The operational bioinstrumentation system provides an amplified electrocardiograph analog signal from either of two designated flight crew members to the orbiter avionics system, where it is converted to digital tape and transmitted to the ground in real time or stored on tape for dump at a later time. The designated flight crew members wear the OBS during the ascent and entry phases. On-orbit use will be limited to contingency situations.

The OBS electrodes are attached to the skin with electrode paste to establish electrical contact. The electrode is composed of a plastic housing containing a non-polarizable pressed pellet. The housing is attached to the skin with double-sided adhesive tape and the pellet contacts the skin. There are three electrodes on the harness marked LC (lower chest), UC (upper chest) and G (ground).

The ECG signal conditioner is a hybrid microcircuit with variable gain (adjusted for each crew member before flight). It provides a zero- to 5-volt output and has an on/off switch within the input plug, which is actuated when the intravehicular activity biomed cable is plugged in. The unit has batteries that will not be replaced in flight.

The IVA biomed cable connects to the signal conditioner and is routed under the IVA clothing to connect to the biomed seat cable. The biomed seat cable is routed to one of the biomed input connectors located on panel A11, A15 or M062M. Rotary control switches on panel R10 provide circuits from the biomed outlets to the orbiter's network signal processor for downlink or recording. The two rotary switches on panel R10 are biomed chan nel 1 and channel 2 . Extravehicular activity positions provide circuits for the EVA UHF transceiver.

The electrode application kit contains components to aid in the application of electrodes. The components include wet wipes, double-sided adhesive tape, overtapes, electrode paste and a cue card illustrating electrode placement.