Preharvest Monitoring of Biomass Production - Engineering and Science of Biomass Feedstock Production and Provision - page 64

The user needs and their required resolution are critical factors in determining
the remote sensing platform and data. To decide the right platform, there are four
key resolution issues involved in the decision process [ 23 ]:

1. Spatial resolution: How small an object do you need to see (pixel size) and how
large an area do you need to cover (swath width)?
2. Spectral resolution: What part of the spectrum do you want to measure?
3. Radiometric resolution: How fi nely do you need to quantify the data?
4. Temporal resolution: How often do you need to look at it?

4.4 Satellite Remote Sensing

Satellite remote sensing is used to obtain remote sensing images with sensors on
earth observation satellites looking down to the earth. They are the “eyes in the sky”
constantly observing the earth as they go around in the orbits. There are various
government and commercial satellites applied to generate the images of the earth by
different sensors with different spectral and spatial resolutions depending on the
intended use of the images the sensors generate. Again, some commercial satellites
(those operated by a satellite/remote sensing company rather than a government
agency) offer very high-resolution imagery (at a correspondingly very high price!)
that look almost exactly the same as an aerial image—but don't require a plane or a
pilot. The company takes an order, points their sensor in the right direction, and
snaps an image, but this increases the cost signifi cantly [ 16 , 17 , 25 ].
The advantages of satellite remote sensing are (1) global dataset of uniform qual-
ity, (2) rapid data acquisition of large area, (3) no need to obtain permission to gather
data, (4) can revisit on a regular basis for lifetime of satellite (5-10 years), and (5)
spacecraft provides stable platforms. The disadvantages of satellite remote sensing
are (1) high cost of satellite systems; (2) takes more than 10 years to develop, build,
test, and launch; (3) possibility of single point failure; (4) relatively coarser spatial
resolution; (5) longer cycle period, usually 14 days; (6) large measurement uncer-
tainty; and (7) require extensive processing as well as storage and analysis [ 20 ].

4.5 Airborne Image-Based Remote Sensing

Airborne remote sensing is common to obtain images of the earth's surface with
downward- or sideward-looking sensors mounted on an aircraft as indicated in
Fig. 4.4a [ 26 ]. The advantage of airborne remote sensing, compared to satellite
remote sensing, is the capability of offering very high spatial resolution images
(20 cm or less) after geo-referencing, as illustrated in Fig. 4.4b . The disadvantages
are low coverage area and high cost per unit area of ground coverage. It is not cost-
effective to map a large area using an airborne remote sensing system. Airborne
remote sensing missions are often carried out as one-time operations, whereas earth
observation satellites offer the possibility of continuous monitoring of the earth.