Is Navigation Satellite System Still Relevant?

The GPS project was launched in the United States in 1973 to overcome the limitations of previous navigation systems,[12] integrating ideas from several predecessors, including classified engineering design studies from the 1960s. The U.S. Department of Defense developed the system, which originally used 24 satellites. It was initially developed for use by the United States military and became fully operational in 1995. Civilian use was allowed from the 1980s. Roger L. Easton of the Naval Research Laboratory, Ivan A. Getting of The Aerospace Corporation, and Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it.
The design of GPS is based partly on similar ground-based radio-navigation systems, such as LORAN and the Decca Navigator, developed in the early 1940s.

When the Soviet Union launched the first artificial satellite (Sputnik 1) in 1957, two American physicists, William Guier and George Weiffenbach, at Johns Hopkins University’s Applied Physics Laboratory (APL) decided to monitor its radio transmissions. Within hours they realized that, because of the Doppler effect, they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their UNIVAC to do the heavy calculations required. Early the next year, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem — pinpointing the user’s location, given that of the satellite. (At the time, the Navy was developing the submarine-launched Polaris missile, which required them to know the submarine’s location.)

All satellites broadcast at the same frequencies, encoding signals using unique code division multiple access (CDMA) so receivers can distinguish individual satellites from each other.

OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals. Then 2 SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at Kwajalein , Ascension Island , Diego Garcia , and Cape Canaveral ). These updates synchronize the atomic clocks on board the satellites to within a few nanoseconds of each other, and adjust the ephemeris of each satellite’s internal orbital model. In practice the receiver position (in three-dimensional Cartesian coordinates with origin at the Earth’s center) and the offset of the receiver clock relative to the GPS time are computed simultaneously, using the navigation equations to process the TOFs.

GPS satellites continuously transmit data about their current time and position. The GPS concept is based on time and the known position of GPS specialized satellites The satellites carry very stable atomic clocks that are synchronized with one another and with the ground clocks. On January 11, 2010, an update of ground control systems caused a software incompatibility with 8,000 to 10,000 military receivers manufactured by a division of Trimble Navigation Limited of Sunnyvale, Calif.

By December 1993, GPS achieved initial operational capability (IOC), indicating a full constellation (24 satellites) was available and providing the Standard Positioning Service (SPS). In 1972, the USAF Central Inertial Guidance Test Facility (Holloman AFB) conducted developmental flight tests of four prototype GPS receivers in a Y configuration over White Sands Missile Range , using ground-based pseudo-satellites. After that the National Space-Based Positioning, Navigation and Timing Executive Committee was established by presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the GPS and related systems.

As of early 2015, high-quality, FAA grade, Standard Positioning Service (SPS) GPS receivers provide horizontal accuracy of better than 3.5 meters, 37 although many factors such as receiver quality and atmospheric issues can affect this accuracy. A fourth ground-based station, at an undetermined position, could then use those signals to fix its location precisely. 24 The SECOR system included three ground-based transmitters from known locations that would send signals to the satellite transponder in orbit.

There were wide needs for accurate navigation in military and civilian sectors, almost none of those was seen as justification for the billions of dollars it would cost in research, development, deployment, and operation for a constellation of navigation satellites. However, closer to earth (<1000km radius) the amount of stuff in space is significantly higher than further out and therefore orbits decay on time scales noticable to us. As someone else mentioned geosynch orbits are not particularly special, except that the angular speed of the satellite happens to match the angular speed of the earth and the satellite appears to sit still above a particular spot on earth. The time difference tells the GPS receiver how far away the satellite is. With distance measurements from a few more satellites, the receiver can determine the user’s position.

DOD is required by law to “maintain a Standard Positioning Service (SPS) (as defined in the Federal Radionavigation Plan and the Standard Positioning Service Signal Specification) that will be available on a continuous, worldwide basis,” and, “develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses.” These strict requirements and current augmentation systems should actually make DOD use of the system transparent to the civil user. Each GPS satellite transmits an accurate position and time signal. The baseline satellite constellation consists of 24 satellites positioned in six earth-centered orbital planes with four operation satellites and a spare satellite slot in each orbital plane.

But if the receiver can access more satellite signals, it will calculate a more accurate position (Tinambunan). The atomic clocks in the satellites are extremely accurate, but the clocks in the GPS receivers are not, which creates a timing error. Although a given satellite receiver is typically designed to use only one of the global systems, there’s no reason why it can’t use signals from two or more at once.

L1 carries the civilian SPS code signal (also known as the C/A code or Coarse Acquisition code), which is relatively short and broadcast about 1000 times a second, and what’s known as the navigation data message, which includes the date and time, satellite orbit details, and other essential data. Comparing the two frequencies allows military grade GPS receivers to calculate precise corrections for radio delays and distortions caused by transmission through the atmosphere, and that still gives military GPS an edge over civilian systems. According to the official website : “The accuracy of the GPS signal in space is actually the same for both the civilian GPS service (SPS) and the military GPS service (PPS).” In practice, while SPS signals are broadcast using only one frequency, PPS uses two.

For that reason, they developed two different “flavors” of GPS: a highly accurate military-grade, known as Precise Positioning Service (PPS), and a somewhat degraded civilian version called Standard Positioning Service (SPS). Radio signals beaming down to us from space satellites aren’t traveling through empty space but through Earth’s atmosphere, including the ionosphere (the upper region of Earth’s atmosphere, containing charged particles, which help radio waves to travel) and the troposphere (the turbulent, uncharged region of the atmosphere, where weather happens, which extends about 50km or 30 miles above Earth’s surface). The best-known satnav system, the Navstar Global Positioning System (GPS), uses about 24 active satellites (including backups).

In addition to heightened security, the United States military also has access to much more accurate positioning by using the Precise Positioning System (PPS). GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km (12,550 miles).

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Global Positioning System

The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Air Force. It is a global navigation satellite system that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. Obstacles such as mountains and buildings block the relatively weak GPS signals.

The GPS does not require the user to transmit any data, and it operates independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the GPS positioning information. The GPS provides critical positioning capabilities to military, civil, and commercial users around the world. The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver.

The GPS project was launched by the U.S. Department of Defense in 1973 for use by the United States military and became fully operational in 1995. It was allowed for civilian use in the 1980s. Advances in technology and new demands on the existing system have now led to efforts to modernize the GPS and implement the next generation of GPS Block IIIA satellites and Next Generation Operational Control System (OCX).

Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. In 2000, the U.S. Congress authorized the modernization effort, GPS III. During the 1990s, GPS quality was degraded by the United States government in a program called “Selective Availability”; this was discontinued in May 2000 by a law signed by President Bill Clinton.[6] New GPS receiver devices using the L5 frequency to begin release in 2018 are expected to have a much higher accuracy and pinpoint a device to within 30 centimeters or just under one foot.

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