GPS satellite in orbit

The basic principle of using the system is to determine the location by measuring the time points of receiving a synchronized signal from navigation satellites to the consumer. The distance is calculated from the signal propagation delay time from sending it by the satellite to receiving it by the GPS receiver antenna. That is, to determine the three-dimensional coordinates, the GPS receiver needs to have four equations: “the distance is equal to the product of the speed of light and the difference between the moments of reception of the consumer signal and the moment of its synchronous radiation from the satellites”:

Here: - the location of the -th satellite, - the moment of reception of the signal from the -th satellite according to the clock of the consumer, - the unknown moment in time of synchronous emission of the signal by all satellites according to the clock of the consumer, - the speed of light, - the unknown three-dimensional position of the consumer.

History

The idea of ​​creating satellite navigation was born in the 50s. At the moment when the USSR launched the first artificial satellite of the Earth, American scientists led by Richard Kershner observed the signal emanating from the Soviet satellite and found that due to the Doppler effect, the frequency of the received signal increases as the satellite approaches and decreases as it moves away. The essence of the discovery was that if you know exactly your coordinates on Earth, then it becomes possible to measure the position and speed of the satellite, and vice versa, knowing the exact position of the satellite, you can determine your own speed and coordinates.

This idea was realized after 20 years. In 1973, the DNSS program was initiated, later renamed Navstar-GPS and then GPS. The first test satellite was launched on July 14, 1974, and the last of all 24 satellites needed to cover the entire earth's surface was launched in 1993, thus GPS came into service. It became possible to use GPS to accurately target missiles to stationary, and then to moving objects in the air and on the ground.

Initially, GPS, the global positioning system, was developed as a purely military project. But after a Korean Airlines plane with 269 passengers on board that invaded the airspace of the Soviet Union was shot down in 1983 due to disorientation of the crew in space, US President Ronald Reagan, in order to prevent similar tragedies in the future, allowed the partial use of the navigation system for civilian purposes. In order to avoid the use of the system for military purposes, the accuracy was reduced by a special algorithm. [ clarify]

Then information appeared that some companies had deciphered the algorithm for reducing the accuracy at the L1 frequency and successfully compensated for this component of the error. In 2000, US President Bill Clinton abolished this coarsening of accuracy by his decree.

satellites

Block	Period launches	Satellite launches				work now
		Start-up puppy	Not successfully	Goto- twists	Plan- rovano
I	1978-1985	10	1	0	0	0
II	1989-1990	9	0	0	0	0
IIA	1990-1997	19	0	0	0	11
IIR	1997-2004	12	1	0	0	12
IIR-M	2005-2009	8	0	0	0	7
IIF	2010-2011	2	0	10	0	2
IIIA	2014-?	0	0	0	12	0
Total		59	2	10	12	31
(Last data update: 9 Oct 2011)

Technical implementation

space satellites

An unlaunched satellite on display at a museum. View from the side of the antennas.

Satellite orbits

Orbits of GPS satellites. An example of the visibility of satellites from one of the points on the surface of the Earth. Visible sat is the number of satellites visible above the observer's horizon under ideal conditions (clear field).

The satellite constellation of the NAVSTAR system revolves around the Earth in circular orbits with the same height and period of revolution for all satellites. A circular orbit with an altitude of about 20,200 km is a daily multiplicity orbit with an orbital period of 11 hours 58 minutes; thus, the satellite makes two orbits around the Earth in one sidereal day (23 hours 56 minutes). The orbital inclination (55°) is also common to all satellites in the system. The only difference between the orbits of the satellites is the longitude of the ascending node, or the point at which the plane of the satellite's orbit intersects the equator: these points are approximately 60 degrees apart. Thus, despite the same (except for the longitude of the ascending node) orbital parameters, the satellites revolve around the Earth in six different planes, 4 satellites in each.

RF characteristics

The satellites radiate open for use signals in the ranges: L1=1575.42 MHz and L2=1227.60 MHz (starting from Block IIR-M), and IIF models will also radiate at L5=1176.45 MHz. Navigation information can be received by an antenna (usually in line of sight to the satellites) and processed using a GPS receiver.

The standard precision code (C/A code - BPSK modulation (1)) transmitted in the L1 band (and the L2C signal (BPSK modulation) in the L2 band starting with IIR-M devices) is distributed without restrictions on use. Initially used on L1, artificial signal coarsening (selective access mode - SA) has been disabled since May 2000. Since 2007, the United States has finally abandoned the technique of artificial coarsening. It is planned to introduce a new L1C signal (BOC(1,1) modulation) in the L1 band with the launch of Block III devices. It will have backwards compatibility, improved path following capability, and is more compatible with Galileo L1 signals.

For military users, signals in the L1 / L2 bands are additionally available, modulated with a noise-resistant crypto-resistant P (Y) code (BPSK (10) modulation). Starting with IIR-M devices, a new M-code was put into operation (modulation BOC (15,10) is used). The use of the M-code makes it possible to ensure the functioning of the system within the framework of the Navwar concept (navigation war). The M-code is transmitted on the existing frequencies L1 and L2. This signal has increased noise immunity, and it is sufficient to determine the exact coordinates (in the case of the P-code, it was also necessary to obtain the C / A code). Another feature of the M-code will be the ability to transmit it for a specific area with a diameter of several hundred kilometers, where the signal strength will be 20 decibels higher. The conventional M signal is already available on IIR-M satellites, while the narrow beam will only be available on GPS-III satellites.

With the launch of the IIF satellite, a new frequency L5 (1176.45 MHz) was introduced. This signal is also called safety of life (protection of human life). The L5 signal is 3 dB stronger than the civilian signal and has a bandwidth 10 times wider. The signal can be used in critical situations associated with a threat to human life. The full signal will be used after 2014.

Signals are modulated with two types of pseudo-random sequences (PRN): C/A-code and P-code. C/A (Clear access) - public code - is a PRN with a repetition period of 1023 cycles and a pulse repetition rate of 1023 MHz. It is with this code that all civilian GPS receivers work. P (Protected/precise)-code is used in systems closed for general use, its repetition period is 2*1014 cycles. P-code modulated signals are transmitted on two frequencies: L1 = 1575.42 MHz and L2 = 1227.6 MHz. The C/A code is transmitted only on the L1 frequency. The carrier, in addition to PRN codes, is also modulated by a navigation message.

Satellite type	GPS II	GPS-IIA	GPS-IIR	GPS IIRM	GPS-IIF
Weight, kg	885	1500	2000	2000	2170
Life span	7.5	7.5	10	10	15
Onboard time	Cs	Cs	Rb	Rb	Rb+Cs
inter-satellite connection	-	+	+	+	+
Autonomous work, days	14	180	180	180	>60
Anti-radiation protection	-	-	+	+	+
Antenna	-	-	improved	improved	improved
Possibility of customization in orbit and power airborne transmitter	+	+	++	+++	++++
navigational signal	L1:C/A+P L2:P	L1:C/A+P L2:P	L1:C/A+P L2:P	L1:C/A+P+M L2:C/A+P+M	L1:C/A+P+M L2:C/A+P+M L5:C

24 satellites provide 100% system operability anywhere in the world, but they cannot always provide reliable reception and good position calculation. Therefore, in order to increase positioning accuracy and reserve in case of failures, the total number of satellites in orbit is maintained in a larger number (31 satellites in March 2010).

