Three Decades of Landsat Instruments

Home , Landsat 1, Landsat 2, Landsat 3, Landsat 6, Landsat 7

Aram M. Mika

Abstract Principal interests of the U~DAincluded synoptic moni- This paper traces the development history of the multispec- toring of agricultural activity to provide additional data for tral sensors for the Landsat series of satellites, from the first acreage control and management of support payments. Forest Multispectral Scanner aboard Landsat 1 to the latest variant inventories and urban land-use assessment were additional of the Enhanced Thematic Mapper for Landsat 7. For each topics of interest. Concurrently, DOI was interested in photo- sensor, we begin with an overview of the design objectives geology, orthophoto mapping, photogrammetry, and map and program context in which the instrument specifications compilation. At the same time, NASA was engaged in a series were established. This is followed by a design description of laboratory and ground studies, airborne measurements that outlines the operation of each sensor and highlights key from a multispectral scanning instrument, high-altitude false- technology features. The discussion for each of these instru- color infrared photography, as well as Earth-observation ex- ments is concluded by a performance summary and opera- periments on the Gemini and Apollo missions. tional history. All of these activities crystallized in the definition of the payload instruments for the Earth Resources Technology Sat- ellite (ERTS), later renamed Landsat 1. The ERTS payload in- Introduction cluded two principal sensing instruments: the Return-Beam The multicolored Landsat images that adorn the walls and Vidicon (REW) system and the Multispectral Scanner (MSS). research papers of nearly everyone in the Earth-science com- The RB~system, consisting of three coaligned television cam- munity originated from a handful of spaceborne sensors de- eras, was initially considered to be the primary sensor on veloped over the past three decades, and several of these ERTS 1, while the MSS would serve as a secondary instru- sensors are still operating today. These instruments have ment. However, concerns about the geometric fidelity and ra- transformed the way that we look at the Earth and have diometric repeatability of the R~V,coupled with the spawned a new remote-sensing industry that is poised to outstanding performance of the changed the operating flourish in the years ahead. ms, protocol once the spacecraft was in orbit: the MSS became While Landsat instruments are fundamentally just electro- the primary imaging instrument on ERTS in fairly short order. optical transducers that ingest photons and eject a digital bit The spatial, spectral, and radiometric performance require- stream, this transduction relies upon the state of the art in nu- ments for both of these payload instruments are summarized merous technologies including optics, precision electrome- in Table 1. chanics, detectors, advanced materials, cryogenics, and signal Spatially, both the REW and MSS on Landsats 1 and 2 processing. The development of these transducers has fol- provided a resolution (instantaneous field-of-view) of approx- lowed a path of progressive sophistication that has exploited imately 80 metres. Spectrally, the three-camera REW system advances in these technologies. Each successive sensor-devel- was designed to cover the blue-green, yellow-red, and red/ opment effort has balanced technical risk, performance, and near-infrared regime, while the MSS was slated to provide a reliability to meet increasingly demanding mission objectives, similar band set, along with a fourth band to extend coverage but this technical progression has been tempered by budget further into the near-infrared spectrum. A later variant of the and schedule pressures that have often played a decisive role RBV system for Landsat 3 utilized two cameras with panchro- in selecting the payload design, particularly for Landsats 6 matic spectral response and higher spatial resolution (40 m) and 7. Nevertheless, infusion of updated technology has to complement the multispectral coverage provided by the added new internal capability and refinement to instruments MSS. that may appear outwardly similar. Spatial resolution, spectral Upon closer examination, the spectral-band specifica- coverage, radiometric sensitivity, calibration accuracy, and re- tions for the MSS seem rather curious (with benefit of 25 liability have all been upgraded over the years. years of hindsight): the band selections progress in multiples of tenth-micrometre intervals. Surely, nature is not so coop- The Multispectral Scanner erative as to arrange the reflective spectral features of vegetation and minerals at these uniform wavelength increments. Design Objectives In actuality, the MSS (and original ~sv)bands were selected The concept for an Earth-resources technology satellite took from a very practical standpoint: to provide image products shape in the early 1960s following the success of early that would approximate false-color infrared aerial photogra- weather-observation spacecraft such as TIROS - the Televi- phy film. Because researchers already had some experience sion-Infrared Observation Satellite. The U.S. Department of in evaluating and interpreting such images from airborne Agriculture (USDA), along with the Department of the Interior platforms, a spaceborne extension of that capability seemed (DOI), began working with NASA to define the instrumentation to be a natural progression; at the time, there was a very lim- for a satellite tailored for observation of the solid Earth.

Hughes Aircraft Company, 2000 E. El Segundo Boulevard, Photogrammetric Engineering & Remote Sensing, Bldg. El, MIS 150, El Segundo, CA 90245. Vol. 63, No. 7, July 1997, pp. 839-852.

The author is presently with Lockheed Martin Missles and 0099-1112/97/6307-839$3.00/0 Space, 1111 Lockheed Martin Way, Bldg. 101, Sunnyvale, O 1997 American Society for Photogrammetry CA 94089. and Remote Sensing

PE&RS July 1997 839 ited experience base for interpreting and analyzing images with other spectral-band combinations. Scan At any rate, the MSS progressed from design studies to subsystem-technology demonstrations to a full-scale flight- hardware development program commencing in 1967.

