VIIRS Instruments Become More Essential As Terra and Aqua Drift from their Traditional Orbits

For more than 20 years, NASA’s Terra and Aqua satellites, launched in 1999 and 2002 respectively, have delivered stunning imagery of Earth and critical data about its atmosphere, ocean, and terrestrial surface. These data have been used in everything from assessments of aerosol content to weather prediction and provide an invaluable record of the Earth system and how it has changed over time.

Designed with a predicted life of six years, both Aqua and Terra continue to perform beyond expectations. However, recent orbital maneuvers in February 2020 and March 2021 indicate that these aging satellites will be retired in the coming years due to declining fuel reserves. When this takes place, the instruments aboard Terra—the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), the Clouds and Earth’s Radiant Energy System (CERES), the Multi-angle Imaging SpectroRadiometer (MISR), the Measurement of Pollution in the Troposphere (MOPITT), and the Moderate Resolution Imaging Spectroradiometer (MODIS)—will be turned off. Similarly, when Aqua is retired, its instruments—MODIS, CERES, the Atmospheric Infrared Sounder (AIRS) and the Advanced Microwave Sounding Unit-A (AMSU-A)—will be turned off as well.

While the loss of any space-based instrument is significant to those who rely on the data it provides, the inevitable end of MODIS will have less of an impact on the remote sensing community given that the Visible Infrared Imaging Radiometer Suite (VIIRS) instruments aboard the joint NASA/NOAA Suomi National Polar-orbiting Partnership (Suomi NPP) and NOAA-20 satellites have been in place to serve in MODIS’s stead.

Suomi NPP launched in 2011 and was designed as a bridging mission from MODIS to the operational instruments in the NOAA series. The first of these, NOAA-20 (formerly the Joint Polar Satellite System-1, or JPSS-1) was launched in 2017. Both were launched to pave the way for the expanded capabilities and high-resolution of the follow-on VIIRS instruments aboard future JPSS missions (i.e., JPSS-2, -3, and -4) that will continue providing critical Earth system observations well into the 2030s.

Origins

Although the first MODIS instrument didn’t go into space until 1999, the MODIS story begins at least a decade earlier with several heritage sensors, including NOAA's Advanced Very High Resolution Radiometer (AVHRR), which was used for meteorology and monitoring sea surface temperature, sea ice, and vegetation; NASA’s Coastal Zone Color Scanner (CZCS) and the Sea-viewing Wide Field of View Sensor (SeaWiFS), which were used to monitor biological activity in the ocean; and the Enhanced Thematic Mapper (ETM+) and Multispectral Scanner System (MSS) instruments aboard the USGS Landsat series of satellites, which were used to monitor terrestrial conditions. MODIS was designed to take measurements in the same spectral wavelengths viewed by these legacy instruments and extend their datasets, thereby promoting the necessary continuity of the essential climate variables required to advance the understanding of both long- and short-term environmental change.

MODIS possesses a unique set of attributes and capabilities for observing Earth’s atmosphere, land, and ocean. It acquires data in 36 spectral bands ranging from 0.4 to 14.4 microns, which makes it a useful tool for detecting land, cloud, snow, ice, and aerosol properties, surface and atmospheric temperatures, ocean color, ozone, water vapor, and much more. It provides imagery in 250-, 500-, and 1,000-meter resolutions, and it has a wide (2,330-kilometer) field of view that provides global coverage every one to two days. 

Terra’s orbit is timed so that its instruments pass from north to south across the equator in the morning, at 10:30 a.m. local time. Conversely, Aqua passes south to north over the equator in the afternoon, at 1:30 p.m. local time. By working in tandem to observe the same areas of Earth in the morning and the afternoon, these platforms provide scientists with a record of the changes in the atmospheric, oceanic, and terrestrial components of the Earth system and maximize opportunities for imagery that is free from clouds or optical phenomena (e.g., glare or sunglint) that can degrade image quality.

