- Email: [email protected]
NEAR cruise and mathilde flyby mission operations — report from the front lines of low cost missions
PII:
Acfa Astronautica Vol. 45, Nos. 4-9, pp. 465-473, 1999 0 1999 Published by Elsevier Science Ltd. AH riehts reserved Printed in Great Britain 0094-5765/99 $ - seefront matter SOO94-5765(99)00166-6
NEAR Cruise and Mathilde Flyby Mission Operations - Report From the Front Lines of Low Cost Missions. Johns Hopkins University,
AlliedSignal
P.D. Carr’ Applied Physics Laboratory,
Laurel, MD 20723
C.T. Kowal Technical Services Corp., Columbia, MD
T.J. Mulich, K. Whittenburg, B. Wishnefsky Old Dominion Systems of Maryland, Inc. A.S. Posner Interface and Control Systems, Inc., Columbia, MD
Abstract NASA’s Near Earth Asteroid Rendezvous (NEAR) mission was launched in February 1996 on a mission to rendezvous with the asteroid 433 Eros in 1999. NEAR, with its five science instruments, is controlled from the NEAR Mission Operations Center at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland by a team of 5-8 sequence planners and flight controllers, with the support of a small engineering group. We examine lessons learned in planning, verifying and executing NEAR cruise and flyby activities. We find that the NEAR prerendezvous phase - so long as science objectives remain tightly focused - has been and can be successfully executed by a small, broadly experienced, highly skilled, closely cooperating team. However, sequence design for the year-long Eros-orbiting phase with multiple, potentially conflicting science activities will require a more robust and tightly integrated structure. 0 1999 Published by Elsevier Science Ltd. All rights reserved.
Introduction The Near Earth Asteroid Rendezvous (NEAR) spacecraft was launched in February 1996 on a mission [I] to rendezvous with the near-Earth asteroid 433 Eros [2] in 1999. Upon arrival, the spacecraft will orbit Eros for one year while studying and mapping the body with a five instrument scientific payload. NEAR was the first Discovery spacecraft to return scientific data - during its June 27, 1997 flyby of the asteroid 253 Mathilde. The NEAR mission is managed for NASA by the Johns Hopkins University Applied Physics Lab in Laurel, Maryland, where the NEAR Mission Operations Center (MOC) is located. With a nearly three year cruise phase, the operations plan for NEAR [3] was to develop most required mission operations capability for the Eros phase during cruise, which was expected to be ‘.\tTo whom correspondence should be addressed. Mail stop: 13N319. E-mail: [email protected] 465
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a period of low activity with the exception of the Mathilde flyby. The reality from February 1996 to the present is that the cruise phase has been a period of moderate to intense Ops activity, with numerous flight software uploads, lengthy Mathilde flyby preparations [4], preparation for 12 trajectory correction maneuvers (10 of which were executed), instrument calibration activities, radio science support during solar conjunction, and a 23 January 1998 Earth Swingby which exceeded the Mathilde flyby in complexity.
The NEAR
Spacecraft
Though a relatively low cost, Delta-class spacecraft [5], NEAR is a fully redundant platform supporting five instruments and six science teams. The instruments are a multi-spectral imager (MSI), an infrared spectrometer (NIS), a magnetometer, an X-ray/gamma ray spectrometer (XGRS), and a LASER Rangetinder (NLR). Each instrument is controlled by a dedicated Data Processing Unit (DPU), with the exceptions that the magnetometer and NIS share a DPU, and a DPU is built into the NLR. The low cost and compressed schedule for NEAR drove a mechanically simple approach. All appendages are statically mounted to the body except for the solar panels, which were deployed by passive hinge mechanisms. The absence of gimbals requires that the spacecraft body be oriented to support science observations, maneuvers or data return while maintaining acceptable sun geometry. NEAR’s attitude is actively stabilized using four reaction wheels as actuators, and gyros in conjunction with a star tracker for attitude determination. The Guidance and Control subsystem [6] uses two sets of functionally distinct processors: the Attitude Interface Units (AIUs) for controlling the G&C 1553 bus and backup safe mode control, and the Flight Computers (FCs) for attitude determination and normal operational mode control. Commands, telemetry and autonomy are handled by redundant Command and Telemetry Processors (CTPs). In all, there are 10 APL-programmed flight processors on NEAR running seven sets of software. The Command and Telemetry Processors support up to 64 time-tagged (delayed action) commands and will execute command macros. The CTP can also check up to 164 autonomy rules at 1 Hz. The autonomy rules are divided into two classes: higher priority safing rules, and utility housekeeping rules, which are used for recorder management, countdown timers, soft telltales, and other lower priority tasks. NEAR’s nominal configuration with the DSN stations is an X-band uplink and downlink at rate l/2 convolutional encoding, with ranging on. The NEAR project began testing with rate l/6 convolutional encoding on the downlink in the second quarter of 1997. Using the l/6 rate improves the downlink margin by approximately 1.7 dl3.
