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Section 3 - Instrument Calibration

3.1    Radiometric Characterization and Calibration Overview

A major objective of the Landsat Project is to upgrade the radiometric quality of the Enhanced Thematic Mapper Plus (ETM+) data to be equivalent to the other sensors in the Earth Observing System (EOS) constellation. Unlike its predecessors, a specific goal of the Landsat 7 mission is to achieve radiometric calibrations of the science data to ± 5 percent uncertainty over the planned five-year life of the satellite. Pre-launch, the mission was designed to support this requirement through sensor hardware changes, as well as pre-launch instrument characterizations. Post-launch while ETM+ is on-orbit, the 5 perecent requirement is supported by a monitoring and calibration program, as well as the implementation of any necessary changes to ground processing of the data.

3.1.1    Radiometric Characteristics

The ground footprint, spatial resolution, and the spectral channels of sensors that were operational at the beginning of the Landsat 7 mission characterize the civilian space-based remote sensing industry around the turn of the century. On one end of the scale are the low-resolution, large footprints, multi-spectral sensors such as NOAA's polar orbiters that have 1 kilometer (km) resolution and a 2000 km swath. On the other end are high resolution, small footprint, and panchromatic sensors such as IKONOS. As depicted in Figure 3-1, Landsat 7 occupies a unique niche between these two extremes.

Figure 3-1 Landsat 7's ETM+ sensor characteristics relative to other satellite systems
Figure 3-1. Landsat 7's ETM+ Sensor Characteristics Relative to Other Satellite Systems

The horizontal bars represent proportionately scaled footprints of the sensors on the left. Listed with each sensor are its spatial resolution, spectral coverage, and radiometric calibration accuracy. The right side of the chart lists the sensor's temporal resolution and pointing capability. Landsat 7 occupies a unique niche. No other sensor can match the characteristics of Landsat 7's ETM+, which includes repetitive, broad-area, and global coverage at moderate spatial resolution in all four passive optical regions of the electromagnetic spectrum (i.e., visible, near-infrared (IR), short-wave IR, and thermal IR), and accurate radiometric calibration. In addition, the full Landsat archive stretches back to 1972 allowing multi-decadal comparisons to be made.

3.2    Pre-Launch Radiometric Characteristics

3.2.1    Spectral Characterization

The measured wavelength locations of the ETM+ spectral bands are compared to all other Landsat sensors in Figure 3-2. Exact bandpass wavelengths can be found on the Landsat Missions Web Site. 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 Band 8. The cold focal plane (CFP) assembly has a filter housing that contains the filters for Band 5 through Band 7. See Section 2.2 for additional information on the ETM+ design.

Figure 3-2 Landsat Bandpass Wavelengths for All Sensors
Figure 3-2. Bandpass Wavelengths for All Landsat Sensors

A discrete spectral shift occurred on the Landsat 5 (Thematic Mapper) sensor that has been largely attributed to filter outgassing on-orbit. The ETM+ filters were made using an Ion Assisted Deposition (IAD) process (see Section 2.2.3.8), which was designed to make the filters resistant to this phenomenon. In addition to this, the new filters have shown significant improvement in band edge responses as compared to the TM sensors on Landsat 4 and Landsat 5.

3.2.2 Radiometric Calibration

Two Spherical Integrating Sources (SIS) (see Section 3.2.3) were used to calibrate the reflective bands (Bands1-5, Bands 7-8) of ETM+ prior to launch. The first approach used a 100 centimeter (cm) diameter source (SIS100) which is equipped with eighteen 200-watt lamps; six 45-watt lamps, and ten 8-watt lamps. This provides radiance levels covering the full dynamic range of the ETM+ 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 Terra MODIS. A second source, a 122 cm (48") SIS with six 200-watt lamps, two 100-watt lamps, and four 25-watt lamps 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 CFP at 105 degrees K. This was only one of three temperature set points for the CFP that could be obtained in ambient pressure and temperature conditions given the available ground testing facilities.

The calibration data reduction is performed as follows for the ETM+ reflective bands. 
The ETM+ band weighted spectral radiances, Lλ (b,s), for band 'b' and sphere level 's' are calculated as:

Lλ (b,s)= ∫RSR(b,λ)Lλ (s,λ)dλ)
∫RSR(b,λ)dλ

Where:
RSR(b,λ)  is the Relative Spectral Response for band 'b' at 'λ' calculated from component level transmission, reflectance, and responsivity measurements, and
Lλ (s,λ)  is the measured spectral radiance of sphere level 's' at 'λ'

The quantized detector (d) responses Q(d,b,s) are regressed against the integrating sphere band-weighted radiance level, Lλ(b,s), per the calibration equation:

Q(d,b,s)= G(d,b)Lλ(b,s)+B(d,b)

The slopes of the resulting 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) at NASA Goddard Space Flight Center (GSFC) has reviewed the various integrating sphere calibrations and their effective transfer to the ETM+ and decided what calibrations should go to the Image Assessment System (IAS) to be used by the pre-launch IAS.

The radiometric calibration of ETM+'s thermal Band 6, 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 Santa Barbara Remote Sensing (SBRS). The calibration for ETM+ Band 6 is formulated as:

Qsc (d)-Qsh(d) = G(d)(Lλ,sc-Lλ,esh)

Where:
Qsc (d) is the quantized response of Band 6 detector ‘d’ to the scene,
Qsh (d) is the quantized response of Band 6 detector ‘d’ to the shutter,
G(d)  is the gain of detector 'd',
Lλ,sc is the spectral radiance of the scene,
Lλ,esh is the scene-equivalent spectral blackbody radiance of the shutter:

Lλ,esh = Lsh +∑a(Lsh-Lj)

and where:

Lsh is the blackbody radiance of the shutter, Lj is the blackbody radiance of the jth component of the ETM+ instrument, and aj is the emissivity-adjusted view factor for the jth ETM+ component where: j = 1 for the scan line corrector; j = 2 for the central baffle; j = 3 for the secondary mirror and mask; j = 4 for the primary mirror and mask; and j = 5 for the scan mirror.