Ground control stations of the space segment

Main article: ground segment of satellite navigation system

The orbital constellation is monitored from the main control station located at Schriever Air Force Base, Colorado, USA and with the help of 10 tracking stations, of which three stations are capable of sending correction data to the satellites in the form of radio signals with a frequency of 2000-4000 MHz. The latest generation of satellites distribute the received data among other satellites.

GPS application

GPS signal receiver

Despite the fact that the GPS project was originally aimed at military purposes, today GPS is widely used for civilian purposes. GPS receivers are sold in many electronics stores and are built into mobile phones, smartphones, PDAs, and onboarders. Consumers are also offered various devices and software products that allow them to see their location on an electronic map; having the ability to lay routes taking into account road signs, permitted turns and even traffic jams; search on the map for specific houses and streets, attractions, cafes, hospitals, gas stations and other infrastructure.

There have been proposals to integrate Iridium and GPS systems.

Accuracy

The components that affect the error of a single satellite in a pseudorange measurement are given below:

Source of error	RMS error, m
Generator instability	6,5
Delay in on-board equipment	1,0
The uncertainty of the spatial position of the satellite	2,0
Other space segment errors	1,0
Ephemeris inaccuracy	8,2
Other ground segment errors	1,8
Ionospheric delay	4,5
Tropospheric delay	3,9
Receiver noise error	2,9
Multipath	2,4
Other User Segment Errors	1,0
Total error	13,1

The total error is not equal to the sum of the components.

The typical accuracy of modern GPS receivers in the horizontal plane is about 6-8 meters with good satellite visibility and the use of correction algorithms. On the territory of the USA, Canada, Japan, China, the European Union and India, there are stations WAAS, EGNOS, MSAS, etc., transmitting corrections for the differential mode, which reduces the error to 1-2 meters in these countries. When using more complex differential modes, the accuracy of determining the coordinates can be increased to 10 cm. The accuracy of any SNS strongly depends on the openness of space, on the height of the satellites used above the horizon.

In the near future, all devices of the current GPS standard will be replaced by a newer version of GPS IIF, which has a number of advantages, including being more resistant to interference.

But the main thing is that GPS IIF provides a much higher accuracy in determining the coordinates. If the current satellites provide an accuracy of 6 meters, then the new satellites will be able to determine the position, as expected, with an accuracy of at least 60-90 cm. If such accuracy is not only for military, but also for civilian applications, then this is good news for owners of GPS navigators.

As of October 2011, the first two satellites from the new version were put into orbit: GPS IIF SV-1 launched in 2010 and GPS IIF-2 launched on July 16, 2011.

In total, the initial contract provided for the launch of 33 new generation GPS satellites, but then, due to technical problems, the start of the launch was postponed from 2006 to 2010, and the number of satellites was reduced from 33 to 12. All of them will be put into orbit in the near future.

The improved accuracy of the new generation of GPS satellites is made possible by the use of more accurate atomic clocks. Because satellites travel at about 14,000 km/h (3.874 km/s) (first escape velocity at 20,200 km), improving time accuracy even in the sixth digit is critical to triangulation.

disadvantages

A common disadvantage of using any radio navigation system is that under certain conditions, the signal may not reach the receiver, or arrive with significant distortion or delay. For example, it is almost impossible to determine your exact location in the depths of an apartment inside a reinforced concrete building, in a basement or in a tunnel, even with professional geodetic receivers. Since the operating frequency of GPS lies in the decimeter range of radio waves, the level of signal reception from satellites can seriously deteriorate under dense foliage of trees or due to very heavy clouds. Normal reception of GPS signals can be damaged by interference from many terrestrial radio sources, as well as (in rare cases) from magnetic storms, or deliberately created by "jammers" (this method of dealing with satellite car alarms is often used by car thieves).

The low inclination of GPS orbits (approximately 55) seriously degrades accuracy in the circumpolar regions of the Earth, since GPS satellites do not rise very high above the horizon.

An essential feature of GPS is the complete dependence of the conditions for receiving a signal from the US Department of Defense.

Now [ when?] The US Department of Defense has decided to start a complete upgrade of the GPS system. It was planned a long time ago, but it was only now that the project could be started. During the upgrade, old satellites will be replaced with new ones designed and manufactured by Lockheed Martin and Boeing. It is claimed that they will be able to provide positioning accuracy with an error of 0.5 meters.

The implementation of this program will take some [ which?] time. The US Department of Defense claims that it will be possible to fully complete the upgrade of the system only after 10 years. The number of satellites will not be changed, they will still be 30: 24 working and 6 standby.

Chronology

1973	Decision to develop a satellite navigation system
1974-1979	System test
1977	Receiving a signal from a ground station simulating a system satellite
1978-1985	Launch of eleven satellites of the first group (Block I)
1979	Reducing program funding. The decision to launch 18 satellites instead of the planned 24.
1980	In connection with the decision to curtail the program of using Vela satellites for tracking nuclear explosions, it was decided to assign these functions to GPS satellites. Launch of the first satellites equipped with sensors for detecting nuclear explosions.
1980-1982	Further cuts in program funding
1983	After the death of the company's aircraft Korean Airlines shot down over the territory of the USSR, a decision was made to provide a signal to civilian services.
1986	The death of the space shuttle Space Shuttle "Challenger" suspended the development of the program, since the latter was planned to launch the second group of satellites into orbit. As a result, the Delta launch vehicle was chosen as the main vehicle.
1988	The decision to deploy an orbital constellation of 24 satellites. 18 satellites are unable to ensure the smooth functioning of the system.
1989	Activation of satellites of the second group
1990-1991	Temporary shutdown SA(English) selective availability- artificially created for unauthorized users rounding the location to 100 meters) due to the Gulf War and the lack of military models of receivers. Inclusion SA June 01, 1991.
08.12.1993	System readiness message Initial Operational Capability ). In the same year, a final decision was made to provide a signal for free use to civil services and individuals.
1994	Satellite constellation completed
17.07.1995	Full system readiness Full Operational Capability)
01.05.2000	Shutdown SA for civilian users, thus the accuracy of determination has increased from 100 to 20 meters
26.06.2004	Signing of a Joint Statement on Complementarity and Interoperability between Galileo and GPS 1
December 2006	Russian-American negotiations on cooperation in the field of ensuring the complementarity of space navigation systems GLONASS and GPS.²

see also

• Transit (first satellite navigation system, 1960s - 1996)
• Galileo (European navigation system)
• GLONASS (Russian navigation system)

Notes

Literature

• Aleksandrov I. Space radio navigation system NAVSTAR (Russian) // Foreign military review. - M ., 1995. - No. 5. - S. 52-63. - ISSN 0134-921X.
• Kozlovsky E. The art of positioning // Around the world. - M ., 2006. - No. 12 (2795). - S. 204-280.
• Shebshaevich V. S., Dmitriev P. P., Ivantsev N. V. et al. Network satellite radio navigation systems / ed. V. S. Shebshaevich. - 2nd ed., revised. and additional - M .: Radio and communication, 1993. - 408 p. - ISBN 5-256-00174-4

Links

Official documents and specifications

• Official site of the US government and the GPS system with the status of a satellite constellation (eng.)

