U.S. GPS satellite, orbiting earth at a distance of 12,550 miles (5% of the distance to the moon). Solar panels provide power. Satellite also contains rocket motors to enable fine repositioning, and nuclear detonation detection "listening" devices so the U.S. military can immediately and precisely locate a nuclear detonation anywhere on earth; useful for test-ban treaty compliance.
Trajectories of the 31 active U.S. GPS satellites circling the earth, twice every 24 hours. At least 6 satellites are visible at any time from any position on the earth's surface. For example, at the head of the arrow 9 satellites are visible. Only 3 visible satellites are theoretically required to determine location, so the additional 6 visible satellites creates a strong redundancy and improved accuracy.
Global Positioning Systems (GPS) have been a familiar consumer technology for the past decade, but the ingenuity and great complexity of GPS is often belied by its intuitively simple principles. This article describes the principles of how GPS in the U.S. has evolved from the early OMEGA Navigation System in the 1970s, into the highly sophisticated 34-satellite GPS system launched in 1989. Current International GPS functionality is based on a combination of U.S. and Russian federation satellites, that results in a potential localization accuracy of approximately 6 meters (about 20 feet). For military applications such as missile guidance, far greater accuracy is obtainable. Although the most frequent use of GPS is in the consumer market, GPS technology, since 2009, has been in extensive use by the U.S. military.
In the 1970s, the Omega Navigation System developed by the U.S. became the progenitor of today’s GPS technology. Omega was based on receivers comparing the time-of-arrival of signal transmissions from pairs of ground-based transmitting stations and calculating the receiver’s (for example, an aircraft) position by the triangulation principle. Omega became the first worldwide radio navigation system; but, as its technology evolved into three-dimensions using space-based satellites, its access became restricted to military use.
The first U.S. GPS satellite was launched in 1989, and the 24th–and last–in 1994. Initially, the civilian sector did have limited access to 3D GPS technology, but the signals available were intentionally degraded by the defense department to restrict the use of the fully accurate GPS system for weapons guidance and other military applications by unfriendly powers.
In 1996, recognizing the importance of GPS for civilian air navigation, President Clinton issued a policy directive declaring GPS to be a “dual-use system” and established an Interagency GPS Executive Board to manage it as a national asset.
With the removal of the intentional degradation of the GPS signals, the accuracy of civilian-accessible GPS improved from about 300 feet to about 60 feet. In 1998, with aviation civil navigation in mind, Vice President Gore announced plans to upgrade the GPS system with two additional civilian channels for enhanced accuracy and reliability, and in 2000 the U.S. Congress authorized and funded the effort.
Current Status of GPS
The upgraded U.S. GPS system, consisting of 24 orbiting satellites, is divided into six orbital planes with four satellites in each plane. An orbital plane is like a circular disk, where the edge of the disk defines the path of the satellite. The centers of all these orbital planes coincide with the center of the Earth (as shown in the 2nd picture above at a moment in time when 9 satellites are visible at the point on the earth's surface shown by the head of the arrow). The orbits were designed so that at least six satellites would always be visible within line-of-sight from anywhere on the earth’s surface. Since only three satellites are theoretically needed to establish a receiver’s three-dimensional location, this provided a lot of redundancy to compensate for transmissions blocked by building structures, malfunctioning satellites, etc. Orbiting at an altitude of 12,550 miles (about 5% of the distance from the earth to the moon), each satellite makes two complete orbits every 24 hours, covering the same projected track on the ground every day. From spherical geometry, this means that 4 satellites are visible from each location on the earth’s surface for a few hours a day. This makes positioning a much faster process. As of March 2008, there have been in fact 33 GPS satellites in orbit--31 active and two in retirement that are maintained as spares. In this new arrangement, about ten rather than six satellites are visible from any location on the ground at any moment in time, providing additional redundancy and increased accuracy of the GPS system.
The flight-paths of GPS satellites are tracked by dedicated U.S. Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs, Colorado, and Cape Canaveral, along with shared monitoring stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington DC.
