Richard C. Oeftering, Martin A. Bradish, Jeffrey R. Juergens, and Michael J. Lewis

Glenn Research Center, Cleveland, Ohio

Component-Level Electronic-Assembly Repair 

(CLEAR) Analysis of the Problem Reporting 

and Corrective Action (PRACA) Database 

of the International Space Station 

On-Orbit Electrical Systems 

NASA/TM—2011-216954

January 2011

CLEAR–DOC–005
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Richard C. Oeftering, Martin A. Bradish, Jeffrey R. Juergens, and Michael J. Lewis

Glenn Research Center, Cleveland, Ohio

Component-Level Electronic-Assembly Repair 

(CLEAR) Analysis of the Problem Reporting 

and Corrective Action (PRACA) Database 

of the International Space Station 

On-Orbit Electrical Systems 

NASA/TM—2011-216954

January 2011

CLEAR–DOC–005

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

NASA Center for Aerospace Information

7115 Standard Drive

Hanover, MD 21076–1320

National Technical Information Service

5301 Shawnee Road

Alexandria, VA 22312

Available electronically at http://gltrs.grc.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2011-216954 iii

Summary

The NASA Constellation Program is investigating and developing technologies to support human 

exploration of the Moon and Mars. The Component-Level Electronic-Assembly Repair (CLEAR) task 

is part of the Supportability Project managed by the Exploration Technology Development Program. 

CLEAR is aimed at enabling a flight crew to diagnose and repair electronic circuits in space yet minimize 

logistics spares, equipment, and crew time and training. For insight into actual space repair needs, in early 

2008 the project examined the operational experience of the International Space Station (ISS) program. 

CLEAR examined the ISS on-orbit Problem Reporting and Corrective Action database for electrical and 

electronic system problems. The ISS has higher than predicted reliability yet, as expected, it has persistent 

problems. A goal was to identify which on-orbit electrical problems could be resolved by a componentlevel replacement. A further goal was to identify problems that could benefit from the additional 

diagnostic and test capability that a component-level repair capability could provide. The study indicated 

that many problems stem from a small set of root causes that also represent distinct component problems. 

The study also determined that there are certain recurring problems where the current telemetry 

instrumentation and built-in tests are unable to completely resolve the problem. As a result, the root cause 

is listed as “unknown.” Overall, roughly 42 percent of on-orbit electrical problems on ISS could be 

addressed with a component-level repair. Furthermore, 63 percent of on-orbit electrical problems on ISS 

could benefit from additional external diagnostic and test capability. These results indicate that in situ 

component-level repair in combination with diagnostic and test capability can be expected to increase 

system availability and reduce logistics. The CLEAR approach can increase the flight crew’s ability to act 

decisively to resolve problems while reducing dependency on Earth-supplied logistics for future 

Constellation Program missions.
NASA/TM—2011-216954 iv
NASA/TM—2011-216954 v

Contents

Summary......................................................................................................................................................iii

1.0 Introduction ..........................................................................................................................................1

2.0 Documents............................................................................................................................................2

2.1 Reference Documents.................................................................................................................2

2.2 Applicable Documents................................................................................................................2

3.0 Problem Reporting and Corrective Action Database Analysis Scope..................................................3

3.1 International Space Station Electrical Systems...........................................................................3

3.2 Electronic Orbital Replaceable Unit Component-Level Repair..................................................4

3.2.1 Rationale ....................................................................................................................... 4

3.2.2 Description of CLEAR Repair Capability .................................................................... 4

3.2.3 Potential Problems Resolved by On-Orbit Component-Level Repair .......................... 5

3.2.4 Screening Criteria—Problems Not Suitable for Component-Level Repair.................. 5

3.3 On-Orbit Diagnostic and Test Capabilities.................................................................................6

3.3.1 Rationale ....................................................................................................................... 6

3.3.2 Description of CLEAR Diagnostic and Test Capability ............................................... 6

3.3.3 Potential Problems Resolved by On-Orbit Component-Level Repair .......................... 6

3.3.4 Screening Criteria—Problems Not Suitable for CLEAR Diagnostics.......................... 7

3.3.5 Software-Hardware Ambiguity..................................................................................... 7

4.0 Problem Reporting and Corrective Action Analysis............................................................................8

4.1 Electrical Power System .............................................................................................................8

4.1.1 Electrical Power System On-Orbit Component-Level Repair...................................... 9

4.1.2 Electrical Power System On-Orbit Diagnostic and Test Operations ............................ 9

4.2 Command and Data Handling...................................................................................................10

4.2.1 Command and Data Handling On-Orbit Component-Level Repair............................ 10

4.2.2 Command and Data Handling On-Orbit Diagnostic and Test Operations.................. 10

4.3 Communications and Tracking .................................................................................................11

4.3.1 Communications and Tracking On-Orbit Component-Level Repair.......................... 11

4.3.2 Communications and Tracking On-Orbit Diagnostic and Test Operations................ 12

4.4 Guidance, Navigation, and Control...........................................................................................13

4.4.1 Guidance, Navigation, and Control On-Orbit Component-Level Repair ................... 13

4.4.2 Guidance, Navigation, and Control On-Orbit Diagnostic and Test Operations.......... 13

4.5 Extravehicular Robotics............................................................................................................13

4.5.1 Extravehicular Robotics On-Orbit Component-Level Repair..................................... 13

4.5.2 Extravehicular Robotics On-Orbit Diagnostic and Test Operations........................... 13

5.0 Summary of Potential of CLEAR Diagnostic and Repair Operations...............................................14

6.0 Conclusions........................................................................................................................................16

Appendix—Acronyms and Abbreviations..................................................................................................17
NASA/TM—2011-216954 vi
NASA/TM—2011-216954 1

Component-Level Electronic-Assembly Repair (CLEAR) Analysis 

of the Problem Reporting and Corrective Action (PRACA) 

Database of the International Space Station

On-Orbit Electrical Systems

Richard C. Oeftering, Martin A. Bradish, Jeffrey R. Juergens, and Michael J. Lewis

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

1.0 Introduction

The Component-Level Electronic-Assembly Repair (CLEAR) task was part of the NASA Exploration 

Technology Development Program (ETDP) Supportability Project from 2006 to 2008. CLEAR’s intent 

was to provide the needed technology to perform in situ repairs of electronic hardware at the component 

level. Potential in situ repair sites include Earth orbit, the Moon, and even Mars. Because of the ongoing 

design of Constellation Program (CxP) vehicles and the ongoing maturation of design information in the 

near term, the CLEAR task chose to examine the design and operational experience of the International 

Space Station (ISS).