Job Explanations

• Global Navigation Satellite Systems (GNSS). How it works? , gps-club.ru

Compatibility with Gallileo and GLONASS

• Galileo and GPS
• Joint Statement on GLONASS and GPS Complementarity and Compatibility ( (unavailable link), copy)

Miscellaneous

Navigation systems
Satellite	historical Transit Cicada/Cyclone Current global GPS GLONASS Current regional Beidou/Compass Galileo Projected IRNSS QZSS
Ground	Omega Alpha Loran-C

As most of us know, the Earth appears in a shape close to a ball, but everyone knows that it is not a ball. A difference that is very significant for accurate navigation and coordinate systems. The complex surface of the Earth was called the geoid in the 19th century. The surface of the geoid coincides with the surface of the seas and oceans in their calm state and virtually extends under the continents.

Coordinate systems for GPS-navigators, geographic coordinates.

Earth, its shape and coordinates.

For practical application, two models of the shape of the Earth are widely used: spherical with a simplified representation of it in the form of a ball with a radius of 6371.1 kilometers and spheroidal in the form of an ellipse of revolution (ellipsoid). It is understood as a geometric figure, which is formed by the rotation of an ellipse around its minor axis. The dimensions of the ellipsoid of revolution, its orientation and location relative to the center of mass of the Earth can be varied to achieve the greatest accuracy of approximation to the real earth's surface. It should be understood that each model used has its own coordinate system.

When we talk about any coordinate system, we also mean the corresponding ellipsoid model. But this is not all the differences that the user of the GPS system needs to know. If the parameters of the ellipsoid are selected for the Earth as a whole, then such an ellipsoid is called the common earth ellipsoid (GGE). In order to describe a local (partial) region of the Earth's surface with greater accuracy, an ellipsoid with other parameters can be used.

Such an ellipsoid, legally accepted for measuring and processing geodetic data, is called a reference ellipsoid (RE), and the coordinate system it forms is called a reference. In the reference ellipsoid, its minor axis does not coincide with the axis of rotation of the Earth, but must be parallel to it. In the OSE, the semi-minor axis always coincides with the axis of rotation, and the center of the ellipsoid coincides with the center of mass of the Earth.

On the territory of the CIS, two global coordinate systems are used, PZ-90 and International WGS-84 (Wordl Geodetic System 1984). The numbers in the designation of the system indicate the year of its creation. Both systems are close to each other. PZ-90 is used in the CIS for geodetic support of orbital flights, and WGS-84 is used worldwide for processing GPS satellite measurements. The Russian reference systems include the SK-42 (Pulkovo) and SK-95 systems. Both systems use the Krasovsky ellipsoid (introduced since 1946) and are used in geodetic and cartographic work.

Coordinate systems for GPS-navigators.

When navigating and using GPS navigators, it is very important to understand that displaying GPS positions on maps with different coordinate systems without recalculating them will lead to large errors. Therefore, mapping programs are used to convert data, for example, from the WGS-84 system into local coordinate systems. Fortunately, users of portable GPS navigators do not have this problem at all. When using a paper map with a coordinate grid together with a GPS navigator, it is necessary to check the coincidence of the coordinate systems of the map and the navigator.

If necessary, you can customize the coordinate system of the navigator by setting parameters in it, called datum, corresponding to the loaded map, or by selecting a custom datum. The navigator will then perform the coordinate transformation automatically. A datum is a geodetic coordinate system, uniquely determined by the size of its ellipsoid and its position relative to the center of the Earth. The number of different datums, or more simply - coordinate systems used in world cartography, is more than a hundred. Different datums have been proposed in order to obtain the best approximation of the model they define to the real surface of the Earth in a given region.

For example, the local NAD-27 North American datum is designed to best represent North America, while the local ED-50 European datum is designed for use in Europe. Local datums cannot be applied outside the area for which they were developed. For the convenience of the user of GPS navigators, the parameters of many datums are stored in their memory, which makes it possible to use electronic maps from various sources in them without any difficulties.

On many papers, an amendment is indicated for the transition from the map coordinate system to the international WGS-84, in which GPS works. For example, in order to plot a point located in the area of ​​the Baltic Sea and Ladoga with coordinates according to the WGS-84 system on a Russian map built in the system of the Pulkovo Observatory in 1942, it is necessary to shift this point by 0.14 minutes to the east. At the latitude of St. Petersburg, this difference corresponds to approximately 130 meters.

Geographical coordinates.

To determine the position of any object on the surface of the Earth, a system of geographical coordinates and two special points is used - the North and South poles. The poles are, as is known, the points of intersection of the axis of rotation of the Earth with the surface of the ellipsoid. Geographic coordinates are most clearly represented in a spherical model of the Earth. In it, geographical coordinates, latitude and longitude, are determined using circles formed when the spherical model of the Earth is cut by planes: for latitude - in the horizontal direction, and for longitude - in the vertical direction.

The circle EQ, formed on the surface of the ball by a horizontal cutting plane perpendicular to the earth's axis and passing through the center of the ball, is called the equator. It divides the globe into northern and southern hemispheres. Circles of small circles, the planes of which are parallel to the plane of the equator, form parallels (PP). The circles formed by planes passing through the earth's axis are called meridians (geographic or true). Among all the meridians, it is necessary to single out the initial (zero) PnGPs, called the Greenwich meridian, since it passes through the astronomical observatory in Greenwich (England). This meridian divides the globe into eastern and western hemispheres.

geographic latitude.

The geographic latitude of some point on the surface of the earth's spheroid is the angle between the equatorial plane and the normal (vertical line) to this surface. For the model of the Earth in the form of a ball, the normal coincides with the earth's radius OM drawn through a given point M to the center of the ball. Latitude is measured by the meridian arc (MOL angle) from the equator to the parallel of the given point. Latitude takes values ​​in the range from 0 to 90 degrees. If the point is in the northern hemisphere, then the latitude is assigned the name N (northern), if in the southern - S.

Geographic longitude.

The geographic longitude of a point is the dihedral angle between the planes of the initial (zero) meridian and the meridian passing through a given point. Thus, the longitude of the point M is determined by the angle GOL. Longitude is measured by the smaller arc of the equator, GL, and, for example, not by the arc GEQL. Longitudes are counted east or west of the prime meridian, from 0 to 180 degrees.

If the point is in the eastern hemisphere, then the longitude is assigned the name E (eastern), if in the western - W (western). Sometimes, to designate the hemispheres of a point, +/- signs are used in coordinates. Moreover, the minus sign is attributed to coordinates located in the southern and western hemispheres. For geographic coordinates in GPS navigators, the following formats are used:

- ddd.mm.ss.s - degrees, minutes, seconds,
- ddd.dddd - degrees, decimal degrees,
— ddd.mm.mmm — degrees, minutes, decimal fractions of minutes.

Based on the materials of the book "All about GPS-navigators".
Naiman V.S., Samoilov A.E., Ilyin N.R., Sheinis A.I.