Per Band Design Description The Mss is a scanning multispectral imaging radiometer that produces radiometrically accurate images of the Earth utilizing a scanning system that covers a 185-km swath across the orbital path of the satellite. This is accomplished by employing an object-space scan mirror - so called because it is placed in front of the sensor's telescope, in the same "space" as the objects to be viewed. Operation of the scanner is dia- grammed schematically in Figure l. Light from the Earth is directed by the scan mirror through a reflective telescope to a fiber-optic array at the focal plane. The MSS operates by re- peatedly scanning this 24-element fiber-optic array from west-to-east across the Earth, while the orbital motion of the spacecraft provides a natural north-to-south scanning motion. The fiber-optic bundle at the focal plane fans out to six detector elements for each of the four spectral bands: 18 of these are photomultiplier tubes, while six are discrete silicon photodiodes. The state of the art in 1966 technology drove the design decisions that led to the ~ss'configuration. At that time, integrated detector arrays had not yet been developed, so fiber optics (very modern in 1966) were the only practical avenue for achieving relatively high effective detector density at the focal plane - a requisite for high spatial resolution. Because detectors were large, expensive, and required extensive support electronics (especially in the case of photomultiplier tubes with their high-voltage power supplies), design tradeoffs were conducted to minimize the number of detectors that would be required. In the limiting case, the ~sscould have been designed with only one detector per spectral band, but such an approach would have resulted in unwork- able scan rates and electronic bandwidths. A sensor design with six detectors per spectral band proved optimal in the context of late 60's technology. The scan mirror assembly was the key to providing wide-field, high-resolution coverage. The mirror follows a (b) "sawtooth" scanning waveform at a frequency of 13.5 Hz Figure 1. MSS scanning approach. (a) Object-space scan (74-millisecond period) with active imaging taking place mirror scans image west-to-east across track while orbital motion provides north-south scan. (b) Fiber-Optic array re- only during the forward scan. The net scan efficiency - the fraction of the total scan period devoted to active imaging - lays scanned image from focal plane to 24 discrete detectors. Active imaging only takes place in one scan is 45 percent. The balance of the scan period is occupied by the retrace interval and settling time required for the mirror direction. to return to high-fidelity linear motion. Controlling the scan mirror to provide a precise, linear scan was a substantial technical challenge. The scan mirror had to be structurally rigid in order to maintain excellent dy- year-after-year of on-orbit operation of these scan mirrors namic flatness to preserve optical performance, yet with min- (without a failure to date) has proven this point. imal mass and rotational inertia to accommodate the rela- While the design of the scan-mirror assembly was eso- tively high scan frequency. These conflicting requirements teric for its day, the optical design of the ~sswas quite were satisfied by use of lightweight beryllium for the scan straightforward, utilizing a Ritchey-Chretien telescope form mirror: beryllium is an extraordinarily rigid and low-density with a 22.9-cm aperture diameter and a focal ratio of ft3.6. metal, and mass was further reduced by removing additional The Ritchey-Chretien is a high-performance variant of the material in the core of the mirror by using electric-discharge conventional Cassegrain design: the Cassegrain utilizes a par- machining. abolic primary mirror, while the Ritchey-Chretien employs a Mindful of the concerns regarding the reliability of elec- hyperboloidal figure for both the primary and secondary mir- tromechanical subsystems, the MSS scan mirror was designed ror to provide excellent resolution over a larger field-of-view. with flexure pivots in lieu of conventional bearings. These This latter feature is important for sensors that have ex- flex pivots are, in effect, torsional springs with high radial tended focal planes with multiple detectors - in this case, stiffness. When designed and utilized properly, they will ex- the %-element fiber-optic bundle of the Mss. The telescope hibit infinite fatigue life, so that the mechanical elements of mirrors are made of fused silica (quartz) glass and held in the scan mirror are not life limiting. Indeed, reliability analy- alignment by an optical metering structure made of Invar - ses indicated that the servo-drive electronics would ulti- a low-expansion steeltnickel alloy. mately prove to be the life-limiting factor, and successful The signal-processing chain for the Mss is straightfor-

July 1997 PE&RS ward, consisting of preamplifiers, analog-to-digital converters, and a multiplexer that merges the digitized data from the 24 detector channels into a 15-megabit-per-second serial bit stream. All of this hardware was packaged into a sensor assembly with dimensions of 53 by 58 by 127 cm, having a mass of 64 kg, and drawing 50W of power during imaging operation, as illustrated in Figure 2. Key performance specifications for the MSS include 80- metre spatial resolution and 6-bit radiometric quantization Although this performance is moderate by today's standards, global multispectral space imagery of that caliber was un- precedented in 1972, and the success of the Mss on Landsat 1 led to similar instruments aboard Landsats 2 and 3 and, later, to the genesis of the Thematic Mapper on Landsats 4 and 5. The radiometric performance for several delivered flight instruments is summarized in Table 2. A total of six Multispectral Scanners were built. The first, a non-flight engineering model, is now on display at the Smithsonian's Air and Space Museum. The first flight model for ERTS was delivered in late 1971 and subsequently launched in 1972, and the second flight model was delivered two years later. Flight model three incorporated a thermal-infrared spectral band in addition to the four visible and infrared bands. This upgrade was initially slated for the second flight model but was deferred to flight three in order to pre- ROTATINO SHIliTER serve the launch schedule for the second spacecraft. The 10.4- to 12.6-ym thermal-infrared band on Mss 3 was designed to provide about 240-m spatial resolution by using two mercury-cadmium-telluride detectors that were radia- tively cooled to 90°K. The thermal band proved problematic and ultimately saw little use on orbit due to failure of one of the detectors and recurring condensation of moisture on the radiative cooler. The MSS design was subsequently modified for the Land- sat D and D' program (Landsat 4 and 5). In contrast to Land- sats 1 through 3, which were designed to fly in a 909-km orbit, Landsat 4 was slated to fly in a 705-km orbit, so the telescope and scan-servo designs were modified to accommo- WN CALIBRATE MIRROR date the different altitude and resulting change in ground- SCAN MONITOR track velocity (and, hence, scan period). The thermal infrared band that had been incorporated experimentally on Landsat TODIODE DETECTORJPREAWP 3 was dropped from the ~sson Landsats 4 and 5 because this spectral coverage would be provided by the new The- (b) matic Mapper. Figure 2. Multispectral scanner hardware. (a) External As an aside, the numbering convention for Mss spectral view of MSS flight hardware. (b) Artist's cutaway view of bands was also changed in the transition from the Landsat 1, MSS internal details. 2, 3 series to the Landsat 4 and 5 series: in the former, the MSS bands were numbered 4 through 7 (4 through 8 for

TABLE1. ERTS (LANOSAT 1) PAYLOADSPEC~FICAT~ONS Instrument Spatial Spectral Radiometric Return-Beam Vidicon (RBV) 80-m IFOV (instantaneous Three spectral bands: Analog video signal System field-of-view) transmitted. 1. 0.48 - 0.58 pm [Three coaligned cameras; one 185-km by 185-km 2. 0.58 - 0.68 pm 33-dB signal-to-noise ratio in for each spectral band.] Framing Cameras 3. 0.70 - 0.83 pm bands 1 and 2; 30-dB in Band 3, all at max radiance (highlights). Multispectral Scanner (MSS) 79-m IFOV Four Spectral Bands: Digital video transmitted:

185-km Swath Scanning 4. 0.5 - 0.6 pm 6 bits per pixel, linear Sensor (Continuous strip 5. 0.6 - 0.7 pm coding; logarithmic coding image) 6. 0.7 - 0.8 pm also available on bands 4, 5 7. 0.8 - 1.1 pm and 6.

I PE&RS July 1997 841 I TABLE2. SIGNAL-TDNOISERATIO MEASUREMENTS FOR DELIVEREDFLIGHT HARDWARE Spectral Spec MSS Spec MSS Band 1-3 MSS - 1 MSS - 2 4 - 5 MSS - 4 129 59 69 98 57 74 76 3 7 62 130 57 84 - - --

TABLE3. MSS LAUNCHAND OPERATINGHISTORY MSS Spacecraft Launch Date Operating Life Remarks - Landsat 1 (ERTS) 23 Jul 72 5.5 years Deactivated after 5.5 years. Landsat 2 19 Jan 75 8.5 years Landsat 3 05 Mar 78 5.5 years Landsat 4 16 Jul 82 14.7+ years Instrument still fully functional; spacecraft on standby. Active operation for first -7 years. Landsat 5 01 Mar 84 13.1+ years Instrument and spacecraft still functional for direct- downlink service. MSS only activated when requested by customers; TM in daily use.

