Voice Loops as Coordination Aids in Space Shuttle Mission Control.
Computer Supported Cooperative Work 8: 353–371, 1999.
© 1999 Kluwer Academic Publishers. Printed in the Netherlands.
Voice Loops as Coordination Aids in Space Shuttle
EMILY S. PATTERSON, JENNIFER WATTS-PEROTTI∗and DAVID D.
Cognitive Systems Engineering Laboratory, Institute for Ergonomics, The Ohio State University
(∗Now at Eastman Kodak Company, Rochester, NY)
(Received December 2 1998)
Abstract. Voice loops, an auditory groupware technology, are essential coordination support tools
for experienced practitioners in domains such as air traffic management, aircraft carrier operations
and space shuttle mission control. They support synchronous communication on multiple channels
among groups of people who are spatially distributed. In this paper, we suggest reasons for why
the voice loop system is a successful medium for supporting coordination in space shuttle mission
control based on over 130 hours of direct observation. Voice loops allow practitioners to listen in
on relevant communications without disrupting their own activities or the activities of others. In
addition, the voice loop system is structured around the mission control organization, and therefore
directly supports the demands of the domain. By understanding how voice loops meet the particular
demands of the mission control environment, insight can be gained for the design of groupware tools
to support cooperative activity in other event-driven domains.
Key words: attention, broadcasting, common ground, coordination, ethnographic study, mission
control, mutual awareness, overhearing, voice loops
In supervisory control, cognitive activities such as monitoring and anomaly
response are often distributed across interdependent sets of practitioners. As
Hughes, Randall, and Shapiro (1992) and others have noted, practitioners in these
domains must be able to coordinate their efforts on a “moment to moment basis,
in response to constantly changing circumstances.” Voice loops, a groupware tech-
nology which allows synchronous communication among groups of people who
are spatially distributed, are used to aid coordination in domains such as air traffic
management, aircraft carrier operations (Rochlin, LaPorte and Roberts, 1987) and
space shuttle mission control.
Voice loops are essential coordination support tools for experienced practi-
tioners in space shuttle mission control. Controllers use the voice loops to directly
communicate with other personnel in mission control. More importantly, however,
controllers use the voice loops to remain aware of the activities of other controllers
and mission events in related shuttle subsystems. Controllers continuously monitor
EMILY S. PATTERSON ET AL.
approximately four voice loops while directly communicating on a primary loop.
By being aware of events when they occur, they can synchronize their activities
with other controllers and with the actions of the astronauts. If something that
is reported on the loops does not match their expectations, they can direct their
attention to that thread of conversation and investigate what the deviations are and
how their own activities might be impacted.
This paper analyzes how voice loops facilitate the coordination of physically
distributed practitioners in space shuttle mission control based on direct observa-
tions of voice loops in use. We describe how mission control is structured and
how the different types of voice loops reflect the organizational structure. Then we
outline the coordination functions that voice loops support and illustrate these func-
tions in an example. We conclude with a discussion of how this analysis provides
insight into the important functions that should be considered in the development
of systems intended to support cooperative work in other event-driven domains.
Our findings are based on direct observations of flight controllers using voice
loops to support their activities during space shuttle operations (see Figure 3
for an excerpt of observed voice loop communications). The observations were
conducted at the Maintenance Mechanical Arm and Crew Systems (MMACS),
Mechanical (Mech), Payloads Officer (Payloads) and Remote Manipulator System
(RMS) flight control consoles. Over 130 hours of observation were conducted
during portions of four actual missions and 27 flight control simulations. Flight
control simulations include a full complement of astronauts and flight controllers
supporting each flight control console. The high-fidelity simulations are used to
train the controllers to respond to unexpected problems.
In addition to these observations, we interviewed controllers during low-tempo
periods in the simulations and missions about how they use voice loops to support
their activities. Controllers described formal and informal protocols that govern
the usage of voice loops in mission control, which loops they monitor, why they
monitor them, and how and when they are expected to speak on the loops. In
addition, we interviewed the personnel who manage the voice loop system to learn
how the specific voice loop assignments are made for each mission, how the loops
are managed during flight operations, and which loops are permanently archived.
3. Cooperative structure of space shuttle mission control
3.1. HIERARCHICAL SUPERVISORY CONTROL STRUCTURE
The voice loop system maps onto the supervisory control structure of mission
control. NASA’s Mission Control Center (MCC) at the Johnson Space Center in
Houston, Texas is responsible for managing space shuttle missions from take-
off to touchdown. During missions, teams of flight controllers monitor spacecraft
SPACE SHUTTLE MISSION CONTROL
Figure 1. The front room in the Mission Control Center.
systems and activities 24 hours a day, 7 days a week. The head flight controller
is the flight director, referred to as “Flight.” Flight is ultimately responsible for all
decisions related to shuttle operations and so must make decisions that trade off
mission goals and safety risks for the various subsystems of the shuttle. Directly
supporting the flight director is a team of approximately sixteen flight controllers
who are co-located in a single location called the “front room” (Figure 1). These
flight controllers have the primary responsibility for monitoring the health and
safety of shuttle functions and subsystems. For example, one controller is respon-
sible for the electrical subsystems (EGIL) and another for the payload systems
(Payloads). These controllers must have a deep knowledge of their own systems as
well as know how their systems are interconnected to other subsystems (e.g. their
heater is powered by a particular electrical bus) in order to recognize and respond
to anomalies despite noisy data and needing to coordinate with other controllers.
