Many car owners use navigators in their cars. However, some of them do not know about the existence of two different satellite systems - the Russian GLONASS and the American GPS. From this article you will learn what their differences are and which one should be preferred.

How does the navigation system work?

The navigation system is mainly used to determine the location of an object (in this case a car) and its speed. Sometimes it is required to determine some other parameters, for example, altitude above sea level.

It calculates these parameters by establishing the distance between the navigator itself and each of several satellites located in Earth orbit. As a rule, for efficient work The system requires synchronization with four satellites. By changing these distances, it determines the coordinates of the object and other characteristics of movement. GLONASS satellites are not synchronized with the rotation of the Earth, which ensures their stability over a long period of time.

Video: GloNaSS vs GPS

What is better GLONASS or GPS and what is their difference

Navigation systems were primarily intended to be used for military purposes, and only then became available to ordinary citizens. Obviously, the military needs to use the developments of their state, because a foreign navigation system can be turned off by the authorities of that country in the event of a conflict situation. Moreover, in Russia military and civil servants are encouraged to use the GLONASS system in everyday life.

In everyday life, an ordinary motorist should not worry at all about choosing a navigation system. Both GLONASS and provide navigation quality sufficient for everyday use. In the northern territories of Russia and other countries located at northern latitudes, GLONASS satellites work more efficiently due to the fact that their travel trajectories are higher above the Earth. That is, in the Arctic, in the Scandinavian countries, GLONASS is more effective, and the Swedes recognized this back in 2011. In other regions, GPS is slightly more accurate than GLONASS in determining location. According to the Russian system of differential correction and monitoring, GPS errors ranged from 2 to 8 meters, GLONASS errors from 4 to 8 meters. But for GPS to determine the location you need to catch from 6 to 11 satellites, GLONASS is enough for 6-7 satellites.

It should also be taken into account that the GPS system appeared 8 years earlier and took a significant lead in the 90s. And over the last decade, GLONASS has reduced this gap almost completely, and by 2020, the developers promise that GLONASS will not be inferior to GPS in any way.

Most modern ones are equipped with a combined system that supports both Russian satellite system, and American. It is these devices that are the most accurate and have the lowest error in determining the vehicle’s coordinates. The stability of received signals also increases, because such a device can “see” more satellites. On the other hand, the prices for such navigators are much higher than their single-system counterparts. This is understandable - two chips are built into them, capable of receiving signals from each type of satellite.

Video: test of GPS and GPS+GLONASS receivers Redpower CarPad3

Thus, the most accurate and reliable navigators are dual-system devices. However, their advantages are associated with one significant drawback - cost. Therefore, when choosing, you need to think - is such high accuracy necessary in everyday use? Also, for a simple car enthusiast, it is not very important which navigation system to use - Russian or American. Neither GPS nor GLONASS will let you get lost and will take you to your desired destination.

Search Lectures

On approval of requirements for accuracy and methods for determining the coordinates of characteristic points of the boundaries of a land plot, as well as characteristic points of the contour of a building, structure or object of unfinished construction on a land plot

Pursuant to Part 7 of Article 38 and Part 10 of Article 41 of the Federal Law of July 24, 2007 No. 221-FZ “On the State Real Estate Cadastre” (Collected Legislation of the Russian Federation, 2007,
No. 31, art. 4017; 2008, No. 30, art. 3597, art. 3616; 2009, No. 1, art. 19; No. 19, art. 2283; No. 29, art. 3582; No. 52, art. 6410, art. 6419) order:

approve the attached requirements for the accuracy and methods of determining the coordinates of characteristic points of the boundaries of a land plot, as well as characteristic points of the contour of a building, structure or unfinished construction site on a land plot.

Minister E.S. Nabiullina

Approved

by order of the Ministry of Economic Development of Russia

from___________ No.___________

Requirements for the accuracy and methods of determining the coordinates of characteristic points of the boundaries of a land plot, as well as characteristic points of the contour of a building, structure or object of unfinished construction on a land plot

1. A characteristic point of the boundary of a land plot is the point at which the description of the boundary of the land plot changes and its division into parts.

A characteristic point of the contour of a building, structure or unfinished construction object on a land plot is the point at which the boundary of the contour of a building, structure or unfinished construction object changes its direction.

2. The location on the ground of characteristic points of the border of a land plot is described by their flat rectangular coordinates in the Gauss-Kruger projection, calculated in the coordinate system adopted for maintaining the state real estate cadastre.

The location of a building, structure or object of unfinished construction on a land plot is established by determining flat rectangular coordinates in the Gauss-Kruger projection of characteristic points of the contour of such a building, structure or object of unfinished construction in the coordinate system adopted for maintaining the state real estate cadastre.

