What is "A-GPS System"? The most important thing about GPS navigation: what is GPS and why is it needed?

GPS satellite navigation has long been a standard for creating positioning systems and is actively used in various trackers and navigators. In Arduino projects, GPS is integrated using various modules that do not require knowledge of theoretical fundamentals. But a real engineer should be interested in understanding the principle and operation of GPS in order to better understand the capabilities and limitations of this technology.

GPS operation scheme

GPS is a satellite navigation system developed by the US Department of Defense that determines precise coordinates and time. Works anywhere on Earth in any weather conditions. GPS consists of three parts - satellites, stations on Earth and signal receivers.

The idea of ​​​​creating a satellite navigation system originated in the 50s of the last century. An American group of scientists observing the launch of Soviet satellites noticed that as the satellite approaches, the frequency of the signal increases and decreases as it moves away. This made it possible to understand that it is possible to measure the position and speed of a satellite knowing its coordinates on Earth, and vice versa. The launch of satellites into low Earth orbit played a huge role in the development of the navigation system. And in 1973, the DNSS (NavStar) program was created, under this program satellites were launched into medium-Earth orbit. The program received its name GPS in the same 1973.

The GPS system is currently used not only in the military field, but also for civilian purposes. There are many applications for GPS:

• Mobile connection;
• Plate tectonics - tracking of plate fluctuations;
• Determination of seismic activity;
• Satellite tracking of transport – you can monitor the position, speed of transport and control their movement;
• Geodesy - determining the exact boundaries of land plots;
• Cartography;
• Navigation;
• Games, geotagging and other entertainment areas.

The most important disadvantage of the system can be considered the inability to receive a signal under certain conditions. GPS operating frequencies are in the decimeter wavelength range. This leads to the fact that the signal level may decrease due to high clouds and dense tree foliage. Radio sources, jammers, and in rare cases even magnetic storms can also interfere with normal signal transmission. The accuracy of data determination will deteriorate in the polar regions, since the satellites rise low above the Earth.

Navigation without GPS

The main competitor to GPS is the Russian GLONASS (Global Navigation Satellite System). The system began its full-fledged operation in 2010, and attempts to actively use it have been made since 1995. There are several differences between the two systems:

• Different encodings - Americans use CDMA, for the Russian system FDMA is used;
• Different dimensions of devices - GLONASS uses a more complex model, which increases power consumption and the size of devices;
• The placement and movement of satellites in orbit - the Russian system provides wider coverage of the territory and a more accurate determination of coordinates and time.
• Satellite lifespan – American satellites are made of higher quality, so they last longer.

In addition to GLONASS and GPS, there are other less popular navigation systems - the European Galileo and the Chinese Beidou.

Description of GPS

How GPS works

The GPS system works as follows: the signal receiver measures the delay in signal propagation from the satellite to the receiver. From the received signal, the receiver obtains data about the location of the satellite. To determine the distance from the satellite to the receiver, the signal delay is multiplied by the speed of light.

From a geometric point of view, the operation of the navigation system can be illustrated as follows: several spheres, in the middle of which there are satellites, intersect and the user is in them. The radius of each sphere is correspondingly equal to the distance to this visible satellite. Signals from three satellites provide information about latitude and longitude; the fourth satellite provides information about the height of an object above the surface. The obtained values ​​can be reduced to a system of equations from which the user’s coordinate can be found. Thus, to obtain an accurate location, it is necessary to carry out 4 measurements of distances to the satellite (if we exclude implausible results, three measurements are sufficient).

Amendments to the resulting equations are introduced by the discrepancy between the calculated and actual position of the satellite. The error that arises as a result of this is called ephemeris and ranges from 1 to 5 meters. Interference, atmospheric pressure, humidity, temperature, and the influence of the ionosphere and atmosphere also contribute. The totality of all errors can bring the error to 100 meters. Some errors can be eliminated mathematically.

To reduce all errors, use differential GPS mode. In it, the receiver receives all the necessary corrections to the coordinates from the base station via a radio channel. The final measurement accuracy reaches 1-5 meters. In differential mode, there are 2 methods for correcting the received data - this is the correction of the coordinates themselves and the correction of navigation parameters. The first method is inconvenient to use, since all users must work using the same satellites. In the second case, the complexity of the location determination equipment itself increases significantly.

There is a new class of systems that increases the measurement accuracy to 1 cm. The angle between the directions to the satellites has a huge impact on the accuracy. At a larger angle, the location will be determined with greater accuracy.

Measurement accuracy may be artificially reduced by the US Department of Defense. To do this, a special S/A mode is installed on navigation devices - limited access. The mode was developed for military purposes in order to not give the enemy an advantage in determining the exact coordinates. Since May 2000, the restricted access regime has been abolished.

