$50SAT, the PICAXE powered satellite, is Silent.

Michael Kirkhart, KD8QBA;

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:

https://www.dropbox.com/sh/l3919wtfiywk2gf/AAD6Q3gJYziGVwavQ1OkSxAia/Latest-Observation

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:

https://www.dropbox.com/sh/l3919wtfiywk2gf/AABKSR5V4cOvEPqPYbs8QYZNa/Telemetry-analysis/Current-Telemetry/Battery-Voltage-MPPT-Current-Chart.pdf

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.

73

The $50SAT/MO-76/Eagle-2 team:

Howie DeFelice, AB2S
Stuart Robinson, GW7HPW
Michael Kirkhart, KD8QBA
Professor Robert Twiggs, KE6QMD

Bravo guys - I've followed this with great interest & also monitored here in NZ during initial passovers. Next launch LoRa™ maybe ? As LoRa™ is more energy efficient, such modulation should also prolong activity when the PV is fading.

Li-ion batteries however are often only good for ~500 charge cycles, so perhaps look at far more durable LiFePO4 types ? Stan. (ZL2APS)

Sad and amazing at the same time. Congrats to the team on a remarkable achievement! KM6DS

manuka said:

Li-ion batteries however are often only good for ~500 charge cycles, so perhaps look at far more durable LiFePO4 types ?

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Well yes, I was aware of that.

But the 'limit' of 500 charge cycles relates to full charge cycles (flat to full) and to full voltage (4.1V). We were not doing that at all, there would be a great many partial charges to less than full voltage (3.9v). How long Li-ion batteries would last versus LiFeP04 under those conditions (and the radiation, vacuum and extreme temperature cycling) was and still is a complete unknown, there is no test data available.

There there are voltage constraints on the components your using, and size and weight.

The Lithium Ion we used was a good fit for the overall design, a LiFeP04 was not.

What about the recent Meteor storms? Could they have contributed to it's silence?

It's quite amazing that a DIY satellite, built from COTS parts which were never intended for the job, has lasted so long. I'm very impressed, and would like to congratulate all involved.

But is the little fella dead, or just dazed ?.

There's 18 months to go before his firey demise, is it possible he could he still give us a shout ?

Buzby said:

But is the little fella dead, or just dazed ?.

There's 18 months to go before his firey demise, is it possible he could he still give us a shout ?

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Unliklely, its doing exactly as planned, if the battery voltage is below 3.3v it wont transmit. At that voltage the battery impedance is fairly high so when you turn TX on (85mA) the battery voltage can fall enough to trigger the seperate low voltage lockout (2.9V) and a reset. So the transmission starts, then stops, it was decided to prevent this happening.

It is very likley still running, but the amount of power coming from the Solar panels is only enough to keep it in idle mode and the battery voltage cant rise enough to allow it to transmit.

ZOR said:

What about the recent Meteor storms? Could they have contributed to it's silence?

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As reported above, $50SAT was last heard from around a month before the recent Meteor storms.