Wireless Sensor Systems for Near-term Space Shuttle Missions
Kevin Champaigne, Program Director, Invocon, Inc., 19221 I-45 South, Suite 530,
Conroe, TX 77385
ABSTRACT
NASA has developed a series of wireless data acquisition and communications systems for monitoring critical
areas of the Shuttle during pre-flight, ascent, on-orbit, and re-entry phases. Wireless sensor and data logger
applications include monitoring the structural dynamics of payloads, collecting data for modal analysis of the
flexhose assemblies, and measuring strain on both the Main Engine struts and the Remote Manipulator System
(Shuttle Arm). Additionally, the Wing Leading Edge Impact Monitoring System is being installed within the Shuttle
wings for the Return to Flight mission, STS-114, and subsequent missions to assist in detecting, locating, and
characterizing the severity of impact events similar to the debris impact that caused the Columbia disaster. In
fact, wireless systems based on Invocon’s line of MicroWIS sensors have been developed to investigate or
mitigate all three of the latest technical causes of Shuttle launch delays. Each battery operated unit interfaces
with various sensors and records data in internal memory for use by analysts post-flight, while some systems
perform local detection algorithms and transfer data through Shuttle data systems for analysis by ground
personnel during the mission. Through the use of a basic system design with considerable flight history and
through the limiting of interfaces with Shuttle resources, the development time and cost, as well as integration
effort for installation, have been significantly reduced, enabling the use of the systems for the first or second
flights. The general design of the systems are discussed in this paper, including sensor interfaces, hardware
design, embedded software algorithms, system operations, and Shuttle integration methods.
BACKGROUND
As part of the Columbia Accident Investigation Board (CAIB) recommendations following the Columbia disaster,
significant recertification of many major Shuttle systems has been undertaken in order to verify that the Shuttle is
as safe as reasonably possible. Discrepancies between the certified usage and environmental limits of various
systems and the actual limits experienced during flight are of major concern. However, sensor data to define the
actual ground processing, launch, on-orbit, and landing stresses and environments is often limited, leading to
overly conservative models. As such, retrofit instrumentation has been requested by a number of organizations
for flight measurements, as well as during Shuttle processing activities. The integration of such a large number of
new sensors into existing Orbiter instrumentation resources likely would have been prohibitively expensive, cause
an excessive increase in vehicle launch weight, and potentially cause significant launch schedule delays.
Alternatively, the Shuttle Program has chosen to utilize a variety of stand-alone, autonomous, miniature data
acquisition systems based on Invocon’s Micro-miniature Wireless Instrumentation System (MicroWIS) concepts
and previous flight developments which minimize the level of vehicle resources or modifications required.
Through the use of flight-proven wireless instrumentation technology developed through a partnership with
Johnson Space Center and Invocon, Inc., battery operated systems either have already been fully developed or
are currently under development for flight applications aboard one or more of the first three Shuttle flights after the
Columbia disaster.
The MicroWIS technology flight applications have been very successful and have enabled the beginning of a
“peel and stick” sensor installation paradigm that promises to revolutionize miniaturized sensor installations on
complex systems. These include wireless sensors that can either record or transmit temperature, triaxial
accelerometer, strain, pressure, tilt, chemical, and ultrasound data. Over 19 NASA missions have included
Invocon wireless instrumentation products comprised of 12 unique hardware designs. The miniature, wireless
instrumentation and sensor products can enable a 10X reduction in the integration and installation costs for the
Shuttle and ISS compared to traditional retrofit wired approaches. Wireless, battery-operated, intelligent sensors,
some less than 1 cubic inch, are easily bonded to vehicle structure and monitor environmental parameters,
enabling real-time monitoring of critical items and providing inexpensive data for model validation.
MicroWIS-based
Extended Life
MicroWIS-based
MicroTAU-based
Wideband
MicroTAU-based
Sample Rate
Up to 1Hz
Up to 1Hz
Up to 500Hz
Up to 20,000Hz
Data Storage
2Mbytes &
Real-time Transmission
1Mbyte
256Mbytes
Local Data Processing
Real-time
Transmission to PC
Only
No
No
Battery Life
9 months
10+ years
Sensors
Resistive sensors
including Temperature,
Strain, Accelerometer,
Pressure
Resistive sensors
including Temperature,
Strain, Accelerometer,
Pressure
Yes
8bit micro-controller
50 hours of data acq.