Each of these components is in front of the shutter and contributes to the apparent scene radiance when the shutter is open.

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

3.2.3 Spherical Integrating Source (SIS)

A SIS is a hollow sphere with the entire inner surface uniformly coated with a material that 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. 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 GSFC, and Japan's National Research Laboratory of Metrology (NRLM, now National Metrology Institute of Japan, NMIJ) radiometric scales. Lastly, 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 SIS sources.

Figure 3-3 The SIS (upper left) in the clean room with ETM+ (middle right)
Figure 3-3. The Spherical Integrating Source (SIS) (upper left) in the Clean Room with ETM+ (middle right)

3.3 Post-Launch Radiometric Characteristics

The post-launch radiometric calibration of the ETM+ is accomplished by the IAS through the regular examination of the instrument's response when illuminated by known sources designed to be relatively stable on orbit. The ETM+ has three 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 used for calibrating all ETM+ bands, while the PASC and FASC are used for the reflective bands. Changes to the ETM+ calibration have occurred since launch (see Section 3.3.5). Ground look calibrations, comparing field observations to ETM+ overpass data are occasionally performed to confirm via the analysis of the USGS Calibration and Validation Team (CVT), the accuracy of the calibration using on-board sources.

3.3.1    On-Orbit Calibration Methods

The three on-board ETM+ calibration devices are the FASC, which is a white painted diffuser panel, a PASC, which is a set of optics that allow the ETM+ to image the sun through small holes, and an IC, which consists of two lamps, a black body, a shutter and optics to transfer the energy from the sources to the focal plane. Details on these devices can be found in the following sections.

The FASC is deployed in front of the ETM+ aperture approximately monthly. Based on the orientation of the panel relative to the sun and instrument, and the pre-launch measured reflectance of the panel, a calibration can be determined. The IC provides a signal to the ETM+ detectors once each scan line as well as a view of the black shutter. The shutter provides the dark reference for the reflective bands and a low temperature source for the thermal band. The lamps and black body provide the high radiance source for the bands. At the short wavelengths, the IC has shown both short-term and long-term instabilities. Performance of the PASC has been anomalous and results are not included here. On-orbit performance of the radiometric calibrators can be found in a paper that covers the subject in greater detail (Markham, et al., 2003).

3.3.2    Internal Calibrator (IC)

The IC consists of a shutter flag, two tungsten lamps, and a blackbody source. The shutter flag, located immediately in front of the prime focal plane (PFP), oscillates in synchronization with the scan mirror. At the end of each scan the shutter mechanically blocks external light to 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 IC 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 IC flag. The light separated for each band is aligned so that it illuminates 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 four 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 each lamp produce a usable signal in all bands. Both lamps "on" 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 PFP, radiation from the blackbody is reflected off 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 to track temperatures over the mission lifetime to assess performance stability. The blackbody has three set point temperatures namely, 30°C, 37°C, and 46°C.

The ETM+ IC, although similar to the IC on Landsat 6, differs from the IC on Landsat 4 and Landsat 5, in several ways: (1) the ETM+ uses two lamps (four states) instead of three lamps (eight 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 use fiber optics in an attempt to improve the uniformity of the calibration flux at the focal plane, and (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 millisecond, or 750 pixels, for the 30-meter channels; the light pulse for the reflective bands, has a width of approximately 40 pixels (see Figure 3-4). For Band 6, the calibration signal is similar in size to the blackbody pulse, about 20 pixels wide.

Figure 3-4 Illustration of ETM+ Band 4, Detector 1, Low gain, IC data acquired on 12/07/1996, using the primary lamp
Figure 3-4. Illustration of ETM+ Band 4, Detector 1, Low Gain, IC Data Acquired on 12/07/1996, using the Primary Lamp

3.3.3    Full Aperture Solar Calibrator (FASC)

The FASC is a white painted panel deployed in front of the ETM+ aperture and diffusely reflects solar radiation into the full aperture of the instrument as illustrated in Figure 3-5. With known surface reflectance, solar irradiance and geometry conditions, this device behaves as an independent, full aperture calibrator. The device consists of an octagon shaped, aluminum honeycomb paddle on a motorized arm.

Figure 3-5 Schematic drawing of a portion of the ETM+ sensor with the FASC deployed in front of the ETM+ aperture
Figure 3-5. Schematic drawing of a portion of the ETM+ sensor with the FASC deployed in front of the ETM+ aperture

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 the FASC is in its 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°, 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 deployed appears to be an essentially flat field with fading cross-track edges. The image increases 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 (~1/6 of each scan line); the design nominally fills the aperture for the central ~50 percent 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° view angle and a 0° view azimuth angle, then at the extreme ends of the scan, the view zenith angle increases by about 1°, and the view azimuth angle varies by +/- 30°. Pre-launch Bidirectional Reflectance Distribution Function (BRDF) measurements indicate that the radiance change across the FASC scan should be a one percent effect across the full scan assuming the aperture is filled. Across the central 1000 pixels, this translates into a 0.1 percent effect.

The FASC calibration is performed every month. Up until mid-2015, the IAS sent FASC requests to the Landsat 7 MOC scheduler. Now, the MOC has been enabled to independently schedule monthly FASC collections, placing them at times of low duty cycle usage to avoid impacts on regular imaging.

3.3.4    Partial Aperture Solar Calibrator (PASC)

The 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+ sunshade and permanently obscures a small portion (~0.5 percent) 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° mirror, and an aperture plate with a precision drilled small aperture (~4 mm). 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., spacecraft sunrise as it orbits from behind the Earth) in the vicinity of the North Pole, at least one facet reflect sunlight directly into the ETM+ aperture and it is able to image the sun.

