Building a soul for Diwata

| Written by Andre DP Encarnacion

Engr. Ariston Gonzalez of the PHL-Microsat bus development team. (Photo by Misael Bacani, UP MPRO)
Engr. Ariston Gonzalez of the PHL-Microsat bus development team. (Photo by Misael Bacani, UP MPRO)

 

For much of his stay at Japan’s Tohoku University, there was little Ariston Gonzalez could do to ease the grind of his daily shift. At times the process of satellite-building seemed more hard labor than scholarship. Even for this UP-trained engineer, each repetitive motion hardly seemed to lead to the near-miracle of creating what essentially became a tumbling box in space that could obey commands.

Take the case of the humble nut and bolt. Where in previous projects, Gonzalez could twist away to his heart’s content, the life cycle of each of his bolts followed strict protocols. There were quality control protocols, cleaning protocols, and even protocols for proper torqueing. “But here was where I learned that every detail matters,” he says. “Because if one bolt fails, everything could fail.”

Indeed, what followed was the opposite of failure. In April 2016 Gonzalez’s little box would rise to fame as Diwata 1, the first ever microsatellite made and designed by Filipinos. Gonzalez, together with three others, was a member of Diwata’s original bus development team. Their crew was tasked not only with building the satellite’s infrastructure, but also providing the sensors that would act as its eyes, the computer that would serve as its brain, and the fans that would serve as its muscles in space.

Expertly tying together all these components made it possible for Gonzalez and the PHL-Microsat team to solve important problems. For instance, how would Diwata know where it was? And once they had a lock on its location, how could they control it? After all, even a satellite with a camera needs to point before it can shoot.

The secret, he says, lies in “attitude control”, the synergistic use of sensors and systems to orient objects like satellites in 3D. And much like the seafarers of old, Gonzalez claims that Diwata’s journey is one that tackles challenges common to all travelers. “How do we get to our destination and where are we even going?”

 

Sea of stars

With his trademark glasses and graphic tee, Gonzalez is fond of his nautical analogies. But instead of a ship on the water, his explanation of attitude control likens Diwata to a vessel in a sea of stars. “Like us, mariners also had tools like telescopes and sextants to chart star relationships. And they had compasses to chart where they were.”

There are multiple phases that go into pointing a satellite. And Gonzalez says, like Moana in the movies, that the first involves using sensing equipment to learn its location and position with respect to certain bodies. Diwata 1, and its successor Diwata 2, have a plethora of such sensors.

First, lining the bodies of both Diwata 1 and 2 are multiple sun aspect sensors (SAS). Holding up a metal square with a reflective center, Gonzalez notes the two purposes of this device. “First it tells us where we are with respect to the sun.” Working like solar cells, the sensors can indicate which side faces the sun when stuck directly by its light.

 

A glimpse at Diwata-2's sun aspect sensor. (Photo by Misael Bacani, UP MPRO)
A glimpse at Diwata-2’s sun aspect sensor. (Photo by Misael Bacani, UP MPRO)

 

Second, even in its tumbling state, the sensors can help calculate Diwata’s angle with respect to the sun using light intensity. “When it shines directly on the sensor, that’s typically when it is most intense,” Gonzalez says. As the angle of sunlight moves off-center, typically the intensity decreases, and the onboard computers can calculate a line tracing the satellite’s orientation based on the information.

Because it orbits the earth lower than larger global positioning system (GPS) satellites, Diwata 2 will be able to receive GPS data, much like a typical mobile phone, to measure both position and velocity. But even this only serves to complement Diwata’s geomagnetic sensor (GAS). “We know that the Earth has a magnetic field of its own,” explains Gonzalez. And with different areas on the planet having fields of different magnitudes, the PHL-Microsat team can use the sensor to map the coordinates of Diwata with a considerable degree of precision.