1 year sleep mode
Resistive or Voltage
sensors including Strain,
Accelerometer, Pressure
Yes
High-speed DSP
80 hours of data acq.
5 years sleep mode
Piezoelectric including
Accelerometer or
Pressure
Flight History
8 Shuttle Flights
Flight Certified – awaiting 9 Shuttle Flights
first flight on Shuttle
Orbital Maneuvering
System
Flight Certified – awaiting
first flight on next mission
STS-114
Table 1. MicroWIS Family System Characteristics
NEAR-TERM SHUTTLE APPLICATIONS
The following applications for various Shuttle instrumentation tasks using wireless sensors and data loggers are
either delivered, under development, or approaching final approval by the Shuttle program for near-term flights.
Many applications use similar technologies, but take advantage of unique implementations to customize the
system for the particular requirements of the system. For each application, the unique portions of the circuit
design that enables the system to meet the requirements are
explained in detail, although most design concepts apply to multiple
systems described below.
Main Engine Flowliner Crack Investigation
During the summer of 2002, significant launch delays were caused by
cracks found in the Space Shuttle Main Engine (SSME) flowliners,
which are part of the piping used to supply liquid hydrogen to the
engines. The Space Shuttle Program funded the development and
application of the Wide-band Micro-Wireless Triaxial Accelerometer
Unit (WB MicroTAU) in order to measure the vibration environment
experienced by the flowliners during launch. This data will be used to
verify the hypothesis of high cycle fatigue as the cause of the cracks
found, and potentially as verification of the environmental
requirements of replacement hardware.
Figure 1. Photographs of Cracks Found in SSME
Flowliners (NASA Photo)
The understanding of how the dynamic behavior of the SSME feedines and SSME low pressure turbo-pump work
together with the pumping fluid to produce cracks in certain areas is critical to continuing flying the Space Shuttle
and to the potential redesign of these components. The WB MicroTAU is intended to take triaxial accelerometer
data on and about the SSME LH2 and LO2 feedlines and their supporting brackets. Units have been installed on
both Discovery and Atlantis vehicles, and are awaiting the first flight of the system.
The upgrade of the original MicroTAU system provided nearly a 2-order of magnitude increase in analysis
bandwidth and data storage capability. The new system was designed, manufactured, qualified for flight, and
installed within just 4 months. System enhancements included:
„
„
„
„
20K samples/second
128Mbyte Flash memory
External cryogenic piezoelectric accelerometers
USB interface for faster downloads post-mission
Due to the extreme vibration and thermal environments of the flowliners, the data
recorders were mounted a short distance from the cryogenic transducer locations.
Integration and installation of the system into the orbiters was simplified by the fact that
there were no power or data interfaces to Shuttle systems. The system is programmed
via RF communication prior to launch to monitor the accelerometer inputs near the
scheduled launch time for a vibration signature indicative of launch. When detected, the
system will store both pre-trigger and post-trigger data into a large non-volatile memory
for the entire launch to orbit phase.
Figure 2. Exploded view
of Wideband MicroTAU.
Each unit is a multiprocessor system with a Network Communication board, a Digital
Signal Processor board, a front-end Data Acquisition board, and a patch antenna. The
system block diagram is shown in the following figure:
ANTENNA
RTD
NETWORK
MODULE
DSP MODULE
DATA
ACQUISITION
MODULE
Channel 1
Channel 2
Channel 3
3.6V
USB
INTERFACE
Figure 3. Block Diagram of Wideband MicroTAU Sensor Unit
Rollout Instrumentation
Although the maximum speed of the crawler during Shuttle Rollout is only 1mph, significant structural modes may
be excited, potentially damaging Shuttle elements or systems. A pair of wireless triaxial accelerometers will be
used to supplement wired ground data acquisition systems to monitor the element interfaces and major modes in
the frequency range of roll-out. Recording of accelerations in a triaxial configuration throughout Rollout is desired,
with the frequencies of interest ranging from 0.3Hz up to 50Hz. The use of the basic design of the Enhanced
Wideband Micro-Triaxial Accelerometer Unit (EWB MicroTAU) is planned, as shown below in Figure 5, but the
accelerometers will be mounted internal to the unit rather than external.
