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The National Aeronautics and Space Administration (NASA)’s Operation IceBridge (OIB) was a 13-year (2009–2021) airborne mission to survey land and sea ice across the Arctic, Antarctic, and Alaska. Here, we review OIB’s goals, instruments, campaigns, key scientific results, and implications for future investigations of the cryosphere. OIB’s primary goal was to use airborne laser altimetry to bridge the gap in fine-resolution elevation measurements of ice from space between the conclusion of NASA’s Ice, Cloud, and land Elevation Satellite (ICESat; 2003–2009) and its follow-on, ICESat-2 (launched 2018). Additional scientific requirements were intended to contextualize observed elevation changes using a multisensor suite of radar sounders, gravimeters, magnetometers, and cameras. Using 15 different aircraft, OIB conducted 968 science flights, of which 42% were repeat surveys of land ice, 42% were surveys of previously unmapped terrain across the Greenland and Antarctic ice sheets, Arctic ice caps, and Alaskan glaciers, and 16% were surveys of sea ice. The combination of an expansive instrument suite and breadth of surveys enabled numerous fundamental advances in our understanding of the Earth’s cryosphere. For land ice, OIB dramatically improved knowledge of interannual outlet-glacier variability, ice-sheet, and outlet-glacier thicknesses, snowfall rates on ice sheets, fjord and sub-ice-shelf bathymetry, and ice-sheet hydrology. Unanticipated discoveries included a reliable method for constraining the thickness within difficult-to-sound incised troughs beneath ice sheets, the extent of the firn aquifer within the Greenland Ice Sheet, the vulnerability of many Greenland and Antarctic outlet glaciers to ocean-driven melting at their grounding zones, and the dominance of surface-melt-driven mass loss of Alaskan glaciers. For sea ice, OIB significantly advanced our understanding of spatiotemporal variability in sea ice freeboard and its snow cover, especially through combined analysis of fine-resolution altimetry, visible imagery, and snow radar measurements of the overlying snow thickness. Such analyses led to the unanticipated discovery of an interdecadal decrease in snow thickness on Arctic sea ice and numerous opportunities to validate sea ice freeboards from satellite radar altimetry. While many of its data sets have yet to be fully explored, OIB’s scientific legacy has already demonstrated the value of sustained investment in reliable airborne platforms, airborne instrument development, interagency and international collaboration, and open and rapid data access to advance our understanding of Earth’s remote polar regions and their role in the Earth system.

For three decades airborne laser-induced fluorescence has demonstrated value for chlorophyll biomass retrieval in wide-area oceanic field experiments, satellite validation, and algorithm development. A new chlorophyll biomass retrieval theory is developed using laser-induced and water Raman normalized fluorescence of both (a) chlorophyll and (b) chromophoric dissolved organic matter (CDOM). This airborne lidar retrieval theory is then independently confirmed by chlorophyll biomass obtained from concurrent (1) ship-cruise retrievals, (2) satellite inherent optical property (IOP) biomass retrievals, and (3) satellite standard band-ratio chlorophyll biomass retrievals. The new airborne lidar chlorophyll and CDOM fluorescence-based chlorophyll biomass retrieval is found to be more robust than prior lidar methods that used chlorophyll fluorescence only. Future research is recommended to further explain the underlying influence of CDOM on chlorophyll production. OCIS codes: 010.4450, 120.0280, 280.3420, 300.2530, 300.6360.

