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. This is a drawing of this venerable instrument, built by the Hughes Aircraft Corp. of Santa Barbara, CA:
Here is a simplified model of the MSS showing how it scans the ground:
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 sideward (lateral) 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) encompasses a swath length across the orbital track of 185 km (115 miles). During a near-instantaneous forward movement of the spacecraft (direction of orbital flight), which takes about 16 milliseconds, the mirror as it sweeps laterally (across track) is also covering a ground strip of about ~ 474 m (1554 ft) from one side of the track to the other. Said another way, this means that in the time it took to oscillate laterally, the spacecraft has advanced 454 m relative to its ground track.
Light reflected from the surface (and atmosphere) as gathered by this scan passes through an optical lens train, during which its beam is split (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.) The radiation is carried by fiber optics to six electronic detectors for each band. Bands 4 through 6 used photomultiplier tubes as detectors, and Band 7 used silicon photodiodes. 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; taken together the width covered is 6 x 259 = 1554 ft, the number stated above). The mirror movement rate (nominally, its instantaneous scan moves across the ground being imaged at a rate of 6.8 m/µsec 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 lateral swing produces a new across-track path of 6 lines (79 x 6 = 474 m) just overlapping the previous group of 6 lines. This is illustrated below:
Note that the individual scan lines in this diagram are slanted relative to lines across track perpendicular to the track boundaries. These are valid traces with respect to the ground, since as the mirror moves sidewards the spacecraft is moving ever forward so that each successive moment in the scan finds its ground target (represented by the pixel) being slightly forward of the previous moment's view.
Perhaps the reader wonders, in examining the previous illustration, about the fact that no data are acquired during the return swing (other than looking then at a light source within the sensor whose known radiometric output helps to calibrate the external reflectances). Textbooks describing the operation of the MSS tend to ignore this conundrum. Here is a simple explanation: During the forward swing (for Landsat, with its southward advancing path, from west to east), the six lines are created. The reverse oscillation leads to no data. But look at the diagram. Line 1 moves left, as do the other lines. When the next forward swing occurs, line 1 is now just below line 6. The very movement of the scanned across-track scene is such that, for the scan rate involved, the next acquisition of the 6 lines starts with where the previous line 6 is located relative to the new line 1.
I-20: Individual scan lines are commonly visible (stand out) in a printed or displayed image of a Landsat scene. Can you think of a technical reason why these may be seen? ANSWER
This question suggests that anomalous scan lines are found in individual scenes. These are usually shown in black (meaning no data received). They are frequent in this Landsat-5 image (a scanner timing failure occurred that presents this problem).
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 minute 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), that at any instant amounts to 0.087 mrad (milliradian, or 0.0573°). At Landsat's orbital altitude of 917 km, the effective resolving power of the instrument is based on the 79 x 79 m2 ground equivalent (pixel) dimensions described above. Each detector is then cleared of its charge so as to produce the next batch of electrons generated from the next IFOV photon inputs during the mirror's lateral sweep. As the scanning continues through the full lateral sweep the set of all IFOV pixels in the line are rapidly read in succession. The onboard computer converts this succession of analog signals (voltages) into digital values which the onboard communication system telemeters (sends) to Earth by radio.
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 ). 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).
Little has been said on this page about the appearance of a basic Landsat image. This is deferred until the next page, although Landsat images have already been shown in the Overview. We comment here about several general characteristics of a Landsat image. Look at this illustration:
In this scene covering part of the southern Asian country of Kyrgyzstan, one sees four strips of imagery, joined to produce a mosaic (considered in Section 7). Each strip is a swath from one orbital pass. Notice that its imagery has a slanted appearance (assuming the vertical is true north). Why this slant: because as the MSS sensor looks down at Earth while moving in its orbit, the Earth's surface underneath has been moving from west to east owing to the planet's general rotation. At the 99° inclination (relative to longitude) of Landsat's orbit, the orbit plane precesses about the Earth at the same angular rate that the Earth moves about the Sun. In the image, each successive line slips slightly westward. The accumulation of these progressive offsets results in a figure that, for any individual scene, would be a parallelgram with inclined sides. This in part also accounts for the orbital inclination of the spacecraft, to compensate somewhat for that rotation.
We said above that a Landsat scene is produced by arbitrarily stopping it at 185 km from top to bottom. But that trimming need not happen. one can continue to produce a more continuous swath scene that is elongate in the direction of orbit. In producing an individual parallelogram scene, it is customary to have about 10% of the top consisting of the bottom 10% of the previous (more northern) scene; there is a similar 10% continuance on the bottom. There is also overlap on the left-right margins; this is called sidelap. Its amount varies with latitude. At the equator, the sidelap ranges from 7% (MSS) to 14% (TM); the amount increases going towards the poles so that at high latitudes the sidelap between adjacent scenes can be as high as 80+%. This north-south and east-west overlap allows a crude form of stereo-viewing to be possible within these margins. In practice, this stereo capability is seldom utilized.