Pre-launch SBRS measurements of the alignment between the PASC and scanner assembly, revealed a small 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 forced the ETM+ to acquire PASC scenes earlier in the 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 percent) to the acquisition of PASC data.

The PASC generates a reduced resolution image of the sun with the resolution being deliberately limited by diffraction from the small apertures. This diffraction effect is wavelength dependent. For example, in Band 1 the blur extends 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 is 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 IFOVs. This equates to 0.039 degrees (16 * 0.0024 degrees) per 72 millisecond scan or ~32.5 degrees/minute. Thus the sun image is 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. The combined effects of oversampling and a non-orthogonal scan pattern, produce an elongated, skewed image of the sun (see Figure 3-6).

Figure 3-6 Simulated ETM+ PASC scene showing a reduced resolution image of the Sun
Figure 3-6. Simulated ETM+ PASC Scene Showing a Reduced Resolution Image of the Sun

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, and Band 7, 105 pixels for Band 6, and 410 pixels for Band 8. Initially the IAS submitted PASC requests to the MOC for scheduling; within a few years of launch, the scheduler was updated to automatically schedule the PASC collects every x orbits, where x has varied over the years since launch:

  • Launch to March 2007—every 17 orbits
  • March 2007 to April 2007—every 34 orbits
  • April 2007 to September 2007—every 17 orbits
  • September 2007 to July 2015—every 25 orbits
  • July 2015 to present—every 29 orbits

3.3.5    ETM+ Calibration Actions

The actions taken by the LPSO at NASA GSFC and the Image Assessment Team at USGS EROS to provide the best possible calibration for Landsat ETM+ imagery are listed in detail on the Landsat Missions Website.

3.3.6    Radiometric Performance

A significant improvement in the Landsat 7 system is the addition of the IAS as part of the ground processing system. The IAS has the role of monitoring the performance and calibration of the ETM+ instrument and providing updates to the CPF. The NASA GSFC LPSO works with the IAS (located at EROS) in analyzing the calibration information and updating the algorithms used within the IAS. Additional funding from NASA supports vicarious radiometric calibration efforts at NASA Jet Propulsion Laboratory (JPL), Rochester Institute of Technology (RIT), South Dakota State University (SDSU), and the University of Arizona. Approximately every 6 months, the scientists and analysts involved in characterizing ETM+ radiometric calibration meet and present their results at the Landsat Science Team Meeting. The results form the basis for updating the radiometric gain calibration parameters in the CPF.

3.3.6.1    Ground Look Calibration (GLC) Methods

There were four investigations evaluating the ETM+ radiometric calibration using GLC or vicarious methods early in the Landsat 7 mission. Each of these investigations predicted the radiance at the sensor aperture using a combination of ground- and/or aircraft-based reflectance, radiance or temperature measurements, coupled with measured and/or modeled atmospheric parameters. Two investigations were looking primarily at the reflective band calibrations: those of Helder and Thome (Thome, et al., 2001). The investigations of Palluconi and Schott were concentrating on the thermal band (Schott, et al., 2000).

3.3.6.2    Radiometric Performance Results

The combined calibration results for Bands 1-5, 7, and 8 are presented in those papers, respectively. The FASC results presented are based on the "best" portion of the FASC panel and have been adjusted based on an apparent 1° difference in the orientation of the panel from pre-launch measurements (Markham, B. L., et al., 2003). The IC results have also been included while recognizing that part of the variability present is related to the IC. In all bands, the vicarious results agree to within 5 percent of the FASC and pre-launch values and that the trends in the GLC results are not significant. The FASC results show significant trends, but the trends are small (less than 1.5 percent/year). The FASC trends are believed to be largely due to changes in the FASCs reflectance and not representative of the instrument. However, there is some consistency between the FASC, GLC, and IC trends, (e.g., in Band 7 all are increasing). If the consistency continues and the vicarious trends become significant, a calibration update will be performed.

In Band 6, the IC is the only on-board calibration device. The slope of the responsivity, though significant, shows a change of less than 0.06 percent/year. This system is remarkably stable, particularly relative to the Landsat 4 and Landsat 5 TM thermal bands. The ETM+ instrument also appears stable relative to the vicarious measurements, though a significant bias was detected. This bias was originally measured as 0.31 (Watts/(m² * sr * µm)) and this correction was applied to the calibration parameter file on October 1, 2000. Updated measurements indicate the bias was closer to 0.29 (see Landsat 7 Calibration Notices). After correction for the bias, the calibrated product radiance has a scatter of about 1percent around the vicarious results.

On-orbit results indicate that the Landsat 7 ETM+ absolute radiometric calibration is stable to better than 1.5 percent/year in the reflective bands and 0.1 percent/year in the thermal band. The uncertainty in the calibration is estimated at <5 percent in the reflective and ~1 percent in the thermal regions. These analyses have been continued to ensure that the ETM+ data are fully characterized through the mission lifetime and that CPFs are updated to provide accurate data processing for all acquired imagery.

3.4 Additional Radiometric Characteristics

3.4.1    Gain States

The ETM+ images are acquired in either a low or high gain state (see Figure 3-7). The MOC controls the gain selection for a scene and is performed by changing the reference voltage for the analog to digital convertor. The science goal in switching gain states is to maximize the ETM+'s 8-bit radiometric resolution without saturating the detectors. This requires matching the gain state for a given scene to the expected brightness conditions. For all bands, the low gain dynamic range is approximately 1.5 times the high gain dynamic range. It makes sense, therefore, to image in low gain mode when surface brightness is high and in high gain mode when surface brightness is lower. Table 3-1 lists the minimum saturation levels for all bands in both the low and high gain states.

