THE MSX COMMAND AND DATA HANDLING SYSTEM

The MSX Command and Data Handling System

David D. Stott, Ronald K. Burek, Peter Eisenreich, Jerry E. Kroutil, Paul D. Schwartz, andGlen F. Sweitzer

he Command and Data Handling System of the Midcourse Space Experimentprovides spacecraft control and data collection, formatting, and onboard storage. It hasembedded encryptors and decryptors to ensure communication security during themission. The Command and Data Handling System also maintains onboard time andperforms autonomy rule checking of housekeeping data for fault protection.

T

INTRODUCTIONOver its 5-year lifetime, the Midcourse Space Exper-

iment (MSX) will carry out hundreds of data collectionevents involving actively tracked targets, celestialbackgrounds, and Earth backgrounds. Data sources in-clude 11 optical sensors that produce images and spec-tra over the infrared, visible, and ultraviolet wave-lengths. These events, which last from 30 min to 6 h,occur while the spacecraft is out of ground contact.Onboard computers execute the event scenario to pointthe spacecraft, control the measurements, and store theresultant data on the spacecraft tape recorders. Afterthe event, the spacecraft returns to its park mode torecover power and thermal balance. Later, data fromthe tape recorders are dumped via the X-band telecom-munication link to the APL ground station. Withinthis framework, the details of individual events arehighly diverse.

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The capability and versatility required by the space-craft provided the design drivers for the MSX Commandand Data Handling (C&DH) System. First among thesedrivers is the high volume of data generated during anevent. System trade-offs resulted in a 25-megabit persecond (Mbps) aggregate real-time science data rate andredundant tape recorders, each capable of storing54 gigabits (Gb). Each event has its own selection ofinstrument operating modes. Therefore, a flexible meth-od of allocating science data bandwidth was required touse this limited resource efficiently. The 5-year missionlife and the criterion that there be “no credible single-point failure” for C&DH functions led to fully redundantC&DH hardware, which implies cross-strapping be-tween units and design of interfaces to ensure fault iso-lation. For spacecraft fault protection, an uploadable,rule-based autonomy capability is required. Relatively

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of these units is discussed in the following sections.Together, they provide the following capabilities for theMSX spacecraft:

• Control of the spacecraft, using pulse commands, datacommands, and more than 200 power switching re-lays. Commands transmitted from ground stations areacquired via the S-band transponders. They may beexecuted immediately or stored in commandprocessor memory for later execution. Commandsare also accepted from the tracking and attitudeprocessors.

2-kbps uplinkcommands

from S-bandreceiver 1

2-kbps uplinkcommands

from S-bandreceiver 2

Commandprocessor 1

Data handlingsystem 1

KG/tempestunit 1

(encryptors)

Taperecorder 1

Taperecorder 2

KG/tempestunit 2

(encryptors)

Data handlingsystem 2

Commandprocessor 2

KG/tempestunit 2

(decryptor)

Ordnance Control

Powerswitching

unit 1

Unit 2

KG/tempestunit 1

(decryptor)

Relaycommands

Ordnancecommands

Commands fromtracking and

attitudeprocessors

Datacommands

Logicpulse

commands

Memoryloads

Datainputs

25 MHzfrom ultra-

stableoscillator

Mission elapsedtime/UniversalTime outputs

25 Mbps X-bandtransmitter 1

S-bandtransmitter 1

X-bandtransmitter 2S-bandtransmitter 2

25 Mbps

16 kbps,1 Mbps

16 kbps,1 Mbps

Commands fromtracking and

attitudeprocessors

Datacommands

Logicpulse

commands

Memoryloads Data

inputs

25 MHzfrom ultra-

stableoscillator

Mission elapsedtime/UniversalTime outputs

Figure 1. Functional block diagram of the MSX Command and Data Handling (C&DH) System, showing cross-strapped interconnectionbetween redundant units (KG = key generator).

simple rules, based on such engineering housekeepingpoints as tape recorder head temperature and power busvoltage, are used to detect and circumvent problemshaving the potential to end the mission. Autonomyrules are stored in and executed by the C&DH com-mand processors. Data security requirements apply tothe telecommunication links between the spacecraft andground, which are all encrypted.

