Spatial Performance of Landsat 8 Instruments
During Landsat 8 prelaunch testing, performance analysis was conducted on the spatial response of the Operational Land Imager (OLI) and Thermal Infrared Sensors (TIRS). This page briefly describes how this performance was measured. Relevant data were received from Ball Aerospace for the OLI sensor, and the TIRS sensor data were collected at NASA Goddard Space Flight Center (GSFC).
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This spreadsheet provides the measured point spread function values for each spectral band in each instrument. Subsequent on-orbit analyses have not detected significant changes to OLI or TIRS spatial performance since launch.
OLI Point Spread Functions
OLI spatial performance was measured prelaunch by scanning edge targets across the OLI detectors in the along- and across-track directions.
These scans were performed in discrete steps so that the target was stationary at each sub-pixel location; so the measurements did not contain detector integration effects.
Edge response measurements were made in all spectral bands and across the full OLI field of view. The figures below respectively show sample across-track and along-track edge response profiles.
The image below displays sample across-track edge response profiles for five consecutive detectors in the first OLI sensor chip assembly (SCA01 detectors 55 through 59) in the red band. The vertical axis is normalized response (dimensionless) and the horizontal axis is target position in units of microradians. Note the sequential detector arrangement in the across-track direction.
Similarily, the image below displays sample along-track edge response profiles for five consecutive detectors in the fourth sensor chip assembly (SCA04 detectors 245 through 249) in the cirrus band. The vertical axis is normalized response (dimensionless) and the horizontal axis is target position in units of microradians. Note the aligned even detector then odd detector arrangement in this direction. OLI even and odd detectors are vertically offset/staggered, creating this effect.
The edge response measurements for each band were differentiated to construct band-average along-track and across-track line spread functions.
Detector integration effects, computed based upon the nominal detector integration times, were then analytically included in the along-track line spread functions to reflect on-orbit performance.
The OLI spatial response for each band is represented in terms of two separable along- and across-track line spread functions which can be combined to construct a two-dimensional point spread function. The line spread functions are sampled at 4 microradian intervals (2 microradians for the panchromatic band) providing approximately ten samples per pixel.
The image below displays the across-track and along-track line spread functions for OLI band 3 (green). Note the effect of detector integration in the along-track function. The OLI multi-spectral detectors integrate for approximately 85 percent of the sample time.
TIRS Point Spread Functions
TIRS spatial performance was measured prelaunch using images of a disk target approximately 16 pixels in diameter.
The TIRS focal plane consists of three sensor chips, each of which contains a two-dimensional detector array from which two science rows in each spectral band are sampled during normal imaging operations. The instrument also has a diagnostic mode in which all detector rows can be read out. It is this mode that was used for spatial characterization. Two-dimensional images of the disk target were acquired as the target was shifted in 0.2 pixel increments across the focal plane.
A sample disk image is shown below. The resulting images of the disk at a range of sub-pixel offsets were then used to estimate the parameters of a two dimensional model of the TIRS spatial response for each spectral band.
A two dimensional formulation was adopted to allow for an optical transfer function that may not be separable or symmetric. The fitted models were then used to calculate the two dimensional point spread function for each band. The TIRS point spread functions were generated in two dimensions (i.e., the functions were not assumed to be separable) at 0.1 pixel sampling, corresponding to 10 meter ground sampling.
A plot of the point spread function for the 10.8 micron band (band 10) is shown below.
