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 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

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)

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)

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

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

Index Mirror Rotating Mirror Angle measurement 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

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

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