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Landsat ETM+ Calibration
8.1 Radiometric Calibration
8.1.1 Overview

A major objective of the Landsat-7 program is to upgrade the radiometric quality of the data to be commensurate with the other sensors in the Earth Observing System (EOS). Unlike its predecessors, a specific goal of the Landsat-7 program is to achieve radiometric calibrations of the data to ± 5% uncertainty over the 5 year life of the mission. Pre-launch, the mission design supports this requirement through hardware design changes, and instrument characterizations. Post-launch or on-orbit, this 5% requirement is supported by a monitoring and calibrations program, and the implementation of any necessary changes to the ground processing of the data.

8.1.2 Pre-Launch

8.1.2.1 Spectral Characterization

The measured wavelength locations of the ETM+ spectral bands are compared to Landat 5's TM in Table 8.1.1 The spectral bandwidths are determined by the combined response of all optical path mirrors (i.e. primary, secondary, scan line corrector, scanning), the spectral filters, and the individual detectors. The spectral filters, located immediately in front of each detector array, are the dominant items that establish the optical bandpass for each spectral band. The prime focal plane assembly has a filter housing that contains filters for bands 1 through 4 and the panchromatic band. The cold focal plane assembly has a filter housing that contains filters for bands 5 through 7.

Table 8.1.1 TM and ETM+ Spectral Bandwidths
Bandwidth (µ) Full Width - Half Maximum
Sensor Band 1
Plot Data
Band 2
Plot Data
Band 3
Plot Data
Band 4
Plot Data
Band 5
Plot Data
Band 6
Plot Data
Band 7
Plot Data
Band 8
Plot Data
TM 0.45 - 0.52 0.52 - 0.60 0.63 - 0.69 0.76 - 0.90 1.55 - 1.75 10.4 - 12.5 2.08 - 2.35 N/A
ETM+ 0.45 - 0.52 0.53 - 0.61 0.63 - 0.69 0.78 - 0.90 1.55 - 1.75 10.4 - 12.5 2.09 - 2.35 .52 - .90

A discrete spectral shift occurred on Landsat 5 TM has been largely attributed to filter outgassing. The ETM+ filters were made using a process called ion assisted deposition (IAD) which presumably makes the filters resistant to this phenomenon. In addition, the new filters have shown significant improvement in band edge responses as compared to Landsats 4 and 5.

8.1.2.2 Radiometric Calibration

Reflective Band Calibration and Monitoring
Two spherical integrating sources (SIS) were used to calibrate the ETM+ prior to launch. The first - a 100 cm source (SIS100) is equipped with 18 200-watt lamps; 6 45-watt lamps, and 10 8-watt lamps. It provides radiance levels covering the full dynamic range of the instrument in all bands, and at least 10 usable radiance levels for each band for each gain state. The SIS100 was used to perform the primary radiometric calibration of the ETM+ in August 1997 and was also used for the pre-launch calibration of AM-1's Moderate Resolution Imaging Spectroradiometer (MODIS). The second source is a 122 cm (48") SIS with 6 200-watt lamps; 2 100-watt lamps, and 4 25-watt lamps. The SIS48 was used for monitoring the radiometric calibration of the ETM+ five times during instrument and spacecraft level testing. During SIS calibrations the Bench Test Cooler (BTC) was used to maintain the temperature of the Cold Focal Plane at 105°K. This was the only one of the three temperature set points for the cold focal plane that could be obtained in ambient pressure and temperature conditions.

The calibration data reduction is performed as follows:

Bands 1,2,3,4,5,7 Calibration Equations

The slopes of these regression lines are the responsivities or gains, (G(d,b), and the intercepts are the biases, B(d,b). The Landsat Project Science Office (LPSO) will review the various integrating sphere calibrations and their effective transfer to the ETM+ before deciding which calibration should go the the IAS to represent the pre-launch IAS.

Thermal Band Calibration
The radiometric calibration of band 6, the thermal band, is fundamentally different than the reflective bands as the instrument itself contributes a large part of the signal. A model of this temperature dependent instrument contribution has been developed by SBRS. The calibration for band 6 is formulated as:

Band 6 Calibration Equations

The pre-launch calibration of band 6 is primarily a calibration of this model. The radiometric calibration of the thermal band occurs during thermal vacuum testing. During this test the ETM+ is aligned to the Thematic Mapper Calibrator (TMC), a collimator with selectable sources at its focus. During the band 6 calibration, blackbody sources will be used in the TMC. The band 6 detectors' responses to combinations of various TMC blackbody and instrument temperatures are used to calibrate the instrument and to refine emitted radiance contributions from various internal ETM+ components. The results of this calibration are nominal gains and biases for band 6, and the emissivity adjusted view factors (a(j)) for the various internal components of the ETM+ that affect the band 6 calibration. The gains and biases are included in the CPF as pre-launch values for band 6.