To meet these requirements, the MSX C&DH Sys-tem was designed using a systematic approach that ledto a partitioned design providing high-performancedata processing and neededmission flexibility while still re-taining high reliability and faulttolerance. The following sectionsdescribe the system design and itscomponents.

OVERVIEWThe MSX C&DH System con-

sists of command processor, datahandling, key generator (KG)/tempest, power switching, and taperecorder components, as shown inFig. 1 and listed in Table 1. Each

Table 1. Components of the MSX Command and Data Handling System.

Function Hardware assemblies ProviderCommand processing Command processors 1 and 2 APLData handling Data handling systems 1 and 2 APLData encryption and Key generator/tempest electronics Cubic Defense

decryption units 1 and 2 Systems, Inc.Power switching Power switching units 1 and 2 APL

Ordnance control unitData storage Tape recorder electronics units Odetics, Inc.

1 and 2

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THE MSX COMMAND AND DATA HANDLING SYSTEM

processors are actively redundant; that is, they arepowered continuously for the life of the mission. Otherunits are selectively powered and utilize commandedcross-strap data paths to route data appropriately.Downlink data streams are routed by selection ofmultiplex switches in the KG/tempest units.

All of the elements of the C&DH System (13 hard-ware assemblies) received thorough stand-alone testingbefore they were integrated with the spacecraft. Specialtest sets were developed for this purpose in parallel withthe flight hardware and software design. The impor-tance of these computer-controlled test sets cannot beoverstated. Their availability was a key milestone in allof the C&DH deliveries. Because interfaces betweenC&DH components are simple and well defined, verylimited tests between flight hardware items were need-ed. Each unit was required to pass component-levelenvironmental tests (vibration and thermal-vacuumcycles) prior to integration with the spacecraft. Anyrepair or rework of a flight unit was followed by spec-ified requalification vibration and temperature testing.

As the details in the following sections show, thespacecraft C&DH System is interface intensive. Ittouches nearly every other box on the spacecraft withat least one electrical interface. Careful design andcomprehensive, exhaustive testing were main ingredi-ents in the successful implementation of the MSXC&DH System.

COMMAND PROCESSORThe MSX command processor receives uplink

command messages, interprets them, and then sends

• Collection, formatting, and serial output of the16-kilobits per second (kbps), 1-Mbps, and 25-Mbpsdata streams. When enabled, these data streams aregenerated simultaneously and in synchronism witheach other.

• Maintenance and distribution of mission elapsed timeand Universal Time on the spacecraft.

• Provision of spacecraft autonomy functions.• Encryption of downlink data, decryption of uplink

(command) data, and command authentication.• Recording of the prime science data stream at 5 Mbps

or 25 Mbps. Data storage capacity is 54 Gb perrecorder. Data playback is at the single rate of25 Mbps to the APL ground station via X-band radiofrequency (RF) link. As an option, the prime sciencedata path can be switched to place the real-time 25-Mbps data stream on the X-band downlink.

Table 2 summarizes the RF telecommunication datalinks between the spacecraft and the ground. There arethree downlinks and one uplink. S-band communica-tion is used for commanding and the lower-rate down-links. The S-band transponders are an industry stan-dard compatible with omnidirectional spacecraftantennas and many ground stations. In contrast, the X-band equipment required at 25 Mbps is unique to theMSX program. A high-gain X-band antenna on thespacecraft is gimbal mounted and pointed at the groundstation during the 25-Mbps downlink operations. Theonly ground station with receiving equipment for thissignal is at APL.

Figure 1 also shows the redundancy and cross-strapping between elements of the MSX C&DH Sys-tem. The uplink receivers, decryptors, and command

Table 2. Summary of MSX telecommunication data links.