How to read GPS coordinates. Before we dive into reading GPS coordinates, it is important that you have a good understanding of the GPS system and have a basic knowledge of geographic lines of latitude and longitude. Once you understand this and read your coordinates, you can practice with the online tools.

Introduction to GPS


GPS stands for Global Positioning System; a system that is used worldwide for navigation and geodesy. It is widely used to accurately determine your location at any point on the surface of the Earth and obtain the current time at a specific location.

This is made possible by a network of 24 artificial satellites called GPS satellites that move around the earth with great speed and precision. Using low-power radio waves, devices can communicate with satellites to pinpoint their exact location on the globe.

Originally used only by the military, the GPS system became available for civilian use almost 30 years ago. It is supported by the US Department of Defense.

Latitude and longitude

The GPS system uses geographic lines of latitude and longitude to provide the coordinates of a person's location or an object's location. Reading and understanding GPS coordinates requires a basic understanding of navigation using lines of latitude and longitude. Using both sets of lines provides coordinates for various locations around the world.


Lines of latitude

Lines of latitude are horizontal lines that run from east to west across the globe. The longest and most important line of latitude is called the equator. The equator is represented as 0° latitude.

Moving north of the equator, each line of latitude increases by 1°. So there will be lines of latitude representing 1°, 2°, 3°, and so on up to 90°. The image above only displays the 15°, 30°, 45°, 60°, 75° and 90° lines of latitude above the equator. You will notice that the 90° line of latitude is depicted as a point at the North Pole.

All lines of latitude above the equator are marked with "N" to indicate north of the equator. So we have 15°N, 30°N, 45°N, and so on.

Moving south of the equator, each line of latitude also increases by 1°. There will be lines of latitude representing 1°, 2°, 3°, etc. up to 90°. The above image only shows the lines of latitude 15°, 30° and 45° below the equator. The 90° line of latitude is represented by a point at the South Pole.

All lines of latitude below the equator are denoted by an 'S' to indicate south of the equator. So we have 15°S, 30°S, 45°S, and so on.

Lines of Longitude

Lines of longitude are vertical lines that extend from the North Pole to the South Pole. The main line of longitude is called null meridian. The meridian is represented as 0° longitude.

Moving east of the prime meridian, each line of latitude increases by 1°. So there will be lines of longitude representing 1°, 2°, 3°, and so on up to 180°. The image only shows the 20°, 40°, 60°, 80°, and 90° lines of longitude east of the meridian.

All lines of longitude east of the meridian are marked with an "E" to indicate east of the Prime Meridian. Thus we have 15°E, 30°E, 45°E, and so on.

Moving west of the meridian, each latitude also increases by 1°. There will be lines of longitude representing 1°, 2°, 3°, etc. up to 180°. The image above only shows the lines 20°, 40° 60°, 80° and 90° west of the prime meridian.

All lines of longitude west of the meridian are marked with a "W" to indicate west of the meridian. Thus we have 15°W, 30°W, 45°W, and so on.

You can view more details about the latitude and longitude line by watching this YouTube video at the link below:

Reading Geographic Coordinates

Global navigation uses lines of latitude and longitude to pinpoint a specific location on the Earth's surface. It is given as geographic coordinates.

Let the Location be along a line of latitude 10°N and along a line of longitude 70°W. When stating the coordinates of a location, the line of latitude is always listed first, followed by the line of longitude. Thus, the coordinates of this place would be: 10° north latitude, 70° west longitude.
The coordinates can simply be written as 10°N, 70°W (10°N, 70°W)

However, most places on Earth are not located along the lines of latitude or longitude, but inside the figures created due to the intersection of horizontal and vertical lines. To accurately identify a person on the surface of the Earth, lines of latitude and longitude are further separated and expressed in one of three general formats:

1 / degrees, minutes and seconds (DMS)

The space between each latitude or longitude line representing 1° is divided into 60 minutes, and each minute is divided into 60 seconds. An example of such a format:

41°24'12.2"N 2°10'26.5"E The line of latitude reads 41 degrees (41°), 24 minutes (24'), 12.2 seconds (12.2') north. The line of longitude is read as 2 degrees (2°), 10 minutes (10'), 26.5 seconds (12.2') east.

2 / Degrees and Decimal Minutes (DMM)

The space between each line of latitude or longitude representing 1° is divided into 60 minutes, and each minute is further divided and expressed in decimals. An example of this format:

41 24,2028, 10,4418 2 The line of latitude is read as 41 degrees (41), 24.2028 minutes (24.2028) north latitude. The coordinate for the line of latitude represents north of the equator because it is positive. If the number is negative, it represents south of the equator.

The line of longitude reads 2 degrees (2), 10.4418 minutes (10.4418) east. The coordinate for the line of longitude represents the east of the Prime Meridian because it is positive. If the number is negative, it represents west of the meridian

3 / Decimal degrees (DD)

The space between each latitude or longitude line representing 1° is divided and expressed in decimals. An example of this format:

41,40338, 2,17403
The line of latitude reads 41.40338 degrees north. The coordinate for the line of latitude represents north of the equator because it is positive. If the number is negative, it represents south of the equator.

The line of longitude reads 2.17403 degrees East. The coordinate for the line of longitude represents the east of the Prime Meridian because it is positive. If the number is negative, it represents west of the meridian.

Reading coordinates on Google Maps

Most GPS devices provide coordinates in degrees, minutes and seconds (DMS) format, or more commonly in decimal degrees (DD) format. Popular Google Maps provide their coordinates in DMS and DD formats.

The picture above shows the location of the Statue of Liberty on Google Maps. Its location coordinates are:

40° 41′ 21.4” N 74° 02′ 40.2” W (DMS) It reads like:
"40 degrees, 41 minutes, 21.4 seconds north and 74 degrees, 2 minutes, 40.2 seconds east"

40.689263 -74.044505 (DD) Recall that decimal (DD) coordinates do not have the letters N or S to indicate whether the latitude coordinate is above or below the equator. If there is no letter W or E to indicate whether the longitude is a coordinate west or east of the Prime Meridian.

This is done using positive and negative numbers. Because the latitude coordinate is positive, the coordinate is above the equator. Because the longitude coordinate is negative, it is west of the prime meridian.

Checking GPS coordinates

Google Maps is an excellent Internet tool for checking the coordinates of places of interest.

Finding coordinates for a specific location
1/ Open Google Maps at https://maps.google.com/ and find the location of your place of interest.

2/ Right click the location and select "What's here?" In the small menu that appears.

3 / A small rectangle will appear at the bottom with the location name and coordinates in decimal degrees (DD) format.

Checking the coordinates of a specific place

Websites are available that provide a quick conversion between DMS and DD. Here is a link to Web site, which provides DMS and DD coordinates for any specific location using Google Maps.

GPS enabled devices

To take advantage of the precise accuracy of the GPS system, you need a GPS-enabled device. These devices communicate directly with GPS satellites using low power radio waves. By communicating with at least three GPS satellites, the device can pinpoint your exact location on Earth.