The wireless triaxial accelerometers will be used to instrument the Orbiter Lift points
at the 582 bulkhead on both the port and starboard sides. A series of wired
accelerometers and strain gauges will be used to complete the instrumentation
suite. These sensors, located inside the payload bay, will have cables routed
through the aft bulkhead and out of the vehicle through the aft compartment. These
signals will be acquired by a standard data acquisition system located on the Mobile
Launch Platform.
Data Acquisition Capabilities
Figure 4.
Shuttle During
Rollout (NASA Photo)
The EWB MicroTAU Remote Unit is a small, battery-powered, autonomous, wireless
device designed for trigger initiated acquisition and recording of acceleration data.
The units interface with three accelerometers, and each unit can be programmed for
a triggered acquisition event. Download and
event setup commands are issued either
wirelessly or through a standard USB
connection from a Graphical User Interface
running on a PC.
Each unit contains 256Mbytes of non-volatile memory, which provides for
over 20hrs of data recording at 500 samples per second. Additionally,
various trigger modes and on-board processing algorithms allow the user
to configure the unit to capture and store only interesting data over an
even longer period of time. MEMS accelerometers featuring low-power
operation and resolution in this frequency band of better than 1mg will be
packaged within the enclosure, along with the circuit boards, battery, and
antenna for a fully self-contained package.
Figure 5. Enhanced Wideband MicroTAU
Synchronization with External Data Acquisition System
582 Location
(Lift Point)
MicroWIS
Base Station
Ground Data
Acquisition System
IRIG-E Time
Generator
Figure 6. Rollout Instrumentation Configuration
The synchronization capabilities of the existing Kennedy
Space Center data acquisition system are based upon an
IRIG-E time generator.
The IRIG-E waveform will be
simultaneously provided to both the ground data acquisition
system and a MicroWIS base station unit. The MicroWIS
base station will record this waveform in non-volatile memory
while also serving as the master unit for synchronizing the
two triaxial accelerometer units via RF communication. This
will provide a high level of synchronization between the
different data systems following a simple post-processing
routine to synchronize the digitized data samples.
During data acquisition mode, the sensor units provide a
means for synchronizing the acquisition of each data sample
between multiple wireless sensors to within ±4µs. This level
of synchronization should easily meet the requirements for
analyzing the low frequency modes of interest. Sensor units
can be configured as master units or slave units. When in
master mode, the unit will transmit a synchronization
message that is used by the slave units to synchronize the
acquisition of the data samples. During a given acquisition,
a slave unit will only listen to one master unit for
synchronization.
Environmental Control Life Support System (ECLSS) Flexhose Analysis
In order to assist in the determination of the root cause of a recent leak in the oxygen and nitrogen flexhoses of
the Shuttle Environmental Control Life Support System (ECLSS), which caused a launch delay for STS-113, the
Space Shuttle Program requested a modified version of the WBMTAU. The modified system provides the
sophisticated scheduling, data processing, and memory capacity necessary to obtain all relevant data during the
following ground processing and flight events w/o battery replacement:
•
•
•
•
•
Roll-over from the Orbiter Processing Facility (OPF) to the Vehicle Assembly Building (VAB);
Lifting and stacking operations in the VAB;
Rollout to the Launch Pad;
Launch and ascent, and
Reentry and Landing
Reverse bending fatigue was identified as a possible failure mechanism by system engineers. However,
available flex-hose vibration data did not support efforts to resolve the root cause of the OV-105, STS-113 ECLSS
flexhose failure. To address the concern that certification tests may not have adequately represented current
processing and flight environments, the MicroWIS system was developed to obtain actual processing and fight
vibration data both on the flexhoses themselves and the bulkheads on which they were mounted. Through
analysis of actual flight data, the Limited Life placed on vehicle Flexhoses may be relaxed from the current limit of
4 flights.
Single Axis
Accelerometer
Triaxial
Accelerometers
Figure 7. ECLSS Flexhose Installation Location and Sensor Placement
Advanced Trigger Mechanisms and Scheduling Capabilities
The ability of a stand-alone system to meet all of the desired data acquisition requirements for monitoring the
flexhoses through many phases of processing and flight was made possible through a number of advanced
triggering and scheduling techniques implemented for this system. A trigger is any event that can cause the
software embedded in the unit to change mode and begin performing a new function. For the Flexhose system, a
trigger can be any of the following:
„
Time
The unit can automatically change data acquisition modes based on GMT time. For example, the unit
can start and stop data recording at programmed times when Roll-out is scheduled to occur.