Each satellite is contacted at regular intervals with orbital and software updates using ground antennas. These updates synchronize atomic clocks on board the satellites and correct the satellites’ orbits when necessary. Atomic clocks, the most accurate timing technology in existence, provide the very high timing accuracy needed to synchronize the transmission of GPS radio signals from different satellites and to account for the so-called “Doppler shift,” a discrepancy in timing that occurs when a satellite transmits its signal while moving toward or away from the receiver (the Doppler shift is the familiar change in pitch of ambulance sirens as they move toward and away from a listener). Additionally, changes in atmospheric conditions and air humidity affect the velocity of the GPS signals as they pass through the earth’s atmosphere. Differences in receiver altitude introduce further discrepancies in timing due to the signals passing through less thickness of atmosphere at higher receiver elevations, but this effect is more relevant to aircraft navigation systems than to land-based navigation systems.
GPS localization accuracy can also be affected when the radio signals reflect off surrounding buildings, canyon walls, hard ground, etc., due to reflective phase-shifts in the radio frequency signals. Finally, the GPS satellites’ atomic clocks also suffer errors due to Einstein’s general relativity principle. When different satellites viewed by a GPS receiver are moving relative to the GPS receiver at different speeds and/or experiencing different gravitational forces by virtue of different altitudes, their clock speeds will slightly vary. With all the above corrections carefully accounted for, the accuracy of civilian GPS systems today is about 6 meters (about 20 ft.), although for strictly military uses such as missile guidance, the accuracy is substantially improved.
Inclusion of WiFi Transmissions in GPS Location
More recently, in addition to receiving GPS signals from orbital satellites, some GPS receivers can also detect WiFi signals originating in their close vicinity. The locational origins of these WiFi transmissions are determined by referral to a publicly accessible WiFi database, and by the process of triangulation, the WiFi-determined location of the GPS receiver is found. This is a better system for positioning when the receiver happens to be indoors, in a tunnel, among very tall buildings, etc., since GPS signals experience degradation when passing through masonry or concrete.
Russian Federation’s GLONAS GPS System
In 1995, the Russian Federation’s satellite navigation system became operational. The system had the acronym GLONAS, which stands for Globalnaya Nvigatsionnaya Sputnikovaya Sistema. Loosely translated into English, this means Global Navigation Satellite System. GLONAS was initially operated by the Russian Federal Space Agency. As with the declassification of the USA’s GPS system, President Vladimir Putin in 2007 ordered all military restrictions to be removed from the GLONOS system so that it could be used by the civilian sector as well as by the military. At the present time, the accuracy of the Russian Federation’s GLONOS system is about 7-8 meters (20-25 feet), similar to the current fully unrestricted civilian accuracy of the US GPS system.
Sweden’s SWEPOS GPS System
In 2011, Sweden’s SWEPOS network of satellite reference stations, which provide data for real-time positioning, became the first known foreign company to use the Russian Federation’s GLONAS system. In Sweden, the accuracy of GLONOS as integrated to the SWEPOS system is about 1 meter (about 3 feet).
An obvious question is how accurately do GLONAS and the U.S. GPS agree with each other. Interestingly enough they don’t! This is because as an absolute origin of its coordinate system, GLONOS uses the North Pole’s global position as measured in 1990 by satellite interferometry, while the U.S. uses the North Pole’s global position as measured in 1984. These two measurements differ by approximately 40 cm (1-1.5 feet).
It is interesting to note that as part of its “Maps” application, Apple iPhones and iPads use both the U.S. GPS and the GLONAS satellites for location.
Military Uses of GPS
The list shown below shows the many applications of GPS by the U.S. military.
Navigation: Soldiers use GPS to find objectives, even in the dark or in unfamiliar territory, and to coordinate troop and supply movement. Commander ranks use the Commanders Digital Assistant, while lower ranks use the Soldier Digital Assistant.
