• Kepler’s First Law: The orbit of a planet is an ellipse with the Sun at one of the two foci.
  • Kepler’s Second Law: A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
  • In simple words, the speed of the planet increases as it nears the sun and decreases as it recedes from the sun.



The varying orbital speed of the earth (in the figure, the orbit of the earth is exaggerated)

  • Kepler’s Third Law: The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.



Orbital period (T): time taken by a plant to complete one revolution around the sun.

Semi Major Axis (a1 and a2): half of the major axis of the ellipse.

T12/a13 = T22/a23

  • In simple terms,the distance of a planet from the sun determines the time it takes for that planet to revolve around the sun (farther the planet is, greater the orbital period).
Planet Orbital Period (T) in years Average Distance (R) in AU T2/R3
Mercury 0.241 0.39 0.98
Venus .615 0.72 1.01
Earth 1.00 1.00 1.00
Mars 1.88 1.52 1.01



  • Most satellites orbit the earth in elliptical patterns.
  • When a satellite is at its farthest point from the earth, it is at the apogee of the orbit.
  • When a satellite is at its closest point to the earth, it is at the perigee of the orbit.
  • In accordance with Kepler’s second law, the satellites are fastest at the perigee and slowest at the apogee.





  • There are two important forces acting on the satellite:
    • the gravitational force which will pull the satellite towards earth and
    • the centrifugal force (due to revolution) which counters the gravitational pull.



  • Revolution causes centrifugal force (the object tends to move away from the centre).
  • Higher the speed of the revolving satellite (orbital velocity), higher the centrifugal force.
  • Thus, by varying the speed (orbital velocity) of the satellite, we can make the satellite
    • fall back to earth by decreasing the orbital velocity (centrifugal force < gravitational force)
    • stay in its orbit by adjusting the speed so that the centrifugal force balances the gravitational pull (centrifugal force = gravitational force). (Lower the orbit, higher should be the orbital velocity).
    • Escape earth’s influence by keeping the orbital velocity above the required speed (centrifugal force > gravitational force).



  • International Space Station (400 km), the Hubble Space Telescope (560 km) and some observation satellites are all rotating the earth in Low Earth Orbit.
  • LEO is high enough to significantly reduce the atmospheric drag yet close enough to observe the earth (remote sensing).
  • In LEO, the satellite’s orbital period is much smaller than the earth’s rotational period (24 hours).
  • That is, the satellites in LEO complete multiple revolutions in 24 hours (Lower the orbit, higher should be the speed).



A) What is the speed required to keep a satellite in LEO?

  • The speed is dependent on the distance from the centre of the Earth.
  • At an altitude of 200 km, the required orbital velocity is a little more than 27,400 kmph.
  • In the case of the space shuttle, it orbits the Earth once every 90 minutes at an altitude of 466 km.

B) Advantages of LEO

  • Low Earth Orbit is used for things that we want to visit often, like the International Space Station, the Hubble Space Telescope and some satellites (usually spy satellites and other observation satellites).
  • This is convenient for installing new instruments, experiments, and return to earth in a relatively short time.

C) Disadvantages of LEO

  • Atmospheric drag will lead to more fuel consumption and constant speed adjustments.
  • A satellite traveling in LEO do not spend very long over any one part of the Earth at a given time.
  • Hence, satellites in LEO are not suitable for communication and weather observation and forecasting.

D) Solution

  • One solution is to put a satellite in a highly elliptical orbit (eccentric orbit ― non-geosynchronous).
  • The other is to place the satellite in a geosynchronous orbit.



E) Space debris in LEO

  • The LEO environment is becoming congested with space debris because of the frequency of object launches. This has caused growing concern in recent years, since collisions at orbital velocities can easily be dangerous, and even deadly. Collisions can produce even more space debris in the process, creating a domino effect, something known as Kessler Syndrome.



  • Kepler’s second law: an object in orbit about Earth moves much faster when it is close to Earth than when it is farther away.
  • Perigee is the closest point and apogee is the farthest.
  • If the orbit is very elliptical, the satellite will spend most of its time near apogee (the furthest point in its orbit) where it moves very slowly.
  • Thus, it can be above a specific location most of the time.



A) Disadvantages of Highly Elliptical Orbits

  • In a highly elliptical orbit, the satellite has long dwell time over one area, but at certain times when the satellite is on the high speed portion of the orbit, there is no coverage over the desired area.

B) Solution

  • We could have two satellites on similar orbits but timed to be on opposite sides at any given time.
  • In this way, there will always be one satellite over the desired coverage area at all times.