„
RF Commanding
The unit can change modes immediately based on a command from the laptop via RF communication.
For example, the user may wish to stop a data acquisition manually if the Rollout is delayed.
„
Primary Sensor Channel Data Signature
The unit can monitor the primary accelerometer channels and trigger if a user defined number of data
samples occur above a programmable threshold. This mode can be used to detect both launch and
landing events.
„
Auxiliary Pressure Sensor
Since Reentry can be at various times, anywhere from a couple days after launch if there is a problem to
16 days for a full mission, a low-power method to detect it was needed. A low-power pressure transducer
is sampled once per minute to detect the change from 0psi to 2psi, which is indicative of reentry into the
atmosphere.
The units were also programmed to perform a large number of unique events, which can all be programmed at
one time pre-launch. The units can be programmed with up to 32 unique Event Acquisitions, and each event can
have a conditional branch to specify the next event. Such a flow-chart based programming sequence enables the
system to handle multiple launch slips without having to be reprogrammed, among other possibilities. The unit
will change the next data acquisition event executed based on whether the launch trigger event was successful or
not successful.
RF Commands:
Start Time
Trigger Setup
Etc.
•
•
•
•
•
Program launch time
Detect Launch through
signal analysis of
Accel/Strain
Record Pre- and PostTrigger data
•
•
•
•
•
RF or GMT Trigger for
Roll-over or Roll-out
Record up to 10 hours
of raw data
Optionally process and
reduce data
Figure 8. ECLSS Example Mission Profile
Program de-orbit time
Detect Pressure Trigger
at low rate
Detect Landing through
signal analysis of Accel
Record Pre- and PostTrigger data
Shuttle Remote Manipulator System Instrumentation
A Wireless Strain Gauge Instrumentation System (WSGIS) is planned to be used to monitor the structural loads
that occur within the mechanical arm portion of the Shuttle Remote Manipulator System (SRMS) during flight
operations in which the arm is being maneuvered.
The instrumentation system will monitor RMS structural loads through the measurement of RMS material strain at
three locations (cross sections): the Shoulder, Elbow and Wrist Pitch/Yaw/Roll Electronic housings as shown
below. At each electronic housing location, four full stain gauge bridges will be bonded to the electronics housing
structure to measure the following parameters:
1. Bending Moments in two orthogonal planes
2. Torsion
3. Axial Force
A wireless unit will be mounted to the RMS at each electronics housing location. The avionics boxes will provide
excitation and conditioning for 4 separate strain gauge circuits, analog to digital conversion of strain gauge
outputs, data processing with stop/start capabilities, data logging in 256Mbytes of non-volatile memory, batteries,
and a wireless radio frequency interface via an external antenna. At the desired sample rate of just 50Hz, there
will be sufficient battery power and memory to acquire up to 120hrs of raw data from all four channels. The
avionic boxes will be mounted at each location under the existing RMS thermal blankets. Each unit will have an
external antenna, which will be connected to the avionics box and will protrude beyond the outer surface of the
thermal blanket.
The system will function as a master/slave system with the synchronization of the data between avionic boxes
maintained via a synchronization signal sent from the master unit to each of the slaves. Start and stop of data
acquisition is initiated via signals received via RF. Control of the system will be conducted from a laptop located
on the aft flight deck of the shuttle by a member of the Crew. The laptop has a wired interface to a Receiver
Assembly, which will be located in the aft flight deck payload bay window. All strain data will be stored within the
units for post flight collection via a wired USB connection.
As an add-on to the current design concept, a forth unit has been discussed in order to measure loads induced by
the EVA crew member on the Arm at the Articulating Portable Foot Restraint (APFR), to which the crew member
is secured to the end of the Arm.
Crew
Transmitter&
Receiver
PGSC
Aft Flight Deck PLB
Window
Figure 9. System Configuration of Shuttle RMS Wireless Strain Gauge System
Wing Leading Edge Impact Detection and Location System
The cause of Columbia accident has been determined to be foam debris from the External Tank left bipod ramp
impacting and damaging the portside wing leading edge Reinforced Carbon Carbon (RCC) panels during ascent
of STS-107. Ground-based cameras captured imagery of the impact, but were not able to reliably characterize
the location or severity of the foam impact and resulting damage. No sensors existed on the vehicle that would
enable ground personnel to determine the extent or location of damage.
