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CHAPTER 8 SECTIONS > Radiometric Calibration Overview | Pre-Launch | Post-Launch
TM Cross Calibration | Geometric Calibration Overview | Alignment | Scan Mirror | Focal Plane

8.2 Pre-Launch

8.2.1 Spectral Characterization

The measured wavelength locations of the ETM+ spectral bands are compared to Landat 5's TM in Table 8.1 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 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
Table 8.1 updated August 2, 2013

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.

8.2.2 Radiometric Calibration

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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 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.

8.2.3 ETM+ Spherical Integrating Source

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Spherical Integrating Source Picture

Figure 8.1 - SIS in clean room

A spherical integrating source is a hollow sphere with the entire inner surface uniformly coated with a material which has a high diffuse reflectance. The basic concept behind the spherical shape is that light from the internal source has a chance to perform multiple bounces thereby randomizing its original direction before it exits a small aperture. The sphere's interior coating is designed to have a very high degree of diffuse reflectance. A perfect diffuse reflector can behave like a perfect (i.e. Lambertian) diffuse source which means energy is distributed in all directions equally. A Lambertian source is a source whose radiance is independent of viewing angle. Radiance is defined as the energy flux per unit projected area per unit solid angle leaving a source, or a surface.

Each SIS is calibrated by SBRS to National Institute of Standards and Technology (NIST) traceable standards of spectral irradiance. In addition, EOS cross calibration activities include comparison of the SBRS radiometric scale to the NIST, University of Arizona, NASA's Goddard Space Flight Center, and Japan's National Research Laboratory of Metrology (NRLM) radiometric scales.

The Landsat Transfer Radiometer (LXR), a visible and near infrared radiometer designed for stability by NIST, is used to monitor the output of each sphere during each calibration and calibration check. This radiometer has also been calibrated by NIST to provide an independent check on the radiometric calibration of the two sources.

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