Global Positioning System

The GPS (Global Positioning System: global positioning system) or NAVSTAR-GPS, is a global navigation satellite system (GNSS or Global Navigation Satellite System ) that allows determining the position of an object, a person, a vehicle or a ship, with an accuracy of up to centimeters (if differential GPS is used), although the usual is a few meters of precision. The system was developed, installed, and currently operated by the United States Department of Defense .

Summary

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  • 1 History
  • 2 GPS in Cuba
    • 1 Technical characteristics and performance
  • 3 Evolution of the GPS system
  • 4 Operation
    • 1 Earth in Space
    • 2 Satellites orbiting the Earth
    • 3 Positioning with GPS
      • 3.1 Latitude: Northern and Southern Hemispheres
      • 3.2 Longitude: East, West
    • 4 Global Positioning System
      • 4.1 Function of a GPS receiver
      • 4.2 Uses of a GPS receiver
    • 5 GPS receiver
  • 5 DGPS or differential GPS
  • 6 Advantages of GPS compared to normal guidance systems
  • 7 Use the data obtained with GPS receivers with PC’s
  • 8 Sources

History

In 1952 , the Soviet Union launched the Sputnik I satellite into space, which was monitored by observing the Doppler effect of the signal it transmitted. Due to this fact, it began to be thought that, in the same way, the position of an observer could be established by studying the Doppler frequency of a signal transmitted by a satellite whose orbit was precisely determined.
The US Navy quickly applied this technology to provide its fleets’ navigation systems with up-to-date and accurate position observations. Thus arose the TRANSIT system, which became operational in 1964 , and by 1967 it was also available for commercial use.
Position updates, at that time, were available every 40 minutes and the observer had to remain almost static in order to obtain adequate information.

Later, in that same decade and thanks to the development of atomic clocks, a constellation of satellites was designed, each carrying one of these clocks and all being synchronized based on a certain time reference.
In 1973 , the programs of the Navy and the United States Air Force were combined (the latter consisting of a coded transmission technique that provided precise data using a signal modulated with a PRN code (Pseudo-Random Noise: pseudo noise). -random), in what became known as the Navigation Technology Program, later renamed NAVSTAR GPS.
Between 1978 and 1985Eleven NAVSTAR experimental prototype satellites were developed and launched, followed by other generations of satellites, until the current constellation was completed, which was declared “initial operational capability” in December 1993 and “full operational capability” in April 1993. 1995 . On May 2, 2000 , Bill Clinton signed an agreement to open A-GPS (Accurate GPS) to all civil uses, which in 2009 led to an agreement with ICAO (International Civil Aviation Organization) on the use of GPS in air navigation. She accepted the offer. [one]

GPS in Cuba

In Cuba ; with charitable purposes (to name it in some way) in conjunction with Chinese Technologies , GPS or On-Board Computers, they resolved to integrate these Systems to the land and maritime environments of our country.
The main purposes of this follow-up, was or is for greater control OVER ALL THINGS to save the fuel used by our mobiles, which before this time, did not have an exact control of this precious and essential treasure. Not only can this be controlled through this System; speed, route deviations among other factors, from then on they have entered into greater and effective control to date.
Currently, we are working under Deferred Systems, which means that the trajectory is analyzed after the mobile performs X round-trip trajectory, it is expected that in time, our country will be able to work in real time in order to carry out an ANALYSIS more exactly.

Technical characteristics and performance

Satellite operator controlling the NAVSTAR-GPS constellation at Schriever Air Base.
Launch of satellites for the NAVSTAR-GPS constellation by means of a Delta rocket.
The Global Navigation Satellite System is made up of:

  • Satellite system: It is made up of 24 units with synchronized trajectories to cover the entire surface of the globe. More specifically, spread over 6 orbital planes of 4 satellites each. The electrical energy they require for their operation is acquired from two panels made up of solar cells attached to their sides.
  • Ground stations: They send control information to the satellites to control the orbits and perform the maintenance of the entire constellation.
  • Receiving terminals: They indicate the position in which they are; Also known as GPS units, they are the ones that we can buy in specialized stores. (on-board computers)

Spatial segment

  • Satellites in constellation: 24 (4 × 6 orbits)

Altitude: 23,200 Km.

Period: 11 h 56 min. (12 sidereal hours)

Inclination: 55 degrees (with respect to the Earth’s equator).

