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SFI: 75 | Ap index:  2  | Kp index:  2  ( n\a @--:--,  n\a @--:--)| X-ray flux:  A8.6 
SW: Density= 2.3 p/cm3 | Speed= 347.6 km/s | Temp=43(x1000)K
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[i]X-ray flares: C=0 M=0 X=0 | Optical flares: M1=0 M2=0 M3=0 | Sub-flares=0
The DSN has been able to extract 5 additional telemetry frames from Friday's IEM power on. These frames are being processed and the telemetry from the 5 critical packets will be distributed. An updated recovery plan was developed and discussed during the daily telecon. On Tuesday, carrier recovery will be commanded, attempting to power on the TWTA. If the downlink signal is detected, the TWTA on time will be limited to ~21 minutes, which is 1.5 times the period of the beat frequency of rotation and nutation. The reaction wheel latching relays will be commanded off. If no signal is detected, the battery recovery commands will be sent for the remainder of the support and the next day's support will be shortened to 4 hours. This operational cadence will continue until BEHIND is restored to active attitude control or the DSS-63 time ends on Sep 11th.

BEHIND observatory status: Unknown state of power, uncontrolled attitude, complex rotation (~14 minute rotation with the previous ~2 minute rotation) about the principal axis of inertia. Current orientation may support communication near the edge of the + Z LGA with some solar array input. Propulsion tanks are frozen.



The DSN detected the carrier signal again from STEREO Behind on the morning of Tuesday, August 30. More details to follow.

On Saturday, August 27, during the evening recovery track, no downlink signal was received after repeated commanding the transmitter on. Battery recovery commands were sent for the last hour of the support.

On Friday evening, as battery voltage was decreasing from the three telemetry packets received, the transmitter was powered off early. However, the downlink signal was lost 6 minutes earlier than expected. It is speculated that with the complex roll, which resulted from powering on the IEM, the battery voltage may have collapsed at some point.

BEHIND observatory status Unknown state of power, uncontrolled attitude, complex rotation (~14 minute rotation with the previous ~2 minute rotation) about the principal axis of inertia. Current orientation may support communication near the edge of the + Z LGA with some solar array input. Propulsion tanks are frozen.



Today, the DSN created a new acquisition sequence for only sweeping a 3 kHz range about the best lock frequency. This was used successfully during the morning Goldstone support to repeatedly sweep and send battery recovery commands. It was agreed to continue battery recovery tonight and on a 4 hour support on Monday. The carrier recovery is scheduled for Tuesday during a 4 hour support with radio science receivers recording.

BEHIND observatory status Unknown state of power, uncontrolled attitude, complex rotation (~14 minute rotation with the previous ~2 minute rotation) about the principal axis of inertia. Current orientation may support communication near the edge of the + Z LGA with some solar array input. Propulsion tanks are frozen.

The primary propulsion tank heaters (~30W) were enabled at 1530z on Friday, Aug 26. During the second track, after the IEM was powered, the downlink signal would drop out periodically. From analysis of the Doppler residual data from the FDF, the rotation is more complex, there is an ~14 minute rotation with the previous ~2 minute rotation. This appears to have shifted the spin axis from 10.5 deg to 22 deg. Three packets of critical telemetry were received. From this very limited data, as expected, the observatory is quite cold, with the battery at 30% state of charge and generating power to support ~150W. From the propulsion tank pressures, the tanks appear to be frozen; however, no temperature data were received. The Sun angle averaged ~ 60 degrees. As the main bus voltage was 24v and falling, the transmitter was powered off early. While early in recovery, 2 of 11 battery cells appear not to be functioning. Verified that the increased battery charge rate command to C/4 was received. From engineering team discussions, it was decided to power off the IEM and power on the secondary battery heater to allow the battery to recharge. The secondary propulsion tank heaters were also powered on to continue thawing of the hydrazine.

BEHIND observatory status: Avionics off, uncontrolled attitude, complex rotation about the principal axis of inertia. Current orientation supports nearly continuous communication near the edge of the + Z LGA with some solar array input. Propulsion tanks are frozen. Secondary battery and propulsion tank heaters are on in between DSN tracks.

