Abstract
The Global Positioning System is a navigation system designed for military use, but subsequently made freely available. It has since been utilized in a wide variety of ways including science, transportation, surveying, financial services, and recreational use. However, the changing and growing demand quickly make it obsolete, and some ways have been proposed to update the system, varying ineffectiveness, complexity, time restrictions, and cost.
Introduction
Global Positioning System (GPS) is a system comprised of satellites capable of broadcasting certain signals designed to provide precise time and location and used primarily for navigation in both the private and the military sectors. It also shows promising perspectives in the fields of robotics, unmanned vehicle navigation, cartography, emergency services, and a wide variety of civil recreational use. However, since its launch in 1972 both the changes in demand and technological progress have established the need for updates, as the GPS is prone to a variety of errors due to its fundamental principles of operation. This paper aims at assessing the range of flaws incorporated into the system, reviewing the current state of updating the system, and outlining the possible directions for further improvements.
The reason for the scope of the paper is the success the system has had so far, the possibilities it offers in a multitude of fields and the impressive list of achievements already made possible by the implementation of GPS, which is properly credited with several awards and can not be dismissed easily. The eradication of errors and flaws characteristic for the current state of the system may not only increase performance in already established fields of use but add new possibilities.
Overview of the Technology
A Brief History of the System
The Global Positioning System was conceived in the 1970s and was essentially a continuation of the earlier works of a similar kind. Its two primary concepts – the constellation of satellites orbiting the Earth to provide constant exposure and the analysis of radio signals from several sources for navigation purposes were borrowed from the TRANSIT military satellite navigation system and the ground-based OMEGA radio navigation systems, respectively [1]. Both systems were fully operational by 1973 and were proven successful. The GPS was initially intended for military use, with its navigational capabilities directed towards operating intercontinental ballistic missiles, strategic bombers, and submarines carrying a nuclear arsenal. The tension between the USA and the Soviet Union helped to accept the budget of the project, which comprised several billion dollars.
However, the system was later given a dual-use status, meaning that it would be available for the civilian as well as military use. The cause most commonly cited is the incident that took place in 1983, when the stray civilian airliner has unintentionally breached the Soviet airspace and was shot down by an interceptor, resulting in 269 deaths. This illustrated the dire need for a readily available navigation system and prompted Ronald Reagan to issue a directive to make GPS publicly accessible as soon as it becomes operational. However, the technology became available that same year, while the full operational capability was announced only in 1995. The dual-use policy meant it was of limited availability to the civilian population while fully accessible to the military. The situation has changed in 2000, when, as a result of a 1996 policy directive issued by Clinton, the limitation was lifted, and the technology became fully accessible worldwide without the need for special equipment or a decryption key.
The Significance of Use in the Contemporary World
Identify applicable sponsor/s here. (sponsors) Since then, the system was implemented in commercial and personal transportation, surveillance, tracking, and for a variety of scientific purposes in astronomy, geology, tectonics, physics, and biology. It is mounted on the aircraft, all kinds of marine vessels, such as commercial fishing boats, military ships, and private recreational yachts, and almost all existing land-based transportation means, including rail systems. The aviation particularly benefits from using the GPS as it makes the waypoint navigation more reliable. The waypoints used by aircraft are three-dimensional points that are ideally not tied to any land-based infrastructure. With a global positioning system, such infrastructure (except for augmentation stations) becomes unnecessary, as GPS essentially uses a Cartesian coordinates system.
The same goes for space vehicles, which demand even more precision and independence from land-based control. This especially concerns automated vehicles, which comprise the majority of space equipment and require constant monitoring. The utilization by automated vehicles opens up additional possibilities for unmanned cars – the technology that is currently in development but already relies heavily on GPS. Surveyors use the technology extensively, as it makes the otherwise time-consuming and tedious process more effective without sacrificing the accuracy or, in some cases, benefiting it.
