Day-in and day-out, for the past 20 years, NASA’s Aqua satellite—one of three flagship satellites in NASA’s Earth Observing System (EOS)—has dutifully maintained a sun synchronous polar orbit 705 kilometers (km) above the Earth, crossing the equator at a mean local time of approximately 1:30 pm. In fact, as of mid-day on May 4, 2022, the 20-year anniversary of the satellite’s launch, Aqua had completed 106,385 orbits of the earth.
These are noteworthy achievements, for when it comes to Earth observation data from satellites, consistency is key. Establishing a credible data record to detect changes in the Earth system over time requires satellites and satellite instruments that are reliable and stable. Aqua has not only met these requirements, it has exceeded them. The satellite has functioned largely without interruption in the harsh environment of space since it launched into orbit on May 4, 2002, and as evidenced by its two-decades-long data record, its instruments have observed the planet in the same way, from the same position almost every day since the satellite was declared operational in July of that same year.
Credit for Aqua’s remarkable longevity in space is readily given to the engineers who built the spacecraft and its instruments and to NASA’s Earth Science Mission Operations (ESMO) Project at NASA's Goddard Space Flight Center, located in Greenbelt, Maryland. ESMO is responsible for the operation and maintenance of Aqua and other EOS spacecraft along with other Earth science missions conducted by NASA’s Earth Science Projects Division (ESPD).
Yet, according to Aqua Mission Director Bill Guit, the way ESMO conducts its operations is unique to the attributes of each spacecraft and the goals of each mission.
“Operations is driven by how frequently you have to communicate with the spacecraft,” said Guit. “[On Aqua] we’re driven by the size of our on-board solid-state recorder (SSR). It can only hold about two-and-a-half orbits-worth of data, so, consequently, we have to dump those data to the ground every orbit.”
There is also a data-latency requirement with the mission such that data from Aqua’s instruments must be distributed to the National Weather Service within a matter of hours from download.
“Getting yesterday’s data is no good for tomorrow’s forecast,” Guit quipped.
Although ESMO contacts the satellite every orbit—and Aqua orbits the Earth every 99 minutes (approximately)—much of the interactions are pre-arranged with a standing schedule that’s made weeks in advance.
“You’re going to have a contact every hour-and-a-half and that contact has all been planned in advance into stored command activities aboard the spacecraft so the amount of human interaction from the console in the control room is somewhat limited,” Guit said.
Each contact starts with a member of the Flight Operations Team (FOT) sending test commands from the control room to ensure they can communicate with the satellite. Once a link is confirmed, the controller then performs a set of routine tasks.
“For the most part, the people who are sitting on-console are monitoring the health and safety of the spacecraft, managing the SSR, performing data-capture activities, looking for alarms, and making sure the daily loads (i.e., the information that tells the spacecraft where it’s going to be for the next 24 to 72 hours) go up to the spacecraft,” Guit said, “That’s usually the extent of the activities during most contacts unless there’s a special activity, such as a spacecraft maneuver to avoid orbital debris or instrument calibration activities.”
Over the years, the process of obtaining data from the satellite has become increasingly automated, both to simplify the retrievals and to minimize the chance of operator error.
“We put the onus on the ground station to capture it,” said Guit.
The phrase “ground station” refers to NASA's EOS Data Operations System (EDOS), a branch of ESMO, which performs data capture and initial (Level 0) processing for Aqua and several other satellite missions. It’s primary tasks are to:
- Capture data from the spacecraft at ground stations around the globe,
- Perform initial data analysis and processing,
- Transfer the science data to the Level Zero Processing Facility at Goddard, and
- Deliver Level 0 data products (in a variety of formats and protocols) to the NASA centers around the country and the discipline-specific Distributed Active Archive Centers (DAACs) affiliated with NASA's Earth Observing System Data and Information System (EOSDIS), for higher level processing.
Aqua’s data are captured at the satellite ground station in Svalbard, Norway, which is close enough to the North Pole that Aqua passes over the station each time it circles the globe. Although at a latitude more south than Svalbard, the Alaska Satellite Facility (part of the University of Alaska-Fairbanks) at North Pole, AK, receives Aqua data as well, but not on every orbit. Once the data are retrieved and analyzed for quality, they are pushed to Goddard in high- and low-rate data streams.
