05 July 2019: Lightsail 2 Mission Manager's Log
The LightSail 2 spacecraft continues to be healthy. Communications with the spacecraft was sporadic, due to degrading accuracy in the two-line element (TLE) orbital state estimate. We have not yet received a TLE specific to LightSail 2, so we are tracking object 44339 which is thought to be Prox-1.
During the evening of July 4, the flight team was able to activate the Payload Interface Board (PIB) gyros, resolving a concern from the previous day’s activities. Due to the poor antenna pointing resulting from TLE errors, we were unable to complete the planned momentum wheel checkout. The attitude control mode was transitioned to Mode 0 (detumble), to keep the spacecraft angular rates low.
In the current configuration with the solar panels stowed, the magnitude of the magnetometer readings are several times higher than the expected magnitude of the Earth’s magnetic field in the LightSail 2 orbit. This is likely due to magnetic field noise generated by the spacecraft components. As a result, the spacecraft attitude determination is inaccurate. Attitude determination should improve when the solar panels are deployed, and the panel-mounted magnetometer sensors are extended away from the spacecraft components. The magnetic field bias in the deployed configuration has been measured during ground testing and is accounted for in the attitude determination software (based upon a configurable gains file). The planned test of attitude control Mode 2, the mode used for control when solar sailing, will be rescheduled to occur after solar panel deployment.
Today’s activities will be focused on the autonomous momentum wheel test.
1. +Y solar panel shown as deployed in telemetry. Resolved: Beacons on July 3 showed all solar panels as stowed.
2. Spacecraft not responding as expected to file downlink commands. Resolved: Communications protocols update allowed multiple successful large file downlinks.
3. Z-axis torque rod showing zero current. Resolved: Transition to ADCS Mode 1 (Z-axis alignment) resulted in non-zero Z-axis torquer current.
4. Cal Poly (Marconi) tracking station unable to decode beacons, despite strong CW signals. Resolved: Ground system connections checked, and several beacons received in subsequent pass.
5. Solar panels indicate spacecraft in eclipse, ADCS algorithm flag (based on TLE) indicate illuminated. Open.
6. Beacon lines 284-286 (computed angular rates) appear to be in rad/s, while the units in the beacon are deg/s. Resolved: This is an error in the ICD & beacon units, but the attitude determination algorithm is correct and internally consistent.
7. Gyro ON command failed. Resolved: Gyron ON command successful on July 5.
8. File uplink for the autonomous wheel test failed. Apparently file uplinks are having the same problem as file downlinks. Open: this may be resolved by the communications protocol update, but not yet confirmed.
9. Norms of +X magnetometer measurements are too high compared to the Earth magnetic field model at the spacecraft orbital altitude (>250 µT compared to the known value of ~50 µT). Open. Possible magnetic field bias due to spacecraft components. Behavior likely to change once solar panels are deployed.
10. Mode 1 behavior not as expected. Angle between +Z axis and magnetic field vector is oscillating around 135 deg +/- 20 deg, based upon computed attitude quaternions. Should be oscillating around 0 deg. Open. Possible attitude determination error due to significant magnetic field bias introduced by spacecraft components in stowed configuration.