All error sources can be divided into several groups:

• Error in orbit calculations;
• Receiver related errors;
• Errors associated with multiple reflections of the signal from obstacles;
• Ionosphere, tropospheric signal delays;
• The geometry of the satellites.

Main characteristics

The GPS system includes 24 artificial Earth satellites, a network of ground-based tracking stations and navigation receivers. Observation stations are required to determine and monitor orbital parameters, calculate ballistic characteristics, adjust deviations from motion trajectories, and monitor equipment onboard spacecraft.

Characteristics of GPS navigation systems:

• Number of satellites – 26, 21 main, 5 spare;
• Number of orbital planes – 6;
• Orbit altitude – 20,000 km;
• The service life of the satellites is 7.5 years;
• Operating frequencies – L1=1575.42 MHz; L2=12275.6 MHz, power 50 W and 8 W, respectively;
• Reliability of navigation determination is 95%.

There are several types of navigation receivers - portable, stationary and aircraft. Receivers are also characterized by a number of parameters:

• Number of channels – modern receivers use from 12 to 20 channels;
• Antenna type;
• Availability of cartographic support;
• Display type;
• Additional functions;
• Various technical characteristics - materials, strength, moisture protection, sensitivity, memory capacity and others.

The operating principle of the navigator itself is that first of all the device tries to communicate with the navigation satellite. As soon as the connection is established, the almanac is transmitted, that is, information about the orbits of satellites located within the same navigation system. Communication with one satellite alone is not enough to obtain an accurate position, so the remaining satellites transmit their ephemeris to the navigator, which is necessary to determine deviations, disturbance coefficients and other parameters.

Cold, warm and hot start of GPS navigator

When you turn on the navigator for the first time or after a long break, a long wait begins to receive data. The long waiting time is due to the fact that the almanac and ephemeris are missing or outdated in the navigator’s memory, so the device must perform a number of actions to obtain or update data. The waiting time, or the so-called cold start time, depends on various indicators - the quality of the receiver, the state of the atmosphere, noise, the number of satellites in the visibility zone.

To start working, the navigator must:

• Find a satellite and establish contact with it;
• Receive the almanac and save it in memory;
• Receive ephemeris from the satellite and save it;
• Find three more satellites and establish contact with them, receive ephemeris from them;
• Calculate coordinates using ephemeris and satellite locations.

Only after going through this entire cycle will the device begin to work. This kind of launch is called cold start.

A hot start is significantly different from a cold start. The navigator's memory already contains the currently relevant almanac and ephemeris. Almanac data is valid for 30 days, ephemeris data is valid for 30 minutes. It follows that the device was turned off for a short time. With a hot start, the algorithm will be simpler - the device establishes a connection with the satellite,, if necessary, updates the ephemeris and calculates the location.

There is a warm start - in this case the almanac is current, but the ephemeris needs to be updated. This takes a little more time than a hot start, but significantly less than a cold start.

Restrictions on the purchase and use of homemade GPS modules

Russian legislation requires manufacturers to reduce the accuracy of receiver detection. Working with uncoarsened precision can only be done if the user has a specialized license.

Special technical means intended for secretly obtaining information (STS NPI) are prohibited in the Russian Federation. These include GPS trackers, which are used for secret control over the movement of vehicles and other objects. The main feature of an illegal technical device is its secrecy. Therefore, before purchasing a device, you need to carefully study its characteristics, appearance, for the presence of hidden functions, and also review the necessary certificates of conformity.

It is also important in what form the device is sold. When disassembled, the device may not belong to the STS NPI. But when assembled, the finished device may already be classified as prohibited.

Smartphones have long ceased to be simple dialers. They opened up a lot of new opportunities for their owners.

In the first place is full-fledged high-speed Internet access and communication on social networks and instant messengers. But GPS positioning is no less in demand, which we will now discuss in detail.

What is GPS?

GPS is a navigation system that determines the location of a smartphone, builds routes and allows you to find the desired object on the map.

Almost every modern gadget has a built-in GPS module. This is an antenna tuned to the GPS geolocation satellite signal. It was originally developed in the USA for military purposes, but later its signal became available to everyone. The gadget's GPS module is a receiving antenna with an amplifier, but it cannot transmit a signal. Receiving a signal from satellites, the smartphone determines the coordinates of its location.

Almost every modern person has used GPS navigation on a smartphone or tablet at least once. The need for it can arise at any time among people of different professions and different types of occupations. It is necessary for drivers, couriers, hunters, fishermen and even ordinary pedestrians who find themselves in an unfamiliar city. Thanks to such navigation, you can determine your location, find the desired object on the map, build a route, and, if you have access to the Internet, avoid traffic jams.

