Agriculture Reference
In-Depth Information
Finally, some sensors can only be directed straight down (i.e., nadir viewing). If
the device can point laterally, the sensor has an off-nadir pointing capability.
The sensor can be placed on a satellite or on an aircraft. Satellite RS presents
many advantages: large area coverage, recurrent coverage of an area of interest,
quantitative measurement of ground characteristics using radiometrically calibrated
sensors, semi-automated processing and analysis, and lower cost per unit area.
Satellite images have a generally lower spatial resolution than aircraft photography.
An advantage of aircraft RS is the capability to offer images with very high
spatial resolution (20 cm or less). Furthermore, the data volume provided by aircraft
scanners can be substantially higher. This makes it possible to have a large number
of spectral bands or channels, and a large number of pixels produced per mission
(or flight). The disadvantages are that it covers a small area and has a high cost per
unit area. Finally, aircraft RS missions are often performed on a one-time basis. An
aircraft RS system was applied to agriculture analysis by Thomson et al. (
2005
), to
detect weeds in early cotton.
There have been several satellite missions to acquire remotely sensed images.
These missions were principally for gathering weather information, and only later
included observations of the Earth resources in their main objectives [see Xie
et al. (
2008
) and Atzberger (
2013
), and the references cited therein]. In the
following, we only describe the main satellites and instruments used for agricultural
monitoring.
The most important weather satellites now in common use are in the National
series. They are low orbit satellites, and the principal sensor of interest for our
purposes is NOAA
s Advanced Very High Resolution Radiometer (AVHRR). The
AVHRR was designed, in particular, to provide information for hydrologic, ocean-
ographic, and meteorological studies. The current version of the AVHRR (NOAA
19) has six wavelength bands.
The Landsat and SPOT expeditions are particularly interesting Earth resource
monitoring missions. The Landsat expeditions are remarkable among the Earth
'
s
resources monitoring missions (see Richards and Jia
2006
). The first three Landsats
(see
http://landsat.usgs.gov/
) had identical orbit features. All the satellites obtained
images nominally at 9:30 a.m. local time, on a descending (north to south) path. The
complete coverage of the Earth
'
s surface was ensured with 251 revolutions in
18 days. The characteristics of the orbits of the second generation Landsats (from
Landsat 4 onward) were different from those of the previous generations. Again, the
images were acquired nominally at 9:30 a.m. local time, but the Earth
'
s surface was
'
covered with a total of 233 revolutions in 16 days.
Landsat 7 is a similar satellite in all aspects. Landsat 7 included the Enhanced
Thematic Mapper + (i.e. ETM+). The Thematic Mapper has improved the spectral,
spatial, and radiometric characteristics.
1
Note that the narrative on satellite mission is updated on July 2014.
Search WWH ::
Custom Search