TABLE4. THEMATICMAPPER SPECTRAL BANDS WERE SELECTED FOR SPECIFIC APPLICATIONS Color Application Blue 0.45 - 0.52 Soillvegetation discrimination, deciduous/coniferous forest differentiation, clear-water bathymetry Green 0.52 - 0.60 Growthlvigor indication for vegetation, sediment estimation, turbid-water bathymetry Red 0.63 - 0.69 Crop classification, ferric iron detection, ice and snow mapping Near Infrared (NIR) 0.76 - 0.90 Biomass surveys, water-body delination Shortwave Infrared (SWIR) 1.55 - 1.75 Vegetation moisture, snow-cloud differentiation Shortwave Infrared (SWIR) 2.08 - 2.35 Hydrothermal mapping, rocklsoil type discrimination for mineral and petroleum geology Thermal Infrared (TIR) 10.4 - 12.5 Thermal mapping, plant stress, urbanlnon-urban land-use differentiation

Landsat 3) because the RBV bands were designated as bands launch of Landsat 1: on-orbit experience with the Mss from 1 through 3; in the latter, the MSS bands were renumbered as research studies such as LACIE (the Large-Area Crop Inven- bands 1 through 4. This change in conventions bears watch- tory Experiment), mission-requirements studies, and a num- ing when comparing historical data from different Landsat ber of seminars and workshops engaging the Earth-science satellites. community led to the specifications for the Thematic Map- per. The Thematic Mapper was named because its images Operational History would be used to produce maps tailored to different Earth- The first launch of the MS~occurred on 23 July 1972 aboard observation themes, such as agriculture, hydrology, geology, ERTS, and the results were spectacular. Originally considered and the like. to be an experimental payload with a one-year operating-life The TM represented a dramatic advancement in every di- requirement, the first MSS operated for 5.5 years before being mension of sensor performance: spatial, spectral, and radio- deactivated. Indeed, all of the MSS instruments have been ex- metric. Spatially, the TM would have a 30-m ground reso- traordinarily long lived, as noted in Table 3. Landsats 2 and lution in nearly all of its spectral bands - a factor of seven 3 have been decommissioned, but MSS instruments aboard improvement in resolved area over the MSS [30 by 30 m (900 both Landsats 4 and 5 are fully functional: Landsat 5 is in square metres) for the TM versus 80 by 80 m (6400 square me- daily use at this writing, but assorted spacecraft infirmities tres) for the MSS],while covering the same 185-kin (100-nauti- have consigned Landsat 4 to standby status (the payload in- cal-mile) swath width. In addition to revealing far greater struments are in fine working order, but the spacecraft's detail in urban areas, the improved spatial resolution would power, communication, and other subsystems on Landsat 4 permit much more accurate agricultural monitoring and crop are marginal). classification of smaller fields under cultivation - a key to accuracy in many regions of the world where agricultural par- cels are much smaller than in the United States. The Thematic Mapper Spectrally, the TM would provide enhanced spectral coverage and spectral resolution. Unlike the MSS band set Design Objectives chosen by analogy to color infrared film, the TM bands were Even as the Multispectral Scanner was under development, selected on the basis of comprehensive study and analysis of the Earth-science community was beginning to define the re- spectral reflection features for a variety of vegetation types mote-sensing objectives for the next generation Landsat in- and surface minerals. Spectral classification accuracy was a strument. This definition process was catalyzed by the key determinant for selecting the specific band edges and

July 1997 PE&RS TABLE5. THEMATICMAPPER DESIGN REQUIREMENTS contrast with the uniform, tenth-micrometre intervals for the Spatial Radiometric MSS bands - the TM bands are anything but arbitrary. Spectral Range Resolution Resolution This band selection process was difficult and conten- Band (pm) [m) (% NEAp) tious, with each scientific discipline lobbying for the band set that would be best suited for its research interests. In- 1 0.45 - 0.52 30 0.8 deed, the process continued even beyond the beginning of 2 0.52 - 0.60 30 0.5 the TM hardware development program. The geological com- 3 0.63 - 0.69 30 0.5 TM 4 0.76 - 0.90 30 0.5 munity felt that the band selections were biased in favor 5 1.55 - 1.75 30 1.0 of agricultural/vegetation applications - to the detriment of 7 2.08 - 2.35 30 2.4 geology and mineralogy. This case was pressed until a sev- 6 10.4 - 12.5 120 0.5K NEAT enth spectral band (2.08 to 2.35 pm in the shortwave infrared) was added to the TM specifications under a contract modification. By this time, development had proceeded to a point where the numbering scheme for the fist six bands was already well entrenched, and this explains one of the bandwidths. Initially, a total of six spectral bands were spec- enduring trivial oddities of TM: the bands are not numbered ified for TM: three in the visible spectrum, one in the near in strict order of increasing wavelength: band 7 is out of se- infrared, one in the shortwave infrared, and one longwave quence because it was added after design was already well (thermal) infrared (the latter spectral band would have a re- underway. duced spatial resolution - 120 m). Each of these bands was The TM specifications also included a substantial im- selected with specific applications and discrimination capa- provement in radiometric performance. Quantization to 256 bilities in mind, as summarized in Table 4. Note the striking levels (8 bits) was specified, corresponding to about 0.5 per-

Optical Scan-Line

Cooled Detectors and Flners (Bands 5-7)

Prime focal Plane

Cooled Focal Plane Bands 5.6. 7

Track Direction

Bands 1 2 3

30m

Scan .)Direction '17-We14 IB MSC (b) Figure 3. Thematic Mapper scanning approach.

PE&RS July 1997 843 cent NEAp in key spectral bands (compared to 6 bits for MSS). Spacecraft Travel The added sensitivity was needed to detect subtle re- I flectance differences that were crucial for improved classifi- t Scan cation accuracy. Overall performance requirements for TM are summarized in Table 5.

Design Description and Performance Like the MSS, the Thematic Mapper is a cross-track scanning instrument utilizing an object-space scan mirror, but a number of significant technological advances mark the TM as a a) Uncompensated second-generation instrument: bidirectional scanning, higher-density detector arrays employing small-scale integra- tion, extensive use of composite materials, as well as ex- b) Correction For Orbital Motion tended spectral coverage, higher spatial resolution, and 8-bit

