Table of Contents 2
Introduction 3
Radio Frequency 4
C-band
Ku-band
Polarization 6
6
6
Earth Station
Antenna
The Uplink
The Downlink
Satellites 10
Bus
Payload
10
10
Transponders 12
Footprint 13
Orbit
Power
14
15
8
8
8
9
Carriers 16
Link Budgets 17
Conclusion 18
Recommended Reading 19
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In 1945, Arthur C. Clarke provided what most consider the initial principles for
Satellite Communications. In a technical paper published in a United Kingdom periodical magazine, he stated that a space-station orbiting 42,000 km above the equator could act as a repeater to relay transmissions between any two points on the hemisphere beneath it. It was not until the early 1960s that the first workable communications satellite was built and launched.
PanAmSat, the first commercial satellite operator in the world, was established in 1984 by the late Rene Anselmo. Fed up with the high cost and poor service by the government-run providers, Rene Anselmo broke the government monopoly on satellite communications and launched PAS-1. By 1992, PAS-1 was effectively sold out and PanAmSat was on its way to become one of the largest satellite providers in the world with a global satellite fleet.
Arthur C. Clarke and Rene Anselmo were bold men who saw growth and potential where many did not. This document was created to provide the basic concepts of satellite communications and how we use it today. The document will give you a better understanding of Radio Frequencies, the earth station components (antennas and amplifiers), PanAmSat satellite components, the two basic carrier types, and link budgets.
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Satellite communications utilizes electromagnetic waves to carry information from the ground to space and back. An electromagnetic wave consists of an electric field and a magnetic field that are perpendicular to each other and to the direction of propagation (figure 1).
Figure 1: Electromagnetic wave
The frequency of an electromagnetic wave is defined as the number of times it cycles in one second and is measured in Hertz (Hz) .
1 Hertz = 1Hz (one hertz)
1,000 Hertz = 1kHz (one kilohertz)
1,000,000 Hertz = 1MHz (one megahertz)
1,000,000,000 Hertz = 1GHz (one gigahertz)
The distance between two similar points on a given wave determines the wavelength of an electromagnetic wave. It is proportional to its frequency and is measured in meters. Every electromagnetic wave exhibits a unique frequency and wavelength (figure 2).
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R A D I O F R E Q U E N C Y ( R F )
Figure 2: Frequency and wavelength
A particular range of frequencies is called a frequency band and the full range of frequencies from zero to infinity is called the electromagnetic spectrum. The radio frequency (RF) segment of the electromagnetic spectrum is the range from
3kHz up to 300GHz and is used by several communications devices including satellites.
Figure 3: radio frequency spectrum
Radio Frequency bands are allocated for various purposes by the International
Telecommunication Union Radiocommunication sector (ITU-R), an agency within the United Nations (UN). The Federal Communications Commission
(FCC) is a member of the ITU-R along with other similar agencies representing their respective government. Their goal is to manage the finite resources of the
RF spectrum and satellite orbital positions. In doing so they have allocated sub
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R A D I O F R E Q U E N C Y ( R F ) bands of the RF spectrum for use in Satellite Communications. For the purpose of this document we will only focus on two of these sub bands: “C” and “Ku”.
They are the most commonly used in commercial satellite communications.
The frequency range allocated for C-band is 3.7GHz – 6.425GHz. It is further divided into separate halves, one for ground-to-space links (Uplink) and one for space-to-ground links (downlink) as shown:
C-band Uplink Frequencies: 5.925GHz – 6.425GHz
C-Band Downlink Frequencies: 3.7GHz – 4.2GHz
The frequency range allocated for Ku-band is 11.7GHz – 14.5GHz. Ku-band, like C-band, is further divided into separate halves, one for ground-to-space links (Uplink) and one for space-to-ground links (downlink) as shown:
Ku-Band Uplink Frequencies: 14GHz – 14.5GHz
Ku-Band Downlink Frequencies: 11.7GHz – 12.2GHz
Although the international satellite communication frequency bands are similar to the U.S. frequency bands, there are some variances; specifically the use of extended C and extended Ku bands that are in use by PanAmSat satellites as well as others.
