Learning Centre

Benefits of Satellite Communications:

  • Universal – available almost anywhere and more coverage than terrestrial.
  • Versatile – can support all of today’s communications needs.
  • Reliable – constant and uniform quality of service.
  • Seamless – as a broadcast medium can service hundreds of locations regardless of geography.
  • Fast – networks can be rolled out quickly, fast to market (very useful in cases of disaster recovery).
  • Expandable – easily scalable for more locations or bandwidth.
  • Flexible – is easily integrated to complement, augment or extend any communications network.

Satellite vs Terrestrial:

  • Terrestrial microwave consists of towers and stations and are limited by line-of-sight.
    • A terrestrial repeater has a limited range due to the curvature of the earth.
    • Repeaters have a maximum range of about 100 kms.
  • Terrestrial copper or fibre could be widely distributed, complicated, different providers, or not exist in the location required.
    • Whereas a terrestrial repeater is point-to-point, the satellite carries signals from anywhere to anywhere; and anywhere to everywhere.
  • A satellite, by virtue of its altitude, can relay signals from one station to another, thousands of miles away.
    • Communications is possible to and from anywhere within the Satellite’s Footprint.
    • Using the example of a mirror in space, anyone who can see this mirror could communicate with each other. Someone in Vancouver could flash a light at the mirror and signal you in Ottawa. In this way the satellite provides multiple access.
    • But of course satellites are doing a lot more than acting like a mirror.

What is a Communications Satellite?

  • A communications satellite is a device used to receive and transmit radio signals in space. The satellite has communications equipment including receive and transmit antennas, power, and electronic components which enable it to receive a signal from a satellite terminal/user and then transmit that same signal to another satellite terminal/user.
  • A vast array of satellites exist with various:
    • Frequencies – C Band, Ku Band, L Band etc.
    • Altitudes – LEO, MEO, GEO
    • Orbital Planes – Equatorial, circular, inclined, polar
  •  There are many functions and services which satellites are designed and used for:
    • Resource surveillance, weather reporting, search and rescue, espionage, global positioning, and telephone communications. In order to provide the type of services desired, the appropriate orbit may be required to provide the required Earth coverage.
    • One note, low orbits reduce the amount of delay associated with sending the signal to the satellite or receiving the signal from the satellite, however line of sight to the satellite will be not available from any location for more than about 2 hours at a time.

Frequencies Used for Satellite Communications:

Designation Range Link Service
Military UHF 240 – 328 MHz Up & Down Mobile Satellite
L Band 1535.0-1543.5 MHz Down Maritime Mobile
1636.5-1645.0 MHz Up
1542.5-1558.5 MHz Down Aeronautical Mobile
1644.0-1660.0 MHz Up
C Band 3700-4200 MHz Down Fixed Satellite
5925-6425 MHz Up
Military SHF 7250-7750 MHz Down Fixed Satellite
7900-8400 MHz Up
Ku Band 10950-11200 MHz Down Fixed Satellite
11450-12700 MHz
14000-14500 MHz Up
Ka Band 17700-21200 MHz Down Fixed Satellite
27500-31000 MHz Up

Life of a Satellite:

  • The design life of geostationary satellites is approximately 10 – 15 years.
    • There are approximately 300 operational geosynchronous satellites.
  • At launch the fuel tanks are filled with hydrazine gas which is used for satellite station keeping.
    • The hydrazine gas is used for small thruster burns which keep the satellite at its correct position for the duration of its life.
  • The satellite is ready for retirement when there is only a small amount of fuel left to be expended.
  • The remaining fuel is used to execute “A final end of life maneuver” which places the satellite as far out of geosynchronous orbit as possible.
    • The satellite gets pushed out to a “disposal orbit“, most satellites end up about 300 km above where they started.
    • Up to a dozen geosynchronous satellites go out of service every year, and there are now several hundred retired satellites in the disposal orbit. They may drift slightly from that altitude under the influence of the sun and moon, but they won’t interfere with the operational satellites below.