Shelf life: 7.5 years

  • Control segment (ground stations)

Main station: 1

Ground antenna: 4

Monitoring station (tracking): 5

  • RF signal

Carrier frequency:

Civil – 1575.42 MHz (L1). Use the Approximate Acquisition Code (C / A).

Military – 1227.60 MHz (L2). Use the Precision Code (P), encrypted.

Signal power level: –160 dBW (on the ground surface).

Polarization: circular clockwise.

  • Accuracy

Position: officially they indicate approximately 15 m (in 95% of the time). In reality, a 12 parallel channel monofrequency portable GPS offers an accuracy of 2.5 to 3 meters in more than 95% of the time. With WAAS / EGNOS / MSAS activated, the accuracy increases from 1 to 2 meters.

Time: 1 ns

  • Global coverage
  • User capacity: unlimited
  • Coordinate system:

World Geodetic System 1984 (WGS84).

Centered on Earth, fixed.

  • Integrity: notification time of 15 minutes or more. It is not enough for civil aviation.
  • Availability: 24 satellites (70%) and 21 satellites (98%). It is not sufficient as a primary means of navigation.

Evolution of the GPS system

Professional GPS station and receiver for centimeter accuracy.

GPS is evolving towards a more robust system (GPS III), with greater availability and that reduces the complexity of GPS augmentations. Some of the planned enhancements include:

  • Incorporation of a new signal in L2 for civil use.
  • Adding a third civil signal (L5): 1176.45 MHz
  • Protection and availability of one of the two new signals for Security for Life (SOL) services.
  • Improved signal structure.
  • Increase in signal power (L5 will have a power level of –154 dB).
  • Improved accuracy (1 – 5 m).
  • Increase in the number of monitoring stations: 12 (double)
  • Enable better interoperability with Galileo L1 frequency

The GPS III program pursues the goal of ensuring that GPS will meet anticipated military and civilian requirements for the next 30 years. This program is being developed to use a 3-stage approach (one of the transition stages is GPS II); very flexible, allows future changes and reduces risks. The development of GPS II satellites began in 2005, and the first of them will be available for launch in 2012, with the goal of achieving the full transition from GPS III in 2017. The challenges are as follows:

  • Represent the requirements of users, both civil and military, regarding GPS.
  • Limit GPS III requirements within operational objectives.
  • Provide flexibility to allow future changes to meet user requirements through 2030.
  • Provide robustness for the growing reliance on precise time and position determination as an international service.

The system has evolved and new positioning systems have been derived from it. IPS-2 refers to the Inertial Positioning System, an inertial positioning system, a data capture system that allows the user to perform measurements in real time and in motion, the so-called Mobile Mapping. This system obtains 3D mobile cartography based on a device that collects a laser scanner, an inertial sensor, GNSS system and an odometer on board a vehicle. Great accuracies are achieved, thanks to the three positioning technologies: IMU + GNSS + odometer, which, working at the same time, give the option of measuring even in areas where the satellite signal is not good.

Functioning

Earth in Space

The galaxy or the Milky Way , is simply one more in the immensity of the Universe. Our closest star, the Sun, is just one of the billions of stars in the Milky Way . The planet Earth is one of 9 satellites that give laps around the sun in an elliptical orbit. These planets, from the closest to the furthest from the Sun, are: Mercury , Venus , Earth, Mars , Jupiter , Saturn , Uranus , Neptune and Pluto. The rules that govern the motion of these solar satellites (the planets) are studied in the discipline of Celestial Mechanics , and were discovered by exceptional scientists like Johannes Kepler and Isaac Newton hundreds of years ago.

The movement of the 9 solar planets is like a fascinating clockwork machinery. The force that holds them together and determines their relative movements is ” gravity .” The closer a planet is to the Sun, the gravitational force it feels is greater and it must move faster in its orbit to avoid falling into the Sun. For example, Earth, located about 150,000,000 km from the Sun, it travels in its orbit at an average speed of about 30 kilometers per second and completes one revolution around the Sun in one year.

Several planets, in turn, have one or more satellites orbiting them. For example, the only natural satellite of the Earth, the Moon , lies at an average distance of about 385,000 kilometers from Earth and makes a complete revolution around it in about 29 days. The different positions of the Moon with respect to the Earth determine its four phases: full moon, new moon, first quarter and last quarter. The definition of satellite is therefore quite easy to guess. It is simply one body orbiting another. Gravity is the force of attraction that makes possible the relative movement of the orbits described by the satellites.

