We extensively covered the AMSAT Fox 1 series Maximum Power Point Tracker in our More Than Radios – AMSAT MPPT blog post. That post detailed the first launch and overall design of the MPPT aboard RadFxSat, Fox-1B. The satellite was designated AO-91 once in orbit. This post covers some fun details about the second launch of the MPPT inside the Fox-1D satellite aboard the PSLV 40 mission, a mission launched out of India with the Polar Satellite Launch Vehicle. This satellite has now been designated AO-92 after successful insertion into its own orbit.
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An Older MPPT
The Maximum Power Point Tracker flying on AO-92 is actually an older version. The board is green instead of the beautiful blue inside AO-91 since it’s the first flight version we built. We dubbed this Rev 1.2 since the PCB’s were Rev 1.1 and there were several PCBA (component) changes which forced a new revision. These changes were largely to stabilize the error amplifiers comprising the analog computer for each individual MPPT. Since we were on a tight schedule we had to use these boards with some capacitors stacked on top of resistors and some passive components DNP’d (Do Not Populate). If you look closely at the PCB below you can see some of the modifications.
The images above were taken the morning I shipped Serial Number 41 (SN41), destined for Fox-1D, from Manhattan Beach, CA. You can see the reused Digi-Key blue static resistant bag holding the flight MPPT and the anti-static bag used to protect it even further. Inside the blue bag we also included a desiccant to keep humidity at least somewhat at bay. It’s important to note that the blue bags, much like pink static bags, are not anti-static, they are static-dissipative bags. This means they are not Faraday cages and you can still zap the PCB inside of the bag by simply handling it incorrectly. The static dissipative bag is designed to not generate static electricity with movement and therefore you still need an anti-static bag as shown for actual protection! Now you know.
The stacked capacitors are certainly not ideal but they are a necessity. Ironically, we were forced to deliver Fox-1D over a year before launch so that it would be integrated onto the launch vehicle. Once you deliver it you cannot touch it. This is a fact of life in the space world and while we certainly could have used a newer MPPT if we didn’t have to deliver the spacecraft a year in advance we were not the primary payload ($$$) so you do what you are told! The image below shows R18 and R12 circled in red. If you look closely you’ll notice there is a capacitor soldered on-top of each one.
Why Were These Capacitors So Important?
Let’s take a look at the schematic and see what these capacitors are doing! Below you can see C168 in parallel with R12 and C162 in parallel with R18 on the Rev 2 schematics. These are both in the RTD circuit which measures the solar panel temperature. Solar panel temperature is very different than the internal MPPT PCB temperature since panels are exposed directly to the Sun and the black of space which is only 3 Kelvin! This doesn’t mean the panels get that cold but it means they loose heat without obtaining any more to warm them up when in eclipse. The MPPT itself is protected inside the CubeSat and heat radiated from adjacent boards warms the MPPT (and vice-versa).
C168 is a 0.1uF capacitor used to slow down the operation of U3A. It’s arbitrarily chosen as a convenient component since we use that value of capacitance extensively elsewhere. We can use this value because a 0.1uF capacitor in parallel with the 249KΩ R12 causes the frequency response of the circuit to be around 6.4 Hz. Solar panels change temperature over the course of seconds so this is more than quick enough for our needs and drastically improves stability.
U3A in the end, is a constant current driver. It forces exactly 1mA through R15 and into the 100Ω RTD by which we can measure the voltage across the RTD using U3D. Since the current is constant, the voltage is directly proportional to the resistance which is directly proportional to the temperature. This relationship allows us to measure temperature using a voltage input to the Analog to Digital Converter (ADC). U3D is our analog computer and C162 slows its computations down such that the circuit is stable while computing Vout = -8.14228*Vin+2.0523. This math equation produces a voltage Vtemp which is the prediction of the maximum power point voltage the MPPT can use as a reference. The corner frequency of computation is about 60Hz due to C162 in parallel with R18 which as shown before is more than fast enough for our needs.
By slowing down the circuits we lose the ability to quickly adjust the Maximum Power Point but we don’t need to do that quickly since it takes several seconds for the panel to change temperature anyways. This is good engineering. We drastically stabilized the circuit by removing most of the computational speed we’ll never use anyways. Unfortunately this was realize after the green Rev 1.1 PCBs were made. However, clever engineering lets us fly in space with properly operating circuits on-schedule!
Powering Digital Images Over Ham Radio
One of the newer and cooler features of AO-92 is its on-board Virginia Tech Camera. This small camera brings imagery from AO-92 to the radio amateur. This is a great step forward in ham radio CubeSat technology and well overdue.
Fox-1D, AO-92, shows that we can have a heck of a lot of fun building satellites and providing decisively non-radio technology such as the MPPT and Virginia Tech Camera. That’s because ham radio is not about the action of communicating. Ham radio is about being a part of an amazing community that is a sandbox of experimentation. I’m incredibly proud to be a part of this hobby and the future is looking bright. Congratulations to AMSAT for an amazing success rate on it’s Fox-1 satellites, you earned it with hard work!
What do you think about AMSAT’s Fox-1 satellites? Are these CubeSats surprising you in their power and sensitivity yet being not much larger than a coffee cup? Let us know in the comments below!
Author: Bryce Salmi
Licensed radio amateur KB1LQC and Co-Founder of FaradayRF. Professional Electrical Engineer designing and building avionics for rockets and spacecraft during the day and developing the future of digital amateur radio experimentation by night. All opinions are my own.