Antenna gain dBi

The gain of a directional antenna is compared to an omni-directional (or isotropic) antenna....more>

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22,300 A Space Odyssey - The Geostationary Clarke Orbit | Some Satellite History | GEO - Geostationary Earth Orbit | Inclined Orbits | LEO: Low Earth Orbit Satellites | MEO: Medium Earth Orbit | Global beams, Hemi beams, Spot beams | Finding Satellite information on the internet | Antenna gain dBi | EIRP or E.I.R.P. | EIRP or dBW on a satellite footprint map. | G/T Satellite Uplink Map | Ka-Band Pointing Accuracy | ESV- Earth Station aboard Vessel | Comsat Mobile Communications | Inmarsat | MEO - Medium Earth Orbit | Satellite Latency | Low Earth Orbit LEO | Medium Earth Orbit MEO | Symbol Rate | SCPC - Single Carrier Per Channel | TDM -Time Division Multiplexing | TDMA - Time Division Multiple Access | Downlink |
In 1945 the renowned science fiction author of "2001 A Space Odyssey", Arthur C Clarke,  proposed that communications satellites placed in orbit at a height of 22300 miles above the earth, would remain in synchronous orbit with the earth, or parked in the same spot above the equator, allowing earth based stations to communicate around the world via satellite. This magic distance causes the satellite to orbit the earth at the same speed as the earths rotation. Satellites that are closer to the earth must orbit faster than the earth to maintain orbit, and satellites that are further from earth orbit  slower than the rotation of the earth.

While Arthur C Clarke may not have been responsible for the science behind the orbit, he was certainly influential with the use of this orbit over the past 65 years for satellite communications and TV distribution as we know it today.
On August 31st, 1962, President John F Kennedy signed into effect the Communications Satellite Act that began the competitive commercial use of satellites and establishing a communications system utilizing space satellites, and contributing to world peace and understanding.

In 1967 President Johnson reported that the Act has brought mankind to the threshold of a full-time global communications service to which all nations of the world may have equal access. Comsat had been joined by 17 other nations for the creation of INTELSAT, (International Telecommunications Satellite Consortium).
In order to order to overcome Earth's gravity, and remain stationary in the sky, synchronized with the rotation of the earth, geostationary satellites must maintain an orbit  of 22,300 miles (37,000Km) above the equator, spaced just 2 degrees apart, and  often even less. This results in the satellite orbiting the earth at exactly the same rate that the earth revolves, thus appearing to the observer to be stationary in the sky.





2 Degree Spacing:
Two degrees may seem like a small separation but they are actually about 700 miles apart. Each satellite is traveling at about 7000 miles per hour to complete a journey of more than 165 000 around the earth each day. The earth is rotating at the same rate, so the satellite appears to be stationary and we can point our antennas at the same spot in the sky.

Because of this vast distance, large, directional,dish reflector antennas are required to communicate with geostationary satellites. In the marine world, these antennas must be stabilized to point precisely at the correct satellite, as the ship moves and turns below.

In reality, the orbits are  elliptical not circular, so the satellite is actually traveling in an ellipse, further away from the earth in apogee and closer to the earth in perigee.

There is also a significant time delay of about half a second  for the signal to travel from earth to the satellite and back.
 
Like a spinning top, the satellite orbits tend to wobble a bit and it is necessary for the satellite operators to periodically command the satellite to fire rocket thrusters to correct the orbit, both in a east -west direction as well as the north south direction in order to maintain the correct position in the sky.


As the satellite ages over 15 or 20 years and begins to run out of fuel, while still fully functional electronically,  the satellite operator can let the satellite go into an inclined orbit, where they use the remaining fuel to keep the satellite in the correct orbital longitude, while letting the orbit drift north and south. The orbit remains consistent but since the Earth is rotating the satellite appears to move north and south in a figure eight pattern in the sky.

This extends the usefulness of the satellite beyond it's scheduled end of life, but the antenna equipment on the ground needs to have the ability to target and search for the satellite that appears from Earth to be moving up and down in a figure 8 pattern. This is not very practical for antennas that are bolted down on land, but can be used very economically for marine antennas that have inherent tracking and searching capabilities.

As the satellite arc fills up with new satellites, satellite slot positions are becoming more and more of a premium, so these days there is a greater tendency to replace aging satellites with the latest and greatest, rather than trying to squeeze the value out of the older equipment.

Inclined satellites can be of value to users in the polar regions for a few hours each orbit, as the satellite dips toward the pole increasing coverage. 
In order for Low Earth Orbit satellites to remain only a few hundred miles above the surface of the earth, it is necessary for them to move much faster with reference to the earth.

Lower orbits appear to move faster than the rotation of the earth, and higher orbits slower. 

Low Earth Orbit (LEO) satellites, like the Iridium fleet, are only a few hundred miles above the surface of the earth, allowing small, handheld terminals (like overgrown cell phones) with omnidirectional antennas to be used. To maintain this low orbit, the satellites are constantly moving, rapidly around the earth.

The disadvantage of LEO satellites is that they are not parked in one spot, relative to the earth, and are actually rotating rapidly around the earth. LEO satellites are rising and setting and zipping across the sky, so to provide uninterrupted service, you need to be able to see more than one satellite at any given time, with the capability to hand off the call from one setting satellite to a new one that might be rising.

Iridium and Globalstar are two of the major players that have pursued this complex technology, not without many commercial and technical problems.

More about LEO systems later.


 Medium Earth Orbit satellites, like the proposed O3B Network have satellites in a circular orbit about 5000 miles above the equator.

The O3B Network uses 8 satellites with 10 steerable spot beams that light up certain areas on the surface of the earth as the satellite revolves around the earth.
The gain of a directional antenna is compared to an omni-directional (or isotropic) antenna.
 
