

   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	

* Air Force One most often uses RF-4.--        