Orbital Location and Footprint:

  • The location of a satellite is referred to its orbital position.
    • All Geostationary Satellites are located in a single ring above the equator in what is known as the Clarke Belt (after the author Arthur C. Clarke). The requirement to space these satellites apart means that there are a limited number of orbital “slots” available, thus only a limited number of satellites can be placed in geostationary orbit.
    • This has led to conflict between different countries wishing access to the same orbital slots. These disputes are addressed through the ITU (International Telecommunication Union) allocation mechanism.
  • The location of a satellite is normally measured in terms of longitudinal degrees East or West from the prime Meridian of 0 degrees.
  • The area of Earth’s surface for coverage of transmit to or receive from is called the footprint.
  • The footprint can be tailored for different frequencies and power levels.

The Satellite Footprint:

  • Each satellite will have its own characteristic footprint based upon the area in which it provides service.
  • Canadian Satellite footprints may be more elliptical east-west than US Satellites due to the layout of our country.
  • A US Satellite may be elliptical north-south to cover from Mexico to Canada for example.
  • Satellites will move in space due to gravitational forces from the sun and moon. But the orbital location can be maintained by control stations on the ground and is called station keeping.
  • The satellite is allowed some deviation from its assigned position but once it hits a defined threshold, thrusters are fired to maintain this deviation allowing for a small amount of pointing error. The allowed pointing error is built into the link budget calculation.
  • The deviation pattern can be thought of as a figure 8 pattern, the centre of the 8 is called centre-of-box.

Typical Satellite Characteristics:

  • The satellite acts as a repeater which receives, amplifies, and frequency translates the signal before beaming it over its footprint.
    • At Ku Band the frequency received from the earth is down-converted by 2300 MHz (the signal received from the satellite will be 2300 MHz lower than the signal transmitted to the satellite) and at C-Band, this frequency translation is 2225 MHz.
      • By knowing the frequency translation performed by the satellite, one can determine the receive frequency (within the range of 3.7 to 4.2 GHz) if the transmit frequency is known (within the 5.9 to 6.4 GHz range) and vise versa.
  • The satellite provides fixed gain – if you need to receive more power from the satellite, the transmitting Earth Station must transmit more power.
  • Normally satellites operate on both the C Band and Ku Band (can also include Ka and X Bands) and will usually have separate antennas for each band.
  • In each band there is 500 MHz of bandwidth in a normal configuration (more with extended band) of frequency spectrum which is “Polarized”.
    • Polarization gives the ability to re-use the frequency spectrum, thereby doubling the capacity.

Polarization:

  • Polarization is the plane of the electric field in a radio wave.
    • All forms of electromagnetic radiation can be modeled as waves.
    • If a point source is radiating a radio wave there are vibrations in all directions perpendicular to the axis of travel. When all directions, or planes, are present we say the energy is unpolarized. We can filter out all but a certain direction of vibration by polarizing the wave.
  • Linear (i.e. fixed) polarization is vertical and horizontal and is generally referred to as the horizontal and vertical polarizations or “pols ”.
  • Circular (i.e. rotating) polarization is the left-hand and right-hand versions.
  • Earth stations have to align the antenna to the proper polarization.
    • This maximizes signal and limit cross-polar interference (cross or x-pol) and is only a concern for linear polarization.
  • Having two polarizations allows us to double the amount of available bandwidth on the satellite.

Uplink and Downlink:

  • Signals transmitted from Earth to Satellite are referred to as uplink signals, and signals received from the Satellite are downlink signals.
    • We use a satellite like a mirror (or reflector) in the sky to effectively bounce the signal over to another part of the country, however, the other part of the country is bouncing signals over to us at the same time.
    • The satellite converts the signal before it retransmits back to earth.
    • The signals going up to the satellite are at one frequency range (within a band of frequencies) and the satellite changes them to a different frequency range coming down so they won’t interfere with the signals going up.
  • The designation ‘C Band’ uplinks at 6 GHz and downlinks at 4 GHz and ‘Ku-Band’ uplinks at 14 GHz and downlinks at 12 GHz.
  • The footprints can be different between C Band and Ku Band.
    • Ku Band is preferred in urban areas since Radio Frequency Interference is less and licensing is easier.
    • In Canada, C Band provides better northern coverage and it better suited for less populated areas since its frequency spectrum is shared with terrestrial microwave.
  • All service types are carried on both bands:
    • By service types, SCPC, Star, video etc.

