Introdución al Posicionamiento con GPS Sistema de Posicionamiento Global This slide presentation is intended for audiences that are interested in the.

Presentación del tema: "Introdución al Posicionamiento con GPS Sistema de Posicionamiento Global This slide presentation is intended for audiences that are interested in the."— Transcripción de la presentación:

1 Introdución al Posicionamiento con GPS Sistema de Posicionamiento Global
This slide presentation is intended for audiences that are interested in the basics of GPS for code phase mapping. Specific products are not shown in this presentation, however, you can find mapping product specific presentations in other parts of this seminar set. Vista general

2 Vista General De dónde son los GPS Como trabajan los GPS
Estado de los GPS Agenda/topics covered This presentation covers where the GPS system came from, how GPS works and the current status of GPS. Antes de los GPS teníamos...

3 Antes de los GPS teníamos...
TRÁNSITO (Efecto Doppler) - Fijación de 16 días o menor - Precisión Sub-métrica casi en 3 días - Covertura mundial - Lat/Long/Altitud LORAN (Triangulación) - Posición fija contínua. - Precisión de 300 metros - Cobertura limitada - Lat/Long This slide shows what positioning systems preceded GPS. GPS was the replacement for the LORAN and TRANSIT positioning systems. LORAN The LORAN positioning system was based on triangulation from radio transmitters. As long as you were within the range of more than one transmitter, you could use the distance and angle between them to calculate your position. The system provided continuous 2D position fixes (lat/long only) to an accuracy of about 300 meters. Accuracy was further degraded by atmospheric conditions and the distance from the transmitters. TRANSIT The TRANSIT positioning system was a satellite based positioning system, based on doppler shift. The signal changed as the satellite rose in the sky and then set (much like the sound of a passing car changing),. The TRANSIT receiver could calculate its position due to the frequency change. It was possible to obtain submeter accurate 3D positions over 3 day of data collection. Ahora

4 …Ahora LORAN (Triangulación) - Posición fija contínua.
- Precisión de 300 metros - Cobertura limitada - Lat/Long TRÁNSITO (Efecto Doppler) - Fijación de 16 días o menor - Precisión Sub-métrica casi en 3 días - Covertura mundial - Lat/Long/Altitud This slide gives a brief introduction to GPS as the replacement for the LORAN and TRANSIT positioning systems. The Global Position System replaces both the LORAN and TRANSIT positioning systems. GPS provides continuous position fixes anywhere in the world. Position accuracy ranges from more than 100 meters to a couple of centimeters depending on the field procedures and the GPS receiver used. The US government claims non-differential GPS positions to be accurate to 100 meters 95% of the time during peacetime. Most mapping grade receivers provide either submeter accuracy or 1- to 5- meter accuracy. Centimeter accuracy can be achieved with survey grade products. GPS - Posición fija contínua - Cobertura mundial - Lat/Long/Altitud - Precición de centímetros en segundos Qué es un GPS?

5 Qué es un GPS? Es un sistema muy preciso
Desarrollado y mantenido por el US Department of Defense Basado en un sistema de Satélites Vendido al Congreso de US como una idea para desarrollar diversas aplicaciones inclusive de uso civil La señal de los GPS es gratuita Ilimitado número de usuarios The US Department of Defense wanted a way of positioning nuclear submarines very accurately anywhere in the world. (GPS does not work under water; submarines drag the antenna behind them on the surface of the water.) The US Congress agreed to investing, because of the potential applications that would follow. However at the time no-one imagined just how many varied applications would follow. The Department of Defense has now paid for the system. GPS can now be used by any civilian free of charge. Porqué utilizamos satélites para hacer mapas.

