Design and Heritage

HICO (Corson et al., 2008, Lucke et al., 2011) is the first spaceborne imaging spectrometer designed to sample the coastal ocean. HICO builds on almost two decades of experience developing hyperspectral imaging for the coastal ocean (Davis et al., 2006). More specifically, the HICO design is based on the PHILLS airborne imaging spectrometers (Davis et al, 2002). HICO samples selected coastal regions at 90 m with full spectral coverage (400 to 900 nm sampled at 5.7 nm) and a very high signal-to-noise ratio to resolve the complexity of the coastal ocean.

HICO was sponsored by the Office of Naval Research (ONR) as an Innovative Naval Prototype (INP), and demonstrates coastal products including water clarity, bottom types, bathymetry and on-shore vegetation maps. As an INP, HICO also demonstrates innovative ways to reduce the cost and schedule of this space mission by adapting proven PHILLS aircraft imager architecture and using Commercial Off-The-Shelf (COTS) components where possible. The HICO program was initiated in February 2006. In January 2007 HICO was selected to fly on the Japanese Experiment Module Exposed Facility (JEM-EF) on the International Space Station. Construction began following the Critical Design Review on Nov. 15, 2007. HICO was completed in July 2008 and it was integrated into the HICO and RAIDS Experimental Payload (HREP) in August 2008. HICO is integrated into HREP and flown with support and direction from DOD’s Space Test Program. HREP has also received support from NASA and JAXA as the first US experiment payload on the JEM-EF. HREP was launched on the H-2 Transfer Vehicle (HTV) Sept. 10, 2009. The HTV rendezvoused with the ISS on Sept. 17, 2009. The first HICO imagery was collected on Sept. 25, 2009. The initial one-year demonstration phase was completed Oct. 1, 2010.

HICO exceeded all its objectives as an INP and continued to operate beyond the 4-year mark. ONR provided support the first three years of operations, including the development and operation of the HICO website at Oregon State University (OSU). With the expiration of ONR funding in December 2012, NASA’s International Space Station (ISS) Program agreed to provide funding such that the operation of HICO, including the OSU HICO website and data distribution, could continue.

Calibration

HICO underwent detailed calibration and characterization at NRL-DC prior to integration and launch. The IGARSS presentation by Dan Korwan and others gives a brief overview of the process and example data. A full description of the laboratory characterization and calibration is in a 2011 paper by Bob Lucke et al., "The Hyperspectral Imager for the Coastal Ocean (HICO): Instrument Description and First Images."

References

Corson, M. R., D. R. Korwan, R. L. Lucke, W. A. Snyder and C. O. Davis, 2008, The Hyperspectral Imager For The Coastal Ocean (HICO) On The International Space Station, IEEE Proceedings of the International Geoscience and Remote Sensing Symposium, 978-1-4244-2808-3/08.

Davis, C. O., J. Bowles, R. A. Leathers, D. Korwan, T. V. Downes, W. A. Snyder, W. J. Rhea, W. Chen, J. Fisher, W. P. Bissett and R. A. Reisse, 2002, Ocean PHILLS hyperspectral imager: design, characterization, and calibration, Optics Express, 10(4): 210–221.

Davis, C. O., K. L. Carder, B.-C. Gao, Z. P. Lee, and W. P. Bissett, 2006, The Development of Imaging Spectrometry of the Coastal Ocean, IEEE Proceedings of the International Geoscience and Remote Sensing Symposium, V4: 1982-1985.

Lucke, R. L., M. Corson, N. R. McGlothlin, S. D. Butcher, D. L. Wood, D. R. Korwan, R. R. Li, W. A. Snyder, C. O. Davis, and D. T. Chen, 2011, Hyperspectral Imager for the Coastal Ocean: instrument description and first images, Applied Optics, 50(11): 1501-1516.

General Properties 

For more information, see: Lucke, R.L. et al. (2011), "Hyperspectral Imager for the Coastal Ocean: instrument description and first images," Appl. Opt. v 50(11), 1501-1516.

ISS Orbit

The International Space Station (ISS) orbital track travels from west to east. It takes approximately 90 minutes to complete each orbit, resulting in approximately 16 orbits per day. The location of the track for each orbit moves to the west as the earth rotates. It has an approximate orbital repeat of 3 days (and a 63-day lighting cycle). More information can be found in the NASA Space Station Orbit Tutorial.

The orbit has a 51.6o inclination - it only travels between 51.6 oN and 51.6 oS. Higher latitudes are not covered.

The altitude of the ISS varies over time, but is approximately 343 km above the earth's surface. Over time the ISS altitude drops due to atmospheric drag. Increased solar activity increases the drag. Reboosts are performed to adjust the orbit. More information on orbital decay can be found in the NASA Science News article "Solar S'Mores."

Wavelengths

The table below lists all 128 wavelengths collected by HICO. The wavelengths shown in bold contain optimal data. The data from the remaining wavelengths (in grey) are less accurate. The HDF5 HICO files include data from all of the wavelengths.

Spectral Resolution

HICO provides data at 5.7 nm spectral resolution. Spectral data are originally collected at 1.9 nm resolution; to increase the signal to noise ratio, three bands are combined on the detector producing wavelength centers 5.7 nm apart. The resulting wavelengths are shown in the table above.

