PACE: Checking Ocean Color Through the Air Above It

Looking at ocean color from orbit requires looking through something else first: the atmosphere. Sunlight changes as it interacts with material in the water, but the light reaching a satellite also encounters particles in the air. Recovering a biological signal from the ocean therefore involves distinguishing it from changes along its atmospheric path. PACE's Ocean Color Instrument offers unusually detailed spectral observations for this task. Yet a richer spectrum does not independently certify its own interpretation. Checking the view from space brings researchers back to aircraft, ships and measurements much closer to the water. [1][2]

PACE launched on February 8, 2024. On April 11, NASA announced public distribution of science-quality data after several weeks of in-orbit testing of the spacecraft and instruments. This account, with an information cutoff of October 13, 2026, examines that data release and subsequent validation work in Monterey Bay. Successful instrument operation and the accuracy of every derived environmental interpretation are different questions. The mission's technological value includes not only a more detailed view, but a measurement chain with which that view can be tested. [1][2]

More than a color name

OCI observes ultraviolet, visible and near-infrared light. NASA's initial data announcement described more than 200 detected wavelengths, compared with the handful available from previous ocean-color satellites. Hyperspectral refers to that fine division of the spectrum. What an eye describes as one color can contain different amounts of light at different wavelengths. Measuring those differences permits a more informative comparison than assigning the ocean a single label such as blue or green. [1]

One important use is distinguishing phytoplankton communities. Different communities of microscopic marine organisms play different roles in ecosystems and the carbon cycle. However, OCI is not taking close-up portraits of individual cells. Community information is inferred from an electromagnetic signal. The ability to distinguish communities does not imply certain identification of every species in every condition, or a direct measurement of the toxicity of every bloom. The additional spectral information enables biological investigation; it does not replace all the analyses that might be needed to characterize a particular event. [1]

Where the ocean signal comes from

NASA's validation account describes a quantity called water-leaving reflectance. Water molecules, phytoplankton, particles such as sand, inorganic materials and even bubbles affect absorption and scattering of sunlight. Ocean color consequently has multiple physical contributors. Biological and nonbiological influences can occur together. Treating the apparent color as an organism's exclusive fingerprint would obscure the mixture of processes that produced the measured light. [2]

The relationship between a surface view and information about the water column must also remain explicit. The measurement supports inferences about material in the water; it does not directly sample every ocean depth. More wavelengths provide more opportunities to distinguish interactions with light, but those opportunities still depend on processing and comparison. The instrument measures radiation. An ecological description is a subsequent use of that measurement, not an object delivered ready-made by the detector. [1][2]

This distinction explains why validation remains valuable after a high-quality data release. A functioning sensor can supply a rich signal while scientific work continues on how accurately derived products describe particular conditions. Returning to a coastal site is not an admission that the satellite has failed. It provides observations against which the route from measured light to an environmental interpretation can be checked. [2]

The atmosphere is inside the measurement problem

Aerosols modify light traveling through the air. The term does not mean only human-made pollution; it describes solid or liquid particles suspended in the atmosphere. Smoke, sea salt, desert dust and pollen are examples. Their optical effects have to be considered when interpreting the signal from the ocean. Putting ocean and atmospheric observations on the same mission therefore reflects the connected physics of the observation rather than an arbitrary combination of subjects. [2]

The validation team measured aerosol optical depth as part of that problem. This quantity concerns extinction of light through absorption or scattering away from the direction being considered. It is not simply total particle mass, nor is it the height of an aerosol layer. An optical measurement cannot be silently substituted for a complete chemical description of the air. Observations at different wavelengths help characterize how the atmospheric particles affect the light that the ocean-color analysis must interpret. [2]

Bringing aircraft and vessel observations together

AirSHARP was deployed to validate OCI products through simultaneous airborne and seaborne campaigns. Work took place in Monterey Bay in October 2024 and May 2025. Returning in another season allowed the team to gather data under different environmental conditions. The autumn period included an unusually large phytoplankton bloom. These contrasting conditions extend the comparison beyond a single visit, but they do not amount to complete validation of every environment around the globe. [2]

NASA Ames, the University of California, Santa Cruz, and the Naval Postgraduate School contributed complementary platforms and instruments. The Twin Otter aircraft carried the C-AIR radiometer, while measurements near the water used C-OPS, with matching technology for water-leaving reflectance. The research vessel R/V Shana Rae participated in the surface observations. Aircraft coverage can span a wider area of the bay, whereas a vessel supplies measurements close to the water. Neither platform is merely a smaller duplicate of the satellite. Each offers another position from which to check the light being interpreted remotely. [2]

The airborne 4STAR-B spectrometer measured aerosol optical depth. The comparison therefore concerned not just whether two views of the sea looked alike, but also how the atmosphere altered the light. NASA reported 13 flight days for the campaign work described. The account does not justify assigning that total exclusively to October or exclusively to May. Keeping the dates attached to their stated scope helps avoid creating a more specific operational history than the source provides. [2]

Three instruments with different questions

In addition to OCI, PACE carries the multi-angle polarimeters HARP2 and SPEXone. They measure polarized light reflected from clouds and small atmospheric particles. NASA's first data release describes SPEXone as combining hyperspectral observations with five viewing angles. HARP2 observes four wavelengths with 60 viewing angles. These values belong to their respective instruments; they should not be combined with OCI's wavelength count as if they described one detector with all the same capabilities. [1]

Polarization provides information different from brightness alone or the distribution of light across wavelengths. Together with multiple viewing angles, it expands the study of cloud and aerosol properties. Complementary measurements, however, do not turn the three instruments into a single automatic answer to every environmental question. Each observation must retain its own interpretation. In particular, the AirSHARP account identifies OCI products as its validation target, not a completed validation of every polarimeter result on the observatory. [1][2]

Validation remains a process

NASA's campaign description explains that satellite measurements will be compared with observations much closer to the ocean, with adjustments made where they do not match. It also announces archived data from the 2024 phase. These statements document data collection and the comparison process. The raw campaign data were not downloaded and independently analyzed for this article. An announcement of available measurements should not be recast as a newly calculated error budget or a guarantee that every derived product is exact. [2]

PACE's contribution is clearer when the steps between light and interpretation stay visible. Many wavelengths provide a richer signal for examining marine communities; atmospheric observations and nearer comparisons help test how that signal should be read. To see the ocean accurately, the satellite must account for the air above it. Aircraft and vessel measurements are consequently not obsolete alternatives left behind by space technology. They are parts of the same effort to turn a distant optical observation into environmental knowledge with a defensible basis. [1][2]

Sources

[1] NASA, April 11, 2024, NASA’s PACE Data on Ocean, Atmosphere, Climate Now Available.

[2] NASA, June 26, 2025, By Air and by Sea: Validating NASA’s PACE Ocean Color Instrument. Page updated March 18, 2026; campaigns described took place in October 2024 and May 2025.