Editor's Note: To view a really cool video of the SuitSat being launched, plus a videotaped teardown of it taken at this year's ESC, click here.
Also, this feature was pulled from the latest Under the Hood EETimes/TechOnline supplement. To see the full digital version of that supplement, click here.
What started out as an innocuous call about their company's line of microcontrollers sucked a small cadre of engineers into a multiyear, space-based design project that would push the boundaries of their professional knowledge and flexibility, while reigniting the sense of wonder, exploration and possibility that led them down the path to engineering in the first place.
That call came in October 2004 from Lou McFadin, hardware manager for Amateur Radio on the International Space Station (ARISS), a volunteer program designed to inspire students
worldwide to pursue careers in science, technology, engineering and math through amateur-radio communications opportunities with the space station on-orbit crew.
McFadin was calling in the wake of an AMSAT Symposium/ARISS International Partner meeting at which it was decided to convert an old Russian Orlan space suit into a satellite by outfitting it with telemetry equipment and antenna--and to toss the suit out of the space station.
The plan, code-named SuitSat, called for the novel "satellite" to transmit spoken greetings from children in multiple languages, as well as slow-scan TV images encoded in audio signals and some spoken telemetry, such as elapsed time, temperature and battery voltage. McFadin was calling Microchip Technology Inc. to see if a PIC microcontroller could handle the job.
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It seemed simple enough, and so a small team from Microchip volunteered to help out. "We got into it thinking it's easy," said Steven Bible, technical staff engineer at the Chandler, Ariz., company. "Kind of like a high-level science fair project." They were mistaken.
What the Microchip engineers didn't fully appreciate at the time was that extreme environmental and safety issues would challenge them at every step of the design. With a vacuum to deal with, they learned about the implications of outgassing and about handling thermal hot spots with no airflow. They also learned how to avoid the Fermi region to prevent arcing, how to handle thermal extremes, and what do about ionizing radiation. They studied power-optimization techniques for system longevity, and with no gravity for stability, they found out the hard way what a tumbling antenna does to radio reception.
The experience proved so interesting that the engineers enthusiastically signed up for SuitSat-2. Leveraging their acquired knowledge, the team has already developed a new solar-conversion technique for the follow-on satellite that will extend system life to months instead of weeks. They have also crafted a software-defined, full-duplex radio with a better antenna for two-way communication and more-refined control.
Electronics the easy part
From a schematic-diagram point of view, the original SuitSat-1 system and its electronics were fairly straightforward. According to Bible, the main design parameters were that it be easy to assemble on the space station and that it work with simple, inexpensive ground receive equipment, so that teachers and hobbyists could readily track the signals. All they should have to do would be to download free software that let them track the audio and decipher the audio-encoded images.
SuitSat-1 comprised a controller box with a control printed-circuit board, dc/dc converter and EMI filter. This connected to a separate radio box that used an off-the-shelf Kenwood TH-K2 amateur handheld two-way VHF radio with an external quarter-wave, ground-plane antenna. The controller also connected to a switch box with one power and two timer switches that were used to start the system upon deployment. The whole system was powered by the suit's own three, very expensive but high-power-density 28-volt silver-oxide batteries.
The controller board was where the messages were stored and where temperature and voltage monitoring took place. At its heart was the PIC18F8722, a 64/80-pin microcontroller with 1 Mbit of enhanced flash, on-board 10-bit analog-to-digital converter and the company's nanoWatt technology. Another reason that processor got picked was its ability to handle voice signals encoded using adaptive pulse-code modulation (ADPCM). Those signals were stored on an 8-Mbit SPI serial flash chip from SST (the SST25LF080A), programmed via an RS-232 interface, and a MAX3232 RS-232 level shifter from either Maxim or Texas Instruments (dual sources).
The ADPCM signal was output from the microcontroller to an MCP6022 op-amp-based, 4-kHz low-pass filter, and from there passed on to the radio, which transmitted at 145.99 MHz. Also on the board were an MCP9800 SPI temperature sensor and three MC14541 programmable timers.
The inclusion of timers on the control board underscored the biggest problem the designers faced: safety. "We had to deal with NASA safety people," said Bible. "It was mind-boggling." Each aspect of the design had its own documentation: snag hazard, outgassing hazard, electric shock and RF hazard. To handle the latter, the timers were used to implement three interlocks to prevent the transmitter from sending out messages before a 30-minute timeout, thereby giving the cosmonauts time to reenter the space station after pushing away the SuitSat.
To keep from exposing the astronauts to toxic fumes, all the components had to be built from materials that could pass a thermal-vacuum test without outgassing. But that wasn't the only selection constraint: Tantalum and ceramic capacitors were chosen instead of electrolytic, since electrolytics are basically a can and are also potential leakers.
All mounting hardware had to be stainless steel, and no lock washers could be used, because they produce microscopic metal shavings that could short out fine-pitch SMT parts. Due to the vibration associated with takeoff, all wires were laced together and glued down. All bare-metal joints were covered with RTV to prevent any debris from shorting connections, and all bolts were glued to prevent loosening. Even the glue itself had to be specially approved by NASA.
One of the more interesting considerations when preparing for a vacuum is the Fermi effect, whereby operation at lower oxygen pressure can cause arcing or a corona effect at voltages of around 40 V. To prevent this hazard, Bible said, the enclosures had extra holes to ensure rapid venting to vacuum. While ionizing radiation and its associated bit-flips and latchups were somewhat of a concern, the low-earth orbit at about 400 to 500 km, and the short life expectancy of the satellite, gave this problem low priority. In the end, it proved to not be an issue.
Thermal, power management
Thermal management did become an issue, however. While convection or forced-air cooling can be used on earth, the vacuum of space meant the designers had to rely on black-body radiation to keep the system cool. To maximize heat extraction, they mounted hot spots, such as the Kenwood radio transmitter, on a large aluminum block that was itself attached to the enclosure. Also, thermal extremes had to be avoided. As it orbited, the suit would be exposed to heat and cold alternately for 45 minutes each.
While the suit was designed to keep an active cosmonaut cool, the team had no information on the environment without a human inside. The suit was not going to be actively cooled. As the electronics were placed inside, the engineers banked on it being an insulator against extreme cold and heat. They used a temperature sensor and spoken telemetry to monitor the variations. It stayed at 12°C for the entire mission.
While the electronics went inside, the switch box and antenna were bolted externally to the helmet, with the cables running inside, taking care to reduce snagging potential. To maximize life expectancy from the used batteries, which would be in an unknown state of charge, the team made sure the transmission was easy to listen to and did not require an excess amount of energy. The result was a 30-seconds-off, 30-second-broadcast mission profile.