Source/Name Bit rate RF carrier destination CommentPrime science 25 Mbps X-band Spacecraft Three modes:

data to ground 1. Playback of data previouslyrecorded at 25 Mbps

2. Playback of data previouslyrecorded at 5 Mbps

3. Real-time 25-Mbps primescience data

Wideband 1 Mbps 2282 MHz Spacecraft Real time onlyscience data to ground

Narrowband data 16 kbps 2282 MHz Spacecraft Real time onlyto ground

Commands 2 kbps 1827 MHz Ground to Commands for immediatespacecraft execution

Commands for delayed executionMemory load data

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appropriate control signals to the spacecraft subsystemsand instruments. Commands may be executed in realtime (immediately after reception) or stored in memoryfor later processing. The command processor can alsoreceive commands from the tracking and attitude pro-cessors, and it has the ability to check the telemetrydata and generate commands autonomously based ona set of uploadable rules.

Figure 2 shows one of the two redundant commandprocessor units in the C&DH System. The 5.9-kg pro-cessor measures 25 3 18 3 31 cm and consumes 1 Wfrom a single 5-V supply. Processing is performed by a0.5 million-instruction-per-second MIL-STD-1750Aprocessor with a real-time multitasking operating sys-tem kernel. The 24 K-word application code is writtenin Ada and assembly language (1 K-word is 102416-bit processor words in the 1750A architecture). Upto 15,000 delayed commands can be stored in the512-K-word, error-detecting and correcting randomaccess memory. Program storage based on programma-ble read-only memory ensures that the processor willfully recover if a single-event upset or other anomalyshould cause a reset. For fault protection, the commandprocessor has its own independent oscillator. UniversalTime is derived from the telemetry data but is main-tained from the internal oscillator should the telemetrynot be available.

Commands are transmitted over a variety of dedicat-ed interfaces. Special interfaces with the power switch-ing units can support 192 relay commands in threepower-switching units. The standard interfaces are

Figure 2. View of the command processor showing the connectorpanel.

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32-bit serial data, 2048-bit serial data, and logic pulses.The 32-bit serial data commands are error checked inthe command processor. If they are error free, they aresent to the subsystem via a dedicated 3-wire interface.The subsystem defines the meaning of the 32 bits.Typically, the bits are used to control modes or setparameters within the subsystem electronics. Long se-rial commands, on the other hand, are passed throughto the subsystem without error checking by the com-mand processor. The purpose of long commands is toload memories in the other onboard processors. Thesubsystem is responsible for error checking the longload message.

The command processor has several unique featuresdesigned to support the complexity of the MSX mis-sion. Multiple sequences of stored commands can beexecuted simultaneously, limited only by the 15,000-command capacity. Rule-based autonomous processingcan check any of the housekeeping telemetry data andexecute stored command sequences in response to log-ical and mathematical tests.

An innovative feature that time-stamps and logscommands as they are executed provides diagnostics tothe flight operations team and the integration and testteam. The log of the most recent 500 commands issubcommutated, at a low rate, into the command pro-cessor status telemetry. A history of the last 500 com-mands, including errors that occurred in processing, canbe reconstructed by the ground support equipmentusing about 4 min of telemetry data.

The processor’s software also provides uplink au-thentication, error checking, delayed command pro-cessing with resolution to 1 s in a year, and backgroundmemory scrubbing.

DATA HANDLING SYSTEMThe data handling system (DHS) collects data, for-

mats downlink telemetry data streams, and keeps space-craft time. It is fully redundant, with two mechanicalassemblies that are mated together for flight. Figure 3shows the redundant sides, DHS 1 and DHS 2, aboutto undergo an x-axis vibration test. The followingparagraphs describe DHS functionality, some highlightsof the 25-Mbps prime science formatter, and packagingdesign.