Polarization is another property of electromagnetic waves. It can be manipulated into two types of polarization: Linear (Vertical and Horizontal) and Circular
(Right-Hand and Left-Hand) polarizations. Linear polarization is commonly
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R A D I O F R E Q U E N C Y ( R F ) used on PanAmSat satellites. Figure 3 shows the orientation that the electric field of an electromagnetic wave would take depending on the capabilities and orientation of an antenna.
Vertical
Right-hand circular
Horizontal
Figure 4 Polarizations
The most important application of polarization is in frequency reuse. This is where two electromagnetic waves, one traveling on the vertical plane and the other in the horizontal plane, are using the same frequency without impacting one another. This gives the ability to essentially double the amount of frequencies available for use.
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Earth Station is the internationally accepted term that includes satellite communications stations located on the ground. They can be configured and utilized in a number of ways but in order for an earth station to transmit or receive a signal it will require uplink and/or downlink equipment.
The antenna provides both the means to transmit the RF signal to the satellite and receive a signal from the satellite. Its design helps minimize Radio
Frequency interference (RFI) by using its reflectors to focus the RF signal on to a single satellite. Its feed, or feed horn, is used to isolate a single polarization for reception or transmission. In order to isolate a single polarization, the antenna and feed must be properly aligned with the satellite’s antenna. For example, a vertically polarized antenna will receive the signal transmitted on the vertical polarization by the satellite. In the case where the antenna is rotated 90 degrees and is oriented horizontally, versus vertically, there will be very little to no reception of the signal. This misalignment of the antenna in respect to the polarization is called “Cross-polarization”.
Properly aligning your antenna to the appropriate satellite and polarization is crucial to the completion of a satellite link. PanAmSat requires an antenna pattern test, or the antenna manufacture’s pattern test, to ensure the antenna’s reflectors will focus the RF energy appropriately and will not interfere with other satellite signals during a transmission. PanAmSat has also required that all transmitting earth stations contact the PanAmSat Network Operations Center
(NOC) prior to transmitting to a PanAmSat satellite to ensure the antenna will be aligned to the appropriate polarization and satellite before transmitting.
The other major components of a typical earth station uplink are the modem, upconverter, and high-powered amplifier. The following is a brief description of each.
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E A R T H S T A T I O N
Modem: Modulates a baseband signal to an Intermediate Frequency (IF).
Usually 70MHz or 140MHz.
Upconverter: Converts IF to RF.
High Power Amplifier (HPA): Increases the power of the RF signal to achieve satisfactory uplink operations.
On the downlink side you would typically have a low noise amplifier, downconverter, and modem. A low noise block downconverter can be used in place of a low noise amplifier and downconverter. The following is a brief description of each.
Low Noise Amplifier (LNA): Amplifies the RF signal received from the satellite.
Downconverter: Converts the RF into IF that is then sent to the modem.
Low Noise Block Downconverter (LNB): Amplifies and converts the RF signal from the satellite into IF. Essentially it is a Low Noise Amplifier (LNA) and downconverter that have been incorporated into a single unit
Modem: Demodulates the IF signal and extracts the data for use.
Keep in mind that depending on several factors, earth stations may use additional equipment that is not listed here.
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The satellite is a very complex communication device and continues to grow in complexity. The following section will cover the very basic components of a satellite and the basic operations of each. It should be noted that any figures used are not a detailed representation but a basic diagram to complement the text within this document.
Every manufactured satellite consists of many different parts, including some that are very specific to its function; the following two components are common to all satellites:
•
Bus
•
Payload
The bus is the platform that supports the payload from launch through the end of its life. The bus is made up of the frame and the bus subsystems which include attitude control, power system, orbital control, thermal control and the TT&C
(Tracking, Telemetry and Command) system.
The payload of a satellite is all the specialized equipment needed to perform its designed function.
A communications payload, like the ones installed on PanAmSat satellites, act like a communications repeater. RF signals to the satellite are received, converted, amplified, and transmitted back to Earth.