Offline maps for GPS

Google has developed a special geolocation application for its Android operating system - Google Maps. It quickly finds satellites, develops routes to objects and offers alternatives. Unfortunately, if there is no cellular network coverage, Google Maps does not work, since geographic maps are downloaded via the Internet.

For offline navigation, the best way is to download applications that support offline maps, such as Maps.me, Navitel and 2GIS. You can also install the Maps: Transportation and Navigation app for Google Maps.

In this case, you won’t have to spend Internet traffic to download maps - they will always be on your device, regardless of location. This is especially true when you are abroad, since the cost of roaming for Internet access is very high.

How to enable GPS on Android?

Activating the GPS module in the Android operating system is possible in two ways:

• Top curtain. Swipe down on the display and in the menu that opens, click the “Location”, “Geolocation” or “Geodata” button (depending on the Android version).
• In the Android settings, find the similar items item and move the checkbox to the “Enabled” position.

During active operation of the smartphone's navigation system, its battery charge begins to be consumed quite actively, so it is worth taking care of additional power sources. For example, when driving you need to use a car charger, and when traveling by bicycle or on foot -.

It is also worth remembering that reliable reception of a satellite signal is possible in open areas, so when you are in a room or tunnel, geolocation becomes impossible. Cloudy weather also has an effect - because of the clouds, the device takes longer to search for satellites and determines its coordinates less accurately.

Not so long ago, GPS was the only geolocation system, so in early versions of Android only it was mentioned, and the service activation button was called that. Since 2010, the Russian one has been fully operational, and since 2012 -.

2 years ago

Global Positioning System GPS(Global Positioning System) was originally planned for use by the US armed forces. It later became the first satellite navigation system used for civilian purposes and is currently used for navigation throughout the world.

The operating principle of GPS is based on the use of a constellation of 30 satellites, which, in addition to the 27 active ones, also includes 3 spare satellites in case of failure of one of the main ones. The working orbit of the satellites is approximately 19,000 km; each satellite makes two revolutions around the Earth per day. The set of satellites is configured in such a way that it ensures round-the-clock reception of a signal from any point on Earth by at least four satellites, that is, the minimum required to determine the exact location. The GPS receiver calculates its location in relation to visible satellites. The greater the number of available satellites in the area and the stronger the signal level from them, the more accurate the coordinate determination results will be.

The GPS receiver determines the distance to each satellite based on the signal transmission delay. Further, having the spatial coordinates of 3 points and 3 distances to the desired point, the location of the receiver on the plane is easily found. Since the system operates in space and not on a plane, a fourth satellite is required, which makes it possible to unambiguously determine the coordinates of a point in three-dimensional space. Compared to solving a theoretical geometric problem, a practical determination differs in the presence of errors in determining the distance to satellites, which leads to the fact that the result of the determination may not be a point, but an area of ​​a certain radius. However, increasing the number of visible satellites will reduce this radius and, therefore, the accuracy of the location will increase. In practice, a civilian GPS system provides accuracy with a radius of 30 meters, while military receivers provide accuracy up to 3 meters. The number of visible satellites depends on the specific receiver model. In addition, for high-quality operation of the GPS system, mutual precise synchronization of the satellite and the GPS receiver is necessary in order to accurately calculate the delay from a predetermined time of sending a signal from the satellite.

GPS navigation has found the widest application in our time. In particular, in navigators, where it is combined and linked to electronic maps. This technology allows not only to determine the coordinates of the subscriber’s location, but also to plan a route of movement in accordance with the method of movement and other initial requirements. Many mobile phone models are equipped with GPS navigators. The combination of mobile communications with the GPS global positioning system has led to the creation of a new assistive technology - A-GPS(Assisted GPS), which involves using the Internet to improve the quality of the basic positioning system in two directions. Firstly, the GPS receiver, after being turned on, first determines the location of the satellites. Sometimes, due to a weak signal, the procedure can take several minutes. Using A-GPS technology, information about the location of satellites is requested via the Internet in special data centers. Secondly, to calculate the location of a large number of satellites under poor conditions for the transmission of signals from satellites, powerful computing power is required, which is not present in all terminals. Sending the obtained preliminary values ​​to data centers and receiving ready-made coordinates can significantly speed up the process of initial positioning. In addition, Internet access can be used for other purposes. This could be, for example, synchronization or obtaining information about the atmospheric state, which can have a significant impact on the calculations.

Recently, many countries have shown interest in creating global positioning systems of their own production. Examples include Glonas in Russia or Galileo in Europe. Such aspirations are caused by the desire to gain independence from the American system, since there remains the possibility of shutting down the system at the initiative of its owner, which could lead to serious disruptions in the functioning of important systems within the state. In such critical civil systems, paired dual systems of 2 or more positioning systems are usually used to increase reliability and accuracy.