In the upper layer of the global ocean, 2082 in situ chlorophyll biomass values are retrieved by concurrent satellite-derived inherent optical properties (IOP). It is found that (1) the phytoplankton absorption coefficient IOP alone does not provide satisfactory retrieval; (2) the chromophoric dissolved organic matter (CDOM) absorption coefficient IOP must also be used to obtain satisfactory retrieval through [varies] [a.sub.ph] + [pa.sub.CDOM] where p is a constant and [a.sub.ph] and [a.sub.CDOM] are, respectively, the phytoplankton and CDOM absorption coefficients; (3) the IOP-based retrieval performance is comparable to standard satellite reflectance ratio retrievals (that have CDOM absorption intrinsically embedded within them); (4) inclusion of the total backscattering coefficient IOP does not contribute significantly to retrieval; and (5) the new IOP-based algorithm may provide the possibility for future research to establish the actual role of extracellular CDOM from all sources in the intracellular production of chlorophyll biomass. OCIS codes: 010.4450, 280.0280, 300.6550.

The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra spacecraft contains spectral bands that allow retrieval of solar-induced phytoplankton chlorophyll fluorescence emission radiance. Concurrent airborne laser-induced (and water-Raman normalized) phytoplankton chlorophyll fluorescence data is used to successfully validate the MODIS chlorophyll fluorescence line height (FLH) retrievals within Gulf Stream, continental slope, shelf, and coastal waters of the Middle Atlantic Bight portion of the western North Atlantic Ocean for 11 March 2002. Over the entire ~480-km flight line a correlation coefficient of [r.sup.2] = 0.85 results from regression of the airborne laser data against the MODIS FLH. It is also shown that the MODIS FLH product is not influenced by blue-absorbing chromophoric dissolved organic matter absorption. These regional results strongly suggest that the FLH methodology is equally valid within similar oceanic provinces of the global oceans. OCIS codes: 010.4450, 120.0280, 280.3640, 280.3420, 300.2530, 300.6360.

We report spatial variability of oceanic phycoerythrin spectral types detected by means of a blue spectral shift in airborne laser-induced fluorescence emission. The blue shift of the phycoerythrobilin fluorescence is known from laboratory studies to be induced by phycourobilin chromophore substitution at phycoerythrobilin chromophore sites in some strains of phycoerythrin-containing marine cyanobacteria. The airborne 532-nm laser-induced phycoerythrin fluorescence of the upper oceanic volume showed distinct segregation of cyanobacterial chromophore types in a flight transect from coastal water to the Sargasso Sea in the western North Atlantic. High phycourobilin levels were restricted to the oceanic (oligotrophic) end of the flight transect, in agreement with historical ship findings. These remotely observed phycoerythrin spectral fluorescence shifts have the potential to permit rapid, wide-area studies of the spatial variability of spectrally distinct cyanobacteria, especially across interfacial regions of coastal and oceanic water masses. Airborne laser-induced phytoplankton spectral fluorescence observations also further the development of satellite algorithms for passive detection of phytoplankton pigments. Optical modifications to the NASA Airborne Oceanographic Lidar are briefly described that permitted observation of the fluorescence spectral shifts.

Airborne laser-induced (and water Raman-normalized) spectral fluorescence emissions from oceanic chlorophyll were obtained during variable downwelling irradiance conditions induced by diurnal variability and patchy clouds. Chlorophyll fluorescence profiles along geographically repeated inbound and outbound flight track lines, separated in time by [approximately] 3-6 h and subject to overlying cloud movement, were found to be identical after corrections made with concurrent downwelling irradiance measurements. The corrections were accomplished by a mathematical model containing an exponential of the ratio of the instantaneous-to-average downwelling irradiance. Concurrent laser-induced phycoerythrin fluorescence and chromophoric dissolved organic matter fluorescence were found to be invariant to downwelling irradiance and thus, along with sea-surface temperature, established the near constancy of the oceanic surface layer during the experiment and validated the need for chlorophyll fluorescence quenching corrections over wide areas of the ocean.