These attributes and capabilities made the MODIS instruments an “order of magnitude better” than its radiometric predecessor, the AVHRR, said Robert Wolfe, chief of the Terrestrial Information Systems Laboratory and manager of NASA’s Level 1 and Atmosphere Archive and Distribution System Distributed Active Archive Center (LAADS DAAC).

“If you look at the spectral range and the number of bands, the number of applications that we’ve focused on for MODIS, or research areas we’ve focused on for MODIS, it’s clear the people who designed MODIS really did a great job of bringing the science community together to identify the spectral bands that are needed to do the science and then building an instrument with those capabilities in mind,” he said. “I would say MODIS is a large leap ahead in terms of technology and capability.”

The significance of that leap is evident in the previously unseen phenomena scientists have been able to observe using MODIS.

“MODIS has made the first observations for determining cloud optical properties from space—how large or small cloud drops, or ice crystals are and whether clouds contain liquid water or ice—those had never been done,” said Dr. Michael King, MODIS scienct team lead. “It also allowed the first quantitative global aerosol observations or measurements over both land and ocean. Previous missions had done the ocean, but the expanded wavelengths offered by MODIS [provided] the ability to do aerosols over the land as well.”

It is also evident in the exponential increase in science quality data products spanning a wide range of Earth science disciplines.

According to figures from NASA's Earth Science Data and Information System (ESDIS) Metrics System (EMS), approximately 15 petabytes (PB) of MODIS data from both Terra and Aqua were in the EOSDIS collection at the end of 2021, making up roughly 25% of the collection's approximately 59 PB of data volume. Since 2000, the year the first MODIS data were publicly available, approximately 66 PB of data have been distributed to global data users. The distribution of data from MODIS remains the highest of any instrument in the EOSDIS collection, with 14 PB distributed during the 2021 fiscal year. MODIS data are available through several NASA's discipline-specific Distributed Active Archive Centers (DAACs).

“When we began to see it grow from only climate modelers using the data to everyone using the data, the products and services began to explode,” said Dr. Miguel Román, chief climate scientist with Leidos and land discipline lead for the MODIS and Suomi NPP VIIRS science teams. “We ended up splitting MODIS data across multiple DAACs and enhancing the data feeds to provide near real-time products through NASA’s Land, Atmosphere Near real-time Capability for [Earth observation] (LANCE). Eventually, we had a diverse international community of scientists and applied practitioners built on this one instrument.”

In the view of Dr. Christopher Justice, a professor in the Department of Geographical Sciences at the University of Maryland, project scientist for NASA's Land-Cover and Land-Use Change (LCLUC) Program, and past land discipline lead for the MODIS and Suomi NPP VIIRS science teams, MODIS’ expansive user community is directly related to the development of standardized MODIS data products, which freed users from the burden of having to generate data products themselves. The MODIS products have improved over time through algorithm enhancements and successive reprocessing of the complete archive.

“That product generation became a hallmark of MODIS and a major contributor to its success in terms of data uptake and use,” he said. “People were able to use peer-reviewed, standard products that are well documented and published and so they could reference them and use them for science and a wide array of applications.”

Most important of all, perhaps, is the impact MODIS’ two-decade-long data record has had on the scientific community’s understanding of the climate and climate change.

“MODIS has an incredible legacy. As of this year, Aqua will be completing its 20th year in orbit and Terra has already completed 22 years. Having over two decades of data from the morning and afternoon instruments together has revolutionized our understanding of the Earth as a system, giving us a much better understanding of the climate,” said Wolfe. “These longer data records have given us a really good understanding of more long-term processes and allowed us to separate the decadal effects from the yearly changes.”

Bryan Franz, ocean discipline lead for the MODIS and VIIRS science teams, agrees.

“The modern record of satellite-based ocean biology and biogeochemistry (i.e., ocean color) started with SeaWiFS in 1997 and continued with MODIS,” he said. “This has allowed us to track the long-term variability and global distribution of marine phytoplankton, understand their seasonal cycles, and measure their response to climate-driven changes in the world’s ocean.”

Now that Aqua and Terra will begin drifting from their original orbits, VIIRS will extend the data records MODIS continued from the heritage sensors like the AVHRR and SeaWiFS.