As part of the Columbia Accident Investigation Board (CAIB) recommendations, monitoring of the Orbiter wing
leading edge during ascent is now a requirement for all future Shuttle missions. Although this can be
accomplished with improved ground based imagery and on-orbit inspections with the Orbiter Boom Sensor
System, the Shuttle Program decided to implement an accelerometer based impact detection and location system
on the wing leading edges for the Return to Flight (RTF) mission. The purpose of this initial system was to enable
ground personnel to place additional emphasis on the on-orbit inspection of particular areas of the wing Thermal
Protection Systems based on possible impact data acquired during ascent. In addition to monitoring for foam
impacts during ascent, the Wing Leading Edge Impact Detection System will also monitor the RCC panels for
Micro-Meteor and Orbital Debris (MMOD) impacts during the on-orbit phase.
Due to the limited time and resources available prior to the RTF mission, the decision was made to implement the
system in such a way as to minimize the level of vehicle resources or modifications required. Through the use of
flight-proven wireless instrumentation technology developed through a partnership with Johnson Space Center
and Invocon, Inc., an autonomous, battery operated system has been fully developed and produced, and is
currently being tested and installed on the Space Shuttle Discovery for the first mission.
Laptop-based Receiver Assembly
collects (via radio frequency) data
from Relay Unit and dumps data
to PC for downlink to Mission
Control
Relay Units collect (via RF) postprocessed data from Sensor Units and
transfer to crew compartment via wired
RS-485 multidrop networked bus
Sensor Units record and post-process
accelerometer
and
temperature
readings during ascent and while onorbit
FIGURE 10. System Overview.
Sensor Selection and Data Acquisition Requirements
Mounting sensors such as accelerometers or acoustic emission (AE) sensors directly on the Wing Leading Edge
RCC panels was considered impossible due to the extremely high temperatures experienced on the back side of
the panels as well as the reluctance to drill any new holes in the leading edge spar to run cabling. Therefore,
instrumenting the back side of the spar was determined to be the optimum solution. Each RCC panel is mounted
to the spar through a series of metallic fittings at 4 bolted locations. Since the load path from the RCC to the spar
was through these bolts, the area around the bolts was determined to be the location with the highest signal level
on the spar. Initial NASA testing during impact testing at Southwest Research Institute showed that relatively low
frequency vibrations and strains were transferred through the potentially loose panel fittings. Since the
accelerometer signal levels observed during impact tests in the 10 – 5000Hz range were significantly above
nominal flight background levels during ascent, accelerometers were selected as the baseline sensor for the
Return to Flight mission. Investigation by NASA into the use of AE sensors at a higher frequency is, however, still
ongoing, and will likely be addressed through the flight of a Development Test Objective (DTO) system to
measure AE background levels with modified EWB MicroTAU units.
Impact Detection Algorithms
Since the amount of data acquired during the 10-minute ascent period is nearly 100Mbytes, the time to download
all data, even from a single unit, via the RF link or serial communications link is excessively long. Therefore, in
order to utilize the data acquired by the EWB MicroTAU shortly after ascent, data reduction algorithms were
required to minimize the quantity of data that must be transmitted via RF. However, care must be taken to not
diminish the amount or quality of information contained in the returned data. The existing hardware contains
significant processing power in the form of a Digital Signal Processor (DSP) to process the acquired data while
expending a minimal amount of battery resources. A variety of algorithms have been identified for feature
extraction and data reduction. Most are currently based on the implementation of a sliding window, root-meansquare (RMS) calculation. After the time period of a likely impact is determined, the raw data for both before and
after the peak for all channels for that period is downloaded to the laptop in the crew compartment and
subsequently downlinked to Mission Control. The following algorithms are available to be executed by the
embedded software during the post-ascent data processing phase or any time after data is acquired. It that way,
new algorithms or settings can be uploaded to the units while on-orbit and run on ascent data if necessary.
¾
¾
¾
Peak G’s
The system is able to generate a summary list of the highest raw acceleration values and the associated
timestamps to be downlinked.
Root Mean Square (RMS) Peaks
The system can perform a sliding window RMS calculation on the raw data according to user selected
window size, window overlap, and digital bandpass filter frequencies, and then return a summary list of
the highest data points. Typically window sizes of 256 samples are used to accentuate the impact
events, which typically excite the structure for on the order of 100ms.
