Landsat Missions

Landsat 8 (L8) Data Users Handbook - Section 2

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Section 1 – Introduction

Section 2 – Observatory Overview

Section 3 – Instrument Calibration

Section 4 – Level-1 Products

Section 5 – Conversion of DNs to Physical Units

Section 6 – Data Search and Access

Appendix A – Known Issues

Appendix B – Metadata File (MTL.txt)

References

Download Landsat 8 (L8) Data User Handbook (.pdf 4.39 MB)


Section 2 - Observatory Overview

Figure 2-1. Illustration of  Landsat 8 Observatory
Figure 2-1. Illustration of Landsat 8 Observatory
The Landsat 8 observatory is designed for a 705 km, sun-synchronous orbit, with a 16-day repeat cycle, completely orbiting the Earth every 98.9 minutes. S-Band is used for commanding and housekeeping telemetry operations while X-Band is used for instrument data downlink. A 3.14 terabit Solid State Recorder (SSR) brings back an unprecedented number of images to the USGS EROS Center archive.

Landsat 8 carries a two-sensor payload: the Operational Land Imager (OLI), built by the Ball Aerospace & Technologies Corporation; and the Thermal Infrared Sensor (TIRS), built by the NASA Goddard Space Flight Center (GSFC). Both the OLI and TIRS sensors simultaneously image every scene, but are capable of independent use should a problem in either sensor arise.  In normal operation the sensors view the Earth at nadir on the sun synchronous WRS-2 orbital path, but special collections may be scheduled off-nadir. Both sensors offer technical advancements over earlier Landsat instruments.  The spacecraft with its two integrated sensors is referred to as the Landsat 8 observatory.

2.1 Concept of Operations

The fundamental Landsat 8 operations concept is to collect, archive, process, and distribute science data in a manner consistent with the operation of the Landsat 7 satellite system. To that end, the Landsat 8 observatory operates in a near-circular, near-polar, sun-synchronous orbit with a 705 km altitude at the equator. The observatory has a 16-day ground track repeat cycle with an equatorial crossing at 10:11 a.m. (+/−15 min) mean local time during the descending node. In this orbit, the Landsat 8 observatory follows a sequence of fixed ground tracks (also known as paths) defined by the second Worldwide Reference System (WRS-2). WRS-2 is a path/row coordinate system used to catalog all the science image data acquired from the Landsat 4 - 8 satellites. The Landsat 8 launch and initial orbit adjustments placed the observatory in an orbit to ensure an eight-day offset between Landsat 7 and Landsat 8 coverage of each WRS-2 path.

The Mission Operation Center (MOC) sends commands to the satellite once every 24 hours via S-band communications from the ground system to schedule daily data collections. A Long Term Acquisition Plan (LTAP-8) sets priorities for collecting data along the WRS-2 ground paths covered in a particular 24-hour period. LTAP-8 is modeled on the systematic data acquisition plan developed for Landsat 7 (Arvidson et al., 2006). OLI and TIRS collect data jointly to provide coincident images of the same surface areas. The MOC nominally schedules the collection of 400 OLI and TIRS scenes per day where each scene covers a 190-by-180 km surface area. The objective of scheduling and data collection is to provide near cloud-free coverage of the global landmass for each season of the year.  Since the 2014 growing season, however, the Landsat 8 mission has been routinely acquiring approximately 725 scenes/day.

The Landsat 8 observatory initially stores OLI and TIRS data on board in a solid-state recorder. The MOC commands the observatory to transmit the stored data to the ground via an X-band data stream from an all-earth omni antenna. The Landsat 8 ground network receives the data at several stations and these stations forward the data to the EROS Center. The ground network includes international stations operated under the sponsorship of foreign governments referred to as International Cooperators (ICs). Data management and distribution by the ICs is in accordance with bilateral agreements between each IC and the U.S. Government.

The data received from the ground network are stored and archived at the EROS Center where also Landsat 8 data products are generated. The OLI and TIRS data for each WRS-2 scene are merged to create a single product containing the data from both sensors. The data from both sensors are radiometrically corrected and co-registered to a cartographic projection with corrections for terrain displacement resulting in a standard orthorectified digital image called the Level-1T product. The interface to the Landsat 8 data archive is called the User Portal and it allows anyone to search the archive, view browse images, and request data products that are distributed electronically through the internet at no charge (see Section 6 - Data Search and Access).

