October 4, 1957 became an important incentive for the United States - after the launch of the first artificial Earth satellite in the USSR, American engineers decided to adapt space to fulfill navigation needs (with practicality typical of the Yankees). At the Applied Physics Laboratory (APL) of Johns Hopkins University, collaborators WG Guyer and J. C. Wiffenbach studied the radio signal from the Soviet Sputnik 1 and drew attention to the strong Doppler frequency shift of the signal emitted by a passing satellite. When our firstborn in space approached, the frequency of the signal increased, and the receding one emitted radio signals of decreasing frequency. The researchers managed to develop a computer program to determine the parameters of the orbit of a passing object from its radio signal in one pass. Naturally, the opposite principle is also possible - the calculation of the already known parameters of the orbit using the same frequency shift of the unknown coordinates of the ground radio receiver. This idea came to the head of APL employee F. T. McClure, and he, together with the director of the laboratory, Richard Kershner, put together a group of researchers to work on a project called Transit.
Richard Kershner (left) is one of the founding fathers of the American Global Positioning System. Source: gpsworld.com
The nuclear submarine "George Washington" is the first user of the Transit system. Source: zonwar.ru
Operational orbits of the Transit constellation. Source: gpsworld.com
The main customer was the US Navy, which needed precision navigation tools for new submarines equipped with Polaris missiles. The need to accurately determine the location of submarines such as "George Washington" was extremely necessary for the then novelty - the launch of missiles with nuclear warheads from anywhere in the oceans.
Transit receiving equipment for submarines. Source: timeandnavigation.si.edu
By 1958, the Americans were able to present the first experimental prototype of the Transit satellite, and on September 17, 1959, it was sent into space. The ground infrastructure was also created - by the time of launch, the complex of the consumer's navigation equipment, as well as ground tracking stations were ready.
Hopkins University engineers assembling and testing the Transit spacecraft. Source: timeandnavigation.si.edu
The Americans worked on a satellite navigation project in full afterburner mode: by 1959, they had constructed as many as five types of Transit satellites, which were later all launched and tested. In operating mode, American navigation began operating in December 1963, that is, in less than five years, it was possible to create a workable system with good accuracy for its time - the root-mean-square error (RMS) for a stationary object was 60 m.
Satellite Transit 5A 1970 model. Source: timeandnavigation.si.edu
A Transit receiver installed in a car used by Smithsonian geologist Ted Maxwell in the Egyptian desert in 1987. The researcher's workhorse turned out to be …
… Soviet "Niva"! Source: gpsworld.com [/center]
Determining the coordinates of a submarine moving on the surface was more problematic: if you make a mistake with the speed value by 0.5 km / h, then the RMS will increase to 500 m. Therefore, it was more expedient to turn to the satellite for help in a stationary position of the vessel, which again was not easy. The low-orbit (1100 km altitude) Transit was adopted by the US Navy in the middle of 64, as part of four satellites, further increasing the orbital grouping to seven vehicles, and from 67, navigation became available to mere mortals. At the moment, the Transit satellite constellation is used to study the ionosphere. The disadvantages of the world's first satellite navigation system were the inability to determine the height of the position of the ground user, the considerable duration of the observation and the accuracy of the object's positioning, which eventually became insufficient. All this led to new searches in the US space industry.
Spacecraft Timation. Source: timeandnavigation.si.edu
The second satellite navigation system was Timation from the Naval Research Laboratory (NRL), which was run by Roger Easton. Within the framework of the project, two satellites were assembled, equipped with ultra-precise clocks for broadcasting time signals to terrestrial consumers and accurately determining their own location.
Experimental satellite Timation NTS-3, equipped with a rubidium clock. Source: gpsworld.com
At Timation, the basic principle of the future GPS systems was formulated: a transmitter worked on the satellite, emitting a coded signal, which recorded the ground subscriber and measured the delay of its passage. Knowing the exact location of the satellite in orbit, the equipment easily calculated the distance to it and, based on this data, determined its own coordinates (ephemeris). Of course, this requires at least three satellites, and preferably four. The first Timations went into space in 1967 and carried quartz clocks at the beginning, and later ultra-precise atomic clocks - rubidium and cesium.
The United States Air Force operated independently of the Navy on its own global positioning system called the Air Force 621B. Three-dimensionality has become an important innovation of this technique - now it is possible to determine the latitude, longitude and long-awaited height of the object. The satellite signals were separated according to a new coding principle based on a pseudo-random noise-like signal. The pseudo-random code increases the noise immunity of the signal and solves the issue of restricting access. Civilian users of navigation equipment have access only to open source code, which can be modified from the ground control center at any time. In this case, all "peaceful" equipment will fail, defining its own coordinates with a significant error. Military locked codes will remain unchanged.
The tests began in 1972 at the New Mexico test site, using transmitters on balloons and airplanes as satellite simulators. "System 612B" showed outstanding positioning accuracy of several meters and it was at that time that the concept of a medium-orbit global navigation system with 16 satellites was born. In this version, a cluster of four satellites (this number is necessary for accurate navigation) provided 24-hour coverage of the entire continent. For a couple of years "System 612B" was in the experimental rank and was not particularly interested in the Pentagon. At the same time, several offices in the United States were working on a "hot" navigation topic: the Applied Physics Laboratory was working on a modification of the Transit, the Navy was "finishing" Timation, and even the ground forces offered their own SECOR (Sequential Correlation of Range, sequential calculation of ranges). This could not but worry the Ministry of Defense, which was at risk of facing unique navigation formats in each type of troops. At a certain moment, one of the American warriors slammed his hand on the table and a GPS was born, which absorbed all the best of its predecessors. In the mid-70s, under the auspices of the US Department of Defense, a tripartite joint committee called NAVSEG (Navigation Satellite Executive Group) was created, which determined the important parameters of the future system - the number of satellites, their heights, signal codes and modulation methods. When they came to the cost figure, they decided to immediately create two options - military and commercial with a predetermined error in positioning accuracy. The Air Force played a leading role in this program, as its Air Force 621B was the most sophisticated model of the future navigation system, from which GPS borrowed practically unchanged pseudo-random noise technology. The signal synchronization system was taken from the Timtation project, but the orbit was raised to 20 thousand kilometers, which provided a 12-hour orbital period instead of the 8-hour one of its predecessor. An experienced satellite was launched into space already in 1978 and, as usual, all the necessary ground infrastructure was prepared in advance - only seven types of receiving equipment were invented. In 1995, GPS was deployed in full - about 30 satellites are constantly in orbit, despite the fact that for operation there are enough 24. Orbital planes for satellites are allocated six, with an inclination of 550… At the moment, GPS surveying applications allow you to determine the position of the consumer with an accuracy of less than one millimeter! Since 1996, Block 2R satellites have appeared, equipped with the AutoNav autonomous navigation system, which allows the vehicle to operate in orbit when the ground control station is destroyed for at least 180 days.
Until the late 1980s, the combat use of GPS was sporadic and insignificant: determining the coordinates of minefields in the Persian Gulf and eliminating imperfections in maps during the invasion of Panama. A full-fledged baptism of fire happened in the Persian Gulf in 1990-1991 during Desert Storm. The troops were able to actively maneuver in a desert area, where it is difficult to find acceptable landmarks, as well as to conduct artillery fire with high accuracy at any time of the day in conditions of sandstorms. Later, GPS proved useful in the peacekeeping operation in Somalia in 1993, in the American landing in Haiti in 1994, and, finally, in the Afghan and Iraqi campaigns of the 21st century.