Functional DescriptionSpacecraft science and housekeeping data are gath-

ered by the DHS and formatted to generate three serialdata outputs. Each stream has selectable formats andcan be turned on and off independently as required bythe mission. These three streams are the following:

1. 25-Mbps (or 5-Mbps) prime science data stream con-taining imager, processor, and housekeeping data;

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THE MSX COMMAND AND DATA HANDLING SYSTEM

spacecraft power bus is 20 W withprime science off and 40 W with theprime science running at 25-Mbps.Each source of data has indepen-dent, isolated interfaces to DHS 1and DHS 2. Operationally, eachdata source has to be commanded toknow which DHS side is powered.This is an important part of theoverall cross-strapped design thateliminates single-point failures andprevents failure propagation.

Prime Science FormatterThe prime science formatter

processes real-time, high-rate imag-er data, as well as stored spacecraftprocessor data, and formats the datainto a serial 25-Mbps output. Anyof several allocations of prime sci-ence bandwidth between instru-ments can be selected by command.

These are chosen to accommodate the expected oper-ating modes of the instruments. They are hardwiredinto the logic of the formatter hardware.

Imager data are transferred using clock and gatesignals generated by the prime science formatter anddistributed to the data sources. To compensate for prop-agation delay effects resulting from the data path (cableand electronics) and to permit formation of a contig-uous output data stream, the formatter automaticallysenses the total data gathering delay for each high-speed imager data source and uses the results to con-tinuously equalize the path delays. (A patent has beenawarded for this innovation.1) High-speed emitter-coupled logic technology is used to implement the25-Mbps processing logic.

Package DesignThe two data handling systems, DHS 1 and DHS 2,

are electrically identical. Mechanically, for better ther-mal design, they are mirror-image assemblies boltedtogether. One DHS flight unit consists of five multilay-er printed circuit boards, a motherboard, and an inter-nal wiring harness. Each unit has dimensions of25 3 22 3 32 cm and a mass of 10 kg. Figure 4 showsthe wide/narrowband telemetry processor board config-ured with connector savers for bench test.

KEY GENERATOR/TEMPESTELECTRONICS UNITS

Figure 5 shows one KG/tempest electronics unit. Itmeasures 10 3 24 3 26 cm, has a mass of 5.7 kg, and

Figure 3. View of the data handling system (DHS) showing redundant DHS 1 and DHS 2 matedin flight configuration. In this photograph, the DHS is mounted on the vibration table with thebench test equipment cables attached.

transmitted in real time or stored on one of thespacecraft tape recorders

2. 1-Mbps snapshot science and housekeeping widebanddownlink data stream; transmitted in real time

3. 16-kbps narrowband downlink data stream contain-ing spacecraft housekeeping and memory dump data;transmitted in real time

A state machine in the DHS collects one completeset of housekeeping data every second. The housekeep-ing data include

1. Single-ended analog monitors (123)2. Differential analog monitors (76)3. Digital discrete (“telltale”) channels (48)4. Temperature transducer channels (96)5. Low-range temperature channels (31)6. Serial digital housekeeping channels (24)

Spacecraft mission elapsed time and Universal Timeare kept in the DHS. They are based on precise 1-Hzpulses derived from the 25-MHz ultrastable crystaloscillators. Copies of these times are placed in eachhousekeeping data set and are distributed to othersubsystems for synchronizing events external to theDHS. Selected critical housekeeping data parametersare stored in a DHS random access memory buffer.Called the “full orbit memory,” this buffer is dumpedby command to a ground station. Finally, the full setof housekeeping data is provided by dedicated interfac-es to the command processors. Software in the com-mand processors uses the contents for synchronizationand for fault protection and autonomous control.

Power is applied to only one of the redundant sidesof the DHS. Power consumption from the 28-V

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Figure 4. Component side view of the wide/narrowband telemetryprocessor board from the data handling system.

Figure 5. Front view of the key generator/tempest electronics unit,showing the connector panel.

consumes a maximum of 14.7 W. In flight, two of theseunits are stacked one on top of the other. Manufacturedby the Cubic Corporation, they provide secure commu-nications between the MSX spacecraft and the ground.Each of the two units consists of a multilevel housingwith three encryptor modules, one decryptor module,and a power converter/regulator module. Table 2 iden-tifies the data links and describes how they are used.

