|3.1 Sensor Overview|
Landsat 7's sensor - the Enhanced Thematic Mapper Plus (ETM+) - was built by SBRS. The sensor is a derivative of the Thematic Mapper (TM) engineered for Landsats 4 and 5, but is more closely related to the Enhanced Thematic Mapper (ETM) lost during the Landsat 6 failure. The primary performance related changes of the ETM+ over the TM's are the addtion of the panchromatic band and two gain ranges (added for Landsat 6), the improved spatial resolution for the thermal band, and the addition of two solar calibrators.
The ETM+ design provides for a nadir-viewing, eight-band multispectral scanning radiometer capable of providing high-resolution image information of the Earth's surface when operated from Landsat 7, a 3 axis stabilized spacecraft located in a near polar, sun-synchronous and circular orbit at a 705 km nominal altitude, with an orbit inclination of 98.2 degrees. The ETM+ is designed to collect, filter and detect radiation from the Earth in a swath 185 km wide as it passes overhead and provides the necessary cross-track scanning motion while the spacecraft orbital motion provides an along-track scan.
|3.2 ETM+ Design|
|Figure 3.2 ETM+ Optical Path.|
The aligned energy encounters the Primary Focal Plane (PFP), where the silicon detectors for bands 1-4 and 8 (panchromatic) are located. A portion of the scene energy is redirected from the PFP by the relay optics to the Cold Focal Plane where the detectors for bands 5, 7, and 6 are located. The temperature of the cold focal plane is maintained at a predetermined temperature of 91 ° K using a radiative cooler. The spectral filters for the bands are located directly in front of the detectors.
Figure 3.4 illustrates the relative position of all the detectors from both focal planes with respect to their actual ground projection geometry. The even-numbered detectors are arranged in a row normal to the scan direction while the odd-numbered detectors are arranged in a parallel row, off exactly one IFOV in the along scan direction. This arrangment provides for a contiguous bank of 32, 16, and 8 detectors for band 8, bands 1-5 and 7, and band 6 respectively. The detector arrays are swept left to right (forward) and right to left (reverse) by the scan mirror which covers a ground swath approximately 185 kilometers wide. With each sweep or scan an additional 480 meters (32, 16, and 8 data lines at a time) of along track image data is added to the acquired subinterval.
During a scan the actual ground observed by each band's detectors is not identical due to the horizontal spacing of detector rows within and between bands. Again referring to the ground projection illustration in figure 3.3 one should note the spacing between bands as measured in 30 meter 42.5 microradian IFOVs. Taken individually, these numbers represent a band's unique leading edge preamble that occurs before coincident data is collected by a band's forward or reverse focal plane neighbor. Taken cumulatively, these numbers represent the first order zero fill offsets that LPS uses during 0R processing to achieve image registration at the level 0 level. Other factors such as detector offsets within a band and sample timing must be considered to calculate registration offsets accurately.
|Figure 3.4 Detector Projection at the Prime Focal Plane|
Band 8 detectors rows are separated by 2 42.5 µrad IFOVs which translates to 4 15 meter samples. The band 8 odd and even detectors are sampled simultaneously, twice per minor frame (i.e. one sample). The registration offsets for the odd and even detectors will therefore always differ by four samples for both forward and reverse scans.
The detector rows within bands 1-5 and 7 are separated by 2.5 42.5 IFOVs. This seemingly curious design makes sense because the multiplexer samples the even detectors .5 IFOV later than the odd detectors within a minor frame of data. The delay effectively separates the odd and even detectors an integral multiple of IFOV's apart in sampling space. A 2 IFOV odd-to-even detector spacing is realized on forward scans while a 3 IFOV spacing occurs on reverse scans. The registration offsets for forward and reverse scans will always differ by these amounts.
The band 6 odd and even detectors are separated by 5 42.5 µrad IFOVs which translates to 2.5 band 6 samples. The odd and evens are sampled, however, in alternating minor frames which separates the odd and even detectors an integral multiple of IFOVs. A 2 band 6 IFOV odd-to-even detector spacing is realized on forward scans while a 3 band 6 IFOV spacing occurs on reverse scans. The registration offsets for forward and reverse scans will always differ by these amounts.
Over the years, different ground system engineers have characterized Landsat focal plane offsets in different ways that resulted in negative and positive offsets depending upon the forward and reverse scan directions and origin of the image grid. For Landsat 7 we have declared all shifts as postive from column 1 in the 0R image buffers. These 8 bit buffers are 3300, 6600, and 13,200 elements in size for the 60, 30, and 15 meter bands respectively.