“The high-rate data include most of the science data and the low-rate data include the rest of the science data and the spacecraft engineering telemetry,” said Guit. “EDOS then gets all the science data and distributes them to the DAACs around the country or the instrument teams themselves. Then the instrument teams and the DAACs do the higher-level processing.”
If there’s a problem with the data, such as a data gap resulting from poor communication between the satellite and the ground station or an excess of low-quality data, and so on, Guit and his colleagues will schedule additional contacts with the satellite to attempt to re-capture data from Aqua within the next two and a half orbits due to the SSR’s limited storage capacity.
In addition to monitoring data dumps, FOT team members monitor the health and safety of the spacecraft and its instruments by looking out for alerts or warnings that suggest something might be amiss.
“We have a very smart ground system and a very smart spacecraft,” said Guit. “It’s able to detect most errors and raise flags.”
Flags are alerts that inform controllers when something on the spacecraft is operating outside of its typical parameters. There are two types of flags, yellow and red (the latter being more severe), and each can be high or low.
“The ground system looks at the satellite’s telemetry data and compares them to the limits predefined within our database parameters,” said Guit. “If something is supposed to be reading 5.2 volts plus or minus 0.2, but goes to 5.5, we would receive a yellow-high. Yellow lights are warnings, like the light on the dashboard of your car. For reds, we have pre-described red limit responses that would be taken in real time, and that might put the instrument in safe mode because there’s something wrong.”
If a controller sees something out of limit (i.e., sees a yellow-low or -high), he or she will start a process to address it and contact the appropriate subsystem or instrument engineer. If the alarm is more serious (i.e., red), the controller will take more significant actions.
For the bulk of Aqua’s 20 years in orbit, FOT team members haven’t had to respond to many major anomalies.
“We’re 20 years in on Aqua and we’re on prime hardware on every spacecraft subsystem, with the exception of some minor anomalies that we’ve had in the electrical power subsystem,” Guit said. “In particular, in the solar array, which is made up of strings and strings of solar cells, we’ve lost 23 of 132, but we’re nowhere near not being able to generate enough power to operate the spacecraft bus and the instruments.”
Aqua has, however, experienced some instrument losses. Its Humidity Sounder for Brazil (HSB) instrument suffered a catastrophic failure 9 months after launch and its Advanced Microwave Scanning Radiometer for EOS (AMSR-E) instrument, which was designed for three years but lasted nine, had a mechanical failure in 2011. Nevertheless, these failures did not threaten the longevity of the mission.
More significant for the mission longevity was an anomaly Aqua experienced earlier this year wherein the spacecraft’s on-board fault management system detected a problem with the primary controller of the satellite’s electrical power subsystem. The system automatically switched to the back-up controller and then put the spacecraft in emergency communications mode. Fortunately, the anomaly occurred while FOT members were in real-time contact with the spacecraft, so controllers immediately saw what happened and were able to start working on the problem. It was resolved in a matter of two weeks.
“Any time you’re not collecting science data it’s a significant anomaly. [During this anomaly], Aqua was not collecting any science data. Zero,” Guit said. “On-board fault management is the thing that protects you from the big stuff and we’ve been really fortunate over the life of the mission.”
However, Guit acknowledges that, sometimes, minor anomalies can be more difficult to address than major ones.
“Months ago, we had a very minor anomaly with our SSR,” he said. “Every two and a half hours, we were hitting a bad set of memory locations on the SSR, which prevented proper data recording, and we’d lose about 17 seconds of data. It took us more than a month to recover.”
Today, Aqua continues to obtain the science-quality data that members of the Earth science community have come rely on for their work. How long this will continue is unknown. Due to limited fuel reserves, Aqua completed all mission maneuvers related to maintaining the approximate 1:30 p.m. equatorial crossing time and 705 km orbit altitude in December 2021. Since then, it has begun drifting to later equatorial crossing times, and, by February 2023, the satellite is expected to reach, and exceed, an equatorial crossing time of 1:45 p.m.
Regardless of when the Aqua mission ends, there’s no denying that the dedicated and well-trained members of ESMO have played a major role in the mission’s longevity.
“It's very demanding work. You need to be able to pay attention to detail,” said Guit. “When everything is going well, routine operations can be boring. But as we always say, boring is good in this business. We don’t want anomalies every day.”