6 Porqué utilizamos satélites para hacer mapas.
No es necesario tener línea vista entre los objetos For traditional surveying line of sight was necessary between points on the ground. With GPS line of sight is not necessary between points on the ground. This allows you to have a longer baseline. However to use GPS it is necessary to have a clear view of the sky, as the GPS receiver needs to be able to ‘see’ the satellites. GPS does not work under water, in buildings or underground. Some GPS receivers can work under partially obstructed views such as under some tree canopies. El sistema de GPS está basado en satélites

7 El sistema de GPS está basado en satélites
Usa un sistema de trilateración desde los satélites 25 satélites - Capacidad inicial de operación Los satélites tienen una órbita muy alta 20,000 Km (12,600 millas) Sólo es posible con la tecnología de hoy día Computadoras y relojes GPS is satellite based, there are 25 satellites operational as of July GPS has reached ‘Initial Operational Capability’ phase according to the U.S. Coast Guard. The rationale for high orbit is for: Maximum coverage of the earth. Survivability of satellites - They are beyond the atmosphere. The satellites were launched with Delta Rockets in a mathematically perfect orbit. The technology is possible due to extremely accurate clocks - Timing is very important. Como es tan exacto el sistema?

8 Como es tan exacto el sistema?
Depende de algunas variables El tiempo tomado en cada medida Alineación del receptor Posición relativa de los satélites Puede ser más de 100 m sin corrección diferencial. GPS Sub-métrico a 5 metros con corrección diferencial. GPS Precisión sub-centímetro desde productos topográficos (survey) El gobierno de US puede degradar la precisión del sistema. It is important to explain the different levels of accuracy that can be obtained. Remember that without differential correction accuracy is 100 meters 95% of the time (2DRMS). The accuracy of a GPS depends on the following: 1. Time Spent on Measurements - Logging a number of positions and averaging them is more accurate than recording a single position. 2. Receiver type - Different receivers are designed to obtain different levels of accuracy. Survey products are designed to achieve sub-centimeter accuracy, while mapping receivers are designed to achieve either submeter accuracy or 1-to 5-meter accuracy with differential correction. 3. Relative positions of the satellites - The actual geometry of the satellites affects accuracy. Satellites which are well spread out in the sky are better than satellites clustered together, however you do not want the satellites to be too low in the sky. Government degradation. The Government can and does degrade the accuracy through Selective Availability (S/A). You can use differential correction to remove the error. 5 pasos de un GPS

9 3 4 2 5 1 5 pasos de un GPS 4 SVs para resolver
Sistema de trilateración 1 Usa mensajes desde los sátelites para su localización. Correcciones por la Troposfera y la Ionosfera 5 Distancia desde los satélites (SV) usando la velocidad de la luz. 2 3 4 SVs para resolver por X,Y,Z,t To understand how GPS works, we can divide the system into five conceptual blocks. Read the ‘Five Steps’ to attendees. The next set of slides discusses each step in detail. Note that SV stands for Space Vehicle (satellite). Trilateración desde los Satélites

10 Trilateración desde los Satélites
1 Trilateración desde los Satélites Midiendo la distancia desde varios satélites usted puede calcular su posición. By measuring the distance from several satellites, you can figure out your position through mathematics. Need four satellites to get a 3D position fix. Trilateración

11 Trilateración Una medida muestra la posición relativa que tendría un observador en una esfera. 17,460 Km Esta es una posición relativa en la esfera This is an example to show you how trilateration works: Imagine you are lost somewhere on earth, but you know that you are 11,000 miles from one satellite. Where would you be? You would be somewhere on the surface of an imaginary sphere with a radius of 11,000 miles from the satellite. Trilateración

12 Trilateración La segunda medición intercepta las dos esferas y muestra un área común. With a second measurement we now know we are 11,000 miles away from one satellite and at the same time we are 12,000 miles from a second satellite. The distance from the second satellite helps us to narrow our possible location. We must be somewhere within the intersection of the two spheres. How can we make this area of unknown even smaller? La intercepción de las dos esferas se da en un círculo Trilateración

13 Trilateración La tercera medida intercepta dos puntos del círculo formado por las dos esferas anteriores Trilateración

14 Trilateración La cuarta medición determina cual de los dos puntos seleccionado en el círculo interior es el válido. Trilateración

15 Trilateración En la práctica 3 mediciones son suficientes, sin embargo
Usted puede descartar un punto, sin embargo, puede cometer errores importantes. Posicionar el punto fuera del espacio. O moverse a muy alta velocidad Por eso es necesaria la cuarta medición para cancelar los errores de reloj. In theory, three measurements are enough to determine a position. Why? One position can be discarded because it will be a ridiculous answer. (It may be in a different hemisphere or nowhere near earth.) Four measurements are important for solving for four components. Latitude Longitude Altitude Time One measurement for each component. Time is important for differential correction.