As presented in Figure 3, the TM'S bidirectional scan mirror sweeps the detector's line of sight in west-to-east and east-to-west directions transversally across track, while the spacecraft's orbital path again provides the north-south motion. The bidirectional scan gives rise to a higher mechanical scan efficiency of 85 percent (versus 45 percent for the MSS) c) Compensated I and a more moderate acceleration profile during mirror re- Figure 4. Scan-line corrector produces parallel bound. The latter factor results in improved linearity, less vi- scans in both directions. bration, and reduced mechanical stress, while the improved scan efficiency translates into additional detector dwell time that can be used to enhance radiometric sensitivity and/or improve spatial resolution. namic flatness and scan-linearity requirements due to the Bidirectional scanning is not as simple as it initially ap- TM's higher spatial resolution. Beryllium was again the mate- pears because the compound effect of along-track orbital mo- rial of choice because of its stiffness-to-weight ratio, but a tion and cross-track scanning leads to significant overlap and more sophisticated lightweighting scheme was used to mini- underlap in ground coverage between successive scans. This mize mass and inertia. A solid billet of beryllium was cut problem was solved by employing a synchronous image-mo- into halves, each of which was electric-discharge machined tion-compensation system utilizing a pair of oscillating mir- with identical egg-crate patterns and then brazed back to- rors in the optical path to introduce a compensatory motion gether. A flex-pivot suspension was again utilized for the that offsets the along-track orbital motion of the spacecraft. scan mirror, in conjunction with a magnetic-compensation This phase-locked mechanism, called a scan-line corrector, system that effectively canceled the spring forces of the piv- rectifies the scan motion so that successive scan lines are par- ots. The mirror's dynamics were controlled by a fully redun- allel, without overlap or underlap (as illustrated in Figure 4). dant digital microprocessor control system. The scan mirror and associated servomechanism for the The larger aperture, longer focal length, and higher reso- TM was even more challenging than that for the MSS because lution of the TM required special consideration to minimize of the TM mirror's larger size (53 cm) and more stringent dy- thermal distortion in order to meet optical performance re-

July 1997 PE&RS TABLE6. MEASUREDRADIOMETRIC AND SPATIALPERFORMANCE FOR THEMATICMAPPER FLIGHTMODELS IS SIGNIFICANTLYB~ERTHAN SPECIFICATIONS. ' [A) Thematic Mauper Radiometric Performance Measurements Scene Radiance Signal-to-Noise Ratio (SNR) Noise-Equivalent (specified) Raflectance Difference [mW/cm2- sr) At Minimum Scene Radiance At Maximum Scene Radiance NUp (%I Measured Measured Measured Performance Performance Performance Landsat Landsat Landsat Landsat Landsat Landsat Band Min Max Spec. 4 5 Spec. 4 5 Spec. 4 5 1 0.28 1.00 32 5 2 60 75 143 143 0.8 0.16 0.16 2 0.24 2.33 3 5 60 59 170 2 79 234 0.5 0.18 0.21 3 0.13 1.35 26 48 46 143 248 215 0.5 0.20 0.23 4 0.19 3.00 32 35 46 240 342 298 0.5 0.19 0.22 5 0.08 0.60 13 40 3 5 75 194 175 1.0 0.23 0.25 7 0.046 0.43 5 2 1 2 8 45 164 180 2.4 0.41 0.37 6 300K 320K NEAT= NEAT= NEAT= NEAT= NEAT= NEAT= 0.5K 0.12K 0.13K 0.42K 0.10K 0.11K

(13)THEMATIC MAPPER SPATIAL-RESOLUTION PERFORMANCE MEASUREMENTS Square-Wave Response at Nyquist Frequency Structurally, graphite-epoxy composites were used [Band Average) throughout the instrument, along with Invar and beryllium. Measured The overall size of the instrument is 2.0 by 1.1 by 0.7 m, with a mass of 258 kg and maximum power consumption of 335 Band Specified Landsat 4 Landsat 5 W. Figure 5 shows external and cutaway views of the TM. Three Thematic Mapper instruments have been built: a ground-based engineering model, a protoflight model for Landsat 4, and a flight model for Landsat 5. The flight instruments have performed markedly better than their specifications: for example, measured NEAp is two to five times better than specified, while NEAT is about four times better than specified, as noted in Table 6.

