The Mars Science Laboratory (MSL) ChemCam instrument can be damaged when the Sun enters or passes through its field of view (FOV). There is no Sun cover, yet other instruments mounted with boresights pointing in the same direction must observe the Sun for scientific observations and for attitude determination. When in a Suntolerant focus range during rover motion and pointing for observations by other remote sensing instruments, the Sun must be allowed to pass through the ChemCam FOV, and when in a Sun-damage focus range for ChemCam observations, the Sun must never be allowed to enter the FOV, even after a rover system fault. Both of these scenarios depend upon knowledge of the attitude of the rover relative to the motion of the Sun. A Sun search that is guaranteed to be Sun-safe for the ChemCam, even when the location of the Sun is unknown, had to be developed.
When the ChemCam is in a Sun-tolerant focus range, the Sun may be tracked for a short allowable tracking time, but after that, the ChemCam is declared to be warm and must be allowed to cool either by allowing the Sun to slowly drift out of its FOV, or by actively repointing the ChemCam away from the Sun. Two cones about the ChemCam boresight are defined. When the Sun is inside the smaller warming cone, the ChemCam is warming. When the Sun is outside the larger cooldown cone, the ChemCam is guaranteed to cool down before the Sun might drift back into the warming cone. The former cone includes the instrument FOV for the Sun-tolerant focal range, margin, and attitude error. The latter cone adds to this provision for planetary rotation during cooldown.
When the ChemCam is in the Sundamage focus range, the Sun must never be allowed to enter the FOV. This applies even after a system fault when the rover is both unable to point away from the Sun, and unable to make the ChemCam focus Sun-tolerant. A single cone is defined, including the narrower instrument FOV for the Sun-damage focal range, margin, attitude error, and provision for planetary motion to provide time for ground-based system fault recovery.
Equivalent to comparing the location of the Sun relative to the Sun-damage cone about the ChemCam boresight, the ChemCam boresight can be compared to the Sun-damage cone centered on the Sun. As the Sun moves across the sky, the Sun-damage cone attached to it traces a ChemCam observation keepout band across the sky. Regions north and south of this band are guaranteed to be Sun-free for several weeks. Also, pointing that places the entire Sun-damage cone (this time about the ChemCam boresight) below the horizon defines a permanent below-the-horizon Sun-free region. Only Sun-free pointing is allowed.
When spacecraft attitude is not known, the location of the Sun relative to the ChemCam is not known. Nevertheless, the NavCam search for the Sun to determine attitude must not overheat the ChemCam. A 360-degree search of the sky could result in the Sun re-entering the ChemCam FOV before fully cooling down, so it must be broken up into segments with intervening cooldown periods. After initial identification of the Sun, a confirmation image is also needed, necessitating an additional intervening cooldown period.
The monitoring of a simple set of constraints on the geometrical relationships between the ChemCam boresight and the position and motion of the Sun in the sky suffices to ensure ChemCam Sun safety. Simplicity enables use of simple graphical displays to check operations plans on the ground before uplink.
This work was done by Stephen F. Peters, Lauren P. DeFlores, Noah Z. Warner, and Todd E. Litwin of Caltech for NASA’s Jet Propulsion Laboratory. This software is available for license through the Jet Propulsion Laboratory, and you may request a license at: https://download.jpl.nasa.gov/ops/request/request_introduction.cfm . NPO-49916