20 July 2019: Lightsail 2 Mission Manager's Log
The flight team completed their analysis of the July 17 test of attitude control in Mode 2, solar sailing mode. The Mode 2 test was performed with full attitude determination capability using five sun sensors and a single magnetometer. Sun sensor voting was enabled, and operated as expected. Analysis shows that the attitude estimate is generally good to within 25-30 deg, which is within the expected performance range.
A system reboot and transition to detumble were performed prior to the Mode 2 test. The Mode 2 sequence was then initiated and maintained for one orbit period. The attitude control performance during the test is shown in Figure 1. For the initial 3500 sec of the test, the attitude control performance was poor. At that point, the momentum wheel speed (Figure 2) changed from zero to approximately -400 rpm and from that point onward the desired attitude profile was followed.
Figure 1. Spacecraft -Z axis angle relative to Sun direction.
Figure 2. Momentum wheel speed during Mode 2 test.
The flight team conducted a thorough review of the test results on July 20, including an ADCS expert from NASA’s NEA Scout mission. A key takeaway from this review is that the momentum wheel performance appears to be compromised near zero wheel speed, and establishing a non-zero spin rate for the wheel is desirable.
The team will perform another test of Mode 2 on July 21. This test will incorporate an additional flight software update, incorporating a filtering of the momentum wheel torque commands to reduce jitter. In additional, the momentum wheel speed will be initialized to -300 rpm, and the spacecraft will detumble (damp angular rates) prior to entering Mode 2.
Sail deployment is scheduled for July 22 at approximately 18:30 UTC. Any additional ADCS updates will be made in the sail deployed configuration.
Acquisition and downlink of images has continued, including the image above showing the Baja peninsula.