2.2 Operational Land Imager (OLI)

Figure 2-2. OLI Instrument
Figure 2-2. OLI Instrument

The OLI sensor, which has a five-year design life, is similar in design to the Advanced Land Imager (ALI) that was included on EO-1, and represents a significant technological advancement over Landsat 7's Enhanced Thematic Mapper Plus (ETM+) sensor.  Instruments on earlier Landsat satellites employed oscillating mirrors to sweep the detectors' field of view across the swath width (“whiskbroom”), but OLI instead uses long linear detector arrays with thousands of detectors per spectral band.  Detectors aligned across the instrument focal planes collect imagery in a “push broom” manner resulting in a more sensitive instrument with fewer moving parts. OLI has a four-mirror telescope and data generated by OLI are quantized to 12 bits, compared to the 8-bit data produced by the TM & ETM+ sensor.

Table 2-1 . OLI and TIRS Spectral Bands Compared to ETM+ Spectral Bands
Table 2-1. OLI and TIRS Spectral Bands Compared to ETM+ Spectral Bands

The OLI sensor collects image data for nine shortwave spectral bands over a 190 km swath with a 30 m spatial resolution for all bands except the 15 m panchromatic band.  The widths of several OLI bands are refined to avoid atmospheric absorption features within ETM+ bands. The biggest change occurs in OLI band 5 (0.845–0.885 μm) to exclude a water vapor absorption feature at 0.825 μm in the middle of the ETM+ near infrared band (band 4; 0.775–0.900 μm). The OLI panchromatic band, band 8, is also narrower relative to the ETM+ panchromatic band to create greater contrast between vegetated areas and land without vegetation cover.  OLI also has two new bands in addition to the legacy Landsat bands (1-5, 7, and Pan).  The Coastal /Aerosol band (band 1; 0.435-0.451 μm), principally for ocean color observations, is similar to ALI's band 1', and the new Cirrus band (band 9; 1.36-1.38 μm) aids in detection of thin clouds comprised of ice crystals (cirrus clouds will appear bright while most land surfaces will appear dark through an otherwise cloud-free atmospheres containing water vapor).

OLI has stringent radiometric performance requirements and is required to produce data calibrated to an uncertainty of less than 5% in terms of absolute, at-aperture spectral radiance and to an uncertainty of less than 3% in terms of top-of-atmosphere spectral reflectance for each of the spectral bands in Table 2‑1. These values are comparable to the uncertainties achieved by ETM+ calibration.

The OLI signal-to-noise ratio (SNR) specifications, however, were set higher than ETM+ performance based on results from the ALI. Table 2‑2 and Figure 2‑3 show the OLI specifications and performance compared to ETM+ performance for signal-to-noise ratios at specified levels of typical spectral radiance (Ltypical) for each spectral band.

Table 2-2 . OLI Specified and Performance  Signal-to-Noise (SNR) Ratios Compared to ETM+ Performance
Table 2-2 . OLI Specified and Performance Signal-to-Noise (SNR) Ratios Compared to ETM+ Performance

Figure 2-3 . OLI  Signal-To-Noise (SNR) Performance at Ltypical
Figure 2-3. OLI Signal-To-Noise (SNR) Performance at Ltypical

The OLI is a push broom sensor that employs a four mirror anastigmatic telescope that focuses incident radiation onto the focal plane while providing a 15-degree field-of-view covering the 190 km across-track ground swath from the nominal Landsat 8 observatory altitude. Periodic sampling of the across-track detectors as the observatory flies forward along a ground track forms the multispectral digital images. The detectors are divided into 14 identical Sensor Chip Assemblies (SCAs) arranged in an alternating pattern along the centerline of the focal plane (Figure 2‑4).

Figure 2 4. OLI Focal Plane
Figure 2-4. OLI Focal Plane

Each SCA consists of rows of detectors, a read-out integrated circuit (ROIC), and a nine-band filter assembly.  Data are acquired from 6916 across-track detectors for each spectral band (494 detector per SCA) with the exception of the 15 m panchromatic band that contains 13,832 detectors. The spectral differentiation is achieved by interference filters arranged in a “butcher-block” pattern over the detector arrays in each module.  Even and odd numbered detector columns are staggered and aligned with the satellite's flight track.  Even-numbered SCAs are the same as odd-numbered SCAs, only the order of the detector arrays is reversed top to bottom.  The detectors on the odd and even SCAs are oriented such that they look slightly off nadir in the forward and aft viewing directions.  This arrangement allows for a contiguous swath of imagery as the push broom sensor flies over the Earth, with no moving parts.  There is one redundant detector per pixel in each VNIR band, and two redundant detectors per pixel in each SWIR band. The spectral response from each unique detector corresponds to an individual column of pixels within the Level-0 product.

