Satellite Basics: Guide To Satellite-based Solutions

What is a communications satellite and how does it work?

A communications satellite is a radio relay station in orbit above the earth that receives, amplifies, and redirects analog and digital signals carried on a specific radio frequency.

In addition to communications satellites, there are other types of satellites:

• Weather satellites: These satellites provide meteorologists with scientific data to predict weather conditions and are equipped with advanced instruments
• Earth observation satellites: These satellites allow scientists to gather valuable data about the earth's ecosystem
• Navigation satellites: Using GPS technology these satellites are able to provide a person's exact location on Earth to within a few meters

The different kinds of orbits:

An orbit is the path that a satellite follows as it revolves around Earth. In terms of commercial satellites, there are three main categories of orbits:

Geosynchronous Orbit (GEO): 35,786 km above the earth

• Orbiting at the height of 22,282 miles above the equator (35,786 km), the satellite travels in the same direction and at the same speed as the Earth's rotation on its axis, taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite is positioned over the equator in an assigned orbital location, it will appear to be "stationary" with respect to a specific location on the Earth.
• A single geostationary satellite can view approximately one third of the Earth's surface. If three satellites are placed at the proper longitude, the height of this orbit allows almost all of the Earth's surface to be covered by the satellites.

Medium Earth Orbit (MEO): 8,000-20,000 km above the earth

• These orbits are primarily reserved for communications satellites that cover the North and South Pole
• Unlike the circular orbit of the geostationary satellites, MEO's are placed in an elliptical (oval-shaped) orbit

Low Earth Orbit (LEO): 500-2,000 km above the earth

• These orbits are much closer to the Earth, requiring satellites to travel at a very high speed in order to avoid being pulled out of orbit by Earth's gravity
• At LEO, a satellite can circle the Earth in approximately one and a half hours

GEO vs. MEO vs. LEO

Most communications satellites in use today for commercial purposes are placed in the geostationary orbit, because of the following advantages:

• One satellite can cover almost 1/3 of Earth's surface, offering a reach far more extensive than what any terrestrial network can achieve
• Communications require the use of fixed antennas. Since geosynchronous satellites remain stationary over the same orbital location, users can point their satellite dishes in the right direction, without costly tracking activities, making communications reliable and secure
• GEO satellites are proven, reliable and secure - with a lifespan of 10-15 years

For a more comprehensive understanding of satellite advantages, see benefits of satellite.

Satellite architecture:

Communications data passes through a satellite using a signal path known as a transponder. Typically satellites have between 24 and 72 transponders. A single transponder is capable of handling up to 155 million bits of information per second. With this immense capacity, today's communication satellites are an ideal medium for transmitting and receiving almost any kind of content - from simple voice or data to the most complex and bandwidth-intensive video, audio and Internet content.

Orbital location and footprint

The location of a geostationary satellite is referred to as its orbital location. International satellites are normally measured in terms of longitudinal degrees East (° E) from the Prime Meridian of 0°.

The geographic area of the Earth's surface over which a satellite can transmit to, or receive from, is called the satellite's "footprint." The footprint can be tailored to include beams with different frequencies and power levels.

Frequency bands and beams

Satellites transmit information within radio frequency bands. The frequency bands most used by satellite communications companies are called C-band and the higher Ku-band. Over the next several years, the use of a higher frequency band known as Ka-band is expected to increase. Modern satellites are designed to focus on different ranges of frequency bands and different power levels at particular geographic areas. These focus areas are called beams. Intelsat offers four beam types:

• Global: covering almost 1/3 of Earth's surface
• Hemi: covering almost 1/6 of Earth's surface
• Zone: covering a large landmass area
• Spot: covering a specific geographic area

 What is installed on the ground

All communications with a geostationary satellite require using an earth station or antenna. Earth Stations may be either fixed (installed at a specific location) or mobile for uses such as Satellite News Gathering (SNG) or maritime applications. Antennas range in size, from large telecommunications carrier dishes of 4.5 to 15 meters in diameter, to VSAT antennas which can be as small as under one meter, designed to support services such as Direct to Home TV (DTH) and rural telephony.

