The UHF system is used as a backup for the S-band PM and Ku-band voice communications primarily during extravehicular activity. For communications with the STDN ground stations, the UHF system operates in a simplex mode, which means that the orbiter flight crew can only transmit or receive, but cannot do both simultaneously. The UHF transceiver takes the voice signal from the audio central control unit and transmits it through the external UHF antenna on the bottom of the orbiter forward fuselage. The incoming UHF signal goes through the external antenna to the UHF transceiver, which sends it to the ACCU for distribution in the orbiter.

UHF transmission is controlled through the UHF mode control knob and the three two-position toggle switches on overhead panel O6 labeled xmit freq, splx pwr ampl and squelch . The xmit freq switch selects one of the two UHF frequencies, 296.8 MHz primary or 259.7 MHz secondary, for external transmission. The splx pwr ampl switch selects the UHF antenna on the external skin of the orbiter's lower forward fuselage or the airlock antenna. The UHF antenna on the lower forward fuselage is covered with reusable thermal protection system. The airlock antenna is used by the EVA astronauts, in extravehicular mobility units, to check out their transceivers before exiting the airlock; it is also used for air-to-air communications during EVA.

The squelch switch permits on or off selection of UHF squelch. A five-position rotary knob on the UHF control panel activates power to the UHF transceiver and selects any of the following modes of UHF transmission. When the knob is positioned to EVA , EVA transmissions are made on one frequency selected by the xmit freq switch, and the message is received on the other frequency. The off position removes all electrical power. When the UHF mode rotary contral knob is positioned to simplex, transmission and reception are both on the frequency selected by the xmit freq switch. Positioned to splx + g rcv, transmission and reception are the same as in simplex except that reception of the UHF guard (emergency) frequency (243.0 MHz) also is possible. In the g t/r position, transmission and reception are both on the UHF guard (emergency) frequency.

Access to transmission and reception of UHF signals is controlled by two-position toggle switches located on the bottom of the audio center panel on panel A1R at the aft station. The switches are labeled t/r for transmission/reception, off for blocking UHF signals to or from the UHF transceiver, a/g for the air-to-ground channel and a/a for air-to-air channel. All three of the UHF frequencies (296.8 MHz, 259.7 MHz and 243.0 MHz) are preset in the UHF transmitter and cannot be altered by the flight crew.

The UHF system is used for EVA operations. The EVA astronaut's UHF communication are through the orbiter UHF airlock antenna. The two existing UHF frequencies of 296.8 MHz and 259.7 MHz are used; an extra UHF of 279.0 MHz is added to the EMU backpack. The 279.0-MHz frequency can transmit or receive only among the two EVA astronauts and the orbiter, not the ground stations.

One EVA astronaut operates in mode A, transmitting data and voice to the orbiter on 259.7 MHz, transmitting voice to the other EVA astronaut on 259.7 MHz, receiving voice from the orbiter on 296.8 MHz and receiving voice from the other EVA astronaut on 279.0 MHz. The other EVA astronaut operates in mode B, transmitting data and voice to the orbiter on 279.0 MHz, transmitting voice to the other EVA astronaut on 279.0 MHz, receiving voice from the orbiter on 296.8 MHz and receiving voice from the other EVA astronaut on 259.7 MHz. The orbiter then communicates through a switch in the orbiter via the UHF EVA relay mode by retransmission over air-to-ground through its S-band system to the STDN ground station, its S-band system to the TDRS or its Ku-band system to the TDRS. As a backup procedure only when the shuttle is over a UHF ground station, the EVA astronauts, orbiter and ground can switch to the 259.7-MHz UHF, simplex. During EVA, the EVA crew members' biomedical data also are transmitted to the airlock antenna and separated from voice signals in the orbiter instrumentation system for transmission to the ground.

The UHF system may be used after entry during the approach and landing phase of the mission. Air-to-ground voice communications take place among the space shuttle, the landing site control tower and chase planes (if used).