Initial Release Notes

The PACE mission launched on 8 February 2024, and spacecraft and instrument commissioning completed on 5 April 2024. This initial release of science data products from PACE (Version 1) provides the science and applications user community with access to the Level-1 data and a limited suite of derived products from the OCI, HARP2, and SPEXone instruments of PACE, with the caveat that the data are in a preliminary state and should be used with caution. Frequent updates and reprocessings to incorporate post-launch calibration knowledge, algorithm refinements, and additional data products should be expected.

All data products are distributed in self-describing netCDF4 format, and are available in the following data processing levels.

• Level-1A, raw instrument data and spacecraft telemetry, reformatted to netCDF4
• Level-1B, calibrated & geolocated instrument data
• Level-1C, calibrated, geolocated, and co-registered to a common grid
• Level-2, derived geophysical science data products
• Level-3, temporally and spatially composited (binned and mapped) global products
• Level-4, geophysical products derived from combined Level-3 inputs and/or models

The Level-1C and Level-2 data files are divided into 5-minute granules that start at the southern terminator crossing of the ascending daylight orbit and end at the northern terminator crossing. For OCI, and HARP2 the Level-1A and Level-1B data are also in 5-minute granules, while SPEXone is organized into continuous daylight orbit periods (~50 minutes).

The products available at this release are categorized into maturity levels as:

• Provisional: results have been reviewed and are in family with heritage data products or other basis of expectation, but which have not yet been validated and may still contain significant errors.
• Test: results have not yet been reviewed by algorithm developers and or may be known to have substantial errors in implementation that are under investigation.
• Diagnostic: products that are produced to support analysis of algorithm behavior, but that are not intended for science.

The notes that follow provide a high-level overview on the status and known issues for the Level-1 products of each instrument and the derived science products currently available.

For a comprehensive list of the science data products planned for PACE and the current status of those products and algorithms see the PACE Data Products Table.

OCI Level-1 Data (Provisional)

OCI data has been calibrated based on prelaunch characterization measurements only. Initial analysis of the on-orbit solar calibration measurements indicates that the absolute instrument radiometric calibration is valid to within a few percent, but comprehensive assessment of all Level-1B data has not yet been performed.

Known Issues and Data Characteristics:

• The optical design of the OCI SWIR detector assembly (SDA) causes the bands to view different locations along-scan at a given time, and the data are packetized by time. The bands are pixel-shifted into alignment with the hyperspectral bands. This results in fill pixels at the start or end of the scan, depending on the required pixel shift. The worst case is the 2260 nm band that has 13 fill pixels at the start of the scan.
• Data below 340nm has not been characterized prelaunch as completely and accurately as the data above 340nm. On-orbit data will be used to fill in the pre-launch characterization gaps. The data below 340nm is only released to facilitate the assessment and improvement of the radiometric accuracy.
• Geolocation errors increase toward the edge of the swath. This will be improved in a future reprocessing through refinement of instrument/spacecraft pointing knowledge.
• There is currently a logic error in the SWIR band L1B processing that results in invalid values in the first scan line of a file. This will be corrected in a software update and the data reprocessed.
• Some SWIR bands show significant signs of apparent band-to-band registration issues. SWIR data (especially bands 1038nm, 1250nm Standard Gain, and 1615nm Standard Gain) with large radiance gradients to neighboring pixels in scan direction will be impacted.

HARP2 Level-1 Data (Provisional)

HARP2 data has been calibrated at the lab during pre-launch characterization. Initial assessment of on-orbit data shows that HARP2 requires additional calibration corrections. On-orbit data has been collected during solar and lunar calibration exercises. The data is currently being analyzed by the HARP2 team and additional corrections are expected in the near future. Data has been processed to levels 1A, 1B, and 1C.

Known Issues:

• Quantitative calibration needs additional corrections and further improvement. Current data set has a known radiance offset. Calibration improvements are being worked on by the HARP2 team.
• Onboard detector alignment can cause false polarization on bright to dark edges. Additional analysis and alignment corrections are needed to minimize noise introduced by false polarization.
• Geolocation is in need of refinement over the wide field of view. Geolocation errors increase towards the larger viewing zenith angles. This will be improved in a future reprocessing after the instrument pointing knowledge is refined and the corrections for optical distortions are improved.
• Some detector pixels present defects that appear in the pushbroom images as vertical lines. These lines will be masked by a future update to the HARP2 defect mask.

SPEXone Level-1 Data (Provisional)

The SPEXone initial data released by the PACE Project has been calibrated based on prelaunch characterization measurements and some of the first flight data. Initial analysis of the on-orbit data indicates that the SPEXone characterization derived from prelaunch measurements is valid for the on-orbit characterization of the instrument.