Diwata’s final two sensors are perhaps its most sensitive. While vessels of old used gyroscopes to measure their orientation, Diwata’s fiberoptic gyroscope (FOG) takes this to another level. Instead of rotating wheels, the FOG features optical fiber coils located on three axes. Light is split and made to travel these coils in opposite directions, with rotation on any of these wheels causing interference that delays one beam relative to the other (called the Sagnac effect). “It is so sensitive to rotations,” Gonzalez says, “that just setting it on the table, it can measure the rotation of the Earth.”

Finally, just as the ancients before it, Diwata 2 also uses the stars. Since stars are fairly static landmarks, the microsatellite can utilize its star tracker telescope (STT) to take a snapshot of a region of space. It then compares this snapshot to the 360-degree star gallery included on board Diwata to estimate the satellite’s current attitude and location.

So accurate is the STT that Gonzalez says it is virtually the standalone system behind Diwata’s most precise pointing capability (called “fine attitude determination”) that locks onto a given location with an accuracy of up to 0.1 degrees.

 

High-altitude photographs

When taking a photo in space, however, knowing one’s location is only half the battle. You need to use all this information to take a position and aim. Once Diwata’s central computer has integrated its sensors’ data, it relies on a pair of clever instruments to move properly.

First, to break it from a high-speed tumble, Diwata 2 features three magnetorquers, which are coils with electric current running though them. When turned on, each magnetorquer generates a magnetic field that aligns with the Earth’s own. And, like typical magnets, both fields will lock onto one another—fixing the satellite in a specific orientation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Once locked in, Gonzalez says finer adjustments can be made. For these, they installed four reaction wheels located diagonally across each other. Because angular momentum is conserved, Gonzalez says paired wheels spinning at equal speeds cancel each other’s influence out. But weaken or turn one off and Diwata can pivot in a controlled fashion suitable to take the high-altitude photographs that have made it famous.

As the public has witnessed since 2016, the images produced by the nation’s microsatellites are as useful as they are remarkable. With the ability to point directly down (called “nadir pointing mode”), to track a specific landmark as it passes, or to aim at an off-center direction, Diwata can take multispectral images to check for environmental health or detailed photos of important targets.

 

A stitched together panorama of multispectral images of Palawan's sediment plumes. (Courtesy of PHL-Microsat project)
A stitched together panorama of multispectral images of Palawan’s sediment plumes. (Courtesy of PHL-Microsat project)

 

With the technical requirements in place, taking, say, a panoramic image of Palawan becomes simple. “So basically what you do when you make a panorama is shoot photos while on the move,” Gonzalez said. “But since Diwata moves on its orbit, you just have to hold it down and take snapshot after snapshot. After stitching these together, you end up with one big mosaic.”

 

A Diwata’s soul

The philosopher Aristotle once described a soul as that which gives a being its primary purpose and contains faculties for it to think, sense and act. In that sense, it might not, therefore, be an exaggeration to say that the sleepless nights Gonzalez and his colleagues spent building and coding were giving Diwata the closest thing it has to a soul.

Fortunately, this larger purpose was never lost on Gonzalez and his team, especially during cold winters. “I always say when I give talks to students that even when you are doing technical work, you should always keep the bigger picture of what you are doing in mind. Looking at it in retrospect, this is really testament to our country’s journey—that we can do it!”

To perk himself up in the lab, there were times when Gonzalez would put his fingerprints on the tiny sensors he made and imagined them being part of something greater. Today, these same sensors are moving at 7 kilometers per second, ensuring that Diwata 2 can watch over the Philippines tirelessly as it looks forward to a future in space.

 

Ariston Gonzalez (seated, center) with members of the PHL-Microsat team prior to the launch of Diwata-2. (Photo by Misael Bacani, UP MPRO)
Ariston Gonzalez (seated, center) with members of the PHL-Microsat team prior to the launch of Diwata-2. (Photo by Misael Bacani, UP MPRO)

 

Read more on the launch of Diwata-2 HERE.