117 lines
6.0 KiB
Plaintext
117 lines
6.0 KiB
Plaintext
Article 14086 in rec.ham-radio:
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From: die@cpoint.UUCP (David I. Emery)
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Newsgroups: rec.ham-radio,sci.electronics
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Subject: More on NightWatch VIP UHF-Mux air/ground circuits
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Keywords: GEP, UHF-MUX, Autovon, Nightwatch, Air Force One, Nationwide wideband
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Message-ID: <2586@cpoint.UUCP>
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Date: 8 Sep 89 08:12:58 GMT
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Reply-To: die@cpoint.UUCP (David I. Emery)
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Distribution: usa
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Organization: Clearpoint Research Corp., Hopkinton Mass.
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Lines: 103
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Some more information on the UHF FM-FDM-SSB system used to link the
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flying command post aircraft and ground communications facilities follows.
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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.
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The airborne radio equipment ( made by ECI), transmits
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a 1kw (or two selectable lower powers) signal to blade antennas on the
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aircraft which are more or less omnidirectional. The ground sites use
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a phased array of four UHF broadband traveling wave antennas arranged in a
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square to give some gain and jamming rejection. The signals are vertically
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polarized.
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The modulation is FM-FDM-SSB with up to 14 voice channels and
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a baseband order wire. Unlike most FDM systems, the channels are
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paired with a lower sideband 4 khz voice channel sharing a carrier
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frequency with an upper sideband voice channel immediately above it.
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The carrier frequencies are used are 8, 16, 24, 32, 40, 48 and 56 khz.
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A 0 TLP tone on a channel is supposed to deviate the transmitter about
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30 some odd khz (although the actual level on real signals appears to be less
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than this).
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The aircraft and ground multiplex equipment use the old fashioned
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2600 hz sf signalling for supervision, this means that idle channels carry
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a -17 db TLP 2600 hz tone which drops when the channel is seized. Dialing
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is DTMF using the Autovon standard phone tones. Since the circuits terminate
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in Autovon switches, they are fully four wire.
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The aircraft transmits a 120 khz pilot tone, ground sites use
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a lower frequency pilot.
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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
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of the command post aircraft are equiped to relay several links at once
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and relay operation is quite often tested both to communicate with
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other command post aircraft and with VIP aircraft such as Air Force One.
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The relay aircraft (particularly the command post aircraft) have manual
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switchboards that allow selected channels from an incoming signal to be cross
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connected to different channels on one or more outgoing signals. VIP
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communications circuits are thus often routed on certain channels of
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command post links that carry military traffic on other channels.
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The frequency from 0-4 khz on the signals is used for a orderwire.
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The radio equipment has appropriate provisions for conference bridging
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on this circuit so the order wire at any point has most all of the
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stations on it. [The order wire on this system has been code named
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ADVENT for at least twenty years.] The order wire is used to coordinate
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circuit switchover between ground stations and/or relay aircraft and to
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coordinate circuit test and maintainence.
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There are a number of Ground Entry Point sites scattered throughout
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the US used with this system, most seem to be at hardened blast resistant
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AT&T microwave sites with deep underground bunkers on springs. These sites are
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probably used because they represent points where hardened underground
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cables connect to each other and radio systems. The ground entry
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sites can be recognized by the distinctive square pattern formed by 4 vertical
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pole antennas (quite long and thick, unlike most UHF antennas) on top of
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an AT&T microwave tower (above the microwave horns) spaced about 8-12
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feet apart. A random sample of sites I am aware of includes Green Hill Rhode
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Island, Waldorf Md, Hillsboro Mo, and Pensuco Florida. There are several
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more.
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Maximum range from an aircraft to a ground site is typically 210-230
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miles depending on altitude, air to air range is closer to 400 miles. With
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powers of 1 kw or more EIRP, the signals are very strong on the ground when
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the aircraft is closer. The high power is supposed to be intended to ensure
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that communications can penatrate nuclear fireballs and other propagation
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disruptions during a nuclear attack.
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The system is used to carry clear voice traffic (including traffic
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from Air Force One), and also various forms of digital transmission of
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a sort that fits in a 4 khz voice channel. This includes slow speed 75 baud
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clear and encrypted tty, 1200/2400 baud data, and full duplex 9600 baud
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data, fax and secure voice.
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The VIP aircraft using the system use only 4 Autovon circuits per
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signal and only use 16 and 24 khz channels from the aircraft (but the ground
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sites 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
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old technology. Much of the information in this article is based on
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material published 20 years ago that I dug up as a college age hacker in that
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era. The signals still seem to be on the air however. I understand that
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Milstar and other sophisticated, secure systems will substantially replace
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these aging links in the near future, both for Air Force One telephone traffic
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and nuclear post attack communications, so there can be little about this
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subject that is truly inappropriate to discuss.
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For those curious to look at the signals, the current VIP frequencies
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are :
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Aircaft Ground
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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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--
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David I. Emery Clearpoint Research Corp.
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35 Parkwood Dr, Hopkinton Ma. 01748 1-508-435-7462
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