Target tracking: Various weapons used by the U.S. military use GPS to track possible ground and air targets before flagging them as hostile. These weapon systems pass target coordinates to precision-guided munitions to allow them to engage targets accurately. Military aircraft, particularly in air-to-ground roles, also use GPS to locate targets.
Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles, precision-guided munitions and artillery projectiles. Embedded GPS receivers, able to withstand accelerations of up to 12,000g have been developed for use in 155-millimeter howitzers.
Reconnaissance: Patrol movement can be managed more closely using GPS.
Nuclear detonation location: U.S. deployed GPS satellites each carry a set of nuclear detonation detectors, consisting of an optical sensor, an x-ray sensor, a dosimeter, and an electromagnetic pulse sensor, that form a major portion of the United States Nuclear Detonation Detection System.
As mentioned earlier, the implementation of GPS technology by the U.S. military, although utilizing the same satellites as those used for civilian GPS, process the satellites' signals differently, resulting in about a factor of 10-50 times greater precision.
Global Positioning Systems have been a familiar consumer technology for the past decade or more. The U.S. GPS system evolved from the early OMEGA Navigation System of the 1970s into the highly sophisticated 34-satellite GPS system launched in 1989. Following the declassification of the U.S. and Russian Federation’s GPS systems by President Clinton and President Putin, and the increased technological collaboration between these two countries, the current International civilian GPS functionality results in potential localization accuracy of approximately 6 meters (about 20 feet). The civilian Swedish SWEPOS system of ground-based GPS receivers, integrated with the Russian Federation’s GLONAS system, produces an impressive localization accuracy of about 40 cm (or about 1-1.5 feet). Military uses of GPS have existed since 2009, and include smart-bomb, howitzer shell, and guided missile guidance, target tracking, and navigation by soldiers in unfamiliar territory or at night. The military GPS receivers process the GPS satellite signals differently than those used by civilians and are able to improve the accuracy of GPS navigation by factors of 10-50 times.
THE JOINT COMPREHENSIVE PLAN OF ACTION (JCPOA) NUCLEAR AGREEMENT WITH IRAN: WHAT iran and the U.S. have AGREED UPON AND THE SCIENCE BEHIND IT--as it was in FEBRUARY 2015
Gas centrifuge "farm" for separating uranium-238 from uranium-235. The vertical tubes are the centrifuges, having internal elements that spin very fast along the vertical axis. Uranium Hexafluoride gas (with natural uranium) is passed into the centrifuges. Uranium-235 and uranium-238 are then separated by virtue of the slightly different centripetal forces acting on the two slightly different nuclear masses. Uranium-238 becomes concentrated at a larger radius within the centrifuge than uranium-235. The two uranium isotopes are then separately extracted from the centrifuges.
Posted on April 3, 2013 by Dr Simple Science
Iran has always claimed that it is enriching uranium as a necessary step toward providing various civilian services, such as radioisotopes for nuclear medicine, a civilian nuclear power program, and a civilian nuclear research program. However, this has clashed with widespread international belief that Iran’s claims are simply a cover for a much more nefarious goal, that of joining the Nuclear Club of nations possessing nuclear weapons–-without being invited, and, more specifically, for "wiping Israel off the face of the earth".
The recently agreed upon Joint Comprehensive Plan of Action (JCPOA) between Iran and the U.S., together with five of the other principal nuclear nations, has caused a kerfuffle in the U.S. Congress and strong condemnation by Israel. Israel's Prime Minister, Bibi Netanyahu, is highly skeptical about the value of the JCPOA agreement and still insists that the U.N. “draw a red line” beyond which Iran’s nuclear development should not be tolerated by the international community and, if violated, might result in preemptive strikes by Israel against Iran's uranium enrichment and plutonium conversion facilities--as has already occurred on two previous occasions. Mr. Netanyahu has also warned that as early as this summer, despite the JCPOA agreement, Iran’s uranium enrichment and nuclear warhead fabrication facilities are expected to be moved to dispersed underground locations, making it far more difficult, if not impossible, to achieve successful verification, therefore rendering the JCPOA agreement largely ineffective.