The complete design and flight test of the next-generation Airborne Oceanographic Lidar (AOL-3) is detailed. The application of new technology has allowed major reductions in weight, volume, and power requirements compared with the earlier AOL sensor. Subsystem designs for the new AOL sensor include new technology in fiber optics, spectrometer detector optical train, miniature photomultiplier modules, dual-laser wavelength excitation from a single small laser source, and new receiver optical configuration. The new design reduced telescope size and maintained the same principal fluorescence and water Raman bands but essentially retained a comparable measurement accuracy. A major advancement is the implementation of single-laser simultaneous excitation of two physically separate oceanic target areas: one stimulated by 532 nm and the other by 355 nm. Backscattered fluorescence and Raman signals from both targets are acquired simultaneously by use of the same telescope and spectrometer-detector system. Two digital oscilloscopes provide temporal- and depth-resolved data from each of seven spectral emission bands. [C] 2001 Optical Society of America OCIS codes: 280.0280, 010.3640, 010.4450.

Oceanic radiance model inversion methods are used to develop a comprehensive algorithm for retrieval of the absorption coefficients of phycourobilin (PUB) pigment, type I phycoerythrobilin (PEB) pigment rich in PUB, and type II PEB deficient in PUB pigment (together with the usual 'big three' inherent optical properties: the total backscattering coefficient and the absorption coefficients of chromophoric dissolved organic matter (CDOM)--detritus and phyoplankton). This fully modeled inversion algorithm is then simplified to yield a hybrid modeled--unmodeled inversion algorithm in which the phycoerythrin (PE) absorption coefficient is retrieved as unmodeled 488-nm absorption (which exceeds the modeled phytoplankton and the CDOM-detritus absorption coefficients). Each algorithm was applied to waterleaving radiances, but only hybrid modeled-unmodeled inversions yielded viable retrievals of the PE absorption coefficient. Validation of the PE absorption coefficient retrieval was achieved by relative comparison with airborne laser-induced PEB fluorescence. The modeled--unmodeled retrieval of four inherent optical properties by direct matrix inversion is rapid and well conditioned, but the accuracy is strongly limited by the accuracy of the three principal inherent optical property models across all four spectral bands. Several research areas are identified to enhance the radiance-model-based retrievals: (a) improved PEB and PUB absorption coefficient models, (b) PE spectral shifts induced by PUB chromophore substitution at chromophore binding sites, (c) specific absorption-sensitive phytoplankton absorption modeling, (d) total constituent backscattering modeling, (e) unmodeled carotinoid and phycocyanin absorption that are not now accounted for in the chlorophyll-dominated phytoplankton absorption coefficient model, and (f) iterative inversion techniques to solve for six constituents with only five radiances. Although considerable progress has been made toward the satellite recovery of PE absorption, the maturity of the retrieval is presently insufficient for routine global application. Instead it must currently be used on a regional basis where localized ship and aircraft validation can be made available. The algorithm was developed for the MODIS (Moderate-Resolution Imaging Spectroradiometer) sensor but is applicable to any sensor having comparable band locations. OCIS codes: 010.4450, 280.0280, 300.6550, 030.5620.

The exchange of trace gases between the Earth's surface and its atmosphere drives atmospheric composition. Airborne eddy covariance can provide observational constraints on surface fluxes at regional scales, helping to bridge the gap between top-down and bottom-up flux estimates and offering novel insights into biophysical and biogeochemical processes. The NASA Carbon Airborne Flux Experiment (CARAFE) utilizes the NASA C-23 Sherpa aircraft with a suite of commercial and custom instrumentation to acquire fluxes of carbon dioxide, methane, sensible heat, and latent heat at high spatial resolution. Key components of the CARAFE payload are described, including the meteorological, greenhouse gas, water vapor, and surface imaging systems. Continuous wavelet transforms deliver spatially-resolved fluxes along aircraft flight tracks. Flux analysis methodology is discussed in depth, with special emphasis on evaluation of uncertainties and vertical flux divergence. CARAFE has successfully flown two missions in the Eastern U.S. in 2016 and 2017, quantifying fluxes over forest, cropland, wetlands, and water. Results from these campaigns highlight the performance of this system and its potential to further our understanding of ecosystem exchange. [ABSTRACT FROM AUTHOR]