The payload includes the antenna, wide-band receivers, input and output multiplexers, programmable attenuation devices, and amplifiers. Satellites designed with a single payload are only able to operate with a single band of frequencies, either C or Ku. Satellites with dual payloads, also known as Hybrid
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S A T E L L I T E S
Satellites, are able to operate with both C and Ku bands (1 band per payload).
Each payload has a set of components that operates with a specific band of frequencies. What follows is an overview of each component and how it affects the RF signal.
Figure 5:Major Components of a typical satellite payload
Receive/Transmit Antenna.
Satellites very often use the same antenna to receive and transmit RF signals. This idea is practical because the satellite receives the uplink signal at a higher frequency and generally sends it back out on the opposite polarization at a lower frequency. When receiving a signal, the antenna routes the 500MHz RF to the appropriate wideband receiver determined by the band and polarization. When transmitting, the satellites antenna feed horn determines the polarization of the signal and directs it onto the antenna to be reflected back to earth.
Wideband Receiver: Receives the full 500MHz RF uplink signal of its assigned band and polarization. The wideband receiver uses a local oscillator
(also known as a frequency downconverter) to convert the signal to a downlink frequency. The output signal is then sent to the input multiplexer (IMUX).
Typically there is a wide-band receiver for each polarization (horizontal and vertical) on each payload (C and Ku).
Input Multiplexer: Takes the 500MHz set of frequencies and separates them into individual channels (also known as transponders). These individual
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S A T E L L I T E S transponder signals are then sent through a programmable attenuation device
(PAD) on its way to its assigned amplifier.
Programmable Attenuation Device (PAD): Adjusts the power of the signal prior to the amplifier. It is used to lower the amount of “noise” being amplified by the spacecraft’s amplifier. Amplifiers with higher attenuation require a stronger signal from the earth station in order to achieve the satellite link.
Amplifiers: Increases the power of each signal sent to the satellite and routed to an output multiplexer. PanAmSat satellites use solid-state-power amplifiers
(SSPA), traveling-wave-tube amplifiers (TWTA), or linear- traveling-wave-tube amplifiers (LTWTA). Each amplifier type has distinct advantages. For example, although the SSPA has a longer life expectancy, the TWTA has a simpler design and is more efficient at higher power levels.
Output Multiplexer: Recombines all transponders into a 500MHz wide-band configuration and is then routed through a wave-guide to the transmitting antenna’s feed horn.
As mentioned earlier, the satellite and all its components simply act as a repeater situated in space. Although many satellites contain only a single payload, some satellites carry dual payloads.
The word “Transponder” is an actual contraction of “transmitter-responder.” It is used to describe a single RF channel that is created at the input multiplexer when it takes the 500MHz set of frequencies and separates them into individual frequency channels. Each transponder is routed to an assigned PAD and amplifier, and then recombined at the output multiplexer.
PanAmSat, along with coordination with the FCC and other satellite providers, added an additional measure to ensure that interference between satellites is minimized. Coordinating each satellite’s transponder frequency plan and the orbital slot it will occupy helps reduce the amount of interference between satellites.
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S A T E L L I T E S
Typically U.S. C-band satellites in operation, whether owned by PanAmSat or any other satellite provider, are coordinated to have the same transponder format as shown in figure 6. Each transponder has 36MHz of usable bandwidth with
2MHz of guard band filter on each side (guard band is an additional measure to minimize interference between adjacent transponders), for a total of 40MHz from the center frequency of one transponder to the center frequency of the adjacent transponder. U.S. C-Band satellites have been coordinated to have 24 transponders for each payload, 12 transponders on each polarization.
H
3700
MHz
V
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4200
MHz
500 MHz
Figure 6: Transponder Layout for a domestic C-band payload
Unfortunately, Ku-band satellites do not follow any standard. On Ku-Band satellites, the number of transponders, assigned frequencies, and usable bandwidth for each transponder may vary.
The transmitting and receiving antennas on the satellite are designed to cover specific regions of the earth’s surface. This is done for several reasons. First, it concentrates the power radiated from the satellite into the desired region.
Second, it increases the sensitivity of its receiving antenna minimizing interference with other adjacent satellite signals. The part of the earth’s surface covered by a satellite is called the satellite’s footprint. The footprint may cover one or more relatively localized regions of the earth or nearly a complete hemisphere. Figure 7 is an example of a footprint for G11 Horizontal Ku-band.