The ISS Problem Reporting and Corrective Action (PRACA) system is used for reporting, 

processing, and tracking space station hardware problems and their resolutions. This system provides 

a wealth of historical space system anomalous incidents from all phases of station development and 

operations, and it is mined by engineers as they design the next generation of exploration spacecraft. This 

document examines the ISS PRACA system to determine the potential number of problems that could be 

resolved with a robust on-orbit electronics diagnostic and repair capability, such as that proposed in 

“Component-Level Electronic-Assembly Repair (CLEAR) Operational Concept” (CLEAR–DOC–007)

and “Component-Level Electronic-Assembly Repair (CLEAR) System Architecture” (CLEAR–DOC–

008). The appendix defines the acronyms and abbreviations used in this document.
NASA/TM—2011-216954 2

2.0 Documents

The following documents contain supplemental information related to the CLEAR task.

2.1 Reference Documents

Document number Document title

CLEAR–DOC–006 Component-Level Electronic-Assembly Repair (CLEAR) Assessment of 

Constellation Program In-Space Electronic Diagnostics and Repair Needs

CLEAR–DOC–007 Component-Level Electronic-Assembly Repair (CLEAR) Operational Concept 

CLEAR–DOC–008 Component-Level Electronic-Assembly Repair (CLEAR) System Architecture

2.2 Applicable Documents

Document number Document title

CLEAR–ANA–001 Component-Level Electronic-Assembly Repair (CLEAR) Life-Cycle Cost 

Impacts of Different Approaches for In Situ Maintenance of Electronics 

Hardware

CLEAR–ANA–002 Component-Level Electronic-Assembly Repair (CLEAR) Noncontact Coupling 

Techniques for Performing Signature Analysis

CLEAR–DOC–001 Component-Level Electronic-Assembly Repair (CLEAR) Component Repair 

Exp-1 (CRE–1) Flight System Requirements Document

CLEAR–DOC–002 Component-Level Electronic-Assembly Repair (CLEAR) Component Repair 

Exp-1 (CRE–1) Concept and Hardware Summary

CLEAR–DOC–003 Component-Level Electronic-Assembly Repair (CLEAR) Recommendations for 

the Design of Electronic Assemblies for NASA's Exploration Program

CLEAR–DOC–004 Component-Level Electronic-Assembly Repair (CLEAR) Trade Study: Current 

Technologies and Instruments for Electronic Fault Diagnosis and Repair

CLEAR–DOC–006 Component-Level Electronic-Assembly Repair (CLEAR) Assessment of 

Constellation Program In-Space Electronic Diagnostics and Repair Needs

CLEAR–DOC–007 Component-Level Electronic-Assembly Repair (CLEAR) Operational Concept 

CLEAR–DOC–008 Component-Level Electronic-Assembly Repair (CLEAR) System Architecture

CLEAR–DOC–009 Component-Level Electronic-Assembly Repair (CLEAR) Recommendations for 

Enabling Manual Component-Level Electronic Repair for Future Space Missions

CLEAR–RPT–001 Component-Level Electronic-Assembly Repair (CLEAR) Component Repair 

Experiment 1 (CRE–1) System Requirements Review (SRR) Panel Report

CLEAR–RPT–002 Component-Level Electronic-Assembly Repair (CLEAR) Soldering in Reduced 

Gravity Experiment (SoRGE) Mission Test Report

CLEAR–RPT–003 Component-Level Electronic-Assembly Repair (CLEAR) Spacecraft Circuit 

Diagnostics by Analog and Complex Signature Analysis

CLEAR–RPT–004 Component-Level Electronic-Assembly Repair (CLEAR) Synthetic Instrument 

Capabilities Assessment and Test Report
NASA/TM—2011-216954 3

3.0 Problem Reporting and Corrective Action Database Analysis Scope

“Component-Level Electronic-Assembly Repair (CLEAR) Assessment of Constellation Program 

In-Space Electronic Diagnostics and Repair Needs” (CLEAR–DOC–006) examines electronic system 

designs for ISS as an analog for CxP. By accessing design information from the ISS Vehicle Master 

Database (VMDB), the project determined the nature of the circuits and the equipment needs to diagnose, 

repair, and test electronics in space. A great deal of circuit drawing documentation was gathered,

including information regarding the materials and processes used in fabricating electronics. This design 

information will allow CLEAR to formulate repair processes.

Design information in the ISS VMDB does not indicate which circuits or components are likely to 

fail or how often physical repairs are needed. For greater insight into the repair history of ISS, the project 

studied the ISS PRACA database. The findings relevant to CLEAR diagnostics and repair are described 

in the following sections.

3.1 International Space Station Electrical Systems 

The CLEAR project investigated ISS PRACA documents to determine what problems were occurring 

on actual flight hardware that were repairable at the component level. Furthermore, the study intended to 

determine diagnostic and test techniques employed in the repair process. Over 4000 ISS PRACA reports 

are in the system, and roughly 1900 are electrical and avionics system problems. The following ISS 

electrical systems were reviewed:

 Electrical Power System (EPS)

 Command and Data Handling (C&DH)

 Communications and Tracking (C&T)

 Guidance, Navigation, and Control (GN&C)

 Extravehicular Robotics (EVR)

The first four systems are considered to be the conventional elements of a space flight power and 

avionics system. EVR is often considered to be mechanical hardware; however, about two-thirds of its 

problems are electrical. At the time of the study, 4076 reports were in the ISS PRACA database with 

1914 reports (47 percent) attributed to the five systems we identified in Table I.

The PRACA system begins tracking problem reports when hardware goes into the acceptance test 

phase. Many reports pertain to issues that are not resolved by repairs, such as relaxing specifications or 

altering documents or procedures. There is much to learn regarding electronic problems reported during 

preflight tests; however, most of these problems are screened out. The problems that occur in flight tend 

to be problems that are difficult to detect in ground-screening tests. They may arise because of unusual 

circumstances that occur in flight or that are operational wear-and-tear problems. Since the objective of 

the CLEAR Project is to develop in situ diagnostic and repair capability, the project chose to focus on the 

on-orbit problems.