GPS navigation devices

Companies such as Garmin and Magellan make dedicated GPS navigation devices. They come in different sizes and can resemble a smartphone or tablet. These devices have special built-in software that uses the GPS system to help people find the shortest path to a certain place, find places of interest, and more. They are commonly used in vehicles, hiking and certain sports.


Above is an image of a GPS navigation device; Magellan RoadMate 2255T-LMB.

Smartphones

Most smartphones, especially high-end phones, support GPS and can be used as a navigation device if you have the right apps installed.

Mobile computers

Some laptops and netbooks support GPS and provide navigational information on the go.

Computer peripherals

Devices that connect to the computer via USB, Bluetooth, or expansion slots allow the computer to use the GPS system.

GPS(Global Positioning System - global positioning system) - a satellite search system, composed of a set of 24 satellites placed into orbit by the US Department of Defense and ground tracking stations, united in a common network. The Global Positioning System works in any meteorological conditions, anywhere in the world, 24 hours a day. There are no restrictions on the use of the coordinate system.

History of GPS development

GPS was originally developed for purely military purposes: the defense system needed, on the one hand, guidance tools for high-precision long-range weapons and, on the other hand, a universal navigation system available for mass use in the army. Combining these tasks into one - the creation of an accurate positioning system - from the 1960s, the US Department of Defense began to work. Seeing the potential of this system not only for military purposes, the developers were tasked with making the equipment available to a wide range of users, but on the condition that the military could at any time limit the operation of the system.

With the basic requirements for the system in place, the US Navy and Air Force began to develop a concept for using radio signals emitted from satellites for navigation purposes. Of course, the reason was the launch of the first artificial satellite. The United States monitored its flight, receiving a signal from an onboard transmitter at ground points with known coordinates in advance. The parameters of the passage of signals through the thickness of the earth's atmosphere and the Doppler frequency shift that occurs when the satellite moves along the orbit, which can be used to calculate the full orbit of the satellite, were studied. Dr. Frank McClure of the Applied Physics Laboratory (APL) pointed out that, conversely, if the full orbit of the satellite is known, then the exact position of the satellite in orbit can be calculated from the Doppler shift. Interest arose in the inverse problem: the calculation of the coordinates of the receiver based on the signals received from the satellite.

The Transit system, developed in 1964, was the forerunner of GPS. It consisted of 7 low-orbit satellites that emitted stable signals. Several ground stations monitored and corrected the orbit parameters. Users determined their coordinates on the earth's surface by measuring the Doppler frequency shift from each satellite. In 1967, the Transit system became available to civilian users. It was very quickly adapted for ship navigation, but due to a large number of shortcomings, it could not be used in aircraft and other fast-moving objects.

The second GPS predecessor, Timation, was developed under the direction of Roger Easton at NRL (Naval Research Laboratory, Naval Research Laboratory). The research program started in 1964 and included the launch of two artificial satellites carrying ultra-stable clocks, the transmission of precise time signals from the satellite, and the determination of the two-dimensional coordinates of the receiver. The main idea was to use synchronized transmitters emitting a coded signal. By measuring the delay of the signal from satellites having pre-known coordinates, it is possible to calculate the distance to the satellites and calculate the coordinates of the receiver based on this. Thus, the basic principle of GPS operation was laid and experimentally tested.

Meanwhile, the US Air Force was working on a three-dimensional system ("System 621B") with continuous access. In 1972, the operation of a system using a new method of separating satellite signals, code division based on a pseudo-random noise-like signal, was demonstrated. In this version, all satellites radiate at the same carrier frequency, which is modulated by an ultra-long pseudo-random code, individual for each satellite, which made it possible to significantly increase the noise immunity and transmit information about the position of the satellites (ephemeris) in the signal, as well as accurate time stamps. In the simplest case, the codes could be both publicly available and secret. Only open codes were available to civilian users, so it was enough to introduce deliberate errors in the information transmitted by open codes, only military equipment would remain operational, and civilian receivers would cease to function with acceptable accuracy. During the tests of this system, the concept of a global system of 16 satellites in geostationary orbits was formulated, whose projections on the earth's surface were extended 30 ° north and south of the equator.

Over the next few years, a committee assembled to coordinate the efforts of all research groups developing various navigation systems finally determined what a satellite navigation system should be. In April 1973, the Air Force was approved as the lead developer of the DNSS (Defensive Navigation Satellite System). In December of that year, the US Department of Defense approved and funded the first of three phases of development of NAVSTAR GPS, a concept-based system.

The first stage implied experimental confirmation of the suitability of the general concept of the satellite navigation system, demonstration of its inherent potential and specification of the further work plan. The second stage included full-scale engineering development, the third - the production and deployment of GPS segments. The first experimental satellites made it possible to test the range measurement method using a broadband radio signal and precision time stamps obtained from atomic clocks. The circular orbits of the satellites were successively increased from 925 km to 13000 km, and then reached the final value of 20145 km. The carrier frequency of the transmitters changed in the same way: first 400 MHz, then 1227 MHz, and later reached the current value of 1575 MHz. The military provided for the dual use of GPS satellites, in addition to the existing positioning and accurate time equipment, the satellites could carry nuclear explosion sensors (NUDET, nuclear detonation) on board, designed to detect the facts of a nuclear weapon test, identify a nuclear attack and assess the extent of destruction.

In August 1979, all the basic components of the system were put into operation, the Joint Center announced the transition to the next stage of work. The period from 1980 to 1989 was marked by attempts to maintain the sustainability of GPS development, with several significant setbacks, mainly due to funding problems. The first satellite of this period was launched into orbit in February 1989 and began operating in April. Then 23 more satellites were launched.

Simultaneously with the satellite segment, the terrestrial and user segments developed. Control was moved to Falcon Air Force Base, Colorado. The system has been fully tested and has demonstrated successful interaction between ground control stations, satellites and terminal equipment.

The first full-scale combat test for the system was the crisis in the Persian Gulf, which occurred in 1990-1991. GPS satellites have enabled the anti-Iraq coalition forces to maneuver, locate and fire with unprecedented accuracy 24 hours a day. The conditions were the most difficult - frequent sandstorms, lack of paved roads, vegetation cover and other landmarks.

In March 1994, the formation of the GPS constellation was completed with the launch of the 24th satellite. The system maintains 100-meter positioning accuracy for civilian users. It was also stated that the US Department of Defense undertakes to notify civilian users 48 hours in advance of the planned shutdown of the standard positioning service and notify of emergencies. Since 1996, satellites of a new type with advanced capabilities, including an autonomous navigation system, have been launched into orbit. It allows the satellite, in case of impossibility of contact with the ground station, to function autonomously without loss of accuracy for at least 180 days.

During the development of the original GPS concept, it was believed that 100 meters accuracy would be sufficient for civilian users. Testing in the late 1970s showed that standard precision codes produced significantly better results. The actual positioning accuracy at that time was within 20-30 m. To ensure the advantage of the military in using GPS, it was decided to introduce a deliberate limitation of accuracy for civilian users (deliberate errors were introduced into the navigation data transmitted by satellites, the accuracy of reference time signals was underestimated). The use of GPS expanded, and soon the standard hundred-meter accuracy ceased to satisfy people. At midnight between May 1 and 2, 2000, the forced accuracy shutdown was disabled.

