An approach guidance method using a single onboard optical measurement
H. A. Hamer, K. G. Johnson · NASA Technical Reports Server (NASA) · 1970
An empirical method is developed for onboard guidance within the sphere of influence of a celestial body.The procedure requires only limited onboard calculations and leads to approach-guidance predictions sufficiently accurate for emergency or backup operations.The method is applied to lunar-approach trajectories and is studied in detail for certain lunar missions.The procedure relies heavily on use of precalculated data and is unique in that only a single angular measurement from the star to the moon is required, provided that it is made at or near the lunar sphere of influence.If the approach guidance is delayed to a time closer to the moon, an additional measurement of the subtended angle of the moon is required.The method is designed specifically to control the magnitude of the perilune radius, but the perilune position and velocity values automatically remain close to the nominal values.An error analysis with an assumed one-sigma error of 10 seconds of a r c in the optical angular measurements and a one-sigma velocity-cutoff error of 0.2 m/sec has shown that the perilune radius can be controlled to a one-sigma accuracy of from 7 to 13 km, depending on the time the approach guidance is performed.The effects of maneuvering errors, star location, and empirical approximation errors on the approach guidance are discussed.For manned flight, the required angular measurements can be readily made from onboard the spacecraft by a sextant-type instrument.For unmanned flight, the measurements can be made automatically by pointing the spacecraft (or tracker instrument) in a predetermined direction to a s t a r and then sighting to the planet with a scanner-type instrument .