The ISS has roughly 770 of the PRACA reports addressing on-orbit problems. Electrical problems 

constitute 328 (42 percent) of these reports. Table I shows the breakdown of the ISS PRACA reports.
NASA/TM—2011-216954 4

TABLE I.—DISTRIBUTION OF INTERNATIONAL SPACE STATION (ISS) 

ELECTRONICS-RELATED PROBLEM REPORTING AND 

CORRECTIVE ACTION (PRACA) REPORTS

ISS system All PRACA

reports

On-orbit PRACA reports

Number Percent of total

Electrical systems overall 1914 328 17.1

Electrical Power System 786 171 21.8

Command and Data Handling 211 30 14.2

Communications and Tracking 498 73 14.7

Guidance, Navigation, and Control 46 19 41.3

Extravehicular Robotics 373 35 9.4

All systems 4076 770 18.9

Of the on-orbit problem reports, Russian hardware was first to be delivered to orbit and, therefore,

accounts for a large portion of the problems in the early phases of assembly. Russia is an international 

partner and, as such, is not required to submit to NASA’s engineering scrutiny. Provided that they do not 

pose an integration risk, Russian problems are handled by Russian contractors. A similar situation will 

arise as the European and Japanese elements are integrated with ISS. As a result, PRACA reports on 

Russian hardware provide little specific information regarding problem root cause or corrective actions.

Thus, for this study, problem reports on Russian systems are included but are not counted as potential 

candidates for CLEAR diagnostics or repair. In reality, the Russian hardware is expected to be repairable 

by component-level replacement. 

3.2 Electronic Orbital Replaceable Unit Component-Level Repair 

3.2.1 Rationale

CLEAR’s premise is that it is technically feasible to make repairs in space by replacing components.

That is, the processes employed for replacing components that are soldered to a circuit assembly on Earth 

can be modified to operate in a spacecraft environment and provide reliable repairs. The examination of 

the PRACA data indicated that there were a number of problems that were or will eventually be repaired 

by replacing components.

For circuits with built-in spare channels, complete orbital replaceable unit (ORU) replacement can 

be deferred. This strategy is used in the EPS where on-orbit replacement can impact many dependent 

systems. The EPS Remote Power Controller Module (RPCM) ORUs have a graceful degradation 

approach, where spare channels can be accessed by swapping the position of two RPCMs. As more 

channels are lost, the ORU must ultimately be repaired at the component level.

A very limited number of ISS ORUs are composed of intermediate-level assemblies often referred 

to as “shop replaceable units” (SRUs) that are intended to be replaceable on orbit. The Multiplexer/

Demultiplexer Module (MDM) of the C&DH system is designed to allow a crew to disassemble an ORU

and quickly perform remove and replace (R&R) operations at the circuit-card level (e.g., SRU-level

repair). The MDM’s backplane design employs a common set of standardized circuit cards that also 

makes component-level repair easier to accomplish. The CLEAR task assumes that, with appropriate 

equipment, the SRU can be repaired in situ at the component level.

3.2.2 Description of CLEAR Repair Capability

Repair of circuit cards at the component level involves several steps, including removal of a 

conformal coating, removal of a component, cleaning, soldering of a new component, and ultimately 

replacing the conformal coating. Component-level repair may be performed manually in many instances, 

however, certain operations are best handled by dedicated preprogrammed equipment designed to 

reproduce many of the conditions used in manufacturing. For example, most high-density devices and 

particularly modern grid-array devices require a solder reflow capability that is beyond the reach of 

simple soldering irons. All of these processes require the containment of vapors, liquids, and debris.
NASA/TM—2011-216954 5

CLEAR–DOC–008 describes a system architecture that employs both manual and semiautomated means 

to accomplish repairs within the confines of a spacecraft, and it includes some teleoperational concepts 

similar to how NASA ISS payloads operate today.

3.2.3 Potential Problems Resolved by On-Orbit Component-Level Repair

Most ORUs are composed of circuit-card assemblies (CCA), where electronic components are 

mounted on printed circuit board substrates. ORUs like the MDM are composed of CCAs that are 

selected from a standard CCA set. Another ORU, the RPCM, has multiple redundant channels that are not 

all used for a given application. If a circuit card on one RPCM fails, then the entire RPCM ORU can be 

swapped out with another from a different location, keeping both systems online. Although not the 

original intent of the design, such swapping is effectively the same as having SRU-level spares available.

Other ISS ORUs, like audio and video equipment, are composed of specialized circuits that do not 

effectively support SRU replacement or have built-in spares. Certain electronic ORUs may be too 

difficult to retrieve from external locations to be repaired inside the cabin. These would normally be 

returned to Earth where component-level repairs would be performed at a depot or at the factory.

Regardless of the design approach, most ORUs can be repaired at the component level.

The majority of electrical ORU assemblies are on a scale that can be repaired within the ISS 

maintenance workstation. Assuming that the internal components are accessible, a properly equipped and 

trained crew member could perform the repairs. The tables in Section 4.0 show instances where an in situ 

component-level repair resolved, or could have resolved, the problem if the repair capability and 

replacement components were made available.

3.2.4 Screening Criteria—Problems Not Suitable for Component-Level Repair

There are many reasons for initiating a problem report that do not require a repair. Sometimes the 

resolution may be as simple as correcting a document or updating a procedure. On-orbit PRACA reports 

tend to have a narrower set of issues, since most trivial problems have been screened out by acceptance 

testing and preflight tests. The following list indicates the most common problems that would not be 

resolved by the repair of an electronic component:

 ISS Lights.—Many lighting assemblies have accumulated hours that are at or in excess of normal 

predicted service life. These are considered to be logistics consumable items.

 Russian Hardware.—The Russians are responsible for their own hardware and are not required 

to provide detailed information to the PRACA database unless there is an impact to the ISS 

integration.

 Mechanical Interfaces.—A number of electrical system problems have been related to 

mechanical interface problems not related to electrical repair.

 Thermal Environment.—Components that are inadvertently exposed to very low temperatures 

that are out of the normal design specification normally do not require repair. 

 Operations and Documentation Errors.—These are simple operational errors that do not harm 

any electronics. 

 Software.—Software problems generally do not require component repair. An exception to this 

rule involves firmware where a physical component must be reprogrammed or replaced to fix the 

firmware.

 Obsolete Hardware Replacement.—When an ORU or SRU is deemed to be obsolete, it is not 

repaired (e.g., the MDM mass data storage disk).

