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4 Navigations from Ground to Space
The SECOR was an all-weather geodetic survey system, consisting of a satellite
and four ground stations, of which three stations’ coordinates had been surveyed
accurately and the fourth station’s coordinates were to be undetermined. By using
the successive positions of the satellites in orbits, the unknown locations on the
Earth’s surface can be determined with exactness over long distances. When radio
waves are flashed from the three stations with the known coordinates to the satellite
and returned, the satellite’s position at any time can be determined by measuring the
ranges between the satellite and the three stations. Using these precisely established
satellite positions as a base, ranges from the satellite to the unknown stations are used
to compute the position of the unknown station. Each ground station was entirely
portable, containing three units: a radio-frequency shelter, a data handling shelter
and a storage shelter. Later, the initial units were replaced by the lighter weight,
solid-state equipment. Between 1964 and 1969, there were a total of 13 SECOR
satellites launched. Each satellite is with a mass of 18 kg and spherical shape or
cubic shape, carrying a transponder, a telemetry system to monitor temperature and
operating voltages, and a power unit comprised of solar panels and batteries. The
SECOR system was in operational use for several years, establishing a global survey
network, and especially allowing continents and islands to be brought within the
same global geodetic grid.
In the early 1970s, although the above four space-based navigation programs
had been developed parallel in the United Sates, even including the built land-based
worldwide radio navigation system Omega, all of the systems were unable to meet
the wide needs for accurate navigation in military and civilian sectors due to their
own limitations. Thereby, it was realized that a superior system would be developed
by synthesizing the best technologies from the multiple systems. However, almost
none of those needs were seen as justification for billions of dollars that would cost
in research, development, deployment and operation for a constellation of navigation
satellites. In the atmosphere of Cold War arms race, the nuclear threat to the existence
of the US was one need that did justify this cost in the view of the U.S. Congress.
In addition, it is also the reason for the ultra-secrecy at that time. The nuclear triad
consisted of the U.S. Navy’s Submarine-Launched Ballistic Missiles (SLBM) along
with the U.S. Air Force strategic bombers and Inter-Continental Ballistic Missiles
(ICBM). Considered vital to the nuclear deterrence posture, accurate determination of
the SLBM launch position was a force multiplier. Meanwhile, the U.S. Air Force with
two-thirds of the nuclear triad also had requirements for a more accurate and reliable
navigation system. In order to increase the survivability of ICBM, the requirements
for using mobile launch platforms were proposed, with the need to fix the launch
position similar to the SLBM situation. Furthermore, the U.S. Army also had the
urgent needs for precisely geodetic surveying by using navigation satellites. As a
result, a meeting of about dozen military officers from the Army, Navy, Marine Corps
and Air Force was held at the Pentagon by the U.S. Department of Defense (DoD)
to discuss the creation of a Defense Navigation Satellite System (DNSS) in May
1973, and a Joint Program Office (JPO) led by the then-Air Force Colonel Branford
Parkinson (1935−) was formed to execute the DNSS program. Later that year, the
DNSS program was named Navigation Satellite Time and Ranging (NAVSTAR),
4 Navigations from Ground to Space
The SECOR was an all-weather geodetic survey system, consisting of a satellite
and four ground stations, of which three stations’ coordinates had been surveyed
accurately and the fourth station’s coordinates were to be undetermined. By using
the successive positions of the satellites in orbits, the unknown locations on the
Earth’s surface can be determined with exactness over long distances. When radio
waves are flashed from the three stations with the known coordinates to the satellite
and returned, the satellite’s position at any time can be determined by measuring the
ranges between the satellite and the three stations. Using these precisely established
satellite positions as a base, ranges from the satellite to the unknown stations are used
to compute the position of the unknown station. Each ground station was entirely
portable, containing three units: a radio-frequency shelter, a data handling shelter
and a storage shelter. Later, the initial units were replaced by the lighter weight,
solid-state equipment. Between 1964 and 1969, there were a total of 13 SECOR
satellites launched. Each satellite is with a mass of 18 kg and spherical shape or
cubic shape, carrying a transponder, a telemetry system to monitor temperature and
operating voltages, and a power unit comprised of solar panels and batteries. The
SECOR system was in operational use for several years, establishing a global survey
network, and especially allowing continents and islands to be brought within the
same global geodetic grid.
In the early 1970s, although the above four space-based navigation programs
had been developed parallel in the United Sates, even including the built land-based
worldwide radio navigation system Omega, all of the systems were unable to meet
the wide needs for accurate navigation in military and civilian sectors due to their
own limitations. Thereby, it was realized that a superior system would be developed
by synthesizing the best technologies from the multiple systems. However, almost
none of those needs were seen as justification for billions of dollars that would cost
in research, development, deployment and operation for a constellation of navigation
satellites. In the atmosphere of Cold War arms race, the nuclear threat to the existence
of the US was one need that did justify this cost in the view of the U.S. Congress.
In addition, it is also the reason for the ultra-secrecy at that time. The nuclear triad
consisted of the U.S. Navy’s Submarine-Launched Ballistic Missiles (SLBM) along
with the U.S. Air Force strategic bombers and Inter-Continental Ballistic Missiles
(ICBM). Considered vital to the nuclear deterrence posture, accurate determination of
the SLBM launch position was a force multiplier. Meanwhile, the U.S. Air Force with
two-thirds of the nuclear triad also had requirements for a more accurate and reliable
navigation system. In order to increase the survivability of ICBM, the requirements
for using mobile launch platforms were proposed, with the need to fix the launch
position similar to the SLBM situation. Furthermore, the U.S. Army also had the
urgent needs for precisely geodetic surveying by using navigation satellites. As a
result, a meeting of about dozen military officers from the Army, Navy, Marine Corps
and Air Force was held at the Pentagon by the U.S. Department of Defense (DoD)
to discuss the creation of a Defense Navigation Satellite System (DNSS) in May
1973, and a Joint Program Office (JPO) led by the then-Air Force Colonel Branford
Parkinson (1935−) was formed to execute the DNSS program. Later that year, the
DNSS program was named Navigation Satellite Time and Ranging (NAVSTAR),