Operational History quirements. A Ritchey-Chretien design with a 40.6-cm aper- The protoflight TM was launched aboard Landsat 4 on 16 ture and 244-cm focal length (f/6)was utilized, with mirrors July 1982, and the second TM was launched aboard Landsat made of ultra-low expansion (ULE] titanium-silicate glass. 5 on 1 March 1984. As of this writing, both of these instru- The primary mirror was extensively lightweighted by using ments are fully functional. Indeed, the Landsat 5 TM is in an egg-crate structure. Further, graphite-epoxy composite ma- daily use providing service to Landsat receiving stations terial was utilized for the optical metering structure: both the throughout the world. These instruments were specified to metering structure and ULE mirrors have extraordinarily low have a two-year operating life, with a design goal of three coefficients of thermal expansion in order to maintain pre- years. To date, they have exhibited a combined longevity of cise optical alignment through temperature changes. over 27 instrument years - nearly seven times their speci- On the focal plane, detector technology had progressed fied life, as summarized in Table 7. At this writing, the to the point that solid-state detector arrays could be em- Landsat 5 TM, for example, has provided over 28 million im- ployed in all spectral bands. Sixteen-element monolithic sili- ages to ground receiving stations throughout the world (My- con photodiode arrays were used for bands 1 through 4. lod, personal communication, 1997). Further, it has not yet During early design tradeoffs, charge-coupled-device detector been necessary to activate any of the internally redundant arrays were also considered for the silicon bands, but the de- systems on either of these instruments. velopment cost and risk was considered prohibitive at the i time of the TM's initial gestation. Enhanced Thematic Mapper (ETM) for Landsat 6 Monolithic indium-antimonide photodiode arrays were specified for the shortwave infrared bands (bands 5 and 7), Design Objectives 1 and the thermal infrared band utilized photoconductive mer- The definition of the Enhanced Thematic Mapper payload for cury-cadmium-telluride detectors. All of these infrared detec- Landsat 6 was the result of a complex series of decisions that tors, which must operate at cryogenic temperatures, were were ultimately driven by policy and economics, rather than cooled to 90°K by a two-stage passive radiative cooler based technology. The story is somewhat convoluted, but its telling upon design techniques derived from the highly successful is necessary in order to reveal the origins of the specifica- coolers developed for the Visible-Infrared Spin-Scan Radiom- tions for the Enhanced Thematic Mapper - indeed, for the eter (VISSR) for geostationary meteorological satellites. very existence of the ETM. Low-noise, wide-bandwidth analog electronics - a com- The first Thematic Mapper was developed in the context bination that is often mutually exclusive - were required for of the Landsat D program - a program that originally encom- the TM'S analog signal processing. These circuits, along with passed the procurement and launch of four identical satellites the analog-to-digital converters and high-speed multiplexers, that would each carry a Thematic Mapper and Multispectral made extensive use of hybrid integrated circuits in order to Scanner. In the same context, long-range plans (extending be- maximize performance while minimizing size, mass, and yond the Landsat D series) called for an advanced Landsat power consumption. Over 250 hybrids were used in the TM system canying third-generation instruments employing new electronics. technology. Concept definition for this third-generation system ernment investment to help underwrite initial development TM costs until the revenue stream from future sales of Landsat Operating data could sustain the commercial enterprise. A number of Launch Design-Life Life to Date proposals were received from industry and several of these Spacecraft Date Requirement [April 19951 Remarks proposals included MLA sensor technology for future Land- sats. Landsat 4 16 Jul 82 2 years 14.7+ years Instrument still fully functional; Unfortunately, the proposals languished because budget spacecraft on constraints made it impossible for the Department of Com- standby. Active merce to proceed with the procurement as originally operation for first planned. The budget for Landsat was subsequently reduced -5 years. by a factor of two, whereupon some of the bidders withdrew. Landsat 5 01 Mar 84 2 years 13.1+ years Instrument fully The reduced budget significantly increased the financial in- functional; vestment and risk associated with fielding new technology spacecraft still in for the next generation of Landsats. In this context, utiliza- daily use. tion of previously developed technology - based upon the Thematic Mapper - emerged as the most workable path for providing continuity of Landsat data, and the Earth-Observa- began even as the Thematic Mapper was under development. tion Satellite Company (Eosat - then a joint venture of NASA and the nascent space remote-sensing community recog- Hughes Aircraft and RCA) was awarded a contract in 1985 to nized that the utility of Landsat data would be further en- proceed with the development of Landsats 6 and 7. hanced by improvements in spatial resolution and radiometric Eosat had originally proposed a dual-sensor spacecraft, performance. with an advanced MLA instrument alongside a TM - a com- There was a broad consensus that this performance im- bination of old and new sensors providing an orderly transi- provement would be delivered by pushbroom Multispectral tion to the advanced higher-performance instrument, much Linear Array (MLA) sensor technology: the MLA approach as NASA had done with the MSS and TM on Landsats 4 and 5. uses linear detector arrays that span the entire cross-track When the procurement budget was reduced, the MLA devel- field of view so that an east-west scanning mirror is not re- opment had to be dropped, and Landsats 6 and 7 would then quired. Only the natural orbital motion of the spacecraft is carry just a Thematic Mapper. However, Landsat 6, with its needed to provide a north-south scan of the detector array 30-metre spatial resolution, would be operating in the late along the ground track - in a fashion that is reminiscent of 1980s and early 1990s when competitive systems would be sweeping with a pushbroom, as illustrated in Figure 6. providing 10-metre panchromatic and 20-metre multispectral The MLA concept offered the promise of improved per- data. Consequently, Eosat proposed to add a 15-metre resolu- formance because of the increase in detector dwell time tion panchromatic band to the TM, and to modify the high- made possible by proliferation of detector elements and elim- speed multiplexer to permit simultaneous transmission of ination of the scan mirror. This is, in essence, a classic serial multispectral and panchromatic data. The combination of versus parallel tradeoff: many parallel detector channels are ETM'S extended spectral coverage (including SWIR and TIR), utilized to simultaneously view the entire cross-track scene, broader swath width, precision radiometry, and excellent op- in contrast to a scanning sensor where a relatively small tical performance at a 15-metre resolution would be highly number of detectors serially sample the cross-track field-of- competitive while still providing fully compatible data conti- view. The resulting increase in detector dwell time permits nuity for Landsat's established (and growing) community of improvements in spatial, spectral, and radiometric resolu- users. Thus, the Enhanced Thematic Mapper was born. tion. Key specifications for the ETM are summarized in Table The feasibility of this sensor concept was driven by the 8. Note that the ETM would produce two 84.9-MBPS data maturation of detector technology: by the mid to late 1970s, streams during its operation, in contrast to the single 84.9- large-scale integrated circuit technology applied to the pro- MBPS data stream hom the TM. This was due to the addition duction of detector arrays made it feasible to produce extended linear focal planes at reasonable cost and technical risk. In this context, NASA sponsored several MLA instrument definition studies in 1981 to develop the advanced sensor concepts that would supplant the TM in future Landsats. Four "Phase-B" instrument definition studies were completed in December 1981 as a prelude to a hardware-development program. However, budget and policy issues quickly came to the forefront to change the course of events. Encouraged by the success of Landsats 1 through 3 and motivated by a desire to transition Landsat to commercial operation, the Administration canceled procurement of two of the four Landsat D series of spacecraft and suspended government sponsorship of MLA sensors for Landsat. Over the next three years, from 1981 through 1984, the Administra- tion devised a commercial transition plan that would gradu- ally shift Landsat from the public sector to commercial operation. This transition included passage of the Land Re- mote Sensing Act of 1982 (Public Law 98-365) that author- ized this action to proceed under the auspices of the Department of Commerce. The Department of Commerce subsequently issued a re- Figure 6. Pushbroom concept utilizes linear quest for proposals to commercialize the Landsat system detector arrays scanned by orbital motion. over a period of 12 years, albeit with a significant initial gov-