 False Error.—This event occurs when the intervening communications make hardware appear to 

be faulty or if telemetry or software falsely indicates a failure. 
NASA/TM—2011-216954 6

3.3 On-Orbit Diagnostic and Test Capabilities

3.3.1 Rationale 

ISS PRACA history shows that telemetry instrumentation and built-in tests (BITs) are used 

extensively but often cannot isolate specific faults. “Root cause unknown because of insufficient data” is 

stated frequently. This is a frustrating outcome when the problem has been investigated extensively, yet 

still cannot be isolated. “Insufficient data” also illustrates the need for more external diagnostics to 

augment existing BITs and telemetry. In general, the PRACA data are not detailed enough to describe the 

diagnostic and test equipment used to diagnose the problem. Often, measurements are listed, but specific 

diagnostic instruments are rarely identified within the reports. Therefore, this analysis on diagnostics and 

test needs should be considered to be preliminary, and further indepth study will be needed. 

3.3.2 Description of CLEAR Diagnostic and Test Capability

In CLEAR–DOC–008, the CLEAR task proposed a suite of test and diagnostic equipment that 

includes the following:

 Imaging tools suitable for circuit board visual examination by both flight crew and ground teams

 Diagnostic instruments such as multifunction meters, oscilloscopes, and signal generators—

which could be stand-alone instruments or, more preferably, could be incorporated as part of a 

synthetic instruments (SI) capability using common hardware that could be reconfigured as an 

instrument on demand

 A signature analysis capability to compare diagnostic probing data with known-good-board 

standards

 An ability to do some limited functional testing using the SI capability, including an analog front 

end with routing of digital signals, analog signals, and power

 A robotic positioning tool for camera positioning and diagnostic probing

 The capability to downlink all the needed diagnostic data for evaluation by system experts

3.3.3 Potential Problems Resolved by On-Orbit Component-Level Repair

There appear to be opportunities to expand diagnostic capability since many problem reports are 

unable to determine the root cause. Often a workaround scheme involving a change in operations, a 

software patch, or a periodic “preemptive refresh” is used to compensate for a persistent hardware 

problem. For this analysis, whenever a BIT and telemetry measurement failed to isolate faults and a rootcause-unknown status persisted, the problem was in clear need of additional external diagnostics. In many 

cases, the availability of a robust diagnostic and test capability would have altered the strategy for 

addressing the problem. For internal ORUs that are relatively easy to access, this capability could have 

resolved issues more quickly or, at minimum, provided additional supporting data. 

For existing hardware on the ISS, the expansion of BIT and telemetry could reduce the diagnostics 

gap, but it would involve a costly design overhaul and a widespread R&R to upgrade ORUs. The cost 

tradeoff between a widespread upgrade versus external diagnostics for an existing ISS system will likely 

favor external diagnostics.

For new designs of CxP hardware, the allocation of BIT and telemetry capabilities is currently in 

the design tradeoff phase. Spacecraft weight, volume, power, and data handling and communications 

bandwidth limitations will constrain built-in capabilities to flight critical measurements. For practical 

design and programmatic reasons, the indepth diagnostics required to isolate faults may, once again, favor 

external diagnostics. Therefore, CLEAR diagnostic and test capability may provide a cost-effective,

flexible solution for both ISS and the next generation of CxP spacecraft.
NASA/TM—2011-216954 7

3.3.4 Screening Criteria—Problems Not Suitable for CLEAR Diagnostics 

As discussed earlier, many problems not suited to component-level repair will still benefit from 

additional diagnostic capabilities. The criteria used to determine if a problem cannot be diagnosed by 

CLEAR diagnostic operations follows:

 Complete BIT and Telemetry Problems.—When existing means of monitoring and BITs are 

successful and unambiguous, then additional CLEAR diagnostic instruments are not needed.

 Russian Hardware Problems.—These problems are out of scope; Russians are responsible for 

their own hardware.

 Mechanical Problems.—These problems are out of scope; most do not require diagnostic 

instruments.

 Software Problems.—Diagnostics may not be needed if self-diagnosing software is effective.

3.3.5 Software-Hardware Ambiguity

There have been a number of instances where a problem has occurred, but it was unclear whether the 

root cause was actually hardware or software. Transients involving multiple interacting software modules 

are particularly difficult to isolate. In some cases, it is necessary to use external diagnostics to locate a 

software flaw or at least exonerate the hardware. There has been at least one occasion where BIT software 

collided when different parts of the system ran BITs concurrently.
NASA/TM—2011-216954 8

4.0 Problem Reporting and Corrective Action Analysis

4.1 Electrical Power System

The EPS includes photovoltaic power generation, power distribution, and power loads. Of the 

171 electrical system problems, 66 were related to lighting. Lighting hardware is life limited, and the 

problems are deemed to be logistics end-of-life (EOL) problems rather than engineering problems. Thus,

lighting problems were not considered for this and subsequent system reviews. Table II shows the EPS 

PRACA report types.

TABLE II.—INTERNATIONAL SPACE STATION ELECTRICAL POWER SYSTEM (EPS)

PROBLEM REPORTING AND CORRECTIVE ACTION (PRACA) REPORTS

EPS subsystem PRACA reports Candidate for 

CLEAR repair

Would benefit from 

Number Percent of CLEAR diagnostics

EPS total

Remote Power Controller Module (RPCM) 60 56.1 56 (93% of RPCM) 57 (95% of RPCM)