The isotropic antenna radiates equally in all directions and has a gain of 1 (0dB). The directional antenna focuses the energy in one direction, thus having a gain  which is expressed as a logarithmic ratio compared to the isotropic antenna as dBi.


Note that the above diagram should be imagined in 3D, with a spherical isotropic radiation and conical directional pattern.

The isotropic antenna is a theoretical antenna (like a ball) , radiating in a perfect sphere in all directions. This is different to a real world omni-directional antenna that radiates in a donut shape with the radiating element at the center.

A typical 1 meter parabolic antenna has a gain of about 40 dBi at Ku-Band  frequencies.
Effective Isotropic Radiated Power which is the effective power radiated  from a directional antenna compared to a theoretical isotropical (omnidirectional) antenna.

This includes the power of the transmitter and the antenna gain and it is expressed in dBW.
The dBW EIRP lines that you see on a satellite coverage footprint map indicate the downlink signal strength that you can expect to receive at that spot on the map.

It is a bit confusing to talk about power ( EIRP = Effective Isotropic Radiated Power) when talking about the downlink receive signal, but it is referring to the effective power radiated from the satellite toward that point on the earth.

The required uplink power needed toward the satellite (the receive characteristics of the satellite) is expressed as G/T in dB/K on a different type of map.

The amount of EIRP will determine the minimum size of antenna you need to receive the signal.

Very basically, a 1 meter antenna will need an minimum EIRP of about 42 dBW and a 45cm antenna would need a minimum EIRP of 49 dBW.


1.5m(60") 1.2m(50") 1m(40") 76cm(30") 60cm(24") 45cm(18") 36cm(14") 33cm(13") 28cm(11")
40 dBW 41 dBW 42 dBW 43 dBW 45 dBW 48 dBW 50 dBW 51 dBW 53 dBW

The uplink power that you need to transmit toward the satellite is expressed as G/T in dB/K on a different type of map. These maps actually show the receive characteristics of the satellite, but from this you can calculate the EIRP needed to satisfy the link.

It is important to remember that the transmit pattern might be different from the receive pattern. The receive characteristics of the satellite are shown as G/T
BEAMWIDTH AND POINTING ACCURACY
I read this somewhere on the web so don't quote me:

Bearing in mind that 3dBs is half the signal strength, at C-band the signal typically drops 3dBs at about 0.75 degrees off the satellite, at Ku band it drops 3dBs at about 0.65 degrees off, but at Ka-band the 3dB  point is as close as 0.4 degrees of the satellite. This means that Ka Band maritime terminals need to point significantly closer in accuracy than C or Ku terminals.

Medium Earth Orbit
The time delay between data leaving the satellite and reaching the ground network and vice versa.

Because the satellite is parked 22, 300 miles above the earth, there is a physical time delay for the signal to travel to and from the satellite. This is a fixed quantity, due to the speed of light, that cannot be changed  (unless Einstein was wrong:-).

Satellite latency is about 240 ms each way, or a total of almost 500ms. Combine that with the terrestrial network latencies and your typical round trip ping times to the satellite are about 720ms. If the satellite hub is located across the ocean from the destination you are pinging, you can expect as much as 1000ms round trip delay. This is due to the added latency of the undersea fiber cable.

Latency can effect the data throughput on TCP/IP traffic because each packet sent needs to wait for an acknowledgement before sending the next. Several spoofing techniques are used to fool the TCP/IP into thinking it received the acknowledgement so that it sends the next packet without waiting. This can speed things up a bit, but you can never overcome the physical delay between the ground and the satellite.

On voice calls, you will sometimes have to wait a second after the other person finishes speaking, to avoid speaking over each other.

There is no way around this satellite delay when using GEO satellites, and don't let any sales folk tell you there is. Low Earth Orbit satellites like Iridium and Globalstar have significantly less latency as they are just a few hundred miles above the Earth.
The symbol rate = Data Rate x Inverse of FEC x Modulation index.

The Modulation index: QPSK=0.5 , 8PSK = 0.333, 16QAM=0.25

Let's take 41250 kbps with 3/4 rate FEC and QPSK as an example:
 
41250 x 4/3 x 0.5 = 27500 ksps (kilo symbols per second)
SCPC - Dedicated, One-on-one satellite circuits.

Single Carrier Per Channel circuits provide a dedicated channel between ship and shore wherein the full bandwidth is always on and available to you alone, whether you are using it or not. Satellite space segment is pricey by any standards, due to the extreme cost of developing, building, deploying, and operating a satellite in space. Therefore, you would need good reasons to justify a dedicated service just for you, unless you were a very heavy, around the clock, bandwidth user with an unlimited budget. There are several variations of SCPC where a shipping company might secure dedicated bandwidth and then share the bandwidth, using various technolgies, across their fleet of ships.
Not to be confused with TDMA, Time-division multiplexing  is a type of digital multiplexing in which two or more signals are transferred  simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel.

The time domain is divided into recurring timeslots, one for each sub-channel. A  byte of data block of channel 1 is transmitted during timeslot 1, channel 2 during timeslot 2, etc.

TDM is Multiplexing technique where each channel is assigned a timeslot in a frame whether they use it or not, and TDMA is multiple access technology when several channels are randomly assigned timeslots on demand, and when needed.
TDMA - Time Division Multiple Access: TDMA is the common form of securely sharing bandwidth, where each second or millisecond is sliced up into microseconds and shared between several users. This is timeshare in the sky except that you are not buying a week or two per year, but rather a few milliseconds every second. While you are downloading your Internet, or speaking on the phone, you don't even realize that there are several other users doing the same thing on the same satellite link. When there are fewer people using the link, there are more timeslots and more bandwidth available for you, and when there are more people, you will have a bit less.

Link from the satellite to the ground station