3. The coordinates of characteristic points of the boundaries of land plots and characteristic points of the boundaries of the contour of a building, structure or object of unfinished construction on a land plot are determined by the following methods:

1) geodetic method (method of triangulation, polygonometry, trilateration, method of direct, back or combined serifs and other geodetic methods);

2) by the method of satellite geodetic measurements (determinations);

3) photogrammetric method;

4) cartometric method.

4. The identification of characteristic points of the boundary of a land plot on the ground with boundary signs is carried out at the request of the customer of cadastral work. The design of the boundary sign is determined by the contract. In the case of fixing characteristic points of the boundary of a land plot with boundary signs, their coordinates refer to the fixed (designated) centers of boundary signs.

5. The method of work to determine the coordinates of characteristic points is established by the cadastral engineer depending on the available initial information and the requirements for the accuracy of determining the coordinates of characteristic points adopted in this document.

6. The geodetic basis for determining the flat rectangular coordinates of the characteristic points of the border of the land plot are the points of the state geodetic network and points of support boundary networks.

The geodetic basis for determining the flat rectangular coordinates of the characteristic points of the contour of a building, structure or object of unfinished construction are the characteristic points of the border of the land plot.

The SKP location of a characteristic point of the contour of a building, structure or object of unfinished construction is determined relative to the nearest characteristic point of the boundary of the land plot.

7. The SKP location of the characteristic point of the border of the land plot should not exceed the standard accuracy of determining the coordinates of the characteristic points of the boundaries of the land plots (Appendix No. 1).

8. The SKP location of a characteristic point of the contour of a building, structure or object of unfinished construction should not exceed the standard accuracy of determining the coordinates of characteristic points of the contour of a building, structure or object of unfinished construction:

for lands of settlements – 1m;

for other lands – 5 m.

If the contour of a building, structure or unfinished construction object coincides with the boundary of a land plot, then the coordinates of the characteristic points of the contour of the building, structure or unfinished construction object are determined with the standard accuracy of determining the coordinates of the characteristic points of the boundaries of land plots.

If a building, structure or unfinished construction object is located on several land plots for which different standard accuracy is established, then the coordinates of the characteristic points of the outline of the building, structure or unfinished construction object are determined with an accuracy corresponding to the accuracy of determining the coordinates of the characteristic points of the outline of the building, structure or unfinished object construction with higher precision.

9. To determine the UPC location of a characteristic point, formulas are used that correspond to the methods for determining the coordinates of characteristic points.

10. Geodetic methods.

Calculation of the UCS location of characteristic points is carried out using software, through which field materials are processed. In this case, a statement (extract) from the software is attached to the boundary plan.

When processing field materials without the use of software to determine the UPC location of a characteristic point, formulas for calculating the UPC are used that correspond to geodetic methods for determining the coordinates of characteristic points.

11. Method of satellite geodetic measurements.

Calculation of the SCP location of characteristic points is carried out using software through which satellite observation materials are processed. In this case, a statement (extract) from the software is attached to the boundary plan.

12. Cartometric and photogrammetric methods.

When determining the location of characteristic points combined with the contours of geographical objects depicted on a map (plan) or aerial photograph, the SKP is taken to be equal to Mt = K*M.

Where M is the denominator of the map or aerial photograph scale.

— for the photogrammetric method, K is taken equal to the graphic accuracy (for example, when determining the location of characteristic points from photographs - 0.0001 m);

— for the cartometric method:

— for populated areas K is taken equal to 0.0005 m;

- for agricultural and other lands
K is taken equal to 0.0007 m.

13. When restoring the boundary of a land plot on the ground based on information from the state real estate cadastre, the position of the characteristic points of the boundary of the land plot is determined with standard accuracy corresponding to the data presented in Appendix No. 1.

14. If adjacent land plots have different categories, then the common characteristic points of the boundaries of the land plots are determined with an accuracy corresponding to the accuracy of determining the coordinates of the land plot with higher accuracy.

15. At the request of the customer, the contract for cadastral work may provide for determining the location of characteristic points of the boundaries of the land plot and the contours of buildings, structures or unfinished construction objects with higher accuracy than established by this procedure. In this case, the determination of the coordinates of characteristic points of the boundaries of the land plot, the contours of buildings, structures or unfinished objects is carried out with the accuracy specified in the contract.

16. Based on the calculated coordinates of the characteristic points of the border of the land plot, a catalog of them is compiled, on the basis of which the area of ​​the land plot is calculated.

