Introduction

Remote sensing is defined as the science which deals with obtaining information about objects on earth surface by analysis of data, received from a remote platform. Since the launch of the first remote sensing weather satellite (TIROS-1) in 1960 and the first Earth resources satellite in 1972 (Landsat-1), various platforms with a variety of remote sensing sensors have been launched to study the Earth land cover, the oceans, the atmosphere or to monitor the weather.

Satellite Remote Sensing and GIS Applications in Agricultural Meteorology pp. 39-65

In the present context, information flows from an object to a receiver (sensor) in the form of radiation transmitted through the atmosphere. The interaction between the radiation and the object of interest conveys information required on the nature of the object. In order for a sensor to collect and record energy reflected or emitted from a target or surface, it must reside on a stable platform away from the target or surface being observed. Important properties of sensor system are the number of spectral bands, the spectral position of these bands, the spatial resolution or pixel size and the orbit of the satellite.

Two satellite orbits are important for remote sensing observation of the Earth: the geo-stationary orbit and the polar orbit. The geo-stationary orbit is such a position for a satellite that it keeps pace with the rotation of the Earth. These platforms are covering the same place and give continuous near hemispheric coverage over the same area day and night. These satellites are put in equatorial plane orbiting from west to east. Its coverage is limited to 70oN to 70oS latitudes and one satellite can view one-third globe (Figure 1). As a result it is continuously located above the same geographical position.

These are mainly used for communication and meteorological applications. Weather satellites such as Meteosat, MSG and GOES are normally positioned in this orbit. It enables the sensor aboard the satellite to take every 30 minutes a picture of the weather conditions over the same locations. This geo-stationary orbit is located at an altitude of 36,000 km above the equator.

The following are the major geo-stationary satellites:

Satellite program Launch Agency

Current Satellite

Country

Operational

Agency

METEOSAT

METEOSAT-7

International

EUMETSAT

ESA

INDOEX

METEOSAT-5

International

EUMETSAT

ESA

GOMS

GOMS-1 (ELEKTRO)

Russia

INSAT

INSAT Series

India

Feng-Yun*

Feng-Yun-2B

China

GMS

GMS-5

Japan

GOES (WEST)

GOES-10

U.S.A.

NOAA

NASA

GOES (EAST)

GOES-8

U.S.A.

NOAA

Figure 1. Geo-stationary Orbit (source CCRS website)

The second important remote sensing orbit is the polar orbit. Satellites in a polar orbit, cycle the Earth from North Pole to South Pole. The polar orbits have an inclination of approximately 99 degrees with the equator to maintain a sun synchronous overpass i.e. the satellite passes over all places on earth having the same latitude twice in each orbit at the same local sun-time. This ensures similar illumination conditions when acquiring images over a particular area over a series of days (Figure 2). Image acquisition mostly takes place in the morning when the sun position is optimal between 9.30 and 11.00 hr local time. The altitude of the polar orbits varies from approximately 650 to 900 km although spy-satellites are in a much lower orbit.

Figure 2. Near Polar Orbits (source CCRS website)

As the satellite orbits the Earth from pole to pole, its east-west position would not change if the Earth did not rotate. However, as seen from the Earth, it seems that the satellite is shifting westward because the Earth is rotating (from west to east) beneath it. This apparent movement allows the satellite swath to cover a new area with each pass (Figure 3). The satellite's orbit and the rotation of the Earth work together to allow complete coverage of the Earth's surface, after it has completed one complete cycle of orbits (Figure 4). Through these satellites the entire globe is covered on regular basis and gives repetitive coverage on periodic basis. All the remote sensing earth resource satellites may be grouped in this category. Few of these satellites are LANDSAT series, SPOT series, IRS series, NOAA, SEASAT, TIROS, HCMM, SKYLAB, SPACE SHUTTLE etc.

Figure 3. Area Coverage on each Consecutive pass (source: CCRS website)

1 BO 0 130

1 BO 0 130

Figure 4. Complete Coverage of Earth Surface by Sun Synchronous Satellites

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