STEREO Weekly Meeting Minutes for 2016-09-13 (Week 37) ANNOUNCEMENTS ------------- * Mission operations has announced that they are suspending Behind recovery attempts until the telemetry situation improves, sometime next year. The spacecraft is now spinning at a rate of approximately once every 52 seconds, which allows only short commands to be uplinked, and does not support telemetry downlink. Communication with the spacecraft degraded over the weekend. Once a month communication attempts will resume, starting with the phased array attempt this weekend, Sep 16-17. Each month will switch between single station and phased array attempts until recovery operations resume. INSTRUMENT and SPACECRAFT STATUS -------------------------------- * Ahead is operating nominally on the main lobe of the high gain antenna. * Behind recovery attempts suspended until communications improve. DISCUSSION AND OPERATIONS CONSTRAINTS ------------------------------------- Week 36 (Sep 5-11) * On day 251 (Sep 7), during the DSS-65 support for Ahead, turbo decoder lock was lost briefly at 1146z and 1245z. This anomaly resulted in the loss of 9 frames of SSR data. * On day 254 (Sep 10), during the DSS-54 support for Ahead, turbo decoder lock was lost briefly at 1241z. This anomaly resulted in the loss of 7 frames of SSR data. * On day 252 (Sep 8), the 91st momentum dump on Ahead was executed successfully at 1930z, which imparted an estimated delta V of 0.098 m/sec. This was the 10th momentum dump that did not use the IMU. After thruster operations completed, there was a 0.82 degree of roll angle error which was dampened out over the next 7 minutes. Fine pointing stabilized 2.5 minutes after completion of the momentum dump. * On day 253 (Sep 9), on Ahead, the SECCHI instrument reset at 07:31:36z. The SECCHI team reconfigured the instrument to operational mode by 254-2000z. Recovery was delayed as the camera electronics boxes did not come online initially as expected. This was the 40th reset of SECCHI on the Ahead observatory. * The average daily science data return for Ahead was 5.0 Gbits during this week. * Behind Observatory Status: Low main bus voltage, 2 (#6 & 9) out of 11 battery cells are currently not functioning, attitude uncontrolled, rotating at a ~52 second period about its principal axis of inertia. Current orientation supports some solar array input; RF communications only supports uplink of short commands to an LGA. All propulsion latch valves are closed, propellant frozen, and transducer #2 not functioning. Power switching boards off, all switched loads off, TWTA in standby. EA mode enabled. The battery charge rate is C/10. Detailed status of the recovery activities to restore operations from the Behind loss of communication anomaly, which occurred on October 1, 2014, are listed below. Active recovery operations began with the carrier detection on August 21, 2016. * On day 249 (Sep 5), the reaction wheels on Behind came on for 1.3 hours after clearing autonomy rule fire counts in preparation for the autonomous momentum dump. While testing on the hardware simulator did not reveal this, due to very limited telemetry, autonomy rule fire counts were unknown. As a result, the attitude state has once again been perturbed. The uplink is still currently usable; however, the rotation of the spacecraft has increased about the momentum vector such that telemetry lock cannot be maintained long enough for a frame to be received. Only one telemetry packet was received before the attitude was disturbed, a spacecraft currents packet which showed all 4 wheels on at 74 W total. As 41 bad frames were received, telemetry extraction has been requested from the DSN. In preparation for the autonomous momentum dump to re-establish attitude control and Sun pointing, primary external propulsion line and valve heaters were powered on and secondary propulsion tank heaters and secondary propulsion internal line heaters were powered off. Testing of the autonomous momentum dump sequence on the hardware simulator was completed with satisfactory results. The sequence powers on the wheels about a minute before the dump to allow the wheels to soak up some momentum from the body, making that portion of the total system momentum more observable. (The proportion of the momentum in the body is imperfectly observable due to the failed gyro axis.) This approach permits the software to point the spacecraft to the Sun and to dump sufficient momentum to give the star tracker an excellent chance of acquiring a solution expeditiously after the promotion to Standby mode. * On day 250 (Sep 6), during 2 brief TWTA sampling periods with Behind this morning, peak carrier signals were -160 dBm dropping carrier lock each cycle with an FDF estimated period of 154 seconds and a spin axis to Earth angle of 41.4 degrees. The DSN radio science receiver team confirmed that the downlink rate is 35 bps indicating the battery bus voltage did not collapse during the night. The uplink is still currently usable for short command lengths; however, a longer duration is necessary to load the updated macro for conducting the autonomous momentum dump. A test macro of the same length containing only the TWTA on/off commands was tested, but did not load successfully. Telemetry lock could