The NEAR Ops Team All NEAR activities have been executed by an Ops team which has typically consisted of five to eight sequence planners and analysts, together with the support of instrument engineers and a subsystem engineering team - with the heaviest engineering load in the disciplines of Attitude Control and RF subsystem engineering.
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For the cruise phase, typical Ops team members have been experienced engineers or scientists with broad backgrounds including engineering design, systems engineering and mission design as well as space operations. The breadth and depth of background has been essential to the flexibility and adaptability of the team working with a minimal toolset and no established procedures for flying planetary spacecraft at APL. Each member can expect to participate in any effort within the Ops charter - from planning momentum management, to assisting in the debug of ground equipment, to scheduling DSN contacts. As the Eros encounter approaches, a more defined division of labor is emerging, with a subset of the Ops team designated as Flight Controllers, whose primary responsibility is for real-time operations. During cruise phase, as Flight Controllers have joined the team, they have developed procedures for routine cruise operations and are in the process of developing procedures for Eros. Specialization is also occurring among the planners and analysts, with some concentrating on the planning and scheduling process itself, with others on subsystem and instrument level activity planning. Training of new Ops team members has been entirely informal in the cruise phase, and mostly consists of supervised on-the-job activities and unsupervised background reading. Veteran Ops team members share an equal responsibility for transmitting the acquired knowledge of NEAR operations to new members.
Activity
Planning,
Analysis
and Simulation
Planning NEAR cruise activity sequences is a largely manual process. Even with no formal command sequence generation or resource management tools in place, all critical cruise phase activities have executed with minimal errors because of extensive preparation, testing and review. Good internal communications and the minimization of functional barriers within the small Ops team have also contributed to the quality of activity preparation. The initial inputs for a cruise phase activity come from the instrument or the mission design teams. These inputs generally take the form of a high level written description of the activity. The NEAR sequence planner will take these inputs and make a rough cut of the Spacecraft Test and Operations Language (STOL) command sequence in a text editor using a command dictionary, personal experience, previous command sequences, and oral tradition. The sequencer then conducts a preliminary design review with all interested parties participating. This review results in an agreed-upon sequence timeline, preliminary resource allocation, and first order sanity check of the sequence. In the case of particularily complex activities, a timeline for one or more flight tests of some portion of the sequence may also be reviewed. After the preliminary review, the Ops team updates the sequence and refines its resource allocation. The Analyst then uses an adjunct utility to the EPOCH 2000 real-time software’ to translate the sequence text file into binary uploadable transfer frames.
‘.\tForm more information on EPOCH 2000, see: http://www.integ.com/isiepoch.htm
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Test runs of an activity are then conducted with a real-time Brassboard simulator, which combines spacecraft development hardware and software simulation to emulate spacecraft dynamics and behavior. The fidelity of the Brassboard simulation varies across subsystems: it is non-existent for the RF subsystem; weak for power, thermal and propulsion, good for C&DH and instruments, and very good for G&C. Achieving even an approximately accurate initial configuration for the Brassboard typically consumes as much time as running the activity itself. Since late 1997, this phase of activity testing has been augmented by a faster than real time software-only simulator still under development. After verifying an activity on the Brassboard, a second sequence planner or supporting engineer conducts a final review of the subject activity, and signs off the command sequence as flight ready. This process is frequently complicated by late activity change requests from the instrument science or engineering teams. NEAR Pre-launch,
Launch
and Early
Orbit
Operations
In addition to routine pre-launch activities (performing systems requirements analyses, supporting design reviews, working with the instrument and engineering teams, etc.), the NEAR Ops team supported the Integration and Test effort. Ops personnel took on responsibilities such as populating and verifying the I&T/flight database, performing I&T functional tests, generating and uploading flight-processor software, and debugging/testing ground-system problems. This helped to educate the flight Ops team on the intricacies of the ground and flight systems. In the months just before launch, the Ops team concentrated on the generation of procedures needed during Launch and Early Operations (LEOps). Contingency and fault-protection procedures and on-board macros received specific attention, as well as Mission-Integrated-Readiness Tests (MIRTs), real-time procedures for early ground contacts, RF configurations, and trajectorycorrection maneuvers. One obstacle to the NEAR Ops development effort was access to simulators and the flight hardware. Despite the oversubscribed nature of the Brassboard simulator, the NEAR Ops team validated all mission and launch-critical software before launch. The NEAR Ops team began participating in around-the-clock launch operations several days before the 17 February, 1996 launch. Each member worked 1 l-hour shifts, seven days a week, performing both real-time console and sequence planning and verification tasks. The NEAR Ops team performed a full set of launch and early orbit activities, including flight software uploads and instrument calibrations. A few days into the launch provided the first of what turned out to be many opportunities to test the Multi-Spectral Imager. The Ops team oriented NEAR with the moon in the MS1 FOV and took images, providing the MS1 team with valuable calibration data. TCM- 1 was the first planned use of the NEAR propulsion system, and was the subject of much simulation and rehearsal. The TCM procedure underwent numerous revisions, as both the engineering and flight-ops teams developed knowledge about the spacecraft subsystems and their interactions. Over the several weeks following launch, all spacecraft subsystems were activated and validated. The interaction of the gyros and accelerometers was characterized and procedures were developed for addressing reaction-wheel-stiction issues. In addition, an updated star catalog was later uploaded to the Flight Computer. Modifications to the Flight Computer software were also uploaded during the first several weeks of the mission. The operational challenge was the size of the upload to be performed with respect to the available upload and download bandwidths of
