Basic Satellite Technology

There are two primary satellite systems in use today: geostationary and low earth orbit (a third, middle earth orbit, is less common). Low earth orbit (LEO), due to its proximity to the Earth's surface, requires considerably less power for transmission, both to and from the satellite. Commercial enterprises, such as Teledesic, intend to use LEO systems to provide high-bandwidth Internet connectivity to areas of the world not readily serviced by terrestrial networks. LEOs can also be used for handheld mobile telephones in any location where there is a reasonably direct line-of-sight to the satellite.

This post focuses on geostationary satellite systems. Satellites in geostationary orbit appear to move only slightly in the sky, hence the term "geostationary." Flying high at 35,800 kilometers (22,000 miles), a geostationary satellite orbits the Earth in the same amount of time it takes the Earth to revolve once. From Earth, therefore, the satellite appears to be stationary, always above the same area of the Earth. The area to which it can transmit is called the satellite footprint.

Exhibit 1 is a simplified diagram of a typical geostationary satellite system using small receiving (end user) VSAT dishes.

Exhibit 1: Geosynchronous Satellite System

Advantages of geosynchronous satellite systems include:

• Quick to deploy at field locations. Once agreements with the satellite and hub providers are in place, additional VSAT dishes can be installed within a few days. Terrestrial lines, on the other hand, can require months to install, depending on the carrier, location, and other logistical factors. Some organizations have mounted VSAT dishes on flatbed trucks so that they can be set up quickly; they can then be moved to another location.

• Only practical solution for very remote locations. For example, in the natural gas industry, pipelines must be run across thousands of miles, some parts of which may be ten to hundreds of miles away from the nearest telecom point of presence (POP). In those cases, satellite transmission of technical data (e.g., pipeline pressure and flow) is the only reasonable way to monitor the pipeline. Similarly, some rural locations cannot get terrestrial services (other than ordinary telephone service) because their usage does not justify the high investment required by the local telephone company to run the conduit.

• Low cost relative to terrestrial lines. In many cases, satellite communications will cost one third to one half that of its terrestrial counterpart. One-way video, in particular, is drastically less expensive than alternatives because of the satellite's ability to carry broadband transmissions. Sometimes, satellite communications are used solely as a backup in case terrestrial lines are down.

  Exhibit 2: Sample Costs to Transfer Internationally 600 Megabytes to 10,000 Subscribers

  
  

  Technology	Transmission Rate (Mbps)	Transfer Time	Transfer Cost (U.S. $)
  Telephone line	0.04	2 days	1,500,000
  ISDN	0.12	12 hours	100,000
  ADSL	1.5	1 hour	20,000
  Cable	4	20 minutes	1,200
  Satellite	40	2 minutes	25

  
  

• Works when parts of the public network are down. As recent world events have reminded us, a temporary slowdown in the public switched telephone network is often a consequence of high-visibility disasters. Telephone lines and cellular phones (which ultimately use terrestrial lines) become swamped. In contrast, VSAT dishes need only electrical power and a clear line of sight with the appropriate satellite. They are unaffected by congestion on the public network or any circuit/equipment breakdown in the POP itself. The one exception to satellite independence from terrestrial carriers is the need to use conventional lines for the backhaul circuit.

• Capable of efficiently multicasting text, images, video, and audio over large geographic areas. In contrast, multicasting via terrestrial lines often equates to repeated transmissions of the same information, wasting valuable band-width. Note that multicasting is a one-way, one-to-many medium.

• Bypasses Internet congestion points when used for Internet transmissions.

• Only one or two providers needed for end-to-end international communications. For terrestrial leased line management of international connections, each individual carrier usually manages its segment of the link. A satellite connection, on the other hand, may use only a single supplier who is responsible for the quality and management of the international backbone connection.

Disadvantages include:

• Weather interference. Bad weather (discussed in detail later) can disrupt satellite communications. Heavy rains, ice storms, and even the vernal/ autumnal equinox can sometimes disrupt the signal.

• Propagation delay. Geosynchronous satellites must be positioned so high above the Earth that even traveling near the speed of light, signals do not instantly traverse from end to end. Voice carried over geosynchronous satellite, while still used for mobile communications such as Inmarsat satellite phones, is considered inferior to landlines for routine business communications. The one-half to one-second delay is disconcerting for most individuals who are used to the full duplex mode of the traditional public network (i.e., both parties can talk at the same time, without delay). From a data communications perspective, this limitation makes both Internet access and highly interactive applications unsuitable for routine use over satellite links.

• Limited two-way bandwidth. While video "downlinks" from the satellite are carried over a large bandwidth, uplinks (from the VSAT dish to the satellite) are narrowband (typically 64 to 256 kbps).