Figure 2-5. Odd/Even SCA Band  Arrangement
Figure 2-5. Odd/Even SCA Band Arrangement

Silicon PIN (SiPIN) detectors collect the data for the visible and near-infrared spectral bands (Bands 1 to 4 and 8) while Mercury–Cadmium–Telluride (MgCdTe) detectors are used for the shortwave infrared bands (Bands 6, 7, and 9).  There is an additional 'blind' band that is shielded from incoming light and used to track small electronic drifts. There are 494 illuminated detectors per SCA, per band (988 for the PAN band); that is a total of 70,672 operating detectors that must be characterized and calibrated during nominal operations.

2.3 Thermal Infrared Sensor (TIRS)

Like OLI, TIRS is also a push broom sensor employing a focal plane with long arrays of photosensitive detectors.  TIRS uses Quantum Well Infrared Photodetectors (QWIPs) to measure longwave thermal infrared (TIR) energy emitted by the Earth’s surface, the intensity of which is a function of surface temperature. The TIRS QWIPs are sensitive to two thermal infrared wavelength bands, enabling separation of the temperature of the Earth’s surface from that of the atmosphere. QWIPs design operates on the complex principles of quantum mechanics. Gallium arsenide semiconductor chips trap electrons in an energy state ‘well’ until the electrons are elevated to a higher state by thermal infrared light of a certain wavelength. The elevated electrons create an electrical signal that can be read out, recorded, translated to physical units, and used to create a digital image.
 

Figure 2-6. TIRS Instrument with Earthshield Deployed
Figure 2-6. TIRS Instrument with Earthshield Deployed

The TIRS sensor, which has a three-year design life, collects image data for two thermal bands with a 100 m spatial resolution over a 190 km swath.  The two thermal infrared bands encompass the wavelength range of the broader TM and ETM+ thermal bands (10.0–12.5 μm), and represent an advancement over the single-band thermal data.  Data generated by TIRS are quantized to 12 bits.  Although TIRS has a lower spatial resolution than the 60 m ETM+ Band 6, the dual thermal bands should theoretically enable retrieval of surface temperature, but stray light issues with band 11 preclude the use of this approach.

Like OLI, the TIRS requirements also specify cross-track spectral uniformity; radiometric performance including absolute calibration uncertainty, polarization sensitivity, and stability; ground sample distance and edge response; image geometry and geolocation including spectral band co-registration. The TIRS noise limits (Table 2‑3) are specified in terms of noise-equivalent-change-in-temperature (NEΔT) rather than the signal-to-noise ratios used for OLI specifications. The radiometric calibration uncertainty is specified to be less than 2% in terms of absolute, at-aperture spectral radiance for targets between 260 K and 330 K (less than 4% for targets between 240 K and 260 K and for targets between 330 K and 360 K), which is much lower than ETM+ measurements between 272 K and 285 K.  Currently the performance of TIRS band 11 is slightly out of specification because of stray light entering the optical path.

Table 2 4. TIRS Noise-Equivalent-Change-in Temperature (NEΔT)
Table 2-4. TIRS Noise-Equivalent-Change-in Temperature (NEΔT)

The TIRS focal plane contains 3 identical SCAs, each with rows of QWIPs (Figure 2‑7).  The QWIP detectors sit between a read-out integrated circuit (ROIC) and a two-band filter assembly.  There is an additional masked or 'dark' band used for calibration purposes.  TIRS has 640 illuminated detectors per SCA, with approximately 27-pixel overlap to ensure there are no spatial gaps.  Each TIRS SCA consists of a 640 column by 512 row grid of QWIP detectors.  Almost all of the detectors are obscured except for two slits that contain the spectral filters for the 12.0 µm and 10.8 µm bands.  These filters provide unvignetted illumination for approximately 30 rows of detectors under each filtered region.

Figure 2-7 .  TIRS Focal Plane
Figure 2-7. TIRS Focal Plane

Thermal energy enters the TIRS instrument through a scene select mirror and a series of four lenses before illuminating the QWIP detectors on the Focal Plane Array (FPA).  Two rows of detector data from each filtered region are collected, with pixels from the second row used only as substitutes for any inoperable detectors in the primary row.  These rows are specified in the calibration parameter file (CPF).