Electrically Erasable Programmable Read-Only 

Memory (EEPROM) bit flip 24 24 24

Field-effect transistor (FET) controller/FET driver 28 28 28

FET charge pump 1 1 1

Electromotive force (EMF) voltage due to fan windmill 1 0 1

Low-temperature exposure 2 0 0

Wire damage 1 1 1

Single event upset (SEU) 1 0 0

Data acquisition hybrid 1 1 1

Lost resistance temperature detector (RTD) 1 1 1

DC–DC Converter Unit (DDCU) 7 6.5 4 6

EMF voltage due to fan windmill 1 0 1

EEPROM bit flip 2 2 2

FET controller 2 2 2

Micrometeoroid and Orbital Debris (MMOD) damage 1 0 0

SEU 1 0 1

Battery Charge/Discharge Unit (BCDU) 4 3.7 1 3

EEPROM bit flip 1 1 1

Incorrectly configured 2 0 1

SEU 1 0 1

Direct-Current Switching Unit (DCSU) 2 1.9 1 1

Inadvertent cold soak 1 0 0

Unknown intermittent 1 1 1

Sequential Shunt Unit (SSU) 2 1.9 1 2

Mechanical adhesive failure 1 1 1

SEU 1 0 1

Main Bus Switching Unit (MBSU) 3 2.8 3 3

Faulty sensor 1 1 1

EEPROM bit flip 2 2 2

Pump Flow Control Subassembly (PFCS) firmware error 1 0.9 0 1

Floating potential measurement unit (FPMU) 1 0.9 1 1

Assembly Power Converter Unit (APCU) 2 1.9 1 1

Heater circuit delamination 1 1 1

Operations error 1 0 0

Plasma contactor unit (PCU) 2 1.9 0 0

Mechanical valve leak 1 0 0

Mechanically insufficient xenon heater 1 0 0

Utility outlet panel 3 2.8 1 2

Circuit failed due to poor construction 1 1 1

Ground fault circuit interrupter (GFCI) trip 1 0 1

Mechanical fastener missing 1 0 0

Rack power switch (RPS) assembly 3 2.8 0 1

Part serial number error 2 0 0

Switch configuration error because of software change 1 0 1

Russian equipment 17 15.9 0 0

Total EPS on-orbit PRACA reportsa 107 100 69 (64% of EPS) 78 (73% of EPS) a

Does not include 66 lighting PRACA reports.
NASA/TM—2011-216954 9

When lighting problems are excluded, over 56 percent of EPS on-orbit PRACA reports are related 

to the RPCM. Of those RPCM problems, 87 percent are dominated by two very persistent problems. 

Russian-related hardware problems account for 16 percent of problems; as noted earlier, these problems 

are too ambiguous to properly analyze. Beyond lighting, RPCM, and Russian hardware, a diverse set of 

problems appear to be distributed over several other ORU packages. Individual or isolated problems tend 

to represent 1 to 2 percent of total EPS on-orbit PRACA reports.

4.1.1 Electrical Power System On-Orbit Component-Level Repair

EPS spans the breadth of all power applications throughout ISS. The varied components require 

unique problem resolutions. For example, the utility outlet panel (UOP) is equivalent to the simple surgeprotected outlet boxes used in homes and offices to protect computers and peripherals from power surges.

The ISS UOP was rather unique in that all units had innate design flaws, workmanship, and material 

flaws from their construction. Because the UOPs were few in number and easy to access, the program 

resolved the problems by simply replacing the entire inventory with new upgraded units. Widespread 

R&R, however, is an extremely rare solution.

In contrast, RPCMs are essential and appear in numerous locations in the ISS power system, both 

internal and external to the ISS cabin environment; thus, wholesale R&R of RPCMs is not feasible.

Despite the relatively high number of RPCM faults, a single fault does not normally disable the entire 

unit. For example, if the RPCM loses only one of several channels, rerouting power through spare 

channels can resolve the problem. Furthermore, a given RPCM may experience multiple faults before 

it is necessary to R&R it.

There are two recurring faults with RPCMs that also appear in other EPS ORUs (i.e., DDCU, BCDU,

MBSU, PCU, and SSU):

 FET Controller Hybrid.—30 problem reports

 RM2667–001 EEPROM.—29 problem reports

Both devices have problems where the root cause is a flaw in the original manufacturing process. The 

nature of the ORU and the specifically identified components make these problems attractive targets for 

CLEAR repair processes. Developing repair solutions for these two problems would have resolved 59 of 

the 107 EPS problem reports.

Overall, the analysis shows that up to 64 percent of EPS problems could have been resolved by a 

component-level repair.

4.1.2 Electrical Power System On-Orbit Diagnostic and Test Operations

As discussed earlier, the PRACA system generally lacks details about diagnostic techniques and 

equipment. Some lower-level failure analysis reports or tear down, test, and evaluations describe 

diagnostic measurements but only on equipment that has been returned from orbit.

For the EPS, there are a significant number of unknown but suspected root causes. Many problem 

reports attribute anomalies to the FET controller or EEPROM problems, but because of insufficient data, 

they are not entirely conclusive. Therefore, regardless of the approach to repair, there is substantial 

evidence that additional diagnostics is needed for power system problems.

SEU incidents, which occur when space radiation causes a semiconductor device to suddenly change 

state, are often blamed for anomalies. Although SEUs are not reproducible events, external diagnostics 

can be used to eliminate other potential causes and provide confidence in the SEU prognosis. 

As shown in Table II, up to 73 percent of EPS problems could employ external diagnostic and test 

capabilities. Unlike adding BIT and telemetry instrumentation, CLEAR diagnostic equipment would 

require no retrofit or design impact. 
NASA/TM—2011-216954 10

4.2 Command and Data Handling

The C&DH system governs the command, data transmission, and processing throughout the ISS. The 

common ORU element is the MDM. MDMs serve as the primary interface and subsystem controller.

Note that MDMs appear at different levels in a system’s hierarchy. That is, multiple redundant MDMs 

may be managing a system at the top level, but subordinate MDMs will also appear at lower levels.

MDMs communicate on an STD 1553 data bus.

The MDMs of the C&DH system employ a chassis that is a backplane connector, or card cage, design 

that provides simplified access, removal, and replacement at the SRU level. The intent was to provide a 

limited number of common, on-orbit SRU spares to reduce the weight and mass of the spare hardware.

ORUs composed of SRUs actually make component-level removal and replacement easier because of the

increased accessibility at the component level.

Low-level MDMs have a variety of analog and digital input/output (I/O) cards for interacting with 

hardware and instruments. The top-level MDMs provide data storage. Early in the program, the most 

common on-orbit MDM problem reports were due to the Mass Storage Device (MSD), which used 

common hard disk technology for data storage and had limited operating life. These problems were 

resolved by a major R&R of MSD disks with Solid State Mass Memory Units (SSMMUs). Table III

shows the C&DH PRACA report types.

TABLE III.—INTERNATIONAL SPACE STATION (ISS) COMMAND AND DATA HANDLING (C&DH) 

PROBLEM REPORTING AND CORRECTIVE ACTION (PRACA) REPORTS

C&DH subsystem PRACA reports Candidate for 

CLEAR repair

Would benefit from 

Number Percent of CLEAR diagnostics

C&DH total

Multiplexer/Demultiplexer Module (MDM) 24 80 4 9

5-V Power supply (component level) 1 1 1

Mass Storage Device (MSD) replaced by Solid State 

Mass Memory Unit (SSMMU) at shop replaceable 

unit (SRU) level

5 0 5

Material issue 2 0 0

Drawing error 2 0 0

Cable, wiring, or connectors 3 3 3

Software 11 0 0

Automated Payload Switch (APS) 3 10 0 3

Single event upset (SEU) 3 0 3

Russian Service Module 3 10 0 0

Total C&DH on-orbit PRACA reports 30 100 4 (13% of C&DH) 12 (40% of C&DH)

4.2.1 Command and Data Handling On-Orbit Component-Level Repair 

Only the 5-V power supply and the cable, wiring, or connector problems appear to be suited to a 

component-level repair. MDMs are configured as a backplane card cage and are very suited to SRU-level 

replacement. The replacement of MSD modules with a SSMMU was the single major SRU replacement

that affected several units. Software fixes and upgrades make up 36 percent of the problem resolutions.