Since uranium enrichment by Iran and its progress towards nuclear weapon acquisition has produced a substantial amount of public fear in past years, I will try to clarify some of the scientific facts behind these issues.
Summary of the JCPOA Agreement Between the U.S. and Allied Nations and Iran
* The primary uranium-235 enrichment site in Iran is Natanz (see map above). Under the JCPOA agreement, Natanz will be permitted to operate 5,060 uranium enrichment centrifuges. This is 25% of Iran's 20,000 currently operating centrifuges and will constitute older, much less efficient models that will be far slower in enriching uranium than the current ones.
* Iran's uranium stockpile of uranium will be reduced by 98% to 300kg (660lbs) for 15 years, and will not be allowed to exceed an enrichment level of 3.67% (see below for further discussion of enrichment).
* The Arak reactor (see map above), which according to its original design would have been a source of fissile plutonium-239 for manufacturing at least one nuclear weapon per year, will be transformed to produce far less plutonium than before and of a poorer quality. Fundamentally, this would limit plutonium-239 production and make it virtually impossible to fabricate any plutonium-239 based nuclear weapons.
* All spent fuel from the Arak reactor that could potentially be reprocessed to recover plutonium-239 will be sent out of the country under a rigorous IAEA inspection protocol. In fact, Iran will ship out all spent fuel from all of its power and research reactors, preventing the accumulation of any spent fuel from which plutonium-239 could be extracted, and will not engage in any activity associated with the reprocessing of spent fuel to obtain plutonium-239, even for research purposes.
U.S. and the 5 Allied Nations' Obligations
* On the part of the U.S. and the five other allied nations in the JCPOA agreement, the commitment given to Iran in response to Iran's acceptance of the above conditions is to end the severe sanctions that were imposed on Iran in 2010. Quoting a memorandum from the U.S. State Department, "These sanctions were designed: (1) to block the transfer of weapons, components, technology, and dual-use items to Iran’s prohibited nuclear and missile programs; (2) to target select sectors of the Iranian economy relevant to its proliferation activities; and (3) to induce Iran to engage constructively, through discussions with the United States, China, France, Germany, the United Kingdom, and Russia in the “E3+3 process,” to fulfill its nonproliferation obligations".
Specific Technology of Uranium and Plutonium Processing for Nuclear Reactor Fuel and Nuclear Weapons Manufacture
Uranium mined from the ground contains various uranium isotopes, including uranium-235, the uranium isotope needed for the manufacture of fission nuclear weapons and nuclear reactor fuel. The "raw" uranium is first combined with hydrofluoric acid, which reacts with the uranium to produce uranium hexafluoride gas. The uranium hexafluoride gas is fed into centrifuges that spin the gas at extremely high speed (see the second picture above). By centripetal force, the heaviest uranium isotope, uranium-238, is forced to the outer wall of the centrifuge where it is extracted, while uranium-235 remains along the central axis of the centrifuge where it is separately extracted. However, this approach to uranium-235 separation is notoriously slow, so to offset this problem a very large number of very large advanced design centrifuges operating simultaneously is required (see third picture above). Limiting the number and efficiency of Iran's centrifuges is the first step in the JCPOA agreement in preventing Iran from enriching uranium to the level where fission nuclear weapons could be manufactured. The next step is limiting the enrichment level of Iran's entire stockpile of uranium to prevent its further enrichment to weapons grade uranium. The final step is to modify the cores of those nuclear reactors which are capable of rapidly producing high quality plutonium-239, which can also potentially be manufactured into fission nuclear weapons, and sending out of the country all spent reactor fuel rods to prevent Iran from extracting plutonium-239 from the burned fuel.
Principles of Uranium Enrichment and Uranium-235 Fission Rates for Nuclear Reactors vs. Fission Weapons
"Raw" or “natural” uranium, as it comes out of the ground, consists of about 0.7% uranium-235 and about 99% uranium-238 (there are a few unimportant additional isotopes of uranium present that account for the remaining 0.3%). Although the uranium-235 and uranium-238 uranium isotopes are chemically identical, uranium-235 is "fissile", whereas uranium-238 is not. Uranium-235 is the only naturally occurring fissile isotope that is suited for the manufacture of nuclear weapons and nuclear reactor fuel.
