Table 1. Comparison of TM and MSS. 
KS 
full range of 
Ln position of 
ig from acidic 
o 
growing in and 
n the red edge 
tion band is 
ng and Collins 
dration state, 
rions is at 
iy the red edge 
m. The exact 
red in plants 
n in Figure 4. 
ed spaceborne 
; Mapper. The 
2nd the 1.0 |im 
d, therefore, 
_e for surface 
ration. Table 
sses and other 
Thematic Mapper (TM) 
Multispectral Scanner (MSS) 
Spectral 
Radiometric 
Radiometric 
Band 
Sensitivity 
Sensitivity 
1 
0.45- 0.52 jim 0.8%(NfAp) 
0.5-0.6 pm 0.57% (NfAp) 
2 
0.52- 0.60 0.5% 
0.6-0.7 0.57% 
3 
0.63- 0.69 0.5% 
0.7-0.8 0.65% 
4 
0.76-0.90 0.5% 
0.8-1.1 0.70% 
5 
1.55- 1.75 1.0% 
6 
10.40-12.50 0.5K (/Vf AT) 
7 
2.08- 2.35 2.4% (NfAp) 
Thematic Mapper (TM) 
Multispectral Scanner (MSS) 
Ground IFOV 
30 m (bands 1-6) 
82 m (bands 1 -4) 
Data rate 
120 m (band 7) 
85 Mbits/s 
15 Mbits/s 
Quantization 
levels 
256 
64 
Weight 
258 kg 
68 kg 
Size 
1.1 x 0.7 X 2.0 m 
0.35 X 0.4 X 0.9 m 
Power 
332 W 
SOW 
The extended spectral coverage of bands 5 and 7 are 
particularly useful in identifying the presence of 
hydrous minerals, such as clays, and carbonates. 
Abrams et al. (1977) showed the value of having a 
broad band in the 2.08-2.35 |im region. 
The region 8.0-12.0 |tm is covered in six bands by 
the airborne thermal infrared multispectral scanner 
(TIMS) (Kahle and Goetz 1983) . At present, this is 
the only such instrument available to acquire 
multispectral data in the thermal infrared region. 
For mineral exploration, the TIMS can play an 
important role since images in the region of the 
reststrahlen features for silicates are particularly 
diagnostic of the presence of quartz in surface 
materials. 
4 NARROWBAND SENSORS 
During the mid-1970s, it was recognized that broader 
spectral coverage and higher spectral resolution was 
necessary to derive meaningful mineralogical 
information. High resolution airborne 
spectroradiometry (Chiu and Collins 1978) has 
provided mineralogical information important to 
mineral exploration (Marsh and McKeon 1983) . The 
Collins spectroradiometer is available for 
proprietary surveys. 
The only narrowband instrument that has been flown 
in earth orbit was the Shuttle Multispectral Infrared 
Radiometer (SMIRR) that acquired profiling 
information in ten spectral bands, including four 
narrow spectral bands in the region 2.0-2.35 |J.m 
underneath the spacecraft (Goetz et al. 1982) . On 
this experiment it was demonstrated that mineral 
identification was possible from orbit using narrow 
spectral band systems. 
5 IMAGING SPECTROMETRY 
The results from narrowband sensors led to the 
development of more ambitious imaging systems broadly 
defined as imaging spectrometers. Imaging 
spectrometry is defined as the acquisition of images 
in a large number of contiguous spectral bands such 
that each picture element (pixel) has associated with 
it a complete reflectance or emittance spectrum. 
Data is acquired with sufficient spectral resolution 
so that all the information available in the returned 
signal can be extracted. In other words, the 
spectrum is sampled often enough to completely define 
the spectral reflectance or emittance of surface 
materials. In the region 0.4-2.5 |tm sampling at 10 
nm intervals satisfies this criterion (Goetz et al. 
1985) . 
Simultaneous imaging in many contiguous spectral 
bands requires a new approach to sensor design. 
Sensors such as the Landsat Multispectral Scanner 
(MSS or TM) are optomechanical systems in which 
Figure 5. Four approaches to sensors for 
multispectral imaging: (a) multispectral imaging with 
discrete detectors; (b) multispectral imaging with 
line arrays (c) imaging spectrometry with line 
arrays; and (d) imaging spectrometry with area 
arrays. 
discrete detector arrays are scanned across the 
surface of the Earth perpendicular to the flight 
path, and these detectors convert the reflected solar 
photons from each pixel in the scene into a sensible 
electronic signal (Figure 5a). 
The detector elements are placed behind filters that 
pass broad portions of the spectrum. The MSS has 
four such sets of filters and detectors whereas TM 
has seven. The primary limitation of this approach 
is the short residence time of the detector in each 
instantaneous field of view (IFOV). To achieve 
adequate signal-to-noise ratio without sacrificing 
spatial resolution, such a sensor must operate in 
broad spectral bands of 100 nm or greater or must use 
optics with unrealistically small ratios of focal 
length to aperture (f no.). 
One approach to increasing the residence time of a 
detector in each IFOV is to use arrays of detector 
elements (Figure 5b) . In this configuration, there 
is a dedicated detector element for each cross track 
pixel, which increases the residence or integration 
time to the interval required to move 1 IFOV along 
the track. The French satellite sensor called SPOT 
uses line array detectors. It provides stereoscopic 
image capability in three spectral bands in the 
region short of 1.0 )im (Chevrel et al. 1981) . 
There are limitations and trade-offs associated 
with the use of multiple line arrays, each having its 
own spectral band pass filter. If all the arrays are 
placed in the focal plane of the telescope, then the 
same ground locations are not imaged simultaneously 
in each spectral band. If beam splitters are used to 
facilitate simultaneous data acquisition, the signal 
is reduced by 50% or more for each additional 
spectral band acquired in a given spectral region. 
Furthermore, instrument complexity increases 
substantially if more than 6 to 10 spectral bands are 
desired. 
Two approaches to imaging spectrometry are shown in 
Figure 5c. The line array approach (Figure 5c) is 
analogous to the scanner approach used for MSS to TM 
except that light from a pixel is passed into a 
spectrometer where it is dispersed and focused onto a 
line array. Thus, each pixel is simultaneously 
sensed in as many spectral bands as* there are 
detector elements in the line array. For high 
spatial resolution imaging (ground IFOVs of 10-30 m), 
this approach is suited only to an airborne sensor 
which flies slowly enough that the readout time of 
the detector array is a small fraction of the 
integration time. Because of the high spacecraft 
velocities, imaging spectrometers designed for Earth 
orbit require the use of two-dimensional area arrays 
of detectors at the focal plane of the spectrometer 
(Figure 5d) , obviating the need for the optical 
scanning mechanism. In this situation, there is a