Each of the flight controllers located in the front room has a support staff that is
located in “back rooms.” The front room and back room controllers communicate
with each other through the voice loop system by activating a voice loop channel
through a touch screen and talking into a headset. The back room support staff are
more specialized than the front room controllers on specific shuttle subsystems and
monitor more detailed information sources. For example, the front room controller
responsible for the Maintenance Mechanical Arm and Crew Systems (MMACS)
has supporting staff members who specialize in: (1) the mechanical systems (Mech
EMILY S. PATTERSON ET AL.
I and II), (2) the photo equipment used by the astronauts (Photo/TV), (3) the escape
hatch that would be used in the event of an aborted take-off (Escape), and (4)
in-flight maintenance for the tools used by the astronauts (IFM).
3.2. EXAMPLE OF COORDINATION ACROSS FLIGHT CONTROLLERS
To illustrate how controllers with different scopes of responsibilities coordinate
their activities, consider the case of the unexpectedly high pressure in an Auxiliary
Power Unit’s (APU’s) fuel pump during the STS-62 mission. Fuel pumps are used
to keep mechanical systems warm in the freezing environment of space. During
orbit, the front room mechanical systems controller (MMACS) and his support
staff (Mech) noticed that pressure cycles in an Auxiliary Power Unit’s (APU’s) fuel
pump inlet pressure were higher than expected. The MMACS controller updated
the flight director about the abnormal readings and requested that the crew respond
to the anomaly by opening the fuel isolation valves to equalize the pressure in
the fuel lines and the fuel tank. The flight director approved the request and then
described the plan to the front room communications controller (CAPCOM) who
then relayed the requested action to the astronauts.
The results from this intervention as well as four subsequent interventions were
unexpected and contradictory. Over several days, the MMACS team coordinated
with other front room controllers and various engineers who designed relevant
shuttle subsystems to generate nineteen competing hypotheses as possible explan-
ations for the anomaly. The selected best explanation, that water in the fuel line had
frozen and then thawed, ended up only minimally disrupting mission plans. Other
competing explanations, such as a hydrazine fuel leak that would start a fire upon
re-entry into the atmosphere, would have had many more implications for other
It is clear in this example that coordination was essential to diagnosing the
fault and creating a response plan. The voice loops were important in support-
ing this coordination. The MMACS and Mech controllers were able to work
through detailed diagnoses without moving to the same physical location. The
sequence of communications, from the MMACS controller to the flight director
to the CAPCOM controller to the astronauts, were less error-prone on the public
voice loops system than if the communications had been private because any of
the mission controllers could have intervened if there were miscommunications.
Other subsystem controllers were able to anticipate questions that they might be
asked by listening in on these communications. The MMACS controller did not
have to explicitly update controllers with inter-connected subsystems because the
other controllers were listening to their public updates to the flight director. In addi-
tion, the MMACS controllers were able to listen for unexpected changes in other
controllers’ sub-systems that might provide new information for their ongoing
SPACE SHUTTLE MISSION CONTROL
4. The voice loop system
4.1. VOICE LOOP INTERFACE AND CONTROLS
As previously stated, a voice loop is a real-time auditory channel that connects
physically distributed people. A controller who speaks on a loop broadcasts to all
controllers who are listening in on that loop. A controller monitoring a loop hears
any communication among other controllers connected to that loop.
Multiple voice loops can be monitored at the same time. The multiple loops
require a mechanism for controllers to select and modify which loops they are
monitoring and which they can speak on. The interface is a map of the available
loops. Any controller can choose to monitor any of the available communication
channels by directly manipulating this representation of the space of channels.
By formal communication protocols in mission control, flight controllers have
privileges to speak on only a subset of the loops they can listen in on. In the voice
loop control interface, each channel can be set either to monitor or talk modes.
Only one channel at a time can be set to the talk mode, although many channels
can be monitored at the same time. In order to talk on a loop set to the talk mode, a
controller presses a button on a hand unit or holds down a foot pedal and talks into
Each controller customizes the set of loops they monitor by manipulating the
visual representation of the loops at their console. The controllers can save a
configuration of multiple voice loops on ‘pages’ under their identification code.
The most commonly used loops are grouped together onto a primary page. The
controllers then reorganize and prioritize the loops to fit the particular operational
situation going on at that time by changing the configuration of loops that are being
monitored and by adjusting the relative volume levels on each loop.
The voice loop interface is generally considered to be easy to use and an appro-
priate communication tool for a dynamic environment like space shuttle mission
control. The fundamental display units are visual representations of each auditory
loop, which captures the waycontrollers think about the system. Inaddition, ifindi-
vidual loops are analogous to windows in a visual interface, then the pages of sets
of loops are analogous to the ‘room’ concept in window management (Henderson
and Card, 1986). Controllers are able to customize the interface by putting their
most commonly used loops together on a single ‘page.’ Active loops on these
pages can be dynamically reconfigured in response to the constantly changing
environment. Dynamic allocations of which loops to listen to are done by directly
selecting loops to turn off and on. Controllers increase or decrease the salience of
particular loops by using loop volume controls to adjust relative loudness.
4.2. VOICE LOOP ORGANIZATION REFLECTS MISSION CONTROL STRUCTURE
The voice loop system design reflects the cooperative structure in mission control
(Figure 2). A primary voice loop, the Flight Director loop, is dedicated to