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Super Star Tracker

Customer's Goals of this Study. Get a good handle on mass/power/cost/size for input into an IMDC study for SI or MAXIM Pathfinder, where 30 microarcsec line-of-sight knowledge is needed.At least better than the WAG we have made in the past IMDC runsDefine other" requirements- jitter, thermal,..U

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Super Star Tracker

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    1. Super Star Tracker

    2. Customer’s Goals of this Study Get a good handle on mass/power/cost/size for input into an IMDC study for SI or MAXIM Pathfinder, where 30 microarcsec line-of-sight knowledge is needed. At least better than the WAG we have made in the past IMDC runs Define “other” requirements- jitter, thermal,.. Understand the scaling laws to make this eventually work for a more complex mission needing 30 nanoarcsec line-of-sight knowledge Identify required technologies: 1) to make possible 2) to make cheaper See what studies are currently under way (eg look through NASA Technology Inventory,…) Where does this fit in? Eg. This should also be a part of the VISNAV system Bring some awareness of these types of issues for our more challenging imaging missions to GSFC people. Better define the problem- the usual “scientist does not quite know how to describe the needs to engineer” difficulty….

    3. Requirements

    4. Alignment

    5. Trade Studies 1) Similar to Hubble Space Telescope: Super Star Tracker Centroids on beacon FOV big enough to catch stars 2) Similar to Gravity Probe-B (GP-B): Telescope only centroids on beacon Gyro provides inertial reference frame 3) Similar to GP-B but use of accelerometers: University of MD like accelerometer (GEOID ESSP proposal using superconducting gravity gradiometer) Use fancy accelerometers with beacon May be linear or angular 4) Kilometric Optical Gyro Uses Sagnac Effect with light- precision ~?/(Area/perimeter) For ? = 630 nm, 4 km perimeter should be adequate Proposed for StarLight, but cost and technical issues 5) Space interferometry Mission - previously ruled out 6) Use of distance between S/C for very long focal length –ruled out

    6. Option 1: Hubble-Like Super Star Tracker Centroids on beacon FOV big enough to catch stars Advantages Simple concept Aberration correction automatic Roll Possible use of superconducting accelerometer Chopper wheel to get small pixels fight 1/f noise Analog solution? Disadvantages FOV vs. resolution 15 magnitude stars => 1/4(arc min)2 Detector pixel size, capacity F-number = huge Integration time = huge

    7. Option 2: Gravity Probe-B-Like Telescope only centroids on beacon Gyro provides inertial reference frame Advantages Gyros exist!! 1/3 micro arc sec/day No need to find stars Just a beacon tracker telescope Cryo-cooler: TRL 5 by 2005 ($2-5M for cryo-cooler (flight model only), FM + EM $3 to $7 million, mass about 20kg Launches on Con-X ~ 2010 Gyro was to launch this year (Oct 2002) $10-100M for both cooler & gyro Disadvantages Must know aberration Delta-V to 3cm/sec (Landis checks) GP-B = expensive Cryogen or coolers/vibrations

    8. Option 2: Gravity Probe-B-Like (cont.) Deltas on GP-B Since 1/3 micro arc sec /day is not required, then we may be able to back off on this capability Cryo-coolers mean normal conductivity launching If negligible magnetic field @ L2, simplifies magnetic shielding design Requirements on magnets in s/c Neutralize cosmic-ray charging Respinning up gyro? (dynamic range) Cryo getters less important? Proof mass? Yes-maybe Squids will be better Venting vs. cooler mechanism Mass, power,size,cost,other req’ts (mag,jitter,thermal) Need to integrate over 10 sec you get 10E-13radians???? ConX, NGST Cooler specs: 150W BOL, 250W EOL, 10 yr lifetime, 20-30kg include electronics, heat syncs 1@100K, 1@room temp, produces 7.5mWatts 6K Selection in March 2002 Cryo cooler mating to Adiabatic Demagnetization Refrigerator (ADR) starts in 2005. Temperature 2 K or less. Estimate 30 watts.

    10. Option 3: Super Accelerometers University of MD like accelerometer (GEOID ESSP proposal using superconducting gravity gradiometer) Laboratory model measured 10E-15 m/s2 acceleration Use Super Accelerometers with beacon and beacon tracker Unable to project, but 1st look is the angular accelerometers will not work. Perhaps looking at angular displacements. Advantages No need to find stars No spin up Less sensitive to magnetics than GP-B option Squids will be better than GP-B Disadvantages Cryogen ? No flight design for accelerometers Delta-V to 3 cm/sec (aberration correction) charging

    11. Option 3a: Super Accelerometers University of MD like accelerometer (GEOID ESSP proposal using superconducting gravity gradiometer) Linear gravity gradiometer. Measure w2R (centripetal) D(w2) =10E-14 rad2/sec2/sqrt(Hz) Noise spectral density of Dw2 Dw = Dw2 /2w for w>>Dw Dw = sqrt(Dw2 )/sqrt2 for w~Dw We would operate with w~Dw

    12. Option 4: KOG & Beacon Kilometric Optic Gyro Resolution ~ lambda/(area/perimeter) Advantages Beacon star tracker only No need to find stars Disadvantages Extra s/c ? Understand “cost/technical” issues for StarLight de-selection Range control ?

    13. Possible Techniques for Tracking Koesters Prism Interferometer Interferometer Move detector in know pattern and AC detecting input signal. Then move s/c Fixed detector - look at fringes (DC). Then move s/c Centroiding Several (up to 5) defocus strips to reduce aberration and clarify point spread functions One centroid Quad cells Optically split beam PSF on 4 detectors Position Sensitive Diodes (for beacon) @ 633 nm (some of 3 and 4) Spatial heterodyne interferometric tracker (moiré pattern).

    14. First Cut at Hardware Approach for Option 2

    15. Coarse Acquisition Procedure

    16. Science Mode Control Logic Sequence – Version Using Science Instrument

    23. Issues and Concerns

    24. Open Issues 1) Hubble-like star tracker with superconducting accelerometer/aperture size trade. Inertial reference stars with beacon. 2) Gravity Probe-B like A) Unavailability of CDR package (wt, power, cost) B) Cryo-cooler option 1) Absorption - no moving parts 2) Isolated mechanical compressor 3) Rotary 5000 Hz (two of the three will be ready 2005) C) Modifications 1) S/C is non-rotating 2) Cold on ground 3) Better squids 4) Eliminate cryogen cooling 5) Spin up gas (Helium is dangerous to PMTs) 6) Magnetic shielding requirements @ L2 7) Lifetime (10 yrs)

    25. Open Issues (cont.) D) Strict dc mag field requirements for s/c components; no field lines thru GP-B gyro E) GP-B telescope modifications 1) Multiple tertiary mirrors for redundant detectors 2) Alternate design using interferometer (with prism) 3) PMT photon counting rate (requires improved technology) 3) Linear or angular superconducting accelerometer (requires improved technology) 1) 0.1 Hz = 1/f knee; so can integrate f @ 10 seconds 2) 7x10E-15 rad2/sec2=w2=linear acceleration=integration if white noise = ?? 3) Angular accelerometer=10E-12 rad/sec2=a=? , Value x time2? 4) Any additional input from U of MD

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