A fissile isotope is one whose nucleus can be induced to break apart, or “fission”, following bombardment by nuclear particles called neutrons. In the case of uranium-235, the nucleus absorbs a neutron that makes it unstable and causes it to break apart. In doing so, it emits 2-3 outgoing neutrons, accompanied by a tremendous amount of energy, mainly as heat and light, but also as ionizing radiation (the kind of radiation that is potentially harmful to humans). Each of the 2-3 outgoing neutrons can now be considered as 2-3 incoming neutrons in the next uranium-235 nuclear interactions, that once again each produce 2-3 more uranium-253 fissions with the emission of 2-3 more outgoing neutrons and more energy. Therefore, the uranium-235 fissions-rate grows "exponentially" with time and, if not controlled, continues until all uranium-235 has been used up.
Uranium-235 for Nuclear Reactor Fuel
Since for each uranium-235 nucleus to fission requires one neutron to be absorbed—i.e., removed from the neutron population--each fission event increases the neutron population by 1-2 neutrons (2-3 neutrons produced in each fission, minus the one neutron that is absorbed to initiate each fission). If each fission event on average absorbed one neutron and emitted only one neutron that could initiate the next fission, the neutron population with time would remain roughly constant; this is the safe, “steady-state” condition under which nuclear reactors typically operate and produce energy. But to ensure that steady-state condition, one of the outgoing neutrons, on average, in each fission event must be "blocked" from causing further fissions. This blocking to some extent occurs naturally, due to the presence of uranium-238, which can absorb neutrons but does not undergo fission. But it is also achieved in a more controlled way using "control rods", which can be inserted into the uranium fuel to various depths. Control rods contain the isotope boron-10, which very strongly absorbs neutrons. Therefore, the fission rate in the uranium fuel is "tuned" by the precise positioning of the control rods in the core to maintain a safe and constant fission rate. If, on the other hand, the fission-rate were to increase exponentially in a nuclear reactor’s core, dangerous overheating could result, with the further possibility of more serious consequences such loss of core coolant followed by core meltdown. This is what happened in the Japanese Fukushima daiichi reactor accidents in 2011.
Uranium-235 for Nuclear Fission Weapons
In contrast, to manufacture uranium-235 fission weapons, we want the maximum amount of fission energy produced in the shortest possible time; which means we need to let the fission rate rapidly increase unchecked until all the uranium-235 has been used up. This means that following "triggering" of the fission process, the uranium-235 fission-rate would increase at a higher and higher rate, i.e., "exponentially". In a nuclear fission weapon, the lack of control rods is insufficient to ensure that the growth in fission-rate is as rapid as possible. But because, as already mentioned, natural uranium is mainly composed of neutron-absorbing--but non-fissile--uranium-238, potential problems are encountered in trying to utilize uranium fission for nuclear weapons. The large amount of uranium-238 significantly “eats up” the fission neutrons produced by the uranium-235, down-regulating the uranium-235 fission-rate just as control rods do in a nuclear reactor core. So the presence of uranium-238 is highly undesirable if the uranium is to be used for a nuclear weapon where the fission-rate must increase as rapidly as possible. Therefore, for the manufacture of nuclear weapons, the uranium-235 enrichment level is increased to at least 20%, but more commonly to 90% or higher, to minimize the deleterious (sometimes referred to as "poisoning") effect of uranium-238.
[For similar reasons, although raw or natural uranium (with a uranium-235 fraction of about 0.7%) could, in principle, be used as a cheap and very safe fuel for nuclear reactors, much better fuel-to-energy conversion efficiency is obtained with slightly higher enrichment levels of 0.9-2%.]
