Agriculture Reference
In-Depth Information
They do not rotate with the earth - as the satellites on geosynchroneous orbits do.
Instead many of these satellites operate on a
sunsynchroneous path
, which means
they pass the globe overhead at essentially the same solar time throughout the whole
year while the earth rotates underneath them. So, theoretically, it is possible to get
snapshots for specifi c places on the planet at the same solar time, which facilitates
multitemporal comparisons. The elevation is between 200 and 900 km, hence much
lower than with geosynchronous orbiting satellites.
For sensing from aerial platforms either a plane, a helicopter or an unmanned
quadrocopter could be used. The latter has rotors like a helicopter, but four of them
and can operate in an autonomous manner. With planes, the vertical distance can be
several km, whereas for helicopters and especially for unmanned quadrocopters it
can go down to 70 m or even less. So the attenuation of the radiation that is refl ected
from soils or crops can be much lower than for satellites.
The spatial- and temporal resolution that can be obtained is important. From the-
ory, it must be expected that the
spatial resolution
decreases in the order farm
machines, aerial platforms, satellites. And in fact, some years ago the spatial resolu-
tion that was obtained from satellites often did not satisfy the needs for site-specifi c
farming. But steady advances in the sensitivity of optical instruments have improved
the spatial resolutions. Today with a clear cloudless sky, satellites on polar orbits can
provide spatial resolutions that make possible a terrestrial cell size of 1 m
2
and even
less. This does not alter the general fact that it is easier to obtain a high spatial resolu-
tion when sensing occurs with smaller distances to soils or crops. Yet the situation is
that with modern techniques and a clear cloudless sky, sensing from every platform
can deliver the spatial resolution that is needed. Especially with optical sensors that
operate from a farm machine, the resolution can be much higher than is even needed.
Concerning
temporal resolution
, sensing from satellites on polar orbits theo-
retically provides for the best prerequisites since data from the same fi eld can be
obtained every day, provided neither a closed atmospheric window nor clouds
impede the radiation. It is practically not feasible to sense from farm machines or
from aerial platforms with such a
temporal frequency
. This holds as long as farm
machines and aerial platforms need drivers or pilots.
However, when sensing occurs from farm machines during a fi eld operation,
another important aspect deserves attention. Since many fi eld operations take place
just once or twice per year, the temporal resolution seems to be extremely low. But
an important point is that with such proximal online and on-the-go control of farming
operations, the sensing can occur exactly at the time when the information is needed.
If there is temporal variation of soil- or crop properties during the growing season -
and in many cases this is the situation - it can be important to sense precisely at the
time when the farming operation takes place. So for these soil- and crop properties it
is
temporal precision
that is needed rather than temporal resolution. A high temporal
resolution might in these cases lead to a huge amount of useless data.
There are farming situations that call for a high temporal resolution or temporal
frequency,
e.g.
when a crop is observed for pest infections. Yet there are also cases
when temporal precision is the most important criterion,
e.g.
when in- season fertilizing
of nitrogen takes place. This distinction between temporal resolution on the one hand
and temporal precision on the other hand is helpful, though both might be needed.
