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Article 14086 in rec.ham-radio:
From: die@cpoint.UUCP (David I. Emery)
Newsgroups: rec.ham-radio,sci.electronics
Subject: More on NightWatch VIP UHF-Mux air/ground circuits
Keywords: GEP, UHF-MUX, Autovon, Nightwatch, Air Force One, Nationwide wideband
Message-ID: <2586@cpoint.UUCP>
Date: 8 Sep 89 08:12:58 GMT
Reply-To: die@cpoint.UUCP (David I. Emery)
Distribution: usa
Organization: Clearpoint Research Corp., Hopkinton Mass.
Lines: 103
Some more information on the UHF FM-FDM-SSB system used to link the
flying command post aircraft and ground communications facilities follows.
This system is also used to provide full duplex telephone grade voice circuits
for the VIP aircraft such as Air Force One.
The airborne radio equipment ( made by ECI), transmits
a 1kw (or two selectable lower powers) signal to blade antennas on the
aircraft which are more or less omnidirectional. The ground sites use
a phased array of four UHF broadband traveling wave antennas arranged in a
square to give some gain and jamming rejection. The signals are vertically
polarized.
The modulation is FM-FDM-SSB with up to 14 voice channels and
a baseband order wire. Unlike most FDM systems, the channels are
paired with a lower sideband 4 khz voice channel sharing a carrier
frequency with an upper sideband voice channel immediately above it.
The carrier frequencies are used are 8, 16, 24, 32, 40, 48 and 56 khz.
A 0 TLP tone on a channel is supposed to deviate the transmitter about
30 some odd khz (although the actual level on real signals appears to be less
than this).
The aircraft and ground multiplex equipment use the old fashioned
2600 hz sf signalling for supervision, this means that idle channels carry
a -17 db TLP 2600 hz tone which drops when the channel is seized. Dialing
is DTMF using the Autovon standard phone tones. Since the circuits terminate
in Autovon switches, they are fully four wire.
The aircraft transmits a 120 khz pilot tone, ground sites use
a lower frequency pilot.
There is considerable provision for air to air relay of communications
and use of relay aircraft is an organic part of the system design. Many
of the command post aircraft are equiped to relay several links at once
and relay operation is quite often tested both to communicate with
other command post aircraft and with VIP aircraft such as Air Force One.
The relay aircraft (particularly the command post aircraft) have manual
switchboards that allow selected channels from an incoming signal to be cross
connected to different channels on one or more outgoing signals. VIP
communications circuits are thus often routed on certain channels of
command post links that carry military traffic on other channels.
The frequency from 0-4 khz on the signals is used for a orderwire.
The radio equipment has appropriate provisions for conference bridging
on this circuit so the order wire at any point has most all of the
stations on it. [The order wire on this system has been code named
ADVENT for at least twenty years.] The order wire is used to coordinate
circuit switchover between ground stations and/or relay aircraft and to
coordinate circuit test and maintainence.
There are a number of Ground Entry Point sites scattered throughout
the US used with this system, most seem to be at hardened blast resistant
AT&T microwave sites with deep underground bunkers on springs. These sites are
probably used because they represent points where hardened underground
cables connect to each other and radio systems. The ground entry
sites can be recognized by the distinctive square pattern formed by 4 vertical
pole antennas (quite long and thick, unlike most UHF antennas) on top of
an AT&T microwave tower (above the microwave horns) spaced about 8-12
feet apart. A random sample of sites I am aware of includes Green Hill Rhode
Island, Waldorf Md, Hillsboro Mo, and Pensuco Florida. There are several
more.
Maximum range from an aircraft to a ground site is typically 210-230
miles depending on altitude, air to air range is closer to 400 miles. With
powers of 1 kw or more EIRP, the signals are very strong on the ground when
the aircraft is closer. The high power is supposed to be intended to ensure
that communications can penatrate nuclear fireballs and other propagation
disruptions during a nuclear attack.
The system is used to carry clear voice traffic (including traffic
from Air Force One), and also various forms of digital transmission of
a sort that fits in a 4 khz voice channel. This includes slow speed 75 baud
clear and encrypted tty, 1200/2400 baud data, and full duplex 9600 baud
data, fax and secure voice.
The VIP aircraft using the system use only 4 Autovon circuits per
signal and only use 16 and 24 khz channels from the aircraft (but the ground
sites transmit a signal with sf tones or traffic on all the upper 12 channels).
The command post aircraft usually use all 14 channels.
The system has been in use since about 1965, and as such it is very
old technology. Much of the information in this article is based on
material published 20 years ago that I dug up as a college age hacker in that
era. The signals still seem to be on the air however. I understand that
Milstar and other sophisticated, secure systems will substantially replace
these aging links in the near future, both for Air Force One telephone traffic
and nuclear post attack communications, so there can be little about this
subject that is truly inappropriate to discuss.
For those curious to look at the signals, the current VIP frequencies
are :
Aircaft Ground
RF Channel 1 382.35 mhz 326.00 mhz
RF Channel 2 305.55 mhz 246.95 mhz
RF Channel 3 336.80 mhz 344.00 mhz
RF Channel 4 * 322.75 mhz 366.00 mhz
RF Channel 5 397.05 mhz 390.00 mhz
* Air Force One most often uses RF-4.
--
David I. Emery Clearpoint Research Corp.
35 Parkwood Dr, Hopkinton Ma. 01748 1-508-435-7462
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