Remote Sensing Technologies
Objectives
- To describe the characteristics of sensors
- To describe the various types of remote sensing platforms and their
advantages
- To describe data reception, transmission, and processing of remotely
sensed data
Introduction
A remote sensing platform is designed with a relatively narrow set
of purposes in mind. Many important decisions must be made when designing
a remote sensing technology. The type of sensor and its capabilities
must be defined. The platform on which the sensors will be mounted must
be determined. The means by which the remotely-sensed data is received,
transmitted, and processed before delivery to its end user must be designed.
All of these decisions are made based on knowledge of the target and
the information about the target that is in demand, balanced by other
factors such as cost, availability of resources, and time constraints.
The end result of this process is a tool that is specifically designed
to perform a task or a set of related tasks that will assist researchers
in better understanding the process that is under investigation.
Sensor Resolution
There are many applications of remote sensing, and each sensor is engineered
for very specific purposes. The design and placement of a sensor is
determined by the unique characteristics of the target that will be
studied and the information that is required from the target. Each remote
sensing application has specific demands on the amount of area to be
covered, the frequency with which measurements will be made, and the
type of energy that will be detected. Thus, a sensor must provide the
spatial, spectral, and temporal resolution necessary to meet the needs
of the application.
Spatial resolution refers to the amount of detail
that can be detected by a sensor. Detailed mapping of land use practices
requires a much greater spatial resolution than observations of a large
scale storm system. Thus, land use satellites such as Landsat generally
have greater spatial resolution than global weather satellites.
Spectral resolution refers to the width or range of
each spectral band measured by a sensor. Detection of some phenomena,
such as vegetative stress, requires a sensor with sensitivity in a narrow
spectral band so that differences in the spectral signatures at a specific
wavelength can be detected. A panchromatic sensor, which covers a wide
spectral range, would not be well suited to such a task. A narrow band
sensor in the red portion of the spectrum would be better at detecting
vegetative stress.
Temporal resolution refers to the time interval between
measurements. For some applications, such as monitoring the development
of a severe thunderstorm, measurements are required at a frequency of
a few minutes. Some applications, such as measuring crop production
or insect infestations, require seasonal measurements, while others,
such as geological mapping, require a single measurement.
Ground-Based Platforms
Remote sensing platforms that position the sensor at the Earth's surface
are called ground-based platforms. These systems are
fixed to the Earth and the sensors are often standard tools used to
measure environmental conditions such as air temperature, wind characteristics,
water salinity, earthquake intensity and such. Ground-based sensors
can be placed on tall structures such as towers, scaffolding, or buildings
to elevate the platform.
Ground-based sensors are generally less expensive to operate and maintain
than aircraft or satellite sensors, but they do not provide the aerial
extent of the airborne platforms. Ground-based sensors are often used
to record detailed information about the surface, which is compared
with information collected from aircraft or satellite sensors.
One example of ground-based remote sensing are sensors mounted on buoys
that make real-time measurements of water temperature, salinity, wind
speed, and wind direction. The buoys are anchored in a body of water
(the target) and they transmit the results of each measurement to receiving
stations to be processed. These sensors can be used to supplement or
"ground truth" measurements made from airborne or satellite
sensors.
Aerial Platforms
Aerial platforms are most often sensors mounted on
fixed-wing aircraft, though other airborne platforms, such as balloons,
rockets, and helicopters can be used. Aircraft are often used to collect
very detailed images of the Earth's surface and facilitate the collection
of data over virtually any portion of the Earth's surface at any time.
Aerial systems elevate the sensor above the Earth's surface in order
to increase its aerial coverage. They also allow researchers to monitor
very large areas of the surface which would be impractical with ground-based
sensors or impossible or dangerous to visit.
Airborne remote sensing dates back to the early 1900's when airplanes
were used during the World Wars to conduct surveillance of the enemy.
More recently, cameras mounted on aircraft have been used to monitor
land use practices, locate forest fires, and produce detailed and accurate
maps of remote or inaccessible locations on our planet. Weather balloons
and rockets are still used by research scientists as a means for obtaining
direct measurements of the properties of the upper atmosphere. These
provide a less expensive and reusable alternative to aircraft and satellite
systems.