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  • If we want continuous coverage over the entire planet at all times, such as the Global Positioning System (GPS satellites are in Medium Earth Orbit though), then we must have a constellation of satellites with orbits that are both different in location and time.
  • In this way, there is a satellite over every part of the Earth at any given time.







  • Another solution to the dwell time problem is to have a satellite whose orbital period is equal to the period of rotation of the earth (24 hrs) (satellite’s revolution is in sync with the earth’s rotation).
  • In this case, the satellite cannot be too close to the Earth because it would not be going fast enough to counteract the pull of gravity.
  • Using Kepler’s third law it is determined that the satellite has to be placed approximately 36,000 km away from the surface of the Earth (~42,000 km from the centre of the Earth) in order to remain in a GSO orbit.
  • By positioning a satellite so that it has infinite dwell time over one spot on the Earth, we can constantly monitor the weather in one location, provide reliable telecommunications service, etc.
  • The downside of a GSO is that it is more expensive to put and maintain something that high up.

A) Geostationary Orbit or Geosynchronous Equatorial Orbit (GEO)

  • A geostationary orbit or geosynchronous equatorial orbit is a circular geosynchronous orbitabove Earth’s equator and following the direction of Earth’s rotation.
  • Because the satellite stays right over the same spot all the time, this kind of orbit is called “geostationary.”



B) Geostationary vs Geosynchronous









  • Medium Earth Orbits (MEO) range in altitude from 2,000 kms up to the geosynchronous orbit at 36,000 km which includes part of the lower and all of the upper Van Allen radiation belts.
  • The Van Allen Radiation Belt is a region of high energy charged particles moving at speeds close to that of light encircling the Earth which can damage solar cells, circuits, and shorten the life of a satellite or spacecraft.
  • Practical orbits therefore avoid these regions.



  • Satellites in these orbits fly over the Earth from pole to pole in an orbit perpendicular to the equatorial plane.
  • This orbit is used in surface mapping and observation satellites since it allows the orbiting satellite to take advantage of the earth’s rotation below to observe the entire surface of the Earth as it passes below.
  • Pictures of the Earth’s surface in applications such as Google Earth come from satellites in polar orbits.





  • Polar orbit and sun-synchronous orbits are low earth orbits.
  • Sun-synchronous orbit is a near polar orbit in which the satellite passes over any given point of the planet’s surface at the same local mean solar time.
  • When a satellite has a sun-synchronous orbit, it means that the satellite has a constant sun illumination.
  • Because of the consistent lighting, the satellites in sun-synchronous orbit are used for remote sensing applications (image the Earth’s surface in visible or infrared wavelengths) like imaging, spying, etc.





  • It is not always possible to launch a space vehicle directly into its desired orbit.
  • The launch site may be in an inconvenient location or the launch window may be very short.
  • In such cases the vehicle may be launched into a temporary orbit called a parking orbit.
  • The parking obit provides more options for realising the ultimate orbit.
  • For manned space missions the parking orbit provides an opportunity to recheck the systems.



  • A geosynchronous transfer orbit is a Hohmann transfer orbit — an elliptical orbit used to transfer between two orbits in the same plane — used to reach geosynchronous or geostationary orbit.





  • Escape velocity is the minimum launch velocity (assuming the object is launched straight up) required for an object to escape earth’s gravitational pull (it doesn’t fall back to earth).
  • One condition is that once launched the object is not supplied with any additional energy nor hindered by external force (like atmospheric drag) other than earth’s gravity.
  • The escape velocity required for an object to escape earth’s gravitational pull is ~11.2 m/s (40,000+ kmph).
  • It is neither feasible (atmospheric friction will turn it into ash) nor desirable (cannot place satellites in desired orbit) to launch rockets at escape velocity.





  • Aryabhata, launched by USSR in 1975. Its main purpose was to develop infrastructure for space programme.
  • Bhaskar, launched by USSR in 1979. It was a prototype remote sensing satellite.
  • APPLE (Ariane Passenger PayLoad Experiment(APPLE), was an experimental communication satellite launched in 1981 by Ariane, a launch vehicle of the European Space Agency (ESA). Satellite served as testbed of the Indian telecommunications space relay infrastructure. It was used in several communication experiments including relay of TV programmes and radio networking.
  • Rohini series of satellites, they were experimental satellites launched by SLV-3
  • SROSS (The Stretched Rohini Satellite Series) are a series of satellites developed by the Indian Space Research Organisation as follow ons to the Rohini Satellites for conducting astrophysics, Earth Remote Sensing, and upper atmospheric monitoring experiments as well as for new and novel application-oriented missions. These satellites were the payload of the developmental flights of the Augmented Satellite Launch Vehicle



Remote sensing is the science of obtaining information about objects or areas from a distance, typically from aircraft or satellites. They are used for resource management. They observe and collect Data through photography as they are fitted with various cameras.