The Behind spacecraft has been successfully commanded to turn on its transmitter to downlink a carrier wave.

Analysis of the carrier wave signal shows that the spacecraft is rotating at the rate of about once every two minutes. Daily communications with the spacecraft are being scheduled to keep the power system online, and to prevent the 3-day command loss timer from activating. Plans for recovering the spacecraft are being developed.


On day Aug 21, during the DSS-14 70m support for Behind for carrier recovery, the downlink carrier was detected at approximately 22:33:23z after contact was lost on October 1, 2014 during testing for a two month long superior solar conjunction. Radio science receivers confirmed a signal was present from 22:27:40z to end of track. Carrier signal level fluctuated between -166 to -179 dBm with an approximate 2 minute rotation. DSS-14 was able to lock up to the 25 kHz subcarrier, confirming the LGA is in use, however, no symbol lock was achieved as expected since commands were sent to power off the avionics for battery charge recovery. The downlink was received for 2.4 hours through end of track. The GSFC mission director declared spacecraft emergency at 235-0030z. No telemetry was received as expected since the avionics were purposely powered off to maintain battery state of charge. While quite early, recovery is expected to proceed slowly to preserve a positive power balance, assess observatory health, re-establish attitude control, and warm all subsystems and instruments.

The frequency segmented acquisition mode which was implemented by the DSN for STEREO Behind was critical for recovering communications with the spacecraft.

At the time of the telecon, Station 14 is observing STEREO Behind. It is still quite early in recovery operations, which will proceed slowly and carefully. Behind is essentially one step above off. The power system is online. The transponder is working, with the spacecraft accepting commands and downlinking the carrier signal. The rotation rate of once every two minutes is not slow enough to be countered using the reaction wheels alone. The MOC is working on refining the draft plan for recovery. The first step is to bring the avionics online to start getting telemetry, but without fault protection coming on, which would cause problems. A plan is being developed, and is expected to be tested and approved for implementation at the end of the week, possibly as early as Friday. After that the plan is to start warming the propulsion, to recover the G&C. There is a very long recovery process with many hurdles to jump through. Daily meetings are being held by the operations team.

Three hours per day of track time has been requested from the DSN for Behind recovery. Recovery of the avionics will require more time.


UPDATE [25.08.2016]

The spin axis is estimated to be 12 to 14 degrees from the Earth-Sun line to spacecraft. Due to the 22 degree/year mean drift rate, the current geometry which provides solar input and continuous RF communications will not occur again till 2022. A separate analysis using the Doppler shift data was conducted by the RF lead and the FDF which was in close agreement with the G&C observations. It was concluded that the solar arrays are generating ~250W and loading during recovery must stay below this to prevent battery collapse. The draft plan for powering on the avionics (IEM) and G&C recovery was reviewed. Command procedures and testing on the hardware simulator are in progress.


UPDATE [26.08.2016]

Based on recent thermal analysis, the propulsion tanks will take from 65 to 308 hours, depending upon current temperature and how many heaters circuits can be used, to warm the hydrazine in the tanks to 2 deg C. As power is available now, the secondary propulsion tank heaters (~30W) have been enabled. Also, selected power switching boards were powered off to prevent unwanted loading in preparations for powering on the avionics. To allow for sufficient time for procedure testing and review, the powering on of the avionics has been moved to the second track on Friday.

BEHIND observatory status: Avionics off, uncontrolled attitude, rotating about the principal axis of inertia. Current orientation supports continuous communication near the edge of the + Z LGA with some solar array input. No telemetry has been received and therefore detailed status is still unknown. Secondary propulsion tank heaters are on in between DSN tracks.


Communications with the STEREO Behind spacecraft were interrupted on October 1, 2014 immediately after a planned reset of the spacecraft performed as part of a test of solar conjunction operations. There have been no successful communications since then, though attempts to recover the spacecraft continue. Here we explain the events that led up to the loss in communications, and the activities that have been carried out in an attempt to recover the spacecraft. As explained below, initial indications are that a series of problems in the guidance and control system led to the anomaly.

