What is GPS?

With the escalating prevalence of automobiles and the surging interest in aerospace information, the significance of the GPS positioning system has garnered considerable attention. GPS, short for Global Positioning System, represents a a global navigation satellite system technology (GNSS) breakthrough initiated by the U.S. Department of Defense. It entails measuring the distance between a satellite, whose location is precisely known, and the receiver utilized by the user. By amalgamating data from multiple satellites, the system calculates the precise location of the receiver. Distinguishing itself from alternative navigation satellite systems, GPS technology exhibits comprehensive, all-weather, and uninterrupted coverage, thus offering users worldwide cost-effective access to highly accurate navigation information.

Table of Contents

Origins of GPS Technology

In 1957, the Soviet Union achieved a significant milestone in space exploration by launching the first satellite capable of orbiting the Earth, known as Sputnik-1. This groundbreaking event led physicists to discover the Doppler effect, which opened up new possibilities and raised the question of whether satellite positions could be calculated based on frequency offsets.

Taking inspiration from this discovery, the United States’ Advanced Research Projects Agency (ARPA) initiated research and development in 1958, aiming to provide discontinuous two-dimensional navigation and positioning services for ships using radio navigation signals.

By 1964, the technology had matured and was officially established and implemented as the Transit system also referred to as the “Navy Satellite Navigation System” (NNSS). Although the positioning accuracy of this early system was limited, it served its purpose until it ceased operation in 1996.

Between 1973 and 1979, a series of four test satellites were successfully launched, accompanied by the development of space-borne atomic clocks, ground receivers, and a comprehensive ground tracking network.

From 1979 to 1984, an additional seven test satellites were launched, alongside the development of receivers tailored to various applications.

A significant milestone was achieved on February 4, 1989, with the successful launch of the first operational GPS satellite.

By June 1993, with the launch of the 24th satellite, the practical GPS network, consisting of 21 operational satellites and three backups, had been established.

In July 1995, full operational capability was achieved, marking a significant milestone in GPS Global Positioning System and replacing the NNSS system with enhanced capabilities.

System CharacteristicsNNSSGPS
Carrier Frequency (GHz)0.15 and 0.401.23 and 1.58
Satellite Altitude (km)Approximately 1000Approximately 20200
Number of Satellites5-624
Orbital Period (min)107718
Satellite Clock Accuracy10^-1110^-12

How the GPS System Works

As stated in the introductory section of the article, the GPS system utilizes data from multiple satellites to precisely determine the user’s location. However, achieving this objective is not a simple task; rather, it involves intricate calculations.

Pseudo Codes

The precise position of the satellite used for determining the user’s location can be obtained from the satellite ephemeris, which is based on the recorded time from the onboard clock. The distance between the user and the satellite is calculated by measuring the time it takes for the satellite signal to travel from transmission to reception. It is important to note that this distance, known as pseudo-range (PR), does not represent the actual distance between the user and the satellite. Instead, it accounts for factors such as atmospheric ionosphere, free space loss, and Earth’s curvature, which can cause interference with the signal. During normal operation, the GPS satellite continuously transmits navigation messages using pseudo-random codes, commonly referred to as pseudo-codes, composed of binary symbols (1s and 0s).

GPS satellite rotation
GPS satellite signal transmission method

In the realm of GPS systems, two primary types of pseudo-code levels are utilized: civilian C/A codes and military P(Y) codes. The C/A code serves as a low-precision signal employed for swift satellite signal search and acquisition. This code operates at a frequency of 1.023MHz, with a repetition period of one millisecond and a code interval of 1 microsecond. Consequently, each code interval represents an approximate distance of 300 meters.

On the other hand, the P code is designed to deliver high-precision GPS signals for applications requiring precise positioning and navigation. Operating at a frequency of 10.23MHz, this code possesses a repetition period of 266.4 days. The code spacing for P code is 0.1 microseconds, corresponding to a distance of approximately 30 meters.

Additionally, the Y code, derived from the P code, offers enhanced security performance. It is specifically tailored to meet stringent military requirements and is known for its superior security features when compared to other code types.

Navigation Message

The navigation message refers to a data package transmitted via satellite navigation system broadcasts. It encompasses crucial information, including satellite ephemeris, operational status, clock correction, ionospheric delay correction, atmospheric refraction correction, and more. This information is demodulated from the satellite signal and transmitted on the carrier frequency with a modulation rate of 50b/s.

Each main frame of the navigation message is comprised of 5 subframes, with each frame lasting 6 seconds. The first three frames consist of 10 characters and repeat every thirty seconds, updating hourly. The last two frames collectively contain 15,000 bits.

The contents of the navigation message primarily comprise the telemetry code, conversion code, first, second, and third data blocks. Of utmost importance is the ephemeris data. When the user receives the navigation message, they extract the satellite time and compare it with their own clock to determine the distance between the satellite and the user. By utilizing the satellite ephemeris data within the navigation message, the user can calculate the position of the satellite at the time of message transmission. Consequently, the user’s position, velocity, and other pertinent information in the WGS-84 geodetic coordinate system can be determined.

Coordinates

The satellite component of the GPS navigation system plays a pivotal role in the continuous transmission of navigation messages. Nevertheless, due to the inherent challenge of synchronizing the user’s receiver clock with the on-board clock of the satellite, an additional unknown variable must be introduced. This variable, denoted as Δt, represents the time difference between the satellite and the receiver.