8.1.3 Post-Launch

The post-launch radiometric calibration of the ETM+ is accomplished by regularly examining the instrument's response when illuminated by known sources that are relatively stable. The ETM+ has 3 on-board calibration devices, namely, the Internal Calibrator (IC), the Partial Aperture Solar Calibrator (PASC), and the Full Aperture Solar Calibrator (FASC). The IC is useful for calibrating all ETM+ bands, while the PASC and FASC are mainly useful for the reflective bands. Changes to the ETM+ calibration have occurred since launch and can be viewed using this graphical timeline.

Ground look calibrations are occasionally performed to confirm, via independent analysis, the accuracy of the calibration using on-board sources.

8.1.3.1 Internal Calibrator

The IC consists of a shutter flag, 2 tungsten lamps, and a blackbody source. The shutter flag, located immediately in front of the prime focal plane, oscillates in synchronization with the scan mirror. At the end of each scan the shutter blocks light from the earth, from the focal planes . In addition, the shutter flag relays light from the IC lamps and blackbody, to the detectors. The two IC lamps are situated near the base of the internal calibrator flag. Light from either or both lamps is directed through optics at the pivot point of the flag, into a sapphire rod contained within the flag. This rod transfers the light up the shutter flag and splits it into separate paths for each of the spectral bands. The light is directed out of the shutter flag and onto the focal planes by additional optics in the head of the instrument. The light separated for each band is aligned so that it impinges on the appropriate detectors.

The IC lamps are supplied with a regulated voltage across a combination of the lamp and a resistor, resulting in quasi-constant power being supplied to the lamp. Each lamp can be commanded "on" or "off", such that 4 lamp states are possible (both "off" [0,0], one "on" [0,1] or [1,0] or both "on" [1,1]). The IC was designed to have one lamp produce a usable signal in all bands. Note, both lamps "on" will saturate some bands particularly in high gain mode.

The IC blackbody is situated off the optical axis of the instrument. When the shutter flag passes in front of the primary focal plane, radiation from the blackbody is reflected off of a toroidal mirror on the flag, into the aft optics of the ETM+ and onto the band 6 detectors. The portion of the shutter flag imaged by band 6, exclusive of the area where the toroidal mirror is located, is coated with a high emissivity paint and acts as the second source for band 6 calibration. This portion of the shutter flag is also instrumented with a thermistor. The blackbody has three set point temperatures namely, 30°C, 37°C and 46°C.

The ETM+ IC, although similar to the IC on the Landsat-6, differs from the IC's on Landsat-4 and Landsat-5, in 5 principal ways: (1) the ETM+ uses 2 lamps (4 states) instead of 3 lamps (8 states), (2) a more compact filament results in a higher flux incident on the IC optics, though the lamps are nearly identical in terms of current and voltage ratings, (3) the control circuit for ETM+ uses voltage regulation in the primary operation mode, whereas TM used radiance stabilization in the primary mode, and voltage regulation in the backup mode, (4) ETM+ uses sapphire rods to transmit the energy from the base of the flag to the head of the shutter flag, while the TM's used fiber optics in an attempt to improve the uniformity of the calibration flux at the focal plane, (5) the ETM+ does not retain the lamp sequencer used on TM to automatically cycle through the lamp states.

When the ETM+ is operating, the shutter flag oscillates in synchronization with the scan mirror. The size of the shutter flag and its speed of movement combine to provide obscuration of the light to each detector for about 8.2 msec, or 750 pixels, for the 30 meter channels; the light pulse for the reflective bands, has a width of approximately 40 pixels (Figure 8.1.9). For band 6, the calibration signal is similar with the blackbody pulse about 20 pixels wide.

8.1.3.2 Full Aperture Solar Calibrator

The Full Aperture Solar Calibrator (FASC) is a white painted panel that is deployed in front of the ETM+ aperture and diffusely reflects solar radiation into the full aperture of the instrument as illustrated in Figure 8.1.10. With known surface reflectance, solar irradiance and geometry conditions, this device behaves as an independent, full aperture calibrator. The device consists of an octagonally shaped, aluminum honeycomb paddle on a motorized arm. On command, the motor rotates the panel from its stowed position away from the ETM+ aperture, to an inclined position in front of the ETM+. When stowed the panel rests adjacent to the stow cover which reduces the exposure of the panel to contaminants and UV radiation. The center 51 cm of the FASC panel is painted with the classic formulation of YB71, an inorganic flat white paint designed for spacecraft thermal control. This paint was selected for its near Lambertian properties, high reflectance, and apparent stability in a space environment. When in the calibrate position the angle between the sensor nadir vector, and the panel normal, is specified to be 23.5°. In use, the panel can be illuminated by the sun from 90° zenith angle (i.e. sunrise on panel) to about 67° zenith angle. Below 67 degs, the instrument begins to shade the panel. Depending on the time of year the solar azimuth angle with respect to the velocity vector of the ETM+, varies from 23° to 37°. The relative azimuth between the nadir view vector and the solar illumination varies across the same range.

