Far above Earth’s ocean NASA satellites peer downward to analyze what’s in the waters below. In the figurative sense, the satellites are measuring concentrations of chlorophyll and other properties, but in the literal sense, that’s not all there is to it.
“In a lot of cases, satellites basically measure light, and based on those measurements we can infer things such as the presence of chlorophyll in water,” said NASA senior support scientist and oceanographer Dr. Jim Acker. “But we’re not actually doing a direct measurement of chlorophyll. Rather, that measurement is derived from a calibration or an algorithm that's based on in situ measurements of what’s in the sea.”
These in situ measurements come from data-gathering campaigns on Earth, such as calibration cruises that use ships to take measurements at the same time a satellite is passing overhead. The data acquired by sensors aboard the satellites will be checked against the ship data to see how well the sensors are working and to learn how to properly interpret their data. In situ data also often include measurements below the water surface beyond what satellites can detect.
“Data from calibration cruises and other NASA sea-truth projects are stored in a great archive called SeaBASS,” said Acker, who has written about SeaBASS and the value of in situ data to NASA's satellite missions for oceanographic research. “Scientists are invited to put their in situ data there so others can pull them out and use them for algorithm and other work.”
SeaBASS stands for SeaWiFS Bio-optical Archive and Storage System. The archive is maintained by NASA’s Ocean Biology Processing Group (OBPG) and is the agency’s primary archive for in situ ocean and water measurements.
The SeaBASS archive includes optical measurements of oceans, lakes, and other bodies of water; phytoplankton pigment concentrations; and oceanographic and atmospheric data, such as water temperature, salinity, and nutrients. The data come from profilers, above-water radiometers, mooring stations, ships, floats, and other platforms and are used primarily for satellite sensor validation and calibration activities along with data-retrieval algorithm development.
SeaBASS was originally created to archive validation data for NASA’s Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Project, which was a small satellite launched in 1997 equipped with NASA’s first global ocean color sensor.
Sean Bailey was a member of the SeaWiFS team and now oversees SeaBASS as the manager of NASA’s Ocean Biology Distributed Active Archive Center (OB.DAAC) located at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
“We knew we needed to be able to tie what we're seeing from the satellite to what we see on in the water,” said Bailey. “It’s really hard to collect data from the ocean, and the volume of data that were available was limited. So, we wanted to try and get as much of that together in one place to allow for validation and algorithm development for SeaWiFS and other satellite sensors."
Since then, the system has grown tremendously and houses data for many projects, including the COral Reef Airborne Laboratory (CORAL), EXport Processes in the Ocean from Remote Sensing (EXPORTS), and the upcoming Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission.
“There's quite a lot of stuff in there and it's continually being updated,” said Bailey.
Bigger and Better
A big reason why SeaBASS continues to thrive and grow after more than 25 years is the spirit of foresight and innovation that the OBPG team have infused into it from the start.
For example, early in the history of SeaBASS, Bailey and then-colleague Dr. Jeremy Werdell teamed up to create a value-added dataset called the NASA bio-Optical Marine Algorithm Dataset (NOMAD). The dataset ties satellite and in situ values, such as chlorophyll concentrations, together so that researchers can use them to develop satellite data retrieval algorithms. NOMAD was used to refine and improve the chlorophyll retrieval algorithm that is still widely used today.
“If we have a coincident measurement of the variable a researcher is interested in comparing to a satellite reflectance, they can use NOMAD to create an algorithm so that the satellite can then retrieve that geophysical product that's useful for their science,” said Bailey.
Another important task the SeaBASS staff addressed was to establish a process for standardizing the data they collected.
“When I started with SeaBASS, the data provided to us by researchers came to us in a lot of different formats; it was mishmash,” said Bailey. The values were labeled inconsistently across datasets. This inconsistency in data formatting and labelling meant it was hard to compare variables, even if they were measurements made from the exact same type of instrument.
Bailey and Werdell set to defining data standards to make the data common and easier to use. They also formatted datasets to be universally accessible and transmittable. What’s more, SeaBASS now attempts to provide users with pointers to related datasets that may be available in other archives, such as the National Science Foundation’s Biological and Chemical Oceanography Data Management Office (BCO-DMO) archive. Finally, all SeaBASS datasets are assigned a permanent Digital Object Identifier (DOI) to give them a permanent presence on the internet.
Keeping PACE with Science
New technologies, missions, and scientific practices continue to push new developments in SeaBASS.
A case in point is the advent of flow cytometry technology. Flow cytometry allows researchers to take a sample of water and rapidly image individual diatoms, plankton, and other objects in the sample and identify their species. The technology can yield thousands of images in just a few minutes, creating massive amounts of very specific data. Hundreds of new data fields to properly label cytometric data have been added to SeaBASS.
The SeaBASS team is also preparing to support the validation of NASA’s PACE mission. PACE's primary sensor, the Ocean Color Instrument (OCI), is a highly advanced hyperspectral spectrometer that will be used to measure properties of light from ultraviolet to near infrared with a few channels in the shortwave infrared as well. OCI will be paired with high-performance liquid chromatography data that will allow researchers to identify the composition of ocean blooms based on subtle variations in their color with more precision than ever.
“PACE is going to give us hyperspectral data, so, there's a lot more information in that spectrum that we can tease out,” said Bailey. “If we’re imaging phytoplankton blooms, we’re hoping we can identify their species from space. But in order to do that, we need in situ pigment data to tie to the satellite measurements, and these data are going to come from SeaBASS.”
In keeping with NASA’s open science practices, data available through SeaBASS are available without restriction and only require an Earthdata Login to explore and download. This easy access aids researchers in developing new or better algorithms and creates the potential for these data to be used for novel and innovative research.
Overall, SeaBASS fills an important niche of archiving in situ aquatic data that complement or close gaps in satellite measurements. And like satellite data, aquatic in situ data are valuable and difficult to obtain, and require careful stewardship to protect and ensure their usefulness and availability.
From its start to this day, the SeaBASS team has proactively and persistently worked to make its datasets relevant, common, and accessible. Their innovative and diligent work equips scientists with the data they need to make important aquatic discoveries, whether from data collected by satellites high in orbit or deep in the water beyond where satellites can see.