themis fdmo review action item status − 1 october 5, 2004 action item status manfred bester

THEMIS FDMO Review Action Item Status − 1 October 5, 2004

Action Item Status

Manfred Bester


Status of Action Items

Review Number of RFAs Open RFAs Due Date

Mission SRR 3 0 N/A

FDMO PDR Peer Review 18 0 N/A

Mission PDR 3 0 N/A

FDMO CDR Peer Review 18 17 Oct 2004

Mission CDR 2 2 Oct 2004

Note: Only Flight Dynamics and Mission Operations Related RFAs Are Listed and Discussed Below


Mission SRR RFAs

RFA Specific Request Supporting Rationale Reviewer Response Status

Rec. 1) Does the spacecraft need to specify a specific circular polarization for the S-band antenna?

2) Need to perform an end-to-end data analysis including data compression, coding, RF link to calculate needed/optimal BER for compressed data.

1) Need to check TDRS capability.

2) Compressed data can be/is sensitive to data loss. An end-to-end data analysis is required to optimize/select data rate.

Schnurr See detailed response submitted to IIRT.

CLOSED

Rec. Provide, at the PDR, an optimized ground station acquisition strategy (a week in the life of THEMIS) that meets all nominal requirements.

With the addition of 5 satellites, the load on BGS and the use of the USN site must be planned to contain costs.

Joyce See detailed response submitted to IIRT.

CLOSED

Rec. Detail, by the PDR, the processes required for providing orbit determination services for THEMIS including implementation of hardware and software for BGS, training of the operations team, MOC automation plans, metric tracking, data evaluation (for BGS and the USN site), the optimized tracking strategy to achieve the required orbital accuracy and test plans.

Capturing and processing metric tracking data to provide the required orbital products must be planned prudently to contain costs. Several “new” systems need to be implemented, tested and integrated into the MOC.


CLOSED


FDMO PDR Peer Review RFAs

RFA Recommended Action Rationale Reviewer Response Status

1 Investigate monthly lunar perturbation effects to determine optimal time during month to launch.

Lunar perturbations can be used to reduce fuel required if judicious selection of launch date is made. However, lunar perturbations can also increase fuel required by decreasing orbit energy & negatively impacting orbit inclination.

Richon The THEMIS team has already built tools which allowed investigation of lunar perturbations on orbits P1, P2 and P3 as well as on the LV insertion orbit in order to minimize potential for re-entry on the science orbits, and to maximize the potential for re-entry within 10 years for the LV to adhere to orbital debris requirements. Lunar effects due to different launch dates on the P3, P4 and P5 orbits were found to be very small.

CLOSED

2 Decide on one pre-injection nomenclature for each probe. Should not be names associated with integers 1-5 since the probes will be numbered Probe 1, Probe 2, etc. post-launch according to their final orbit positions (similar to TDRSS).

Currently probes are designated as “Themis- 1, Themis-2,…”, “red,green…”, and “A,B, C…”. Multiple names for same probe causes confusion & misidentification of specific probes.

Richon The team agreed that the pre-launch assignment of probe names shall be THEMIS A-E. These names are uniquely related to the CCSDS telemetry and command IDs for each probe bus. Probe constellation IDs P1-P5 are assigned post-launch, once the probes and their instruments have been checked out and the probe placement decision has been made.

CLOSED

3 Resolve time tag issue. Harman The THEMIS attitude sensor read-out timing and attitude determination system is closely modeled after the FAST system which has been operational on orbit for 7.5 years.

CLOSED




4 Get initial 3rd-stage separation states (attitude & orbit) for each probe from Boeing.

Needed for orbit injection analysis. If Delta PMA is being done, these states should be available soon.

Richon UCB received the nominal launch trajectory ephemeris from KSC. These data are used to perform subsequent studies such as communications coverage and probe re-contact analyses.

CLOSED

5 GMAN modeling issues: define each ‘maneuver mode’ including type of maneuver, which thrusters used, pulsed/continuous, etc. Then determine if GMAN can model maneuver correctly.

Several questions regarding whether GMAN can be used to model finite maneuvers in all maneuver configurations.

Richon All anticipated orbit and attitude maneuvers have been summarized and analyzed. Bob DeFazio verified that all planned maneuvers can be modeled with GMAN.

CLOSED

6 Determine ground station contact tracking requirements such as pass frequency & duration, number of stations, for maneuver planning & recovery versus regular OD, etc.

