Lelia Vann (l.b.vann@larc.nasa.gov), SAGE Science managerh, NASA Langley Research Center
On July 12 and 13, a systems Preliminary Design Review (PDR) of the Stratospheric Aerosol and Gas Experiment (SAGE) III was conducted at the instrument developer's (Ball Aerospace) facility in Boulder, CO. The PDR team consisted of the following people, all from NASA Langley except for John Loiacono from NASA Goddard: Len McMaster, Chair; Lelia Vann, Executive Secretary; Mike Blythe; Richard Foss; John Greco; John Gustafson; Reginald Holloway; Sam Joplin; John Loiacono; Jim Miller; and Pamela Rinsland.
The PDR objectives were to:
To help ensure that the PDR objectives were met, the SAGE III Project Manager, Ed Mauldin, held subsystem "table-top" reviews of the instrument two weeks prior to this system-level review for a more-detailed review of each of the major subsystems. In addition, where possible these table-top reviews were chaired by the subsystem technical expert from the system-level PDR review team to enhance continuity between the reviews. The table-top reviews that were held and the respective leads are identified below:
Optical Wes Lawrence Electrical Mike Blythe Pointer/Scanner Jim Miller Mechanical/Thermal Richard Foss Software Pamela Rinsland
During the system-level PDR each table-top lead summarized the respective table-top review and highlighted any concerns as a result of the review. The corresponding Ball subsystem manager followed each lead with a summary of the subsystem and an explanation of the plans to resolve any concerns which were identified.
The SAGE III Project Manager gave a brief overview of the Master schedule. Several of the major milestones were the Critical Design Review (CDR) one year from now, the first instrument delivery to the Russians in December 1997, the Meteor-3M flight in mid-1998, the second instrument delivery (Flight of Opportunity (FOO)) in mid-1998, and the third instrument delivery (International Space Station Alpha [ISSA]) in December 1998.
In addition, Mauldin pointed out that this PDR was to review the technical portions of the Meteor-3M mission. A delta PDR will be conducted for the FOO and the ISSA mission at a later time because of undefined interfaces for those missions at this time. Also, a delta PDR will be held this fall after contract negotiations with Ball Aerospace to baseline the program cost and schedule.
Co-Investigator, Joseph Zawodny, presented the SAGE III science objectives, including justification and heritage, importance to the Earth Observing System (EOS) program with its global coverage and long-term trend measurements, and predictions. He identified the main differences between the SAGE III and the SAGE II instrument, which were to enhance measurement capability and provide additional nighttime measurements (namely, NO3 and OClO, which are key to the O3 chemistry) via lunar occultation.
Jim Miller characterized the overall pointer/scanner mechanical and electrical design as being mature due to strong SAGE II heritage. The SAGE II-specific components will be procured to SAGE II specifications from the original vendor or alternative sources which he identified. His concerns were issues related to the elevation motor sizing, the azimuth slew rate (5 deg/sec to 10 deg/sec), the flex-pivots, and the sun/lunar sensor ambiguity.
Art Ray, Ball Instrument Manager, followed Miller with a description of the sensor design. The ISSA mission is currently driving the mirror scan motor development because it requires the mirror to scan +/-3.7 degrees. The Meteor-3M mission only requires the mirror to scan +/- 1.8 degrees, which is within the SAGE II scan motor design. The SAGE III/Meteor-3M orbital parameters are 1020-km altitude, 99.53 degrees inclination, and 9:15 a.m. ascending node. The viewing angles on Meteor-3M for both elevation and azimuth axes were shown and seemed adequate for both the solar and lunar events. The Meteor-3M has a 105-minute period. The 99.53 degrees inclination places SAGE III over latitudes between +/- 80 degrees. Each orbit offers a sunrise and sunset event of approximately 2 minutes; approximately 15% of the orbits offer a lunar event; and approximately 1% of the orbits offer both a moonrise and moonset.
The science and low-rate data are stored in the data storage unit (DSU). The DSU is dumped once every 12 hours over both Russia and the U.S.
To predict the characteristics of the SAGE III sun/lunar viewing events, Ball has developed an analytic tool which will determine the frequency of the sun/lunar limb viewing, the duration of opportunities, the azimuth/elevation line-of-sight directions of each opportunity, the sensitivity analysis for solar zenith angles for 95-to-98 degrees, and the translation of GPS data on-orbit to initialize on-orbit event prediction. In summary, there are ample sun viewing opportunities (twice per orbit revolution for 120 seconds of viewing). The lunar viewing is limited by season and moon phase. There are simultaneous dual sun/lunar viewing opportunities that exist which require targeting preference of one or the other, but not both. The solar zenith angle 98 degrees versus 95 degrees has small impact on the number of lunar-viewing opportunities. Lastly, there are no field-of-view interferences with the sun/lunar azimuth line-of-sight directions and the METEOR-3M hardware.