16 Posición de los Satélites
2 Posición de los Satélites Medida de la distancia desde los satélites. La posición es por medio de la medida del viaje de una señal de radio. The distance to a satellite is determined by measuring how long it takes for a signal to reach us. How do we know when the signal left the satellite? You can use the following Baseball/Football stadium example to help explain: One person is at one end of the stadium, and one is at the opposite end of the stadium. Both people begin to count at the same time. When one person shouts the number ‘one,’ will the other person hear it at the same time? No, because sound takes some time to travel. As one person is shouting the number ‘three,’ he/she may hear the other person’s voice say ‘one.’ This tells us that it takes 2 seconds for sound to travel across the stadium. With that information, we are able to calculate the distance between the 2 people by multiplying this time by the speed of sound. The same thing occurs with the signals of the satellites, only they deal with the speed of light rather than the speed of sound. We are not actually measuring the distance between earth and the satellite. We are measuring the time it takes for the signal to reach the earth from the satellite.

17 Mediciones a la velocidad de la luz
Midiendo cuanto tarda la señal en ser recibida por el GPS usted puede: Muiltiplique el tiempo por 300,000 Km/sec Tiempo (seg) x 300,000 = Km Si usted tiene un buen reloj en el recibidor, usted necesita conocer todas las posiciones exactas por medio de la señal enviada por los satélites. Once we know how long it took the signal to reach us, we can work out the distance. To work out the distance we multiply the travel time by the speed of light, to get the distance between our position and the satellite. time x 300,000 = kilometers, or time x 186,000 = miles The hard part is knowing exactly when the signal left the satellite.

18 ¿Cómo podemos saber cuándo la señal esta fuera del rango del satélite?
Una de las ideas inteligentes el GPS: Usa el mismo código para el recividor y el satélite Se sincroniza los satélites y los recividores para poder generar en mismo código al mismo tiempo A continuación, nos fijamos en el código de entrada desde el satélite y ver cuánto tiempo hace que nuestro receptor genera el mismo código To work out when the signal left the satellite the GPS receiver uses the incoming code. Each satellite has its own unique code called a Pseudo-Random Code. The codes are not really random. They are carefully selected and repeat every millisecond. The codes are binary (string of 1's and 0's) generated and broadcasted by the satellites. Both the satellites in space and the receivers on the ground are generating the same code at exactly the same time. (Assume for now that the satellite and receiver are exactly synchronized.) The receiver looks at the incoming code and compares it to the code generated internally. The receiver calculates the lapse or delay experienced and multiplies by the speed of light. That's why Pathfinder receivers are called code phase receivers. The code is actually being measured. Desde el satélite Desde el recibidor en tierra Medición de tiempos diferentes entre partes iguales de código

19 Porqué un Código? Un Código le permite operar en cualquier lugar
El Código permite a muchos satélites operar en una misma frecuencia Código nos da una manera de aumentar la relación de ruido de una señal Using known codes allows the receiver to jump in anywhere at any time. The receivers are programmed to know the sequence of the binary code. If the receiver knows exactly what code to listen for, many satellites can operate on the same frequency. Example: You’re in a crowded room with lots of people talking and you hear your friend, who is on the other side of the room, mention your name. We are sensitive to certain voices (frequencies). With known codes, very weak signals can be heard. This combats potentially noisy frequencies. For example: If AM radio stations broadcast their signals as a binary code, they would not be affected by static.