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125bps and 4Obps, respectively. Flight Computer software uploads spanned consecutive DSN contacts and often took multiple contacts to download for verification. The 253 Mathilde
Flyby
The first major science activity of the NEAR mission] was the flyby of asteroid 253 Mathilde [4], on June 27, 1997. Mission Operations preparations for the Mathilde flyby began in August 1996. The first of five rehearsals was run on the spacecraft on September 25. The STOL scripts for all tests, and for the flyby itself, were written by hand, using a text editor. The script for the flyby was 60 pages long. Although all scripts were tested on the Brassboard, several of the tests had problems due to typographical errors. Nevertheless, after numerous tests and rehearsals, the flyby itself was executed flawlessly, The Mathilde flyby activities were divided into three separate phases: optical navigation, trajectory and orbit correction, and the flyby itself. Starting 42 hours before the flyby, images were taken of the star field at the expected location of Mathilde. When the asteroid was detected on the images, its location was used to compute a corrected asteroid orbit, which was then uploaded to the spacecraft. Ops had prepared a contingency TCM for twelve hours before closest approach, but mission management elected not to perform the maneuver. Since the Mathilde imaging sequence was pre-programmed with specific spacecraft clock times, it was not possible to change those times to adjust for errors in the time of closest approach. From the optical navigation, it was determined that the spacecraft would reach Mathilde 9 seconds earlier than originally expected, and the spacecraft clock was incremented accordingly. Because of power constraints at the large heliocentric distance of the solar-powered spacecraft, the only instrument that was turned on for the flyby was the Multi-Spectral Imager, (MSI). Doppler tracking data taken just before and after closest approach was used to determine the mass of the asteroid [7]. During the 25 minutes of the flyby, the Multi-Spectral Imager obtained 534 images of the asteroid and its vicinity. Every aspect of the Mathilde flyby was a total success. NEAR
Instrument
Calibration
All instruments aboard NEAR required calibration during cruise phase (see the timeline in Figure 1). The detailed planning of these calibration activities is largely carried out by the Ops team, with support from instrument and G&C engineering. The Multispectral Imager (MSI) was calibrated several times using the star Canopus. The MS1 was also calibrated during the January 23rd, 1998 Earth Swingby (ESB) along with the NIS using both the Moon and the Earth as targets. With the MSI, NIS, magnetometer and XGRS operating simultaneously, the ESB provided a partial simulation of Eros operations.
‘.\tRadio Science data was taken a few months earlier during a solar conjunction event.
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NEAR Major Cruise Phase Instrument
Maneuver
Planning
Planetary
Missions
Calibrations to February 1998.
and Execution
The NEAR Mission Design group at APL generates maneuver inputs, with considerable refinement by G&C engineering to achieve high precision execution. Ops uses spreadsheet-based tools to assemble a detailed timehne for each Trajectory Correction Maneuver (TCM). Maintaining consistency between the timeline and the text files used to generate the command load is done by actually generating certain STOL source files as linked worksheets and by generating aliases for the preprocessor on linked worksheets. For example, spreadsheet macros were used to generate lengthy sequences of fuel and oxidizer valve cycling commands for the July 1997 Deep Space Maneuver (TCM-7). The use of spreadsheets in this way has been the model for other G&C activities and some instrument activities as well. The process remains largely manual whenever rearchitecting of the TCM sequence takes place, as has often been the case. The use of spreadsheets unavoidably makes traceability and configuration management difficult, and the Planner’s memory is relied upon to prevent serious errors. Minor errors are removed after Brassboard testing and iteration with subsystem engineering. DSN Support
Planning
NEAR uses the Deep Space Network for tracking, telemetry, and commanding. Allocated station times are agreed upon at the DSN user level by all NASA projects. This is accomplished through attendance at a Resource Allocation Planning (RAP) Team Meeting, DSN Scheduling Meeting, or Resource Allocation Review Board (RARB) Meeting. The Deep Space Network (DSN) requires that Sequence of Events (SOE) files be generated by each project utilizing the DSN. For the NEAR project, the SOE file contains Deep Space Station (DSS) specific configuration and tracking directives coordinated with NEAR Mission Operations Center (MOC) supported spacecraft contacts. These files are generated by the NEAR project on a weekly basis at a minimum. The files are typically built to include one DSN weeks worth of configuration information. APL has developed software to assist NEAR Ops personnel in the generation of SOE data. The main software product is APLKey, the NEAR DSN SOE Keyword File Generator. A DSN contact planning tool (CONTACT) aids the NEAR DSN scheduler in keeping track of when NEAR has access to DSN antennas to command/receive data. A copy of the SOE is placed on a shared volume in the NEAR MOC, for generation of the STOL script of timetagged commands for the contacts using the READ_SOE tool developed by the Ops team.