Satellites orbiting the Earth

Detailed knowledge of the rules of Celestial Mechanics and the study of the movement of natural satellites has allowed scientists to design and put into orbit artificial satellites around Earth and Mars (such as the Viking ). Powerful rockets are used to launch satellites into space . If the launch speed is very low, the satellite will fall back to Earth attracted by the force of gravity., in the same way that when throwing a stone it falls back to the earth’s surface. On the other hand, if the launch speed is too high, the Earth’s gravitational force will not be enough to keep the satellite in orbit and it will escape into space. Today there are many artificial satellites orbiting the Earth with different purposes:

  • Telecommunications
  • Weather forecast
  • Military applications
  • Scientific investigation
  • Geophysics

The orbits of some satellites are synchronized with the rotation period of the Earth. If their speeds exactly coincide with that of the Earth’s rotation, the satellites are called geostationary and they always remain at the same point in the sky with respect to the Earth. If the speeds are different from the Earth’s rotation, then the satellites “rise” and “set” at different times, just like the Moon . Some go off and on multiple times throughout a day. Some form of communication is needed to send orders and receive responses from satellites to Earth. Although there are many ways to do this, the basic alphabet of communication consists of radio waves, such as those used to broadcast television and radio programs .

GPS positioning

Determining our position on Earth means providing the latitude and longitude of a certain point on the Earth’s surface. Therefore, most receivers provide the values ​​of these coordinates in units of degrees (°) and minutes (‘). Both latitude and longitude are angles and therefore must be measured with respect to a well-defined 0 ° reference.

Latitude: Northern and Southern Hemispheres

Latitude is measured relative to the Equator (latitude 0 °). If a certain point is in the northern hemisphere or southern hemisphere , its latitude coordinate will be accompanied by the letter N or S in each case. Another type of nomenclature refers north latitudes with positive numbers and south latitudes with negative numbers.

Length: East, West

For historical reasons, longitude is measured relative to the Greenwich meridian . If we measure an angle to the east or west of the Greenwich meridian, we write the letter E or W depending on the case accompanying the number that gives the longitude. Negative numbers are sometimes used. For example, the following longitude values ​​are equivalent: W 90 °; E 270 °; and -90 °.

Global Positioning System

GPS (Global Positioning System) is a constellation of 24 artificial satellites uniformly distributed in a total of 6 orbits, so that there are 4 satellites per orbit. This configuration ensures that at least 8 satellites can always be “seen” from almost anywhere on the earth’s surface. GPS satellites orbit the Earth at an altitude of about 20,000 km and travel two full orbits every day. They describe a type of orbit such that they “rise” and “set” twice a day. Each satellite transmits radio signals to Earthwith information about its position and when the signal is emitted. We can receive this information with GPS receivers (GPS receivers), which decode the signals sent by several satellites simultaneously and combine their information to calculate their own position on Earth, that is, their latitude and longitude coordinates with an accuracy of about 10 meters. There are more sophisticated receivers that can determine position with an accuracy of a few millimeters.

Function of a GPS receiver

As we have said before, the GPS receivers receive the precise information of the time and the position of the satellite . Exactly, it receives two types of data, the Almanac data, which consists of a series of general parameters on the location and operation of each satellite in relation to the rest of the satellites in the network, this information can be received from any satellite, and once the GPS receiver has the information from the last Almanac received and the precise time, knows where to look for satellites in space; The other series of data, also known as Ephemeris, refers to the precise data, only, of the satellite that is being captured by the GPS receiver, they are exclusive orbital parameters of that satellite and are used to calculate the exact distance from the receiver to the satellite . When the receiver has captured the signal from at least three satellites, it calculates its own position on Earth using trilateration [2]of the position of the captured satellites, and they present us with the calculated Longitude , Latitude and Altitude data . GPS receivers can and usually do receive signals from more than three satellites to calculate their position. In principle, the more signals they receive, the more accurate the position calculation. Taking into account that the initial conception of this system was to make military use of it, it should be noted that the receivers found on the market are for civilian use, and that they are subject to a precision degradation that ranges from 15 at 100 meters RMS or 2DRMS depending on the current geostrategic circumstances, as interpreted by the US Department of Defense, who manages and provides this service. This degradation is regulated by the Selective Availability Program of the US Department of Defense or SA (Selective Availability) and, as we have indicated before, introduces an error in the transmission of the position for the receivers of civil use. This is, naturally, to maintain a strategic advantage during military operations that require it. It follows from all this that GPS receivers usually have a nominal error in the calculation of the position of approximately 15 m. RMS that can increase up to 100 m. RMS when the Government of the USA. deems it appropriate. If the use given to the GPS receiver requires even more precision, such as topographic work, cartographic surveys, orientation races, location of beacons, etc., almost all firms have optional antennas with DGPS devices for some of their receivers that correct this error by differential calculation, reducing it to a margin of 1 to 3 meters RMS.

