Spaceborne Remote Sensing

## Spaceborne Remote Sensing

In spaceborne remote sensing, sensors are mounted on-board a spacecraft (space shuttle or satellite) orbiting the earth. At present, there are several remote sensing satellites providing imagery for research and operational applications. Spaceborne remote sensing provides the following advantages:

- Large area coverage;
- Frequent and repetitive coverage of an area of interest;
- Quantitative measurement of ground features using radiometrically calibrated sensors;
- Semiautomated computerised processing and analysis;
- Relatively lower cost per unit area of coverage.

Satellite imagery has a generally lower resolution compared to aerial photography. However, ground resolution of 1 m or less will be commercially available in the near future with the launch of commercial very high resolution remote sensing satellites.

## Satellite Orbits

A satellite follows a generally elliptical orbit around the earth. The time taken to complete one revolution of the orbit is called the orbital period. The satellite traces out a path on the earth surface, called its ground track, as it moves across the sky. As the earth below is rotating, the satellite traces out a different path on the ground in each subsequent cycle. Remote sensing satellites are often launched into special orbits such that the satellite repeats its path after a fixed time interval. This time interval is called the repeat cycle of the satellite.

## Geostationary Orbits

Geostationary Orbit: The satellite appears stationary with respect to the Earth's surface.

If a satellite follows an orbit parallel to the equator in the same direction as the earth's rotation and with the same period of 24 hours, the satellite will appear stationary with respect to the earth surface. This orbit is a geostationary orbit. Satellites in the geostationary orbits are located at a high altitude of 36,000 km. These orbits enable a satellite to always view the same area on the earth. A large area of the earth can also be covered by the satellite. The geostationary orbits are commonly used by meteorological satellites.

## Near Polar Orbits

A near polar orbit is one with the orbital plane inclined at a small angle with respect to the earth's rotation axis. A satellite following a properly designed near polar orbit passes close to the poles and is able to cover nearly the whole earth surface in a repeat cycle.

## Sun Synchronous Orbits

A near-polar sun synchronous orbit

Earth observation satellites usually follow the sun synchronous orbits. A sun synchronous orbit is a near polar orbit whose altitude is such that the satellite will always pass over a location at a given latitude at the same local solar time. In this way, the same solar illumination condition (except for seasonal variation) can be achieved for the images of a given location taken by the satellite.

## Remote Sensing Satellites

Descriptions of some of the operational remote sensing sattelite platforms and sensors are provided in the appendix of this tutorial.

Optical/Infrared Remote Sensing Satellites

- LANDSAT
- SPOT
- MOS
- ADEOS
- IRS
- RESURS

Microwave Remote Sensing Satellites

- ERS
- JERS
- RADARSAT

Space Shuttles

- Shuttle Imaging Radar

Meteorological Satellites

Airborne Remote Sensing Digital Image

Go to Main Index