Figure 3-7 Design of the high and low dynamic ranges of the ETM+ reflective bands
Figure 3-7. Design of the High and Low Dynamic Ranges of the ETM+ Reflective Bands

 

Band Low Gain High Gain
Minimum Saturation Level
(W/(m² * sr * µm))
Minimum Saturation Level
(W/(m² * sr * µm))
1 285.7 190.0
2 291.3 193.7
3 225.0 149.6
4 225.0 149.6
5 47.30 31.50
6 17.21 12.78
7 16.70 11.10
8 235.0 156.3

Table 3-1. ETM+ Minimum Saturation Levels for All Bands

3.4.2    At-Launch Gain Setting Strategy Rationale

The following paragraph was extracted from a paper that describes the rationale behind the at-launch gain setting strategy (Goward, et. al., 1999):

"The 8-year monthly averages of AVHRR visible and near infrared planetary reflectance measurements from the AVHRR data set, at the original 15 km spatial resolution, were used to evaluate the gain settings. Visible (Bands 1-3), near infrared (Band 4), and shortwave infrared (Band 5 and 7) gains were considered separately. The at-satellite planetary reflectance was converted to at-satellite radiance, based on the solar zenith angle at the time of satellite overpass. For each Landsat WRS-2 scene, the observed spectral radiance was subjected to the two gain states. For each gain state, an entropy statistic was calculated to determine the potential scene contrast in each setting. Where low gain was found to provide substantially greater scene contrast (e.g., glaciers in summer), this setting was selected. For all other cases the high gain was selected. This decision process will no doubt yield less than optimum results for some applications, but it was the best compromise to meet the requirement to minimize gain changes while providing generally high quality measurements".

Initial ETM+ gain settings are depicted graphically in maps linked to Table 3-2. These lasted from launch until late 2000 when they were updated with per band gain calculations. This list shows the gain setting changes for every Landsat 7 ETM+ WRS-2 path/row in the year 2000.

Initial ETM+ Monthly Gain Settings for Daytime Scenes
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Table 3-2. Initial ETM+ Gain Settings

3.4.3    Gain Settings on Orbit

The gain setting strategy for Landsat 7 scheduling has undergone several revisions since launch. On December 2, 1999, in response to feedback from data users about saturation issues with Band 4 over agricultural targets, the decision was made to acquire Band 4 data in low gain when the sun elevation angle exceeds 45 degrees. On July 13, 2000, a strategy was implemented consisting of a fixed categorization of the land cover types of the Earth, and associated gain setting rules that are land-cover and sun-angle based. Each Landsat 7 WRS-2 path/row location is categorized into one of six land cover types. The land cover types considered and the relevant gain change rules are detailed as follows. (Within the LTAP, the gains for Bands 1-3 are always changed together, as are the gains for Band 5 and Band 7. The gains for Band 6 and Band 8 are static.)

Land (non-desert, non-ice):

  • Bands 1-3 set to high gain
  • Band 4 set to high gain except where sun elevation is greater than 45° (set to low gain) in order to avoid dense vegetation (reflectance > 0.66) saturation. (At this sun angle, high gain in Band 4 causes saturation at or above a reflectance of 0.66, therefore, switching to low gain keeps targets from saturating.)
  • Band 5 and Band 7 set to high gain
  • Band 8 set to low gain

Desert:

  • Bands 1-3 set to high gain except where sun elevation is greater than 28° to avoid bright desert target (reflectance >0.65 in Band 3, >0.66 in Band 1, >0.71 in Band 2) saturation
  • Band 4 set to high gain except where sun elevation is greater than 45°(set to low gain) to avoid bright desert (reflectance > 0.66) saturation
  • Band 5 and Band 7 set to high gain except where sun elevation is greater than 38° to avoid bright desert target (reflectance >0.70 in Band 5, >0.68 in Band 7) saturation
  • Band 8 set to low gain

Ice/Snow and Sea Ice:

  • Bands 1-3 set to high gain except where sun elevation is greater than 19° to avoid snow ice (reflectance > 0.95 in Band 3, >0.94 in Band 1, >1.03 in Band 2) saturation
  • Band 4 set to high gain except where sun elevation is greater than 31° to avoid snow/ice (reflectance >0.92) saturation.
  • Band 5 and Band 7 set to high gain
  • Band 8 set to low gain

Water/Coral Reefs:

  • Bands 1-5 and Band 7 set to high gain
  • Band 8 set to low gain

Volcano/Night – nighttime imaging (sun elevation angle < 0°) is only routinely performed for targets identified as "Volcano" (see Section 4.1.2).

  • Bands 1-4 set to high gain
  • Band 5 and Band 7 set to low gain to reduce saturation of volcanic hot spots
  • Band 8 set to low gain

The actual saturation reflectance for daytime imagery corresponding to a given sun angle is influenced by the Earth-Sun distance, which varies by ±1.5 percent over the year producing a ±3 percent irradiance variation. The current gain setting rules do not account for this variability. Band 8 is in low gain for all routine acquisitions as the noise level in this band is such that high gain provides very little improvement in performance. This implementation is currently under review and the Band 8 gain settings may be altered in the future.

Maps applying to the first day of each month are linked in Table 3-3. The nominal gain file (American Standard Code for Information Interchange (ASCII)) contains a complete list of gain states, organized by month and WRS location, for daytime scenes.

ETM+ Monthly Gain Settings for Daytime Scenes
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Table 3-3. ETM+ Gain Settings

On April 20, 2001, the strategy was updated such that gain changes for scenes in northern Africa were made over the Mediterranean, so that land images were not impacted. There is a small discontinuity in the image wherein gain changes take effect, and some processing systems are not able to work through them. On May 8, 2002, the scheduler software was modified to incorporate the ability to shift gain changes, when not over the U.S., into flywheel scenes, scenes with high cloud cover predictions, or prior to the start of an imaging interval. Over the U.S., gain changes are made during the cloudiest scene prior to the scene requiring the change. In both cases, no single scene has a gain change two acquisitions in a row. Again, this was to make sure the same scenes were not impacted by gain changes and to place the changes in scenes of low priority.