System Composition

The GPS system consists of three segments:

• space segment
• control and management segment
• consumer navigation equipment/satellite navigation equipment segment

Space segment

At present, the GPS constellation has 32 satellites.

Work is underway to update the GPS system by replacing the orbital constellation with the fifth generation satellites (GPS-III), for which:

In Waterton, pc. Colorado, Lockheed Martin completed the assembly of 10 GPS-III satellites;

The GPS-IIISV01 satellite was launched in December 2018 by a Falcon-9 launch vehicle (the first of ten GPS-III satellites that should gradually replace other vehicles currently in orbit);

The US Air Force in September 2018 signed an additional contract with Lockheed Martin for the manufacture of 22 GPS-IIIF satellites;

Raytheon has delivered a next-generation ground control system (GCC) front-end system, known as Block 0, to support launches and orbital checks of GPS-III satellites.

The use of GPS-III satellites should provide an increase in consumer characteristics, including: power, stability, reliability, noise immunity, positioning accuracy, expand the possibilities of using the civilian L1C signal and the M-Code signal for military consumers.

The introduction of new GPS navigation signals ensures the improvement of the structure of digital information and the use of new types of modulation, as well as the transition from the structure of the navigation message of the NAV type to structures of the CNAV and CNAV-2 types, which more accurately transmit information about the GPS state in the new format (current time, KA, ephemeris-temporal information, system almanac, etc.). In this case, messages are transmitted in the form of packets of various durations instead of the superframe/frame architecture used.

A significant change in the structure of CNAV is an increase in the number of spacecraft used for their intended purpose, from 32 to 63, as well as the ability to quickly transmit data on the performance of a particular device (integrity) with a delay of no more than 6 s.

Additional features

Nuclear explosion detection system.

Satellite emergency warning system DASS.

KA blocks

Principle of operation

The satellites of the system move in a precise orbit with a period of revolution of 11 hours 58 minutes and transmit information to the Earth. GPS receivers take this information and use triangulation to calculate the exact location of the user. Essentially, the GPS receiver compares the time transmitted by the satellite with the time that the time was sent. The time difference tells the receiver how far away the satellite is. Having measured such a distance to several more satellites, the receiver can determine the position of the user and, for example, show it on the electronic map of the navigation receiver (navigator).

The navigator must be locked to the signals of at least three satellites to determine two coordinates (latitude and longitude). With four or more satellites in view, the receiver can determine three user coordinates (latitude, longitude, and altitude). Once the user's position is determined, the system can calculate other information such as speed, heading, distance traveled, distance to destination, sunrise and sunset time, and so on.

Today's GPS receivers are extremely accurate due to their parallel multichannel nature. 12 parallel GPS receivers are able to maintain satellite signals even in dense foliage or city buildings. Certain atmospheric factors and other sources of error can affect the accuracy of global system receivers.

Newer models of GPS receivers with the system WAAS(Wide Area Augmentation System) are able to improve the accuracy of determining coordinates up to 2-3 meters. This space-based system transmits information that ensures the continuity of satellite signals, as well as correction data determined by ground stations. The governments of the United States, Canada and other countries have installed differential GPS stations ( DGPS) designed to transmit corrective signals. These stations operate in coastal areas, as well as in navigable river basins. The use of the DGPS system is free. The signals transmitted by the DGPS stations not only correct errors in the calculation of the position, but also compensate for the deterioration in GPS accuracy caused by the use of the SA (Selective Availability) program run by the US Department of Defense. Additional hardware is required to use DGPS.

Sources of GPS Signal Errors

Factors that can degrade the GPS signal and thus affect accuracy are:

• Ionosphere and troposphere delays- the satellite signal weakens as it passes through the atmosphere. The GPS system uses a built-in program that calculates the average amount of delay to partially correct for this type of error.
• Branched signal- This occurs when the GNSS signal is reflected off objects such as tall buildings or rock surfaces before it reaches the receiver. This increases the signal travel time, thus causing errors.
• Receiver clock errors- The receiver's built-in clock is not as accurate as the GPS satellite's onboard atomic clock. Therefore, it may have very small timing errors.
• Orbital errors- Also known as ephemeris errors, are satellite position errors.
• Number of satellites- The more satellites the receiver can see, the better the accuracy. The global system modules will not normally operate indoors, underwater or underground.
• satellite geometry- It refers to the relative position of the satellites at any given time. The ideal satellite geometry exists when the satellites are at wide angles relative to each other.
• Intentional degradation of satellite signal performance- The main source was the presence of the so-called "restricted access" regime. In this mode, the US Department of Defense a priori introduced an error into the satellite signals, which made it possible to determine the location with an accuracy of 30-100 m. From May 1, 2000, the "limited access" mode was disabled.

GPS application

Despite the fact that the GPS project was originally aimed at military purposes, today GPS is widely used for civilian purposes. GPS receivers are sold in many electronics stores and built into mobile phones, smartphones, PDAs, and onboarders. Consumers are also offered various devices and software products that allow them to see their location on an electronic map; having the ability to lay routes taking into account road signs, permitted turns and even traffic jams; search on the map for specific houses and streets, attractions, cafes, hospitals, gas stations and other infrastructure.

• Geodesy: using GPS, the exact coordinates of points and land boundaries are determined
• Cartography: GPS is used in civilian and military cartography
• Navigation: using GPS, both sea and road navigation is carried out
• Satellite monitoring of transport: with the help of GPS, the position and speed of cars are monitored, and their movement is controlled
• Cellular: The first mobile phones with GPS appeared in the 90s. In some countries, such as the United States, this is used to quickly determine the location of a person calling 911. In Russia, in 2010, a similar project, Era-glonass, was launched.
• Tectonics, Plate Tectonics: GPS Observations of Plate Movements and Vibrations
• Outdoor Activities: There are various games that use GPS, such as Geocaching, etc.
• Geotagging: information, such as photographs, are “attached” to coordinates thanks to built-in or external GPS receivers

As is often the case with high-tech projects, the initiators of the development and implementation of the GPS (Global Positioning System - global positioning system) were the military. The satellite network project for determining real-time coordinates anywhere in the world was called Navstar (Navigation system with timing and timing - navigation system for determining time and range), while the abbreviation GPS appeared later, when the system began to be used not only in defense, but also for civilian purposes.

The first steps to deploy the navigation network were taken in the mid-seventies, while the commercial operation of the system in its current form began in 1995. Currently, 28 satellites are in operation, evenly distributed in orbits with an altitude of 20,350 km (24 satellites are enough for full-function operation).

Running a little ahead, I will say that a truly key moment in the history of GPS was the decision of the US President to cancel the so-called selective access (SA - selective availability) regime from May 1, 2000 - an error artificially introduced into satellite signals for inaccurate operation of civilian GPS receivers . From now on, the amateur terminal can determine the coordinates with an accuracy of several meters (previously, the error was tens of meters)! Figure 1 shows errors in navigation before and after disabling the selective access mode (data ).

Let's try to understand in general terms how the global positioning system works, and then touch on a number of user aspects. Consideration will begin with the principle of determining the range, which underlies the operation of the space navigation system.

Algorithm for measuring the distance from the observation point to the satellite.

