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:
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
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.
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.
