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Unit 12: Space-based Wireless WANs

that directed its radio energy at the earth. It was rather sophisticated and heavy. At 500-1000 Notes
pounds it could only be launched by the ATLAS- CENTAUR launch vehicle. ADVENT never
flew, primarily because the CENTAUR stage was not fully reliable until 1968, but also because
of problems with the satellite. When the program was canceled in 1962 it was seen as the death
knell for geosynchronous satellites, three-axis stabilization, the ATLAS-CENTAUR, and complex
communications satellites generally. Geosynchronous satellites became a reality in 1963, and
became the only choice in 1965. The other ADVENT characteristics also became commonplace in
the years to follow.

In the early 1960s, converted intercontinental ballistic missiles (ICBMs) and intermediate range
ballistic missiles (IRBMs) were used as launch vehicles. These all had a common problem: they
were designed to deliver an object to the earth’s surface, not to place an object in orbit. Upper
stages had to be designed to provide a delta-Vee (velocity change) at apogee to circularize the
orbit. The DELTA launch vehicles, which placed all of the early communications satellites in
orbit, were THOR IRBMs that used the VANGUARD upper stage to provide this delta-Vee. It
was recognized that the DELTA was relatively small and a project to develop CENTAUR, a high-
energy upper stage for the ATLAS ICBM, was begun. ATLAS-CENTAUR became reliable in 1968
and the fourth generation of INTELSAT satellites used this launch vehicle. The fifth generation
used ATLAS-CENTAUR and a new launch-vehicle, the European ARIANE. Since that time
other entries, including the Russian PROTON launch vehicle and the Chinese LONG MARCH
have entered the market. All are capable of launching satellites almost thirty times the weight of
EARLY BIRD.
In the mid-1970s several satellites were built using three-axis stabilization. They were more
complex than the spinners, but they provided more despun surface to mount antennas and they
made it possible to deploy very large solar arrays. The greater the mass and power, the greater the
advantage of three-axis stabilization appears to be. Perhaps the surest indication of the success
of this form of stabilization was the switch of Hughes, closely identified with spinning satellites,
to this form of stabilization in the early 1990s. The latest products from the manufacturers of
SYNCOM look quite similar to the discredited ADVENT design of the late 1950s.
Much of the technology for communications satellites existed in 1960, but would be improved
with time. The basic communications component of the satellite was thr traveling-wave-tube
(TWT). These had been invented in England by Rudoph Kompfner, but they had been perfected
at Bell Labs by Kompfner and J. R. Pierce. All three early satellites used TWTs built by a Bell Labs
alumnus. These early tubes had power outputs as low as 1 watt. Higher- power (50-300 watts)
TWTs are available today for standard satellite services and for direct-broadcast applications. An
even more important improvement was the use of high-gain antennas. Focusing the energy from
a 1-watt transmitter on the surface of the earth is equivalent to having a 100-watt transmitter
radiating in all directions. Focusing this energy on the Eastern U.S. is like having a 1000-watt
transmitter radiating in all directions. The principal effect of this increase in actual and effective
power is that earth stations are no longer 100-foot dish reflectors with cryogenically-cooled maser
amplifiers costing as much as $10 million (1960 dollars) to build. Antennas for normal satellite
services are typically 15-foot dish reflectors costing $30,000 (1990 dollars). Direct-broadcast
antennas will be only a foot in diameter and cost a few hundred dollars.

Mobile Services

In February of 1976 COMSAT launched a new kind of satellite, MARISAT, to provide mobile
services to the United States Navy and other maritime customers. In the early 1980s the Europeans
launched the MARECS series to provide the same services. In 1979 the UN International Maritime
Organization sponsored the establishment of the International Maritime Satellite Organization
(INMARSAT) in a manner similar to INTELSAT. INMARSAT initially leased the MARISAT and
MARECS satellite transponders, but in October of 1990 it launched the first of its own satellites,
INMARSAT II F-1. The third generation, INMARSAT III, has already been launched.

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