A) Types of Remote Sensing Instruments

Passive Instruments

  • They detect natural energy that is reflected or emitted from the observed scene. Passive instruments sense only radiation emitted by the object being viewed or reflected by the object from a source other than the instrument. Reflected sunlight is the most common external source of radiation sensed by passive instruments. Scientists use a variety of passive remote sensors.

Various Passive Remote Sensing Instruments

  • Radiometer: An instrument that quantitatively measures the intensity of electromagnetic radiation in some band of wavelengths in the spectrum. Usually a radiometer is further identified by the portion of the spectrum it covers; for example, visible, infrared, or microwave.
  • Imaging Radiometer: It includes a scanning capability to provide a two-dimensional array of pixels from which an image can be produced is called an imaging radiometer. Scanning can be performed mechanically or electronically by using an array of detectors.
  • Spectrometer: A device designed to detect, measure and analyze the spectral content of the incident electromagnetic radiation is called a spectrometer. Conventional, imaging spectrometers use gratings or prisms to disperse the radiation for spectral discrimination.
  • Spectroradiometer: A radiometer that can measure the intensity of radiation in multiple wavelength bands (i.e. multispectral). Often the bands are of a high spectral resolution- designed for the remote sensing of specific parameters such as sea surface temperature, cloud characteristics, ocean colour, vegetation, trace chemical species in the atmosphere, etc.

Active Remote Sensing Instruments

  • They provide their own energy (electromagnetic radiation) to illuminate the object or scene they observe. They send a pulse of energy from the sensor to the object and then receive the radiation that is reflected from that object. Scientists use several types of active remote sensors:
  • RADAR (Radio Detection and Ranging): A radar uses a transmitter operating at either radio or microwave frequencies to emit electromagnetic radiation and a directional antenna or receiver to measure the time of arrival of reflected radiation from distant objects. Distance to the object can be determined since electromagnetic radiation propagates at the speed of light.
  • Scatter meter: A scatter meter is a high frequency microwave radar designed specifically to measure reflected radiation. Over ocean surfaces, measurements of reflected radiation in the microwave spectral region can be used to derive maps of surface wind speed and direction.
  • LIDAR (Light Detection and Ranging): A Lidar uses a laser (light amplification by stimulated emission of radiation) to transmit a light pulse and a receiver with sensitive detectors to measure the reflected light. Distance to the object is determined by recording the time between the transmitted and reflected pulses and using the speed of light to calculate the distance travelled. Lidars can determine atmospheric profiles of aerosols, clouds, and other constituents of the atmosphere.
  • Laser Altimeter: A laser altimeter uses a lidar to measure the height of the instrument platform above the surface. By independently knowing the height of the platform with respect to the mean Earth’s surface, the topography of the underlying surface can be determined.

B) Types of Remote Sensing

Satellite Remote Sensing

  • The remote sensing satellites are equipped with sensors looking down to the earth. They are “the eyes in the sky” constantly observing the earth as they go round in orbits.
  • In satellite remote sensing of the earth, the sensors are looking through a layer of atmosphere separating the sensors from the Earth’s surface being observed. So, the analysis of the effects of atmosphere on the electromagnetic radiation travelling from the earth to the sensor through the atmosphere provides vital inputs. The atmospheric constituents cause wavelength dependent absorption and scattering of radiation. These effects lead to the deterioration of quality of images.
  • An important consequence of atmospheric absorption is that certain wavelength bands in the electromagnetic spectrum are strongly absorbed and effectively blocked by the atmosphere. The wavelength regions in the electromagnetic spectrum usable for remote sensing are determined by their ability to penetrate atmosphere. These regions are known as the atmospheric transmission windows.
  • Remote sensing systems are often designed to operate within one or more of the atmospheric windows. These windows exist in the microwave region, some wavelength bands in the infrared, the entire visible region and part of the near ultraviolet regions. Although the atmosphere is practically transparent to x-rays and gamma rays, these radiations are not normally used in remote sensing of the earth.

Optical and Infrared Remote Sensing

  • In optical remote sensing, optical sensors detect solar radiation reflected or scattered from the earth, resembling photographs taken by a camera high up in space. The wavelength region usually extends from the visible and near-infrared to the short-wave infrared.
  • There are also infrared sensors measuring the thermal infrared radiation emitted from the earth, from which the land or the sea surface temperature can be derived.