July 1997 PE&RS TABLE8. ENHANCEDTHEMATIC MAPPER (ETM) DESIGNREQUIREMENTS bands was carried forward without change. Indeed, spare Spatial components from the Landsat 415 program were available for Performance Radiometric On 6' Spatial (MTF at Sensitivity There were significant changes to the electronics to ac- Specbal Range Resolution Nyquist (SNR) commodate the addition of the panchromatic band, provide Band (~m) (m) Frequency) [high-gain state] improved radiometric performance, and upgrade redundancy for greater predicted reliability. The addition of a second 85- %bit quantization, MBPS multiplexer to handle the added data rate, due to the 8 bits transmitted P 0.50 - 0.90 15 0.124 15 pan band, required repackaging the electronics into two mod- 1 0.45 - 0.52 30 0.275 32 ules: the main electronics module mounted on the scanner as- 2 0.52 - 0.60 3 0 0.275 3 5 sembly (as before for the TM), along with a new auxiliary 3 0.63 - 0.69 30 0.275 26 electronics module mounted remotely. New analog-to-digital 4 0.76 - 0.90 30 0.275 3 2 converters were designed to provide 9-bit radiometric preci- 5 1.55 - 1.75 30 0.275 13 sion, and two gain states were incorporated so that only 8 bits 7 2.08 - 2.35 30 0.275 5 were transmitted. In this fashion, the sensitivity and dynamic 6 10.4 - 12.5 120 0.275 0.5K NEAT range of the instrument were extended: ground controllers could select the gain characteristics to optimize radiometric performance for each scene. Numerous other rehements on a of the 15-m panchromatic band: this band would produce subsystem-by-subsystembasis, such as enhanced redundancy data at four times the rate of a 30-m multispectral band - in the power supplies, made the ETM a more robust and capa- approximately two thirds of the total data rate of the TM. Al- ble instrument than its predecessor. Table 9 summarizes the though this could have been handled by a second data physical characteristics of the ETM, while Figures 7 and 8 stream of approximately 56 MBPS, it was more straightfor- show the flight instrument during assembly-and-test and, sub- ward and economical to produce two identical multiplexers sequently, as mounted on the Landsat 6 spacecraft. operating at 85 MBPS and utilize the additional capacity to Test measurements on the completed ETM showed that provide redundant transmission of two of the 30-m multi- the instrument's performance was substantially better than its spectral bands. Further refinements were also specified, such specifications, as noted in Table 10. The modulation transfer as %bit radiometric resolution by using two gain states. function for the panchromatic band was particularly notewor- thy: the measured MTF at the nyquist frequency was 0.38, ver- ETM Design Description and Performance sus a requirement of 0.124. This outstanding performance Although many features and subsystems of the Thematic portended superb images from space. Note also that the MTF Mapper carried forward to the ETM, there were a figures as listed are the worst-case data in the Scan direction; number of modifications and refinements associated with the performance is even better in the along-track d~ection- enhancements for Landsat 6. The telescope optics and scan-mirror assembly were sub- ETM Operational History stantially similar to the TM, but the optical field-of-view was The operational history of the ETM for Landsat 6 is disap- slightly increased to accommodate the additional spatial ex- pointingly nonexistent because the spacecraft failed to reach tent of the focal plane that now included the detectors for its orbit. Although the launch on 5 October 1993 aboard a the panchromatic band. Additionally, the resolution require- Titan 11 booster proceeded smoothly, a probable failurelmal- ments associated with the 15-m panchromatic band, corre- function in the spacecraft's propulsion system led to orbit-in- sponding to a 21-yrad angular subtense, placed more strin- jection failure and loss of the spacecraft. The spacecraft was gent tolerances on telescope manufacture and alignment, as never located, but it is presumably somewhere on the ocean well as on the linearity and dynamic flatness of the scan mir- floor in the South Pacific. ror. The larger focal plane and increased field-of-view also affected the design of optical baffles. Landsat 7 Payload The prime focal plane for the ETM was substantially upgraded by the use of a single, monolithic silicon detector ar- Design Objectives ray for all of the visible and near-infrared spectral bands The Landsat Program changed substantially between the (including the new 15-m panchromatic band). This approach commercialization initiatives of the early to mid 1980s and provided better band-to-band geometric registration and sta- the definition and development of Landsat 7 in the 90s. bility: because all of these detectors are on a common silicon The challenges associated with commercialization of the pro- substrate, their geometry is established with photolitho- gram led to a reevaluation of government priorities and poli- graphic accuracy. In contrast, the TM used four separate de- cies. Landsat's value in providing benchmark data sets for tector arrays for bands 1 through 4, and this required global-change research was becoming evident, so continuity precision OP~O-mechanicalalignment. Although on-orbit per- of such data became increasingly important to the scientific formance has validated the original TM design approach, the ~ommunity.Further, there was increasing interest in Landsat monolithic ETM focal plane is inherently more producible by the Department of Defense: Landsat had proven valuable and stable. The cooled focal plane with indium antimonide for updated during the Gulf War, and the prom- and mercury cadmium telluride detectors for the infrared ise of a more capable Landsat system was of considerable interest, so a "dual-use" (civil and defense) strategy was adopted for Landsat. All of these factors crystallized in the TABLE9. ETM PHYSICALCHARACTERISTICS passage of the Land Remote Sensing Policy Act of 1992 (Pub- Mass Scanner Assembly: 288 kg lic Law 102-555) and the formation of a joint Air-FO~C~/NASA Auxiliary electronic Module: 81 kg program office that was charged with the procurement, de- Dimensions Scanner Assembly: 1.3 by 0.7 by 2.0 m velopment, and operation of Landsat 7. Auxiliary electronic Module: 0.5 by 0.7 by 1.0 rn This led to a request for proposal that included specific Power (maximum) 490 W requirements for data continuity, along with a list of desired Date Rate 2 by 84.9 MBPS enhancements to Landsat's capability - enhancements such as improved spatial resolution (5 m), stereo imaging capabil-

PE&RS July 1997 847 Figure 7. ETM flight hardware during assembly and test.

Figure 8. ETM on the Landsat 6 spacecraft. ity, and cross-track pointing for more frequent revisit oppor- I I tunities. Additionally, there was a requirement for upgraded absolute radiometric accuracy. This latter specification came from the recognition that Landsat 7 would effectively become viewing. This approach effectively partitioned the develop- a part of the Mission to Planet Earth sensor suite, with in- ment risk for the program by using proven technology for the creasing reliance on the fidelity and accuracy of its data for data-continuity mission, and a new sensor for the added ca- global-change research purposes. The Landsat 7 design pabilities. Again, this was a reinforcement of NASA's success- objectives established by the joint Air Forcel~~S~program office are summarized in Table 11. TABLE11. LANDSAT7 DESIGN OBJECTIVES Landsat 7 Payload Design Description 1. Data continuity - with data at least equal in quality and kind to In order to satisfy the needs of the broad user community for that offered by the Thematic Mappers and Enhanced Thematic Landsat 7, the General ElectricIHughes Aircraft team pro- Mapper of Landsats 4, 5, and 6: posed a system with two payload instruments: a further up- One Panchromatic Band with 215-metre resolution (Landsat 6) grade to the Enhanced Thematic Mapper (i.e., the ETM+) for Seven multispectral bands spanning the visible through long- data continuity with earlier Landsats, and a second sensor, wave infrared spectrum Six Visible through shortwave infrared bands at 230-metre the High Resolution Multispectral Stereo Imager (HRMSI) to resolution provide high spatial resolution (5-m pan, 10-m multispectral) One Longwave infrared band at 5120-metre resolution and agile pointing capability for both stereo and cross-track Radiometric performance (signal-to-noise ratio and calibration) at least equal to that provided bv Landsats 5 and 6 185-km (~OOnautical'mile) swath width TABLE10. ETM MEASUREDPERFORMANCE WAS SIGNIFICANTLY BETTERTHAN 16-day revisit cycle THE SPECIFICATIONS 5-year on-orbit reliability Specified Measured Specified Measured Band SNR SNR MTF MTF 2. Enhancements: The following, in priority order, represent additional capabilities desired for Landsat 7, after meeting the data-con- 20 tinuity requirements: 45 Improved spatial resolution 55 Improved absolute calibration 40 Stereo mapping capability 4 7 Additional spectral bands 2 1 Improved revisit time (cross-track pointing) 19 Improved radiometric sensitivity 0.13K NEAT Improved line-of-sight accuracy

July 1997 PE&RS SPACECRAFT .Landant 6 upgndo ElMc DATA CONTINUITY 16 m Panehrwnatlc .30 m VWlft18W1~ .SO m LWIR

Cstlbntfon .txfSMpM

HRUSI: HIWRIDRfFY ENHANCEMENTS .6m~rowtutIm .10 m VNIR: 4 Muitlspectni WMIs .BOkJnswmh .AlOtl#&& IltM.0 imPl)h .Crasstrack pdntIna id+'Jw mwtt .2x')bMbps .WaksteppdarterdsaienphBM - ixtwmr budqrt oonstrslnEb Figure 9. Landsat 7 payload as originally planned. I ful pattern of carrying old and new instruments alongside ment over other systems that rely on multiple-orbit side-look- one another in order to effect a smooth transition and mini- ing images to acquire a stereo image pair. mize risk to data continuity. Figure 9 depicts the originally Cross-track pointing: Cross-track pointing at f 38" provides a 3-day or less revisit frequency at the equator and more fre- planned payload suite for Landsat 7. quent revisit at higher latitudes. 60-km swath: A 112- by 112-km stereo image area can be ac- High-Resolution Multispectral Stereo Imager (HRMSI) quired well within 90 days. Utilizing pushbroom sensor technology, the High-Resolution The HRMSI development effort exploited over a decade of de- Multispectral Stereo Imager (HRMSI)was designed to provide sign work on multispectral linear array instruments. The tel- several high-priority performance enhancements for Landsat escope utilized an all-reflective three-mirror anastigmatic 7: form that provided excellent optical performance over the 5" Improved spatial resolution: 5-metre ground-sampling dis- field-of-view, and the entire telescope assembly was mounted tance (GSD) panchromatic band and four 10-metre GSD Visible in a two-axis gimbal to provide the requisite pointing agility. and Near-Infrared (VNIR) bands. Linear photodiode arrays were specified for all of the detec- Improved radiometric sensitivity: The pushbroom sensor con- tor bands, and on-board data compression was utilized in or- cept provides increased dwell time and improved signal-to- noise ratios compared to equivalent channels of the ETM, even at higher spatial resolution. Stereo imaging: Stereo imaging at a 5-metre spatial resolution with a variable in-track stereo angle from nadir to f 30". This same-pass stereo capability represents a significant improve-

