93 lines
5.4 KiB
Plaintext
93 lines
5.4 KiB
Plaintext
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Emergency Communications Systems of the U.S. Government
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This system is also used to provide full duplex telephone grade voice circuits
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for the VIP aircraft such as Air Force One. The airborne radio equipment
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( made by ECI), transmits a 1kw (or two selectable lower powers) signal to
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blade antennas on the aircraft which are more or less omnidirectional. The
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ground sites use a phased array of four UHF broadband traveling wave antennas
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arranged in a square to give some gain and jamming rejection. The signals are
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vertically polarized.
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The modulation is FM-FDM-SSB with up to 14 voice channels and a baseband order
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wire. Unlike most FDM systems, the channels are paired with a lower sideband
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4 khz voice channel sharing a carrier frequency with an upper sideband voice
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channel immediately above it. The carrier frequencies used are 8, 16, 24, 32,
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40, 48 and 56 khz. A 0 TLP tone on a channel is supposed to deviate the
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transmitter about 30 some odd khz (although the actual level on real signals
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appears to be less than this).
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The aircraft and ground multiplex equipment use the old fashioned 2600 hz sf
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signalling for supervision, this means that idle channels carry a -17 db TLP
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2600 hz tone which drops when the channel is seized. Dialing is DTMF using
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the Autovon standard phone tones. Since the circuits terminate in Autovon
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switches, they are fully four wire.
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The aircraft transmits a 120 khz pilot tone, ground sites use a lower frequency
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pilot. There is considerable provision for air to air relay of communications
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and use of relay aircraft is an organic part of the system design. Many of the
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command post aircraft are equiped to relay several links at once and relay
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operation is quite often tested both to communicate with other command post
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aircraft and with VIP aircraft such as Air Force One. The relay aircraft
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(particularly the command post aircraft) have manual switchboards that allow
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selected channels from an incoming signal to be cross connected to different
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channels on one or more outgoing signals. VIP communications circuits are
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thus often routed on certain channels of command post links that carry military
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traffic on other channels.
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The frequency from 0-4 khz on the signals is used for a orderwire. The radio
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equipment has appropriate provisions for conference bridging on this circuit
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so the order wire at any point has most all of the stations on it. [The order
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wire on this system has been code named ADVENT for at least twenty years.] The
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order wire is used to coordinate circuit switchover between ground stations
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and/or relay aircraft and to coordinate circuit test and maintainence.
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There are a number of Ground Entry Point sites scattered throughout the US
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used with this system, most seem to be at hardened blast resistant AT&T
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microwave sites with deep underground bunkers on springs. These sites are
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probably used because they represent points where hardened underground cables
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connect to each other and radio systems. The ground entry sites can be
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recognized by the distinctive square pattern formed by 4 vertical pole antennas
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(quite long and thick, unlike most UHF antennas) on top of an AT&T microwave
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tower (above the microwave horns) spaced about 8-12 feet apart. A random sample
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of sites I am aware of includes Green Hill Rhode Island, Waldorf Md, Hillsboro
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Mo, and Pensuco Florida. There are several more.Maximum range from an aircraft
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to a ground site is typically 210-230 miles depending on altitude, air to air
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range is closer to 400 miles. With powers of 1 kw or more EIRP, the signals are
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very strong on the ground when the aircraft is closer. The high power is
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supposed to be intended to ensure that communications can penatrate nuclear
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fireballs and other propagation disruptions during a nuclear attack.
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The system is used to carry clear voice traffic (including traffic from Air
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Force One), and also various forms of digital transmission of a sort that fits
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in a 4 khz voice channel. This includes slow speed 75 baud clear and encrypted
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tty, 1200/2400 baud data, and full duplex 9600 baud data, fax and secure voice.
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The VIP aircraft using the system use only 4 Autovon circuits per signal and
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only use 16 and 24 khz channels from the aircraft (but the ground sites
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transmit a signal with sf tones or traffic on all the upper 12 channels).
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The command post aircraft usually use all 14 channels.
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The system has been in use since about 1965, and as such it is very old
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technology. Much of the information in this article is based on material
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published 20 years ago that I dug up as a college age hacker in that era. The
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signals still seem to be on the air however. I understand that Milstar and
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other sophisticated, secure systems will substantially replace these aging
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links in the near future, both for Air Force One telephone traffic and nuclear
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post attack communications, so there can be little about this subject that is
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truly inappropriate to discuss.
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For those curious to look at the signals, the current VIP frequencies are :
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Aircaft
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Ground RF Channel 1 382.35 mhz 326.00 mhz
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RF Channel 2 305.55 mhz 246.95 mhz
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RF Channel 3 336.80 mhz 344.00 mhz
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RF Channel 4 * 322.75 mhz 366.00 mhz
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RF Channel 5 397.05 mhz 390.00 mhz
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* Air Force One most often uses RF-4.--
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