Michael Kirkhart KD8QBA provides an update on the $50SAT amateur radio spacecraft which measures just 5x5x7.5 cm.
Sunday, June 21, 2015 marked the 19 month anniversary of the launch of $50SAT/MO-76/Eagle-2. The good news is it still operating. The bad news is the power situation has been degrading, with an apparent step change on or near May 12, 2015, followed by another on Tuesday, June 23, 2015. The last full telemetry capture made here in EN82 land was on Wednesday, May 27, 2015, and the last time it was heard was on Friday, June 6, 2015. I continued to attempt to listen for it for another week or so, and heard nothing. Has anybody heard it since then?
At this point, I have been monitoring it using Anton’s (ZR6AIC) WebSDR as it makes daytime passes over South Africa. These occur between 7:30 and 9:00 UTC, which translates to 3:30 and 5:00 AM here in EN82 land. This is tough, as I am not a morning person. Sometimes, however, you have to do these things; helping build a satellite might be a once-in-a-lifetime event. During these passes, where it has already spent a significant amount of time in sunlight, the battery voltage is below 3400 mV. Is the battery going bad? While it is certainly possible the battery has suffered from some loss of charge capacity, one has to remember it is does not generate energy; it merely stores it. Since it is the solar power system that generates the power used by the satellite and stored in the battery, could the drop in battery voltage be due to a degradation in solar power generation?
Back around May 12, I noticed the MPPT (solar) current readings were typically less than 10 mA. This much lower than it should be. To better understand what might be going on, a new chart was added to the telemetry spreadsheet which shows both the battery voltage and the MPPT (solar) current (with the zero readings removed), each with its own linear regression line. This chart can be seen from the following URL:
Notice how the MPPT current trend line has been sloping downward, similar to that of the battery voltage. Moreover, starting 2 weeks before June 4, 2015 (each X axis division on the chart represents 2 weeks time), each reading has been at or below the trend line.
A more striking comparison can be seen by doing the following:
1. Zoom in of the Battery-Voltage-MPPT-Current-Chart to show the 4 week interval starting May 7, 2015, and ending June 4, 2015
(see https://www.dropbox.com/sh/l3919wtfiywk2gf/AACdQtySHZW3kVl7UMgSrxfHa/Telemetry-analysis/Battery-Voltage-MPPT-Current-Comparison-2015-05/Battery-Voltage-MPPT-Current-2015-05.pdf )
2. Zoom in of the Battery-Voltage-MPPT-Current-Chart to show the 4 week interval starting May 8, 2014, and ending June 5, 2014
(see https://www.dropbox.com/sh/l3919wtfiywk2gf/AACracUWkivilfsKGBUFkmDXa/Telemetry-analysis/Battery-Voltage-MPPT-Current-Comparison-2015-05/Battery-Voltage-MPPT-Current-2014-05.pdf )
In comparing these charts, it is clear both the battery voltage and the MPPT current were significantly lower this year that during the same period last year. On June 5, 2014, the trend line value for battery voltage was about 3610 mV, and for MPPT current was about 30 mA. One year later (June 4, 2015), the trend line value for battery voltage was about 3380 mV, and for MMPT current was about 14 mA. Therefore, while it is likely the battery has suffered some loss of capacity, it appears the low battery voltage is due to low solar power output. There are many possible reasons for this, including:
* Solar cell damage due to sputtering: since there was no protective covering on the solar cells, impacts from high energy particles can damage the cells, causing a drop in output.
* Solar cell damage due to thermal cycling: We know from telemetry data the interior of the satellite cycle between +30 degrees C and -30 degrees C each orbit. It is likely the exterior temperatures experienced higher extremes, and this periodic thermal cycling may have caused the solar cells to fracture, thus leading to a drop in output.
* Short circuit failure of one or more of the diodes which isolate each MPP tracker output, which can cause an inactive MPP tracker (one whose corresponding solar panel is not facing the sun) to load an active one (one whose corresponding solar panel is facing the sun).
Because of the limited amount of telemetry gathered, it may not be possible to determine the exact cause. If the solar output power continues to drop, the battery voltage may never get above the 3300 mV threshold needed to enable the transmitter, at which point we will lose the ability to monitor its status. Even if this does happen, however, we never really thought it would last this long. We would have been happy if it just worked, and really happy if it lasted a month or two. 19 months – this is way beyond what any of us expected!
As of June 25, 2015, the orbit has decayed by about 73 km since launch. Since April 21, 2015, it has been decaying at a rate of about 1 km per week. Apogee is now at 561 km, and perigee is at 529 km.
The following are the TLEs from 2015-06-25:
1 39436U 13066W 15176.16386703 .00013608 00000-0 90105-3 0 9991
2 39436 97.7444 252.3622 0022818 80.2035 280.1767 15.07230510 86697
Again, if anyone wants to make an attempt at predicting when it will de-orbit, here is some useful information:
Average cross-sectional area = 0.014252 m^2
Mass = 210 g
Area/mass ratio = 0.06787 m^2/kg
From the 2015-06-25 TLEs:
Semi-major axis: 6922.8 km
As always, please post any telemetry, or for that matter, any reception reports to the Yahoo discussion group. We would especially like to encourage our friends in the southern hemisphere to listen for $50SAT/MO-76/Eagle-2. We really appreciate everyone who has provided reception reports and telemetry as well as access to their WebSDRs. To date, we have 3,098 individual error-free telemetry captures, and the vast majority of these did not come from Stuart, Howie, or I.
73 Michael Kirkhart KD8QBA
$50SAT is one of the smallest amateur radio satellites ever launched at 5x5x7.5 cm and weighs only 210 grams. Transmitter power is just 100 mW on 437.505 MHz (+/-9 kHz Doppler shift) FM CW/RTTY. It uses the low cost Hope RFM22B single chip radio and PICaxe 40X2 processor.
$50SAT has been a collaborative education project between Professor Bob Twiggs, KE6QMD, Morehead State University and three other radio amateurs, Howie DeFelice, AB2S, Michael Kirkhart, KD8QBA, and Stuart Robinson, GW7HPW.
Further information in the $50SAT Dropbox https://www.dropbox.com/sh/l3919wtfiywk2gf/-HxyXNsIr8
There is a discussion group for $50SAT http://groups.yahoo.com/groups/50dollarsat/