A smoothing filter (Gaussian) is applied to the uncalibrated spectral data to fix etaloning at the longer wavelengths. The size of the filter is 10 nm for the shorter wavelengths (400 - 745 nm) and 20 nm for the longer wavelengths (746 - 900 nm). Thus the data are 10 (or 20) nm width data centered on 5.7 nm wavelength centers. The filter sizes for each wavelength can be found in the header files accompanying the data, under the parameter name "fwhm".

Target Spatial Coordinates

Multiple scenes from the same target do not cover identical spatial coordinates. This is due to a number of reasons outlined below.

Targeting is NOT (and in fact for HICO cannot be) determined based on ISS attitude and position at the instant of image acquisition.  Targeting is determined from hours to days ahead of time using a predicted ISS ephemeris and a predicted mean-attitude (i.e., the Torque Equilibrium Attitude -TEA).  Therefor, changes in the ephemeris, oscillations about the TEA, and time precision and offset issues can affect the center coordinates of each collected scene in both the along-track and cross-track directions. In some cases, the resulting spatial offset can be more than 10 km.

Changes in the ISS altitude can also affect the target spatial coverage by varying the pixel size. Atmospheric drag gradually decreases the altitude. Periodic reboosts are performed to compensate for the loss in altitude. Altitude changes are also frequently made by the ISS during docking maneuvers.

The HICO pointing angle (the angle HICO is tilted to the side to view the target) also affects the pixel size. HICO can vary its pointing angle from -45 degrees to +30 degrees. Looks at 20 degrees (for example) have larger pixel sizes than looks at nadir. When the ISS is travelling in its standard forward mode (+XVV), negative pointing angles are to the north and positive angles are to the south. The opposite is true when travelling in -XVV mode.

On rare occasions the ISS orientation is temporarily changed from its standard forward +XVV mode to the reverse -XVV mode (flying backwards). This orientation change usually happens during shuttle docking maneuvers. During the first year of HICO operations, the targeting calculations were incorrect for this infrequent -XVV mode, and the scenes in this mode were collected from the wrong places. The ISS orientation is listed in the header file.

Filename Conventions

HDF5 Files

An example NASA HDF5 HICO filename is:

H2013020173455.L1B_ISS

They follow the convention in the table below.

Scale Factor

The user must apply a scale factor to all radiance data to transform them to appropriate units.

For HDF5 Level 1b data, the scale factor is in the Metadata-Variable_Attributes-products/Lt-slope parameter. The data in the HDF5 Level 1b file must be multiplied by the slope before use. For Level 1b data the slope is currently 0.02.

Why is there a scale factor at all?

By scaling the data they can be stored more efficiently in the data file. For example, by multiplying the Level 1b radiances by 50, it is possible to store the data as two-byte signed integers.

Flags

Some rough data quality flags are provided in the Level 1b rad_flag files. There are eight flags, stored together as one byte per pixel (one flag per bit). Flags have a value of zero if they are not set (false), otherwise they are one (true). In the table below, the bit positions assume little-endian bit order (least significant bit is bit 0).

An example is illustrated in the table below.

NASA HDF5 Files

In January 2013, NASA began funding the HICO project and providing data to the public from the NASA Ocean Color website. NASA provides the data in HDF5 format to conform with the other ocean color satellite data that they manage. No wavelength or pixel cropping is done to these HDF5 files. Currently only Level 1B data are available.

The NASA HDF5 HICO data are freely available to the public, however an EOSDIS user account is required to access the data. To register for an account, please visit: https://earthdata.nasa.gov/urs/register.

The NASA Level 1B HDF5 files contain the datasets listed in the table below.

Opening NASA HDF5 Files

Software Requirements¹

HICO HDF5 files are compatible with NASA's Sea, earth, atmosphere Data Analysis System (SeaDAS) 7.0 and higher software. With some scripting, they can also be opened using programming languages such as Fortran, IDL, and MATLAB, and in atmospheric correction programs such as Tafkaa (NRL), ATREM (Atmospheric Removal Program, University of Colorado), and FLAASH (Fast Line-of-Sight Atmospheric Analysis of Spectral Hypercubes).

Scaling the Data

The user must apply a scale factor to the data to transform them to appropriate units before use.

Using SeaDAS

1. Launch SeaDAS (7.0 or higher).
2. To load a HICO file, use File > Open Product from the main menu.

Using MATLAB Programming Language

Sample matlab code is available demonstrating how to work with HICO HDF5 data in the MATLAB programming language. This code loads the data, creates a true-color (RGB) image, and plots a spectrum.

Using an HDF5 Reader

Various tools for opening and viewing HDF5 files are available. These tools allow you to see inside the files and find the names of the various datasets.

¹ Software products listed on this webpage are not meant to imply endorsement by the HICO team, the Office of Naval Research, or NASA. The products named are provided only as examples of software that may be used.

While HICO was in operation, all HICO data users were asked to attend an annual HICO team meeting to present their results and discuss HICO data and its uses and applications.

• HICO First Users Meeting - Alexandria, Virginia, Feb. 2011
• HICO Second Users Meeting - Glasgow, Scotland, Oct. 2012
• HICO Third Users Meeting - Washington D.C., May 2014

View a list of HICO team members.

An alphabetical list of targets collected by HICO and distributed through this website are shown below.

Each target covers an area 50 km wide by 200 km long. There are two possible orientations for each target (northwest to southeast, or southwest to northeast). The target view depends on whether the satellite was in the ascending or descending portion of the orbit. During the ascending portion of the orbit, the satellite travels from southwest to northeast. During the descending portion it travels from northwest to southeast.