Remarkable recovery of satellite studying the sun

August 25th, 2016, Published in Articles: EE Publishers, Articles: EngineerIT

 

After 22 months, on 21 August 2016, NASA scientists re-established contact with STEREO-B, one of NASA’s solar terrestrial relations observatories, after communication was lost on 1 October 2014. Scientists recently started a monthly recovery operation, using NASA’s deep space network, or DSN, which tracks and communicates with missions throughout space. Their persistence was crowned with success.

Artist's concept showing a coronal mass ejection (CME) sweeping past STEREO. Image credit: NASA

Artist’s concept showing a coronal mass ejection sweeping past STEREO. Image credit: NASA

The DSN established a lock on the STEREO-B downlink carrier. The downlink signal was monitored by the mission operations team over several hours to characterise the attitude of the spacecraft and then the transmitter was powered down to save battery power. The STEREO missions operations team plans further recovery processes to assess observatory health, re-establish attitude control, and evaluate all subsystems and instruments.

Communication with STEREO-B was lost during a test of the spacecraft’s command loss timer, a hard reset that is triggered once the spacecraft has been without communications from Earth for 72 hours. The STEREO team was testing this function in preparation for something known as solar conjunction, when STEREO-B’s line of sight to Earth – and therefore all communication – was blocked by the sun.

The two STEREO spacecraft, launched in October 2006, were originally designed to complete a two-year mission, ending in 2008. But — like many NASA spacecraft — they lasted much longer. The long lives of the two STEREO spacecraft, now nine years old, have been a boon for scientists studying the sun and its influence throughout the solar system.

The two STEREOs slowly drifted away from Earth as they orbited the sun, one ahead and one behind our home planet, giving scientists constantly-improving views of the sun’s far side, allowing scientists for the first time to see the whole sun at once.

However, there are always challenges associated with operating a spacecraft for more than four times its original lifespan. In STEREO’s case, its orbit was the biggest hurdle. The same slow drift that lets the two STEREO spacecraft give widely varying views of the sun means that each spacecraft eventually lay on the other side of the sun from Earth, leading to a three-month period where communication was impossible due to the sun’s interference.

“The sun emits strongly in nearly every wavelength, making it the biggest source of noise in the sky,” said Dan Ossing, mission operations manager for the STEREO mission at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “Most deep space missions only have to deal with sun interference for a day or so, but for each of the STEREO spacecraft, this period lasted nearly four months.”

When the team realised that the STEREO spacecraft would operate long enough to enter this interference zone, they began planning.

The STEREO spacecraft were designed with a command loss timer, an automatic reset button that restarts the spacecraft after 72 hours without contact. This reset is intended to correct any issues that could be preventing communication. The command loss timer can’t be changed—meaning that during its phase on the other side of the sun the two STEREO spacecraft would be rebooting every three days for over three months straight.

Regardless, mission operators planned to make it work. They were in the middle of testing the reset by intentionally withholding communications from STEREO-B for three days — a test that had already been completed with great success on STEREO-A — when communications were lost.

STEREO-B captured this view of an erupting coronal mass ejection on July 23, 2012. The unique vantage points of the two STEREO spacecraft gave us unprecedented simultaneous views of the entire sun. Credits: NASA/STEREO

STEREO-B captured this view of an erupting coronal mass ejection on July 23, 2012. The unique vantage points of the two STEREO spacecraft gave us unprecedented simultaneous views of the entire sun. Credits: NASA/STEREO

The hard reset happened as expected, 72 hours and 20 minutes after operators stopped communications with the spacecraft. After the reset, STEREO-B was supposed to power itself  back on, identify  certain stars so it could point its antenna at Earth, and send down a status report.

At first, everything went well. Seconds after the reset, the STEREO team received a signal from STEREO-B — but it was much weaker than they expected, and it quickly faded away. That was the last time any things was heard from STEREO-B.

The signal received was so weak and so brief that the team was only able to extract a few packets of data to form a partial status report. From that small amount of information, the STEREO team was able to extrapolate the most likely case for where spacecraft is and what it’s doing.

“The telemetry showed that the inertial measurement unit, or IMU — which tells the spacecraft if and how fast it’s rotating — failed in a way that was not expected. Rather than cutting out altogether, it was feeding incorrect information into the guidance and control computer.”

The STEREO team believed that this bad information led the guidance and control computer down a path that eventually sent the spacecraft spinning, leaving its solar panels dark most of the time and its battery only intermittently charged.

As of 30 November 2015, spacecraft operators have had three three-hour blocks of time on the deep space network each week to try and contact STEREO-B. The first two blocks are dedicated to building up the charge in the spacecraft’s battery by telling it turn off the flight systems that boot up automatically. If STEREO-B can reduce the amount of power it consumes during those brief periods when its battery is charging, the spacecraft could remain on long enough to talk to and receive commands from Earth. The third block in each sequence was spent sending commands to turn on the spacecraft’s transmitter.

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