Distribution17 April 1969G 38-69A. Laats and G. EdmondsReduction in Number of P52 Alignments During Apollo Missions
References: 1. RAPSIC Memo 2. Apollo 10 Flight Plan, March 12, 1969
Experience in Apollo 8 has established that if the G&N system is left in operate continuously during a mission accurate gyro drift measurements can be obtained from realignments and the gyro block data terms are stable. (Ref. 1) As a re- sult, the number of P52 realignments can be reduced if the gyro drift compensa- tion is updated during the mission.
The following criterion are recommended. All realignments used for drift measure- ment must use the REFSMMAT option.
1. Two realignments to REFSMMAT after TLI. The second realignment to be at least six hours after TLI. If the average of these two measurements is more than 0.36 meru different from the prelaunch load for any gyro, that NBD compensation is updated to the measured value. The 0.36 meru is the largest standard deviation of the measurements for Apollo 8 (Ref. 1). (Two measurements are suggested to reduce the chance of gross errors.
2. At least 1 alignment to REFSMMAT per day. If more than 8 hours have elap- sed since the last alignment, the results should not differ by more than 0.72 meru from the value obtained ��� above. If the change is greater than 0.72 meru consideration will be given to updating the compensation or performing additional drift measurements such as described in 5 below. (This criterion would not have been exceeded on Apollo 8 if a good value had been loaded as in 1 above).
* Apollo 9 data is not considered [note: rest of footnote is unreadable]
3. Realignments before every major burn (i.e., before all burns in- cluding entry but not before MCC burns). Generall these alignments will be to a new preferred orientation, and so gyro drift measurements will not be possible. (If the gyro torquing option is used approximate gyro drift measurements can be obtained.)
4. Special alignment requirements may be required:(a) the use with P22 around the moon in Apollo 10; (b) before LM docked alignment; (c) accurate alignment for rendezvous back up. Update drift compensation on same basis as in 2 above. The ������ do not know how the requirements for these alignments were obtained. Perhaps they should be reviewed also.
5. Additional measurements will be required if the above measurements indicate unusual gyro performance. This would include a bias change of more than 5 meru ������������������ that obtained in 1 above and drift measurements ������������ significantly larger than the 0.35 meru experienced in Apollo 8.
Error Analysis Using Reference 1, it can be seen ������������������� above will give the ���� orbit to about 0.36 meru ���� if this error exists for 21 hours, the platform misalignment on a 3σ basis will be
3 x 0.36 x 10⁻³ x 15 x 21 = 0.220°
This will be accurate enough so that auto pilots can place a star within the SXT field-of-view. This is also an acceptable error for nominal MCC burns.