Sequence
Planning
for the Eros Orbiting
Phase
The cruise method of constructing an activity sequence can take weeks of effort to produce a sequence that executes for a few hours. This process is inadequate for Eros operations, when
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activity sequences will be expected to run for as long as 16 hours a day every day. For this reason Ops, with a team of software developers, has begun developing a new tool suite which will replace the cruise phase process. This new process of sequence construction uses a generic operations sequencing and scheduling tool, SEQ_GEN, developed by the Multimisson Ground Systems Office at JPL. SEQ_GEN requires adaptation for NEAR. This adaptation includes loading a copy of the NEAR command database into SEQ_GEN, along with modeling of the NEAR subsystems. NEAR operations staff, in conjunction with the science teams and instrument engineers, are building small, reusable Brassboard tested fragments of command sequences, which once loaded into SEQ_GEN will become the building blocks for activity sequence generation. The Eros process flow will make science teams responsible for their own opportunity analysis. The Science Operations Planning Group [3] will reallocate resources such as data bandwidth and ownership of NEAR pointing amongst concurrent investigations as the orbit about Eros changes. Using the approved list of fragments, each of the science teams will provide the operations schedulers with a SEQ_GEN formatted request file for each load period. The Ops scheduler will input all requests for a given load period into SEQGEN, resolving any conflicts or constraint violations found in the detailed modeling. The time-ordered list of primitive commands SEQ_GEN produces are then processed into command macros with their associated execution times. The resulting command load is processed by the faster than real-time simulator. This entire process should take about 3 weeks for a command load that would take one week to execute, with three command loads concurrently under construction at different stages of development.
Conclusions Cruise mode activity estimates tend to be optimistically low. Planning for low-cost missions should allow for this reality, but there is a balance to be struck - flight experience will lead to important changes in operations procedures, so large expense in preparation of detailed sciencemode procedures before launch may not be justifiable - especially not for a first-of-a-kind spacecraft with an extended cruise phase. What is important in the early stages of a Discovery mission
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Planetary
Missions
is to work through the architecture of how Ops will proceed upon arrival at the target body. A key to stabilizing this architecture in a low cost environment is maximizing the use of COTS/GOTS software tools for planning, scheduling, simulation, and for spacecraft configuration and resource management. Science and engineering teams working with Ops need to be educated as early as possible about the risks and demands that spacecraft activities impose. NEAR Ops has enjoyed excellent working relationships with all the science teams, but there have been unrealistic expectations about the space and ground resources required to coordinate and perform activities. It is also important to inculcate in the science teams the need for a modicum of discipline in scheduling and planning. An important lesson learned has been the composition and organization of the small team needed to perform low-cost operations. A flexible, unlayered team with minimal functional barriers has been essential to the outstanding success of NEAR Mission Operations.
Acknowledgments The authors wish to acknowledge R.L. Nelson and R.B. Dickey of APL for their editorial support and their contributions to NEAR operations; the NEAR science, mission design, subsystem engineering and navigation teams, for their stalwart support, patience, and willingness to communicate, and M.G. Carr for her editorial assistance.
References [l] Farquhar,R.W., D.W. Dunham, and J.V. McAdams, “NEAR Mission Overview and Trajectory Design,” Journal ofthe Astronautical Sciences Volume 43, No. 4, October-December 1995, pp. 353-372. [2] Yeomans, D.K.,“Asteroid 433 Eros: The Target Body of the NEAR Mission,” Journal of the Astronautical Sciences Volume 43, No. 4, October-December 1995, pp. 417-426.
[3] Landshof, J. and A. Cheng, “NEAR Mission and Science Operations,” Journal ofthe 1995, pp. 477-490.
Astronautical Sciences Volume 43, No. 4, October-December
[4] Kowal, C.T. and A.S. Posner, “Planning and Execution of the Asteroid 253 Mathilde Encounter for the Near Earth Asteroid Rendezvous (NEAR) Mission”; Fifth International Symposium on Space Mission Operations and Ground Data Systems; Tokyo, Japan; June 1998. [5] Santo, A.G., S.C. Lee, and R.E. Gold, “NEAR Spacecraft and Instrumentation,” Journal 1995, pp. 373-398.
ofthe Astronautical Sciences Volume 43, No. 4, October-December
[6] Strikwerda, T. E., J. C. Ray, D. R. Haley, G. A. Heyler, H. L. Fisher, and R.T. Pham, “NEAR Guidance and Control System,” AAS 97-077,2Oth Annual AAS Guidance and Control Conference, Breckenridge, CO, February 5-9, 1997.