17. To calculate the maximum error in determining the area of ​​a land plot, the formula is used:

∆Р — maximum error in determining the area of ​​a land plot (sq.m);

M t — the maximum value of the mean square error of the location of characteristic points of the border of the land plot, calculated taking into account the technology and accuracy of the work (m);

R - land area (sq.m);

k— coefficient of elongation of the land plot, i.e. the ratio of the greatest length of a section to its smallest width.

Appendix No. 1

Standard accuracy of determining the coordinates of characteristic points of land boundaries

Item no.	Category of land, area of ​​land plots	Mean square error, (m)
1.	Agricultural land
	land area up to 1 hectare	0,2
	land area up to 100 hectares
	land area more than 100 hectares	2,5
2.	Lands of settlements	0,2
3.	Lands of industry, energy, transport, communications, radio broadcasting, television, computer science, lands supporting space activities, lands of defense, security and other lands special purpose	0,5
4.	Lands of specially protected natural territories and objects, lands of the forest fund, lands of the water fund and reserve lands	5,0

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Testing the accuracy of GPS receivers on mobile phones

During work on one project, we needed to find out the real (and not declared) accuracy of geopositioning for various smartphones.

For this purpose, a stationary receiver from Topcon was used, the readings of which were taken as a standard. The tested devices were located in the same place. After a cold start, an additional 2 minutes were kept for a more accurate determination of the coordinates.

The following devices took part in testing:

• Fly IQ447 ($80);
• Nokia Lumia 625 ($100);
• Samsung Galaxy Tab 2;
• Industrial smartphone Motorola TC-55 – ($1500);
• Industrial smartphone Coppernic C-One ($1500);

It looked like this:

As a result, the results (the discrepancy between the coordinates of smartphones and the coordinates of a stationary receiver) were as follows:

• Fly IQ447 (GPS) – 1-3 meters;
• Coppernic C-One (GPS + GLONASS) – 2 meters;
• Motorola TC-55 (GPS + GLONASS) – 6 meters;
• Samsung Galaxy Tab 2 (GPS) – 8 meters;
• Nokia Lumia 625 (GPS) – 30 meters.

Motorola was a bit disappointed - for its price the results were expected to be better.

But most of all I was surprised Fly phone. For its price of 3,000 rubles, it turned out to be the most accurate; despite the fact that it does not have a Glonass receiver. We rechecked the results several times, but they always turned out to be excellent.

By the way, this phone- the only one who always and everywhere on an airplane from a cold start finds satellites and calculates coordinates. Despite the apparent good conditions reception, most other phones do not always find a signal from a sufficient number of satellites in flight - sometimes you can wait 20 minutes, but still not be able to determine the coordinates.

By the way, we initially did not want to take the coordinates of a point on a map (for example, Yandex) as a standard. We are aware of the possible discrepancies between maps and real coordinates. At our location near Yandex, the magnitude of this discrepancy was about 5 meters.

Special error

main reason GPS data errors are no longer a problem. On May 2, 2000, at 5:05 a.m. (MEZ), the so-called Special Error (SA) was turned off. A special error is an artificial falsification of time in the L1 signal transmitted by the satellite. For civilian GPS receivers, this error led to less accurate determination of coordinates. (error of approximately 50 m within a few minutes).

In addition, the received data was transmitted with less accuracy, which means that the transmitted position of the satellite was not correct. Thus, within a few hours there is an inaccuracy of 50-150 m in position data. In the days when the special error was active, civilian GPS devices had an inaccuracy of approximately 10 meters, and today it is 20 or usually even less. Turning off sampling error mainly improved the accuracy of the elevation data.

The reason for the special error was safety. For example, terrorists should not be able to detect important construction sites using weapons on remote control

. During the first Gulf War in 1990, the special error was partially disabled because... American troops lacked military GPS receivers.

10,000 civilian GPS devices (Magellan and Trimble) were purchased, which made it possible to freely and accurately navigate desert terrain. The special error has been deactivated due to the widespread use of GPS systems around the world. The next two graphs show how the accuracy of determining coordinates has changed after turning off the special error. The length of the boundary of the diagrams is 200 meters, the data were obtained on May 1, 2000 and May 3, 2000, within a period of 24 hours each. While coordinates with a special error are within a radius of 45 meters, without it, 95 percent of all points are within a radius of 6.3 meters.

"Geometry of satellites"

If two satellites are in the best position relative to the receiver, then the angle between the receiver and the satellites is 90 degrees. The signal travel time cannot be absolutely certain, as discussed earlier. Therefore, possible positions are marked with black circles. The intersection point (A) of the two circles is quite small and is indicated by a blue square field, which means that the determined coordinates will be quite accurate.

If the satellites are located almost in one line relative to the receiver, then, as you can see, we will get a larger area at the crosshairs, and therefore less accuracy.