In order to determine the receiver’s three-dimensional coordinates (x, y, z), along with the time difference Δt, a system of four equations is employed to solve these four unknowns. Consequently, it is imperative to receive signals from a minimum of four satellites to accurately ascertain the receiver’s location.

Receiver

The GPS receiver undertakes the decoding of the received satellite signal or employs alternative techniques to extract the modulated information from the carrier. Subsequently, the carrier can be restored. Strictly speaking, the carrier phase should be referred to as the carrier beat phase. This phase signifies the disparity between the carrier phase of the received satellite signal, influenced by the Doppler frequency shift, and the phase of the signal generated by the local oscillation of the receiver.

Typically, the carrier beat phase is measured at the epoch time established by the receiver clock. The phase change value can be recorded by continuously tracking the satellite signal. However, certain variables remain unknown at the onset of observation. These include the initial phase value of the receiver and the satellite oscillator, as well as the integer ambiguity associated with the first epoch. The integer ambiguity represents an undetermined parameter, necessitating resolution during the data processing phase.

Phase observations offer exceptional accuracy, reaching the millimeter level. However, this high precision is contingent upon resolving the ambiguity encompassing the entire phase circle. As a result, phase observation values find utility in scenarios involving relative positioning and continuous observation values. Achieving positioning accuracy better than the meter level is only feasible when employing phase observations.

Types of Global Navigation Satellite System

gnss

GPS is one of the most widely used satellite navigation systems, there are several other global positioning systems in the world:

GLONASS

The satellite navigation system developed and operated by Russia, known as GLONASS, bears striking similarities to GPS. However, notable distinctions exist in terms of satellite quantity, satellite arrangement, and data transmission methodology.

GLONASS employs a satellite constellation consisting of satellites placed in three orbital planes at an inclination angle of approximately 64.8° to the equator. These satellites revolve asynchronously with a period of 11 hours, 15 minutes, and 44 seconds. This configuration results in a stable orbital system that requires minimal correction to maintain the spacecraft’s trajectory. Even if a few satellites deviate from their intended orbits, it does not impact the availability of navigation services within the territory of the Russian Federation. In contrast, GPS satellites are arranged in six orbital planes, inclined at around 55°. These satellites rotate synchronously and necessitate ground correction stations to ensure accurate geolocation.

Regarding data transmission, GLONASS utilizes the Frequency Division Multiple Access (FDMA) protocol. This protocol offers heightened confidentiality but consumes relatively more energy. On the other hand, GPS utilizes the Code Division Multiple Access (CDMA) method, which is more economical but provides lower security compared to FDMA.

BDS

Development of the Beidou system

The Beidou navigation system, developed and operated by the China National Space Administration, encompasses a constellation of satellites for global positioning. It comprises 5 geostationary orbit satellites and 30 medium to high orbit satellites. Notably, the Beidou system utilizes a mixed constellation, combining satellites in three orbital planes, which confers enhanced anti-occlusion capabilities, particularly in low-latitude regions, when compared to other technologies that rely solely on high-orbit satellites.

The Beidou system offers navigation signals across multiple frequency points, enabling improved service accuracy through the utilization of multi-frequency signals. This feature enhances positioning precision and reliability, catering to evolving user demands.

Recognizing the evolving landscape, the China Satellite Navigation Commission and the Russian Space Agency are actively fostering collaboration to ensure complementarity between the Beidou and GLONASS systems. This collaborative effort aims to provide higher precision and more stable navigation services globally in the near future.

Galileo

Galileo

In response to concerns regarding the safety and reliability of the global positioning system, the Galileo satellite navigation system was collaboratively developed by the European Space Agency and the European Union. Since its official launch in 2002, Galileo has joined the ranks of the world’s major satellite navigation systems alongside the US GPS, Russia’s GLONASS, and China’s Beidou system.

This system encompasses two ground control centers and a constellation of 30 satellites, with 27 operational satellites and 3 spares. With a positioning and navigation accuracy of 10 to 15 meters, Galileo offers precise location-based services. However, it should be noted that currently, only four satellites are operational, which means that the system is unable to provide continuous global positioning and navigation services around the clock. Further deployment of satellites is necessary to achieve this goal.

Global Positioning System Applications

In the realm of urban management, GPS technology is still in its nascent stage. Governments envision leveraging this precise positioning technology to enhance resident management. For instance, equipping vehicles responsible for clearing and transporting construction waste with GPS systems allows authorities to monitor the final destination of such waste. Similarly, during urban planning, GPS aids in accurate land and area assessment. Notably, some regions like Spain, Belgium, and the United States have begun utilizing GPS positioning watches as an alternative to conventional correctional facilities.

GPS for urban planning

In terms of civilian applications, GPS technology has long been adeptly employed. Apart from conventional vehicle navigation, enables remote monitoring and care for the elderly and children. When the designated location is breached, alarms are triggered, and the lost individuals’ locations can be tracked via mobile phones. Additionally, for avid travelers, GPS serves as a crucial lifeline, facilitating swift rescue operations by support teams.

Of course, the military sector is where GPS truly shines, as it was initially developed to serve armed forces. Within military operations, autonomous navigation capabilities provided by the global positioning system enhance combat troop mobility. Moreover, the system’s specialized surveillance mechanisms enable command and monitoring of moving targets within theaters of operation. Most critically, GPS ensures precise positioning for guided weapons, thus evaluating target hit rates with exceptional accuracy.

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