Mike Blythe summarized the electrical subsystem table top as being a "good thing." The architectures are established and the interfaces are well defined. There were no show stoppers in sight with the possible exception of meeting the power budget for a FOO. His primary concerns were the power budget, no limit check on the elevation motor, charge-coupled device (CCD) and analog/digital (A/D) converter not on the same board as the CCD, potential noise due to long wires, termination of the 1553 bus, time alignment of science and engineering data, and the ground support equipment (GSE) design and staffing.
Richard Tarde, the Ball Electrical Subsystem Lead, gave an overview of the electrical subsystem and concluded by specifically addressing the concerns which were presented by the table-top reviewers. The time alignment of the science and engineering will be studied. There is most likely a time delay on the elevation scan mirror. Zawodny pointed out that the scientists need to know the elevation position at a known time. They can live with a known time delay.
Pamela Rinsland summarized the software subsystem table top as being successful and highlighted that the subsystem requirements, detailed top level diagrams and the process flows, and operational scenarios were well presented. Her concerns included the definition of detection and recovery from error conditions, requirements clarification/enhancements, short-term specifications, long-term design goals (patchability on orbit and that the bench checkout unit [BCU] software should be designed to be reused for the mission operations), and that the GSE may need more staffing. Overall, however, she felt that the Ball team was ready to move on to detailed design for the Meteor-3M mission.
Larry Zimmerman, the Ball Software Subsystem Manager, followed Rinsland with a summary of the flight software capabilities, which include autonomous science-event prediction, controlling the science data collection, and on-orbit reprogramming. SAGE III also has two flight computer software configuration items (CSCIs), the instrument controller (master) and CCD controller (slave). The GSE software capabilities include the BCU and the instrument simulator unit (ISU). Ball plans to simulate the 1553 serial data bus interface with the Russian spacecraft. Also the Russians have agreed to provide a spacecraft simulator to Ball. The interface between the MIL Std 1553B dual redundant bus and the Russian triple bus is critical to the instrument operation and should be checked out prior to spacecraft integration and test (I&T).
Richard Foss summarized the mechanical/thermal subsystem table-top review and stated that the overall mechanical design and structural/thermal analysis relied heavily on SAGE II heritage and that the majority of the design and analysis was based on the Meteor-3M mission requirements because the ISSA and FOO are not yet fully defined. He had concerns such as the power budget, undefined Russian interfaces, and some modeling updates and analysis needed but saw "no show stoppers."
Dane Schnal, the Ball Mechanical/Structural Subsystem Manager, followed Foss with an overview of the mechanical subsystem. SAGE III consists of the following mechanical assemblies: spectrometer/telescope/lunar sensor, the azimuth system, and the scan head. Areas of greatest concern include the elevation motor trade, the flex pivots, and the Russian interfaces (specifically the Russian interface definition document [IDD] indicates a 0.1 g wide spectrum vibration possible).
Lenny McMaster summarized the spectrometer/telescope subsystem table-top review. The concerns highlighted included the linearity of the CCD array responsivity, the quantum efficiency stability of the CCD array, and the spectrometer stray light analysis, which thus far indicates minimum margin.
Jim Baer, the Ball Aerospace Spectrometer/Telescope Subsystem Manager, followed McMaster with an overview of the spectrometer/telescope assembly. The changes to the design since the System Requirements Review (SRR) include an all-aluminum telescope, including the primary and secondary mirrors; the secondary mirror monolithic with a spider support ring; the telescope, spectrometer, slit mounting, and registration have been detailed; and an improved grating substrate is under consideration (all aluminum grating). The primary issues identified and plans discussed for potential resolution were the need to measure stray light levels internal to the spectrometer (out-of-band rejection is the primary concern); the stray light effect on the lunar azimuth sensor; the contamination level effects on the scan mirror and slit; the location and use of the witness samples; and the CCD stability and linearity.
James Flores, Ball Aerospace, presented the effects of radiation on the CCD arrays. The silicon damage and the dielectric damage were analyzed. The results from the analyses were that the CCDs will need to be warmed to +20 degrees C for a few hours once a week (or CCDs could be controlled to 20 degrees C) to meet the radiation model projections; the end of life (EOL) linearity issues can be controlled with clock voltage optimization; and it is likely that the overall radiation damage rate can be mitigated by reducing the gate e-fields between operations.
