Lord Kelvin had been the first to discover in 1879 that the state transitions of
caesium atoms could be used to measure time. Later in the early twentieth century,
physicist Niels Bohr determined precisely that electrons modify their orbit around
the nucleus by so-called quantum leaps, every billionth of a second with a bias
every ten million years. This physical principle was later exploited by the US
National Bureau of Standards to build the first atomic clock in the 1940s, which
was to be equipped at astronomical observatories with ultra-precise calibration
instruments.
21
Three decades later, the technology was mature enough to go into space. Any
terrestrial receiver could measure the delay between the instant of emission and
that of the reception of the signal to calculate its distance from the satellite, simply
by multiplying the delay by the speed of light. By repeating this operation with the
synchronous signals of at least four satellites, the receiver could determine its
longitude, latitude and height from Earth surface.
In 1973, the Pentagon gave mandate to Col. Bradford Parkinson to unify both
Navy and Air Force projects under his command in order to build the Navigation
Satellite Timing and Ranging Global Positioning System (NAVSTAR-GPS). Five
years later, the first satellites were launched into orbit, and in 1991 during the Gulf
War in Kuwait, the whole system became operational.
While the satellite signals were intended for military use, the Pentagon granted
some frequencies for civilian use. In 1983, a Korean airliner
22
was shot down by
Russian fighter planes due to an error in its inertial navigation system, which
caused it to fly over Soviet territory. The media pointed out that if the airplane had
had a GPS receiver, the tragedy could have been avoided,
23
and so President
Reagan decided to free a slot of GPS radio signals for this purpose.
From that moment onward, satellite navigators started being sold all over the
world, despite not providing the same accuracy as the military ones. In fact, the
NORAD control centre intentionally altered civilian signals to decrease their performance and prevent enemy countries or terrorists from exploiting GPS to attack
the US. The so-called selective availability (SA) was an intentional degradation of
public GPS signals implemented for national security reasons. Yet it also ended up
reducing the quality of service for federal agencies and US companies that wanted
to use the system to improve the general safety of the commercial and industrial
transportation sector even within US territory. Research institutions went on to
21
Audoin, Claude, and Bernard Guinot. 2001. The Measurement of Time. Cambridge University
Press, 129.
22
https://www.nytimes.com/1983/09/02/world/us-says-soviet-downed-korean-airliner269-lost-reagan-denounces-wanton-act.html.
23
Hillstrom, Laurie Collier. 2011. Global Positioning System. Gale Cengage Learning, 24.
58 GPS: The Stellar Clock
caesium atoms could be used to measure time. Later in the early twentieth century,
physicist Niels Bohr determined precisely that electrons modify their orbit around
the nucleus by so-called quantum leaps, every billionth of a second with a bias
every ten million years. This physical principle was later exploited by the US
National Bureau of Standards to build the first atomic clock in the 1940s, which
was to be equipped at astronomical observatories with ultra-precise calibration
instruments.
21
Three decades later, the technology was mature enough to go into space. Any
terrestrial receiver could measure the delay between the instant of emission and
that of the reception of the signal to calculate its distance from the satellite, simply
by multiplying the delay by the speed of light. By repeating this operation with the
synchronous signals of at least four satellites, the receiver could determine its
longitude, latitude and height from Earth surface.
In 1973, the Pentagon gave mandate to Col. Bradford Parkinson to unify both
Navy and Air Force projects under his command in order to build the Navigation
Satellite Timing and Ranging Global Positioning System (NAVSTAR-GPS). Five
years later, the first satellites were launched into orbit, and in 1991 during the Gulf
War in Kuwait, the whole system became operational.
While the satellite signals were intended for military use, the Pentagon granted
some frequencies for civilian use. In 1983, a Korean airliner
22
was shot down by
Russian fighter planes due to an error in its inertial navigation system, which
caused it to fly over Soviet territory. The media pointed out that if the airplane had
had a GPS receiver, the tragedy could have been avoided,
23
and so President
Reagan decided to free a slot of GPS radio signals for this purpose.
From that moment onward, satellite navigators started being sold all over the
world, despite not providing the same accuracy as the military ones. In fact, the
NORAD control centre intentionally altered civilian signals to decrease their performance and prevent enemy countries or terrorists from exploiting GPS to attack
the US. The so-called selective availability (SA) was an intentional degradation of
public GPS signals implemented for national security reasons. Yet it also ended up
reducing the quality of service for federal agencies and US companies that wanted
to use the system to improve the general safety of the commercial and industrial
transportation sector even within US territory. Research institutions went on to
21
Audoin, Claude, and Bernard Guinot. 2001. The Measurement of Time. Cambridge University
Press, 129.
22
https://www.nytimes.com/1983/09/02/world/us-says-soviet-downed-korean-airliner269-lost-reagan-denounces-wanton-act.html.
23
Hillstrom, Laurie Collier. 2011. Global Positioning System. Gale Cengage Learning, 24.
58 GPS: The Stellar Clock