Component-level repairs on these SRUs would be easily accommodated because of the ease of access 

that the backplane design provides. In addition, the standard circuit card makes it easier to develop cardhandling fixtures that simplify the repair process. So far, there has been little need for component-level 

repairs, but MDM circuits would be quite amenable to such future repairs.

4.2.2 Command and Data Handling On-Orbit Diagnostic and Test Operations

The C&DH system has strong telemetry and self-diagnostic capability. Since data storage or software 

codes were the primary problems, BIT diagnostics proved to be quite adequate in isolating most problems. Only 27 percent appear to be problems that would benefit from the external diagnostics provided by 

CLEAR. The MDM is the only ORU that is supported by additional diagnostic capability known as the 

MDM on-orbit tester (MOOT). 
NASA/TM—2011-216954 11

Figure 1.—The Multiplexer/Demultiplexer Module (MDM) on-orbit tester (MOOT) serves as an interface kit 

between an MDM and a space station computer and utility power strip. 

The MOOT is not a traditional test instrument. It would be more accurate to describe it as an interface kit.

It is composed of a power supply along with an RS422 and 1553 data bus cables and an adapter. The MOOT 

provides a special keying plate that assures that only the correct pins are connected so that the crew cannot 

damage the MDM. The MOOT also includes an existing space station computer (SSC) that acts as an external 

terminal to command BITs and acquire test data. Figure 1 shows a simple schematic of the MOOT. 

Data systems, by nature, are most suited to self-diagnostics since they can easily expand data

handling to incorporate internal measurements and system states. If the main controller card and power 

supply are operational, BIT diagnostics can be used. The weakness is an inability to resolve problems 

where the power or main processors are inoperative. In those cases, external diagnostics is required.

4.3 Communications and Tracking

C&T is composed of a very diverse set of hardware ranging from audiofrequency circuits, to video, to

ultrahigh frequency (UHF), S-band, and Ku-band radiofrequency (RF) communications. 

Audio is composed of low-frequency analog circuits with channel switching linked by a 1553 data 

bus. Video is composed of numerous video cameras mounted internally and externally, including some 

used in critical EVR activities. Video is also supported by videotape recorders (VTRs) and display 

monitors. RF communications includes antennas and their related pointing mechanisms. RF thus has a 

substantial number of mechanical problems. 

4.3.1 Communications and Tracking On-Orbit Component-Level Repair 

Analysis of the PRACA C&T data indicated that 21 percent of the problems would have been suited 

to in situ repairs that CLEAR could perform on orbit. Most C&T ORUs have problems where the root 

cause remains unknown. There would be more opportunities for repairs if these causes could be isolated.

C&T mechanical and thermal problems have been exclusively in the RF portion of the system.

The audio subsystem appears to be plagued by hardware quality problems. Video support equipment 

like VTRs and video monitors are experiencing EOL failures. Some VTR problems were resolved by 
NASA/TM—2011-216954 12

performing periodic recording head cleaning. This is a recurring service item that consumes both crew 

time and logistics. Tape recorders wear out and become obsolete; thus, an upgrade to durable solid-state 

memory is preferred over component repairs.

4.3.2 Communications and Tracking On-Orbit Diagnostic and Test Operations

Problems with an unknown root cause represent a full 25 percent of the C&T problem reports, and 

this is due in part to lack of adequate telemetry data and in part to the intermittent nature of many C&T

problems. Loss-of-communication faults that occurred for audio, video, and RF equipment often resulted

in a loss of data that would help diagnose the problem.

Since C&T is the main conduit for telemetry measurements, it is natural that a C&T fault could 

interfere with the diagnosis of a problem. There is some evidence that BITs ran, but efforts to reestablish 

communications often required a reset of hardware resulting in the loss of diagnostic data records. 

Without tangible data, engineering was forced to speculate on the root cause of problems. Of the 

problems listed, 63 percent would benefit from independent external diagnostics in isolating the root 

cause of faults. Table IV shows the C&T PRACA report types.

TABLE IV.—INTERNATIONAL SPACE STATION (ISS) COMMUNICATIONS AND TRACKING (C&T) 

PROBLEM REPORTING AND CORRECTIVE ACTION (PRACA) REPORTS

C&T subsystem PRACA reports Candidate for 

CLEAR repair

Would benefit from 

Number Percent of CLEAR diagnostics

C&T total

Audio 18 25 6 17

Wire and connector 1 1 1

Connector pin 1 1 1

Connector contamination 1 1 1

Root cause unknown 8 ? 8

Operations error 1 0 0

1553 bus (suspect) 2 1 2

Microphone electromagnetic interference (EMI) 1 0 1

Earpiece receiver 1 1 1

Data glitch 1 0 1

Defective switch 1 1 1

Video 22 31 5 20

Wire and connectors 1 1 1

Videotape recorder (VTR) cleaning 4 1 4

Root cause unknown 7 ? 7

Suspect single event upset (SEU) 1 0 1

Software fault 1 0 0

1553 hybrid 2 1 2

High-frame-rate circuit-card assembly (CCA) 1 1 1

VTR operations 1 0 0

End of life (EOL) 1 0 1

SEU 1 0 1

VTR reel motor binding 1 1 1

VTR EOL 1 0 1

S-band 13 18 1 3

Mechanical 10 0 0

SEU 1 0 1

Solder 1 1 1

Root cause unknown 1 ? 1

Ku-band 10 14 3 5

Mechanical 3 0 0

Root cause unknown 2 ? 2

Software 1 0 0

Heater 1 1 1

Components 2 2 2

Flight operations 1 0 0

Ultrahigh frequency (UHF) 1 1 0 0

Mechanical 1 0 0

Russian equipment 7 10 0 0

Total C&T on-orbit PRACA reports 71 15 (21% of C&T) 45 (63% of C&T)
NASA/TM—2011-216954 13

4.4 Guidance, Navigation, and Control

The GN&C system has a low occurrence of problems in comparison to other systems. The control 

moment gyroscope (CMG) system represents a full 60 percent of the problems. This is an important and 

rather large electromechanical ORU with spinning gyros. The CMG system is essential to maintaining 

orbital attitude of the entire ISS. 