Essentially any commercial activity that requires accuracy over large areas has utilized it successfully, such as precision farming, environmental studies, meteorology, mapping, including its coastal and other underwater variations, and disaster relief. The latter has made extensive use of GPS tracking capabilities – for finding the distressed individuals, and surveillance – for mapping the areas and assessing the damages, especially when the areas are not easily reachable, i.e. in the case of forest fires. Due to its principles of operation, GPS can provide accurate time data in addition to positioning, which opens up additional commercial and scientific possibilities. The projects which require high-precision timing utilize it as a formidable alternative to the more costly atomic clocks, both by direct time acquisition and by communicating the delay results to synchronize geographically distant points in time.
Financial networks and companies around the world use timestamps to track and record business activities. Both data networks and wireless stations depend on accurate timing to ensure the clarity and fidelity of signals. Electrical power grids also require synchronized and precise clocks to effectively detect and handle electrical anomalies that lead to blackouts. Finally, GPS has found its use in various recreational activities, such as hiking, cycling, tourism, and hunting. Several distinct types of orienteering are based on technology, like geocaching and waymarking. Also, it has spawned a marginal art form, known as GPS drawing, which uses a GPS tracking signal to draw simple forms.
Principles of operation
The principles of GPS operation can be fundamentally described as the constellation of satellites emitting modulated radio signals with coded data which, when received and processed by a receiving device, can provide the three-dimensional coordinates. These coordinates are then applied to the Earth model and transformed into the object’s position relative to the Earth.
Types of Signals
The data in the message is a pseudo-random set of characters which contain the information about the source of the signal (the number of the satellite) and the time of transmission (TOT). The information about the satellite allows us to determine the time of arrival of the signal (TOA), which then can be compared to TOT to calculate the relative distance of the receiver from the satellite [2]. The information from one satellite is insufficient for positioning the receiver on the three-dimensional Cartesian coordinates, so at least four simultaneous signals from different satellites are needed for the precise placement – three for the three corresponding dimensions and the fourth for determining the clock offset of the device.
Signal characteristics
The signal transmitted by the satellites carries the message which consists of several components serving different purposes. The components are encoded into five different subframes ten words each, with each word being 30 bits long, so the total frame length is 1500 bits. Each subframe carries the unique piece of information in words three through ten. The first word is reserved for the Telemetry Word (TLM), which indicates the beginning of the subframe and communicates the time at which the receiving device begins the encoding. The second word carries the Handover Word (HOW), which contains the precise time of the beginning of the next subframe (basically thus identifying when the current one ends) and specifies the position of the subframe within the complete frame (in other words, signals the subframe’s number).
The first subframe carries the time data obtained from the satellite’s atomic clock, represented by the number of the week followed by the number of seconds since the week’s inception, and the service information regarding the functionality of the unit. The second and third subframes provide the information about the satellite’s detailed orbit, known as the ephemeris. The two final subframes carry the data necessary for error amendments – the information on the approximate positions of other satellites in the constellation, known as the almanac. The fourth and fifth subframes are subcommutated 25 times each, which makes the total length of the transmitted message 37,500 bits (25 times 1500 bits of each frame). The bit rate of the transmission is 50 bits per second, so the total length of each message is 750 seconds or 12.5 minutes.
The transmissions are triggered by the onboard atomic clock every 30 seconds. This time signifies the minimum time required for the retrieval of the position by the navigation device since the beginning of the reception. All the satellites use the same frequency, so for the receiving device to be able to distinguish the individual signal, a multiplexing method is used, known as code division multiple access, or CDMA. As the system was originally intended to utilize the dual-use scheme, it uses two distinctive encoding methods: the coarse acquisition method, which was planned for distortion and degradation of the signal intended for civilians, and the precise method, or P(Y), encrypted in a manner that requires the code key to be entered into a navigation device to obtain the signal of maximum quality and accuracy. While the practice of limiting the accuracy for civilian use has been discontinued since the year 2000, the system has retained the encryption of the P(Y) signal.
