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EHF - Extremely High Frequency

In the DoD, the 30/20 GHz bands are referred to collectively as "Ka-band."

Use of extremely high frequency (EHF) and other advanced techniques enable satellite communications systems to achieve a high degree of survivability under both electronic warfare and physical attack.(1) Unlike systems dependent on lower frequencies, EHF satellite communications recover quickly from the scintillation caused by a high-altitude nuclear detonation. The use of higher frequencies offers a number of advantages -- assurance of reliable communications in a nuclear environment, minimal susceptibility to enemy jamming and eavesdropping, and the ability to achieve smaller secure beams with modest-sized antennas.

Besides the obvious benefit of smaller terminal equipment for already overcrowded topsides, K/Ka-band SATCOM is also attractive from an electromagnetic interference (EMI) standpoint in the maritime environment, particularly aboard military ships. The 30/20 GHz bands are well above the bands used by current maritime radar systems. The same cannot be said about the C- and X-bands. Today, naval C- and X-band shipboard SATCOM terminals require supplemental EMI rejection filters to allow them to operate in a battle group environment, especially in close proximity with Aegis surface combatants. Another warship concern for which K/Ka-band SATCOM can provide benefit is reduced radar cross section (RCS). It is not the case that navies expect to make their warships entirely invisible to radar. Rather the elimination of highly reflective hot spots in a warship's topside is of paramount importance in the face of threats from increasingly sophisticated sea-skimming anti-ship cruise missiles. The smaller a topside SATCOM antenna system is, the less it will contribute to a ship's RCS.

An obvious benefit of K/Ka-band SATCOM for all users, commercial and military alike is the additional bandwidth. The C- and X-bands each have 500 MHz of bandwidth and they are already crowded with users. The government and non-government K/Ka-band allocations form a contiguous 3.5 GHz band. The entire government K/Ka-band allocation (30.0-31.0 GHz Earth-to-space and 20.2-21.2 GHz space-to-Earth) is designated for fixed and mobile SATCOM services (FSS and MSS) on a co-primary basis. Portions of the non-government K/Ka-bands are also allocated for MSS. In the space-to-Earth band (17.7-20.2 GHz) a 500 MHz segment (19.7-20.2 GHz) has been allocated for FSS and MSS on a co-primary basis. However, in the Earth-to-space band, only a 100 MHz segment (29.9-30.0 GHz) has been allocated for MSS uplinks on a co-primary basis with FSS uplinks. In the 400 MHz segment from 29.5 to 29.9 GHz, MSS uplinks are allowed, but they have secondary status whereas FSS has primary status. See [18] for further discussion of this MSS/FSS allocation issue.

It should be recognized that the ability to support higher data rates with smaller shipboard terminals comes at the expense of the need to point spot beams. For instance, while it is necessary to use a large shipboard antenna at C-band, it is not necessary to schedule the movements of C-band spot beams. On the other hand, narrow spot beams allow frequency re-use that is not possible with Earth coverage beams.

The Military, Strategic, and Tactical Relay Satellite (Milstar) is a joint service satellite communications system that will provide secure, jam resistant worldwide communications to meet essential wartime requirements for high priority military users. The multi-satellite constellation will link command authorities with a wide variety of resources, including ships, submarines, aircraft and ground stations. Milstar will be the first major space-based communications effort using EHF technology (30-300 gigahertz) to overcome crowding and interference in other frequencies.(3)

The system will use a variety of new technologies, onboard signal processing, adaptive antennas, uplink nulling, steerable downlinks, and cross-links to provide satellite-to-satellite inter-connectivity.(4)

Milstar will be the first defense communication satellite system to use frequency hopping on the uplink to frustrate enemy eavesdropping and jamming. Additional protection against jammers will be obtained by using a phased-array antenna on the satellite that can minimize sensitivity in the direction of a jamming signal.

The Milstar system is composed of three segments: space (the satellites), terminal (the users) and mission control. Air Force Materiel Command's Space and Missile Systems Center at Los Angeles Air Force Base, CA, is responsible for development and acquisition of the Milstar space and mission control segments. The Electronics Systems Center at Hanscom AFB, MA, is responsible for the Air Force portion of the terminal segment development and acquisition.

The Milstar space segment will serve priority users in all the services through a variety of ground terminals. Although each service manages a program to develop terminals suited to its unique operational needs, channelization and standardized signal formats will ensure system integrity and control. Two requirements for these terminals are mobility and compatibility. Rapid movement of communication terminals to the operational area, rapid setup, and quick circuit configuration are essential for timely support of the initial stages of deployment.

Each Milstar satellite serves as a smart switchboard in space by directing traffic from terminal to terminal anywhere on the Earth. It establishes communications networks for users. Satellite-to-satellite cross-link capability will assure global coverage. The cross-link network will route the appropriate communication traffic from terminals in view of one satellite to another terminal located at other parts of the world not covered by that satellite's field of view. The cross-link capability will provide near-real-time connectivity without extensive relay and circuit patching.

Since the satellite actually processes the communications signal and can link with other satellites through crosslinks, the requirement for ground controlled switching is significantly reduced. The satellite establishes, maintains, reconfigures and disassembles required communications circuits. Milstar terminals will provide encrypted voice, data, teletype, or facsimile communications. A key goal of Milstar is to support interoperability between users of Army, Navy, and Air Force Milstar terminals. Geographically dispersed mobile and fixed control stations provide survivable and enduring operational command and control for the Milstar constellation.


1. Adapted from: Maj Michael J. Muolo, Maj Richard A. Hand, Maj Bonnie Houchen and Maj Lou Larson, Space Handbook A War Fighter's Guide to Space -- Volume One, AU-18, Air University Air Command and Staff College, (Air University Press, Maxwell Air Force Base, Alabama, December 1993).

2. Philip J. Klass, "First Milstar Satellite to Undergo Final Integration Tests in 1990," Aviation Week & Space Technology, 3 April 1989, page 61.

3. Kostas Liopiros and Edward Lam, "Extremely High Frequency Satellites Offer Flexibility," Signal, vol. 44, no. 11, July 1990, page 77.

4. James W. Rawles, "Milstar Fights for Survival," Defense Electronics, vol. 22, no. 3, March 1990, page 51.

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