20 Precisión de los relojes
3 Precisión de los relojes Es necesario para la medición del tiempo de viaje de la señal. Asegurarse de que tanto el receptor y el satélite están sincronizados Todo el sistema depende de relojes muy precisos Cada satélite tiene un reloj atómico Presición muy cara pero necesaria Receptores en tierra tienen relojes consistentes y precisos. El secreto está en el satélite extra ya que mide y ajusta los relojes de receptores. So that the whole code thing works, the satellites and our ground receivers must be generating their codes at exactly the same time. Discuss importance of accurate clocks. satellites have atomic clocks which means that they measure time by using the oscillations of a particular atom as a metronome. It is the most accurate method for time-keeping ever developed. BUT it is very expensive ($100,000 per clock). Each satellite has four atomic clocks. With this expense in mind, it's not practical to have atomic clocks in the receivers. All that is needed on the ground are very consistent clocks. Ground receivers use quartz clocks that are consistent for short periods of time and are fairly inexpensive, much like the quartz watches you wear on your wrist. The ground receiver clocks can be perfectly synched to universal time by using an extra ranging measurement (the 4th measurement) can be used to calculate the slowness or fastness of our clocks here on the ground. The receiver clock is constantly being calibrated by the satellite. Eventually, the receiver clock will update itself no matter what time you set.

21 La idea de situación Precisión de relojes
4 seg's 6 seg's Example of perfectly accurate clocks. This diagram is in 2D for the sake of drawing. Sometimes we refer to the distance from a satellite in terms of time. In this example, we are 4 seconds away from one satellite, and 6 seconds away from another satellite. This makes our location one of two positions. We can discard one position. OR ... (next slide) En 2 dimensiones por motivo del dibujo

22 Adicionando una tercer Medición
4 seg's 6 seg's 8 seg's Precición de relojes By adding a third measurement, we get our exact location in 2D. We are 4 seconds away from one satellite, 6 seconds away from a second satellite, and 8 seconds away from a third satellite. La tercera medición pasaría por nuestra posición si es correcta En 2 dimensiones

23 Con relojes rápidos Mala posición porque el reloj esta fuera
5 seg error de tiempo 7 seg However if the clock in our receiver is not accurate but is one second fast. What will happen to the resulting position? The resulting position will be altered significantly. Mala posición porque el reloj esta fuera de posición por un segundo En 2 dimensiones

24 Tercera Medición Relojes rápidos
The error is quite obvious now when the three measurements do not intersect. This proves how important accurate clocks are. The receiver will then adjust its clock by the amount of time that will allow the spheres of all three satellites to intersect at one point. No pasará por los otros dos dando un error En 2 dimensiones

25 Se necesitan 4 Satélites para tener una posición en 3D
Si buscamos en las diapositivas anteriores en 3d Entonces tenemos que: 4 satélites para posiciones en 3d (X, Y, Z, tiempo) 3 satélites para posiciones en 2d (X, Y, tiempo) - el usuario puede introducir el valor Z Problema - si el usuario tiene una posición no precisa Z, entonces X e Y será incorrecto! Solución - sólo trabajar en 3D! We need four measurements to get an accurate position in a real 3D situation. This means the GPS receiver must have at least 4 channels or be able to sequence among 4 satellites. Explain what is meant by a channel. The following scenario can help: Say you wanted to watch six TV programs at once? There are two ways of doing this: 1. Get six TVs and watch six different programs in parallel at once. 2. Flick stations very rapidly on a single TV with a channel changer. These are the two types of receivers/channels: parallel and sequential. Receivers have channels also. A parallel GPS receiver can track one satellite for each channel. So a receiver with 8 parallel channels can track up to 8 satellites. A sequential GPS receiver is much like the channel-flicking option. This receiver will actually cycle through all the different satellites. Some receivers have a mixture of parallel and sequential channels. These receivers track the best satellites on the parallel channels and cycle through the other satellites with the sequential channels.

26 Saber dónde están los satélites
4 Saber dónde están los satélites Después de todo, son hasta kilómetros Órbita Alta Órbita muy estable No arrastre atmosférico Supervivencia de los equipos Cobertura en toda la Tierra Monitoreado por el US Defense Department DOD transmite la corrección desde un satélite Correción transmitida desde los satélites para mensajes de estado The satellites are in a very high orbit. This is important because: the orbit is very stable, the satellites are completely out of the earth's atmosphere, so that their movement through space is not affected by atmospheric drag, they will survive longer there and they can provide maximum coverage. The 12-hour orbital period means the satellites will pass over one of the monitoring stations at least 2 times per day. The orbit is monitored by the Department of Defense. When the Department of Defense measures and finds a satellite out of position, it transmits a message to the satellite. The satellite then broadcasts its new/adjusted position information.