Satellites and Orbits:

  • Geosynchronous Orbit (GEO) are located 35,786 Km above Earth.
    • A single satellite can view approximately 1/3 of Earth’s surface.
    • They travel in the same direction and speed as Earth’s rotation so they appear “stationary”.
      • The benefit: Earth station antennas do not need to track the satellite.
    • Takes approximately 250 msec for the signal from earth to reach the satellite and come back down to earth, round trip then is about 500 msec (source to satellite to destination to satellite and back to source).
  • Medium Earth Orbit (MEO) are located 8,000-20,000 Km above Earth.
    • Typically, they have an elliptical (oval-shaped) orbit, but some travel in near perfect circles.
    • The orbital period is anywhere from 2 to 12 hours.
      • The most common use for satellites in this region is for navigation, communication, and geodetic/space environment science.
      • They are used by GPS satellites.
      • Communications satellites that cover the North and South Pole use MEO satellites.
  • Low Earth Orbit (LEO) are located 500-2,000 Km above Earth.
    • LEOs are much closer to earth and travel at high speed to avoid being pulled out of orbit by Earth’s gravity.
    • They orbit Earth about every 90 minutes.
      • The international space station is a LEO.

What is Installed on the Ground?

  • All communications with a geostationary satellite requires the use of Earth stations.
  • They may be fixed or mobile, from small to very large antennas.
  • The Earth station typically consists of an antenna, RF equipment to Tx/Rx, indoor unit, and the final communications devices.
    • Final communications devices could be local or off site via terrestrial network infrastructure.
  • A teleport or super hub is essentially a large version of a typical Earth station.
    • Teleports have similar equipment to a remotes but the equipment will be hub centric since it is looking at many remotes, rather than the remote just looking at the hub.
    • As well teleports will also have extra reliability by means of backup power, redundancy of equipment, and sometimes the ability to counteract the effects of fading (uplink power control).

What is a Backhaul?

  • A backhaul (or local loop) is the intermediate link between a core network (teleport or hub) to smaller networks or devices at the edge of the network.
    • It is the physical link or circuit that connects the customers premises to an Earth station.
    • A backhaul is usually more cost-effective than a customer having their own hub or teleport.

Why use a backhaul?

  • It may be more cost-effective to use a multi-user Teleport rather than own the hub Earth station.
  • The cost of the backhaul to the Teleport is offset by the reduction in Earth station equipment costs.
  • Customers share support and standby facilities at a Teleport.
  • For short-haul links it is usually cheaper to rent a land line from another carrier.
    • Infosat does not normally provide private backhaul services, but will assist customers in their procurement or rental.
    • A virtual private network (VPN) can be used.
    • Infosat does provide internet access which can be used to create a quasi-private backhaul using a VPN.
    • Depending on the type of traffic, distance between locations and customer requirements, there are a number of considerations for the type of backhaul.

Fading:

  • Satellite Services are subject to fading.
    • The higher the frequency the more the signal may be affected. C Band is less affected than Ku. Ku is less affected than Ka.
  • Fading can severely affect service when heavy rain or snow is present.
  • Tolerances are built into the power levels of the transmitted services to minimize the effect.
  • These tolerances are referred to as fade margins.
    • That means we transmit more power than what is needed during clear sky conditions and the amount of this power is determined by the link budget analysis.
    • System design will include a margin to accommodate some signal reduction by precipitation. How much fade margin is used will be determined by the customer’s service availability requirements.
    • Even with fade margin there still will be some instances where the density of clouds and rain reduces the signal enough that it affects data with errors, or a voice call gets noisy, or it affects a TV channel.
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