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S A T E L L I L T E S
Figure 7: G11 Ku Horizontal footprint
Most communications satellites are in a geo-synchronous orbit. A satellite in geo-synchronous orbit must be positioned 22,300 miles above the equator. At this distance, it takes the satellite 24 hours to circle the Earth, which is the same amount of time it takes for the earth to rotate one time hence the Earth and satellite are in sync.
Achieving and maintaining a correct orbital attitude requires ongoing coordination between ground tracking and command functions and the satellite’s attitude control, telemetry, and orbital control subsystems.
The engineers at the PanAmSat Satellite Operations Control Center (OCC) are responsible for guiding satellites to their orbital slots after launch and for keeping the satellites within their orbital slot until the end of its life (usually about 12 to 14 years). As mentioned earlier, The ITU-R coordinates who will
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S A T E L L I T E S occupy each orbital slot. Each orbital slot corresponds to the longitudinal position directly above the earth’s equator.
Once the satellite is in orbit, and throughout the satellite’s life span, periodic adjustments must be made to keep the satellite within its assigned orbital location, also known as “center of box”. These adjustments, or maneuvers, are usually needed every two to three weeks. Some of the newer satellites calculate their own position and fire thrusters numerous times each day. They are considered to always be at “center of box”. Collectively, these adjustments are called station keeping.
Because satellites have a limited amount of fuel, every maneuver must be calculated precisely in terms of fuel consumption. A satellite’s life span depends upon its fuel supply and its ability to be maneuvered. Once the fuel is almost depleted, the satellite can no longer be maintained and must be taken out of orbit by burning off the remaining fuel to push the satellite to a higher (super-sync) orbit.
A communication satellite’s primary source of power is its solar array. Arrays of solar cells convert sunlight to electrical energy. Batteries are used as back up and during times of eclipse, which is when the satellite is in the earth’s shadow and it is unable to utilize the sun for energy.
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An RF signal centered on a specific frequency is called a carrier. The carrier may be a continuous wave (CW), also known as a clean carrier, or it may contain modulation. The two general forms of modulation are Analog and
Digital. The information, whether it is video, data or voice, is carried within the modulation of the carrier. Depending on the amount of information, the rate and type of modulation, and the quality desired would determine how much bandwidth the carrier will utilize or occupy.
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A link budget involves addition and subtraction of gains and losses within an
RF link. When these gains and losses of various components are determined and summed, the result is an estimation of end-to-end system performance in the real world. To arrive at an accurate answer, factors such as the uplink power amplifier gain and noise factors, transmit antenna gain, slant angles and corresponding atmospheric loss over distance, satellite transponder noise levels and power gains, receive antenna and amplifier gains and noise factors, cable losses, adjacent satellite interference levels, and climatic attenuation factors must be taken into account.
Fortunately with the help of PanAmSat’s Customer Support Engineers (CSE) and computer programs, a link budget can be calculated to help in the design of your RF link.
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All man made Satellites serve the same basic purpose: To communicate information. The type of information usually determines the satellite's design and orbit. Today, there are numerous satellites orbiting the earth at various altitudes used for a wide range of specialized functions.
Communications satellites are primarily used as repeaters in space. An earth station would transmit an RF carrier (video or data) to the satellite. The satellite would then receive and transmit the RF signal back down to earth onto a specific footprint where it may be received and demodulated by one or several earth stations.
Today’s satellites are complex devices that are continuing to grow in their complexity. This guide was intended to give you, our customer, a basic understanding of satellite communications. We hope that it has been helpful and look forward to continuing to provide you with excellent customer service.
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Elbert, Bruce R. Introduction to Satellite Communications Second Edition
Norwood, MA: Artech House Inc., 1999
Roddey, Dennis Satellite Communications Second Edition New York, NY:
McGraw-Hill, 1996
Maral, Bousquet, Satellite Communications Systems: Systems, Techniques and
Technology Hobokon, NJ: John Wiley & Sons Ltd, 2002
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