producing an umbrella-type coverage. By appropriate selection of the operating frequency in 1 to 2 MHz passbands (windows) located in the 2 to 10 MHz range, NVIS provides gapless coverage over distances up to 300 km.


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    Helicopter HF Communications Using the NVIS Mode

    Proia, M. / Advisory Group for Aerospace Research and Development / Mission Systems Panel | British Library Conference Proceedings | 1995


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