Orbits

Polar Orbits

The more correct term would be near polar orbits. These orbits have an inclination near 90 degrees.

This allows the satellite to see virtually every part of the Earth as the Earth rotates underneath it.

A satellite in a polar orbit will pass over the equator at a different longitude on each of its orbits

It takes approximately 90 minutes for the satellite to complete one orbit.

These satellites have many uses

As measuring ozone concentrations in the stratosphere or measuring temperatures in the atmosphere.
Earth Mapping and Observation
Reconnaissance
Weather satellites

Sun Synchronous Orbits

These orbits allow a satellite to pass over a section of the Earth at the same time of day. Since there are 365 days in a year and 360 degrees in a circle, it means that the satellite has to shift its orbit by approximately one degree per day.

These satellites orbit at an altitude between 700 to 800 km. These satellites use the fact since the Earth is not perfectly round (the Earth bulges in the center), the bulge near the equator will cause additional gravitational forces to act on the satellite. This causes the satellite's orbit to either proceed or recede.

These orbits are used for satellites that need a constant amount of sunlight. Satellites that take pictures of the Earth would work best with bright sunlight, while satellites that measure long wave radiation would work best in complete darkness.

These satellites are very important for military and remote sensing purposes.

Geosynchronous Orbits

Also known as geostationary orbits, satellites in these orbits circle the Earth at the same rate as the Earth spins. The Earth actually takes 23 hours, 56 minutes, and 4.09 seconds to make one full revolution.

Geosynchronous orbits allow the satellite to observe almost a full hemisphere of the Earth. These satellites are used to study large scale phenomenon such as hurricanes, or cyclones.

These orbits are also used for communication satellites. The disadvantage of this type of orbit is that since these satellites are very far away, they have poor resolution. The other disadvantage is that these satellites have trouble monitoring activities near the poles.

Difference between Geostationary and Geosynchronous Satellites

In geostationary, the orbit is circular and in equatorial plane thus the inclination is zero. So there is only one geostationary orbit. Also, the angular velocities of these satellites are equal to angular velocity of the earth and hence these appear to be stationary with respect to earth all the time.

On the other hand, in geosynchronous satellites, the orbit is neither circular nor equatorial and hence is inclined. It also has angular velocity equals to earths and period of revolution equals to rotation of earth as geostationary satellites. But there are many geosynchronous orbits unlike only one geostationary orbit.

Parameter	LEO	MEO	GEO
Satellite Height	500-1500 km	5000-12000 km	35800 km
Orbital Period	10-40 min	2-8 hrs	24 hrs
Number of Satellites	40-80	8-20	3
Satellite Life	Short	Long	Long
Number of Handoffs	High	Low	Least (none)
Gateway Cost	Very Expensive	Expensive	Cheap
Propagation Loss	Least	High	Highest

IRNSS

IRNSS is an independent regional navigation satellite system being developed by India.

It is designed to provide accurate position information service to users in India as well as the region extending up to 1500 km from its boundary, which is its primary service area.

The IRNSS System is expected to provide a position accuracy of better than 20 m in the primary service area.

Two types of services

Standard Positioning Service (SPS), which is provided to all the users and

Restricted Service (RS), which is an encrypted service provided only to the authorized users.

Components of IRNSS

IRNSS Space Segment consists of seven satellites, with three satellites in geostationary orbit and four satellites in inclined geosynchronous orbit.

IRNSS Ground Segment is responsible for navigation parameter generation and transmission, satellite control, ranging and integrity monitoring and time keeping .

Applications

Terrestrial, Aerial and Marine Navigation

Disaster Management

Vehicle tracking and fleet management

Integration with mobile phones

Precise Timing

Mapping and Geodetic data capture

Terrestrial navigation aid for hikers and travelers

Visual and voice navigation for drivers

Orbital Decay

In orbital mechanics, decay is a process that leads to gradual decrease of the distance between two orbiting bodies at their closest approach over many orbital periods.

These orbiting bodies can be a planet and its satellite, a star and any object orbiting it, or components of any binary system.

The orbital decay can be caused by a multitude of mechanical, gravitational, and electromagnetic effects. For bodies in a low Earth orbit, the most significant effect is the atmospheric drag.

If left unchecked, the decay eventually results in termination of the orbit where the smaller object strikes the surface of the primary; or for objects where the primary has an atmosphere, it burns, explodes, or otherwise breaks up in its atmosphere; or for objects where the primary is a star, ends with incineration by the star's radiation (such as for comets), and so on.

Causes of Orbital Decay includes Atmospheric drag, Tidal effects, Mass concentration, light and thermal radiation and gravitational radiation

Graveyard Orbit

A graveyard orbit, also called a junk orbit or disposal orbit, is a super synchronous orbit that lies significantly above synchronous orbit, where spacecraft are intentionally placed at the end of their operational life.

It is a measure performed in order to lower the probability of collisions with operational spacecraft and of the generation of additional space debris (known as Kessler syndrome).

A graveyard orbit is used when the change in velocity required to perform a de-orbit maneuver is too high.

For satellites in geostationary orbit and geosynchronous orbits, the graveyard orbit is a few hundred kilometers above the operational orbit.

The transfer to a graveyard orbit above geostationary orbit requires the same amount of fuel that a satellite needs for approximately three months of station keeping.

It also requires a reliable attitude control during the transfer maneuver.

While most satellite operators try to perform such a maneuver at the end of the operational life, only one-third succeed in doing so.

PSLV

The Polar Satellite Launch Vehicle, usually known as PSLV is the first operational launch vehicle of ISRO.

PSLV is capable of launching 1600 kg satellites in 620 km sun-synchronous polar orbit and 1050 kg satellite in geo-synchronous transfer orbit (GTO).

In the standard configuration, it measures 44.4 m tall, with a lift off weight of 295 tonnes.

PSLV has four stages using solid and liquid propulsion systems alternately.

The first stage is one of the largest solid propellant boosters in the world and carries 139 tonnes of propellant.

A cluster of six strap-ons attached to the first stage motor, four of which are ignited on the ground and two are air-lit.

The reliability rate of PSLV has been superb. With its variant configurations, PSLV has proved its multi-payload, multi-mission capability in a single launch and its geosynchronous launch capability.

PSLV has launched many foreign satellites such as SARAL-ARGOS and ALTIKA.

In the Chandrayaan-mission, another variant of PSLV with an extended version of strap-on motors, PSOM-XL, the payload haul was enhanced to 1750 kg in 620 km SSPO.

PSLV has rightfully earned the status of workhorse launch vehicle of ISRO.