This is a good explanation of what your are hearing. I found this nice write up in a google search from carmachicago milair PDF.
Northstar C3 Wideband Multiplexed System:
The system onboard the E-6B TACAMO, Air Force 1, The E4B NAOC and other aircraft that provides
connectivity into the Northstar network via Ground Entry Points (GEP) is the UHF frequency division
multiplexed (FDM) command, control, and communications (C3) system. Both commercial and military
networks can be accessed. The onboard components consist of three UHF AN/ARC-171-1H transceivers,
five UHF antennas, three MD-1172 modems, three multiplexers, and associated hardware. Currently, the
equipment provides three of any combination of the following four services:
- UHF FDM (up to three full-duplex groups of up to15 channels each)
- Airborne Launch Control System (ALCS) data links
- Conventional UHF AM line-of-sight (LOS) (up to three half-duplex channels)
- UHF SATCOM phase shift keying (PSK) receive-only channel (TACAMO broadcast)
A UHF LOS air-to-ground configuration is used in conjunction with the ALCS for communications to
intercontinental ballistic missiles. In peacetime, the ALCS is used only during exercises in certain
geographic regions within LOS of missile silos. The 75 baud UHF SATCOM data (TACAMO broadcast) is
simultaneously broadcast on HF and can be received via that medium. Therefore, much of the time, the
third ARC-171 is available for other missions or functions. Nominally, at 25,000 feet, connectivity is
provided at an aircraft distance of up to 180 nmi from a GEP. GEP stations are usually located at elevated
sites for maximum unobstructed view. Towers are used to elevate antennas in flat areas. Separate transmit
and receive omnidirectional antennas are used. All sites are terrestrially connected via AT&T T-1 lines.
The current Air-to-GEP interface is a full duplex UHF FDM link that provides up to 1000 Watts of power
and 12 full duplex voice channels per FDM group. GEP sites have at least two radios and two compatible
multiplexers. The channels are frequency multiplexed into a 60 kHz baseband group consisting of twelve 3
kHz full duplex voice channels and three orderwire channels plus guard bands and then frequency
modulated for transmission on a UHF channel.
Principal sites at Waldorf, MD, Omaha, NE and Lamar, CO are connected via multiple T-1 lines. The
primary ground control site for the entire network is the Special Government Operations Center (SGOC) at
Waldorf, MD. The secondary ground control site is at Offutt AFB, NE