“The MODIS instrument was meant as a research instrument that would be used to develop the next operational NOAA instrument,” said Wolfe. “The idea was NASA would build a research instrument, identify what you can do with it, and then NOAA would either build a similar instrument or use that information to decide what the next-generation instrument would be. That instrument is VIIRS.”

Differences Large and Small

Although VIIRS and MODIS fulfill the same role and serve the same function, they have notable differences. VIIRS has fewer spectral bands—22 to MODIS’ 36, a more consistent spatial resolution throughout a scan, and offers finer overall resolution with 15 bands at 750 meters and seven bands at 375 meters. Just how significant these differences are depends on the capabilities required by each Earth science discipline.

“In terms of aerosols, VIIRS is not really missing anything that MODIS had with the exception of the water vapor band around 940 nanometers,” said Dr. Robert Levy, research physical scientist in NASA's Climate & Radiation Laboratory and the lead of the Dark Target aerosol retrieval algorithm team. “It lacks a couple of thermal infrared bands used in tests, which we call cloud masking, where we use different bands, like thermal infrared, and different combinations of ratios in different bands to differentiate between clouds and other phenomena. So, the cloud masking is a little bit weaker for VIIRS and we can see that. It creates a little bit more noise in the product.”

The reduced number of VIIRS spectral bands presents challenges for the ocean color community as well.

“VIIRS is missing some channels in the red [wavelength of light], and without them we can’t measure the phytoplankton chlorophyll fluorescence signal, which is a valuable indicator of phytoplankton health and nutrient stress,” said Franz.

However, he acknowledged that these limitations can be overcome with data from other missions, such as the upcoming Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) platform. For researchers studying other aspects of the Earth system, such as land processes, VIIRS’ capabilities are an improvement.

“VIIRS has some upgrades that are a major step forward in the state of the art and the state of technology with respect to observations of critical phenomena like wildfires and city lights,” said Román. “The 375-meter channel on VIIRS allows us to detect a larger frequency of thermal anomalies—anomalies not caught by the MODIS thermal channels, which have a coarser resolution.”

Another characteristic of VIIRS that distinguishes it from MODIS is its Day-Night Band, which allows for observations of light emission sources under varying conditions. The same capability was available from the Operational Linescan System (OLS) sensor aboard Defense Meteorological Satellite Program (DMSP) platforms, but VIIRS improves on it with higher spatial resolution and radiometric accuracy.

“I would argue, the most important new capability that VIIRS provides is the Day-Night Band. Since the release of NASA’s Black Marble product, the scientific community has exploded in terms of the new research and applications on light pollution, illegal fishing, disaster impacts and recovery, and human settlements and their associated energy infrastructure,” Román said. “NASA’s Black Marble product suite has also helped diversify and increase the number of end users who did not initially see MODIS as a major source of data to address their questions, especially around new strategies to track sustainable development from a global perspective.”

Yet, the most significant and far-reaching difference between MODIS and VIIRS may have less to do with capability than the orbits in which Terra, Aqua, Suomi NPP, and NOAA-20 travel. Of the four satellites, only Terra is in the morning orbit and, currently, no NASA or NOAA satellite is slated to assume Terra’s orbit when it retires. This is a concern to the remote sensing community given that in some parts of the world there are fewer clouds in the morning over land, which makes the land and whatever may be occurring on it easier to see by optical instruments.

“If you look at diurnal effects like fire regimes, having early morning measurements really help in terms of understanding the diurnal cycle of fires,” said Wolfe. “Not having a VIIRS instrument in the a.m. orbit is going to have a significant impact on the ability for some science to be done for understanding that early morning regime and also understanding some of the diurnal effects.”

Román concurred and noted that data from the morning orbit is also critical for fire prevention.

“Early fire detection and building data systems to deliver low-latency, high-quality data to fire managers as soon as possible has been one of the biggest contributions that NASA has made to society,” he said. “If you look at NASA's Fire Information for Resource Management System (FIRMS) and other near real-time applications, they are built upon the need to handle early morning data and provide [these data] to decision-makers.”

