The Radio Amateur Satellite Corporation, AMSAT, recently designated RadFxSat as AO-91 after its successful deployment from a Delta II rocket as a secondary payload to NASA’s JPSS-1. RadFxSat is the first of several AMSAT satellites which are flying a Maximum Power Point Tracker (MPPT) designed and built by Brent and I as a continuation of our Rochester Institute of Technology (RIT) senior design project. The story of the Fox-1 MPPT is a great example of how amateur radio is what you want it to be.
The MPPT symbolizes the vast nature of amateur radio which spans far beyond just communicating over the airwaves. The Maximum Power Point Tracker has no radio inside of it yet it is profoundly amateur radio. It’s sole purpose is to enable scientific and ham radio payloads on-board the spacecraft. Amateur radio is about learning, having fun, and applying technology to accomplish great things. Let’s explore this side of the hobby!
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Maximum Power Point Tracking
Let’s cover the basics first. A Maximum Power Point Tracker is an intermediate circuit placed between a solar panel and the load being powered. Solar panels produce the maximum amount of power at a specific voltage which varies with light irradiance as well as board temperature. The MPPT interfaces the solar panel with the load in such a way that the conditions for maximum power are always met when needed. In the case of RadFxSat the MPPT is temperature based since most of the change occurs with solar panel temperature and much smaller effects are created by irradiance from the Sun. Read the Fox-1 MPPT Technical Document if you are interested in the details of how the Fox-1 MPPT works.
It All Started at RIT
During the Dayton Ohio Hamfest in May 2012 Anthony Montiero, AA2TX, agreed to sponsor a senior design project for us during the next academic year at the Rochester Institute of Technology. Tony was the Vice President of AMSAT engineering at the time. He wanted us to design a new Maximum Power Point Tracker that could be used by AMSAT on future missions which were flying higher power radios and larger scientific payloads. The original intent was to fly this MPPT on a 3U cubesat named Fox-2.
The senior design team consisted of just four students: Ian Mackenzie (KB3OCF), Dan Corriero, Brenton Salmi (KB1LQD), and myself (KB1LQC). We had chosen to work together as we were all members of K2GXT, the RIT Amateur Radio Club, and had recently built and launched RITchie-1 (a high altitude balloon) together. We were guided by two Rochester, NY area engineers, Vincent Burolla and Leo Farnand, who had agreed to help us out and were obviously skeptical of the pure amount of work we had signed-up for. However, our team was motivated and ready for the challenge.
P13271: AMSAT Maximum Power Point Tracker
At the time RIT was still on the quarter system which meant that the two quarter Senior Design course only gave us 20 weeks to design, build, and test a working MPPT. Every one of us still had normal coursework and were active members of K2GXT. Great lessons of descoping projects, good communication, and teamwork were tested during this time. I vividly remember being one of the only teams who would put feelings aside in effort to make quick progress. For example, we never perfected our documentation before sending it to our guides. Instead our team wanted their feedback as soon as possible if were unsure about something. We knew we could do it but we also knew that we were learning and acknowledging that let us focus on the goal of getting an analog MPPT designed, built and tested in such a short time-frame.
Below is a video of the Fox-2 MPPT at RIT during a test on campus in April 2013. During this test we walk through the voltage and current set-points that prove the senior design project performed maximum power point tracking and output voltage regulation under varying loads.
The board was built by hand as shown in the time-lapse video below. Our project had a budget of $500 which didn’t allow for automated board assembly. Therefore the board was designed with larger surface mount electronics so hand-assembly was easier. Still, we spent several days building each sub-circuit and carefully testing it before allowing it to working with the rest of the board. Thus, we were isolating the circuits during bring-up to ensure each worked as expected and any issues were immediately obvious.
Our working senior design MPPT board is shown below:
Downsizing for AMSAT Fox-1
Following graduation Brent and I moved to Los Angeles, CA and were contacted by Jerry Buxton, N0JY, with a request to update the RIT MPPT for use on a smaller 1U cubesat called Fox-1. Jerry had just become the Vice President of Engineering at AMSAT and they were having trouble with their smaller MPPT circuit. Fox-1 was flying well before Fox-2 so this needed to be fixed quickly. The RIT design worked and could be scaled much larger if needed. However, it was tricky to scale for smaller solar cells. In the end we did it but it was not a straightforward redesign.
The first step was to downsize the PCB to fit inside the cubesat and put all six MPPT’s on one board. A single MPPT was needed for each side of the satellite as there are solar panels on all sides of the Fox-1 design. A cubesat only allows for the PCB to be 95mm x 95mm and this board had over 400 parts on it connected by four layers of copper internally. This first revision barely fit but it worked. The second board design (Rev 1.1) shown below used smaller components and was aimed at being flight worthy. It did however suffer from a few circuit stability issues which required one more revision to get right.
Going Flight – AMSAT MPPT
RadFxSat has the beautiful board below installed inside of it. This is the second revision and it fixed all circuit stability issues found during the two years of development after graduating from RIT. Yes, this took two years of work to complete. Hundreds of hours were spent pouring over this board so that Brent and I were sure it would operate in orbit. The entire satellite is literally dependent on this board to charge its batteries. Without power the satellite would not operate. The antenna deployment signals also go through the MPPT on their way to the solar panels adding anxiety inducing reliance on the MPPT board to work.
The AMSAT MPPT is an analog computer. It measures solar panel temperature and performs a Y=mX+b computations which predict the maximum power point voltage. It must operate with solar panels that can vary from -60°C to +60°C and the MPPT board temperature itself can vary widely above and below 0°C. Just because it works on a lab bench doesn’t mean it will work in orbit. The design is simple, radiation tolerant, and fails gracefully. That is what you want when designing spacecraft. The hope is that we can get at least five years of operation out of the satellite.
To put this into perspective the MPPT will be subjected to one orbit every 90 minutes. An orbit every 90 minutes means 16 orbits per day and a 5 year mission means the board will revolve around earth about 29,200 times during its mission. That’s 29,200 heating and cooling cycles where a solder can crack or component could fail. Cracks in pavement are caused by heating and cooling from the Sun for example, the MPPT must not crack.
To learn more about the final flight design of the MPPT please look at the following resources
This is Amateur Radio
I can’t begin to explain how rewarding this experience has been. There were certainly tough times and lots of stress along the way too. However, thanks to amateur radio I have a circuit board in space. It takes hard work but anyone can do this, including you. AMSAT did not require me to be an electrical engineer in the aerospace industry! AMSAT is a volunteer organization that has been putting satellites into space with ham radio payloads since the 1960’s. Consider checking them out.
Just as FaradayRF aims to promote the technical advancement of amateur radio we see that AMSAT is right there doing the same. It’s easy as a radio amateur communicating through an AMSAT satellite to forget there is an army of volunteers that worked to build all the invisible systems necessary for the satellite to operate. If talking through a radio isn’t all that interesting then consider helping build a satellite. Consider becoming a ham and exploring this aspect of the hobby. This is amateur radio, have fun!
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.