ETM+ image data acquired with the FASC will appear to be an essentially flat field with vignetted cross track edges. The image will increase in brightness along track as the solar zenith angle (SZA) on the panel decreases (roughly at 1/cos(SZA)). Specifications require the FASC to fill the ETM+ aperture for the central 1000 pixels (approx 1/6 of each scan line); the design nominally fills the aperture for the central ~50% of the scan line. As the mirror scans, the view angles to the FASC panel change. If the nadir viewing pixel has the nominal 23.5 deg view angle and a 0 deg view azimuth angle, then at the extreme ends of the scan, the view zenith angle increases by about 1 deg, and the view azimuth angle varies by +/- 30 degs. Pre-launch BRDF measurements indicate that the radiance change across the scan, should be a 1% effect across the full scan assuming the aperture is filled. Across the central 1000 pixels, this translates into a 0.1% effect.

8.1.3.3 Partial Aperture Solar Calibrator

The Partial Aperture Solar Calibrator (PASC) is used for calibrating bands 1-5, 7 and 8 and consists of a small passive device that allows the ETM+ to image the sun while viewing a 'dark earth'. It is attached to the ETM+ sun shade and permanently obscures a small portion (~0.5%) of the aperture. It consists of four essentially identical sets of optical elements each in a slightly different orientation. Each set (or facet) consists of an uncoated silica reflector, a 45 degree mirror, and an aperture plate with a precision drilled small aperture (~4 mm) (Figure 7). The combination of the small aperture and the uncoated silica reflector reduces the signal amplitude sufficiently to bring it into the ETM+ dynamic range. The four facets are duplicated to account for angular variations of Sun position with season. They are oriented such that in any given orbit, as the satellite passes out of solar eclipse (i.e space vehicle sunrise) in the vicinity of the north pole, at least one facet will reflect sunlight directly into the ETM+ aperture and the ETM+ will image the sun.

Simulated ETM+ PASC Scene
Figure 8.1.11 Simulated ETM+ PASC scene.

Recent SBRS measurements of the alignment between the PASC and scanner assembly, have revealed a design misalignment which resulted in a nominal declination angle of the PASC (relative to spacecraft nadir) of 20 degrees, versus the prescribed 18 degrees. This increase in declination effectively forces the Spacecraft to acquire PASC scenes earlier in its orbit (i.e. closer to spacecraft sunrise). Although the spacecraft solar panel undergoes a period of thermal instability during sunrise, an analysis of the resultant spacecraft jitter has shown minimal impact (< 1%) to the acquisition of PASC data.

The PASC will generate a reduced resolution image of the sun, the resolution being limited by diffraction from the small apertures. This diffraction effect is wavelength dependent. For example, in band 1 the blur will extend across about 7 pixels (at the first dark ring of the diffraction pattern), and in band 7 the extent is about 32 pixels. In addition to the blur, the image will be elongated in the "along-track" direction. The "along-track" movement across the solar disk can best be expressed in terms of the spacecraft pitch rate (i.e. 360 degrees in ~100 minutes or ~3.6 degrees/minute). By comparison, the ground is normally scanned at 16 instaneous field of views (IFOV's). This equates to .039 degrees (16 * .0024 degrees) per 72 msec scan or ~32.5 degrees/minute. Thus the sun image will be oversampled along track by a factor of about 9. One other contributor to the rendition of the solar image, is the scanning direction which is not perpendicular to the motion of the sun - the angle between the two can be as small at 45 degrees. These combined effects of oversampling, and a non-orthogonal scan pattern, produce an elongated, skewed image of the sun as seen in Figure 8.1.11.

Within a PASC processed image, it is anticipated that most uniform portions of the solar disk center will be approximately 200 pixels in width for bands 1-5, 7, 105 pixels for band 6, and 410 pixels for band 8.

PASC calibration is performed once a day, every day, on the orbit specified by the IAS. The IAS orders the resulting data from the EDC DAAC for calibration processing and assessment.

Source: http://landsathandbook.gsfc.nasa.gov