Not having a MODIS-like instrument in the morning orbit will likely have an effect on atmospheric observations as well.

“Over the ocean, we do not expect diurnal dependence with aerosols. Where you do have diurnal dependence is over land, where you have a valley and so there are altitude differences and things move up and down vertically. Those are places where we expect to observe diurnal differences between morning and afternoon,” said Levy. “In most of the rest of the world, the issue comes back to coverage, where clouds can be different in the morning and afternoon. The ability to have [morning and afternoon observations] gives you a better chance of seeing a clear retrieval on a particular day.”

There are, of course, good reasons the Suomi NPP and NOAA-20 platforms travel in an afternoon orbit: data continuity for weather prediction. NOAA has a prime requirement to provide data for weather forecasting, so having a backup platform should a problem arise with the primary platform is of great importance.

“The entire suite of [platforms] from the JPSS era will, for the foreseeable future, be in the afternoon because the American economy depends on the five-day weather forecast,” Román said. “You must feed data from not just VIIRS, but from the key workhorse weather instruments aboard Suomi NPP and NOAA-20, like the microwave instrument sounders, into the European and NOAA [weather] models to tell us if there is going to be four inches of snow tomorrow. From a leadership perspective, we try to balance the benefits of maintaining an early morning capability with the operational needs for observations.”

One way to address the gap in morning orbit is to take advantage of the measurements and observations from the satellites operated by NASA’s international partners, ESA (European Space Agency) and the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT).

“NOAA has a relationship with their European equivalent, EUMETSAT, which also has an observing system. In 2023, they’ll launch the METimage instrument (a multi-spectral imaging passive radiometer) aboard the operational Metop-Second Generation [platform] series that will run from 2024 to 2031,” said Justice. “METImage has 20 spectral bands at 500-meter resolution and a 9:30 [a.m. equator] crossing time, which is close to the MODIS Terra overpass. So, one option is that we could have [a data-sharing] arrangement with these agencies and for NASA to establish continuity products with data from the European a.m. platform.”

Data from ESA’s Sentinel-3A and -3B platforms, which launched in 2016 and 2018, respectively, could also provide data from the morning orbit.

“ESA is contributing by way of the Sentinel-3 [platforms], which have a 10:00 a.m. orbit,” said Román. “It’s 30 minutes earlier, so there are some delicacies in trying to match that data with Terra MODIS, but it’s useful; only the data are not managed by a NASA mission. Thanks in part to NASA’s early championing of open access and open data during the MODIS-era, data from Sentinel-3 and other non-U.S. [instruments] are now freely accessible to the global science community.”

Ensuring Product Continuity

MODIS will exit NASA’s A-Train in September 2023 as the orbital equator overpass will have drifted past its lower limit of 10:15 a.m. The withdrawal of MODIS Terra from collecting data won’t take place for several years, but the planning to address this gap in observation is already underway. So too are efforts to ensure the continuity of MODIS data products prior to the transition to VIIRS.

“We need to help users understand what’s happening in terms of the transition. They need to be aware that it’s already happening, that orbits are changing,” said Wolfe. “People who’ve been using MODIS products all these years and are happy with MODIS products will need to transition to VIIRS if they want to continue their science or their applications.”

Wolfe contends that, for most algorithms and most products, that transition will be “fairly painless” because many of the same people who worked on the MODIS products, or their successors in the same working groups, have also been working on products from VIIRS.

Román agreed.

“The same people who have developed the MODIS science data products are now transitioning them to VIIRS. The majority of them are employing similar algorithms, have fine-tuned them down to the file specifications, accounting for changes in the VIIRS calibration, and all the software tools necessary to ensure consistency between the products,” he said. “We’re right around that stage where you can use VIIRS data from Suomi NPP or NOAA-20 or Aqua MODIS data because they’re in the same afternoon orbit."

The Ocean Biology Processing Group at NASA’s Goddard Space Flight Center has been able to distribute a suite of ocean color products from VIIRS that are consistent in content and format with the MODIS products, albeit with some exceptions.