Each of the three encryptors accepts plain text datainput at the specified data rate and provides an encrypt-ed cipher text data output. The single decryptor acceptsencrypted (cipher text) data input at 2 kbps and pro-vides plain text command data output. Multiple out-puts for the cross-strapped data paths are switched andbuffered within the encryptor/decryptor modules. Theredundant data paths are indicated on the C&DH

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System block diagram of Fig. 1. In addition, the encryp-tors and the decryptor contain serial data and house-keeping logic for command and status information.Finally, the power converter accepts 28-V spacecraftbus power and provides nine isolated, regulated,and filtered outputs that are distributed to the othermodules.

POWER SWITCHING UNITSThe MSX spacecraft has three power switching

units: units 1 and 2 and the ordnance control unit. Eachis capable of controlling 64-relay commands. Together,the units provide switched power and signals for space-craft operation. The ordnance control unit is dedicatedto spacecraft ordnance functions. Separate ordnancefunction is required to protect against spurious ignitionof ordnance devices. Measures taken include inter-locked relay contact circuits, electromagnetic interfer-ence shielding, and dedicated connectors and harnesswiring for ordnance functions. A total of 252 relays areused in the power switching units.

All three units contain a motherboard and six18 3 18 cm plug-in cards (two driver/telemetry boardsand four relay boards) mounted in a 23 3 23 3 25 cmchassis. Each unit has a mass of 7.3 kg. Two 50-pin “D”connectors are used on each relay board, one for powerin and the other for switched power out. Four types oflatching relays are used for the various switching func-tions and are selected according to the power require-ments of the various users. The contact ratings rangefrom 2 to 25 A. In addition, 2-A nonlatching relays areused to provide pulse commands. Figure 6 is a photo-graph of a completed power switching unit, ready forinstallation on the spacecraft. Figure 7 is a componentside view of a typical relay board.

The two driver/telemetry boards in each unit pro-vide redundant relay coil drive and relay contactposition telemetry. Each board accepts an 8-bit parallelcommand word from one of the two commandprocessors and decodes the command into the appro-priate signal to operate the relays, whose drive circuitsare configured in an 8 3 8 matrix. Latching relaycontrol circuitry is made fully redundant by driving thetwo latching relay coils from separate matrix decoders.The boards also contain parallel-to-serial shift registerswith output interfaces for the telemetry of relay contactposition.

All latching relays are monitored. The telemetrydata streams are daisy-chained between the three unitssuch that one 160-bit data stream from each of thedriver/telemetry boards is fed to one of the redundantdata handling systems. Command decoding and telem-etry logic is implemented using CD4000B series inte-grated circuits. Discrete transistors are used in the relaycoil drive circuits.

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Figure 6 . Front view of power switching unit 1, showing the connectorpanel.

The units operate with 28-V and 5-V inputs to eachdriver/telemetry board. The 28-V power, supplied bythe spacecraft battery, is used for command decodingand relay operation, and the 5-V power is used for thetelemetry circuits. Power consumption per unit is630 mW.

Figure 7. Component side view of relay 2 board from power switchingunit 1.

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THE MSX COMMAND AND DATA HANDLING SYSTEM

SPACEBORNE TAPE RECORDERS

PurposeRecording the prime science data ensures that no

information is lost while MSX is out of contact withthe ground receiving station. Each of the spacecraft’stwo tape recorders can continuously accumulate dataat 25 Mbps for up to 36 min, or else data can beintermittently recorded at 5 Mbps for a maximum totalrecord time of 3 h.

Upon command, the recorded data are played backto the ground at 25 Mbps. This process can take 36 minif the entire tape has been recorded. Since the MSXsatellite takes about 13 min at most to pass over theAPL ground station, the data are played back piece-meal until all information has been recovered.

