Data acquisition level

The figure presented below shows the remote sensing data acquisition levels, because the sensor's height related to the image surface is also an interference factor, not only the intensity and signal quality, as in the registration techniques and data analysis.


Acquisition data levels.

While changing the level, the dimensions of the area being imaged also changes, consequently it is obtained images with different resolutions, which requires studies for different methodology analysis.

    A-) Laboratory Level (0 to 20 m) - it works with reduced portions of the material and it is used to study the spectral behavior almost without environmental factors interference. The area being analyzed by these methods is reduced.

    B-) Airplane Level (300 to 3.000 m) - the energy registered by the sensor does not refer to a determined object, but to an arrangement of objects in the scene. Some objects can be individually detected by its configuration.

    C-) Orbital Level (400 to 920 Km) - in each element of the resolution in the terrain, the energy registered is the integration of the answer of different objects.

The remote sensing techniques evolved first for military purpose. Nowadays these techniques and the research have been producing sophisticated tools, broadening its applicability to a large number of ecological, urban, and natural resources areas. However, when selecting a certain remote sensing technique, the user has to have knowledge of the meaning of the Remote Sensing measured related to the problem being investigated.

Basically, one has to pay attention to two sides of remote sensing: the data acquisition and the data usage.

The data acquisition phase can be divided into:

  • Platform: aerial or spatial its selection depends on the program main goals. The combination of both (aerial and spatial) gives excellent results.
  • Sensor: there are several types of sensors and its selection is closely related to the type of the phenomenon being investigated.
  • Trajectory: it depends on the location of the area of interest, necessity of spatial and temporal coverage.
  • Terrestrial truth: field data (vital factor). It is through sampling that it is possible to train, verify, and evaluate all the remote sensing system.
  • Pre-processing: it considers all the modifications of the collected information by the sensors, before the platform returns to earth or before being transmitted to the tracking station. Here it is included corrections for radiometric, geometric, conversion from analogical to digital, etc. The several products required for interpretation are generated or processed as it is the case for images and tapes compatible with the computer (CCT).
Introduction to Remote Sensing

Orbital Systems

This item will be treated deeper for the Landsat system then the SPOT and Ers-1 systems. These systems, in the actual stage, represents the major source of data for remote sensing with higher continuity potential through out time. Besides this, it is available in the country through the INPE - The Brazilian Institute for Space Research, a whole infra-structure to receive, process and distribute images captured by these systems.

Before mentioning each of these systems, let's present the orbital characteristics for each of them, which are based on the following premises:

  • the orbit has to be circular, to guarantee that images taken in different regions on Earth have all the same scale and resolution;
  • the orbit has to allow the surface cyclical imaging, to guarantee the periodical observation an repeatable in the same places.
  • the orbit has to be synchronous with the sun (sun-synchronous), so the illumination conditions of the earth surface are kept constant;
  • the satellite passage time has to supply the requests of different application areas (geology, geomorphology, agricultural, etc).

The table below shows the characteristics for the Landsat, SPOT and Ers-1.

Landsat 4 and 5
SPOT 1 and 2
Ers-1
Orbit
circular

98,2 degrees

sun-synchronous
circular

98,7 degrees

sun-synchronous
circular

98,5 degrees

sun-synchronous
Period
99 minutes
97 minutes
100,467 minutes
Height
705 km
832 km
785 km
Crossing
9:45 hours
10:39 hours
10:30 hours (desc.)
Cycle
16 days
26 days
35 days (SAR)
Adj. Orbit
172 km
108 km
100 km
Suc. Orbit
2.750 km
2.700 km
-

LANDSAT

The LANDSAT system, today, is composed by 5 satellites developed by NASA (National Aeronautics and Space Administration), receiving, initially, the Earth Resources Technology Satellite-1 (ErsT-1) name, changed in January 1975 to LANDSAT.

The Landsat 1 and 2 carry on board two sensor systems with the same spatial resolution, but with different imaging concept: the RBV system with instantaneous imaging of the whole scene and the MSS system, with terrain imaging by line-scanner.

The purpose of both systems were to get multispectral data, but the performance of the MSS system, in terms of radiometric fidelity, changed the RBV system in the 3º satellite of the series, such that it provides data with a higher spatial resolution in a single spectrum band. It was also added a spectral band to the MSS system in order to work in the thermal infrared band.

For the Landsat 4 and 5, instead of the RBV sensor, the satellite pay-load had a TM (Thematic Mapper) sensor, working in 7 spectral bands. This sensor is conceptually similar to the MSS because it is a line-scanner system. However, it incorporates a series of upgrades in both the optical components and the electronic components.

RBV IMAGING

RBV (Return Beam Vidicon): it is a system similar to a tv camera and allows an instantaneous registration of a certain area in the terrain. The energy from the whole scene goes to the photosensitive surface inside the camera tube and, during certain time, the energy input is interrupted by an obturator, so that the terrain image is scanned by an electron beam. The video signal can be transmitted telemetrically.