3.4.4 Gain Change Timing Rationale

The gain change commands at the time of launch were issued at the start time of a scene, which due to various error accumulations was placing the gain change one to two seconds into the start of the scene. The MOC was informed that the U.S. ground processing system could not handle this band change location. A scheduler modification was subsequently made to move the gain change commanding back by four seconds, so it would occur during the trailing end of the preceding scene. This was initially incorporated between July 14 and July 26, 1999. This change was made permanent starting August 2, 1999.

The rational for setting the offset to -4 seconds was:

  1. Payload command timing error: up to 1-second quantization error because the onboard computer executes commands on integer second boundaries.
  2. Gain change command execution time: 0.1 second/band x 8 bands = 0.8 seconds
  3. Orbital along-track position uncertainty: typically <1.0 km, equivalent to 0.2 seconds (except on days following delta-V orbit correction maneuver, in which the along-track error could reach two seconds).
  4. Scene center-to-center time variance: typically 23.92 +/- 0.09 seconds due to orbital eccentricity.

The -4 second gain change offset exists for all scenes acquired after August 2, 1999. This is a different issue than the gain change rationale in the LTAP (see Section 4).

3.5 Geometric Calibration Overview

This section describes the geometric characterization and calibration activities performed over the life of the Landsat 7 mission using the software tools developed as part of the Landsat 7 IAS. Along with the radiometric corrections previously discussed, the IAS provides the capability to routinely perform four types of 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-1 Systematic (Corrected) (L1G) products
  2. Geometric accuracy assessment to qualitatively and quantitatively evaluate residual internal geometric distortions within L1G images
  3. Band-to-band registration assessment to measure and monitor the relative alignment of the eight ETM+ spectral bands
  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
  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 involved measuring and verifying the Landsat 7 ETM+ system performance during the Initial On-orbit Checkout (IOC) period that lasted until mid-June 1999. This required the use of the available geodetic, geometric, band-to-band, and image-to-image characterization capabilities, and performance of the initial sensor alignment calibration. Refining the pre-launch sensor alignment knowledge was critical to ensure that the Level-1 product geodetic accuracy specification could be met. Sufficient supporting datasets (e.g., ground control, terrain data) to perform these characterization and calibration activities had to be available at launch. The second priority during the IOC period was to verify and, as necessary, update the pre-launch focal plane, particularly band placement, as well as scan mirror profile calibrations. The results of these initial calibration activities were used to verify that the system was performing within specifications and to create the initial post-launch release of the CPF,(see Section 3.10), which was used by the IAS to create Level-1 products that met the Landsat 7 geodetic accuracy requirements.

After the IOC period, ongoing calibration activities 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 includes processing additional calibration scenes under a variety of acquisition conditions (e.g., orbital position, ETM+ time on, as instrument performance changes) to measure the system's geometric performance as a function of time, temperature, and location. The next section details the current instrument status.

3.5.1 Geometric Performance

3.5.1.1 Requirements

The geometric performance of the ETM+ is judged against three key requirements placed on the Landsat 7 system. These requirements are:

  • Absolute Geodetic Accuracy
  • Band-to-Band Registration
  • Image-to-Image Registration

3.5.1.2 Absolute Geodetic Accuracy

This requirement ensures that geometrically corrected products be accurate to 250 meters (1 sigma), excluding terrain effects, without ground control. The geodetic accuracy is limited by spacecraft / instrument geometric model accuracy (e.g. ephemeris, attitude, alignment knowledge). Geodetic accuracy is monitored using calibration scenes containing Ground Control Points (GCPs). Scenes are first radiometrically and geometrically corrected. Control point locations are then measured on the processed imagery and compared to precisely known ground locations. Any terrain effects are removed analytically in the comparison. The product's geodetic accuracy depends on the accuracy of four data inputs. These are:

The ephemeris is estimated post-pass using satellite tracking data. On-board star trackers and gyros measure attitude. The clock performance is monitored by the Landsat 7 MOC, and the ETM+ alignment is determined by a ground processing calibration that determines the orientation of the ETM+ payload relative to the L7 spacecraft Attitude Control System (ACS) reference. Multiple scenes with ground control are used to measure the systematic biases attributable to ETM+ alignment. During the initial on-orbit calibration during the first ninety days, seven different calibration scenes were used. Periodic geodetic accuracy testing showed a slow build-up of along-track bias after July 1999. A sensor alignment calibration update was performed in June of 2000.

Twenty-four scenes acquired since July 1999 (~1 per cycle) were used to perform the initial calibration. A separate and independent set of eighteen scenes covering the same time span were used to verify the results. The ETM+ alignment shows time-varying behavior and will continue to be monitored during the course of the mission. Current trends reveal post-calibration geodetic accuracy of systematic ETM+ products of approximately 80 meters (1 sigma), which is much better than the 250 meter specification, placed on the satellite system.

3.5.1.3 Band-to-Band Registration

Band-to-band registration assessment is performed periodically throughout the mission's life. The purpose of this assessment is to measure the relative alignment of the eight ETM+ spectral bands after processing to Level-1s to verify that the 0.17 pixel band-to-band registration requirement is being met. If not, the IAS remedies the band alignment by deriving new band center locations via band-to-band registration calibration and subsequently updating the CPF.    

Band registration is monitored using desert scenes because they provide the best cross-band correlation performance. The band center locations measured pre-launch were evaluated using on-orbit data and then updated using these calibration scenes during the on-orbit checkout period. The measured band registration accuracy was 0.06-0.08 pixels. However, the registration accuracy degraded slightly after July, 1999. Measurements revealed that the registration offset between the primary PFP and CFP in the scan line direction increased to 0.08-0.10 pixels.