Uses of a GPS receiver

You can use a GPS receiver for whatever you think might be helpful. However, it must be taken into account that they are exclusively data receivers that calculate the exact position and that they do not work with any analog data ( temperatures , pressure , humidity …). They are extremely useful devices for any navigation task, route tracking, point storage for later studies, … but in no case can we expect to deduce atmospheric data from them. However, it should also be appreciated that even the “smaller” models that GPS manufacturers make available for personal navigation are an evolution of aeronautical navigation systems.and maritime that have been perfected daily for years. This supposes a series of important advantages for the users of GPS’s for the terrestrial personal navigation. First, a question of scale. It is clear that the dimensions of aeronautical and maritime navigation relative to the dimensions of land navigation, even with motorized vehicles, are much greater. This means that the “small” receivers also have the navigation resources and accuracy of the large ones, only that the former have less sophisticated functions than the latter for the navigation itself. The displays and graphical functions required by the pilot of a boat incorporated into his GPS receiver must be much more and more sophisticated than those required to orient himself in smaller dimensions. But the reception system, and the calculation of the position is the same in one case as in another. A GPS receiver provides many more features for land navigation than is needed for orientation. Course deviation monitoring, route monitoring,Electronic compasses , etc., are functions that can be found in a “small” GPS’s.

GPS receiver

  • The situation of the satellites can be determined in advance by the receiver with the information of the so-called almanac (a set of values ​​with 24 orbital elements), parameters that are transmitted by the satellites themselves. The collection of almanacs for the entire constellation is completed every 5-20 minutes and stored in the GPS receiver.
  • Information that is useful to the GPS receiver in determining your position is called ephemeris. In this case, each satellite emits its own ephemeris, which includes the health of the satellite (whether or not it should be considered for taking the position), its position in space, its atomic time, Doppler information, etc.
  • The GPS receiver uses the information sent by the satellites (time at which they emitted the signals, their location) and tries to synchronize its internal clock with the atomic clock that the satellites have. Synchronization is a trial and error process that occurs once every second on a portable receiver. Once the clock is synchronized, you can determine your distance to the satellites, and use that information to calculate your position on earth.
  • Each satellite indicates that the receiver is at a point on the surface of the sphere, centered on the satellite itself and the total distance from the receiver radius.
  • Obtaining information from two satellites tells us that the receiver is on the circumference that results when the two spheres intersect.
  • If we acquire the same information from a third satellite we notice that the new sphere only cuts the previous circumference in two points. One of them can be ruled out because it offers an absurd position. In this way we would already have the position in 3D. However, since the clock in the GPS receivers is not synchronized with the atomic clocks of the GPS satellites, the two points determined are not precise.
  • By having information from a fourth satellite, we eliminate the inconvenience of a lack of synchronization between the clocks of the GPS receivers and the clocks of the satellites. And it is at this time that the GPS receiver can determine an exact 3D position (latitude, longitude and altitude). As the clocks between the receiver and the satellites are not synchronized, the intersection of the four spheres centered on these satellites is a small volume instead of being a point. The correction consists of adjusting the receiver’s time in such a way that this volume becomes a point.

DGPS or differential GPS

Field team conducting seismic data surveying using a Navcom SF-2040G StarFire GPS receiver mounted on a mast.
DGPS (Differential GPS), or differential GPS, is a system that provides GPS receivers with corrections of data received from GPS satellites, in order to provide greater accuracy in the calculated position. It was conceived primarily due to the introduction of selective availability (SA).
The rationale lies in the fact that the errors produced by the GPS system affect the receivers in close proximity equally (or very similarly). Errors are strongly correlated in neighboring receivers.
A GPS receiver fixed on the ground (reference) that knows its position exactly based on other techniques, receives the position given by the GPS system, and can calculate the errors produced by the GPS system, comparing it with its own, known in advance. This receiver transmits the error correction to the receivers close to it, and thus these can, in turn, also correct the errors produced by the system within the signal transmission coverage area of ​​the reference GPS equipment.
In short, the DGPS structure would be as follows:

  • Monitored station (reference), which knows its position with very high precision. This station is made up of:

A GPS receiver.
A microprocessor, to calculate the errors of the GPS system and to generate the structure of the message that is sent to the receivers.
Transmitter, to establish a unidirectional data link towards end user receivers.