Disadvantages of GPS

The following problems may occur when using the GPS global positioning system:

• When the coordinates are first determined, the time depends on the orbital data and the relevance of the history stored in the receiver. In other words, the longer a device has been turned off, the more information it must acquire before a position can be determined. For example, if the device has been unplugged for 2 - 6 hours, it will need approximately 45 seconds. If the device did not work for several days, or when driving more than 300 km without receiving information, up to 12.5 minutes.
• There are severe limitations on the visibility of GPS satellites in urban environments, and in tunnels or enclosed spaces visibility is not possible at all.
• High power consumption of the GPS receiver.

A-GPS functions

A-GPS system algorithms require a communication channel with a remote server that provides information to the receiver. Typically for mobile devices this channel is cellular communication. To exchange information, the device must be located within the coverage area of ​​a mobile operator's base station and have access to the Internet.

There are two modes of A-GPS operation:

• Basic On-line mode, in which the receiver receives information about satellite orbits through the infrastructure and calculates the location based on data received from users. This mode requires a high coverage density from the cellular operator.
• An auxiliary Off-line mode, which speeds up the cold and hot start time of the A-GPS receiver, updating the almanac, ephemeris and list of available satellites. Moreover, the GPS receiver independently receives satellite signals and determines its own location. However, some A-GPS receivers cannot operate in this mode.

Benefits of A-GPS

Among the advantages of A-GPS, it is worth noting the quick location acquisition immediately after switching on and the increased sensitivity of receiving weak signals in problem areas (tunnels, depressions, indoors, on narrow city streets, in dense deciduous forests).

Disadvantages of A-GPS

A-GPS cannot function outside of cellular coverage.

There are receivers with an A-GPS module combined with a GSM radio module, which cannot start when the radio module is turned off. To start the A-GPS module itself, a GSM network is not required. A-GPS modules consume a small traffic of 5-7 kB at startup, but if the signal is lost, it is necessary to re-synchronize, which can increase the client’s costs, especially in roaming.
While looking through various types of GPS equipment, you have probably come across the abbreviation A-GPS (most often found on smartphones). What is A-GPS and why would it be good to have this feature?

A-GPS stands for Assisted GPS, a function that helps the GPS module establish a satellite signal. The so-called “cold start time” is the time required to determine coordinates from the moment the GPS is turned on with the device completely turned off. There is a so-called TTFF parameter (time to determine coordinates), this period increases significantly in difficult conditions when there is no direct visibility of all satellites. Most often, this happens in dense urban areas, when satellite signals are reflected from all nearby buildings.

It must be said that the AGPS function only works when connected to the mobile Internet. This can be GPRS data transmission or WCDMA (3g mobile Internet). By transmitting and receiving location-related data from the Internet, the technology allows you to speed up the process of determining your coordinates. What data is transmitted via the Internet will be discussed below.

The first start of a GPS device is called a cold start or factory start. At this time, three types of data are loaded into the device: satellite signals, almanac data and ephemeris data) to determine its location. A cold start occurs after a prolonged shutdown, moving over long distances, or when resetting the cache data (saved data about the last location of the module.

For standalone GPS receivers, a cold start can take up to 10 minutes. However, if the signal is uncertain and is interrupted during the start of coordinate determination (due to low signal level, weather conditions, buildings, etc.), a cold start may take longer.

A-GPS helps speed up coordinate determination by connecting via the Internet to a web server (called an Assisted Server), which already contains up-to-date information about all satellites. This information is transmitted via GPRS to the phone.

In addition, it has been shown that even a warm start is faster using A-GPS, by an average of a minute.

What A-GPS technology doesn't do

Despite the fact that the described function remarkably improves the performance of the satellite module, there are certain limitations. The satellite signal will not be detected inside large and reinforced concrete buildings (far from windows). Also, the GPS signal will not work underwater or even underground. By the way, in the absence of mobile communications (Internet), this function also does not work.

In addition, do not confuse the A-GPS function with Wi-Fi positioning or the “triangulation” method for cell phones, when determining coordinates using GPS is impossible.

Some devices combine these positioning methods. Such location determination methods are called: hybrid positioning systems.

Where is Intelli LLC heading with its developments? First of all, these are integrated positioning systems.

Under ideal conditions, GPS or Glonass is the fastest and most accurate way to determine the location of an object. At the same time, there are certain positioning restrictions: inside buildings, underground (underground parking, for example), under water, etc. The positioning technology we have come to is to provide our clients with a comprehensive solution.

At the 2010 FIG Congress, the main trend was ubiquitous positioning using all available technologies: GPS/Glonass, Wi-Fi positioning and triangulation method for cell phones.

Combining the advantages of these technologies will improve not only the accuracy and stability of GPS monitoring systems, but will also be used universally in all popular mobile devices.

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