The information about the GPS time from the TLM, the correction information from HOW and the ephemeris data are all that is required to determine the distance between the satellite and the receiving device, or the time of flight (TOF). The almanac data is used to shorten the time needed to search for the additional satellites and calculate their position. In essence, it is used as a shortcut to avoid unnecessary additional calculations.
The precise nature of the ephemeris requires frequent corrections, so it is updated every two hours (one update in four hours is sufficient to maintain accurate coordinates). The almanac, on the other hand, contains relatively coarse data, so it suffices to update once every twenty-four hours. As an additional precaution, the data for a few weeks forward is also timely updated as a risk mitigation plan.
Satellite Generations
There are currently three major groups of GPS satellites in operation, with the fourth one being completely discarded as obsolete, and the fifth being planned for launch in 2017. The first group, also commonly referred to as the first-generation satellites, or block I, is now out absent from the constellation due to age and declining hardware. The so-called second generation, or Block II, is divided into three broad categories: IIR, IIR(M), and IIF. These three groups currently form the constellation, represented by the twelve, seven, and twelve satellites, respectively. Blocks I and IIR both utilize two types of codes, C/A (coarse acquisition code, for civilian use) and P/Y (the encrypted precise code, for military use). These codes are also known as the legacy signals and are present in all of the current satellites. Starting from the IIR(M) generation, an additional signal was introduced for civilian usage, known as L2C. Finally, the newest generation, the IIF, has retained all the previous signals and added the third improved civilian signal, the L5. The currently planned third generation, scheduled for launch in 2017, will have all the signals of the IIF block units, and the fourth civil signal, the L1C. Naturally, every generation sports the improved reliability, lifespan, signal clarity and precision, and hardware upgrades.
Signal Frequencies
The most common legacy signals, L1 C/A and L2 P/Y utilize two frequencies. The L1 signal is broadcast on the frequency of 1575.42 MHz. The same range is used for the newest L1C civil code of the IIF generation. The L2 uses the 1227.60 MHz band. The L2C, which was introduced on the IIR(M) units, also shares this frequency. The P(Y) code is the most complex of these, as it uses encryption for the prevention of unauthorized use. The original P, or precision signal, which contains the data required for positioning, is modulated using the W code. The resulting code is referred to as Y, or, more commonly, P(Y)-code, and requires a key to be decrypted. The key is constantly changed to prevent forced decryption. The P/Y signal is transmitted on both L1 and L2 frequencies. Its structure allows for better reliability and precision than that of the civil signals.
The P-code is 235,000,000,000,000 bits in length, which gives it an advantage in terms of correlation gain and eliminates the range ambiguities, but makes it impossible to transmit, let alone lock-on by the receiver. Thus, the code is broken into segments, and each segment is broadcast by the predefined satellite. This allows the receiving device to determine the space vehicle number (SVN) of the transmitting unit as well as its pseudorandom noise number (PRN) used to distinguish the signals of each satellite. The resulting segment of the master P-code is 6,187,100,000,000 bits in length, which is still very long, so the receiver usually locks on a much shorter C/A code first and, after determining the SVN and the trajectory of the satellite, starts receiving the P-code.
Very little detail is known of the P/Y signal’s encryption methods. It is presumed that the encrypting W code’s frequency is about 20 times slower than the P signal. This allowed several parties to develop the semi-codeless methods of tracking the P/Y without decrypting it.
The C/A differs from P/Y in structure. The first difference is signal nature. The C/A signal is shared by all satellites of the constellation but is modulated differently for each unit. The modulation technique used is the binary phase-shift keying, the most common method for digital data transmission via the modulated messages. The modulation is done using the pseudorandom sequence, with each satellite using its own PRN key for modulation.
Finally, the newest signal, the L5, uses the frequency of 1.17645 GHz, which was previously dedicated to aeronautics and Safety of Life (SoL), and remains within the internationally protected range. The protected status grants the clarity of the signal.