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[7] Yeomans, D.K., et. al., “Estimating the mass of asteroid 253 Mathilde from tracking data during the NEAR flyby,” Science 1997 Dee 19;278(5346):2106-2109.
Acfa Astronautica Vol. 45, Nos. 4-9, pp. 465-473, 1999 0 1999 Published by Elsevier Science Ltd. AH riehts reserved Printed in Great Britain 0094-5765/99 $ - seefront matter SOO94-5765(99)00166-6
NEAR Cruise and Mathilde Flyby Mission Operations - Report From the Front Lines of Low Cost Missions. Johns Hopkins University,
AlliedSignal
P.D. Carr’ Applied Physics Laboratory,
Laurel, MD 20723
C.T. Kowal Technical Services Corp., Columbia, MD
T.J. Mulich, K. Whittenburg, B. Wishnefsky Old Dominion Systems of Maryland, Inc. A.S. Posner Interface and Control Systems, Inc., Columbia, MD
Abstract NASA’s Near Earth Asteroid Rendezvous (NEAR) mission was launched in February 1996 on a mission to rendezvous with the asteroid 433 Eros in 1999. NEAR, with its five science instruments, is controlled from the NEAR Mission Operations Center at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland by a team of 5-8 sequence planners and flight controllers, with the support of a small engineering group. We examine lessons learned in planning, verifying and executing NEAR cruise and flyby activities. We find that the NEAR prerendezvous phase - so long as science objectives remain tightly focused - has been and can be successfully executed by a small, broadly experienced, highly skilled, closely cooperating team. However, sequence design for the year-long Eros-orbiting phase with multiple, potentially conflicting science activities will require a more robust and tightly integrated structure. 0 1999 Published by Elsevier Science Ltd. All rights reserved.
Introduction The Near Earth Asteroid Rendezvous (NEAR) spacecraft was launched in February 1996 on a mission [I] to rendezvous with the near-Earth asteroid 433 Eros [2] in 1999. Upon arrival, the spacecraft will orbit Eros for one year while studying and mapping the body with a five instrument scientific payload. NEAR was the first Discovery spacecraft to return scientific data - during its June 27, 1997 flyby of the asteroid 253 Mathilde. The NEAR mission is managed for NASA by the Johns Hopkins University Applied Physics Lab in Laurel, Maryland, where the NEAR Mission Operations Center (MOC) is located. With a nearly three year cruise phase, the operations plan for NEAR [3] was to develop most required mission operations capability for the Eros phase during cruise, which was expected to be ‘.\tTo whom correspondence should be addressed. Mail stop: 13N319. E-mail: [email protected] 465
466
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a period of low activity with the exception of the Mathilde flyby. The reality from February 1996 to the present is that the cruise phase has been a period of moderate to intense Ops activity, with numerous flight software uploads, lengthy Mathilde flyby preparations [4], preparation for 12 trajectory correction maneuvers (10 of which were executed), instrument calibration activities, radio science support during solar conjunction, and a 23 January 1998 Earth Swingby which exceeded the Mathilde flyby in complexity.
The NEAR
Spacecraft
Though a relatively low cost, Delta-class spacecraft [5], NEAR is a fully redundant platform supporting five instruments and six science teams. The instruments are a multi-spectral imager (MSI), an infrared spectrometer (NIS), a magnetometer, an X-ray/gamma ray spectrometer (XGRS), and a LASER Rangetinder (NLR). Each instrument is controlled by a dedicated Data Processing Unit (DPU), with the exceptions that the magnetometer and NIS share a DPU, and a DPU is built into the NLR. The low cost and compressed schedule for NEAR drove a mechanically simple approach. All appendages are statically mounted to the body except for the solar panels, which were deployed by passive hinge mechanisms. The absence of gimbals requires that the spacecraft body be oriented to support science observations, maneuvers or data return while maintaining acceptable sun geometry. NEAR’s attitude is actively stabilized using four reaction wheels as actuators, and gyros in conjunction with a star tracker for attitude determination. The Guidance and Control subsystem [6] uses two sets of functionally distinct processors: the Attitude Interface Units (AIUs) for controlling the G&C 1553 bus and backup safe mode control, and the Flight Computers (FCs) for attitude determination and normal operational mode control. Commands, telemetry and autonomy are handled by redundant Command and Telemetry Processors (CTPs). In all, there are 10 APL-programmed flight processors on NEAR running seven sets of software. The Command and Telemetry Processors support up to 64 time-tagged (delayed action) commands and will execute command macros. The CTP can also check up to 164 autonomy rules at 1 Hz. The autonomy rules are divided into two classes: higher priority safing rules, and utility housekeeping rules, which are used for recorder management, countdown timers, soft telltales, and other lower priority tasks. NEAR’s nominal configuration with the DSN stations is an X-band uplink and downlink at rate l/2 convolutional encoding, with ranging on. The NEAR project began testing with rate l/6 convolutional encoding on the downlink in the second quarter of 1997. Using the l/6 rate improves the downlink margin by approximately 1.7 dl3.
