Low Earth Orbit and Non-Geostationary Satellite systems

Presentation on theme: "Low Earth Orbit and Non-Geostationary Satellite systems"— Presentation transcript:

1 Low Earth Orbit and Non-Geostationary Satellite systems

2 Introduction DTH is one of major application of GEO orbit
Non geostationary satellites used for- Surveillance of earth surface Military data gathering Earth resources data gathering GPS LEO satellites are small,simple and low cost.Multibeam antennas can be used with them. Exa. Iridium system uses 66 LEO satellites,to provide continuous global coverage.

3 Orbit Considerations There are different NGSO (Non-GeoStationary Orbit) orbits that are used for Scientific,military and commercial satellite missions. 1)Equatorial Orbits 2)Inclined Orbits 3)Elliptical orbits 4)Molniya orbits 5)Sun Synchronous Orbits

4 1)Equatorial Orbits Two methods to launch satellite in this orbit:
Prograde Retrograde Equatorial orbits lie exactly in the plane of the geographical equator of the earth i.e. above the equator all the time Eastward directed orbit is called as Prograde orbit . Westerly directed orbit is called as Retrograde orbit. P= (24T) / (24-T) where,P=The apparent orbital period to the observer on the euator T=Real orbital period Two methods to launch satellite in this orbit: 1)To locate the launch site on the equator and to launch the spacecraft toward the east along the equatorial plane. 2)To launch the satellite into an inclined orbit and to execute a maneuver either during the launch trajectory or when the satellite is in an inclined orbit that changes the plane of the initial orbit so that the final orbit is in the plane of the equator

5 2)Inclined Orbits In order to extend the life of a satellite some satellite operators decide to put the satellite in an inclined orbit. The greater the inclination of the orbit is, the larger the surface area of the earth that the satellite will pass over at some time in its flight

6 3)Elliptical orbits An elliptical orbit, also called an eccentric orbit, is in the shape of an ellipse. In an elliptical orbit, the satellite's velocity changes depending on where it is in its orbital path. When the satellite is in the part of its orbit closest to the Earth, it moves faster because the Earth's gravitational pull is stronger. The satellite is moving the fastest at the low point of an elliptical orbit. The low point of the orbit is called the perigee. The high point of the orbit, when the satellite is moving the slowest, is called the apogee. An elliptical orbit can be useful to a communications satellite because it allows the satellite to travel over a specific region for a long portion of its orbit, and it is only out of contact with that region for a short time when it is zipping quickly around the other side of the Earth. 𝒆 𝟐 =𝟏− 𝒃 𝟐 𝒂 𝟐 Where, e is orbit eccentricity, a is semimajor axis and b is semiminor axis

7 4)Molniya orbits Molniya means lightning in Russian
Used as communications satellites Highly elliptical orbit Apogee = 39,152 km Perigee = 500 km Orbital Period = 11 h 38 min Orbital Inclination = 62.9˚ Signals from the geostationary satellite cant reach to extreme north and Russia has 11 time zone so to achieve 24 hour coverage molniya satellite used.they are separated by 180 degree.

8 5)Sun Synchronous Orbits
This orbit is a special case of the polar orbit.In a sun-synchronous orbit, though, the satellite passes over the same part of the Earth at roughly the same local time each day. There is a special kind of sun-synchronous orbit called a dawn-to-dusk orbit. In a dawn-to-dusk orbit, the satellite trails the Earth's shadow. When the sun shines on one side of the Earth, it casts a shadow on the opposite side of the Earth. (This shadow is night-time.) Because the satellite never moves into this shadow, the sun's light is always on it (sort of like perpetual daytime). Since the satellite is close to the shadow, the part of the Earth the satellite is directly above is always at sunset or sunrise. That is why this kind of orbit is called a dawn-dusk orbit. This allows the satellite to always have its solar panels in the sun.

9 Coverage & Frequency Consideration
Θ angle tell the coverage area on the surface of the earth d distance determines the free space path loss along the propagation path and a key factor in link budget design Φ elevation angle will influence the G/T ratio of the antenna, the blockage probability from terrain and buildings near the antenna FREQUENCY BAND Depending on the selected orbit, and also type of services, frequency band is selected. Ex. Lowest RF is selected for LEO for data and voice services. Also C/N of mobile receivers is proportional to square of the RF frequency of downlink. The power that must be transmitted by mobile transmitter is also proportional to RF frequency squared, when the mobile uses an omni-directional antenna. By choosing lower frequency EIRP can be kept low, thus cost can be kept low.

10 Delay and Throughput Consideration
Delay in communication is dependent on distance of satellite from earth • Delay for GEO link is a half second • Delay for MEO link is a 79 ms • Delay in LEO link is 5.4 ms • Delay can cause adverse effect on throughput

11 Throughput Considerations
• Delay must be minimized • Proper communication protocol must be used to minimize loss of throughput • If ISL is to be used then proper matching across seam of links must be considered.

12 Iridium The Iridium satellite constellation is a system of 66 active communication satellites and spares around the Earth. It allows worldwide voice and data communications using handheld devices. The Iridium network is unique in that it covers the whole earth, including poles, oceans and airways. The satellites used are frequently visible in the night sky as short-lived bright flashes, known as Iridium flares. The Iridium satellites orbit at an altitude of approximately 483 miles above the earth (and travel at approximately 16,689 mph resulting in a complete orbit of the earth approximately every 100 minutes).

13 Named IRIDIUM after element Iridium with atomic number 77 and the satellites evoking the Bohr’s Model of electrons orbiting around the Earth as its nucleus. The constellation of 66 active satellites has 6 orbital planes spaced 30 degrees apart, with 11 satellites in each plane. Iridium system is a satellite-based, wireless personal communications network providing voice and data features all over the globe. It is comprised of three principal components the satellite network the ground network Iridium subscriber products, including phones and data modems Iridium network allows voice and data messages to be routed anywhere in the world. Voice and data calls are relayed from one satellite to another until they reach the satellite above the Iridium handset or terminal and the signal is relayed back to Earth. When an Iridium customer places a call from a handset or terminal, it connects to satellite overhead, and the signal is relayed among satellites around the globe to whatever satellite is above the appropriate Earth gateway, which downlinks the call and transfers it to the global public voice network or Internet so that it reaches the recipient.

14 Teledesic Internet in the sky. Teledesic officially suspended its satellite construction work on October 1, 2002. Teledesic has 288 satellites in 12 LEO orbits, each at an altitude of 1350 km. It has been designed for Internet-like data traffic rather than voice communication Any user can access any other user or ISP(Internet Service provider) independent of the location and the exixting telecommunications infrastructure. The concept of Teledesic is to provide a complete worldwide data communications system above the surface of the earth using satellites,instead of on the earth’s surface using fiber-optic cables.

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16 References 1)Satellite Communications by Timothy Pratt,Charles Bostian and Jeremy Allnutt 2) 3)

17 Thank You