Ranging is based on calculating the distance from the time delay of the propagation of a radio signal from a satellite to a receiver. If you know the propagation time of a radio signal, then it is easy to calculate the path it has traveled by simply multiplying the time by the speed of light.

Each satellite of the GPS system continuously generates radio waves of two frequencies - L1=1575.42MHz and L2=1227.60MHz. The transmitter power is 50 and 8 watts, respectively. The navigation signal is a phase-shift keyed PRN code (Pseudo Random Number code). PRN is of two types: the first, C / A-code (Coarse Acquisition code - coarse code) is used in civilian receivers, the second P-code (Precision code - exact code), is used for military purposes, and also, sometimes, for solving problems geodesy and cartography. The L1 frequency is modulated with both C/A and P-code, the L2 frequency exists only for transmitting the P-code. In addition to those described, there is also a Y-code, which is an encrypted P-code (in wartime, the encryption system may change).

The code repetition period is quite large (for example, for a P-code it is 267 days). Each GPS receiver has its own oscillator, operating at the same frequency and modulating the signal according to the same law as the satellite's oscillator. Thus, from the delay time between the same sections of the code received from the satellite and generated independently, it is possible to calculate the signal propagation time, and, consequently, the distance to the satellite.

One of the main technical difficulties of the method described above is the synchronization of clocks on the satellite and in the receiver. Even a tiny error by ordinary standards can lead to a huge error in determining the distance. Each satellite carries a high-precision atomic clock on board. It is clear that it is impossible to install such a thing in every receiver. Therefore, in order to correct errors in determining the coordinates due to the errors of the clock built into the receiver, some redundancy is used in the data necessary for unambiguous binding to the terrain (more on this later).

In addition to the navigation signals themselves, the satellite continuously transmits various kinds of service information. The receiver receives, for example, ephemeris (accurate data on the satellite's orbit), a forecast of the propagation delay of a radio signal in the ionosphere (since the speed of light changes when passing through different layers of the atmosphere), as well as information about the satellite's performance (the so-called "almanac", containing updates every 12.5 minutes information about the status and orbits of all satellites). This data is transmitted at 50 bit/s on L1 or L2 frequencies.

General principles for determining coordinates using GPS.

The basis of the idea of ​​determining the coordinates of a GPS receiver is to calculate the distance from it to several satellites, the location of which is considered known (these data are contained in the almanac received from the satellite). In geodesy, the method of calculating the position of an object by measuring its distance from points with given coordinates is called trilateration. Fig2.

If the distance A to one satellite is known, then the coordinates of the receiver cannot be determined (it can be located at any point of the sphere with radius A, described around the satellite). Let the distance B of the receiver from the second satellite be known. In this case, the determination of the coordinates is also not possible - the object is somewhere on a circle (shown in blue in Fig. 2), which is the intersection of two spheres. The distance C to the third satellite reduces the uncertainty in coordinates to two points (indicated by two bold blue dots in Fig. 2). This is already enough to unambiguously determine the coordinates - the fact is that of the two possible locations of the receiver, only one is located on the surface of the Earth (or in close proximity to it), and the second, false, turns out to be either deep inside the Earth or very high above it surface. Thus, theoretically, for three-dimensional navigation, it is sufficient to know the distances from the receiver to three satellites.

However, life is not so simple. The above considerations were made for the case when the distances from the observation point to the satellites are known with absolute accuracy. Of course, no matter how sophisticated engineers are, some error always takes place (at least according to the inaccurate synchronization of the receiver and satellite clocks indicated in the previous section, the dependence of the speed of light on the state of the atmosphere, etc.). Therefore, to determine the three-dimensional coordinates of the receiver, not three, but at least four satellites are involved.

Having received a signal from four (or more) satellites, the receiver looks for the intersection point of the corresponding spheres. If there is no such point, the receiver processor begins to correct its clock by successive approximations until it achieves the intersection of all spheres at one point.

It should be noted that the accuracy of determining the coordinates is associated not only with a precise calculation of the distance from the receiver to the satellites, but also with the magnitude of the error in setting the location of the satellites themselves. To control the orbits and coordinates of the satellites, there are four ground tracking stations, communications systems and a control center controlled by the US Department of Defense. Tracking stations constantly monitor all satellites in the system and transmit data on their orbits to the control center, where refined trajectory elements and satellite clock corrections are calculated. These parameters are entered in the almanac and transmitted to the satellites, which, in turn, send this information to all operating receivers.

In addition to those listed, there are a lot of special systems that increase the accuracy of navigation - for example, special signal processing schemes reduce errors from interference (interaction of a direct satellite signal with a reflected one, for example, from buildings). We will not delve into the features of the functioning of these devices, so as not to unnecessarily complicate the text.

After the abolition of the selective access mode described above, civil receivers are "bound to the terrain" with an error of 3-5 meters (the height is determined with an accuracy of about 10 meters). The above figures correspond to the simultaneous reception of a signal from 6-8 satellites (most modern devices have a 12-channel receiver that allows you to simultaneously process information from 12 satellites).

Qualitatively reduce the error (up to several centimeters) in the measurement of coordinates allows the mode of the so-called differential correction (DGPS - Differential GPS). The differential mode consists in the use of two receivers - one is fixed at a point with known coordinates and is called the "base", and the second, as before, is mobile. The data received by the base receiver is used to correct the information collected by the rover. Correction can be carried out both in real time and with "offline" data processing, for example, on a computer.

Typically, the base receiver is a professional receiver owned by a navigation or surveying company. For example, in February 1998, near St. Petersburg, NavGeoCom installed Russia's first differential GPS ground station. The transmitter power of the station is 100 watts (frequency 298.5 kHz), which makes it possible to use DGPS at a distance of up to 300 km from the station by sea and up to 150 km by land. In addition to terrestrial base receivers, OmniStar's satellite-based differential service system can be used for GPS differential correction. Data for correction is transmitted from several geostationary satellites of the company.

It should be noted that the main customers of differential correction are geodetic and topographic services - for a private user DGPS is of no interest because of the high cost (the OmniStar service package in Europe costs more than $1,500 per year) and bulky equipment. And it is unlikely that situations arise in everyday life when you need to know your absolute geographical coordinates with an error of 10-30 cm.

In conclusion of the part that tells about the "theoretical" aspects of the functioning of GPS, I will say that Russia, in the case of space navigation, has gone its own way and is developing its own GLONASS (Global Navigation Satellite System). But due to the lack of proper investment, only seven satellites are currently in orbit out of the twenty-four required for the normal functioning of the system ...

Brief subjective notes of a GPS user.

It just so happened that I learned about the possibility of determining my location using a wearable device the size of a cell phone in the ninety-seventh year from some magazine. However, the wonderful perspectives drawn by the authors of the article were ruthlessly broken by the price of the navigation apparatus stated in the text - almost 400 dollars!