The geometry of the satellites also depends a lot on tall cars or whether you are using the instrument in a car. If any of the signals are blocked, the remaining satellites will try to determine the coordinates, if this is possible at all. This can often happen in buildings when you are close to windows. If location determination is possible, in most cases it will not be accurate. The more part of the sky is blocked by any object, the more difficult it becomes to determine the coordinates.

Most GPS receivers not only show the number of satellites "caught", but also their position in the sky. This allows the user to judge whether a particular satellite is being obscured by an object and whether the data will become inaccurate when moving just a couple of meters.

Manufacturers of most instruments provide their own formulation of the accuracy of the measured values, which mainly depends on various factors. (which the manufacturer is reluctant to talk about).

DOP (Dilution of Precision) values ​​are primarily used to determine the quality of satellite geometry. Depending on what factors are used to calculate DOP values, different options are possible:

• GDOP(Geometrical Dilution Of Precision); Complete accuracy; 3D coordinates and time
• PDOP(Positional Dilution Of Precision) ; Position accuracy; 3D coordinates
• HDOP(Horizontal Dilution Of Precision); Horizontal accuracy; 2D coordinates
• VDOP(Vertical Dilution Of Precision); Vertical accuracy; height
• TDOP(Time Dilution Of Precision); temporal accuracy; time

HDOP values ​​below 4 are good, above 8 are bad. HDOP values ​​become worse if the "caught" satellites are high in the sky above the receiver. On the other hand, VDOP values ​​get worse the closer the satellites are to the horizon, and PDOP values ​​are good when there are satellites directly overhead and three more spread out on the horizon. For accurate location determination, the GDOP value should not be less than 5. The PDOP, HDOP and VDOP values ​​are part of the NMEA data of the GPGSA.

The geometry of the satellites does not cause error in position determination, which can be measured in meters. In fact, the DOP value amplifies other inaccuracies.

High DOP values ​​increase other errors more than low DOP values.

The error that occurs in position determination due to the geometry of the satellites also depends on the latitude at which the receiver is located. This is shown in the diagrams below. The diagram on the left shows the height uncertainty (the curve is shown with a special error at the beginning) which was recorded in Wuhan (China). Wuhan is located at 30.5° north latitude and is the best place where the constellation of satellites is always perfect. The diagram on the right shows the same recorded interval taken at Kasei station in Antarctica (66.3°S latitude). Due to the less than ideal constellation of satellites at this latitude, more severe errors occurred from time to time. In addition, the error occurs due to the influence of the atmosphere - the closer to the poles, the greater the error.

Satellite orbits Although the satellites are in fairly well-defined orbits, slight deviations from the orbits are still possible due to gravity. The Sun and Moon have little influence on the orbits.

The orbit data is constantly adjusted and corrected and is regularly sent to the receiver in the empirical memory. Therefore, the impact on accuracy

location determination is quite small and if an error occurs, it is no more than 2 meters.

Effects of signal reflections

Another source of inaccuracy is a decrease in the speed of signal propagation in the troposphere and ionosphere. The speed of signal propagation in outer space is equal to the speed of light, but in the ionosphere and troposphere it is less. In the atmosphere at an altitude of 80 - 400 km, solar energy creates a large number of positively charged ions. Electrons and ions are concentrated in the four conductive layers of the ionosphere (D-, E-, F1-, and F2 layers).
These layers refract electromagnetic waves emanating from satellites, which increases the travel time of signals. Basically, these errors are corrected by the computational actions of the receiver. Various speed options when passing through the ionosphere for low and high frequencies are well known for normal conditions. These values ​​are used when calculating location coordinates. However, civilian receivers are unable to adjust for unexpected changes in signal transmission, which can be caused by strong solar winds.

It is known that during the passage of the ionosphere, electromagnetic waves slow down in inverse proportion to the area of ​​their frequency (1/f2). This means that low frequency electromagnetic waves decelerate faster than high frequency electromagnetic waves. high frequencies. If the high and low frequency signals that reached the receiver allowed the difference in their arrival times to be analyzed, then the time of passage through the ionosphere would also be calculated. Military GPS receivers use signals of two frequencies (L1 and L2), which behave differently in the ionosphere, and this eliminates another error in the calculations.

The influence of the troposphere is the next reason why the signal travel time increases due to refraction. The causes of refraction are different concentrations of water vapor in the troposphere, depending on the weather. This error is not as large as the error that occurs when passing through the ionosphere, but it cannot be eliminated by calculation. To correct this error, an approximate correction is used in the calculation.

The next two graphs show the ionospheric error.