The antenna, itself, will generally be connected to equipment indoors called an indoor unit (IDU), which then connects either to the actual communications devices being used, to a Local Area Network (LAN), or to additional terrestrial network infrastructure.

Network topologies

Depending on the application, satellites can be used with different ground network designs or network topologies. At its simplest, satellite can support one-direction or two-direction links between two earth stations (called respectively simplex transmission and duplex transmission). More complex communications needs can also be addressed with more sophisticated network topologies, such as star and mesh.

The following examples show some of the options available to customers for configuring their satellite networks:

Simplex Transmission

Applications for simplex services include broadcast transmissions such as:

• TV and video services
• Radio services

Point-to-Point Duplex Transmission

Applications for duplex services include:

• Voice Telephony transport
• Data and IP transport (especially in asymmetric configurations)
• Corporate networks
• TV and Broadcast program contribution and distribution

Point-to-Multipoint Transmission

(May be simplex or duplex, symmetric or asymmetric).

Applications for point-to-multipoint services include:

• Corporate networks, including VSAT services and business television
• Video and broadcast distribution, including Direct-to-Home Internet services

Mobile Antenna Service

Applications for mobile antenna services include:

• Satellite News Gathering
• Special Event Backhaul and Broadcasting
• Maritime services

Star Network

Applications for Star Networks include:

• Corporate Networks
• Distance Learning

Mesh Network

Applications for Mesh Networks include:

• National and International Telephony and Data networks
• Rural Telephony

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MONOSTABLE MULTIVIBRATOR USING 555 TIMER IC

INTRODUCTION: 

A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network,connected externally to the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capaci­tor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accomplished by external triggering. The schematic of a 555 timer in monostable mode of operation is shown in figure.

Monostable Multivibrator Circuit details:

Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of output this input is kept at + VCC. To obtain transition of output from stable state to quasi-stable state, a negative-going pulse of narrow width (a width smaller than expected pulse width of output waveform)  and  amplitude of greater than + 2/3 VCC is applied to pin 2. Output is taken from pin 3. Pin 4 is usually connected to + VCC to avoid accidental reset. Pin 5 is grounded through a 0.01 u F capacitor to avoid noise problem. Pin 6 (threshold) is shorted to pin 7. A resistor RA is connected between pins 6 and 8. At pins 7 a discharge capacitor is connected while pin 8 is connected to supply VCC.

 

555 monostable-multivibrator-operation

For explaining the operation of timer 555 as a monostable multivibrator, necessary internal circuitry with external connections are shown in figure.
The operation of the circuit is explained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable to quasi-stable state, as shown in figure. As the discharge transistor is cutoff, the capacitor C begins charging toward +VCC through resistance RA with a time constant equal to RAC. When the increasing capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1 goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the capacitor C and the output of the timer goes low, as illustrated in figure.


Thus the output returns back to stable state from quasi-stable state.

The output of the Monostable Multivibrator remains low until a trigger pulse is again applied. Then the cycle repeats. Trigger input, output voltage and capacitor voltage waveforms are shown in figure above.

Monostable Multivibrator Design Using 555 timer IC:

The capacitor C has to charge through resistance RA. The larger the time constant RAC, the longer it takes for the capacitor voltage to reach +2/3VCC.

In other words, the RC time constant controls the width of the output pulse. The time during which the timer output remains high is given as

tp = 1.0986 RAC
where RA is in ohms and C is in farads. The above relation is derived as below. Voltage across the capacitor at any instant during charging period is given as

vc = VCC (1- e-t/RAC)

Substituting vc = 2/3 VCC in above equation we get the time taken by the capacitor to charge from 0 to +2/3VCC.

So +2/3VCC. = VCC. (1 – e-t/RAC)   or   t – RAC loge 3 = 1.0986 RAC

So pulse width, tP = 1.0986 RAC s 1.1 RAC

The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is widely used in industry for many different timing applications.   

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