A year and a half later (in August 1998), fate brought me to a small sports shop in the American city of Boston. What was my surprise and joy when on one of the windows I accidentally noticed several different navigators, the most expensive of which cost $ 250 (simple models were offered for $ 99). Of course, I could no longer leave the store without the device, so I began to torture the sellers about the characteristics, advantages and disadvantages of each model. I didn’t hear anything intelligible from them (and by no means because I don’t know English well), so I had to figure everything out myself. And as a result, as often happens, the most advanced and expensive model was purchased - Garmin GPS II +, as well as a special case for it and a cord for power from the car's cigarette lighter socket. The store had two more accessories for my now device - a device for attaching a navigator to a bicycle handlebar and a cord for connecting to a PC. I turned the last one in my hands for a long time, but, in the end, I decided not to buy it because of the considerable price (a little over $30). As it turned out later, I didn’t buy the cord quite correctly, because all the interaction of the device with the computer comes down to the “cream” in the computer of the route traveled (and also, I think, the coordinates in real time, but there are certain doubts about this), and even then condition of purchasing software from Garmin. Unfortunately, there is no possibility to load maps into the device.

I will not give a detailed description of my device, if only because it has already been discontinued (those who want to get acquainted with the detailed technical characteristics can do this). I will only note that the weight of the navigator is 255 grams, the dimensions are 59x127x41 mm. Due to its triangular cross-section, the device is extremely stable on the table or on the dashboard of the car (Velcro is included for a stronger fixation). Power is supplied from four AA batteries (they only last for 24 hours of continuous operation) or an external source. I will try to tell you about the main features of my device, which, I think, the vast majority of navigators on the market have.

At first glance, GPS II + can be mistaken for a mobile phone released a couple of years ago. Just looking closely, you notice an unusually thick antenna, a huge display (56x38 mm!) and a small number of keys by telephone standards.

When the device is turned on, the process of collecting information from satellites begins, and a simple animation (a rotating globe) appears on the screen. After the initial initialization (which takes a couple of minutes in an open place), a primitive sky map appears on the display with the numbers of visible satellites, and next to it is a histogram indicating the signal level from each satellite. In addition, the navigation error is indicated (in meters) - the more satellites the device sees, the more accurate the determination of coordinates, of course.

The GPS II+ interface is built on the principle of "flipping" pages (there is even a special PAGE button for this). The "satellite page" was described above, and besides it, there is a "navigation page", "map", "return page", "menu page" and a number of others. It should be noted that the described device is not Russified, but even with poor knowledge of English, you can understand its work.

The navigation page displays: absolute geographic coordinates, distance travelled, instantaneous and average movement speeds, altitude, movement time and, at the top of the screen, an electronic compass. I must say that the height is determined with a much greater error than two horizontal coordinates (there is even a special note in the user manual on this subject), which does not allow using GPS, for example, to determine the height of paragliders. But the instantaneous speed is calculated extremely accurately (especially for fast-moving objects), which makes it possible to use the device to determine the speed of snowmobiles (whose speedometers tend to lie significantly). I can give "bad advice" - when renting a car, turn off its speedometer (so that it counts fewer kilometers - after all, payment is often proportional to the mileage), and determine the speed and distance traveled using GPS (fortunately, it can measure both in miles and in kilometers ).

The average speed is determined by a somewhat strange algorithm - idle time (when the instantaneous speed is zero) is not taken into account in the calculations (in my opinion, it would be more logical to simply divide the distance traveled by the total travel time, but the creators of GPS II + were guided by some other considerations).

The distance traveled is displayed on the "map" (the memory of the device is enough for 800 kilometers - with a higher mileage, the oldest marks are automatically erased), so if you wish, you can see the scheme of your wanderings. The scale of the map changes from tens of meters to hundreds of kilometers, which is undoubtedly extremely convenient. The most remarkable thing is that in the memory of the device there are coordinates of the main settlements of the whole world! The USA, of course, is presented in more detail (for example, all areas of Boston are present on the map with names) than Russia (the location of only such cities as Moscow, Tver, Podolsk, etc. is indicated here). Imagine, for example, that you are heading from Moscow to Brest. Find "Brest" in the memory of the navigator, press the special "GO TO" button, and the local direction of your movement appears on the screen; global direction to Brest; the number of kilometers (in a straight line, of course) remaining to the destination; average speed and estimated time of arrival. And so anywhere in the world - even in the Czech Republic, even in Australia, even in Thailand ...

Equally useful is the so-called return function. The memory of the device allows you to record up to 500 key points (waypoints). The user can name each point at his own discretion (for example, DOM, DACHA, etc.), various icons are also provided for displaying information on the display. By turning on the function of returning to the point (any of the pre-recorded ones), the owner of the navigator gets the same options as in the case described above with Brest (i.e. distance to the point, estimated time of arrival and everything else). For example, I had such a case. Arriving in Prague by car and settling in a hotel, my friend and I went to the city center. Leaving the car in the parking lot, we went for a walk. After an aimless three-hour walk and dinner at a restaurant, we realized that we did not remember where we left the car. It is night outside, we are in one of the small streets of an unfamiliar city ... Fortunately, before leaving the car, I wrote down its location in the navigator. Now, by pressing a couple of buttons on the device, I found out that the car was 500 meters away from us, and after 15 minutes we were already listening to quiet music, heading by car to the hotel.

In addition to moving to the recorded mark in a straight line, which is not always convenient in city conditions, Garmin offers the TrackBack function - returning along your own path. Roughly speaking, the motion curve is approximated by a series of straight sections, and marks are placed at the break points. On each straight section, the navigator guides the user to the nearest label, and when it is reached, it automatically switches to the next label. An extremely convenient function when driving a car in unfamiliar terrain (the signal from satellites, of course, does not pass through buildings, therefore, in order to obtain data on your coordinates in densely built-up conditions, you have to look for a more or less open place).

I will not delve further into the description of the capabilities of the device - believe me that in addition to those described, it also has a lot of pleasant and necessary lotions. One change of display orientation is worth something - you can use the device both in a horizontal (car) and vertical (pedestrian) position (see Fig. 3).

One of the main charms of GPS for the user, I consider the absence of any fee for using the system. Bought once the device - and enjoy!

Conclusion.

I think there is no need to enumerate the areas of application of the considered global positioning system. GPS receivers are built into cars, cell phones and even wristwatches! Recently I came across a message about the development of a chip that combines a miniature GPS receiver and a GSM module - it is proposed to equip dog collars with devices based on it so that the owner can easily find a lost dog via a cellular network.

But in any barrel of honey there is a fly in the ointment. In this case, Russian laws act as the latter. I will not discuss in detail the legal aspects of using GPS navigators in Russia (something about this can be found), I will only note that theoretically high-precision navigation devices (which, without a doubt, are even amateur GPS receivers) are prohibited here, and their the owners are waiting for the confiscation of the device and a considerable fine.

Fortunately for users, in Russia the severity of laws is compensated by the optionality of their implementation - for example, a huge number of limousines with a washer-antenna of GPS receivers on the trunk lid drive around Moscow. All more or less serious marine vessels are equipped with GPS (and a whole generation of yachtsmen has already grown up, having difficulty navigating in space using a compass and other traditional navigational aids). I hope that the authorities will not put a spoke in the wheels of technical progress and will soon legalize the use of GPS receivers in our country (they have canceled permissions for cell phones), and will also give the go-ahead to declassify and replicate detailed maps of the area necessary for the full use of car navigation systems.