The data shown on the left was obtained with a single-frequency receiver, which cannot correct for ionospheric error. The graph on the right was obtained with a dual-frequency receiver that can correct for ionospheric error. Both diagrams have approximately the same scale (Left: Latitude -15m to +10m, Longitude -10m to +20m. Right: Latitude -12m to +8m, Longitude -10m to +20m). The right graph shows higher accuracy.

Using WAAS and EGNOS you can set up "maps" of weather conditions over different regions. The corrected data is sent to the receiver and significantly improves accuracy.

Clock inaccuracy and rounding errors

Even though the receiver time is synchronized with the satellite time during position determination, there is still a time inaccuracy, which leads to a 2m error in position determination. Rounding and receiver computational errors have an error of approximately 1m.

Relativistic effects

This section does not provide a complete explanation of the theory of relativity. In everyday life we ​​are not aware of the importance of the theory of relativity. However, this theory affects many processes, including the proper functioning of the GPS system. This influence will be briefly explained below.

As we know, time is one of the main factors in GPS navigation and should be equal to 20-30 nanoseconds to ensure the necessary accuracy. Therefore, it is necessary to take into account the speed of the satellites (approximately 12,000 km/h)

In general, the clocks on the satellites seem to move a little faster. The time deviation for an observer on Earth would be 38 microseconds per day and would result in a total error of 10 km per day. To avoid this mistake there is no need to constantly make adjustments. The clock frequency on the satellites was set to 10.229999995453 MHz instead of 10.23 MHz, but the data is used as if it had a standard frequency of 10.23 MHz. This trick solved the problem of the relativistic effect once and for all.

But there is another relativistic effect that is not taken into account when determining location using the GPS system. This is the so-called Sagnak effect and is caused by the fact that the observer on the surface of the Earth is also constantly moving at a speed of 500 m/s (speed at the equator) due to the fact that the planet rotates. But the influence of this effect is small and its adjustment is difficult to calculate, because depends on the direction of movement. Therefore, this effect is taken into account only in special cases.

GPS system errors are shown in the following table. Partial values ​​are not constant values, but are subject to differences. All numbers are approximate values.

Lecture on the anatomy of mobile devicesV. Navigation (GPS, GLONASS, etc.) in smartphones and tablets. Sources of errors. Testing methods.

Until recently, it was possible to buy devices called “Navigators” in retail chains. Main function The performance of these devices fully corresponded to their name, and they usually performed it well.

At that time, practically the only normally functioning navigation system in the world was the American GPS (Global Positioning System), and it was enough for all needs. Actually, the words “navigation” (navigator) and GPS were synonymous at that time.

Everything changed when manufacturers of PDAs (handheld computers), and then smartphones and tablets, began to build navigation support into their devices. Physically, it was implemented in the form of built-in receivers of navigation signals. Sometimes navigation support could be found even in push-button phones.

From that moment on, everything changed. Navigators, as separate devices, have almost disappeared from both production and sale. Consumers have switched en masse to using smartphones and tablets as navigators.
In the meantime, two more navigation systems were successfully put into operation - the Russian GLONASS and the Chinese Beidou (Beidou, BDS).

But this does not mean that the quality of navigation has improved. The navigation function in these devices (smartphones and tablets) has no longer become the main one, but one of many.

As a result, many users began to notice that not all smartphones are “equally useful” for navigation purposes.

This is where we come to the problem of identifying the sources of errors in navigation, including the question of the role of dishonesty of device manufacturers in this matter. Sad but true.

But before blaming manufacturers for all their sins, let’s first look at the sources of errors in navigation. For producers, as we will find out later, are not to blame for all sins, but only for half. :)

Navigation errors can be divided into two main classes: caused by reasons external to the navigation device, and internal.

Let's start with external reasons . They arise mainly due to the unevenness of the atmosphere and the natural technical error of measuring instruments.

Their approximate contributions are:

Signal refraction in the ionosphere ± 5 meters;
- Satellite orbit fluctuations ± 2.5 meters;
- Satellite clock error ± 2 meters;
- Tropospheric unevenness ± 0.5 meters;
- The influence of reflections from objects± 1 meter;
- Measurement errors in the receiver ± 1 meter.

These errors have a random sign and direction, so the final error is calculated in accordance with probability theory as the root of the sum of squares and is 6.12 meters. This does not mean that the error will always be this way. It depends on the number of visible satellites, their relative position, and most of all, on the level of reflections from surrounding objects and the influence of obstacles on the weakening of satellite signals. As a result, the error may be either higher or lower than the given “averaged” value.

Signals from satellites may weaken, for example, in the following cases:
- when indoors;
- when located between closely spaced high objects (between high-rise buildings, in a narrow mountain gorge, etc.);
- while in the forest. Experience shows that dense, tall forest can make navigation significantly more difficult.

These problems are due to the fact that high-frequency radio signals travel like light - that is, only within a line of sight.