not be maintained long enough for a frame to be received. As 30 bad frames were received, telemetry extraction has been requested from the DSN. In preparation for the autonomous momentum dump to re-establish attitude control and Sun pointing, primary external propulsion line and valve heaters, primary propulsion tank heaters, and primary propulsion internal line heaters remain on. * On day 251 (Sep 7), telemetry was received again from Behind for 8 minutes out of 14 minutes during 5 brief TWTA sampling periods, peak carrier signals were -158 dBm, dropping carrier lock for 6 minutes. As battery pressure is quite good, battery temperature was high (33 deg C) and some propulsion lines are still cold, two additional heaters were powered on: secondary propulsion line internal heaters and TWTA replacement heater to ensure all propulsion lines and components are warmed. As the battery is degraded (2 out of 11 cells not functioning), to monitor battery performance with 5 heaters on to thaw the hydrazine, a 6 hour DSS-43 support was added starting at 2100z. Later, after evaluating the latest telemetry, the propulsion lines were warm except the A2 and A3 thrusters which will not be used in EA mode. All subsystems were polled and GSFC made the decision to proceed. All heaters were powered off and the command to execute the sequence for the autonomous momentum in EA mode as sent at 2316z. At 0004z, carrier lock was received from the HGA at a -155 dBm peak signal with modulation for ~1 minute, and then faded. The radio science receiver team confirmed the expected 633 bps downlink rate. Carrier lock was again received at 0110z at a -162 dBm peak signal for ~1 minute, and then faded. As expected, at 0158z, carrier lock was received on the -Z LGA at -162 dBm, however, then slowly dropped after 8 minutes. At 0225z, a stable carrier lock was received at -157 dBm, however, after 10 minutes it began to decrease. At 0239z, the first telemetry frame was received. System momentum was high at 22.5 Nms, battery pressure was 510 psi and decreasing, main bus volt was 21.7v with the main bus current at 18.5A. As system momentum was high, commanded a momentum dump. As battery pressure was dropping, commanded the TWTA and operational and survival heaters off. At 0330z, the sweep did not work; the uplink was re-swept and sent all commands again. As the TWTA powered off on this time, it is assumed the subsequent momentum dump command executed. A total of 6 telemetry frames were received. Telemetry extraction was requested from the DSN. * On day 252 (Sep 8), two ~1 minute carrier locks with Behind at a peak signal level of -176 dBm were received this morning from commanding the TWTA on. As the momentum dump in EA mode was not as effective as we had hoped, from only 6 housekeeping packets that were received yesterday after promoting to C&DH standby mode, system momentum remains high causing instability with attitude control. Commands were sent repeatedly today to conduct an autonomous momentum dump and to power off 14 operational heaters to preserve battery state of charge. Before the next track tomorrow, the soft command timer will expire resulting in switching to the +Z LGA, powering on the TWTA and rotate, if wheels allow, at 5 degrees/minute. * On day 253 (Sep 9), while the Behind spacecraft was configured for switching to the +Z LGA and powering on the TWTA before the track this morning, no signal was received today after repeatedly attempting to power on the TWTA. As the spacecraft rotation will settle out about its principal axis of inertia which should continue to provide limited solar array input and communications, battery state of charge recovery has commenced today. As it appears that the momentum dump in EA mode was not as effective as we had hoped for during the Behind recovery into C&DH standby mode late day 251, it is suspected that battery voltage collapsed on day 252 due to sustained high wheel speeds. The 2nd spacecraft emergency ended at 253-0000z. * On day 254 (Sep 10), once again the Behind carrier was detected by the DSN this morning. During 3 brief TWTA sampling periods, the carrier signal had a lower peak signal of -166 dBm and a period of ~60 seconds continuously. Subcarrier lock did not occur. While the uplink can support short critical commands, the downlink cannot support telemetry. The spacecraft was configured for a low power mode by powering off all power switching boards and loads. EA mode was enforced as to prevent the use of the HGA in C&DH standby mode. * On day 255 (Sep 11), during 2 brief TWTA sampling periods with Behind, the carrier signal was similar to yesterdays with a continuously fluctuating signal of -166 dBm to -180 dB. From the FDF Doppler analysis, the period is now ~52 seconds, and has increased 3 seconds over the last 30 hours. Subcarrier lock did not occur. While the uplink can support short critical commands, the downlink cannot support telemetry. The spacecraft was configured for a low power mode by powering off all power switching boards and loads. EA mode was enforced as to prevent the use of the HGA in C&DH standby mode. Week 37 (Sep 12-18) * Ahead has 8 passes during the week, and overall coverage is nominal. There are four passes at 720 kbps, one at 360 kbps, and three at 