Richon Mark Beckman performed an OD covariance analysis showing the number of passes and arc lengths required to meet THEMIS OD 3-σ position requirements of 100 km near apogee and 10 km near perigee. In case of the P1 orbit, the required position accuracy can be achieved with 4 BGS passes taken over a 5.5-day arc. Some of the orbit fine tuning maneuvers require a delta V of 30-50 cm/s. An adequate pre-maneuver orbit solution can be obtained by collecting two-way Doppler data from three different ground stations, e.g. Berkeley, Wallops Island and Santiago. Further analyses are under way.

CLOSED




7 Determine UCB station capabilities including support for perigee passes.

Mission depends heavily on UCB antenna for tracking. Need to know if orbit determination constraints can be met.

Richon Two-way Doppler tracking capabilities of the Berkeley Ground Station were demonstrated with equipment on loan from Wallops Flight Facility. The 1-σ error of range rate measurements is expected to be 3.5 mm/s in a 10-s integration time. Due to the low orbital inclination the maximum azimuth and elevation rates during a pass support will be less than 0.5 deg/s. The Berkeley Ground Station is capable of tracking with maximum azimuth and elevation rates of 8.0 and 3.0 deg/s, respectively.

CLOSED

8 ITOS-to-MSASS interface code to be provided to UCB.

Harman used successfully for other mission.

Richon UCB will provide interface code to extract attitude sensor data from ITOS and reformat these to be compatible with MSASS/MTASS input.

CLOSED

9 Real-time attitude determination - is this a requirement? If so, which software? Possibly use MTASS with modifications.

Harman Real-time attitude determination is required for maneuver support. A ground-based real-time ADS will be implemented, based upon the quaternions that are derived from sun sensor and IRU data and are propagated on-board, and are in turn sent to the ground.

CLOSED




10 Multi-satellite critical operations: need to have at least two prime workstations supporting, plus at least one hot back-up for emergencies.

Normally one has one prime and one back-up for any critical operation for a single-satellite mission. UCB will have to be able to support multiple satellites with critical operations.

Richon The ground systems design includes at least 10 ITOS workstations, including a designated prime and back-up for each probe. Furthermore, any of these workstations can be assigned to any probe. In addition, two ITOS workstations will be connected to the Closed IONet for communications via TDRSS with one probe at a time.

CLOSED

11 MSASS is not capable of performing magnetometer alignment. Determine if UCB needs to acquire MTASS in addition to MSASS for this function.

Harman UCB will obtain ADS which includes both MSASS and MTASS. A software usage agreement is in the process of being finalized.

CLOSED

12 Include pre-launch operations (Simulation planning & support, launch preparations) in future FD&MO reviews.

Ensure resource requirements are identified & included in planning.

Richon Pre-launch operations and simulations including green card exercises were included in the MPDR presentations already and will be presented in more detail in upcoming reviews.

CLOSED

13 Provide access to latest mission information to peer panel at least two weeks before review.

Richon The review team will receive most recent mission documentation at least two weeks prior to future peer reviews.

CLOSED




14 Maneuver calibration procedures need to be documented.

Each orbit & attitude maneuver should be analyzed post-maneuver for efficiency & accuracy. Results should be applied to future maneuver planning. Also needed for bookkeeping of fuel used.

Richon The UCB team has received sample procedures from GSFC FDAB and from Swales for EO-1 maneuver planning and execution. Applicable elements of these procedures will be incorporated into the THEMIS procedures.

CLOSED

15 Document who is responsible for fuel-used bookkeeping (propulsion engineer or Flight Dynamics Engineer or other FOT member).

One entity should have responsibility for this function. It is particularly critical at the end of life when probes must perform a controlled reentry.

Richon A dedicated member of the THEMIS flight dynamics team, namely the propulsion engineer will be responsible for bookkeeping the fuel usage.

CLOSED

16 Institute Command Authorization Meetings (CAMs) for all orbit & attitude maneuvers. Requires participation & ‘signoff’ from all relevant subsystems & systems (FOT, flight dynamics, thermal, power, C&DH, scientists, etc.).

Best practices. Reduces risk.

Richon Command Authorization Meetings (CAMs) will be held prior to any critical command operation, including execution of orbit and attitude maneuvers as well as critical instrument operations such as boom deployment or HV turn-on. Such meetings are common practice already with all missions currently operated by UCB, i.e. FAST, RHESSI and CHIPSAT.