Additionally, the P/Y code is transmitted on both the L1 and the L2 frequencies. While utilizing the same frequency as the C/A, it is transmitted as a 10.23 MHz signal, as opposed to the 1.023 MHz of the former. Besides, the modulation of the P/Y is ninety degrees out of phase with the civil signal, making it in quadrature to the civilian L1.
The L1 and L2 frequencies utilized by Global Positioning System offer several benefits. The first is the obvious resistance to jamming and interference. The second is the possibility to account for the possible ionospheric interference by comparing the two signals, calculating the delay, and removing it. This can only be done by a certain range of receiving devices, known as dual-frequency receivers. Despite the improvements in both the transmitting and the receiving equipment, the ionospheric delay remains the single biggest issue which disrupts the accuracy of the system.
Modulation
The method of modulation is different from the AM and FM techniques commonly used in the majority of wireless technologies. It is referred to as a normalized sinc function. The main difference is the SNR (the signal-noise ratio). While most of the terrestrial sources use signals with positive SNR, the GPS satellites emit the signal characterized by negative SNR. This means their signal is weaker than the noise in their band. At the receiving end (the surface of the Earth) the signal is about 26dB lower than the noise floor of the planet [3]. This makes the antenna a requirement for receiving the satellite signal. The reason for such an approach is the distance: the more traditional AM and FM signals with positive SNR will not be able to reach Earth and remain precise enough to carry any meaningful information.
Demodulation Techniques
As all of the satellites utilize the same frequency range, and the receiver is required to receive at least four signals simultaneously, they need to be separated before the data can be extracted from them. This is done after the receiver determines all sources of the signals. As each of the transmitting devices has its unique ID, known as the Gold Code, once the receiving device determines the code, it can be applied to the received PRN-modulated signal to decrypt it. The acquiring of the Golden Code is possible in two ways. During the beginning of the operation, when the receiver is powered up with no previous data available, it searches for the available C/A signal and locks on it. By matching the SVN to the information from the database, it applies the Golden Code (each SVN corresponds to a single code) and proceeds to download the ephemeris and the almanac data. This may take up to 30 seconds. Once the almanac is acquired and loaded into the memory of the device, the receiver can pre-calculate the estimated positions of all the satellites, and match the data it receives to the predicted Golden Code. This second phase is significantly faster.
Accuracy
As the system fails when less than four satellites are in the direct vicinity of the receiver, the constellation is organized in such a way that at least four of them remain visible from any point of the globe at all times. Such a system requires at least 24 satellites orbiting the Earth. The current number of satellites in Earth’s orbit is 31 [4], which adds to the reliability of the data and brings the chance of losing a signal to the minimum. The positioning data also gains in accuracy and reliability proportionally to the number of simultaneously received signals. While the first devices were capable of four to five receptions – just above the required minimum – modern equipment commonly has twelve to twenty channels – the number that exceeds the possible maximum number of satellites in the vicinity. In some cases, the required minimum is even lower, such as in a situation when one of the coordinates is known and constant. In these cases, two satellites suffice to calculate the remaining two unknown coordinates and acquire the object’s position. This is primarily used in marine navigation, as the vessel’s height above the sea level is always known.
The current state of the system allows for an accuracy of about 3 meters, although this number varies depending on weather conditions, the strength of interference, and the quality of equipment. Multiple augmentation techniques exist which increase the accuracy of positioning to up to ten centimeters in some cases.
Flaws in the System
However, the system is still prone to flaws and errors, which are mostly accounted for to a varying degree. Some of them are negligible while others present sufficient concern and demand attention. First, the GPS is highly dependent on the precision of both the signals and data calculation. The clocks used in the satellites are highly accurate atomic clocks that are synchronized across the constellation. The clocks on the receiving end are usually not synchronized with satellites, using the transmission to extrapolate the offset. This greatly reduces the complexity of the device but introduces additional error, which is negligible in most cases of civilian use but can be crucial in scientific applications relying on precise timing.