  • User equipment, composed of a DGPS receiver (GPS + receiver of the data link from the monitored station).

There are several ways to obtain DGPS corrections. The most used are:

  • Received by radio, through a channel prepared for it, such as RDS on an FM station.
  • Downloaded from the Internet , or with a wireless connection .
  • Provided by a satellite system designed for this purpose. There is WAAS in the United States, EGNOS in Europe and MSAS in Japan, all compatible with each other.

Two types of corrections can be included in messages sent to nearby recipients:

  • A correction directly applied to the position. This has the disadvantage that both the user and the monitoring station must use the same satellites, since the corrections are based on those same satellites.

A correction applied to the pseudo-ranges of each of the visible satellites. In this case, the user will be able to make the correction with the 4 satellites with the best signal-to-noise ratio (S / N). This correction is more flexible.
The selective availability (SA) error varies even faster than the data transmission speed. Therefore, along with the message that is sent of corrections, the validity time of the corrections and their trends is also sent. Therefore, the receiver must do some kind of interpolation to correct the errors produced.
If it is desired to increase the coverage area of ​​DGPS corrections and, at the same time, minimize the number of fixed reference receivers, it will be necessary to model the spatial and temporal variations of the errors. In this case we would be talking about the wide area differential GPS.
With DGPS, errors due to:

  • Selective availability (eliminated from the year 2000 ).
  • Propagation through the ionosphere – troposphere.
  • Errors in the position of the satellite (ephemeris).
  • Errors caused by problems in the satellite clock.

For DGPS corrections to be valid, the receiver has to be relatively close to some DGPS station; generally less than 1000 km. The accuracies handled by differential receivers are centimetric, so they can be used in engineering.

Advantages of GPS over normal guidance systems

It is very important to understand that the calculation of position and altitude is not made from data from analog pressure, humidity or temperature sensors (or a combination of these) as in analog altimeters or altimeters-barometers, or even as In the most sophisticated digital altimeters, it is made from the data sent by a constellation of satellites and orbit that, despite being simple as satellites, provide the reliability of making use of the most sophisticated and precise technology from which man currently has. It should also be noted that the evolution of those analog data, which, in effect, will be very useful for forecasting atmospheric changes and environmental conditions for the development of the activity to be carried out, they are relatively reliable in calculating exact position and altitude. In addition, all GPS’s incorporate really sophisticated navigation functions that will change the very concept of orientation. For example, routes can be drawn up on maps, registering on the device the points through which you want or must pass and, on the ground, activating that route, a graphic screen will indicate if you are on the correct course or if there is a deviation in some direction; or use the same function in reversible routes, that is, to record points through what is passed and then be able to return through those same points safely. With all these data, you can also deduce the speed at which you are traveling accurately, while maintaining a straight line course, or deduct the speed at which you are traveling if all course change points have been recorded … and a long etc. of very useful and interesting functions that can be discovered when using these devices.

Use the data obtained with GPS receivers with PC’s

If we need to export the data obtained with a GPS receiver to a computer to make the necessary calculations, it is good to remember that, usually, kits for data transfer between PC’s and GPS’s, as well as power supply kits, tend to be devices Optional when acquiring a GPS receiver, at least up to mid-range receivers, which are already beginning to incorporate functions that may make it necessary to include these kits in series. In addition, it must not be forgotten that specific software is necessary to import this data in a more or less standard way, which allows it to be used in a versatile way. The most common interfaces with NMEA 0180, 0181 and 0183, so it is necessary a software that includes these interfaces, to make transfers through a serial port. It is also common to find interfaces with RS232 corrections that allow transferring through parallel ports. In addition, there are interfaces of many GPS manufacturers’ firms that create their own protocols. The software for these tasks is relatively cheap (if what you want is simply to obtain that data, of course), and there are even many applications ofshareware and freeware that can be found.

 

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