DescriptionThe two recorders were manufactured for APL by

Odetics Corporation. Each carries 2.2 km of 2.54-cmtape that can record up to 54 Gb of data. The 36-cmdiameter reels are stacked one above the other andcontrarotate to minimize disturbances to the spacecraftattitude from recorder speed changes. To further reduceangular momentum transients and to eliminate thetime and energy of rewinding, the tapes are playedout backwards so the last datum recorded is the firstrecovered.

An error detection and correction system achievesan average bit error rate of less than 1 3 1027. Eachrecorder has three capstans, which are coupled by re-dundant Kapton belts to a servo-controlled, brushlessDC motor with optical commutation. Four additionalservo-controlled motors maintain tension and lateralposition of the tape. The housing of each transport unitis pressurized with nitrogen to protect the tape, heads,and lubricated bearings from the vacuum of space.

Table 3 lists the characteristics of the MSX space-borne tape recorders. Figure 8 shows a tape recorderwith cables interconnecting the transport and elec-tronics units, and Fig. 9 shows the transport unit withits cover removed.

SUMMARYBy any of the usual benchmarks of complexity, such

as data volume, number of relays, harness complexity,stored command capacity, and number of lines of flightcode, the MSX C&DH System was an enormous un-dertaking. The C&DH System meets the program re-quirements for high speed, massive onboard storage,and precise timing. Some innovative techniques wererequired to synchronize data from distributed sources atthe 25-Mbps rate. A high level of mission operation

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Table 3. Characteristics of the MSX spaceborne taperecorders.

Record speeds 18.8 cm/s and 94 cm/sPlayback speed 94 cm/sTape dimensions 2.54 cm 3 2.2 kmTape format 42-track IRIG standardPower consumption 45 to 225 W, mode dependentMass, transport unit 57.6 kgMass, electronics unit 21.8 kgTotal mass 79.4 kgSize, transport unit 50.8 3 50.8 3 33.0 cmSize, electronics unit 30.5 3 50.8 3 38.1 cmNumber of parts 16,732

Figure 8. Spaceborne tape recorder, showing transport unit and elec-tronics unit with interconnecting cables.

Transport unit

Electronicsunit

flexibility is incorporated into the command systemsoftware. Multiple command sources, uploadable au-tonomy rules, and stored commands are all accommo-dated. Hardware and software were fully tested anddelivered on time to the MSX spacecraft for integrationand test.

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Figure 9. Spaceborne tape recorder transport unit with coverremoved. The coaxial tape reels, tape path, and capstan assem-blies are clearly visible.

REFERENCE1Schwartz, P. D., “High Speed Propagation Delay Compensation Network,”U.S. Patent No. 5,379,299 (3 Jan 1995).

ACKNOWLEDGMENTS: We wish to acknowledge APL staff members JosephBogdanski, Brian Heggestad, Christopher Hersman, and Binh Le for their contri-butions to the design of the MSX C&DH System. Former staff members TyroneFloryanzia and Norman Mortensen were also substantial contributors. Residentsubcontract software engineers Greg Gallant and Jay Offutt wrote flight code forembedded processors as well as much of the ground support software. CubicCorporation, San Diego, California, manufactured the KG/tempest units under theleadership of program manager Peter Nelson, now retired. Odetics, Inc., Anaheim,California, manufactured the spaceborne tape recorders, under the leadership ofprogram manager Greg Horn. Telenetics, Inc., Columbia, Maryland, developedbench test equipment for the data handling system under the technical leadershipof Robert Krueger. The MSX mission is sponsored by the Ballistic Missile DefenseOrganization. This work was supported under contract N00039-94-C-0001.