CLOSED




17 Provide MAP Anomalous Forces technical paper to UCB.

Outgassing of instruments & other hardware may introduce anomalous forces creating torques on probes).

Richon UCB received a copy of the MAP anomalous force paper.

CLOSED

18 Provide JON/WBS to Code 595 for THEMIS support.

Code 595 flight dynamics engineers cannot support THEMIS without charge number.

Richon GNCD has received a JON for support of THEMIS flight dynamics work.

CLOSED


Mission PDR RFAs


15 Detail the roles of GSFC flight dynamics support to THEMIS mission operations. This shall include use of GSFC tools (GTDS, GMAN, etc.) and configuration management of these tools and GSFC personnel support during Phases C/D and E.

Significant new capabilities are required of UCB to support this mission including orbit determination and maneuver planning and maneuver operation.


CLOSED

16 Document the criteria for selection of probes for their operational orbits (designation as P1, P2, P3, P4, P5) to prepare for the initial conjunction observations. Discuss potential probe anomaly scenarios and non-nominal instrument complements that may be part of this selection process. Define the team members and positions that will support the PI in the probe placement decision.

The selection process should be well defined to enable the team to optimally implement the probe placements to prepare for the initial prime science observing season.


CLOSED


Mission PDR RFAs


30 Consider adding a requirement to upload the Probe numerical P# to the Probe once it is (re-) positioned in its orbit, and a requirement to include the P# in all telemetry packets (extension to secondary header??). An additional benefit of this is that the on-board P# could be coupled with a P# embedded in uplink commands, which could cross-check to make sure the ground ops team didn’t inadvertently uplink a maneuver command to the wrong probe.

Without this information being embedded in the downlink packets, the science team will have to have some additional table in order to correctly interpret the results. Having it embedded in the data avoids any possibility of confusion. While the CCSDS S/C ID ensures that a Probe doesn’t process a command not intended for it, it doesn’t do that in terms of the Probe orbit position. What is probably a very small investment in adding this information to telemetry and command formats now might save a lot of headache (and maybe heartache) later.

Killough See detailed response submitted to IIRT.

CLOSED


FDMO CDR Peer Review RFAs

RFA Statement of Concern Recommended Action Reviewer Response Status

1 The previously discussed support by GSFC to train UCB/THEMIS personnel in the use of GTDS for end-to-end orbit determination operations has not been implemented. This training is vital to the development of this key mission function.

1. An agreement between appropriate GSFC and UCB/THEMIS personnel should document the required orbit determination training and the technical personnel to support the training.

2. A training plan as part of an overall orbit determination operations schedule is to be provided one month prior to MOR.

Joyce Following recommendations of the GNCD Branch Head and the FDF Operations Director, UCB is now planning on contracting directly with AI Solutions to obtain mission critical training of the THEMIS Flight Dynamics Team in GTDS based orbit determination. GNCD FDAB/FDF will not participate in the orbit determination training.

A statement of work and a request for quotation will be submitted to AI Solutions shortly. AI Solutions already has developed material for suitable GTDS training courses that are applicable. As part of the statement of work, AI Solutions will also be contracted to provide assistance with configuring GTDS for THEMIS orbit determination at the Berkeley Mission Operations and Flight Dynamics Center.

Details of the training plan and the corresponding implementation schedule with various milestones are outlined in THEMIS FDMO CDR Peer Review RFA Reponses.

OPEN




2 There may be no documented requirement that an updated post-injection vector be provided to the THEMIS Project based on Boeing Delta Redundant Inertial Measurement System data. This data is required to maintain communications with the probes until an accurate orbit determination is available. It is even more important in the event of a non-nominal injection.

The Project should pursue levying this requirement with additional details provided by GSFC/FDF on Boeing prior to the MOR.

Joyce Requirements for delivery of real-time telemetry as well as second stage burn modeling from Boeing to GSFC/FDF to generate state vector updates are outlined in the Delta (NASA) Launch Vehicle PSLA (451-PSLA-Delta (NASA)). Only guidance data from the second stage are available. In the absence of a navigation system on the third stage, a post-insertion state vector for the THEMIS probes will be generated by GSFC/FDF based on observed second-stage guidance data and a predicted nominal third-stage performance. GSFC/FDF support for THEMIS for the first 48 hours of the mission is documented in the THEMIS PSLA (451-PSLA-THEMIS).