Art Ray presented the trade-offs and changes to the azimuth tube, azimuth sensor, universal asynchronous receiver/transmitter (UART), scan head parts, calibration, and telescope from the SRR design approach. For the azimuth tube, material trade-offs are being made to consider the thermal properties. For the azimuth sensor, a large FOV azimuth sensor was added to ensure solar acquisition and a study to combine this function with the small FOV is being made; a small FOV azimuth sensor was moved to ease the alignment and installation; and a study was made of moving the science slit alignment toward the limb edge of the azimuth sensor to accommodate small limb angles on the Meteor-3M mission.
Next, Art Ray presented the "acceptable risk" for the power estimate, CCD performance, lunar acquisition, and the spacecraft disturbances. Power: Ball recommends proceeding with the current design and negotiating with the Russians for additional power. CCD: A waiver request has been submitted to retest the Scientific Imaging Technology CCD arrays for quantum efficiency (QE) repeatability and linearity. Also provision will be made for warm-up to anneal out dark current and radiation effects. Plans are to calibrate linearity on-orbit. Lunar Acquisition: For the Meteor-3M mission, offset the science slit toward the limb edge of FOV to avoid its brightness. For the ISSA mission, use the large FOV to do a better job with the small FOV and develop an "expert" system to take out ISSA deterministic pointing errors. Meteor Disturbances: Work with the Russians to understand the interface. The 0.05 g to 150 Hz is unbelievable, especially if this is applied laterally.
External interfaces were shown and discussed. Richard Tarde presented the electrical external interfaces. Fred Hausle presented the mechanical external interfaces. Armen Melikian presented the thermal external interfaces. He reviewed the requirements, the thermal design approach, and the results of the analysis. The Meteor-3M mission thermal predictions indicate that all the thermal requirements are being met.
Mike Cisewski, Lockheed/Martin, presented the mission operations concept for the Meteor-3M mission, which is based on the Meteor-3M/TOMS mission. Commands to the instrument are transmitted from the SAGE III mission operations to Russia via Internet every 2 weeks. These commands will be uplinked to Meteor-3M/SAGE III once every 2 weeks.
Tim Torphy, Ball Aerospace, presented both the electrical and mechanical ground support equipment (GSE). He also presented the system performance requirements verification and validation plan and demonstrated the system used to track these requirements. The performance requirements come from the Statement of Work and the Instrument Design and Performance Specification. He then presented the integrated test plan for the SAGE III project. The integration and testing is divided into the development testing, the flight unit testing, and the post-shipping testing. The development testing is dry running the flight testing and will use the verification unit (VU). The calibration plan includes developing a transfer reference spectral radiometer (TRSR) which will be characterized and calibrated by doing a side-by-side viewing of the sun with a "calibrated" radiometer. The TRSR will then be used at Ball to look at the sources for the calibration tests. The SAGE III instrument will be tested by looking at both the sun and the Radiometric Source Unit (RSU).
Dave Wilson, Ball Aerospace, presented the radiation effects analysis for electical, electronic, electromechanical (EEE) parts. The worst case radiation environment (Meteor-3M) was used in the analysis. Of the 130 parts evaluated, four were considered risk items. Of the four, one was considered moderate risk and the other three were considered low risk.
Don Alderman, Ball Aerospace, presented the reliability analysis. Attention areas, which have been mentioned previously, are the flex pivots and the attenuator mechanism. The preliminary failure modes and effects analysis (FMEA) has been completed. The most critical failure mode is the potential for shorts on the -27 V input power bus lines.
A project descope plan was presented by Ed Mauldin. The PI defined the minimum success criteria at the SRR, and the Project Manager developed a plan based on the minimum success criteria. For severe unexpected problems, the science and engineering team will consider cancellation of one of the instruments for one of the three missions. For known problems that start causing cost/schedule creep, a case-by-case descope plan will be developed by the science and engineering team and parallel efforts to recover the science will be implemented as well.
In conclusion, the PDR panel felt that the SAGE III program had demonstrated that the preliminary designs for the Meteor-3M mission met the system requirements with acceptable risk as defined by the payload classification and that all verification methodologies and interfaces had been identified and satisfactorily addressed. The panel consensus was that the SAGE III Project is ready to proceed into detailed design for the Meteor-3M mission.