Known Issues:

• No comprehensive assessment of all the L1B/L1C data has been performed so far. Users should expect data to be reprocessed in the coming months as improvements to the characterization and calibration are derived.
• The current data have reduced polarimetric accuracy for wavelengths below 400 nm. The SPEXone team is investigating improvements.
• The L1C pixels at the swath edges have a reduced accuracy

OCI Level-2 Products

They are organized into product suites, with each suite in one file per granule. In some cases, the Level-2 products suites may contain a mix of Provisional, Test, and Diagnostic products.

OC_AOP, Ocean Color Apparent Optical Properties

Provisional

• Remote Sensing Reflectance (Rrs)
• Rrs Uncertainties (Rrs_unc)

Test

• Apparent Visible Wavelength (avw)

Diagnostic

• Aerosol Optical Thickness (aot)
• Aerosol Angstrom Exponent (angstrom)

Known Issues:

• No vicarious calibration has been applied.
• A bias in Rrs has been observed, including negative values in very clear waters (especially in red) and highly absorbing waters (especially in blue).
• Performance of Rrs retrieval in the UV (below 400nm) has not yet been meaningfully reviewed and is likely to contain significant biases and erroneous variability.
• Corrections for absorbing gases have been applied, but they will require further refinement. Rrs variability, especially in the red, is likely to contain residual artifacts from water vapor and oxygen absorption.
• Due to the advanced capabilities of OCI the limits of the atmospheric correction approach are being tested. Notably the current processing extends to higher view zenith angles than the heritage sensors. The atmospheric correction becomes increasingly difficult at these extreme geometries, and erroneously elevated reflectance are evident at those angles.
• Chlorophyll Fluorescence Line Height will be included in a future release.

OC_IOP (Ocean Inherent Optical Properties)

Provisional

• Total absorption coefficient (a)
• Total backscatter coefficient (bb)
• Phytoplankton absorption coefficient (aph)
• Phytoplankton absorption coefficient standard uncertainty (aph_unc)
• Non-algal particle plus chromophoric dissolved organic matter absorption coefficient (adg)
• Non-algal particle plus chromophoric dissolved organic matter absorption coefficient standard uncertainty (adg_unc)
• Particle backscattering coefficient (bbp)
• Particle backscattering coefficient standard uncertainty (bbp_unc)

  Test

  • Spectral diffuse attenuation coefficient (Kd)

  Diagnostic

  • Mean residual of model fit (rrsdiff)

  Known Issues:

  • Products are derived from Rrs. See OC_AOP: Rrs known issues.
  • Some algorithm failure and artifacts noted in highly productive and near-shore waters. Future refinement will improve these missing data.

  OC_BGC (Ocean Biogeochmical Properties)

  Provisional

• Phytoplankton Chlorophyll-a (chlor_a)
• Phytoplankton Chlorophyll-a uncertainties (chlor_a_unc)

  Test

  • Phytoplankton Carbon (carbon_phyto)
  • Phytoplankton Carbon uncertainties (carbon_phyto_unc)
  • Particulate Organic Carbon (poc)

  Known Issues:

  • Products are derived from Rrs. See OC_AOP: Rrs known issues.
  • Circular artifacts observed near subsolar point, where OCI is tilting through nadir and glint is extreme. Future refinement will improve or mask these artifacts.
  • Particulate Inorganic Carbon (pic) and poc_unc will be included in a future release.

  PAR (Photosynthetically Available Radiation)

  Test

  • Daily Mean PAR, Below-surface, Scalar (par_day_scalar_below)
  • Daily Mean PAR, Above-surface, Planar (par_day_planar_above)
  • Daily Mean PAR, Below-surface, Planar (par_day_planar_below)
  • Instantaneous PAR, Below-surface, Planar (ipar_planar_above)
  • Instantaneous PAR, Below-surface, Planar (ipar_planar_below)

  SFREFL (Surface Reflectance)

  Test

  • Spectral Surface Reflectance (rhos)

  Known Issues:

  • Data is limited to 50 spectral wavelengths between 339nm to 2260nm.
  • Contribution from aerosols is not removed.
  • Residual artifacts in the spectral vicinity of strong atmospheric gas absorption (e.g., water vapor and oxygen) is expected.

  LAND (Land Surface Indices)

  Test

  • Enhanced Vegetation Index (evi)
  • Normalized Difference Vegetation Index (ndvi)
  • Normalized Difference Water Index (ndwi)
  • Normalized Difference Infrared Index (ndii)
  • Normalized Difference Snow Index (ndsi)
  • Carotenoid Content Index (car)
  • Chlorophyll-Carotenoid Index (cci)
  • Chlorophyll Index Red Edge (cire)
  • Photochemical Reflectance Index (pri)
  • Modified Anthocyanin Reflectance Index (mari)

  Notes:

  Multi-band indices (NDVI, EVI, NDWI, NDII, CCI, NDSI) are calculated using aggregates of OCI spectral reflectances corresponding to relevant MODIS bandwidths.

  Known Issues:

  • The NDSI algorithm does not currently incorporate a snow impossible mask.
  • Contribution from aerosols is not currently removed from surface reflectance data, which may cause residual artefacts in vegetation index calculations.