Band calibration analysis showed a systematic shift in the Band 5, 6, 7 locations after July 1999. The primary focal plane band centers are very stable but the cold focal plane band centers are much more variable with a 3-4 microradian mean shift. The CFP offsets coincided with changes in ETM+ operating temperature range, which is hotter than during the 90-day checkout period. The band center calibration was updated for data acquired after July, 1999 although analysis revealed that band center estimates from individual scenes are still highly correlated with temperature telemetry. The current calibration is time-dependent pending development of a temperature dependent model. Nonetheless, registration performance (see Figure 3-8) was well within the 0.17 pixel specification early in the mission.

 
Figure 3-8 ETM+ band center offsets vs. specifications
Figure 3-8. ETM+ Band Center Offsets vs. Specifications

3.5.1.4 Image-to-Image Registration

The image-to-image (or multi-temporal image) registration accuracy requirement assures that geometrically corrected images from multiple dates shall be capable of being registered to an accuracy of 7.3 meters (1 sigma). This requirement was determined using cloud-free scenes of the geometric calibration "super-sites". The term "geometric super-site" is used to describe those pre-selected WRS-2 path-row locations for which ground control, digital terrain data, and reference imagery have all been collected as opposed to the more widely distributed areas with GCPs  used in the previous tests. This supporting data set makes it possible to produce precision and terrain corrected ETM+ images, and to compare them to accurately geo-registered reference images.

The required ground control, terrain models, and reference images were derived from Digital Orthophoto Quadrangle (DOQ) data (see Section 3.7). The one-meter resolution DOQs were mosaicked and reduced to the equivalent of 15-meter resolution. Five cloud-free images of two separate calibration sites were used to measure registration accuracy. The image registration assessment was performed in two ways. First, the ETM+ images were compared against the DOQ reference images. This provides a measure of image distortion relative to an absolute reference. Second, two ETM+ scenes were cross-correlated. This provides a measure of image distortion that may change from scene-to-scene although systematic calibration distortions may cancel out.

Coupled with the image registration analysis is the need to measure the ETM+ scan mirror performance to ensure the pre-launch profile and modeling is correct. The geometric calibration super-site scenes are also used for this purpose. A DOQ reference image was constructed to provide full-width coverage of a Landsat 7 scene so that measurements at all scan angles could be obtained. Mirror deviations as a function of scan angle were obtained by cross-correlating the ETM+ scene to the reference image.

No apparent problem was observed with the along-scan mirror profile. A minor adjustment to the cross-scan profiles was made to model slightly non-linear scan line corrector behavior. Also, an adjustment to the prelaunch scan angle monitor start/stop angles was made to improve image-to-image registration accuracy. The scan mirror calibration update was made to the CPF in the fourth quarter of 1999, during ETM+'s first year of operation.

 Figure 3-9 Pre- and post-scan mirror calibration registration accuracy
Figure 3-9.  Pre- and Post-Scan Mirror Calibration Registration Accuracy

Image registration accuracy (see Figure 3-9) was measured before and after the scan mirror calibration. Results revealed that the required image registration accuracy was achieved using the baseline pre-launch scan mirror calibration parameters. Specifically:

  • All-scene average registration to DOQ:
    • 5.8 meters along-scan and 4.7 meters across-scan (1 sigma)
  • All-scene average ETM+ to ETM+ registration:
    • 3.9 meters along-scan and 1.8 meters across-scan (1 sigma)
  • Two individual scenes fell outside specification versus DOQ

The image registration also improved using the updated scan mirror calibration parameters. Analysis yielded the following results.

  • Registration to DOQ:
    • 4.3 meters along-scan and 4.2 meters across-scan (1 sigma)
  • ETM+ to ETM+ registration:
    • 3.2 meters along-scan and 1.9 meters across-scan (1 sigma)
  • All scenes were within specification

3.6 Sensor Alignment Calibration

The goal of the sensor alignment calibration is to improve the on-orbit 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 calendar quarter over the mission lifetime. This calibration uses discrete GCPs 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 or simple, static offset) utilizing a series of precision correction solutions, which measure a combination of orbit, attitude, and alignment errors. Landsat 7 has more accurate (estimated to be in the 10 to 50-meter range versus 133 meter accuracy for the ephemeris downlinked in the Payload Correction Data (PCD)) post-pass definitive ephemeris data available for the alignment calibration test scenes, reducing the uncertainty due to orbital errors. The GSFC Flight Dynamics Facility (FDF) provides this precise ephemeris from the IAS through additional processing of the down-linked data. Multiple precision correction solutions will be integrated using a Kalman filter algorithm to estimate the best-fit systematic alignment bias. As the Kalman processes additional precision correction solutions filter, the filter's estimates of the alignment biases will improve.

Periodically, the IAS decides that the alignment knowledge has changed enough to warrant generating an updated sensor alignment matrix for inclusion in the CPF. 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 standard deviation moves below the 24-arc second alignment accuracy requirement threshold. During normal operations, a new alignment matrix is generated whenever a new version of the CPF was scheduled for release.

As it becomes available, the Landsat 7 definitive ephemeris is used for geometrically correcting ETM+ data. Definitive ephemeris substantially improves the positional accuracy of the Level-1 product over predicted ephemeris. An ephemeris is a set of data that provides the assigned places of a celestial body (including a manmade satellite) for regular intervals. In the context of Landsat 7, ephemeris data shows the position and velocity of the spacecraft at the time imagery is collected. The position and velocity information are used during product generation within the Landsat Product Generation System (LPGS).