Microwave Remote Sensing

  • There are some remote sensing satellites which carry passive or active microwave sensors. The active sensors emit pulses of microwave radiation to illuminate the areas to be imaged. The images of the earth surface are formed by measuring the microwave energy scattered by the ground or sea back to the sensors.
  • These satellites carry their own flashlight emitting microwaves to illuminate their targets. So, the images can be acquired day and night.
  • Microwaves have an additional advantage as they can also penetrate clouds. Images can be acquired even when there are clouds covering the earth surface. A microwave imaging system which can produce high resolution image of the earth is the synthetic aperture radar (SAR). The intensity in a SAR image depends on the amount of microwave reflected by the target and received by the SAR antenna.
  • Remote sensing satellites are of various uses some are:
  • Measuring forest cover and tree cover, assessing the health of the forest cover
  • Measuring crop acreage and farm productivity
  • Wasteland mapping
  • Used in watershed development programme
  • Used in drought prone area programme
  • Command area development programme
  • Ground water management
  • Coastal zone management
  • Mangroves management
  • Mineral exploration
  • Water, sea, land and air pollution
  • Continental shelf management
  • Tsunami early warning
  • Urban planning
  • Encroachment, illegal construction
  • Sustainable development is fragile ecosystems

C) Placement of remote sensing satellites:

  • These satellites are place in polar orbits, which have a constant angle with respect to the sun i.e.polar sun synchronous orbit. So it is easy to compare two photographs. As if the angle is different than it won’t be easy to compare two photographs as the illumination would be different.

D) Indian remote sensing satellites:


  • The initial versions are composed of the 1 (A,B,C,D). The later versions are named based on their area of application including Ocean Sat, Carto Sat, Resource Sat. Some of the satellites have alternate designations based on the launch number and vehicle (P series for PSLV).
Serial No. Satellite Date of Launch Launch Vehicle Status
1 IRS-1A 17 March 1988 Vostok, USSR Mission Completed
2 IRS-1B 29 August 1991 Vostok, USSR Mission Completed
3 IRS 1D 29 September 1997 PSLV-C1 Mission Completed
4 IRS-P4 (Oceansat-1) 27 May 1999 PSLV-C2 Mission Completed
5 Technology Experiment Satellite (TES) 22 October 2001 PSLV-C3 Mission Completed
6 IRS P6 (Resourcesat-1) 17 October 2003 PSLV-C5 In Service
7 IRS P5 (Cartosat 1) 5 May 2005 PSLV-C6 In Service
8 IRS P7 (Cartosat 2) 10 January 2007 PSLV-C7 In Service
9 Cartosat 2A 28 April 2008 PSLV-C9 In Service
10 RISAT-2 20 April 2009 PSLV-C12 In Service
11 Oceansat-2 23 September 2009 PSLV-C14 In Service
12 Cartosat-2B 12 July 2010 PSLV-C15 In Service
13 Megha-Tropiques 12 October 2011 PSLV-C18 In Service
14 RISAT-1 26 April 2012 PSLV-C19 In Service
15 SARAL 25 Feb 2013 PSLV-C20 In Service
16 Cartosat-2C 22 June 2016 PSLV-C34 In Service
17 Cartosat-2D 15 Feb 2017 PSLV-C37 In Service
18 Cartosat-2E 23 June 2017 PSLV-C38 In Service
19 Cartosat-2F 12 Jan 2018 PSLV-C40 In Service
20 RISAT-2B 22 May 2019 PSLV-C46 In Service
21 Cartosat-3 27 Nov 2019 PSLV-C47 In Service


Oceansat-1; 1999

  • It was an ocean resource management sat for finding potential fishing zones for fishermen, coastal region and continental shelf management. Payload was known as OCM (ocean colour monitoring) camera. In 2009 ocean sat 2 was launched to replace oceansat1.

Carto sat 1, 2….3

  • They are used for map making purposes, as a payload they have a pan-chromatic camera which provides high resolution pictures. These pictures are used for urban planning, encroachment control etc

Technology experiment satellite (TES)

  • Launched in 2001 it was India’s first spy satellite


  • Launched after Mumbai attacks, these are official spy satellites to keep track of oceans around India, these are radar imaging type, they send microwave signals on earth when they hit, they are reflected back thus working as a radar. They have cloud penetration capabilities and can-do imaging without sunlight because of microwave remote sensing capabilities.


  • Satellite with ARgos and ALtiKa is a cooperative altimetrytechnology mission ofISROand CNES (Space Agency of France). SARAL performs altimetric measurements designed to study ocean circulation and sea surface elevation.


Megha-Tropiques is a satellite mission to study the water cycle in the tropical atmosphere in the context of climate change.A collaborative effort between India and France. Megha-Tropiques was successfully deployed into orbit by a PSLV rocket in October

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