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Emergency Communications Systems of the U.S. Government
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 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