The MOC receives tracking data on a daily basis that shows the position and velocity of the Landsat 7 spacecraft. This information comes from the three U.S. operated ground-receiving stations and is augmented by similar data from NASA's Tracking and Data Relay Satellites System (TDRSS). The Flight Operations Team (FOT) processes this information to produce a refined or "definitive" ephemeris that shows the position and velocity of Landsat 7 in one-minute intervals. Tracking data are used to compute the actual spacecraft position and velocity for the last 61 hours and to predict these values for the next 72 hours. The predicted ephemeris data are uploaded to the spacecraft daily. On-board software interpolates this data to generate the positional information contained in the PCD.

Engineers with the Landsat Project have completed a predicted versus definitive ephemeris analysis. Comparisons to GCPs demonstrated the definitive ephemeris is reliably more accurate than the predicted ephemeris. Geometric accuracy on the order of 30-50 (1 sigma) meters, excluding terrain effects, can be achieved when the definitive ephemeris is used to process the data. Level-1 products produced after March 29, 2000 use definitive ephemeris, if available. The .MTL field "ephemeris_type" in the product metadata files identifies whether a product was created with definitive or predicted ephemeris. Daily definitive ephemeris profiles have been archived since 1999 and are available for download.

3.7 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, in order 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 is possible to estimate corrections to the pre-launch scan mirror profiles, as needed. Any significant deviations detected are folded back into the CPF through updates to the mirror profile polynomial coefficients.

Scan mirror calibration applies to both the along and across scan directions so it detects and compensates for Scan Line Corrector (SLC) deviations as well (see Section 2.2.3.1). In practice, SLC deviations were indistinguishable from scan mirror deviations; therefore, they were modeled as deviations of the scan mirror motion. Detecting systematic deviations that can be attributed to mirror motion requires reference points which can be uniquely associated with individual forward and reverse ETM+ scans and provide a good distribution of data as a function of scan angle.

The current approach to acquiring such a control reference uses spatially accurate reference imagery for one or more calibration areas. The scan mirror calibration procedure compares a precision and terrain corrected ETM+ panchromatic band image with the reference image constructed from USGS 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 are then correlated with the reference image to measure the (sub-pixel) residual distortion. The distortion patterns from many scans are analyzed to detect systematic deviations from the pre-launch forward and reverse scan mirror profiles.

On April 1, 2007, ETM+ was switched to 'bumper operational mode'. The ETM+ was designed to have a fixed line length on every scan controlled by the Scan Angle Monitor (SAM). However, due to the long duration of the mission, the wear on the bumpers became sufficiently large that it was no longer possible to maintain control in the SAM-mode; therefore, ETM+ was put in a backup mode where the mirror is allowed to scan at a fixed frequency of 14 Hertz (Hz) as a free pendulum. The calibration shutter is still synchronized with the scan mirror so that calibration pulse can be obtained at the end of each scan. The net result is that the variability of the scan length increases and the geometric accuracy decreases.

3.8 Focal Plane Calibration

The focal plane calibration operations involve measuring the alignment of all eight ETM+ bands to ensure that band registration accuracy meets a 0.28 pixel requirement. If the band-to-band comparisons detect any uncompensated misalignment the band placement calibration procedure is used to update the band center location parameters in the CPF accordingly. Detector-to-detector alignment is also monitored to ensure that image discontinuities are not introduced by using incorrect detector locations as that may cause delays in the Level-1 image resampling process.

Landsat 7 ETM+ images of focal plane calibration test sites are 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 ground data. Band to band registration assessment requires scenes that contain significant high spatial frequency content 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.

3.9 Modulation Transfer Function Characterization

Pre-launch modeling of the ETM+ optical system predicted that Modulation Transfer Function (MTF) performance would change on-orbit. A method was developed to monitor the along-scan MTF performance of the ETM+ sensor system using on-orbit data of the Lake Pontchartrain Causeway in Louisiana. The MTF of the ETM+ is regularly measured for comparison to the pre-launch test results and for monitoring long-term instrument performance. The MTF characterization methodology involves the analysis of cloud free scenes over the Lake Ponchartrain Bridge in Louisiana (see Figure 3-10). The bridge is a long, straight, double-spanned structure (two 10-meter wide spans, separated by 24.4 meters), is approximately aligned with the Landsat ground track (<1°), and offers high signal contrast to the waters below.

Figure 3-10 ETM+ image of the MTF characterization site. Lake Ponchartrain is the largest discrete water body in the area with the bridge shown with the red arrow.
Figure 3-10. ETM+ Image of the MTF Characterization Site. 

ETM+ image scan lines crossing the bridge are treated as multiple measurements of the target taken at varying sampling phases. These line measurements are interleaved to construct an over-sampled target profile for each ETM+ scan direction. Corresponding profiles are simulated using analytical models of the bridge and of the ETM+ system transfer function. Model parameters are adjusted to achieve the best fit between the simulated profiles and the image measurements. The ETM+ modulation at the Nyquist frequency and the full width at half maximum of the point spread function are computed from the best-fit system transfer function model.

Tracking these parameters over time has revealed apparent MTF performance degradation although this is observed mainly in the 15-meter resolution ETM+ panchromatic band. These tests confirmed the pre-launch model prediction that the panchromatic band was the most sensitive to changes in ETM+ optical performance (see Figure 3-11). The consistency between the prelaunch model,the on-orbit measurements, and the relative stability of the on-orbit measurements across the spectral bands and from date to date, suggest that the MTF estimation method is yielding useful results, and is achieving the goal of providing a means of monitoring the changing ETM+ MTF performance.

Figure 3-11 ETM+ images of Lake Ponchartrain Bridge
Figure 3-11. ETM+ Images of Lake Pontchartrain Bridge

3.10 Calibration Parameter Files

3.10.1 File Description

The IAS is responsible for the sustained radiometric and geometric calibration of the Landsat 7 satellite and its ETM+ sensor and passing this knowledge to the user community. This is achieved by assessing new imagery on a daily basis, performing both radiometric and geometric calibration when needed, and developing new processing parameters for creating Level-1 products. Processing parameters are stored in the CPF, which is stamped with applicability dates and sent to EROS for storage and utilization with outbound products. As appropriate, the CPF is also sent to International Ground Stations (IGS) via the Landsat 7 MOC. The CPF provides all the radiometric and geometric calibration coefficients needed for processing raw, uncorrected Landsat 7 ETM+ image data. In addition, the IAS documents and disseminates any significant changes to CPFs through online Calibration Notices as needed to alert data users to changes in ETM+ performance and or standard processing.

