TV satellite

Life of a TV satellite starts with its launch to its orbit around the Earth. Not just any orbit - it has to be a stationary geosynchronous orbit. This means that it has to take to a TV satellite exactly as long to make a single revolution, as it does to the Earth herself. Also, that it orbits right above the equator. Otherwise, its orbit would have to be inclined to the equatorial plane (a stable orbit is only possible with the Earth center coinciding with the orbital center). This would result in a daily back-and-forth change of the height of a TV satellite above the horizon along the north-south line. Likewise, if the period of its revolution would differ from the Earth's, its position as seen from a given ground point would vary azimuthally. In other words, any other than stationary geosynchronous orbit would require receiving dish with tracking capability in order to ensure continuous signal reception.

The height of Earth's geosynchronous orbit - determined by a point where the gravitational force equals force of repulsion of a small body rotating at the geosynchronous speed - is over 22,000 miles. That is how high above the ground are TV satellites. After the satellite has been brought to the "low Earth orbit" (124 to 1240 miles above ground), it is moved up to its final orbit by firing an engine, and "parked" there by firing another one, at a precise moment and angle. Ready for its first day at work...

Large ground based antenna focuses high-frequency uplink signal carrying programming from various sources onto the satellite. The signal is received and emitted back by satellite transponders - a series of one to four dozen of receivers/emitters operating at different frequencies. Each transponder can transmit a number of frequencies, adding up to a large potential volume of channel capacity per satellite.

High-frequency downlink signal is sent from the satellite back to Earth in a diverging beam, covering large ground areas. Typical area of coverage - so called footprint - is 40 to 60 degrees mid-latitude. However, the signal strength, and so the reception quality, weakens toward the outer footprint area, often making necessary to use larger dish sizes for efficient signal reception.

Another factor affecting signal strength is height of the satellite in the sky. The lower it is, the more of the atmosphere to go through, and the more of Earth's thermal noise finds its way into receiving dish antenna. The height of a TV satellite above horizon depends on both, satellite orbital position and local ground coordinates. Ideally, satellite's orbital location should be close to the local longitude. That would be around 80West for the East Coast, around 100W for the Central US, and around 120W for the West Coast. That would place the satellite in the proximity of the southern meridian, at the highest possible highest altitude.

In reality, it is impossible to accomplish such an ideal satellite distribution. While relatively small deviations from the southern line are not significant, satellites positioned far out toward east, or west, will likely face more of a challenge to deliver steady signal to the opposite side of the continent, due to their low height above the horizon (they are positioned slightly bellow the local celestial equator).

Orbital positions of TV satellites are regulated according to international agreements in general and, specifically, by the FCC (Federal Communication Commission). All DBS satellites serving US territories are located between 61.5 and 148 degrees western longitude. The satellite is often identified by its orbital position. For instance, TV satellite located at 110 degrees western longitude is referred to as 110W, or simply 110. At present, the two US DBS giants - DirecTV and Dish Network - use fleets of 11 and 13 TV satellites, respectively, with more planned to be launched before the end of 2006, and in the following year.

Active life of a TV satellite lasts as long as its fuel, usually 10 to 15 years. After that, it usually uses last of its fuel to move out of the active orbiting belt. 

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