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Mars Sextant PDR Opti 523 Paul Ward-Dolkas

Mars Sextant PDR Opti 523 Paul Ward-Dolkas. Requirements. Provide backup navigation for a manned Moon or Mars rover No magnetic field for compass Re-align inertial measurement gyrocompass, or: Provide simplest backup for total system failure

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Mars Sextant PDR Opti 523 Paul Ward-Dolkas

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  1. Mars Sextant PDR Opti 523 Paul Ward-Dolkas

  2. Requirements • Provide backup navigation for a manned Moon or Mars rover • No magnetic field for compass • Re-align inertial measurement gyrocompass, or: • Provide simplest backup for total system failure • Multi-week dust storms prevent use of conventional fixed 3-star tracker approach • The whole purpose of a sextant is to: • Measure the angle (elevation) of sun/star above horizon and/or: • Measure the angle between 2 stars • Use this (and time of measurement) to determine longitude and/or: • Use elevation @ noon to determine latitude • Accuracy required (for .5 km accuracy): • .5 arc min between sextant & clock (1 sec accuracy assumed) • .26 arc min for sextant • Rover assumptions: • +/- 20o tilt of rover • Rover can be repositioned to view sun/star • Window positioned at 45o to allow horizon-zenith viewing

  3. Overall Layout • Initial layout patterned after traditional sextant • Range of use: 0o to 90o • I.e.: view sun/star from horizon to overhead • Telescope axis changed from horizontal to 30o upward to reduce overall envelope • Artificial horizon eliminates need to see horizon directly (not available anyway)

  4. Trade Summary - chosen option underlined • Overall Layout • Horizontal telescope vs. 30o up-tilt • Artificial horizon • Projected horizon line vs. bubble • Float mirror vs. pendulum • Pendulum pivot type • Bearings vs. flexure vs. knife edge • Dichroic beam splitter • Prism vs. flat filter (may be revisited when fn is determined) • Index mirror type • Split Poro-prism vs. periscope • Horizon mirror type • Partial horizon (half clear, half-mirrored) vs. full horizon (entire mirror half-silvered)

  5. Pendulum • Provides artificial horizon by projecting a horizontal line to telescope • Pivots to remain aligned vertically • Instrument ball bearings • CG adjustment via set screws in base • Magnetically dampened • PB board-mounted dark field retical projector • Hinges (integral flexure) to trim projection angle • Adjustable level illumination provided by LEDs • Dichroic Prism beam splitter (not shown) • Reflects retical image (horizon line) to telescope • Transmits image from Index/Horizon mirrors to telescope

  6. Horizon Mirror • Fixed position fold mirror • Named after similar mirror in regular sextant – a bit of a misnomer since it’s not used here to view horizon • Half silvered design (“partial horizon” type) • LH side reflects image from Index Mirror • RH side transparent: used for star-star measurement • Alignment of primary surface provided by pins (not shown) mounted in match drilled holes in side frames • Nylon-tipped set screws used to provide pressure

  7. Index Mirror • Rotating Mirror • Catches light from sun/star and projects it to fixed Horizon Mirror • Fully silvered design • Angle measurement • Digital encoder on LH shaft (primary system) • Vernier dial on RH side (manual backup) • .26 arc min accuracy req’d • Non-vinietted for <50o • Compromise between full FOV and mirror size

  8. PC Board • Mount & circuit for Retical Projector (not shown) • Fixed on one side; integral flexure allows rest of board to tip/tilt to tweak projection angle • Mounts in bottom of Pendulum Assy., projects retical beam vertically upward

  9. Trades to be done • Telescope design • Fn vs. FOV • System FOV & vinietting analysis • Retical projector • Laser line generator vs. illuminated dark field retical

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