The NEAR Ops Team All NEAR activities have been executed by an Ops team which has typically consisted of five to eight sequence planners and analysts, together with the support of instrument engineers and a subsystem engineering team - with the heaviest engineering load in the disciplines of Attitude Control and RF subsystem engineering.
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For the cruise phase, typical Ops team members have been experienced engineers or scientists with broad backgrounds including engineering design, systems engineering and mission design as well as space operations. The breadth and depth of background has been essential to the flexibility and adaptability of the team working with a minimal toolset and no established procedures for flying planetary spacecraft at APL. Each member can expect to participate in any effort within the Ops charter - from planning momentum management, to assisting in the debug of ground equipment, to scheduling DSN contacts. As the Eros encounter approaches, a more defined division of labor is emerging, with a subset of the Ops team designated as Flight Controllers, whose primary responsibility is for real-time operations. During cruise phase, as Flight Controllers have joined the team, they have developed procedures for routine cruise operations and are in the process of developing procedures for Eros. Specialization is also occurring among the planners and analysts, with some concentrating on the planning and scheduling process itself, with others on subsystem and instrument level activity planning. Training of new Ops team members has been entirely informal in the cruise phase, and mostly consists of supervised on-the-job activities and unsupervised background reading. Veteran Ops team members share an equal responsibility for transmitting the acquired knowledge of NEAR operations to new members.
Activity
Planning,
Analysis
and Simulation
Planning NEAR cruise activity sequences is a largely manual process. Even with no formal command sequence generation or resource management tools in place, all critical cruise phase activities have executed with minimal errors because of extensive preparation, testing and review. Good internal communications and the minimization of functional barriers within the small Ops team have also contributed to the quality of activity preparation. The initial inputs for a cruise phase activity come from the instrument or the mission design teams. These inputs generally take the form of a high level written description of the activity. The NEAR sequence planner will take these inputs and make a rough cut of the Spacecraft Test and Operations Language (STOL) command sequence in a text editor using a command dictionary, personal experience, previous command sequences, and oral tradition. The sequencer then conducts a preliminary design review with all interested parties participating. This review results in an agreed-upon sequence timeline, preliminary resource allocation, and first order sanity check of the sequence. In the case of particularily complex activities, a timeline for one or more flight tests of some portion of the sequence may also be reviewed. After the preliminary review, the Ops team updates the sequence and refines its resource allocation. The Analyst then uses an adjunct utility to the EPOCH 2000 real-time software’ to translate the sequence text file into binary uploadable transfer frames.
‘.\tForm more information on EPOCH 2000, see: http://www.integ.com/isiepoch.htm
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Test runs of an activity are then conducted with a real-time Brassboard simulator, which combines spacecraft development hardware and software simulation to emulate spacecraft dynamics and behavior. The fidelity of the Brassboard simulation varies across subsystems: it is non-existent for the RF subsystem; weak for power, thermal and propulsion, good for C&DH and instruments, and very good for G&C. Achieving even an approximately accurate initial configuration for the Brassboard typically consumes as much time as running the activity itself. Since late 1997, this phase of activity testing has been augmented by a faster than real time software-only simulator still under development. After verifying an activity on the Brassboard, a second sequence planner or supporting engineer conducts a final review of the subject activity, and signs off the command sequence as flight ready. This process is frequently complicated by late activity change requests from the instrument science or engineering teams. NEAR Pre-launch,
Launch
and Early
Orbit
Operations
In addition to routine pre-launch activities (performing systems requirements analyses, supporting design reviews, working with the instrument and engineering teams, etc.), the NEAR Ops team supported the Integration and Test effort. Ops personnel took on responsibilities such as populating and verifying the I&T/flight database, performing I&T functional tests, generating and uploading flight-processor software, and debugging/testing ground-system problems. This helped to educate the flight Ops team on the intricacies of the ground and flight systems. In the months just before launch, the Ops team concentrated on the generation of procedures needed during Launch and Early Operations (LEOps). Contingency and fault-protection procedures and on-board macros received specific attention, as well as Mission-Integrated-Readiness Tests (MIRTs), real-time procedures for early ground contacts, RF configurations, and trajectorycorrection maneuvers. One obstacle to the NEAR Ops development effort was access to simulators and the flight hardware. Despite the oversubscribed nature of the Brassboard simulator, the NEAR Ops team validated all mission and launch-critical software before launch. The NEAR Ops team began participating in around-the-clock launch operations several days before the 17 February, 1996 launch. Each member worked 1 l-hour shifts, seven days a week, performing both real-time console and sequence planning and verification tasks. The NEAR Ops team performed a full set of launch and early orbit activities, including flight software uploads and instrument calibrations. A few days into the launch provided the first of what turned out to be many opportunities to test the Multi-Spectral Imager. The Ops team oriented NEAR with the moon in the MS1 FOV and took images, providing the MS1 team with valuable calibration data. TCM- 1 was the first planned use of the NEAR propulsion system, and was the subject of much simulation and rehearsal. The TCM procedure underwent numerous revisions, as both the engineering and flight-ops teams developed knowledge about the spacecraft subsystems and their interactions. Over the several weeks following launch, all spacecraft subsystems were activated and validated. The interaction of the gyros and accelerometers was characterized and procedures were developed for addressing reaction-wheel-stiction issues. In addition, an updated star catalog was later uploaded to the Flight Computer. Modifications to the Flight Computer software were also uploaded during the first several weeks of the mission. The operational challenge was the size of the upload to be performed with respect to the available upload and download bandwidths of