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THE MSX COMMAND AND DATA HANDLING SYSTEM

THE AUTHORS

DAVID D. STOTT supervises the Control Electronics Section of the APL SpaceDepartment’s Spacecraft Subsystems Group and is a member of the PrincipalProfessional Staff. He received B.S. and M.S. degrees in electrical engineeringfrom the University of Michigan in 1963 and 1966, respectively. Since joiningAPL in 1966, he has worked on spacecraft command and telemetry systems,ground support equipment, aircraft avionics systems, and biomedical instrumen-tation. He was lead engineer for the MSX Command and Data Handling Systemand had the same assignment for the NEAR Program. His e-mail address isDavid.Stott@jhuapl.edu.

PETER EISENREICH is a Senior Engineer in the APL Space Department’sSpacecraft Subsystems Group. He received a BSEE degree from Geneva Collegein 1982 and an MSEE degree from The Johns Hopkins University in 1987. Sincejoining APL in 1982, he has designed flight- and ground-based electronicssystems on several projects, including AMPTE, GEOSAT, Polar BEAR, Delta181, MSX, and NEAR. His specialty is the design of high-reliability, radiation-hardened flight electronics. Mr. Eisenreich was the lead engineer for the MSXcommand processor and is currently the spacecraft systems engineer for AGRE.His e-mail address is Peter.Eisenreich@jhuapl.edu.

JERRY E. KROUTIL is a Senior Staff member of the APL Space Department’sSpacecraft Subsystems Group. He holds an Associate of Applied Science degreefrom the Capitol Radio Engineering Institute. Since joining APL in 1965, Mr.Kroutil has developed power switching components for numerous spacecraftprograms, including TIP, HUT, Polar BEAR, Delta Star 181 and 183, MSX, andNEAR. He is currently the lead engineer for the ACE power switchingcomponents. His e-mail address is Jerry.Kroutil@jhuapl.edu

RONALD K. BUREK is a Senior Staff member of the Signal Processing Sectionof the APL Space Department’s Spacecraft Subsystems Group. In 1964 hereceived his undergraduate degree in electrical engineering from the Universityof Detroit and joined APL. He began his career by helping with the introductionof satellite navigation computers into the surface navy. Subsequently, he hasbeen involved in electronic or software design for spaceborne radar altimetry,ship navigation, spacecraft ground support, and biomedical data acquisition. Inaddition, he has been lead engineer for the data recorders on 10 spacecraft.Currently, he is the technical contract engineer for magnetic tape and solid-staterecorders on the MSX, NEAR, and ACE spacecraft. His e-mail address isRonald.Burek@jhuapl.edu.

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PAUL D. SCHWARTZ received his B.S. and M. Eng. degrees in electricalengineering from Cornell University. Since joining the APL Space Departmentin 1973, Mr. Schwartz has been the lead electronics design engineer for thedevelopment of numerous spacecraft subsystems including the AMPTE com-mand system, the COBE momentum management assembly, the MSX datahandling system, and the NEAR command telemetry processor. He designedand developed power conditioning electronics for several space instrumentsincluding the Galileo energetic particle detector and the Ulysses Lan Experi-ment. He is currently the system engineer for the development of a miniaturizedvisible imager for the Glacier spacecraft. His e-mail address isPaul.Schwartz@jhuapl.edu.

GLEN F. SWEITZER is an Associate Engineer in the Flight Processors/Computers Section of the APL Space Department’s Spacecraft SubsystemsGroup. He studied electronics in the U.S. Air Force in 1957–58, and he hastaken numerous electronics and computer courses since joining APL in 1961. Hebegan his APL career as part of the development and testing team for the SPG59 radar system. He has developed satellite pre- and postlaunch test andoperations scenarios; assisted in development of TDRSS signal processingequipment and satellite test and operations equipment; provided test support forbiomedical research during the development of the implantable/rechargeablepacemaker; and was the on-site technical representative from 1985 to 1990 forthe Transit program SOOS spacecraft development at RCA Astro-Electronics inPrinceton, New Jersey. He was lead engineer for the MSX KG/tempest unit andtechnical contract engineer for the NEAR flight computer. Currently, Mr.Sweitzer is project manager for the Transit Program. His e-mail address isGlen.Sweitzer@jhuapl.edu.

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