OTA transmission is more complicated than it looks.
Satellites first were used in television for short-duration transatlantic interconnection. Short could mean a few minutes, but at the time it was a technological wonder. Early Telstar transmissions required substantial antennas and large amounts of transmit power due to the low sensitivity and large-transmit beam size with which the satellites were equipped. Though it seems hard to believe today, the satellites were not in geostationary orbits. Dishes had only seconds to acquire a signal and had to track rapid motion — 1.5 degrees per second.
Telstar had one transponder, making each pass usable by only one transmission of about 20 minutes. The cost of transmissions was prohibitively expensive and accessible at only very specific times. That made for a tough business case. Compelling content and experimental use provided for uses. Only two earth stations were available, in Maine and at Goonhilly in southwest England.
Soviet Molniya satellites
Other satellites also were used for communications that were not in stationary orbits. Some of the most remarkable were the Soviet Molniya satellites, which were in highly elliptical inclined orbits. At apogee (farthest from the Earth) they were essentially stationary for long periods, but as they moved closer they sped up considerably, making them practical for most of each orbit, but then useless.
The Soviet national network used them for many years for time zone delays for their large distribution system. In later years, they were used for some occasional feeds by Western broadcasters.
Today we use exclusively geostationary satellites. Each orbital slot occupies a highly controlled location over the equator (essentially a box). Inside this box, the spacecraft moves in a figure-8 pattern that is highly predictable, making unmotorized antennas practical. Antennas on modern spacecraft are extremely complex, making patterns that waste little energy over water or unpopulated areas on land.
Satellites excel when transmissions are one to many, broadcasting the same signal to many receivers, and thus multiplying the economic advantage manyfold. Though terrestrial delivery over fiber has certainly achieved economies that rival one-to-one satellite service today, satellites will likely enjoy an advantage for high-quality broadcast distribution for some time.
In this respect, satellite technology resembles compression. The complexity and cost is pushed to the transmit end so inexpensive and technologically simple receivers can be deployed. An earth station must be licensed for transmission, and the hardware is not inexpensive. Occasional use can be accomplished with transportable or vehicle-mounted systems, but for permanent use a fixed, licensed antenna is the only practical answer. It would be nice if systems could be inexpensive, but there are so many elements that must be part of an uplink that low cost is hardly an option at high bandwidth.
First, the antenna (aperture) and feed must be well designed to achieve adequate performance in the center of the beam, but at the same time the energy that misses the reflector must be extremely well controlled to avoid irradiating areas around the antenna. In addition, the beam itself must have (by regulation) well controlled side lobes to ensure that only one satellite is illuminated at a time. The standards for U.S. communications satellites and others are not necessarily the same in this regard, and each satellite operator has specs for their spacecraft.
Second, the RF sections of the transmission chain need to provide well-filtered signals with sufficient power to drive the transponder in the satellite to saturation. This is a complex calculation, termed link budgeting. Many factors must be taken into account, including free space loss over the length of the path to the satellite using the actual location on the earth and the location of the satellite in space. The distance is nominally 23,000mi each way, so even a pencil-thin beam loses a lot of intensity by the time it reaches the satellite. The calculations for the link budget must also take into account the gain of the antenna on the transmission end, as well as the gain and power of the spacecraft, plus the gain of the antenna on the ground. All of the figures are easily obtained, and the link budget can actually be calculated online using a variety of available tools.
The constraints on the overall system effectively mean that you cannot simply blast the spacecraft with as much power as you can buy (large aperture and big amplifiers). Interference with terrestrial uses in some bands restricts the actions operators can take — particularly in C band, where the frequencies are shared with terrestrial microwave services. It is often the case that certain frequencies on certain azimuths cannot be used to avoid interference.
The burden of proof is on the operator of the newer service. If the transmission system has been licensed for the full arc of available satellites and all frequencies (transponders), any new services — terrestrial or satellite service — must coordinate their use to avoid interfering with the existing system's licensed use. Of course the reverse is also true.
With Ku (10.95GHz to 14.5GHz) and Ka (26.5GHz to 40GHz) satellites, coordination with other uses is not an issue because there are no sources of terrestrial interference. There are other issues to be understood, the most important of which is that precipitation degrades higher frequencies to a more serious degree. This “rain fade” may make high-reliability service impractical without geographic diversity plans to transmit from a second location. Of course, having diverse receive sites is seldom an option.
Next month, I'll discuss other aspects of the technology, including modulation schemes, the use of compression in links and downlink/receiver issues.
John Luff is a television technology consultant.
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