The geometric algorithms used by Level 1 processing system at EDC were originally developed for the Landsat 7 Image Assessment System (IAS). The overall purpose of the IAS geometric algorithms is to use Earth ellipsoid and terrain surface information in conjunction with spacecraft ephemeris and attitude data, and knowledge of the Enhanced Thematic Mapper Plus (ETM+) instrument and Landsat 7 satellite geometry to relate locations in ETM+ image space (band, scan, detector, sample) to geodetic object space (latitude, longitude, and height). These algorithms are used for purposes of creating accurate Level 1 output products, characterizing the ETM+ absolute and relative geometric accuracy, and deriving improved estimates of geometric calibration parameters such as the sensor to spacecraft alignment.
|10.2 Level 1 Algorithms|
The Level 1 processing algorithms include:
1. Payload Correction Data (PCD) processing
2. Mirror Scan Correction Data (MSCD) processing
3. ETM+/Landsat 7 sensor/platform geometric model creation
4. sensor line of sight generation and projection
5. output space/input space correction grid generation
6. image resampling
7. geometric model precision correction using ground control
8. terrain correction.
The diagrams that follow describe the high-level processing flows for the IAS Level 1 processing algorithms. Figure 10.1 describes the process involved in initialization and creation of the Landsat 7 geometry model. Figure 10.2 shows the process of creating a geometric correction grid and the application of that grid in the resampling process. Figure 10.3 describes the process of refining the Landsat 7 geometry model with ground control, resulting in a precision geometry model. Figure 10.4 again describes the creation of a geometric correction grid (this time precision), and resampling with terrain correction.
Detailed algorithm descriptions for each of main process boxes in these diagrams can be found in the Landsat 7 Image Assessment System (IAS) Geometric Algorithm Theoretical Basis Document (PDF). In this document are supporting theoretical concepts and mathematics of the IAS geometric algorithms, a review of the Landsat 7 ETM+ viewing geometry, a discussion of the coordinate and time systems used by the algorithms and the relationships between them, the mathematical development of, and solution procedures for the Level 1 processing, geometric calibration, and geometric characterization algorithms and finally an examiniation of the estimates of uncertainty (error analysis) associated with each of the algorithms.
|10.3 Ancillary Data|
The Landsat 7 ETM+ geometric correction algorithms are applied to the wideband (image plus supporting PCD and MSCD) data contained in level 0R (raw) or 1R (radiometrically corrected) products. Some of these algorithms also require additional ancillary input data sets. These include:
1. Precise ephemeris from the Flight Dynamics Facility - used to minimize ephemeris error when performing sensor to spacecraft alignment calibration.
2. Ground control/reference images for geometric test sites - used in precision correction, geodetic accuracy assessment, and geometric calibration algorithms.
3. Digital elevation data for geometric test sites - used in terrain correction and geometric calibration.
4. Pre-launch ground calibration results including band/detector placement and timing, scan mirror profiles, and attitude sensor data transfer functions (gyro and ADS), to be used in the generation of the initial Calibration Parameter File.
5. Earth parameters including: static Earth model parameters (e.g., ellipsoid axes, gravity constants) and dynamic Earth model parameters (e.g., polar wander offsets, UT1-UTC time corrections) - used in systematic model creation and incorporated into the Calibration Parameter File.