Sometimes navigation, albeit with errors, can also work on signals reflected from obstacles; but when repeatedly reflected, they become so weak that navigation stops working with them.

Now let's move on to the "internal" causes of errors in navigation; those. which are created by the smartphone or tablet itself.

Actually, there are only two problems here. Firstly, poor sensitivity of the navigation receiver (or problems with the antenna); secondly, the “crooked” software of a smartphone or tablet.

Before looking at specific examples, let's talk about ways to check the quality of navigation.

Navigation testing methods.

1. Testing navigation in “static” mode (with the smartphone/tablet in a stationary position).

This check allows you to determine the following parameters:
- speed of initial determination of coordinates during a “cold start” (measured by the clock);
- list navigation systems, with which this smartphone/tablet works (GPS, GLONASS, etc.);
- estimated accuracy of coordinate determination;
- speed of determining coordinates during a “hot start”.

These parameters can be determined using both regular navigation programs and special test programs (which is more convenient).

The rules for static testing are very simple: testing must be done in open space(wide street, square, field, etc.) and when the Internet is turned off. If the last requirement is violated, the “cold start” time can be significantly accelerated due to direct downloading of satellite orbits from the Internet (A-GPS, assisted GPS) instead of determining them from signals from the satellites themselves; but it will no longer be “fair”, since this will no longer be the pure work of the navigation system itself.

Let's look at an example of how the AndroiTS navigation testing program works (there are analogues):

(click to enlarge)

The picture just presented shows that the smartphone works with three navigation systems: American GPS, Russian GLONASS and Chinese Beidou (BDS).

At the bottom of the screenshot you can see the successfully determined coordinates of the current location. The value of one degree in latitude is approximately 100 km; accordingly, the price of a unit of the lowest rank is 10 cm.

The value of one degree in longitude is different for different geographical locations. At the equator it is also about 100 km, and near the poles it decreases to 0 (at the poles the meridians come closer together).

To the right of the column indicating the nationality of the satellites is a column with satellite numbers. These numbers are strictly attached to them and do not change.

Next come columns with colored bars. The size of the bars indicates the signal level, and the color indicates whether or not they are being used by the navigation system. Unused satellites are indicated by gray bars. The color of those used depends on their signal level.

The next column is also the signal level from navigation satellites, but in numbers (“conventional units”).

Then there is a column with green checkmarks and red dashes - this is a repetition of information about whether the satellite is being used or not.

In the top line, the word "ON" indicates the status of the navigation state; in this case, this means that the determination of coordinates is allowed in the smartphone settings and they are determined. If the status is “WAIT”, then determination of coordinates is allowed, but the required number of satellites has not yet been found. The "OFF" status means that coordinate determination is prohibited in the smartphone settings.

Then a circle with concentric circles and the number 5 indicates the estimated accuracy of determining coordinates in this moment- 5 m. This value is calculated based on the number and “quality” of satellites used and assumes that the processing of data from satellites in a smartphone is done without errors; but, as we will see later, this is not always the case.

As the satellites move, all this data should change, but the coordinates (in the bottom line) should change slightly.

Unfortunately, this application does not show the time spent on the initial determination of coordinates ("cold start"), and neither do other similar applications. This time must be “timed” manually. If the “cold start” time was less than a minute, then this is an excellent result; up to 5 minutes – good; up to 15 minutes – average; more than 15 minutes – bad.

To determine the “hot start” speed, just exit the testing program and log in again after a few minutes. As a rule, during the launch of the test program, it manages to determine the coordinates and immediately presents them to the user. If the delay in presenting coordinates during a “hot start” exceeds 10 seconds, then this is already suspiciously long.

Effect quick definition coordinates during a “hot start” is due to the fact that the navigation system remembers the last calculated satellite orbits and does not need to re-determine them.

So, we’ve sorted out testing navigation in “static” mode.

Let's move on to the 2nd point of testing navigation - in motion.

The main purpose of navigation is to lead us to the right place while moving, and without testing while moving, the test would be incomplete.

In the process of movement, from a navigation point of view, there are three types of terrain: open terrain, urban areas and forest.

Open areas are ideal navigation conditions; there are no problems here (except for very “sucky” devices).

Urban development in most cases is characterized by the presence of a high level of reflections and a slight decrease in the signal level.

The forest “works” the other way around – a significant weakening of the signal and a low level of reflections.

First, let's look at a sample of an almost "ideal" track:

The picture shows two tracks: there/back (this will continue to be the case in almost all pictures). Such pictures allow you to make a reliable conclusion about the quality of navigation, since you can compare two almost identical tracks with each other and with the road. Everything is fine in this picture - the track vibrations are within the limits of natural error. In the upper part, the passage on different sides of the roundabout is adequately drawn. In some places there is a noticeable discrepancy between the tracks, probably caused by reflections of the signal from water surface and from the metal structures of the bridge over the river. And in some - an almost perfect coincidence.