240 kbps. * Behind starts the week at an SPE angle of 13.8 degrees. * The uplink array attempts for Behind on Sep 16,17 will be carried out as originally scheduled. Each attempt will begin with the Ahead spacecraft to calibrate the phasing between the stations. There will be no SSR playback or instrument commanding during these passes. Following the calibration, there will be a 3.3 hour pass for Behind on Sep 17 for battery recovery, and a 5.3 hour pass on Sep 18 for carrier recovery. Station 14 will listen for the Behind carrier, along with the Allen Telescope Array which will join 1 hour and 15 minutes later. A waiver has been requested and received for operating below the 47.5 degree limit for the 80 kW transmitter. Week 38 (Sep 19-25) * A 2.6 hour return-to-service track for station 63 with STEREO Ahead is scheduled for Saturday, Sep 24. There will be no SSR playback or instrument commanding during this track. Given that the Madrid maintenance has been delayed, this event may be rescheduled. * Ahead has 9 passes during the week, and overall coverage is nominal. There are four passes at 720 kbps, one at 360 kbps, one at 240 kbps, and three at 160 kbps. * Behind starts the week at an SPE angle of 13.7 degrees. There are no passes for Behind scheduled during the week. Week 39 (Sep 26-Oct 2) * The 36th SECCHI stepped calibration roll is scheduled for Tuesday, Sep 27, starting at 11:55 UT. This is the fifth calibration roll without using the gyros. System momentum is expected to be about 1 N-m-s. * Ahead has 11 passes during the week, and 70 meter time is short about four hours between Thursday and Friday. There are two passes at 720 kbps, five at 240 kbps, and four at 160 kbps. The in situ instruments are expected to ride out the shortage without any problems, but SECCHI will need to reduce their image cadence. * Behind starts the week at an SPE angle of 13.6 degrees. There are no passes for Behind scheduled during the week. Week 40 (Oct 3-9) * Ahead has 12 passes during the week, and overall coverage is nominal. There are five passes at 720 kbps, three at 240 kbps, and four at 160 kbps. * Single station recovery attempts for Behind are scheduled for Friday-Sunday, Oct 7-9. The battery recovery passes will be carried out with the 80 kW transmitter on station 26. Week 41 (Oct 10-16) * The 92nd momentum dump on Ahead is scheduled for Monday, Oct 10, with ignition at 23:45 UT. * Ahead has 11 passes during the week, and overall coverage is nominal. There are seven passes at 720 kbps, and four at 160 kbps. * There are no passes for Behind scheduled during the week. Week 42 (Oct 17-23) * Ahead has 12 passes during the week, and overall coverage is nominal. There are three passes at 720 kbps, six at 240 kbps, and three at 160 kbps. * There are no passes for Behind scheduled during the week. COORDINATED OBSERVATIONS ------------------------------------------------------------------------------- Scheduled activities for Week 37: M Sep 12 (256) T Sep 13 (257) W Sep 14 (258) T Sep 15 (259) F Sep 16 (260) S Sep 17 (261) Behind: Phased uplink array battery recovery attempt S Sep 18 (262) Behind: Phased uplink array carrier recovery attempt ------------------------------------------------------------------------------- Scheduled activities for Week 38: M Sep 19 (263) T Sep 20 (264) W Sep 21 (265) T Sep 22 (266) F Sep 23 (267) S Sep 24 (268) Ahead: Return-to-service track with DSS-63 (TBC) S Sep 25 (269) ------------------------------------------------------------------------------- Scheduled activities for Week 39: M Sep 26 (270) T Sep 27 (271) Ahead: SECCHI stepped calibration roll, 11:55 UT W Sep 28 (272) T Sep 29 (273) F Sep 30 (274) S Oct 01 (275) S Oct 02 (276) ------------------------------------------------------------------------------- Scheduled activities for Week 40: M Oct 03 (277) T Oct 04 (278) W Oct 05 (279) T Oct 06 (280) F Oct 07 (281) Behind: Battery recovery pass with station 26 S Oct 08 (282) Behind: Battery recovery pass with station 26 S Oct 09 (283) Behind: Carrier recovery pass with station 14 ------------------------------------------------------------------------------- Scheduled activities for Week 41: M Oct 10 (284) Ahead: Momentum dump, 23:45 UT T Oct 11 (285) W Oct 12 (286) T Oct 13 (287) F Oct 14 (288) S Oct 15 (289) S Oct 16 (290) ------------------------------------------------------------------------------- Scheduled activities for Week 42: M Oct 17 (291) T Oct 18 (292) W Oct 19 (293) T Oct 20 (294) F Oct 21 (295) S Oct 22 (296) S Oct 23 (297) ------------------------------------------------------------------------------- Upcoming events: Behind events are contingent on recovering the spacecraft. 2016: * Dec Behind: Resume LGA downlink capability on 70 m DSN 2017: 2018: * Mar 5 Ahead: Mercury transit 2019: * Mar Ahead: Switch downlink rate to 240 kbps for 34 meter * Apr Behind: Switch downlink rate to 240 kbps for 34 meter 2020: * May Behind: Switch downlink rate to 360 kbps for 34 meter * Jun Ahead: Switch downlink rate to 360 kbps for 34 meter * Jun Behind: Switch downlink rate to 480 kbps for 34 meter * Sep Ahead: Switch downlink rate to 480 kbps for 34 meter 2021: * Jul Behind: Switch downlink rate to 720 kbps for 34 meter * Aug Ahead: Switch downlink rate to 720 kbps for 34 meter