The following images depict Atlanta, Georgia as seen from an airborne
sensor mounted on a specially equiped Lear Jet. These images were part
of a study on the effect of urban sprawl on the temperature within a
city.
Satellite platforms
In the early 1960's researchers started mounting sensors on satellites
placed into orbit over the Earth and ushered in a new era of environmental
remote sensing that continues to grow at a rapid pace today. The vantage
point of space allows researchers to observe and measure phenomena on
a time and spatial scale that was previously impossible. Today, satellites
provide us with views of the Earth that allow us to monitor global change
and understand our planet.
This wealth of data comes with a price, however. To build a satellite
and place it into orbit is a very difficult and expensive endeavor,
often coming with a price tag that approaches billions of dollars. Satellites
must be operated remotely from the ground and data from the satellite
sensors must be transmitted to the surface. The communications technologies
in remote sensing satellites can be very complex and expensive to engineer
and maintain. A number of satellites have failed to reach orbit, or
failed to operate once in orbit around the earth, which is a testament
to the incredible complexity involved in designing, building, and operating
a satellite.
All of these difficulties not withstanding, environmental satellites
have contributed greatly to our understanding of the Earth's environment
and continue to be used extensively for remote sensing research. For
example, weather satellite technology, one of the first practical applications
of satellite remote sensing, has vastly expanded our understanding of
the Earth's weather by providing a synoptic (large scale) view of our
weather systems that was previously impossible. It was only after the
advent of satellites that weather patterns such as hurricanes and mid-latitude
cyclones were fully understood. Prior to satellites, any knowledge of
these storms was collected through ground level observations that unfortunately
did not provide the information necessary to adequately understand them.
The contribution of satellites to our understanding of dangerous weather
events has saved countless numbers of lives since the early 1960's.
Land use satellites such as those in the Landsat program have monitored
land use change for decades, providing detailed insight into how development
has affected tropical rain forests, how climatic changes have affected
agricultural production, how deserts advance and withdraw, and how the
polar ice caps have retreated. The Landsat program, which started in
1972 with the launch of the Landsat-1 satellite, continues even today
with Landsat-7, which provides us with daily images of parts of the
Earth's surface.
Communications and Data Collection
Data collected from a remote sensing system must be retrieved and delivered
to the end users. Often, this must be done quickly for the data to be
of any use, such as in the case of severe thunderstorm forecasting where
storms develop into severe storms within minutes. Thus the transmission,
reception, processing, and distribution of data from a satellite sensor
must be carefully designed to meet the users' needs.
Ground-based remote sensing platforms can transmit data using ground-based
communication systems, such as radio and microwave transmissions or
computer networks. Some systems can store data on the platform, allowing
researchers to manually collect the data from the platform. Data collected
in an aircraft can be stored on board and retrieved once the aircraft
lands. Satellite data, however, is very difficult to obtain since the
satellite remains in space during its entire operational lifetime. This
data must be transmitted back to the Earth to a ground receiving
station, which can receive the data and process it for distribution
to the end user.
Data collected from a satellite platform can be transmitted to Earth
in a variety of ways. A satellite can transmit data directly to a ground
receiving station that is within its line of sight. When the satellite
is not in sight of a ground station, it can store its data on board
and "dump" the data later, when it is back in sight of a ground
station. Finally, for immediate transmission, a satellite can relay
data to the ground receiving station through a series of communications
satellites in orbit around the Earth, transferring data from one satellite
to the next until it is able to reach the ground receiving station desired.
Many satellites use a combination of the methods described above. The
TIROS class meteorological satellites (NOAA satellites) use a continuous
transmission of lower resolution data that can be received from any
ground station within radio range while also using on-board storage
to store higher resolution data that is transmitted to specific ground
stations capable of receiving it.
The data received at the ground station are in a raw digital format.
They may then, if required, be processed to correct systematic, geometric
and atmospheric distortions to the imagery, and be translated into a
standardized format. The data are written to some form of storage medium
such as tape, disk, or CD. The data are typically archived at most receiving
and processing stations, and full libraries of data are managed by government
agencies as well as commercial companies responsible for each sensor's
archives.
References
- Spaceborne Imaging
Radar-C/X-Band Synthetic Aperture Radar (SIR-C/X-SAR) Images of the
Earth
- http://www.jpl.nasa.gov/radar/sircxsar/
