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that verify each system requirement. If an economical,
practical test cannot be defined, the requirement
should be rewritten.
is the system specification, discussed in Section V.
This paper focuses on tradeoff studies as the heart
of system engineering, during the early phases of the
system life cycle.
The most important product of the system engineer
11. EXAMPLES OF LARGE-SCALE SYSTEMS
1) UnpZanned Some systems evolve without
an initial plan for change. For example, water and
sewer supplies are designed for a target-sized city.
The system grows as building permits are issued.
When the system nears capacity or when pollution
regulations are instituted, the system is replanned
and then continues to evolve at the whim of land
developers. New York City’s third water-supply tunnel
was begun several decades after the point when a
failure of either of the original two tunnels would
have caused severe water shortages.
grids are examples. Thomas Edison, George
Westinghouse, and Alexander Bell did not foresee
The early stages of electric-power and telephone
IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. 33, NO. 2 APRIL 1997 579
500-KVDC power lines, fiber-optic cables, and
communication satellite relays.
Other examples are pre-1990 automobiles and
commercial airliners, which were built by subsystem
experts with commercially available parts without a
system architecture.
2) Evolutionary: Most large-scale systems fall
into this category. They are planned to evolve in
stages, grow until physical limits are reached, then
are replanned and grow again.
Integrated national power and telephone grids are
examples. Other examples are the US. Air Force’s
and NASA’s spacecraft tracking and communication
systems. Both began as missile-tracking ranges and
evolved into ground-based satellite-tracking networks.
NASA’s network for low-orbit spacecraft evolved
further into a space-based system using tracking and
data relay satellites (TDRS). Still another example
is the International Space Station, which is being
designed to be constructed in stages from modules
furnished by several countries. The design will
change in unforeseen ways if new nations join the
consortium.
These systems are traditionally planned by a
central organization. In most countries, government
organizations plan the infrastructure: NASA, Post and
Telephone companies, power companies. In countries
where the infrastructure is privately owned, voluntary
committees and nonprofit organizations are created:
the North American Reliability Council, the Electric
Power Research Institute. Bell Laboratories planned
the North American telephone system when ATT had
a monopoly on long-distance service.
Occasionally a system is fully
planned from the beginning. The Apollo expedition to
the Moon is an example [6]. The boosters, two 2-stage
spacecraft, the launch complex, fuel-fabrication plants,
tracking network, and lunar trajectory were designed
as an entity and remained essentially unchanged
throughout the 10 year program. Rail rapid transit
systems are usually fully planned, though they
are built in increments. In the 1990s, America’s
long-haul railroads commissioned the design of a
position-location, control and communication system
that would be used universally north of the Rio
Grande Ri