Low-Cost Planetary Missions
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125bps and 4Obps, respectively. Flight Computer software uploads spanned consecutive DSN contacts and often took multiple contacts to download for verification. The 253 Mathilde
Flyby
The first major science activity of the NEAR mission] was the flyby of asteroid 253 Mathilde [4], on June 27, 1997. Mission Operations preparations for the Mathilde flyby began in August 1996. The first of five rehearsals was run on the spacecraft on September 25. The STOL scripts for all tests, and for the flyby itself, were written by hand, using a text editor. The script for the flyby was 60 pages long. Although all scripts were tested on the Brassboard, several of the tests had problems due to typographical errors. Nevertheless, after numerous tests and rehearsals, the flyby itself was executed flawlessly, The Mathilde flyby activities were divided into three separate phases: optical navigation, trajectory and orbit correction, and the flyby itself. Starting 42 hours before the flyby, images were taken of the star field at the expected location of Mathilde. When the asteroid was detected on the images, its location was used to compute a corrected asteroid orbit, which was then uploaded to the spacecraft. Ops had prepared a contingency TCM for twelve hours before closest approach, but mission management elected not to perform the maneuver. Since the Mathilde imaging sequence was pre-programmed with specific spacecraft clock times, it was not possible to change those times to adjust for errors in the time of closest approach. From the optical navigation, it was determined that the spacecraft would reach Mathilde 9 seconds earlier than originally expected, and the spacecraft clock was incremented accordingly. Because of power constraints at the large heliocentric distance of the solar-powered spacecraft, the only instrument that was turned on for the flyby was the Multi-Spectral Imager, (MSI). Doppler tracking data taken just before and after closest approach was used to determine the mass of the asteroid [7]. During the 25 minutes of the flyby, the Multi-Spectral Imager obtained 534 images of the asteroid and its vicinity. Every aspect of the Mathilde flyby was a total success. NEAR
Instrument
Calibration
All instruments aboard NEAR required calibration during cruise phase (see the timeline in Figure 1). The detailed planning of these calibration activities is largely carried out by the Ops team, with support from instrument and G&C engineering. The Multispectral Imager (MSI) was calibrated several times using the star Canopus. The MS1 was also calibrated during the January 23rd, 1998 Earth Swingby (ESB) along with the NIS using both the Moon and the Earth as targets. With the MSI, NIS, magnetometer and XGRS operating simultaneously, the ESB provided a partial simulation of Eros operations.
‘.\tRadio Science data was taken a few months earlier during a solar conjunction event.
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NEAR Major Cruise Phase Instrument
Maneuver
Planning
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Calibrations to February 1998.
and Execution
The NEAR Mission Design group at APL generates maneuver inputs, with considerable refinement by G&C engineering to achieve high precision execution. Ops uses spreadsheet-based tools to assemble a detailed timehne for each Trajectory Correction Maneuver (TCM). Maintaining consistency between the timeline and the text files used to generate the command load is done by actually generating certain STOL source files as linked worksheets and by generating aliases for the preprocessor on linked worksheets. For example, spreadsheet macros were used to generate lengthy sequences of fuel and oxidizer valve cycling commands for the July 1997 Deep Space Maneuver (TCM-7). The use of spreadsheets in this way has been the model for other G&C activities and some instrument activities as well. The process remains largely manual whenever rearchitecting of the TCM sequence takes place, as has often been the case. The use of spreadsheets unavoidably makes traceability and configuration management difficult, and the Planner’s memory is relied upon to prevent serious errors. Minor errors are removed after Brassboard testing and iteration with subsystem engineering. DSN Support
Planning
NEAR uses the Deep Space Network for tracking, telemetry, and commanding. Allocated station times are agreed upon at the DSN user level by all NASA projects. This is accomplished through attendance at a Resource Allocation Planning (RAP) Team Meeting, DSN Scheduling Meeting, or Resource Allocation Review Board (RARB) Meeting. The Deep Space Network (DSN) requires that Sequence of Events (SOE) files be generated by each project utilizing the DSN. For the NEAR project, the SOE file contains Deep Space Station (DSS) specific configuration and tracking directives coordinated with NEAR Mission Operations Center (MOC) supported spacecraft contacts. These files are generated by the NEAR project on a weekly basis at a minimum. The files are typically built to include one DSN weeks worth of configuration information. APL has developed software to assist NEAR Ops personnel in the generation of SOE data. The main software product is APLKey, the NEAR DSN SOE Keyword File Generator. A DSN contact planning tool (CONTACT) aids the NEAR DSN scheduler in keeping track of when NEAR has access to DSN antennas to command/receive data. A copy of the SOE is placed on a shared volume in the NEAR MOC, for generation of the STOL script of timetagged commands for the contacts using the READ_SOE tool developed by the Ops team.