4.4.1 Guidance, Navigation, and Control On-Orbit Component-Level Repair 

On the basis of the data from the PRACA database, communication-related faults and gyro spin 

motor controllers appear to be the best candidates for on-orbit repair because of the electronic nature of 

these problems.

4.4.2 Guidance, Navigation, and Control On-Orbit Diagnostic and Test Operations

Roughly 50 percent of the problems appear to benefit from external diagnostics in isolating the root 

cause of faults. Table V shows the GN&C PRACA report types.

TABLE V.—INTERNATIONAL SPACE STATION (ISS) GUIDANCE, NAVIGATION, AND CONTROL (GN&C) 

PROBLEM REPORTING AND CORRECTIVE ACTION (PRACA) REPORTS

GN&C subsystem PRACA reports Candidate for CLEAR 

repair

Would benefit from 

Number Percent of CLEAR diagnostics

GN&C total

Control moment gyroscope (CMG) 11 61 6 8

Mechanical 3 0 0

Loss of CMG communications 4 4 4

Angular bias 1 0 1

Spin motor control 2 2 2

Over rate error 1 0 1

Rate Gyro Assembly (RGA) 2 11 0 1

Attitude walk off 1 0 1

Mechanical pressure leak 1 0 0

Russian equipment 5 28 0 0

Total GN&C on-orbit PRACA reports 18 100 6 (33% of GN&C) 9 (50% of GN&C)

4.5 Extravehicular Robotics

EVR involves a very large remote manipulator arm that can move around the ISS truss, move 

payloads, and recover and install external ORUs. Although mechanical in nature, it is surprisingly 

complex in terms of instrumentation and imaging support. Over 66 percent of EVR PRACAs are 

electrical.

4.5.1 Extravehicular Robotics On-Orbit Component-Level Repair 

This system has a mix of controllers and instruments. Motor controllers for joint drives (and braking) 

are sophisticated closed loops with careful monitoring of motor currents. For teleoperations, a number of 

cameras and related video support are required. Roughly 39 percent of EVR problems appear to be 

suitable for on-orbit repair.

4.5.2 Extravehicular Robotics On-Orbit Diagnostic and Test Operations

Roughly 55 percent of the problems appear to benefit from external diagnostics in isolating the root 

cause of faults. Table VI shows the EVA PRACA report types.
NASA/TM—2011-216954 14

TABLE VI.—INTERNATIONAL SPACE STATION (ISS) EXTRAVEHICULAR ROBOTICS (EVA) 

PROBLEM REPORTING AND CORRECTIVE ACTION (PRACA) REPORTS

EVR subsystem PRACA reports Candidate for 

CLEAR repair

Would benefit from 

Number Percent of CLEAR diagnostics

total

Lighting 3 9 1 1

Camera and video 6 18 4 6

Software control and telemetry 6 18 0 3

Motor and controller 4 12 4 4

Electromechanical switches 2 6 2 2

Connector cable 2 6 2 2

Mechanical 10 30 0 0

Total EVR on-orbit PRACA reports 33 13 (39% of EVR) 18 (55% of EVR)

5.0 Summary of Potential of CLEAR Diagnostic and Repair Operations

CLEAR reviewed PRACA reports for ISS electrical systems to help define the repair and diagnostic 

needs. CLEAR is aimed at developing repair and diagnostic capabilities for the CxP. The ISS is the first 

spacecraft with a relatively long on-orbit experience base, and it is the first instance where an ORU 

logistics strategy has been used. The CLEAR Project was also interested in determining how often an 

in situ repair and diagnostics capability could be applied. Table VII summarizes the ISS PRACA reports.

TABLE VII.—SUMMARY OF INTERNATIONAL SPACE 

STATION (ISS) PROBLEM REPORTING AND 

CORRECTIVE ACTION (PRACA) REPORTS

ISS system PRACA

reports

On-orbit PRACA 

reports

ISS (all systems) 4076 770

Electrical system 1914 328

Electrical system PRACA reports compose about 42 percent of all on-orbit PRACA reports. This is 

not to say that all electrical system problems are purely electrical. However, mechanical systems will also 

have electrical problems. To keep the analysis within time and cost constraints, the CLEAR team limited 

the scope of the effort to electrical systems. The EVR system was included because of the high percentage 

of electrical problems.

To further limit the analysis set of this review, the CLEAR team did not consider the 66 ISS light 

fixture problems from the EPS since they involve items that had reached or exceeded their predicted 

EOL. Lighting has been described as a logistics issue rather than a repair issue. In addition, two duplicate 

problem reports were extracted from the C&DH set. The remaining 259 problem reports were evaluated 

to determine if a potential CLEAR repair or diagnostics capability could have resolved the problem.

Table VIII and the chart in Figure 2 show the number of potential repairs and diagnostic capabilities 

for each system. The percentage indicates the fraction of the system’s problem set that could employ 

CLEAR repair or diagnostics. Overall, in situ CLEAR repair appears to cover 42 percent of the electrical 

problems. CLEAR diagnostic and test capabilities appear to cover 63 percent of the electrical problems.

TABLE VIII.—POTENTIAL IMPACT ON INTERNATIONAL SPACE STATION (ISS) ELECTRICAL 

PROBLEMS BY A ROBUST COMPONENT-LEVEL DIAGNOSTIC AND REPAIR CAPABILITY

ISS system On-orbit 

PRACA reports

Candidate for 

CLEAR repair

Would benefit from 

CLEAR diagnostics

Number Percent of 

total

Number Percent of 

total

Number Percent of 

total

Electrical Power System (EPS) 107 41 69 64 78 73

Command and Data Handling (C&DH) 30 12 4 13 12 40

Communications and Tracking (C&T) 71 27 15 21 45 63

Guidance, Navigation, and Control (GN&C) 18 7 6 33 9 50

Extravehicular Robotics (EVR) 33 13 16 48 18 55

All electrical on-orbit PRACA reports 259 100 110 42 162 63
NASA/TM—2011-216954 15

Figure 2.—This chart shows that the portion of problems that benefit from component-level repair varies from 

system to system. External diagnostics, however, appears to provide a substantial impact across all systems.

