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radio-navigation systems

radio-navigation systems

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he design of GPS is based partly on similar ground-based radio-navigation systems, such

asLORAN and the Decca Navigator developed in the early 1940s, and used during World War

II. In 1956, Friedwardt Winterberg

[3]

proposed a test of general relativity (for time slowing in a

stronggravitational field) using accurate atomic clocks placed in orbit inside artificial satellites.

(To achieve accuracy requirements, GPS uses principles of general relativity to correct the

satellites' atomic clocks.

[4]

) Additional inspiration for GPS came when the Soviet Union launched

the first man-made satellite, Sputnik in 1957. Two American physicists, William Guier and

George Weiffenbach, at Johns Hopkins's Applied Physics Laboratory (APL), decided on their

own to monitor Sputnik's radio transmissions. They soon realized that, because of the Doppler

effect, they could pinpoint where the satellite was along its orbit from the Doppler shift. The

Director of the APL gave them access to their brand new UNIVAC II to do the heavy

calculations required. When they released the orbit of Sputnik to the media, the Russians were

dumbfounded to learn how powerful American computers had become, as they would not have

been able to calculate the orbit themselves. The following spring, Frank McClure, the deputy

director of the APL, asked Guier and Weiffenbach to look at the inverse problem where you

know the location of the satellite and you want to find your own location. (The Navy was

developing the submarine-launched Polaris missile, which required them to know the

submarine's location.) This led them and APL to develop the Transit system.

[5]

Official logo for

NAVSTAR GPS

Emblem of the50th

Space Wing

The first satellite navigation system, Transit (satellite), used by the United States Navy, was first

successfully tested in 1960. It used a constellation of five satellites and could provide a

navigational fix approximately once per hour. In 1967, the U.S. Navy developed

the Timationsatellite that proved the ability to place accurate clocks in space, a technology

required by GPS. In the 1970s, the ground-based Omega Navigation System, based on phase

comparison of signal transmission from pairs of stations,

[6]

became the first worldwide radio

navigation system. Limitations of these systems drove the need for a more universal navigation

solution with greater accuracy.

While there were wide needs for accurate navigation in military and civilian sectors, almost none

of those were seen as justification for the billions of dollars it would cost in research,

development, deployment, and operation for a constellation of navigation satellites. During

the Cold War arms race, the nuclear threat to the existence of the United States was the one

need that did justify this cost in the view of the United States Congress. This deterrent effect is

why GPS was funded. It is also the reason for the ultra secrecy at that time. The nuclear

triad consisted of the United States Navy's submarine-launched ballistic missiles (SLBMs) along

with United States Air Force (USAF) strategic bombers and intercontinental ballistic

missiles (ICBMs). Considered vital to the nuclear deterrence posture, accurate determination of

the SLBM launch position was a force multiplier.

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Precise navigation would enable United States submarines to get an accurate fix of their

positions prior to launching their SLBMs.

[7]

The USAF with two-thirds of the nuclear triad also

had requirements for a more accurate and reliable navigation system. The Navy and Air Force

were developing their own technologies in parallel to solve what was essentially the same

problem. To increase the survivability of ICBMs, there was a proposal to use mobile launch

platforms (such as Russian SS-24 and SS-25) and so the need to fix the launch position had

similarity to the SLBM situation.

In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System for

Accurate ICBM Control) that was essentially a 3-D LORAN. A follow-on study called Project 57

was worked in 1963 and it was "in this study that the GPS concept was born." That same year

the concept was pursued as Project 621B, which had "many of the attributes that you now see

in GPS"

[8]

and promised increased accuracy for Air Force bombers as well as ICBMs. Updates

from the Navy Transit system were too slow for the high speeds of Air Force operation. The

Navy Research Laboratory continued advancements with their Timation (Time Navigation)

satellites, first launched in 1967, and with the third one in 1974 carrying the first atomic clock

into orbit.

[9]

With these parallel developments in the 1960s, it was realized that a superior system could be

developed by synthesizing the best technologies from 621B, Transit, Timation, and SECOR in a

multi-service program.

During Labor Day weekend in 1973, a meeting of about 12 military officers at the Pentagon

discussed the creation of a Defense Navigation Satellite System (DNSS). It was at this meeting

that "the real synthesis that became GPS was created." Later that year, the DNSS program was

named Navstar. With the individual satellites being associated with the name Navstar (as with

the predecessors Transit and Timation), a more fully encompassing name was used to identify

the constellation of Navstar satellites, Navstar-GPS, which was later shortened simply to

GPS.

[10]

After Korean Air Lines Flight 007, carrying 269 people, was shot down in 1983 after straying into

the USSR's prohibited airspace,

[11]

in the vicinity of Sakhalin and Moneron Islands,

President Ronald Reagan issued a directive making GPS freely available for civilian use, once it

was sufficiently developed, as a common good.

[12]

The first satellite was launched in 1989, and

the 24th satellite was launched in 1994.

Initially, the highest quality signal was reserved for military use, and the signal available for

civilian use was intentionally degraded (Selective Availability). This changed with President Bill

Clinton ordering Selective Availability to be turned off at midnight May 1, 2000, improving the

precision of civilian GPS from 100 meters (330 ft) to 20 meters (66 ft). The executive order

signed in 1996 to turn off Selective Availability in 2000 was proposed by the US Secretary of

Defense, William Perry, because of the widespread growth of differential GPS services to

improve civilian accuracy and eliminate the US military advantage. Moreover, the US military

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he design of GPS is based partly on similar ground-based radio-navigation systems, such asLORAN and the Decca Navigator developed in the early 1940s, and used during World War II. In 1956, Friedwardt Winterberg[3] proposed a test of general relativity (for time slowing in a stronggravitational field) using accurate atomic clocks placed in orbit inside artificial satellites. (To achieve accuracy requirements, GPS uses principles of general relativity to correct the satellites' atomic clocks.[4]) Additional inspiration for GPS came when the Soviet Union launched the first man-made satellite, Sputnik in 1957. Two American physicists, William Guier and George Weiffenbach, at Johns Hopkins's Applied Physics Laboratory (APL), decided on their own to monitor Sputnik's radio transmissions. They soon realized that, because of the Doppler effect, they could pinpoint where the ...
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