History of Remote Sensing

Landsat's Multi-Spectral Scanner (MSS)

The MSS instrument has operated on the first five Landsat spacecraft. Although the basics of scanning spectroradiometric sensors were reviewed earlier in this Section, because of MSS's important role in these missions which extended over 31 years some of this information is repeated and expanded on this page. A simplified model of this optical-mechanical sensor appears in the next figure.

This is a drawing of this venerable instrument, built by the Hughes Aircraft Corp. of Santa Barbara, CA:

The MSS gathers light through a ground-pointing telescope (not shown). The scan mirror oscillates (1 cycle every 33 milliseconds) over an angular displacement of ± 2.89 degrees that is perpendicular to the orbital track. In the forward scan, the mirror covers an angle of 11.56 degrees (Angular Field of View or AFOV) that from an orbital altitude of 917 km ( about ~570 miles) covers a swath length across the orbital track of 185 km (115 miles). During a forward scan, which takes about 16 milliseconds, it sweeps a ground strip of about ~ 474 m (1555 ft) from one side of the track to the other. Light reflected from the surface (and atmosphere) as gathered by this scan passes through an optical lens train, during which its beam is divided so as to pass through 4 bandpass filters that produce images in spectral bands at MSS 4 = 0.5 - 0.6 µm (green), MSS 5 = 0.6 - 0.7 µm (red), MSS 6 = 0.7 - 0.8 µm (photo-IR), and MSS 7 = 0.8 - 1.1 µm (near-IR). (The band numbering begins with 4 because bands 1-3 were assigned to the RBV sensor.) Light through each filter reaches its set of six electronic detectors (24 in all, for the 4 bands) that subdivide the across-track scan into 6 parallel lines, each equivalent to a ground width of 79 m (259 ft). The mirror movement rate (nominally, its instantaneous image moves across the ground it sees at a rate of 6.8 m/µmsec along a scan line) is such that, at the orbital speed of 26,611 kph (16,525 mph), after the return oscillation during which no photons are collected, the next forward swing produces a new path of 6 lines (79 x 6 = 474 m) just overlapping the previous group of 6 lines. This is illustrated below:

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At each detector, the incoming light (photons) from the target frees electrons in numbers proportional to the number of photons striking the detector. These electrons move as a continuous current that passes through a counting system, which measures the quantity of electrons released (thus, indicating radiation intensity) during each nine microsecond detection interval. Over that time interval (called the dwell time) the advancing mirror picks up light coming from a lateral ground distance of 79 m (259 ft). The detector thus images a two-dimensional, instantaneous field of view (IFOV), usually expressed in steradians, which denotes the solid angle that subtends a spherical surface and, in scanning, connotes the tiny area, within the total area being scanned, viewed at any instant of 0.087 mrad (milliradian, or 0.0573°), which, at Landsat's orbital altitude of 917 km, means the effective resolving power of the instrument is based on the 79 x 79 m² ground equivalent dimensions described above. Each detector is then cleared of its charge to receive the next batch of electrons from the next IFOV input during the forward sweep, and so as the scanning continues through the full forward sweep the set of all IFOV pixels in the line are read in succession. The onboard computer converts this succession of analog signals into digital values which the onboard communication system telemeters (sends) to Earth by radio.

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For each band detector, the electronic signal from this IFOV results in a single digital value (called its DN or digital number, which, for the MSS, can range from 0 - 255 [2⁸]). The value relates to the proportionally averaged reflectances from all materials within the each IFOV. Since the mix of objects on the ground constantly changes, the DN numbers vary from one IFOV to the next. Each IFOV is represented in a b & w image as a tiny point of uniform gray-level tone, the pixel described earlier in this Section, whose brightness is determined by its DN value. In a Landsat MSS band image, owing to a sampling rate (every nine microseconds) effect in which there is some overlap between successive spatial intervals on the ground, a pixel has an

effective ground-equivalent dimension of 79 x 57 m (259 x 187 ft) but contains the reflectances of the full 79 m² actually viewed. This "peculiarity", illustrated in this diagram, needs further explanation:

The wider rectangle (a square for the MSS), which can be designated the Ground Resolution Cell (GRC) size, is established by the IFOV of the scanner. But because the sampling interval Δt is finite, i.e., cannot be zero, the previous and next cells contribute parts of the their represented ground scene that overlap (by 11.5 m) into each individual GRC rectangle/square. This requires removal (by resampling) of the overlap effects leading to a new resolution cell that represents the actual Ground Sampled Distance (GSD). Thus, for the Landsat MSS the GRD of 79 x 79 m becomes a GSD of 79 x 57 m. Each GSD contains all the radiation sent from the GRC for each band spectral interval, integrated into single values expressed by the DNs.

The average number of pixels within a full scan line (representing 185 km) across the orbital track is 3240 (185 km/ 0.057 km). In order to image an equi-dimensional square scene, which requires 185 km of down track coverage, the average total number of lines to do this is set at 2340 (185 km/0.079 km). Each band image therefore consists of approximately (again variable) 7,581,600 (3240 x 2340) pixels - a lot to handle during computer processing, over 30 million pixels when the 4 bands are considered. The number of pixels actually does change somewhat owing to satellite attitude (shifts in orientation (wobble) called pitch, roll, and yaw) and instrument performance that lead to slight variations in the pixel total.

Image producers can use the continuous stream of pixel values to drive an electronic device that generates a uninterrupted light beam of varying intensity, which sweeps systematically over film to produce a b & w photo image. The resulting tone variations on the image are proportional to the DNs in the array. In a different process, we can display the pixels generated from these sampling
intervals as an image of each band by storing their DN values sequentially in an electronic signal array. We can then project this array line by line on to a TV monitor, and get an image made of light-sensitive spots (also called pixels) of varying brightnesses. Or, these DNs can be handled numerically, not to produce images, but to be inputs for data analysis programs (such as scene classifications as described in Section 1).

One final comment: The 79 x 57 IFOV dimensions given above are those often quoted for the Landsat-1 MSS. The official Landsat Web site gives values of 83 x 68 m but does not specify the particular mission associated with those numbers. There is thus an information gap here. The

writer assumes (??? with uncertainty) that this second number applies to a later MSS and is not a revision of the spatial resolution assigned to the first MSS. Also, the 79 m value is sometimes given as 80 m.

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