TABLE12. HRMSI DESIGNCHARACTERISTICS Scanning Method Pushbroom, Multispectral Linear Array Swath Width 60-km (5" field-of-view from 705-km orbit) Pointing Capability f 38" cross track for rapid revisit + 30" along-track for stereo imaging Telescope 18-cm aperture, unobscured reflecting triplet Size Sensor Assembly: 82.5 by 81.3 by 64.8 cm Electronics Module: 0.1 m3 Mass Sensor Assembly: 45.4 kg Electronics Module: 54.5 kg Power 125 w, 148W when slewing telescope Data Rate Panchromatic Band: 75 Mbps at 4 bits per pixel (Compressed) VNIR Bands: 75 Mbps at 5 or 6 bits per pixel Spectral Spatial Bandwidth Resolution Band (pm) Detectors (m) SNR Pan 0.50 - 0.90 6100 silicon photodiodes 5 20 1 0.45 - 0.52 3050 silicon photodiodes 10 32 2 0.52 - 0.60 3050 silicon photodiodes 10 35 3 0.63 - 0.69 3050 silicon photodiodes 10 2 6 4 0.76 - 0.90 3050 silicon photodiodes 10 32

PE&RS July 1997 Rad-

- (Stowed) r=IElectronics Module Analog FAC = Full Aperture Callbrator Two 75 Mbps Postamps PAC = Partbal Aperture Calibrator SLC = Scan Line Corrector Command 8 OBC = On Board Calibrator Streams to Telemetry PFPA = Prime Focal Plane Assembly CFPA = Cold Focal Plane Assembly Pavload Correction PS = Power Supply -ETM+ NewIModlfied Designs Figure 11. ETM+ block diagram highlights new or modified subsystems in bold out- line.

der to maintain compatibility with the 2- by 75-MBPS months short of the critical design review that would have capacity of the Tracking and Data Relay Satellite. These and marked the completion of the detailed design of the instru- other design features are summarized in Table 12, followed bv" an artist's illustration of the instrument in Figure 10. Design of the HRMSI instrument began in ~eitember 1992, progressing through a preliminary design review and well into the detailed design phase. Unfortunately, budgetary pressures and differing priorities placed the Landsat 7 program in a considerable state of flux in early 1994. This ultimately led to the Air Force's withdrawal from the program, and Landsat 7 emer~edas a NASA-sponsoredproject under the aegis of ~issiontoPlanet ~arth:In this context, The- matic vMapper data continuity for global-change research became the raison d'etre for Landsat 7. and budeetarv constraints precluded further development of The ~SI.As a result, work on HRMSI ceased in May of 1994, just a few

Scanning Method Bidirectional cross-track, Scan Frequency: 7 Hz Swath Width 185 krn (15" field-of-view from 705 km orbit) Telescope 40.6-cm aperture, Ritchey Chretikn Size Scanner Assembly: 1.5 by 0.7 by 2.5 m Auxiliary Electronics Module: 0.4 by 0.7 by 0.9 m Mass Scanner Assembly: 298 kg Auxiliar~Electronics Module: 103 kg- Cable ~Gness:20 kg Power 510 W Quantization 9 bit AID conversion, 8 bitslpixel transmitted (2 gain states) Data Rate 2 by 75 MBPS, CCSDS format SNR Spectral Spatial (at min Bandwidth Resolution signal Band (pm) Detectors (m) radrance) Pan 0.50 - 0.90 32 Si photodiodes 15 1 0.45 - 0.52 16 Si photodiodes 30 2 0.52 - 0.60 16 Si photodiodes 30 3 0.63 - 0.69 16 Si photodiodes 30 4 0.76 - 0.90 16 Si photodiodes 30 5 1.55 - 1.75 16 InSb photodiodes 30 manufacturing processes for enhanced producibility and 7 2.08 - 2.35 16 InSb photodiodes 30 6 10.4 - 12.5 8 HgCdTe photoconductors 60 reliability.

July 1997 PE&RS LaunchLock

Figure 13. Partial-aperture solar calibrator uses faceted prism assembly to provide calibration-reference irradiance when spacecraft crosses the terminator. ment. Landsat 7 was subsequently restructured as a single- payload program, carrying only the ETM+ for the data-continuity mission.

Enhanced Thematic Mapper Upgrade (ETM+) (b) The ETM+ represents a further refinement of the ETM devel- Figure 14. Full-aperture solardiffuser calibration refer- oped for Landsat 6, with the addition of end-to-end on-orbit ence. (a) Full-aperture solar diffuser is designed to pro- calibration and a 60-m, rather than a 120-m, long-wavelength vide an end-toend calibration reference upon ground (LWIR) infrared band. The 15-m panchromatic band and the command. (b) ETM+ hardware photo shows full-aperture six 30-m multispectral bands are carried forward from the calibrator in stowed position. Multilayer protective blan- ETM. Additional upgrades for ETM+ are focused on enhanced kets cover back sid of diffuser; stacked rotary actuator is reliability. While this may seem unnecessary in view of the at lower right side of diffuser. extraordinary on-orbit longevity of the Thematic Mappers on Landsats 4 and 5, the statistical probability of long-term operation can be bolstered by increasing redundancy in subsystems or by incorporating additional flexibility in on-orbit switching and cross-strapping of existing subsystems, and data buffers within the multiplexers allow continuous data these upgrades become important in the context of a five- transmission during the scan-turnaround interval, whereas year design-life requirement. The ETM+ block diagram illus- previous TM instruments transmitted actual image data only trated in Figure 11 highlights the new or modified during the active portion of the scan. The new multiplexers subsystems that differentiate the ETM+ from its predecessor, are also fully redundant with additional cross-strap switch- and Table 13 summarizes the key design characteristics of ing to enhance reliability. the instrument. Recognizing Landsat's importance in providing accurate While the ETM+ can trace its lineage to the Thematic measurements in support of Mission to Planet Earth, calibration accuracy has been an area of special emphasis on ETM+. Mappers of Landsat 4 and 5, and the ETM of Landsat 6, many of the subsystems and components such as detectors, spec- Accordingly, three independent on-board calibration systems tral-bandpass filters, high-speed electronics, and on-board are used to calibrate the panchromatic, visible and near-in- calibration subsystems have been upgraded. The cooled focal frared (VNIR), and short-wavelength infrared (SWIR) bands. plane assembly, incorporating the detectors for bands 5, 6, They consist of and 7, has been updated to reflect modern practice in detec- A full-aperture solar diffuser on the inner surface of the aper- tor producibility. This includes backside-illuminated indium ture door that illuminates the focal planes with diffusely re- antimonide detectors for bands 5 and 7 and a quartz sub- flected solar energy when commanded into position; strate for the focal plane, as shown in Figure 12. The spec- A partial-aperture solar reflector that illuminates the focal tral-bandpass filters have been manufactured with planes with attenuated solar energy, once per orbit; and ion-deposition techniques to provide better control of band- Calibration lamps that project calibrated energy onto the focal planes via the main calibration shutter, once per scan, during edge characteristics and to minimize airlvacuum shifts in the scan mirror turnaround. spectral response. Elsewhere in the system, the high-speed multiplexers have been redesigned to provide two 75 MBPS Note that only the calibration lamps and shutter were uti- data streams in CCSDS (Consultative Committee for Space lized in the earlier TM and ETM instruments, so the ETM+ Data Systems) format for compatibility with the Earth Ob- represents a significant step forward in absolute radiometric serving System communication and data processing protocol. calibration accuracy. Figures 13 and 14 show the additional The reduction from 85 MBPS to 75 MBPS is feasible because on-board calibrators for the ETM+.