3.10.2    CPF Updates

IAS updates and distributes the CPF at least every 90 days. Updates were more frequent during early orbit checkout and also occur between the regular 90-day cycles when necessary. Any irregular updates, however, do not affect the regular 90-day schedule. The timed release of a new CPF must be maintained because of the Universal Time Code (UTC) corrected (UT1)  time corrections and pole wander predictions included in the file. These parameters span a 180-day interval time centered on the effective start date of the new IAS CPF. The IAS maintains a CPF archive. All Landsat CPFs are available for download from the Landsat Calibration web page. Only the most recent CPFs are used in ongoing ETM+ data processing.

Prior to switching ETM+ to “bumper operational mode”, CPFs needed to be released on a regular quarterly basis, primarily because of the UT1 time corrections and the pole wander predictions included in the updated file. Following the mode switch on April 1, 2007, multiple version updates are expected during any given quarter due to a hardly predictive nature of the scanning mirror bumper parameters. The irregular (mid­quarter) updates do not affect the three­month CPF release schedule.

The CPF is time stamped by IAS with an effective date range. The first two parameters in the file, “Effective_Date_Begin” and “Effective_Date_End”, designate the range and are of the form YYYY-MM-DD. The “Effective_End_Date” for the most recent parameter file is its “Effective_Date_Begin” plus 90 days. After this date, the file is without applicable UT1 time predictions. The parameter file that accompanies an order has an effective date range that includes the acquisition date of the image ordered.

Through the course of the mission, a serial collection of CPFs is generated and sent to EROS for coupling to Level-0R products. A distinct probability exists that a CPF will be replaced due to improved calibration parameters for a given period or perhaps due to file error. The need for unique file sequence numbers becomes necessary as file contents change. In order to generate a unique file name, the IAS uses the following file naming procedure:

L7CPFy1y1y1y1m1m1d1d1_y2y2y2y2m2m2d2d2.nn

where:    

L7 = Constant for Landsat 7
 CPF = Three-letter CPF designator
 y1y1y1y1 = Four-digit effectivity starting year
 m1m1 = Two-digit effectivity starting month
 d1d1 = Two-digit effectivity starting day
 _ = Effectivity starting/ending date separator
 y2y2y2y2 = Four-digit effectivity ending year
 m2m2 = Two-digit effectivity ending month
 d2d2 = Two-digit effectivity ending day
 . = Ending day/version number separator
 nn = Sequence number for this file (starts with 01)

As an example, suppose four calibration files were created by the IAS on 90-day intervals and sent to EROS during a given year of the mission. Further suppose that the first file was updated twice and the second and third files were updated once. The assigned filenames would be as follows:

File 1 L7CPF19980601_19980829.00
  L7CPF19980601_19980829.01
  L7CPF19980601_19980829.02
File 2 L7CPF19980830_19981127.01
  L7CPF19980830_19981127.02
File 3 L7CPF19981128_19990225.01
  L7CPF19981128_19990225.02
File 4 L7CPF19990226_19990526.01

The 00 sequence number assigned to the original CPF (File 1) uniquely identifies this as a pre-launch CPF. Sequence numbers for subsequent time periods all begin with 01. New versions or updates are incremented by one.

This example assumes the effectivity dates do not change. The effectivity date range for a file can change, however, if a specific problem (e.g., detector outage) is discovered somewhere within the nominal 90-day effectivity range. Assuming this scenario, two CPFs with new names and effectivity date ranges are generated for the time period under consideration. The “Effective_Date_End” for a new pre-problem CPF would change to the day before the problem occurred. The “Effective_Date_Begin” remains unchanged. A post-problem CPF with a new file name would be created with an “Effective_Date_Begin” corresponding to the imaging date when the problem occurred. The “Effective_Date_End” assigned would be the original “Effective_Date_End” for the time period under consideration. New versions of all other CPFs affected by the erroneous parameter also would be created.

Using this example, suppose a dead detector is discovered to have occurred on January 31, 1999. Two new CPFs are created that supersede the time period represented by file number three, version 2, and a new version of file number four is created. The new file names and sequence numbers become:

File 3 L7CPF19981128_19990225.01
  L7CPF19981128_19990225.02
  L7CPF19981128_19990131.03
  L7CPF19990201_19990225.03
File 4 L7CPF19990226_19990526.01
  L7CPF19990226_19990526.02

3.10.3    File Structure

All calibration parameters are stored as ASCII text using the Object Description Language (ODL) syntax developed by NASA’s Jet Propulsion Laboratory (JPL). ODL is a tagged keyword language developed to provide a human-readable data structure to encode data for simplified interchange. The body of the file is composed of two statement types:

  1. Attribute assignment statement used to assign values to parameters.
  2. Group statements used to aid in file organization and enhance parsing granularity of parameter sets.

To illustrate, consider the first three parameters in the file: “Effective_Date_Begin”, “Effective_Date_End”, and the “CPF_File_Name”. These three parameters form their own group, which is called “FILE_ATTRIBUTES”. The syntax employed for this collection of parameters in the CPF appears as:

GROUP = FILE_ATTRIBUTES
Effective_Date_Begin = 1999-02-26
Effective_Date_End = 1999-05-26
CPF_File_Name = L7CPF19990226_19990526.0
1
END_GROUP = FILE_ATTRIBUTES

A description of the CPF contents is in Appendix B.


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