Sequence
Planning
for the Eros Orbiting
Phase
The cruise method of constructing an activity sequence can take weeks of effort to produce a sequence that executes for a few hours. This process is inadequate for Eros operations, when
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activity sequences will be expected to run for as long as 16 hours a day every day. For this reason Ops, with a team of software developers, has begun developing a new tool suite which will replace the cruise phase process. This new process of sequence construction uses a generic operations sequencing and scheduling tool, SEQ_GEN, developed by the Multimisson Ground Systems Office at JPL. SEQ_GEN requires adaptation for NEAR. This adaptation includes loading a copy of the NEAR command database into SEQ_GEN, along with modeling of the NEAR subsystems. NEAR operations staff, in conjunction with the science teams and instrument engineers, are building small, reusable Brassboard tested fragments of command sequences, which once loaded into SEQ_GEN will become the building blocks for activity sequence generation. The Eros process flow will make science teams responsible for their own opportunity analysis. The Science Operations Planning Group [3] will reallocate resources such as data bandwidth and ownership of NEAR pointing amongst concurrent investigations as the orbit about Eros changes. Using the approved list of fragments, each of the science teams will provide the operations schedulers with a SEQ_GEN formatted request file for each load period. The Ops scheduler will input all requests for a given load period into SEQGEN, resolving any conflicts or constraint violations found in the detailed modeling. The time-ordered list of primitive commands SEQ_GEN produces are then processed into command macros with their associated execution times. The resulting command load is processed by the faster than real-time simulator. This entire process should take about 3 weeks for a command load that would take one week to execute, with three command loads concurrently under construction at different stages of development.
Conclusions Cruise mode activity estimates tend to be optimistically low. Planning for low-cost missions should allow for this reality, but there is a balance to be struck - flight experience will lead to important changes in operations procedures, so large expense in preparation of detailed sciencemode procedures before launch may not be justifiable - especially not for a first-of-a-kind spacecraft with an extended cruise phase. What is important in the early stages of a Discovery mission
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is to work through the architecture of how Ops will proceed upon arrival at the target body. A key to stabilizing this architecture in a low cost environment is maximizing the use of COTS/GOTS software tools for planning, scheduling, simulation, and for spacecraft configuration and resource management. Science and engineering teams working with Ops need to be educated as early as possible about the risks and demands that spacecraft activities impose. NEAR Ops has enjoyed excellent working relationships with all the science teams, but there have been unrealistic expectations about the space and ground resources required to coordinate and perform activities. It is also important to inculcate in the science teams the need for a modicum of discipline in scheduling and planning. An important lesson learned has been the composition and organization of the small team needed to perform low-cost operations. A flexible, unlayered team with minimal functional barriers has been essential to the outstanding success of NEAR Mission Operations.
Acknowledgments The authors wish to acknowledge R.L. Nelson and R.B. Dickey of APL for their editorial support and their contributions to NEAR operations; the NEAR science, mission design, subsystem engineering and navigation teams, for their stalwart support, patience, and willingness to communicate, and M.G. Carr for her editorial assistance.
References [l] Farquhar,R.W., D.W. Dunham, and J.V. McAdams, “NEAR Mission Overview and Trajectory Design,” Journal ofthe Astronautical Sciences Volume 43, No. 4, October-December 1995, pp. 353-372. [2] Yeomans, D.K.,“Asteroid 433 Eros: The Target Body of the NEAR Mission,” Journal of the Astronautical Sciences Volume 43, No. 4, October-December 1995, pp. 417-426.
[3] Landshof, J. and A. Cheng, “NEAR Mission and Science Operations,” Journal ofthe 1995, pp. 477-490.
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[4] Kowal, C.T. and A.S. Posner, “Planning and Execution of the Asteroid 253 Mathilde Encounter for the Near Earth Asteroid Rendezvous (NEAR) Mission”; Fifth International Symposium on Space Mission Operations and Ground Data Systems; Tokyo, Japan; June 1998. [5] Santo, A.G., S.C. Lee, and R.E. Gold, “NEAR Spacecraft and Instrumentation,” Journal 1995, pp. 373-398.
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[6] Strikwerda, T. E., J. C. Ray, D. R. Haley, G. A. Heyler, H. L. Fisher, and R.T. Pham, “NEAR Guidance and Control System,” AAS 97-077,2Oth Annual AAS Guidance and Control Conference, Breckenridge, CO, February 5-9, 1997.
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[7] Yeomans, D.K., et. al., “Estimating the mass of asteroid 253 Mathilde from tracking data during the NEAR flyby,” Science 1997 Dee 19;278(5346):2106-2109.