satellite orbit

8/12/2019 Satellite Orbit

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1.3 Satellite Orbit Configurations 21

Figure 1.18 Illustration of Keplers laws of planetary motion.

3. The square of the period of a satellite is proportional to the cube of itsmean distance from the center of the Earth. That relationship is plotted in

Figure 1.19 to give an idea of the relationship between altitude and orbital

period.

Figure 1.20 provides three classical Earth-orbit schemes and their basic proper-

ties. LEO systems employ satellites at altitudes ranging from 500 to 1,000 km.Over that range, the orbit period is between 1.6 and 1.8 hours, the higher orbit

resulting in a slightly longer period of revolution. The reason for that small changein period is that it is the distance from the center of the Earth that determines the

period, not the elevation above sea level. The altitude of a medium Earth orbit

(MEO) is around 10,000 km (a period of about 6 hours). Between 2,000 and 8,000km, there is an inhospitable environment for electronic components produced by

the Van Allen radiation belt.The principal advantage of LEO satellites is the shorter range that the radio

signal has to traverse, requiring less power and minimizing propagation delay. The

range of delay for a single up-down hop is shown at the top of the right-handgraph in Figure 1.19. Their short orbital period produces relatively brief durations

when a given satellite can serve a particular user. For altitudes in the range of

8,000 to 10,000 km, a MEO satellite has a much longer period and thus tends to

Figure 1.19 Orbit period (in hours) versus altitude, based on Keplers third law.


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22 Fundamentals of Satellite Systems

Figure 1.20 The three most popular orbits for communication satellites are LEO, MEO, and GEO.The respective altitude ranges are 500 km to 1,000 km for LEO; 5,000 km to 12,000km for MEO; and 36,000 for GEO. Only one orbit per altitude is illustrated, eventhough there is a requirement for multiple orbits for LEO and MEO satellites to providecontinuous service.

hang over a given region on the Earth for a few hours. Transmission distanceand propagation delay are greater than for LEO but still significantly less than for

GEO. In the case of the latter, there are really two classes of orbits that have a

24-hour period. A geosynchronous orbit could be elliptical or inclined with respectto the equator (or both). The special case of an equatorial 24-hour circular orbit,

in which the satellite appears to remain over a point on the ground (which is on

the equator at the same longitude where the satellite is maintained), is called GEO.A 24-hour circular geosynchronous orbit that is inclined with respect to the equator

is not GEO because the satellite appears to move relative to the fixed point on the

Earth. A GEO satellite would not require ground antennas that track the satellite,while an inclined geosynchronous orbit satellite might. Other non-GEO orbits have

been used at various times, such as the highly elliptical Earth orbit (HEO) to allowcoverage of northern latitudes.

1.4 Frequency Spectrum Allocations

Satellite communications employ electromagnetic waves to carry informationbetween ground and space. The frequency of an electromagnetic wave is the rate

of reversal of its polarity (not polarization) in cycles per second, defined to be unitsof hertz (Hz). Alternating current in a copper wire also has that frequency property;

if the frequency is sufficiently high, the wire becomes an antenna, radiating electro-

magnetic energy at the same frequency. Recall that wavelength is inversely propor-tional to frequency, with the proportionality constant being the speed of light

(approximately 300 million meters per second in a vacuum).

A particular range of frequencies is called a frequency band, while the fullextent of all frequencies for zero to infinity is called the electromagnetic spectrum.

In particular, the radio frequency (RF) part of the electromagnetic spectrum permitsthe efficient generation of signal power, its radiation into free space, and reception

at a distant point. The most useful RF frequencies lie in the 300-MHz and300,000-MHz range, although lower frequencies (longer wavelengths) are attractivefor certain applications. While frequencies are all expressed either in hertz or meters

satellite orbit

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