Because both STEREO spacecraft are now on the far side of the Sun, new modes of operations needed to be developed to protect the High Gain Antenna from the elevated temperatures caused by pointing near the Sun, and also to put the spacecraft into an autonomous safe mode for the period when solar radio interference will prevent communication. (More information) Collectively, these new modes are known as solar conjunction operations. A series of tests were conducted to make sure that the spacecraft perform as planned with these new operations modes. The first spacecraft to be affected was STEREO Ahead, and these tests were all carried out successfully on that spacecraft. On August 20, 2014, Ahead entered the first phase of the solar conjuction period, which will continue in its various phases until early 2016.

On September 27, 2014, the STEREO Behind spacecraft began one of the test sequences that had already been carried out on Ahead. This sequence was designed to test putting the Behind spacecraft into the safe mode that it will be in during the solar radio interference period, and then to bring it back out again into normal operations. One part of this test was to observe the firing of the spacecraft hard command loss timer, which resets the spacecraft if no commands are received after three days. The purpose of this is to correct any problems that might be preventing the spacecraft from receiving commands from the ground. While the spacecraft is out of contact on the far side of the Sun, this reset will occur every three days.

On October 1, 2014, the hard command loss timer on the spacecraft was observed to fire at the expected time, causing the spacecraft to reset. However, the radio signal observed on the ground immediately after the reset was very weak, and then quickly faded away. That was the last signal received from the Behind spacecraft. The only information that we have is from the limited telemetry extracted from the radio signal just before and just after the reset. Here is what is known so far:

The telemetry from just before the reset shows no problems with the spacecraft.
Only three packets could be successfully recovered from the very faint telemetry received just after the reset. These data show that the Inertial Measurement Unit (IMU) was powered on. The IMU uses ring laser gyros to measure the rotational rates and orientation of the spacecraft. At this stage of the mission, it's normally powered off. The IMU being powered on indicates that the Star Tracker, which normally provides this information, had not acquired a set of guide stars in a predetermined length of time.
This is not unexpected—there have been other occasions when it took the Star Tracker several minutes, or even a few days, to start determining the spacecraft orientation based on star images. In fact, on September 28, as part of the same test sequence, the spacecraft was reset, and it took 12 minutes for the Star Tracker to start providing an attitude solution. When the Star Tracker is offline, the spacecraft will automatically turn on the IMU to provide rotational rate information.
The lack of data from the Star Tracker would explain why the radio signal after the reset was initially very weak and then faded away. Without the Star Tracker, the high gain antenna cannot be properly pointed at Earth, resulting in the initial faint signal, which then drifted even further so that the signal was lost completely.
However, the limited telemetry received after the spacecraft reset also indicates that one of the laser gyros in the IMU had failed, and was providing bad data to the attitude control system. Thus, two simultaneous failures had occured in the attitude control system—the Star Tracker and the IMU—and the ability of the spacecraft's guidance and control electronics to cope with multiple failures is limited.
With only that limited amount of information, it's unclear what happened next. If the spacecraft managed to recognize that the IMU had an anomaly, it would have disabled it, and fallen back on the only system left for determining orientation, the solar aspect sensors. There are five solar aspect sensors on the spacecraft, among them covering the full sky. With just these sensors operating, the Behind spacecraft should at least have kept itself well enough pointed to keep the solar panels pointed at the Sun, and maintain power on the spacecraft.

However, if the spacecraft did not detect that the IMU was providing bad data, it may have fired its thrusters to stop the roll that it thought it was in. With bad data coming from the IMU, instead of stabilizing the spacecraft, this would have sent it into a spin about its principal axis of inertia. The solar panels may have then stopped getting enough sunlight to keep the spacecraft powered, draining the batteries and shutting the spacecraft down.