PE&RS July 1997 TABLE14. THREEDECADES OF LANDSATINSTRUMENTS MSS TM ETM ETM+ Spectral Bands 1 0.5 - 0.6 pm 1 0.45 - 0.52 pm P 0.52 - 0.90 pm P 0.52 - 0.90 pm 2 0.6 - 0.7 pm 2 0.52 - 0.60 pm 1 0.45 - 0.52 lm 1 0.45 - 0.52 pm 3 0.7 - 0.8 pm 3 0.63 - 0.69 pm 2 0.52 - 0.60 pm 2 0.53 - 0.61 pm 4 0.8 - 1.1 pm 4 0.76 - 0.90 pm 3 0.63 - 0.69 pm 3 0.63 - 0.69 pm 5 1.55 - 1.75 pm 4 0.76 - 0.90 pm 4 0.78 - 0.90 pm 7 2.08 - 2.35 pm 5 1.55 - 1.75 pm 5 1.55 - 1.75 pm 6 10.4 - 12.5 wm 7 2.08 - 2.35 pm 7 2.09 - 2.35 lm 6 10.4 - 12.5 pm 6 10.4 - 12.5 pm Spatial Resolution 30 m VNIR/SWIR 15 m Pan 15 m Pan 120 m TIR 30 m VNIR/SWIR 30 m VNIR/SWIR 120 m TIR 60 m TIR Radiometric Resolution 6 bits 8 bits 9 bits (8 bits transmitted, 9 bits (8 bits transmitted, 2 gain states) 2 gain states) Data Rate 15 Mbps 85 Mbps 2 X 85 Mbps 2 X 75 Mbps Mass 64 kg 258 kg 288 kg Scanner 318 kg Scanner 81 kg AEM 103 kg AEM Average Imaging Power 490 W 510 W

Envelope 1.3 X 0.7 X 2.0 m Scanner 1.5 X 0.7 X 2.5 m Scanner 0.5 X 0.7 X 1.0 m AEM 0.4 X 0.7 X 0.9 m AEM Aperture 23 cm 40.6 cm 40.6 cm 40.6 cm

At this writing, the ETM+ is in the final assembly and ship of Oscar Weinstein and Jack Engel, with the late Warren test process. Interim test results indicate that this instrument Nichols providing program management and executive lead- will meetlexceed its specifications, as has been the case for ership of the team leading to the delivery of the Protoflight its predecessors, but definitive acceptance-test data are not Model for Landsat 4. Dr. Vince Salomonson led the outstand- yet available for the ETM+. ing Landsat Thematic Mapper science team at NASA. The Landsat 5 TM was completed and delivered by a team led by Operational History Dr. Fletcher Phillips. For the ETM, technical development The ETM+ for Landsat 7 has nearly completed its manufac- was again led by Jack Engel with contributions by Frank ture and test at Hughes Santa Barbara Remote Sensing, and Malinowski and program management by Richard Ruiz. Lee will subsequently be delivered to Lockheed-Martin for inte- Tessmer, Richard Roberts, and Roberto Diffoot have all been gration with the spacecraft. Launch of Landsat 7 is scheduled key figures in the development of the ETM+ for Landsat 7, for 1998. with Rick Obenschain, Ken Dolan, and Dr. Darrel Williams providing project leadership at NASA. Finally, Virginia Trau- Summary twein, Sharon Fullmer, Barbara Hoffman, Margaret Finlay, Three decades of Landsat sensor development have produced Kathy Cremeen, Greg Krueger, and Nancy Richards all con- progressively more capable instruments for this important tributed to exhuming the historical records that served as Earth-observation mission, as delineated in Table 14. The source material for this paper. The author extends apologies MSS first launched in 1972 led in turn to the Thematic Map- in advance for any errors of omission in these acknowledg- per, and three successive generations of Thematic Mapper in- ments. struments have been produced for Landsats 4. The latest of these, the upgraded Enhanced Thematic Mapper (ETM+) is References nearing completion. All of the sensors built to date have performed better than their specifications, and those that have Colwell, R.N., D.S. Simonett, and F.T. Ulaby (editors), 1983. Manual successfully reached orbit have exhibited extraordinary on- of Remote Sensing, Second Edition, Volume 1, American Society orbit reliability. of Photogrammetry, Chapter 12. Jones, C.R., and J.L. Engel, 1981. Multispectral Scanner, Thematic Mapper and Beyond, Proceedings of Remote Sensing Sympo- Acknowledgments sium. The author gratefully acknowledges the contributions of nu- Lansing, J.C., Jr., and R.W. Cline, 1975. The Four and Five Band merous colleagues at Hughes and NASA. Literally hundreds Multispectral Scanners for Landsat, Optical Engineering, 14(4): of talented engineers and scientists contributed to the design 312-322. and development of the instruments described in this paper, Santa Barbara Research Center, 1981. MSS-D 4-Band 52000 F-1 and it is a privilege to serve as the chronicler of this collec- Model, Serial No. 3 Consent to Ship Data, SBRC Reference No. tive work. Key contributors to the development of the Mss HS248-6692. included Virginia Norwood and Jack Lansing, with addi- , 1984. Thematic Mapper: Design Through Flight Evaluation, tional engineering and project management by Ralph Wen- Final Report, SBRC Reference No.RPT41741. gler, Leroy Barncastle, and the late Tony Lauletta. The UC Santa Barbara Extension and Santa Barbara Research Center, Thematic Mapper was developed under the technical leader- 1983. Landsat Short Course.

July 1997 PE&RS