MSS IMAGING

MSS (Multispectral Scanner): it is a sensor system that allows the lines imaging of the terrain with 185 Km range, perpendicularly to the satellite orbit. The terrain scan is performed using a mirror which oscillates perpendicularly to the satellite trajectory. During the mirror oscillation, the terrain image through the range is focused over a detectors matrix. The dimension of each detector in the matrix is responsible for its instantaneous field of view (the area observed by each detector in the Earth surface). The energy registered by each detector is transformed in an electrical signal and it is transmitted to the stations on Earth.

To each mirror oscillation, the satellite moves through its orbit, to give a continuous imaging of the terrain. However, the Earth rotation movement causes a small displacement of the scanning initial point to west after each mirror oscillation, that is, after six lines have been imaged. If considered the displacement of 185 Km through out the satellite orbit, there is a displacement of 12.5 cm between the first and the last pixel column.

TM IMAGING

TM (Thematic Mapper): it is an advanced system for multispectral scanning designed to provide: a better spatial resolution, better spectral discrimination between objects on the terrain surface, higher geometrical fidelity and better radiometric precision related to the MSS sensor.

The energy from the scene gets the scanning mirror which oscillates perpendicularly to the direction of the satellite orbit in the east-west and west-east directions. The signal traverses the telescope and a set of mirrors, whose main purpose is to correct the collected signal by the scanning mirror. In this way, the detected signal in each detector matrix for each channel is transferred to an amplifier and converted to digital signal through an A/D (analogical/digital) system. The data output is transmitted through telemetry.


SPOT

The SPOT system is a French spatial program similar to the Landsat, which was designed by the Centre National d'Etudes Spatiales (CNES) and carries on board two high resolution sensors (HRV - HAUT Resolution Visible). These sensors were conceived to operate in two different modes. The multispectral mode allows the data acquisition in three different electromagnetic spectral bands with a spatial resolution of 20 m. And the panchromatic with a spatial resolution of 10 meters.

One of the remarkable characteristics of the instruments on board of the SPOT is the possibility of observation "off-nadir" (directional pointing). The sensor can be directed to observe lateral scenes to the orbit where the satellite is placed in a given moment. This observation possibility "off-nadir" enhances the chances to get a repeatable coverage in certain areas. Another advantage in the "off-nadir" view is the possibility to be acquired in stereoscopic pairs in certain areas.

The light from the scene gets the mirror plane, that can be controlled from the earth stations varying in angles of +/- 0.6 up to 27o related to the vertical axis.

The energy that gets the mirror plane is focused on a detectors linear matrix of the CCD (charge-Coupled Device) type. Each matrix is composed by 6000 detectors linearly arranged, forming which is called of "push-broom scanner" or electronic scanning system. This system allows the instantaneous imaging of a complete line on the terrain, perpendicular to the satellite orbit path.


Ers-1

This satellite was built by the European Space Agency (ESA), the Ers-1 was launched from the spatial center in the French Guiana using the Ariane 4 in July 16, 1991. With a nominal mission of two years, the main purpose of this satellite was related to the studies of ocean and glacier areas, in several natural science areas.

Among the equipment on board of the satellite, there is the AMI (Active Microwave Instruments), which is a Synthetic Aperture Radar (SAR) system and a scatterometer (a wind measurement equipment). The images acquired by the SAR gives data in a 100 Km x 100 Km range areas, with a spatial resolution of 30 meters.

An antenna of 10 x 1 meters emits and receive a microwave beam in the 5.3 GHz band (C band), with polarization VV and an incidence angle of 23 degrees.

The SAR operation in the image mode produce a very high data rate (105 Mbps), so the images can be generated in zones equipped with reception stations. The Earth surface can be entirely covered and imaged in 35 days cycles.


See also:
Image Format


Introduction to Remote Sensing

Images Reception System

The Brazilian Satellite Data Reception System is composed of one reception station in Cuiaba which is operational since 1973. This reception station is located in the South America geographical center, which allows the data acquisition over the whole Brazilian territory and part of the territory of neighbor countries.

Another component of the Brazilian Satellite Data Reception System is the laboratory of electronic and photographic processing of data collected by Remote Sensing on board of the satellites, located at Cachoeira Paulista - SP.

At Cuiaba, the data is received through a parabolic antenna and recorded in High Density Digital Magnetic Tape (HDDT). These tapes are sent to Cachoeira Paulista - São Paulo.

The laboratory of image processing at Cachoeira Paulista has the purpose to transform data received by the receiving stations in photographic images, magnetic tapes compatible to the computer (CCT-Computer Compatible Tapes, Streamer, Dat) or optical disks (CD-ROM).

Introduction to Remote Sensing