Attempts to reestablish communications with the STEREO Behind spacecraft are ongoing. Since the loss of communications on October 1, the operations team has been broadcasting commands designed to correct a number of possible scenarios as to why no communications have been heard. Some of these commands are intended to aid the Star Tracker come back online. Other commands are intended to reactivate the spacecraft transmitter, in case it somehow got deactivated. Commands have also been broadcast to disable the failed IMU. So far, none of these activities have been successful. Attempts were also made to detect the signal from Behind with the Green Bank 100 meter radio telescope, without success.

Even if the STEREO Behind spacecraft is currently drifting without power, not all hope is lost. A similar situation occured with the SOHO mission in 1998, where the spacecraft ended up in spin with the solar panels pointed away from the Sun. As SOHO continued to move along in its orbit, it eventually got to a point where sunlight started falling on the solar panels again. The operations team (with a considerable amount of effort) was able to recover the spacecraft into operating mode. Now, sixteen years later, SOHO is still providing valuable data to the solar and space weather communities.

Once communications are restored and the anomaly resolved, the operational plan for exiting the solar conjunction testing will continue to return the Behind observatory back to nominal science data collection as soon as safely possible.

Launched in 2006, the STEREO mission achieved its prime science goals within the two-year prime mission, but continues to explore solar and heliospheric activity through the current solar maxmimum and beyond. STEREO Ahead continues to operate nominally, and is currently providing our only views of the far side of the Sun.

[Update: 09-Dec-2014]

If the Behind spacecraft has been recovered by Jan 19 2015, preparations for superior solar conjunction would begin on Jan 19, starting with the instruments powering down. The SSR would be played back on Jan 20, and on Jan 22, the spacecraft would be reset to rotate at 5 degrees per minute. Regardless of the state of the spacecraft, DSN coverage for Behind ends on Jan 26.

There was some discussion of a test of phasing together several dishes at Goldstone to increase the uplink signal power in the attempt to recover the STEREO Behind spacecraft. This should either be done before Jan 26, or else in April, after superior solar conjunction.

[Update: 08-Jul-2015]

On day May 4th, through June 27th, Behind recovery operations were conducted as the observatory has cleared solar interference for LGA communications. Based on the Failure Review Board recommendations, the recovery operations consist of recovering the battery state of charge for the first two days. On the third day, the transmitter will be commanded on to receive the carrier. Recovery operations were significantly different than what was conducted in January. This included a faster frequency segmented acquisition sequence, sweeping a lower frequency range, battery state of charge recovery and powering on the transmitter using only short critical commands. The frequency segmented acquisition sequence is a significant departure for deep space communications which typically require a slow sweep of the entire best lock frequency range. The DSN staff cleverly developed a 10 second automated uplink sweep consisting of 18 one kHz frequency segments followed by repeatedly sending short commands. The JPL radio science receiver capability for the carrier recovery tracks was not used due to a funding issue.

On Jun 25, during the DSS-14 70m support for Behind, 225 commands were sent for battery state of charge recovery.

On Jun 26, the Behind DSS-14 70m support for battery state of charge recovery was moved to Ahead for the first opportunity for telemetry post solar conjunction.

On Jun 27, during the DSS-63 70m station support for Behind, 320 commands were sent for powering on the transmitter. No downlink signal was detected. Due to the duration of the support and the increased commands for each step, only 16 of the 18 frequency segments were commanded. Three commands must be received sequentially to power on the transmitter. The Green Bank Radio Telescope also observed, with reduced sensitivity due to a cryogenics anomaly, for 3.25 hours beginning at 0945z, however, it did not detect the downlink.

The Green Bank and Arecibo telescopes have not detected any signal from Behind, though Green Bank did detect Ahead's low gain antenna.

Operations to recover Behind are now suspended, due to the spacecraft's retrograde motion back toward the Sun.


The acquisition sequence currently being used for Behind will be tested with Ahead, tentatively scheduled for the first week of September. Uplink array tests will also be carried out with Ahead later in the year. The eventual plan is to combine the signal from three DSN stations, with one of the stations having significantly stronger uplink power than the other two. This capability is still under development.


The following movie illustrates one possible model of the loss of contact with the STEREO Behind spacecraft: DOWNLOAD

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