Landsat 7

Chapter 9 - Geometric Calibration

9.1 Overview

This chapter describes the geometric characterization and calibration activities that will be performed over the life of the Landsat 7 mission using the software tools developed as part of the Landsat 7 Image Assessment System . The IAS provides the capability to routinely perform four types of geometric characterization to verify and monitor system geometric performance, and three types of geometric calibration to estimate improved values for key system geometric parameters.

The geometric characterizations include:

1.geodetic accuracy assessment to measure the absolute accuracy of Level 1Gs (systematic) corrected products;

2.geometric accuracy assessment to qualitatively and quantitatively evaluate residual internal geometric distortions within Level 1Gs images;

3.band to band registration assessment to measure and monitor the relative alignment of the eight ETM+ spectral bands; and

4.image-to-image registration assessment to measure and monitor multi-temporal image registration accuracy.

The geometric calibration capabilities provided by the IAS include:

1.sensor alignment calibration to provide improved knowledge of the geometric relationship between the ETM+ optical axis and the Landsat 7 attitude control reference system;

2.scan mirror calibration to measure and correct any systematic deviations in the ETM+ scan mirror along and across scan profiles; and

3.focal plane calibration to measure and provide improved estimates of the eight band center locations on the two ETM+ focal planes relative to the ETM+ optical axis. Techniques for measuring and estimating improved values for individual detector locations and delays are being researched and may be added to the IAS as a post-launch capability.

The most critical geometric calibration activities involve measuring and verifying the Landsat 7 ETM+ system performance during the Initial On-orbit Checkout (IOC) period using the geodetic, geometric, band-to-band, and image-to-image characterization capabilities, and to perform the initial sensor alignment calibration. Refining the pre-launch sensor alignment knowledge is critical to ensure that the Level 1Gs product geodetic accuracy specification can be met. Sufficient supporting data sets (e.g., ground control, terrain data) to perform these characterization and calibration activities must be available at launch.

The second priority during the IOC period will be to verify and, if necessary, update the pre-launch focal plane (particularly band placement) and scan mirror profile calibrations. The results of these initial calibration activities will be used to verify that the system is performing within specifications and to create an initial post-launch release of the Calibration Parameter File which can be used by the IAS or the Landsat 7 Level-1 Product Generation System (LPGS) to create Level 1G products which meet the Landsat 7 geodetic accuracy requirements.

After the IOC period, ongoing calibration activities will monitor the stability of the Landsat 7 ETM+ system's geometric performance and attempt to identify and characterize any systematic variations in the system's geometric parameters. This will include processing additional calibration scenes under a variety of acquisition conditions (e.g., orbital position, ETM+ time on) to measure the system's geometric performance as a function of time, temperature, and location.

9.2 Sensor Alignment Calibration

The goal of the sensor alignment calibration is to improve the in-flight knowledge of the relationship between the ETM+ instrument and the Landsat 7 navigation base reference. The IAS is required to estimate this alignment to an accuracy of 24 arc seconds (per axis) at least once per calender quarter. This calibration will use discrete ground control points in a set of pre-defined calibration reference scenes.

The primary challenge in alignment calibration is the need to estimate the underlying alignment trend (assumed initially to be a bias) from a series of precision correction solutions which measure a combination of orbit, attitude, and alignment errors.

Landsat 7 will have more accurate (estimated to be in the 10-50 meter range versus 133 meter accuracy for the ephemeris downlinked in the Payload Correction Data) post-pass definitive ephemeris data available for the alignment calibration test

scenes, reducing the uncertainty due to orbital errors. This precise ephemeris is provided by the GSFC Flight Dynamics Facility (FDF) upon request from the IAS. Multiple precision correction solutions will be integrated using a Kalman filter algorithm to estimate the best fit systematic alignment bias. As additional precision correction solutions are processed by the Kalman filter, the filter's estimates of the alignment biases will improve.

Periodically, an IAS analyst will decide that the alignment knowledge has changed enough to warrant generating an updated sensor alignment matrix for inclusion in the Calibration Parameter File. Initially, this decision is based on the alignment bias covariance estimates generated by the Kalman filter. A new set of CPF parameters are generated as soon as the bias estimate tandard deviation move below the 24-arc second alignment accuracy requirement threshold. During normal operations, a new lignment matrix is generated whenever a new version of the CPF was scheduled for release.

9.3 Scan Mirror Calibration

The behavior of the ETM+ scan mirror is measured and, if necessary, calibrated using the IAS scan mirror calibration capability.

This process compares a terrain corrected image to a high accuracy reference image constructed from a higher resolution source, to detect systematic deviations of the scan mirror motion from its nominal profile. The support data used to construct the terrain corrected image is used to generate test points which can be related to a particular time within a particular forward or reverse scan. By comparing these test points to the reference image and analyzing the measured deviations as a function of scan direction and scan time, it will be possible to estimate corrections to the pre-launch scan mirror profiles, if needed. Any significant deviations detected will be folded back into the Calibration Parameter File as updates to the mirror profile polynomial coefficients.

Scan mirror calibration applies to both the along and across scan directions so it will detect and compensate for Scan Line Corrector (SLC) deviations as well. In practice, SLC deviations will be indistinguishable from scan mirror deviations so we have chosen to model the deviations as part of the scan mirror motion. Detecting systematic deviations which can be attributed to mirror motion requires reference points which can be uniquely associated with individual forward and reverse ETM+ scans and which provide a good distribution of observations as a function of scan angle. The current approach to acquiring such a control reference is to use spatially accurate reference imagery for one or more calibration areas. The scan mirror calibration procedure will compare a precision and terrain corrected ETM+ panchromatic band image with the reference image constructed from USGS Digital Orthophoto Quadrangle (DOQ) data to detect within-scan mirror deviations. This involves constructing an array of points in the ETM+ scan geometry which are mapped to the output terrain corrected product. These points, with known scan number and time in scan coordinates will be correlated with the reference image to measure the (sub-pixel) residual distortion. The distortion patterns from many scans will be analyzed to detect systematic deviations from the pre-launch forward and reverse scan mirror profiles.

9.4 Focal Plane Calibration

The focal plane calibration operations involve measuring the alignment of the eight ETM+ bands to ensure that band registration accuracy meets the 0.28 pixel requirement as stated in the system specification. If the band-to-band comparisons detect any uncompensated misalignment the band placement calibration procedure will be used to update the band center location parameters in the Calibration Parameter File accordingly. Detector to detector alignment will also be monitored to ensure that image discontinuities are not introduced by using incorrect detector locations and delays in the Level 1G image resampling process.

Landsat 7 ETM+ images of focal plane calibration test sites will be used to measure and calibrate the internal alignment of the detectors on the two ETM+ focal planes. These test sites are selected based on image content rather than the availability of supporting data. Band to band registration assessment requires scenes which contain significant high spatial frequency content that is common to all eight ETM+ bands. Although it is anticipated that scenes with long linear features would be used to assess the alignment of individual detectors, detector placement calibration techniques are still under investigation and at this time are not a part of the focal plan calibration procedure.