OPEN




3 Currently, the mission design work centers on the evolution of Keplerian elements. However, it has been claimed that despite their individual motions the line of apsides of each orbit remains constant for 1st order.

A set of plots showing the right ascension and declination of the line of apsides is suggested to support this claim and to clarify the reasons for selecting a constant right ascension of perigee (RAP).

Schiff The annual RAP drifts for all five probes during T1 and D1 are summarized in Table 3-1. Launch RAP is equal to launch RAAN for an APER of 0 deg.

The plots shown in Figures 3-1 through 3-5 in THEMIS FDMO CDR Peer Review RFA Reponses show the evolution of the line of apsides, i.e. the declination of apogee versus right ascension of apogee (black traces) for each of the five probes for tail season 1 (21-Oct-2006 – 22-Apr-2007). In addition, the plots also show the development of the inclination (blue traces), APER (green traces) and RAP = RAAN + APER (red traces).

OPEN

4 A ΔV budget with allocations to account for daily / time-of-day launch window and ELV dispersions was not presented and may not be available as of this date. A complete ΔV budget is needed to determine how stable the current design is.

A technical approach and schedule should be drafted as soon as possible and be submitted for a review at MOR.

Schiff The ΔV budget accounting for launch window and ELV dispersions is available, but was not presented at the peer review.

See detailed response in THEMIS FDMO CDR Peer Review RFA Reponses.

OPEN




5 On Slide 7 of the Propellant Budget, four ΔV inefficiencies were considered. However, these inefficiencies are all associated with the geometry of the thruster, either in the body frame (i.e. misalignment) or inertially (rotational impulse loss sin(η/2) / (η/2), the beta inefficiency, and finite arc losses). However, performance loss is not considered.

The project should take a specific position on an Isp inefficiency which, if considered, will lower the overall ΔV capability of the spacecraft. Consider the rocket equation:

ΔV = g Isp ln (mi / mf)

A change in Isp of δIsp will cause a change in ΔV. If the Project decides against considering this possible inefficiency then they should be clear on all ΔV budgets that such an inefficiency is not being used.

Schiff The THEMIS thruster performance is known to be very repeatable with a 3-σ variance of 2.8% based on 26 firings of test thrusters over the last 3 years. Therefore a 2.8% Isp

inefficiency is carried in the maneuver calculator spreadsheet.


OPEN




6 Clarify requirements in mission for close approach/debris per NASA Safety Standard 1740.14 and NASA Policy Directive 8710.3B. NASA satellites are required to provide predicted ephemeris to Cheyenne Mountain for every orbit-changing maneuver of >1 km, and for launch and early orbit operations.

Review NSS 1740.14 & NPD 8710.3B to determine requirements applicable to THEMIS. Incorporate into procedures and analysis.

Richon Requirements for close approach and orbital debris generation are addressed in the THEMIS Final Orbital Debris Report (CDRL #37) which was submitted by the Project for review by NASA HQ, GSFC and KSC. UCB will work with Adrienne Davis at GSFC/FDF to prepare the required paperwork to notify DoD about THEMIS orbit maneuvers in accordance with NPD 8710.3B, Section 5.b.7. Notification of DoD will be part of the operations procedures.

OPEN

7 Need verification / QC of trajectory design and maneuver plan for all probes. Currently one person (Frey) doing all analysis. Need capability for targeting and optimizing trajectory design. FreeFlyer (AI Solutions) can do this.

Contract with GSFC, AI Solutions, other trajectory design group to verify trajectory design and maneuver plan. If switch from GMAN to FreeFlyer, AI Solutions is logical choice.

Richon A consulting contract will be set up between UCB and AI Solutions to verify GMAN/FreeFlyer compatibility. UCB purchased a FreeFlyer Engineer run-time license in the meantime. AI Solutions will likely be contracted by the Explorer’s Office to independently verify key scenarios in the THEMIS mission design.


OPEN




8 There is a requirement for use of MSASS and MTASS for attitude determination. As of now, there is no formal agreement between GSFC/FDAB for any requirement or modifications to software, delivery of software, version (CM) for software, or testing to meet launch schedule.

Agreement on requirements for software, delivery schedules, versions and testing needs to be done as soon as possible with GSFC/FDAB. Resource (funding) to make any modifications or do testing needs to be agreed upon.

Hoge The requirements document for the THEMIS Real-time Attitude Determination System (RTADS) which includes MSASS and MTASS are in the draft stage and are currently reviewed by UCB and Swales.

