ETM+ Description
The ETM+ is a fixed position, nadir viewing, "whisk-broom", multispectral scanning radiometer and is capable of providing high-resolution imaging information of the earth's surface. Radiation in both the visible and infrared regions of the spectrum are detected by the instrument in eight distinct bands. The ETM+ is an improved version of the Landsat 4/5 Thematic Mapper (TM) payloads, but still provides data continuity with all prior Landsat missions. Improvements in the instrument include increased spatial resolution of the thermal IR band (Band 6), improvement of the radiometric calibration equipment, and the addition of a panchromatic band (Band 8). Below is a simplified diagram of the ETM+.
The ETM+ is designed to collect, filter, and detect radiation from the earth in a 185 km wide swath as it passes overhead. The viewing swath is produced by means of an oscillating mirror system that sweeps across track as the sensor field of view moves forward along track due to space vehicle motion. The data from the ETM+ is output on two channels, each at 75 Mbps. Each of the channels is multiplexed to contain data from several detectors along with Payload Correction Data (PCD), time stamp, and instrument status. The data channels contain:
Channel 1 = Band 1-3 (Visible), Band 4 (VNIR), Band 5 (SWIR), Band 6 (LWIR), time, PCD, status data
Channel 2 = Band 6 (LWIR), Band 7 (SWIR) and Band 8 (Pan), time, PCD, status data
Data from each band can be selected for output in High or Low gain, a commandable setting to adjust the multiplexor reference voltage. Band Six appears in both channels, with the data in Channel One fixed in High gain and the Band Six data in Channel Two fixed in low gain.
The spectral range of the ETM+ is as follows:
| Spectral Band |
Half Amplitude Bandwidth (micro-m) |
IFOV Size (micro-r) |
Instantaneaous Geometric FOV (m) |
Nominal Ground Sample Distance (m) |
| Panchromatic |
0.522-0.90 |
18.5x21.3 |
13x15 |
15 |
| 1 |
0.45-0.52 |
42.6 |
30 |
30 |
| 2 |
0.52-0.60 |
42.6 |
30 |
30 |
| 3 |
0.63-0.69 |
42.6 |
30 |
30 |
| 4 |
0.76-0.90 |
42.6 |
30 |
30 |
| 5 |
1.55-1.75 |
42.6 |
30 |
30 |
| 6 |
10.4-12.5 |
85.2 |
30 |
60 |
| 7 |
2.08-2.35 |
42.6 |
30 |
30 |
The ETM+ contains two pieces of external moving hardware: the Full Aperture Calibration Door and the Cold Focal Plane Radiative Cooler Door. Both are restrained during launch and ascent and will be freed within the first two days after launch. The Cooler door will be moved to its Outgas position, and the FAC door will be left in place and not moved until the ETM+ has fully outgassed (approximately 45 days after launch).
The ETM+ optics contain the Scan mirror and Scan Line Correction Assembly among other components. The Scan mirror, scanning at 7Hz, provides the across track motion for the imaging while the forward velocity of the spacecraft provides the along track motion. The Scan Line Correction Assembly is used to remove the 'zig-zag' motion of the imaging FOV produced by the combination of the along and across track motion.
Below is a simplified drawing of the optics within the ETM+. Focal planes are also shown.
There are two focal planes used by the ETM+. The Primary Focal plane uses Silicon photodiode material as a sensor and detects radiation for Bands 1-8. The Cold Focal Plane uses detectors of Indium Antimonide (InSb) and Mercury Cadmium Telluride (HgCdTe), (which operate best at temperatures in the area of 77 deg K) and senses radiation for Bands 5, 6, and 7. Because the Cold Focal Plane must be radiated into space to maintain proper themal balance, certain considerations must be made during non-routine operations. Exposure of the radiator to sunlight may cause permanent damage to the ETM+.
The ETM+ contains several survival heaters that are necessary during backup attitude control modes to keep it thermally stable. These heaters will not be turned off during normal operations.
Calibration of ETM+ data is accomplished using several types of calibration schemes. The ETM+ is equipped with an internal calibration paddle that moves into the path of the incoming radiation once each scan line. This paddle has several calibration lamps with known energy signatures that, when turned on, will be sensed by the detectors and supply calibration data at the end of every scan line. In addition, the ETM+ is built to perform two types of calibration that use the Sun as a radiation source. Full Aperture Solar Calibration and Partial Aperture Solar Calibration will be performed periodically (at IAS request). Ground Look Calibration (GLC) will also be performed on scenes identified and requested by the IAS.
ETM+ - Calibration
There will be two special types of operations associated with the calibration of the ETM+. They are Full Aperture Solar Calibration (FASC) and Partial Aperture Solar Calibration (PASC). Both of these operations must take place in specific orbital positions and involve special calibration equipment on-board the spacecraft.
PASC will be scheduled once every day during normal operations and is used for radiance calibration of Bands 1-5, 7, and 8. Approximately one minute after the spacecraft passes into sunlight and while its subsatellite point is still in darkness (see Figure), the ETM+ is commanded to image. The PASC consists of a small device housed in the ETM+ sunshade. This device reflects sunlight off of two of its surfaces through a small aperture when the spacecraft is in the orbital position described above. Sunlight is used because the Sun is a well known, well characterized source of radiation. The ETM+ is then commanded to image just as it would be during normal operations and effectively creates an image of the Sun. The operation lasts just over two minutes. The IAS will be notified daily via the MOC Events Schedule Report of the operation so that they may obtain the image data from EDC-DAAC for processing. Additional operations will be done at the request of the IAS.
FASC will be scheduled approximately once every four to six weeks during normal operations, and at least three times during on-orbit checkout. FASC performs the full aperture radiance calibration for Bands 1-5, 7, and 8. FASC operations involves positioning the FASC paddle in front of the ETM+ aperture to reflect sunlight into the field of view. The ETM+ will then image for at least 10 scans in low gain, switched to high gain for at least 10 scans, and switched back again to low gain for at least 10 scans. This operation will take place over the North Pole terminator. The IAS will be notified of the operation so that they may obtain the image data from EDC-DAAC for processing. Additional operations will be done at the request of the IAS.
In addition to PASC and FASC operations, calibration data will be extracted from normal ETM+ images taken over specified calibration targets. These targets consist of landmass scenes with well known, well characterized features and/or properties and will be supplied to the FOT by the IAS. Ground Look Calibration (GLC) will be done using ground truth images to help characterize the effective radiance seen by the ETM+. GLC will occur approximately every two to six months during on-orbit operations. Geometric calibration will also be done using normal ETM+ images of specified scenes. Calibration data is also injected into the normal data stream during all imaging operations. This data includes internal shutter, lamp, blackbody, and cold focal plane array temperatures.
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