Mission Overview ▸ Instrument Description ▸ Sensor and Data Characteristics
Sensor and Data Characteristics
- General Properties
- ISS Orbit
- Wavelengths
- Spectral Resolution
- Target Spatial Coordinates
- Filename Conventions
- Scale Factor
- Flags
- NASA HDF5 Files
- Launch SeaDAS (7.0 or higher).
- To load a HICO file, use File > Open Product from the main menu.
General Properties
| platform | International Space Station (ISS) |
| HICO launch | September 10, 2009 |
| HICO installed on ISS | September 24, 2009 |
| first image date | September 25, 2009 |
| last image date | September 13, 2014 |
| HICO end of operations | September 13, 2014 |
| on-orbit lifetime | one year minimum |
| orbit | near circular (see the ISS Orbit) section. |
| inclination | 51.6o |
| altitude | 343 km (varies) |
| ISS orientation | +XVV (standard forward orientation) -XVV (reverse orientation - infrequent) |
| orbit repeat time | 3 days (approximate) |
| orbit lighting cycle | 63 days |
| orbit period | 90 minutes |
| scenes per orbit | 1 maximum |
| scenes per day | 15 maximum |
| cross-track pointing | varies from -45 to +30 degrees -45: 45 degrees port (north if +XVV orientation) +30: 30 degrees starboard (south if +XVV orientation) |
| swath orientation | varies depending on the orbit path: NW to SE (descending) SW to NE (ascending) |
| ground sample distance (GSD) | 90 m (varies with altitude and angle) |
| scene size (km) | 42 x 192 km (varies with altitude and angle) |
| scene size (pixels) | 512 x 2000 pixels |
| sensor field of view (FOV) | 6.92o (i.e. +/- 3.46o from the center) covering 512 cross-track pixels |
| wavebands | 128 bands |
| wavelengths | 353 - 1080 nm |
| RGB bands | R : 638.9 nm (band 42) G : 553.0 nm (band 27) B : 461.4 nm (band 11) |
| spectral resolution | 5.7 nm (details below) |
| spectral FWHM | 10 nm (400 - 745 nm) 20 nm (746 - 900 nm) (details below) |
| signal-to-noise ratio (SNR) | > 200:1 for water-penetrating wavelengths and assuming 5% albedo |
| polarization sensitivity | < 5% (430 - 1000 nm) |
| data format | HDF5 |
| disk space required per scene | L1B: 120 MB (varies) / 230 MB (uncompressed) L2A: 400 MB (varies) / 696 MB (uncompressed) |
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 blue 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.
| 352.528 358.256 363.984 369.712 375.440 381.168 386.896 392.624 398.352 404.080 409.808 415.536 421.264 426.992 432.720 438.448 |
444.176 449.904 455.632 461.360 467.088 472.816 478.544 484.272 490.000 495.728 501.456 507.184 512.912 518.640 524.368 530.096 |
535.824 541.552 547.280 553.008 558.736 564.464 570.192 575.920 581.648 587.376 593.104 598.832 604.560 610.288 616.016 621.744 |
627.472 633.200 638.928 644.656 650.384 656.112 661.840 667.568 673.296 679.024 684.752 690.480 696.208 701.936 707.664 713.392 |
719.120 724.848 730.576 736.304 742.032 747.760 753.488 759.216 764.944 770.672 776.400 782.128 787.856 793.584 799.312 805.040 |
810.768 816.496 822.224 827.952 833.680 839.408 845.136 850.864 856.592 862.320 868.048 873.776 879.504 885.232 890.960 896.688 |
902.416 908.144 913.872 919.600 925.328 931.056 936.784 942.512 948.240 953.968 959.696 965.424 971.152 976.880 982.608 988.336 |
994.064 999.790 1005.52 1011.25 1016.98 1022.70 1028.43 1034.16 1039.89 1045.62 1051.34 1057.07 1062.80 1068.53 1074.26 1079.98 |
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.
| HYYYYDDDHHMMSS.L*_ISS | |
| H | HICO |
| YYYYDDDHHMMSS | collection datetime in UTC (year, day of year, hour, minute, second) |
| L* | processing level (L1B, L2A, ...) |
| ISS | HICO platform name |
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).
| Bit | Flag | Description |
|---|---|---|
| 0 | 1 - LAND | land (or possibly glint or clouds) (ρNIR > 0.02) |
| 1 | 2 - NAVWARN | latitude or longitude out of bounds |
| 2 | 3 - NAVFAIL | navigation is rough (currently always set to 1) |
| 3 | 4 - HISATZEN | satellite view angle > 60o |
| 4 | 5 - HISOLZEN | solar zenith angle at estimated position > 75o |
| 5 | 6 - SATURATE | pixel has ≥ 1 saturated bands |
| 6 | 7 - CALFAIL | pixel has ≥ bands from a dropped packet |
| 7 | 8 - CLOUD | rough cloud mask (ρNIR > 0.05 and ρRED > 0.5) or (0.8 < ρNIR/ρRED < 1.1) |
An example is illustrated in the table below.
| Flag example ASCII value 79 |
||||||||
|---|---|---|---|---|---|---|---|---|
| Bit number | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| Flag | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 |
| Binary 79 | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 |
| The flags set are LAND, NAVWARN, NAVFAIL, HISATZEN, and CALFAIL. | ||||||||
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.
| products | ||
| Lt | top of atmosphere radiance (W/m2/um/sr) | |
| 2000 x 512 x 128 | 16-bit unsigned integer | |
| images | ||
| true_color | true color (638.9, 553.0, 461.3 nm) | |
| 2000 x 512 x 3 | 8-bit unsigned character | |
| navigation | ||
| latitudes | latitude (degrees N) | |
| 2000 x 512 | 32-bit floating point | |
| longitudes | longitude (degress E) | |
| 2000 x 512 | 32-bit floating point | |
| sensor_azimuth | sensor azimuth (degrees) | |
| 2000 x 512 | 32-bit floating point | |
| sensor_zenith | sensor zenith (degrees) | |
| 2000 x 512 | 32-bit floating point | |
| solar_azimuth | solar azimuth (degrees) | |
| 2000 x 512 | 32-bit floating point | |
| solar_zenith | solar zenith (degrees) | |
| 2000 x 512 | 32-bit floating point | |
| quality | ||
| flags | scan-line quality flags | |
| 2000 x 512 | 8-bit unsigned character | |
| data | ||
| The "hico" dataset below was accidentally included in some of the earlier files. It is absent from more recent files. | ||
| hico | Level 0 data (W/m2/um/sr) | |
| 2000 x 512 x 128 | 16-bit unsigned integer | |
Opening NASA HDF5 Files
Software Requirements1
HICO HDF5 files are compatible with NASA SeaDAS 7.0 and higher software. With some scripting, they can also be opened using programming languages such as Fortran, IDLTM, 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
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 from http://www.hdfgroup.org/downloads. 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.