Once the review at UCB and Swales is completed (target date: October 8, 2004), the document will be submitted to GSFC/GNCD for review and comments.

GNCD has agreed to provide upgrade of MSASS including new nutation estimation utility. This upgrade is already covered as an institutionally funded activity. This task covers the transfer of the upgraded software and a small level of consultation for its use. It is assumed UCB has experience in the use of MSASS for the FAST mission.

OPEN




9 Version of GTDS software is planned for upgrade to latest version. GSFC/FDF maintains only an HP version of software. A Solaris version is used for THEMIS operations and analysis. When updated version is delivered will need to be compiled under Solaris. This will require time and resources to compile and test software that may not be in schedule. Additionally, CM process for upgrade is not clear.

Determine effort for testing and verification of software. Implement a CM process for GTDS software if one does not exist. Work with GSFC/FDF for software upgrade via Commercialization Office at GSFC.

Hoge UCB currently uses GTDS version 96.03 Delta 4, 09/11/1997 (HP-UX V10.2 source code, compiled under Solaris). FDF currently uses version 2002.01 for operations. A new version 2003.01 will be available in October 2004.

UCB has been working with the GSFC Commercialization Office to obtain a transfer license for version 2003.01. A preliminary 90-day license has been granted already.

Upon delivery, version 2003.01 will be installed under a dedicated flight dynamics operations account at UCB and placed under SCCS version control. Subsequently the source code will be compiled under Solaris.

Upon compilation, a series of tests will be performed to compare state vector propagation results against test cases and ephemeris files generated with the same version of the code at GSFC. Also, the new version will be compared against the version that is currently used at UCB for THEMIS mission design work.


OPEN




10 Given flexibility and priority of data available from day to day of the mission, is there a science data collection statistic that can be agreed upon as a measure of mission success?

Identify statistics and metric and include in operations plan to monitor performance.

Crouse Conjunction science data up to 750 Mbits per orbit collected during the wedding season (February 21 ± 2 months) and during the dayside season (August 21 ± 2 months) are considered mission critical, and 100% recovery will be attempted. In addition, up to 40 Mbits of data collected in slow survey mode during radiation belt crossings of each orbit are to be recovered at the 100% level also. Also, all probe state-of-health and engineering data are to be recovered at the 100% level. Other data are of lesser importance. Statistics will be generated against the above metrics.

OPEN




11 Understand implications (if any) of the NISN mission operations voice enhancement activity.

Review Project Management Plan from NISN and identify impact (schedule, cost, etc.).

Crouse The THEMIS Network Operations Manager (John Ervin) states there is no problem with UCB’s SCAMA system w.r.t. planned voice system upgrades at GSFC.

OPEN

12 Current analysis uses 50% penumbra to determine eclipse. What impact is there for penumbra less than 50% on both power and thermal?

Look at eclipses defined as 100% (umbra) from beginning of penumbra to end of penumbra. Evaluate recharge period as well.

Crouse A refined shadow analysis has been performed to determine the penumbra and umbra durations for all encountered shadows throughout the mission. A Power Management Tool (PMT) is currently under development at Swales. Additional analyses will be performed when the development of this tool has been completed. The PMT will be available to the THEMIS Flight Operations Team to manage the probe power budgets during on-orbit operations.

Swales has performed a begin-of-life, end-of-life and worst case power analysis, showing that the power system will not experience lockout, and battery performance meets our current worst case requirements at EOL.


OPEN




13 Are maneuvers needed specifically to calibrate the gyros? Will maneuvers already in the orbit plan be enough to calibrate the gyros?

A study is needed to see if the gyros can be calibrated sufficiently through the nominal course of maneuvers. Is there enough observability of the attitude about these maneuvers plus enough maneuvering about different axes? Can bias, scale factor and misalignment be solved? If there are errors in these, how much attitude error will there be and will this be more than the knowledge requirement for maneuvers?

Ottenstein The Inertial Reference Units (IRUs) a.k.a. gyros are calibrated as a byproduct of already planned-for maneuvers. Swales has performed an analysis showing that special maneuvers to calibrate the gyros are not required.

OPEN

14 The health-and-safety check of THEMIS B is currently scheduled during shadow without any check before ATS load in sunlight.

Insert a 10-minute delay in BGS acquisition health-and-safety checks so all five probes are in sunlight.

Ottenstein A 10-minute delay was inserted into the L&EO timeline so that all probes are in sunlight during the first set of AGO and BGS passes.

