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Orbits - Where Do You Want it?

Why do satellites need special orbits?

“A place for everything and everything in its place.” That probably sums up satellite orbit planning. Some satellites only need to be above the atmosphere, so as to take advantage of the space environment. Most however have a job to do that either is related to the Earth or is affected by it and so different orbits are needed by different satellites depending on their purpose.

Equatorial LEO

The vast majority of satellites that are launched are placed into Equatorial “Low Earth Orbits” (LEO). This means that they are normally at an altitude of between 400 and 1000 km and orbit the Earth close to, though not necessarily directly over, the Equator. This is a general purpose orbit. Basically, unless there is a good reason to have a different orbit the satellite is put here.

All equatorial LEOs are not the same though. The International Space Station (ISS) at an altitude of 400 km is tilted quite a lot from the equator so that it can be reached from launches from both the Kennedy Space Centre in Florida and Star City in Russia. The Hubble Space Telescope (HST) is at an altitude of 600 km and at a far lower tilt to the equator and so shuttle missions to the HST do not go on to dock with the ISS.

Polar LEO

Most Earth Observation satellites, such as weather satellites and other “remote sensing” missions are placed in polar orbits. If they were placed in equatorial orbits then they could never observe the higher latitudes such as Western Europe, Russia and much of The United States of America. Also by being in a polar orbit the Earth rotates underneath the satellite once a day. This means that a single satellite can scan the surface of the entire Earth. Most civil remote sensing satellites orbit somewhere between 600 and 900 km above the Earth. Military spy satellites though will normally orbit much lower in order to get clearer pictures. The penalty for this is that the lower the orbit the greater the drag from the atmosphere is and so very low orbit satellites will re-enter the atmosphere and burn up after a relatively short time.

GPS Orbits

The orbits of the GPS constellation of satellites (a constellation is group of satelites that work together) need to be set so that anyone on the Earth can ‘see’ at least four satellites at the same time. This allows users of GPS receivers to determine their latitude, longitude, altitude and speed with great accuracy whenever they need to. The solution is to have 24 satellites in six different orbits. Each orbit is inclined at 55° to the equator and at an altitude of 20,200 km giving them an orbital period of twelve hours. The European ‘Galileo’ constellation will use a similar set of orbits.

Geostationary Orbits

If your satellite TV transmitter were place in LEO then it would move across the sky. This would mean that your dish would have to track the satellites and every time it set below the horizon your dish would have to look for another one. The result would not make for relaxing viewing. Just as England were ready to score the deciding goal you would lose the signal. Imagine it!
Back in the 1949 however the science fiction author, Arthur C. Clarke, ‘invented’ the idea of the geostationary orbit. If a satellite were to be placed a little less than 36,000 km above the Earth it would take 24 hours to complete one orbit. If that orbit were directly over the Equator then the satellite would seem to hang in the sky, unmoving, never setting. A ring of three such satellites, equally spaced about the planet, could cover almost the entire surface of the Earth (apart from the extreme polar regions). Also, as the satellites would be able to see each other they could form a huge communications network linking the world's population. This vision is now a reality. The diagram shows three satellites that between them cover the entire equator (and everything up to 71.2° north and south). If, for example, two places on opposite sides of the globe need to talk, the first site contacts satellite one, which relays the message to satellite two that sends it down to the final site. Satellite three is not needed for this call. The only problem with making a telephone call over a geostationary link is that there can be long…   …delays of up to half a second. This doesn’t matter though if the link is for data.

Odd Orbits

There are some satellites that don’t match these simple orbits and these tend to be very special purpose ones. One example is the communications satellites that are needed for polar regions. These have very stretched out orbits that (for North Pole communications) come in close to the Earth at the South Pole. Whilst there they move quickly across the sky but at the North Pole (where they are at a much higher altitude) they move slowly across the sky. As these satellites do rise and set more than one is needed in the same orbit, equally spaced out so that at least one is in the sky at all times.

Class Task

Using the Satellite Orbits Scale Model spreadsheet try to work out the exact height of a geostationary satellite. Once you have done this decide on a scale for your model (a 30 cm diameter Earth works well. Work out where various Earth satellites are (for example the International Space Station, the Hubble Space Telescope, GPS satellites, geostationary communications satellites and the Moon). Now, as a class, build a scale model of the Earth satellite system. How quickly do the various satellites move?



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