Tuesday, July 21, 2015 marked the 20 month anniversary of the launch of $50SAT/MO-76/Eagle2, and unfortunately, it appears to have gone silent. The last time I heard it was on Sunday, July 19, 2015, 08:42 UTC from Anton’s (ZR6AIC) WebSDR. A screenshot of the WebSDR while $50SAT was transmitting RTTY, a screenshot of gpredict showing its location during the transmission, and the captured RTTY audio are up on the Dropbox; they are accessible via the following URL:
Unfortunately, there was a fade starting in the middle of the capture, so I was only able to do a partial decode:
2015-07-19,08:42,KG43,ZR6AIC/KD8QBA,$50SAT,,2990,15719,,,84,3,,22,?,?,?,?,?,?,*? (NO CHECKSUM – MISSING DATA)
Has anyone else has heard $50SAT since July 19? If so, please let us know.
The likely cause of failure was a near complete loss of solar power. Looking at the last 5 complete RTTY telemetry messages, the amount of solar power being generated was very low:
2015-06-21,08:23,KG43,ZR6AIC/KD8QBA,$50SAT,,2990,15688,,,82,3,,22,16,78,,3435,1572,3319,*4A (3435 mV, 0 mA, 0 mW)
2015-06-22,08:15,KG43,ZR6AIC/KD8QBA,$50SAT,,2990,15689,,,81,3,,21,15,78,4,3536,1573,3339,*7D (3536 mV, 4 mA, 14 mW)
2015-06-22,08:20,KG43,ZR6AIC/KD8QBA,$50SAT,,2990,15689,,,84,3,,22,17,78,3,3556,1573,3339,*78 (3556 mV, 3 mA, 11 mW)
2015-07-07,08:25,KG43,ZR6AIC/KD8QBA,$50SAT,,2990,15706,,,84,3,,22,18,78,1,3475,1590,3319,*7C (3475 mV, 1 mA, 3 mW)
2015-07-09,08:07,KG43,ZR6AIC/KD8QBA,$50SAT,,2990,15708,,,81,3,,22,15,78,3,3516,1592,3319,*7E (3516 mV, 3 mA, 11 mW)
Moreover, when looking at the Battery Voltage/MPPT Current chart, the last set of MPPT (solar) current measurements are well below the trendline, which itself has a negative slope:
Because $50SAT will not transmit if the battery voltage is below 3300 mV, we do not know if it is completely dead or the battery voltage is almost always too low to enable the transmitter.
Our best guess as to what happened is the solar cells have been slowly damaged due to sputtering. All the high energy particles from the solar wind can effectively “sandblast” the satellite, and since our cells had no protective cover glass, this will cause the surface to slowly become diffused, and thus cause the output of the cells to drop. We knew this would happen. What we did not know is $50SAT would operate long enough where this would become a problem; our bets were on the battery failing first. Since many other CubeSats used the same Spectrolab TASC cells as we did, we are curious about how long it took for the solar output to degrade on these other satellites. If any of them are reading this post, we would love to hear from you!
We are now in the “how long will it stay in orbit?” phase of the mission. Thanks to James DeYoung, N8OQ, we have a de-orbit prediction of May of 2017. From July 6, 2015 to August 28, 2015, the orbital decay rate was about 0.79 km/week. Earlier this year, it was about 1.2 km/week. As of August 28, 2015, apogee was at 554 km, and perigee was at 525 km. We will continue to monitor the TLEs from Celestrak and periodically update the “Orbital-Analysis” folder on the Dropbox.
We would like to extend a very big THANK YOU to the worldwide amateur satellite community! You helped make our mission a resounding success! We were able to determine it was possible to make a satellite this small which could generate and store its own power as well as have two-way radio communication capability. We were able to do this using commercially available parts, including a $10 Li-Ion camera battery, a $10 ISM band radio, and a microcontroller programmed in interpreted BASIC. We look forward to seeing what the next generation of PocketQubes can do.
The $50SAT/MO-76/Eagle-2 team:
Howie DeFelice, AB2S
Stuart Robinson, GW7HPW
Michael Kirkhart, KD8QBA
Professor Robert Twiggs, KE6QMD