27 Saber dónde están los satélites - Efemérides
Estaciones de Monitoreo Diego Garcia Ascension Island Kwajalein Hawaii Segmento espacial GPS Control Colorado Springs Efemérides actuales se transmite a los usuarios Ephemeris The position information broadcast from the satellite is referred to as the ephemeris. The ephemeris contains precise information about the satellite’s orbit. Each satellite has an ephemeris.

28 Correcciones Atmosféricas
5 Correcciones Atmosféricas Correcciones Aplicadas estimadas Las señales se retrasan por la ionosfera y la troposfera El receptor hace correcciones estimadas para estos retrasos Ionosfera Troposfera Atmospheric conditions can affect the accuracy of positions. GPS does not operate in a vacuum. When the signal travels through any real medium, like air or water, it slows down. The GPS signals are affected by the ionosphere - a band of charged particles. Signals bounce around when traveling through charged particles. As the signal bounces, the amount of time it takes to reach the earth becomes longer. This can alter the calculated position. Ionospheric error is the greatest during the heat of the day. The GPS receiver takes these delays into account before it calculates a position. Our weather occurs in the troposphere. The tropospheric effects are only significant if the rover and base are greater than 300 miles apart. Yes, GPS works in the rain.

29 3 4 2 5 1 GPS en 5 Pasos 4 SVs para resolver
Basado en Trilateración 1 Uso de mensajes desde los satélites para la localización Corrección por la Troposfera y la Ionosfera 5 Distancia desde los satélites (SV) usando la velocidad de la luz 2 3 4 SVs para resolver para X,Y,Z,t To understand how GPS works, we can divide the system into five conceptual blocks. Read the ‘Five Steps’ to attendees. The next set of slides discusses each step in detail. Note that SV stands for Space Vehicle (satellite).

30 Estado de los GPS En desarrollo desde 1973
Primer satélite lanzado en 1978 Todos los satéllites GPS construidos y probados La siguiente generación de (Block II R) están en construcción Administrados por el US Department of Defense All satellites have been built and tested (even though some have not been launched yet). All satellites have already been paid for - new satellite launches will not affect the current budget. Next-generation satellites (Block IIRs) are currently under construction. Block IIR satellites have the capability of communicating with each other. This capability eliminates the problem of one satellite having a different almanac than another.

31 Descripción de la Segmentación del Espacio
25 satélites 6 planos con 55° de rotación Cada plano son 4 o 5 satélites Órbita Muy Alta 20,000 km 1 revolución en aproximadamente 12 horas Para precisión Survivability Cobertura Read the slide, point to image of constellation. This is what the satellites look like orbiting the earth. Point out the 6 orbits with 4 satellites per orbit. Remember the satellites are 12,600 miles away from the earth. Copiado desde ‘GPS Navstar User’s Overview’ preparado por GPS Joint Program Office, 1984

32 Satélites Disponibles (Sept. 1995)
SV number *10 14 13 16 19 17 18 20 21 PRN 12 2 Launch date m-d-yr 15 23 24 25 28 26 27 32 29 1 22 31 37 39 35 34 36 07 09 05 04 06 SV Number - Each satellite has a unique number (like a name). Like a manufacturing serial number, sometimes this number is the same as the PRN and sometimes not. Block 1 - Note the * next to satellite 10. This is a Block 1 satellites. Block 1 satellites were the first satellites launched and are gradually being phased out. The expected life of a satellite was 4 to 5 years. As you can see, they have outlasted that estimation. Block 1 satellites cannot be programmed with S/A (Selective Availability). Block 2 - The remaining satellites are Block 2. These satellites can be programmed for S/A. PRN - Pseudo Random Number, this is the unique code discussed in the ‘5 Steps to GPS’. Sequence of 1's and 0's (binary code). Each satellite has its own code, limited to The satellites are known by this PRN number. Launch Dates - Notice the pattern of launches: first of current satellites launched in ‘84 ’85; the Space Shuttle disaster in ‘86 caused a set-back until ‘92, when launching took off again. As of December 1993, the system achieved Initial Operational Capability (IOC). *Bloque I

33 Preguntas ?