Now let's look at several typical cases of "problem" tracks.

Let's look at the GPS track of a smartphone, which was affected by a decrease in signal level in a high forest:

The divergence of the tracks from each other and from the road is noticeable, but far from catastrophic. In this case, the accuracy of smartphone navigation decreased within the limits of “natural decline” for such conditions. Such a smartphone must be considered suitable for navigation purposes.

On the right side of the screenshot, the discrepancies between the tracks and the road are clearly visible. Such discrepancies in the conditions of such a “well-shaped” development are almost inevitable, and in this case they do not in any way indicate against the smartphone being tested.

Theoretically, the more navigation systems a smartphone (tablet) supports, the more satellites it uses for navigation and the smaller the error should be.
In practice, this is not always the case. Quite often, due to crooked software, a smartphone cannot correctly connect data from different systems and, as a result, abnormal errors occur. Let's look at a few examples.

Take, for example, this track:

The screenshot just shown shows a needle-shaped ejection, which could not be the result of any interference: the path passed through a low-rise building without dense forested plantings. This release is entirely on the conscience of the “crooked” software.

But these were still “flowers”. There are smartphones where abnormal navigation errors are no longer flowers, but berries:

When recording of this track anomalous errors of “crooked” software were combined with weakening of signals in a high forest. The result is a track from which it is simply impossible to guess that the path there and back was taken along the same path by a sober person. :)
And the thick bunch of lines at the top is the “path” of a motionless smartphone during a rest stop. :)

There is another type of anomalous error associated with a pause in the flow of data coming from the navigation receiver to the computing part of the smartphone:

This picture shows that part of the path (about 300 m) passed in a straight line, and partly directly through the water. :)

In this case, the smartphone simply connected the points where the coordinate stream disappeared and appeared with a straight line. Their loss could be associated either with a decrease in the number of visible satellites below a critical number, or with “crooked” software and even hardware problems (although the latter is unlikely).

In the event of a complete loss of signals from satellites, navigation programs usually do not connect the points of loss and appearance with straight lines, but simply leave an “empty space” (this results in a gap in the track):

This picture shows a break in the track in the place where part of the path passed through an underground passage with a complete loss of visibility of all satellites.

After studying the causes and typical navigation errors, it’s time go to conclusions.

The best navigation, as you would expect, is found in smartphones and tablets of “high” brands. Problems in the form of anomalous errors have not yet been detected with them. And, of course, the more navigation systems a device supports, the better. True, support for the Chinese Beidou still makes sense when using the device in regions and countries located near the Middle Kingdom. The Chinese navigation system is not global, but “local” (for now). So support for GPS and GLONASS will be quite enough.

If a smartphone or tablet is not of very “renowned” origin, then there may or may not be problems with navigation. Before using it in combat, it is recommended to test it both statically and in motion in different environments, so that later it does not present any unpleasant surprise. In most cases mobile devices those with GPS support alone bring fewer problems, although their accuracy is lower than that of multi-system systems.

Unfortunately, when choosing a smartphone (tablet) with good navigation, it is quite difficult to navigate through reviews of devices on the Internet. The overwhelming number of IT portals ignore checking navigation on the move and in difficult conditions. This check is done only on this portal () and literally on a couple of others.

Finally It must be said that not only smartphones and tablets, but also many other devices are now equipped with navigation aids. They are installed, for example, in cameras, video cameras, GPS trackers, car video recorders, smart watches, some specialized types of devices, and even in electronic system taxation of drivers of Russian heavy trucks "Platon".

Your Doctor.
20.01.2017

The user of a GPS navigator is always interested in the real accuracy of GPS navigation and the degree of confidence in its readings. How close can you get to any navigational hazard relying solely on your GPS receiver? Unfortunately, there is no clear answer to this question. This is due to the statistical nature of GPS navigation errors. Let's take a closer look at them.

The speed of propagation of radio waves is influenced by the ionosphere and troposphere, ionospheric and tropospheric refraction. This is the main source of errors after turning off SA. The speed of radio waves in vacuum is constant, but changes when the signal enters the atmosphere. The time delay is different for signals from different satellites. Radio wave propagation delays depend on the state of the atmosphere and the satellite's altitude above the horizon. The lower , the longer the path its signal travels through the atmosphere and the greater the distortion. Most receivers exclude signals from satellites with an elevation of less than 7.5 degrees above the horizon.