Relative RMS Peaks
The system can also optionally remove the background noise-floor level from the RMS calculation above.
This is done by taking another sliding window RMS calculation with a much wider window size and
subtracting this background from the narrower window. In this way, varying background noise levels,
such as occur throughout ascent due to SRB ignition, etc., can be removed.
Examples of these algorithms are shown below as applied to a set of representative data. This data set was
created summing actual flight accelerometer data acquired on the Shuttle wing with test data for representative
foam impacts acquired during testing.
Figure 11. Raw Flight Data Set (Prior to Adding Impact Signals).
(a)
(b)
RMS Peaks Algorithm Results with Impact Signatures Added
Relative RMS Peaks Algorithm Results with Impact Signatures Added
Figure 12. Multiple Techniques for Identifying Impact Events.
Additional Applications
Three additional applications using wireless, autonomous sensor devices will also be flown on some or all of the
initial three flights. The STS-114 will be the first flight of the Extended Life MicroWIS, which was developed
initially for measuring temperature on external portions of the ISS for the life of the vehicle. The C-cell battery will
provide for a calculated 17-year life while sampling, storing, and transmitting a temperature sample once per
minute. Units will be located within the Orbital Maneuvering System (OMS) pod for temperature monitoring of an
OMS engine during all phases of the mission.
The MicroTAU system will be making its third flight aboard the Shuttle on STS-114. The original MicroTAU is a
network of small, battery powered, wirelessly programmable, tri-axial acceleration data recorders, designed to
monitor the relative movements of the Multipurpose Logistics Module (MPLM) payload and the Shuttle sill
longeron at the interface trunnion pin. In order to meet this requirement,
the MicroTAU units have a wireless data synchronization capability of
±30uS at 250Hz sampling.
As part of its eighth mission on Shuttle, Micro-Strain Gauge Units will be
used to acquire strain data on each of three thrust structure support
beams for the SSME pitch actuators in the aft of the orbiter. The data
recording will be triggered on the launch event, and will continue through
ET separation (~820 seconds), including 10 seconds of buffered strain
data prior to the launch event. Orbiter Structures Division will compare
the measured data with certification analyses to extend the limited life of
these parts. The Micro-Strain Gauge unit will store.
CONCLUSIONS
NASA has traditionally used models for predicting various spacecraft performance, remaining life, and
environmental parameters, typically using high safety margins and assuming worst case conditions. Through the
use of low-cost Invocon wireless sensors and data recorders, actual flight measurements have been made which
have enabled NASA to refine models and reduce safety margins effectively. Invocon systems have been used to
support an increase the design life of the Shuttle Main Engine Struts, to correlate models of Shuttle payload
launch dynamics, and soon to analyze the Shuttle Main Engine flow-liners for high-cycle fatigue-causing vibration
and to monitor the Shuttle Wing Leading Edges for impacts from foam, ice, or ablator material during launch.
Determining efficient and effective methods for implementing similar retrofit instrumentation functions for the
lowest cost in terms of mass, volume, development effort, and installation effort needs to be a NASA priority as it
attempts to complete the International Space Station and continue on the Moon and Mars as part of future
Exploration programs. Overall, wireless and wired networked sensor/actuator technologies have been recognized
by NASA, as well as other space agencies, as being critical for improving spacecraft reliability, reducing
spacecraft wire harness weight, and drastically reducing spacecraft development, testing, and life-cycle costs.
Invocon has been at the forefront of wireless data acquisition and communication for space applications since the
beginning, flying the first automated wireless sensor network in space. Enormous challenges exist with regard to
the new Exploration Vision to further reduce size, weight, power, and infrastructure requirements for vehicle and
payload sensor systems. Invocon, in partnership with NASA, seeks to build upon the current technology by
providing improved bandwidth, communication robustness, temperature range, radiation tolerance, and battery
life while minimizing size and mass for systems with highly capable local data processing within self-configuring,
self-repairing communication networks.
ACKNOWLEDGMENTS
The author would like to thank John Saiz, Robert Villarreal, George Studor, and George James of NASA Johnson
Space Center, John Coates of Lockheed Martin, and Frank Graffagino of Dynacs, Inc for contributing graphical
material for this paper.