The disparity between diagnostics and repair is due to the number of problems where symptoms are 

observed but a repair cannot be defined because of the root-cause-unknown status. Often the root cause 

remained unknown because the problem involved a loss of communications needed to transmit health 

data or the problem prevented the execution and storage of BIT diagnostics. With external in situ 

diagnostics, many of the root causes could be established and appropriate corrective actions, including 

repair could be performed. In general, CLEAR diagnostic capabilities have more widespread application 

than repair because of the need for diagnosing problems that are out of reach of existing BIT diagnostics 

or health-monitoring telemetry. 

In situ electronics repair was not applied to mechanical, thermal, or structural problems. Furthermore,

problems that stem from software, operations, or documentation errors are not applicable. Another 

category is the SEU problem that occurs when digital electronics are exposed to space radiation.

Generally, a memory register or a logic gate is corrupted, and this can be cleared by a register refresh or 

by executing an error correction code. If no permanent damage occurs, then a component repair is not 

needed. 
NASA/TM—2011-216954 16

6.0 Conclusions

International Space Station (ISS) electronics is proving to be somewhat more reliable than predicted.

The on-orbit Problem Reporting and Corrective Action (PRACA) history indicates, however, that tens of 

thousands of operating hours are accumulating on ISS and that failures are inevitable. The ISS PRACA 

history also indicates that a number of persistent problems seem to evade normal telemetry measurements 

and built-in tests. The number of “root cause unknown due to insufficient data” statements would be 

alarming if it was not for the robustness of the system design and the resourcefulness of operations and 

engineering. It also indicates that telemetry and built-in tests need to be augmented with external 

diagnostic capabilities to unveil the elusive root causes. It appears that as high as 63 percent of electrical 

problems would benefit from additional external diagnostics and test capability. In most cases, the 

instruments would augment existing capability; in other cases, the external instruments could quickly 

isolate a problem that currently has an unknown root cause. 

PRACA history only describes component-level repairs in Earth-based depot and vendor facilities.

Multiplexer/Demultiplexer Modules are the only hardware where actual shop replaceable unit-level 

replacements of circuit-card assemblies have been performed. In regards to problems that can be repaired 

by in situ component-level repair, it appears that 42 percent of the problems are linked to component 

faults. PRACAs represent instances of problems, and thus, many component repairs may be addressed 

by very few executable repair procedures. For example, roughly 56 percent of Electrical Power System

problems can be traced to only two components: the field-effect transistor controller hybrid and 

Electrically Erasable Programmable Read-Only Memory (EEPROM). It may be practical to perform 

both repairs with a single orbital replaceable unit processing procedure.

The Component-Level Electronic-Assembly Repair (CLEAR) team concludes that problems 

amenable to component-level repairs exist on ISS and in significant numbers. Many nagging problems 

hang unresolved while hardware continues to age and degrade. With the shuttle flights ending in the near 

future, diagnostic and repair capabilities could meet real needs and could be employed immediately.

Further definition of specific solutions requires direct interaction with the ISS system managers, 

engineers, and contractors.
NASA/TM—2011-216954 17

Appendix.—Acronyms and Abbreviations

APCU Assembly Power Converter Unit

APS Automated Payload Switch

BCDU Battery Charge/Discharge Unit

BIT built-in test

C&DH Command and Data Handling

C&T Communications and Tracking

CCA circuit-card assembly

CLEAR Component-Level Electronic-Assembly Repair

CMG control moment gyroscope

CRE–1 Component Repair Experiment 1

CxP Constellation Program

DCSU Direct-Current Switching Unit

DDCU DC–DC (direct-current to direct-current) Converter Unit

EEPROM Electrically Erasable Programmable Read-Only Memory

EMF electromotive force

EMI electromagnetic interference

EOL end of life

EPS Electrical Power System

ETDP NASA Exploration Technology Development Program

EVR Extravehicular Robotics

FET field-effect transistor

FPMU floating potential measurement unit

GFCI ground fault circuit interrupter

GN&C Guidance, Navigation, and Control

I/O input/output

ISS International Space Station

MBSU Main Bus Switching Unit

MDM Multiplexer/Demultiplexer Module

MMOD Micrometeoroid and Orbital Debris

MOOT MDM On-Orbit Tester

MSD Mass Storage Device

ORU orbital replaceable unit

PCU plasma contactor unit

PFCS Pump Flow Control Subassembly

PRACA Problem Reporting and Corrective Action

RF radiofrequency

R&R remove and replace

RGA Rate Gyro Assembly

RPCM Remote Power Controller Module

RPS rack power switch
NASA/TM—2011-216954 18

RTD resistance temperature detector

SEU single event upset

SI synthetic instrument

SoRGE Soldering in Reduced Gravity Experiment

SRR System Requirements Review

SRU shop replaceable unit

SSC space station computer

SSMMU Solid State Mass Memory Unit

SSU Sequential Shunt Unit

UHF ultrahigh frequency

UOP utility outlet panel

VMDB Vehicle Master Database

VTR videotape recorder
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14. ABSTRACT

The NASA Constellation Program is investigating and developing technologies to support human exploration of the Moon and Mars. The Component-Level ElectronicAssembly Repair (CLEAR) task is part of the Supportability Project managed by the Exploration Technology Development Program. CLEAR is aimed at enabling a flight 

crew to diagnose and repair electronic circuits in space yet minimize logistics spares, equipment, and crew time and training. For insight into actual space repair needs, in early 

2008 the project examined the operational experience of the International Space Station (ISS) program. CLEAR examined the ISS on-orbit Problem Reporting and Corrective 

Action database for electrical and electronic system problems. The ISS has higher than predicted reliability yet, as expected, it has persistent problems. A goal was to identify 

which on-orbit electrical problems could be resolved by a component-level replacement. A further goal was to identify problems that could benefit from the additional 

diagnostic and test capability that a component-level repair capability could provide. The study indicated that many problems stem from a small set of root causes that also 

represent distinct component problems. The study also determined that there are certain recurring problems where the current telemetry instrumentation and built-in tests are 

unable to completely resolve the problem. As a result, the root cause is listed as “unknown.” Overall, roughly 42 percent of on-orbit electrical problems on ISS could be 

addressed with a component-level repair. Furthermore, 63 percent of on-orbit electrical problems on ISS could benefit from additional external diagnostic and test capability. 

These results indicate that in situ component-level repair in combination with diagnostic and test capability can be expected to increase system availability and reduce logistics. 

The CLEAR approach can increase the flight crew’s ability to act decisively to resolve problems while reducing dependency on Earth-supplied logistics for future Constellation 

Program missions.

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Space operations; Space logistics 

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