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 |
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.
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).
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.
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.
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.
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 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.
This includes the power of the transmitter and the antenna gain and it is expressed in dBW.
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.
|40 dBW||41 dBW||42 dBW||43 dBW||45 dBW||48 dBW||50 dBW||51 dBW||53 dBW|
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
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.
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.
Let's take 41250 kbps with 3/4 rate FEC and QPSK as an example:
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.
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.