The Politics of Uranium Enrichment
From the perspective of the recent JCPOA agreement, if Iran were committed to maintaining its 660 lb stockpile of uranium at an enrichment level of 20% or lower--as it was permitted to do in a previous deal proposed by Russia and initially accepted by Iran about 10 years ago--there is a theoretical possibility that Iran still could manufacture nuclear fission weapons; but more realistically, it could use its outdated 5,060 permitted centrifuges to moderately elevate the enrichment of the stockpiled uranium to bring it into a range where nuclear fission weapons could more easily be manufactured. To prevent this from possibly occurring, as part of the JCPOA agreement Iran's stockpile of uranium is not permitted to exceed an enrichment factor of 3.67%; from which, as already mentioned, it would be impossible to manufacture nuclear fission weapons.
However, for the operation of nuclear reactors that can produce plutonium-239 from uranium-238 (plutonium-239 is an alternative fissile isotope suitable for nuclear fission weapon manufacture), a 3-4% uranium enrichment level is ideal, maximizing the efficiency and, therefore, the speed of uranium-to-plutonium conversion. That is why a component of the JCPOA agreement consists of disabling the ability of Iran's nuclear reactors from the production of plutonium-239.
Iran’s uranium enrichment issue, together with the “hair trigger” of Israel’s decision of possibly preempting an Iranian nuclear attack on Israel by a destruction of all of Iran’s nuclear capabilities, is probably the most dangerous military crisis the U.S. has faced since the Cuban missile affair. We can only hope that the recent diplomatic progress between the U.S. and Iran will lead to a nuclear stand-down between Iran and the Western powers and Israel to give the world another decade or two of nuclear peace.
Uranium-235 can be enriched to higher percentages that the 0.7% level in naturally occurring uranium. Up to about 20% enrichment, the uranium can only be used as fuel for nuclear fission reactors, but at enrichments levels greater than 20%, the possibility exists that the uranium can be used to manufacture nuclear fission weapons. Much lower uranium enrichment levels of 3-4%--permitted under the current JCPOA agreement--though precluding the direct manufacture of nuclear fission weapons, if used as nuclear reactor fuel could, in principle, lead to the rapid production of plutonium-239, a fissile isotope of plutonium from which nuclear fission weapons can be manufactured as they can from uranium-235. Under the JCPOA agreement "Export" of used nuclear fuel rods out of Iran should, in principle, prevent this from happening.
The concepts of 1) a steady-state fission-rate, maintained through the use of control rods in nuclear reactors, and 2) an exponentially increasing fission-rate, desirable in nuclear fission weapons, are both determined by the fate of the additional neutron produced with each uranium-235 fission, over and above the one outgoing one neutron that is necessary in a steady-state fission rate. With that "extra" neutron absorbed either by control rods or by the uranium-238 present in the uranium, a steady-state fission-rate is achieved, which is the operation mode of nuclear reactors. However, if that extra neutron is not removed and becomes available to cause further uranium-235 fissions, an exponentially increasing fission-rate results results which, if allowed to proceed unhindered, results in a nuclear fission explosion.
The 3.67% enrichment level of Iran's stockpile of uranium-235, permitted under the JCPOA agreement, although too low to enable nuclear fission weapon manufacturer (as well as being precluded from being further enriched by the restrictions on the number and type of the centrifuges Iran is permitted to retain) can, nevertheless, be manufactured into very efficient nuclear reactor fuel. Such efficiently fueled nuclear reactors can potentially rapidly produce plutonium-239, from which nuclear fission weapons can also be manufactured. However, one component of the JCPOA agreement consists of disabling the nuclear reactors that are capable of the rapid production of high-quality plutonium-239, and in addition requiring the export of all burnt up fuel to prevent the covert extracting of plutonium-239 from the uranium fuel residue.
Many countries, in particular Israel, consider the JCPOA agreement much too favorable to Iran, with too many loopholes permitting Iran's continued development of nuclear weapons; but, as president Obama has stated, "It was either this deal or no deal". The world can only hope and pray (if that is our disposition) that this agreement will be honored by both sides, which would greatly alleviate the international nuclear tensions that presently exist.
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