The term Remote Sensing was firstused in the early 1960s. It is defined as the total processesused to acquire and measure information of some property of objects and phenomena by a recording device (sensor) that is not in physical contact with the objects and phenomenaunder study. It primarily involves an object surface, a recording device, and information-carrying energy waves.
Remote sensing data acquisition involves several sequential processes that help collect information about the properties of the Earth's surface:
The Sun is the most important source of energyused in remote sensing. Energy may also be artificially generated, such as flashguns or energy beamsused in RADAR (Radio Detection and Ranging).
Energy propagates in the form of waves at the speed of light: $$c \thickapprox 300,000 \text{ km/s}$$ This propagation is called Electromagnetic Radiation (EMR). The plotting of these variations is known as the Electromagnetic Spectrum, which is grouped into Gamma rays, X-rays, Ultraviolet rays, Visible rays, Infrared rays, Microwaves, and Radio waves. Remote sensing primarilyutilizes the visible, infrared, and microwave regions.
Propagating energy interacts with Earth features, leading to absorption, transmission, reflection, or emission. Different objects responduniquely to different regions of the spectrum. For example, a fresh water body absorbs more energy in the red and infrared regions (appearing dark/black), while a turbid water body reflects more in the blue and green regions (appearing light-toned).
Atmospheric constituents like gases, water molecules, and dust particles modify the reflected energy. Carbon dioxide ($CO_2$), Hydrogen ($H$), and water molecules absorb energy in the middle infrared region, while dust particles scatter blue light, preventing some energy from reaching the sensor.
Sensors are placed on satellite platforms operating in specific orbits:
| Orbital Characteristics | Sun-Synchronous Satellites | Geostationary Satellites |
|---|---|---|
| Altitude | 700 – 900 km | @ 36,000 km |
| Coverage | 81°N to 81°S | 1/3rd of the Globe |
| Orbital period | @ 14 orbits per day | 24 hours |
| Resolution | Fine (182 metres to 1 metre) | Coarse (1 km x 1 km) |
| Uses | Earth Resources Applications | Telecommunication and Weather monitoring |
The received radiations are electronically converted into a digital image composed of digital numbers arranged in rows and columns. This data is transmitted to Earth Receiving Stations (e.g., Shadnagar near Hyderabad in India). Once corrected for errors, information is extractedsing digital image processing or visual interpretation, and finally converted into thematic maps or tabular data.
Sensors are classified into photographic (analogue) and non-photographic (digital) sensors. Non-photographic sensors are known as scanners.
MSS are designed to obtain images of objects by sweeping across the field of view. The oscillation of a scanning mirror directs received energy to detectors, converting it into electrical signals and then into Digital Numbers (DN). Scanners are divided into two main types:
Remote sensors are characterized by three primary types of resolutions:
| Satellite/Sensor | Spatial Resolution (in metres) | Number of Bands | Radiometric Range (Grey Level Variations) |
|---|---|---|---|
| Landsat MSS (USA) | 80.0 × 80.0 | 4 | 0 - 64 |
| IRS LISS – I (India) | 72.5 × 72.5 | 4 | 0 - 127 |
| IRS LISS – II (India) | 36.25 × 36.25 | 4 | 0 - 127 |
| Landsat TM (USA) | 30.00 × 30.00 | 4 | 0 - 255 |
| IRS LISS III (India) | 23.00 × 23.00 | 4 | 0 - 127 |
| SPOT HRV - I (France) | 20.00 × 20.00 | 3 | 0 - 255 |
| SPOT HRV – II (France) | 10.00 × 10.00 | 1 | 0 - 255 |
| IRS PAN (India) | 5.80 × 5.80 | 1 | 0 - 127 |
Remotely sensed data products are classified into Photographic Images (recorded on light-sensitive film, typically 0.3 – 0.9 µm) and Digital Images (composed of discrete picture elements called pixels, each with a specific DN value).
Visual interpretation is a manual exercise thatses several image and terrain characteristics to identify objects:
| S. No. | Earth Surface Feature | Colour (In Standard FCC) |
|---|---|---|
| 1. | Healthy Vegetation and Cultivated Areas • Evergreen • Deciduous • Scrubs • Cropped land • Fallow land | Red to magenta Brown to red Light brown with red patches Bright red Light blue to white |
| 2. | Waterbody • Clear water • Turbid waterbody | Dark blue to black Light blue |
| 3. | Built –up area • High density • Low density | Dark blue to bluish green Light blue |
| 4. | Waste lands/Rock outcrops • Rock outcrops • Sandy deserts/River sand/Salt affected • Deep ravines • Shallow ravines • Water logged/Wet lands | Light brown Light blue to white Dark green Light green Mottled black |