In addition, atmospheric interference depends on the time of day. After sunset, the density of the ionosphere and its influence on radio signals decreases, a phenomenon well known to shortwave radio operators. Military and civilian GPS receivers can autonomously determine atmospheric signal delay by comparing delays at different frequencies. Single-frequency consumer receivers make an approximate correction based on the forecast transmitted as part of the navigation message. The quality of this information is Lately increased, which further increased the accuracy of GPS navigation.

SA mode.

To maintain the advantage of high accuracy for military GPS navigators, the SA (Selective Availability) access restriction mode was introduced in March 1990, artificially reducing the accuracy of a civilian GPS navigator. When the SA mode is enabled, an error of several tens of meters is added in peacetime. In special cases, errors of hundreds of meters may be introduced. The US government is responsible for the performance of the GPS system to millions of users, and it can be assumed that the re-enablement of SA, much less such a significant reduction in accuracy, will not be introduced without sufficiently serious reasons.

Precision coarsening is achieved by chaotic shifting the transmission time of the pseudo-random code. Errors arising from SA are random and equally probable in each direction. SA also affects the GPS heading and speed accuracy. For this reason, a stationary receiver will often show slightly varying speed and heading. So the degree of impact of SA can be assessed by periodic changes in course and speed according to GPS.

Errors in ephemeris data during GPS navigation.

First of all, these are errors associated with the deviation of the satellite from the calculated orbit, clock inaccuracies, signal delays in electronic circuits. These data are corrected from the Earth periodically, and errors accumulate in the intervals between communication sessions. Due to its small size, this group of errors is not significant for civilian users.

Extremely rare, larger errors may occur due to sudden information failures in the satellite's memory devices. If such a failure is not detected by self-diagnosis, then until the ground service detects the error and transmits a command about the failure, the satellite may transmit incorrect information for some time. There is a so-called violation of continuity or, as the term integrity is often translated, the integrity of navigation.

The influence of the reflected signal on the accuracy of GPS navigation.

In addition to the direct signal from the satellite, the GPS receiver can also receive signals reflected from rocks, buildings, passing ships - the so-called multipath propagation. If the direct signal is blocked from the receiver by the ship's superstructure or rigging, the reflected signal may be stronger. This signal travels a longer path, and the receiver “thinks” it is further from the satellite than it actually is. As a rule, these errors are much less than 100 meters, since only nearby objects can produce a strong enough echo.

Satellite geometry for GPS navigation.

Depends on the location of the receiver relative to the satellites by which the position is determined. If the receiver has picked up four satellites and they are all in the north, the satellite geometry is bad. The result is an error of up to 50-100 meters or even the inability to determine coordinates.

All four dimensions are from the same direction, and the area where the position lines intersect is too large. But if 4 satellites are located evenly on the sides of the horizon, then the accuracy will increase significantly. Satellite geometry is measured by the geometric factor PDOP (Position Dilution Of Precision). The ideal satellite location corresponds to PDOP = 1. Large values ​​indicate poor satellite geometry.

PDOP values ​​less than 6.0 are considered suitable for navigation. In 2D navigation, HDOP (Horizontal Dilution Of Precision) is used, less than 4.0. A vertical geometric factor VDOP less than 4.5 and a temporal TDOP less than 2.0 are also used. PDOP serves as a multiplier to account for errors from other sources. Each pseudo-range measured by the receiver has its own error, depending on atmospheric interference, errors in ephemeris, SA mode, reflected signal, and so on.

So, if the expected values ​​of the total signal delays for these reasons, URE - User Range Error or UERE - User Equivalent Range Error, in Russian EDP - equivalent rangefinder error, add up to 20 meters and HDOP = 1.5, then the expected location error will be 20 x 1.5 = 30 meters. GPS receivers present information differently to evaluate accuracy using PDOP.

In addition to PDOP or HDOP, GQ (Geometric Quality) is used - the inverse value of HDOP, or a qualitative assessment in points. Many modern receivers display EPE (Estimated Position Error) directly in distance units. EPE takes into account the location of the satellites and the forecast of signal errors for each satellite depending on SA, the state of the atmosphere, and satellite clock errors transmitted as part of the ephemeris information.

Satellite geometry also becomes an issue when using a GPS receiver inside Vehicle, in a dense forest, mountains, near tall buildings. When signals from individual satellites are blocked, the position of the remaining satellites will determine how accurate the GPS position will be, and their number will indicate whether the position can be determined at all. A good GPS receiver will show you not only which satellites are in use, but also their location, azimuth and elevation, so you can determine if a given satellite is having difficulty receiving.

Based on materials from the book “All about GPS navigators.”
Naiman V.S., Samoilov A.E., Ilyin N.R., Sheinis A.I.