Figure 2-8.  TIRS Optical Sensor Unit
Figure 2-8. TIRS Optical Sensor Unit

2.4 Spacecraft Overview

The Landsat 8 spacecraft was built by Orbital Sciences Corporation at their spacecraft manufacturing facility in Gilbert, Arizona. It was originally awarded to General Dynamics Advanced Information Systems (GDAIS) in April 2008, but was subsequently acquired by Orbital Sciences Corporation in 2010.  Orbital assumed responsibility for the design and fabrication of the Landsat 8 spacecraft bus, integration of the two sensors onto the bus, satellite level testing, on-orbit satellite check-out, and continuing on-orbit engineering support under GSFC contract management (Irons & Dwyer, 2010). The specified design life is five years with an additional requirement to carry sufficient fuel to maintain the Landsat 8 orbit for 10 years.  However, the hope is that the operational lives of the sensors and spacecraft will exceed the design lives and fuel will not limit extended operations.

The spacecraft supplies power, orbit and attitude control, communications, and data storage for OLI and TIRS. The spacecraft consists of the mechanical subsystem (primary structure and deployable mechanisms), command and data handling subsystem, attitude control subsystem, electrical power subsystem, radio frequency
(RF) communications subsystem, the hydrazine propulsion subsystem and thermal control subsystem. All the components, except for the propulsion module, are mounted on the exterior of the primary structure. A 9×0.4 m deployable sun-tracking solar array generates power that charges the spacecraft's 125 amp-hour nickel–hydrogen
(Ni–H2) battery. A 3.14-terabit solid-state data recorder provides data storage aboard the spacecraft and an earth-coverage X-band antenna transmits OLI and TIRS data either in real time or played back from the data recorder. The OLI and TIRS are mounted on an optical bench at the forward end of the spacecraft. Fully assembled, the spacecraft without the instruments is approximately 3 m high and 2.4×2.4 m across with a mass of 2071 kg fully loaded with fuel.

2.4.1 Spacecraft Data Flow Operations

The Landsat 8 observatory receives a daily load of software commands transmitted from the ground. These command loads tell the observatory when to capture, store, and transmit image data from the OLI and TIRS. The daily command load covers the subsequent 72 hours of operations with the commands for the overlapping 48 hours overwritten each day. This precaution is taken to ensure that sensor and spacecraft operations continue in the event of a one or two day failure to successfully transmit or receive commands. The observatory's Payload Interface Electronics (PIE) ensures that image intervals are captured in accordance with the daily command loads. The OLI and TIRS are powered on continuously during nominal operations to maintain the thermal balance of the two instruments. The two sensors' detectors continuously produce signals that are digitized and sent to the PIE at an average rate of 265 megabits per second (Mbps) for the OLI and 26.2 Mbps for TIRS.

Ancillary data such as sensor and select spacecraft housekeeping telemetry, calibration data, and other data necessary for image processing are also sent to the PIE. The PIE receives the OLI, TIRS, and ancillary data, merges these data into a mission data stream, identifies the mission data intervals scheduled for collection, performs a lossless compression of the OLI data (TIRS data are not compressed) using the Rice algorithm (Rice et al., 1993), and then sends the compressed OLI data and the uncompressed TIRS data to the 3.14 terabit SSR. The PIE also identifies those image intervals scheduled for real time transmission and sends those data directly to the observatory's X-band transmitter. The International Cooperator receiving stations only receive real time transmissions and the PIE also sends a copy of these data to the on-board SSR for playback and transmission to the Landsat 8 GNE receiving stations (USGS captures all of the data transmitted to International Cooperators). Recall that OLI and TIRS collect data coincidently and therefore the mission data streams from the PIE contain both OLI and TIRS data as well as ancillary data.

The observatory broadcasts mission data files from its X-band, Earth-coverage antenna. The transmitter sends data to the antenna on multiple virtual channels providing for a total data rate of 384 Mbps. The observatory transmits real time data, SSR playback data, or both real-time data and SSR data depending on the time of day and the ground stations within view of the satellite. Transmissions from the Earth coverage antenna allow a ground station to receive mission data as long as the observatory is within view of the station antenna.  OLI and TIRS collect the Landsat 8 science data. The spacecraft bus stores the OLI and TIRS data on an onboard solid-state recorder and then transmits the data to ground receiving stations. 

The ground system provides the capabilities necessary for planning and scheduling the operations of the Landsat 8 observatory and the capabilities necessary to manage the science data following transmission from the spacecraft. The real-time command and control sub-system for observatory operations is known as the Mission Operations Element (MOE). A primary and back-up Mission Operations Center (MOC) houses the MOE with the primary MOC residing at NASA GSFC. The Data Processing and Archiving System (DPAS) at the EROS Center ingests, processes, and archives all Landsat 8 science and mission data returned from the observatory. The DPAS also provides a public interface to allow users to search for and receive data products over the internet (see Section 6 - Data Search and Access).

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