Asynchronous Satellite Orbits

Asynchronous Orbits

Asynchronous Satellite Orbits are lower than geostationary orbits. Satellites in asynchronous orbit will pass overhead at different times of the day. Asynchronous Orbits are in different altitudes.

(80 to 1,200 miles )480 to 970 km - Asynchronous Orbits
Main type of satellite in these altitudes is Observation satellite. Observation satellites are used for tasks like photography. Observation satellites perform tasks such as:
  • Mapping
  • Ice and sand movement
  • Locating environmental situations
  • Locating mineral deposits
  • Finding crop problems
Search-and-rescue satellites are also works in these altitudes. Search-and-rescue satellites act as relay stations to rebroadcast emergency radio-beacon signals from a downed aircraft or ship in trouble.
The Space Shuttles and other man mission capsules are also operating in these altitudes.

3,000 to 6,000 miles(4,800 to 9,700 km). 
Some of the Science satellites are operating in altitudes of 3,000 to 6,000 miles (4,800 to 9,700 km). They send their research data to Earth via radio telemetry signals. Scientific satellite applications include:
  • Researching plants and animals
  • Earth science, such as monitoring volcanoes
  • Tracking wildlife
  • Astronomy, using the Infrared Astronomy Satellite  

6,000 to 12,000 miles - Asynchronous Orbits
Navigation satellites are operating at altitudes of 6,000 to 12,000 miles. Most popular navigation satellite is the satellite used for Global Positioning System (GPS). These satellites can determine the exact location of the GPS receiver. The GPS receiver may be located: 
  • In a ship at sea
  • In another spacecraft
  • In an airplane
  • In an automobile
  • In your pocket 

Navigation satellites

Types of Satellite Orbits

Types of Satellite Orbits

Satellite orbits are mainly classified into three based on the satellite's position relative to Earth's surface.
 First one is called Geostationary Orbits or Geosynchronous or Synchronous orbits.  In this orbit the satellite is always positioned over the same spot on Earth. 

Geostationary Orbits

Geostationary satellites are positioned in a band along the equator, at an altitude of about 35,786 km / 22,223 miles. Television, communications and weather satellites use geostationary orbits.
Second types of orbits are called Asynchronous Orbit. This orbit is much lower than geostationary orbits. Satellites in asynchronous orbit will pass overhead at different times of the day. The space stations and many other satellites are positioned in an asynchronous orbit at various  altitudes depending of the application of satellite. 
 Asynchronous Orbit

Third type of orbit is called Polar Orbit. In polar orbit, the satellite generally flies at a low altitude. The satellite in this orbit passes over the planet's poles on each revolution. 

  Polar Orbit
The polar orbit remains fixed in space and Earth rotates inside the polar orbit. As a result almost all part of earth passes under a satellite in a polar orbit and satellites in polar orbits achieve excellent coverage of the planet.  They are often used for remote sensing satellites.

Indian Remote Sensing Satellites

Indian Remote Sensing Satellites

Remote Sensing effort in India started in late sixties with the aerial survey using multi-data instrument and aircraft carrying variety of sensors like infra-red scanner and multi-specialty scanner, radio meter etc. for monitoring different crops, soil degradation, water pollution, land usage etc. Considering the need to have a global picture on a larger scale, development efforts for remote sensing satellite were pursued. Based on the successful launch of Aryabhata and Bhaskara, new series of remote sensing satellite called IRS were developed. 
ISRO has designed and developed high performance remote sensing satellite system for India and the first one, IRS-1A was launched on March 17, 1988 on board a Soviet Vostok Rocket.
The second satellite IRS-1B identical to IRS-1A was launched August 29, 1991. This carried complex payloads like linear imaging Self Scanner – LISS-1 with the resolution of 72.5 m and LISS -2A, LISS -2B with a resolution of 36.25 m. The satellite was placed in 904 km polar sun synchronous orbit with an orbital period of 103 minutes. The satellite return to their original orbital trace every 22 days enabling repeated collection of data over the same place and at the same local time. This was followed by IRS-1C class satellites.

With the PSLV getting operational, IRS-P2 satellites were placed in sun-synchronous orbit on October 15, 1994.
 The satellite control center located at Bangalore and other ground stations at different locations regularly track and monitor IRS satellite. National Remote Sensing Agency (NRSA) at Hyderabad receives the data from the IRS satellites. After processing & distribute it to the user agencies.

IRS data has been extensively used to evaluate agricultural crops yield estimate, drought monitoring and assessment, flood mapping, land use and land cover mapping, waste land management, water resource land management, ocean/marine resource survey, urban planning, mineral prospecting, various resource survey etc. The IRS system has become one of the main service sectors of ISRO for the nation.
 The TES-Satellite has provided a resolution of better than 1 m in which one could clearly identify the vehicles moving on the road. 
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