Time Offset
Another widely publicized concern with timing is the offset of clocks in orbit and those on Earth’s surface because of the difference in relative speed. This effect was predicted by the general and special theory of relativity and is accounted for by intentionally slowing the clock on board the satellite before the launch. The internal clock of the satellites is set to 10.22999999543 MHz, which results in 10.23 MHz for an observer from Earth [5]. Thus, the error is timely fixed and does not interfere with the functionality of the service, serving instead as a scientific instrument.
Current Solution
Most of the errors in time discrepancies and lack of synchronicity are already solved with the help of the approximation technique called tracking. An additional algorithm called a tracker records the obtained positioning information and predicts further positions based on the available information. These predictions are then compared to new coordinates and corrected if necessary. The use of the tracker, assuming it can accurately predict the movement, excludes the incorrect calculations, and equalizes the time data to allow smoother transitions. Such approximation is only applicable to navigation which requires precision, but it increases the smoothness of navigation, conserves resources, and allows the calculation of traveling time estimates.
Foreseeable Solutions
Next-Generation Satellites
The factor responsible for the majority of erroneous calculations is the atmospheric interference, occurring when signals from satellites pass through the ionosphere and the troposphere. Various weather conditions, like atmospheric pressure and humidity, factor in and compromise the accuracy of timing. Two known methods to amend atmospheric error exist. The first method suggests updating the transmitting equipment. This essentially means launching new satellites that would take the place of the outdated ones. Such a program is already underway, with third-generation satellites, GPS Block IIIA will transmit two additional signals for civilian use: the L2C, on an L2 frequency of 1227.6 MHz in addition to the L1C signal at the L1 frequency of 1575.42 MHz present in the current generation.
Two more frequency bands, coded L4 and L5, are being developed. The L5 band with a frequency of 1176 MHz will contain a safety-of-life (SoL) signal currently reserved for aviation safety measures and is scheduled for implementation in third-generation satellites. An L4 with a frequency of 1379.913 MHz is being studied for overcoming ionospheric interference but is currently not approved for implementation on IIIA [6]. The broadcast on two different frequencies is predicted to minimize the ionospheric correction, which is the biggest source of inaccuracy in GPS.
The main concern with this method is its cost. The hardware and the launch of satellites are quite expensive and time-consuming, require long-term planning, and eliminate the possibility of dynamic tuning.
Ground-Based Augmentations
The second method lies in using augmenting techniques. These techniques are ground-based and either introduce additional data to the equation used in calculations or eliminate the unknown data based on its redundancy. The example of the latter is marine navigation, which excludes the height of the ship on the premise of it being constant. The former can be divided into two broad categories. The first is a device mounted on the vehicle or another moving object, in which case it collects data, like speed and orientation, independently and introduces it to the GPS receiver to increase accuracy. The second is a set of ground-based installations that assess the data from controlled receivers, compare the resulting data to the independently collected control data, and accumulating the offset to correct information. Thus, the first method is limited by the narrow range of data it has access to while the second can only calculate the inconsistencies based on secondary information.
The Preferred Method
Nevertheless, two major positive points need to be stressed. Firstly, the results of the already operational augmentation programs show promising results as the accuracy of measurements rises tremendously, sometimes more than tenfold [7]. Secondly, the cost of implementation for such programs is sufficiently lower than updating the satellite hardware. The availability of maintenance is also a decisive factor. Thus, while having their limitations, the augmenting programs offer the potential for further development to improve certain characteristics of GPS and its usability in certain fields.
Conclusion
GPS has already proven to be useful in both professional and private fields. It greatly contributed to safety, reliability, cost-efficiency, and comfort in commerce, science, and even sports and arts. The further development of the system will be beneficial for society, and developing perspective methods for enhancing it should not be neglected. At the same time, the recent progress in science and technology opens new possibilities for such development, and prioritizing them accordingly will allow for timely and dynamic upgrades to the system.
Acknowledgment
Author thanks his mentor for the help in maintaining the scientific approach and for the support on all stages of the research.
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