OPEN




15 It appears that more attitude analysis is needed to prepare for the mission. The presentation does not show enough analysis yet.

1. Derived requirements.

Are there thermal or instrument constraints which will derive requirements on attitude determination or control? Are there Sun angles to be avoided? Consider the problem of the EFI on POLAR at certain Sun angles and determine whether such a problem will apply to THEMIS. Determine what the range of Sun angles is during the mission.

Ottenstein Quote from Prof. Mozer, EFI PI: “The only Sun angle problem with EFI occurs when SPB spheres pass through the spacecraft shadow. This puts a large glitch in the sphere output signal. THEMIS is planning for this.”

The nominal range of angles between the spin axis and the direction toward the Sun is 80-100 deg for all probes.

OPEN

2. Torque analysis.

How much change to gravity gradient, solar radiation pressure and dipole moment torques affect the attitude and Sun angle per orbit and over the course of a month?

With all booms deployed, the THEMIS probes carry an angular momentum of 829.4 N m s, which provides a huge “gyroscopic stiffness”.

Gravity-gradient (GG) torques cause a maximum spin axis tilt of 0.038 deg per orbit.

Torques due to solar radiation pressure cause a worst case tipping of 0.064 deg per month.

Magnetic moments cause a worst case tipping of the spin axis by 0.037 deg per month.





3. Nutation damping.

How quickly will nutation be damped after a maneuver and separation? If it does not damp as expected, are there contingency plans to actively damp it?

The dominant source of nutation damping is fuel slosh. A 1st linear model plot of damping versus tank fill is shown below. Nutation damping is being confirmed by detailed models at both Swales and UCB.

Nutation Time Constant (NTC) results from simulations (presented at Mission CDR) were obtained using pendulum models for slosh with analytical parameters (mass, length, hinge point, residual mass and location) and empirical parameters (damping) from SLOSH software (Dodge). The dimensionless time constants (DTC) from these simulation results are greater than three times the DTC from empirical Neer-Salvatore scaling (Hubert 2003) for comparable cases (same inertia ratio & fill fraction). Therefore, the results are considered conservative. See detailed response in THEMIS FDMO CDR Peer Review RFA Reponses.

There are no plans to actively damp nutation.




4. Shadow analysis.

How will long shadow periods affect the attitude of the spacecraft? Will there be perturbations to contraction of the booms? How will the shadows affect the ability to perform attitude determination? Are maneuvers planned with respect to knowledge of the shadows? How are gyros affected? How is the system affected by not having a Sun pulse during the shadows? Data from WIND, POLAR and FAST of spin rate changes near eclipse may be of use.

The wire boom cables are a multi-conductor construction designed for low CTE (< 1 ppm/C). The thermal mass is ~10x WIND / IMAGE / DE-1 wires, while the CTE is 20x lower, which makes shadow troubles almost benign. A plot of the POLAR spin period during a two-hour shadow is shown below.

Attitude determination during shadows will be based on FGM data for near-earth shadows. Otherwise, attitude determination will not be performed during shadows. The probe attitude vector is assumed to be inertially stable.

Maneuvers are planned to be performed outside of shadows. During shadows the probes generate artificial sun pulses for onboard timing purposes.

5. Attitude error budget.

How was the 1deg science error budget obtained? How stringent is it? What elements of error go into the budget?

The 1 deg science attitude budget requirement is derived from the minimum science requirement to measure the magnetic field (to within 10% or 1 nT, whichever is larger.





6. MSASS training.

If a copy of MSASS using Sun and FGM is available before the version with the Kalman filter, this should be used with FAST data to see how well Sun and magnetometer agree with Sun and Earth for training with use of FGM data.

The THEMIS team is looking into using FAST Sun sensor and magnetometer data to test MSASS. Furthermore, the Kalman filter MSASS version of MSASS with magnetometer and MSSS should be compared to the Sun and Earth data version with data from FAST.

7. MSASS studies.

At each planned attitude examine observability for Sun sensor and magnetometer. Generate simulated data (Matlab can probably generate this to go directly to the DA system). One group should generate the data while another group should determine the attitude with an a priori knowledge of no better than 5 deg or so. Generate clean data first, but at proper bin size, then later include noise and misalignments and offsets that can be expected.

The THEMIS team wishes to thank the reviewer for this excellent suggestion.




