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4.7 Ultrahigh Frequency Subsystem

The ISS UCS is one of the subsystems of the Space-to-Space Communication System (SSCS) and operates in the UHF frequency range. The other parts of the SSCS are the orbiter and Extravehicular Mobility Unit (EMU) space-to-space UHF Subsystems. The UCS, commonly referred to as the UHF Subsystem or UHF, provides the SSCS link for the ISS.

4.7.1 Purpose

The purpose of the Station UHF Subsystem is to provide for space-to-space communication in and around the ISS when hardline communication is not possible. This space-to-space communication is between the ISS and the orbiter for voice, commands, and telemetry; EVA-suited crewmembers for voice, biomedical, and EMU data; and to accommodate future Free Flyer (FF) payloads for commands and telemetry.

4.7.2 Ultrahigh Frequency Subsystem Operations and Components

The UHF Subsystem is designed to support up to five simultaneous users. It is a digital data system operating on a RF network. It consists of a Space-to-Space Station Radio (SSSR), enclosing two transceivers, two sets of external double antennas, and internal antennas that are found in every USOS habitable module. (See Figure 4-7, UHF Subsystem overview). The UHF Subsystem supports not only traditional EVA functions of voice, EMU, and biomedical data transmission, it also supports the space-to-space transmission of commands and telemetry. This is used during rendezvous and docking operations when the Station must be configured remotely. The orbiter sends commands to the ISS UHF Subsystem, which passes them to the
C&C MDM for execution. The ISS UHF returns only telemetry data. FFs (remotely controlled vehicles) use the UHF Subsystem for approach and docking too. However, with FFs, the ISS sends commands to the FF through the UHF Subsystem and receives telemetry. The European ATV is such a vehicle.

Figure 4-7. UHF Subsystem overview

The UHF Subsystem can be operated by the crew or by flight controllers. Flight controllers can configure the system or run system tests by commands sent to the Station. These commands are sent via the ISS S-band or the shuttle OIU to the C&C MDM and then to the UHF radio. The ground also supports UHF troubleshooting. The crew uses the Portable Computer System (PCS) laptop to command the UHF Subsystem and verify power modes.

4.7.2.1 Space-to-Space Station Radio

The SSSR consists of one ORU, containing two transceivers. Both transceivers are contained in one housing and are serviced by one coldplate; but, each is powered by separate EPS RPCs. Similar to RF systems, the SSSR transceivers (radios) consist of a 1553 module that receives and transmits commands and telemetry from and to the C&C MDM. The SSSR also formats digital audio and multiplexes it with 1553 data and sends it to the internal modem. The modem R-S encodes the data and turns the data into an RF signal that is amplified before it is sent to the antennas for broadcast. Conversely, the SSSR amplifies and demodulates the received signal. The modem decodes and decrypts the data. The signal is then processed to separate the audio data from the commands or EMU data. It sends the commands and EMU data to the C&C MDM over the 1553 bus. It sends the digital audio to the IAS AUAI for distribution internally through the IAS and externally through the same AUAI to the S-band, which transports the voice to the ground. The loss of both transceivers in the ORU results in loss of all UHF Subsystem RF communications for the Station. A fire in one rack could destroy both radios.

4.7.2.2 UHF External Antennas

The UHF external antennas consist of two pairs of antennas mounted on the U. S. Lab Module and truss of the ISS. The antennas are designed to receive signals up to 7 kilometers away. For EVA activity, communication availability provided by these antennas is nearly 100 percent (with
all four antennas functional). If the orbiter is present, the orbiter UHF Subsystem can also communicate with the EVA crew. The IVA crew communicates with the EVA crew via the IAS AUAI and the UHF Subsystem. The loss of an external antenna lowers the ability to transmit and receive in that antenna’s range. Before the second external antenna pair is assembled on
Flight TBD, a loss of one Station external antenna can cause about 30 percent of coverage to be lost.

4.7.2.3 UHF Internal Antennas

The UHF Subsystem also supports the periods before and after EVA operations. Before the EVA crewmembers unplug from the EMU Audio Control Panel (EACP) and open the airlock hatch to egress the Station, the USOS EMU-suited crewmembers can communicate using the Airlock antenna while still in the airlock. They can also communicate with each other in the airlock when they return to the airlock completing the EVA or plug back into the EACP. Voice, biomedical, and EMU communication is thus uninterrupted. If the Airlock antenna is lost, the EVA crew must connect to the EACP to communicate. The UHF Subsystem not only supports EVAs occurring external to the ISS, but also EVAs occurring within a depressurized Station module. The internal antennas consist of an Intravehicular Antenna Assembly (IAA) and the Joint Airlock antenna. The IAA’s antennas are located throughout the U.S. pressurized modules. This capability is necessary to carry out repairs and restore nominal ISS operations. After this second external antenna pair arrives, a loss of one Station external antenna does not cause any coverage to be lost.

4.7.3 UHF Software

In the case of UHF Extended Loss of Communication (ELOC), an auto recovery command is issued by the UHF application software located in the C&C MDM. The crew may also execute manual recovery procedures. The autorecovery command swaps transmission from one transceiver to the other transceiver. This autorecovery routine is initiated by an expiration of a timer.

4.7.4 Comparable Russian Segment Communication Systems

The Russian VHF System is analogous to the U.S. UHF Subsystem. The VHF-2 channel is part of the TTC Subsystem (see IAS subsection) and is used primarily for space-to-space communication. Communication with the EVA cosmonauts is via the VHF-2 channel. The VHF-2 also uses duplex as well as simplex audio communication that occur between the Station and approaching manned vehicles. This voice communications link can also be routed directly to the ground through the VHF-1, Lira or Regul voice channels, or can be recorded to a voice recorder for playback at a more convenient time. The Transit Autonomous Radio System is used for the spacesuit parameters and control during egress into space and consists of the Transit-A System contained in the cosmonauts spacesuits and the Transit-B System in the SM. This system is part of the Onboard Measuring Subsystem.

Table 4-3 shows the expansion of the UHF Subsystem capabilities during assembly of the ISS