“Generally speaking, in terms of continuity, the products that we derive from VIIRS are created with common algorithms and common calibration methods, which helps ensure the continuity of the long-term [MODIS] record into the VIIRS era,” said Franz.

“The largest issue to date has been the temporal calibration of the VIIRS instrument on Suomi NPP due to some pre-launch damage to the primary mirror that made the instrument less stable than MODIS. The changes were pretty severe in the beginning and have settled out to some degree, but that’s been a challenge for continuity.”

Franz noted, however, that the VIIRS instrument aboard NOAA-20 appears to be very stable and can likely provide a consistent record going forward.

“The promise of future versions of that [instrument] is very appealing,” he said.

For Levy and his colleagues in the aerosol community, the VIIRS instrument aboard Suomi NPP presented different challenges.

“VIIRS on Suomi NPP appears to have a slight causative or high bias in some of the visible bands,” said Levy. “So, the question is, do we use it as is and live with the results, do we try to correct them upstream, or do we try and correct them within our algorithms?”

Levy and his colleagues are awaiting the release of Version 2 of VIIRS calibration data, as they were curious to see whether it would reduce the bias.

“We think that if we can find the calibration that works then we could stitch the records together very well,” he said.

For members of the land community, product continuity seems less of a concern.

“We did some studies, and the early assessment is that the VIIRS instrument provides continuity with MODIS for land science and applications. The [VIIRS] instrument isn’t the same and there are some differences, but in terms of the product continuity, it’s not a big issue,” Justice said. “One of the things that needed more work was differences due to the scan angle observation and the frequency of those differences, because if you look at the coverage it’s really rather different. But it’s not enough to really impact the utilization. There was some concern earlier that we would need to have an overlap between the instruments to figure out the continuity and people were talking about having two or three years of overlap. But we’ve had 10 years now for overlap, so the intercomparison of the instrument is quite mature.”

Efforts to ensure the continuity of the near real-time (NRT) MODIS products available from LANCE have been underway for some time as well.

“The NRT products are largely based off of the science products” said Justice. “They’ve been streamlined a little bit to allow rapid processing, but they’re basically using the same algorithm as the standard product. The NRT data from LANCE is meant for time-sensitive applications, such as fire monitoring. We’ve sacrificed a little bit of information with the NRT product, while the standard fire product is available for science studies.”

Indeed, the VIIRS NRT products currently available in LANCE provide continuity from MODIS products used for a range of applications, including fire detection; air quality, agricultural, and surface monitoring; and snow and sea ice cover.

“A lot of the transition to what I call the essential capabilities provided by LANCE, especially fire detection and agricultural monitoring, have not only been fully transitioned but also fully vetted through operational communities and partners involved in international work,” said Román.

Beyond the NRT products, impacts to the user community should be small, said Justice.

“If you talk about the sort of standard products that are used—the surface reflectance, the vegetation indices, leaf-area index—I think there’s enough of a continuity there for users, so I don’t think it’s significant enough to impact their current use.”

There are, however, some minor issues that users should be aware of, such as the difference in file formats between MODIS and VIIRS.

“MODIS data have been processed in HDF format and it’s going to go to NetCDF format,” said King. “VIIRS is already done in NetCDF, so this may impact people depending on the codes they use to analyze data, but people who are savvy in this sort of thing should be fine.”

In addition to changes in data format, users may also encounter changes in resolution and size.

“There are differences in data size and resolution and things like that, at least for the Level 2 products, and it may be tricky to merge data because of the different break-ups in terms of products times,” said Levy. “It’s more than just the quality of the product, it’s also how the data are archived and where the data are processed and what users need to read [the data]. So, users may need different tools.”

Nevertheless, Román believes there is a good balance between what will be given up in the transition from MODIS to VIIRS and what is being gained in terms of advancements and new capabilities.

“Are people going to miss MODIS? Some will, but they’re just going to miss the nostalgia of the good-old days,” he said. “I think VIIRS is going to deliver. The people who are using the data and doing science, they are going to be just fine.”