8. Magnetometer.

Magnetometer observations are critically affected by timing errors. How will offsets in time be communicated to the attitude determination team? Will additional MSASS development be needed? Note that analysis with Matlab for magnetometer only attitude solutions for a spinning spacecraft (FAST and IMAGE) was made for GSFC in the past and these studies should be examined.

Flux gate magnetometer data are time tagged by the IDPU with an accuracy that is negligible with respect to the clock drift. The latter can be as large as ± 500 ms/day. However, the probe clock will be adjusted periodically to an accuracy of a few milliseconds with respect to UTC. If such a clock adjustment is performed just prior to a critical attitude determination event near perigee, the clock offset may be kept below 10-20 ms. At a spin rate of 20 rpm this timing error translates into an attitude error of 1.2 – 2.4 deg, which is less than the 5 deg requirement for maneuver operations. A modification of MSASS may not be required to take clock offsets into account.




16 TDRSS SSA support is baselined. TDRS HIJ SMA services may be able to support telemetry and command and have greater availability.

UCB/SSL should investigate use of SMA services from TDRS HIJ and that Doppler rates at perigee passes are within TDRSS limits.

Gramling Code 451 performed a link analysis for TDRSS, showing that only TDRSS SSA mode is feasible. In addition, the GN style THEMIS transponder is not compatible with SMA mode in terms of RF frequency and modulation.

The maximum range rates between the THEMIS probes and TDRSS are 10.5 km/s and 2.4 m/s2 near perigee of the insertion orbit. The maximum rates that TDRSS can support are 12.0 km/s and 15.0 m/s2, respectively.

OPEN

17 There may not be sufficient tracking data (from more than one station) to allow for post-launch orbit determination (may not have adequate acquisition data). There may be a time gap in the timeline at 02:15 to 03:50 MET where another ground station, preferably Santiago, to get two-way Doppler data. Or, can second set of round-robins (starting at 04:00 MET) be done through another ground station?

Look at timeline to see if another station could be brought up to get additional Doppler data sooner after launch. Develop timeline for early orbit solution updates and determine OD support and station requirements. Timeline should go through the first apogee maneuvers (perigee raising) for all five probes and include time after maneuvers to do OD.

Gramling Once the probes have been maneuvered to a Sun normal attitude, one pass set per day scheduled for support by the Berkeley Ground Station has been shifted over to the Wallops Ground Station to allow for optimal tracking data acquisition for early orbit determination.


OPEN

18 Is there a requirement for a back-up MOC?

If yes, pre-launch testing would be required.

Hoge According to the Mission Manager (Frank Snow), a back-up MOC is not required for THEMIS.

CLOSED


Mission CDR RFAs


3 Revisit TDRS link margin to establish sufficient forward link margin and ensure any new operational constraints are included in the operations plan.

Current forward link margin (1.6 dB) is not sufficient at CDR.

Schnurr The TDRSS link budget has been revisited and an effort has been made to find ways to improve the forward link margin. On the probe side, the receiver G/T performance is fixed. The modulation index cannot be increased because the resulting lower RF carrier signal level would impact receiver lock performance negatively. There are no hidden losses on the TDRSS side that are too pessimistic. The TDRS-to-probe range cannot be restricted to retain optimum coverage for monitoring orbit insertion and maneuvers. However, by far the largest variation in forward link margin is due to the probe attitude in combination with a non-omni directional probe antenna pattern. The only way to increase the forward link margin from 1.6 to 3.0 dB would be by restricting the attitude angles, and correspondingly maneuver scenarios such that the forward link margin is 3.0 dB or larger during critical operations. The constraint analysis depends very strongly on the details of the probe antenna pattern, and we need to wait until a radiation pattern of a high-fidelity engineering model can be obtained from the manufacturer – New Mexico State University / Physical Sciences Laboratory (NMSU/PSL), presumably by end of September 2004.

OPEN


Mission CDR RFAs


Rec Project should provide periodic mission design/propellant budgeting oversight to help assure propellant usage is being best used to satisfy science mission, contingency operations, extended mission and end-of-life operations requirements and objections.

Delta V is a critical resource which needs considerable attention through the design cycle through mission operations.

Ford The Explorer’s Office has initiated a task to independently verify UCB’s mission design work, including verification of the delta V budget. The UCB Flight Dynamics Team provides insight into their mission design and propellant budgeting details in various reviews focusing on flight dynamics and mission operations.

OPEN

themis fdmo review action item status − 1 october 5, 2004 action item status manfred bester

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