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 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 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

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 System FOV & vinietting analysis
Fn vs. FOV
System FOV & vinietting analysis
Retical projector
Laser line generator vs. illuminated dark field retical