Each satellite sends the data on its precise location (position and elevation) and the starting point for the transmission of such data. A GPS receiver interprets. the signal, then measures the time elapsed between the time of signal transmission and reception to determine the distance that exists' between it and the satellite.
The NAVSTAR satellites (NAVigation Satellite Timing And Ranging) are spread over six orbital planes (4 per plan) with a slope of 55 Ëš from the equator. They orbit at an altitude of about 20,000 km above the Earth's surface (or 3 times the radius of the Earth), which gives them a period of revolution of about 12 hours. The high altitude allows users far away (several hundreds of kilometers) to capture signals simultaneously from the same satellites. latitudes For Quebec, a satellite is, at most, six hours above the local horizon, between sunrise and sunset. At least 4 satellites (sometimes even 12) are always available in all parts of the globe 24 hours a day, regardless of weather.
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The tracking stations of the control component whose main function to calculate the trajectory of the GPS satellites and to estimate the errors of time clocks on board satellites. The 5 tracking stations of origin are located on the islands of Ascension (Atlantic Ocean), Diego Garcia (Indian Ocean), Kwajalein and Hawaii (Pacific Ocean) and Colorado Springs (master station) . Today, a couple of stations further up the network. The tracking stations are equipped, among others, GPS receivers stationed on geodetic points whose coordinates are precisely known. The comments received are used to calculate the position of satellites in the form of ephemeris and calculate corrections to the clocks on board satellites. This information is transmitted to satellites, which store it in memory of their onboard computer, for later rebroadcast to users. This information is transmitted to users through the signals from the satellites themselves.
Finally, the user component includes the receptors used for positioning. These are only passive receivers receive signals from GPS satellites. Their functions to measure distances between the antenna and the satellite-receiver-transmitter, decoding the broadcast messages containing the satellite clock corrections and ephemerides used in calculating the satellite position at time of observations , and calculate the position of the user. Several types of receptors provide navigational features and ability to save the calculated coordinates and observations. there is no mention of costs incurred by using signals GPS (excluding the purchase or lease of receivers).
The GPS system calculates the three-dimensional position (latitude, longitude and altitude) of a user, continuously and instantaneously, anywhere on Earth. When a mobile GPS receiver is its speed and direction of its movement can also be determined. In addition, GPS provides thetiming information, ie, a user can associate a time indicator to all information that is collected or at all events that occur during land-surveying.
Originally developed for military navigation purposes, the GPS system was quickly used for purposes of localization and positioning for both civilian and military. The GPS system is a potential solution to almost any application requiring a spatial reference (address) such as geodesy, thehydrography, the management of fleets of transport, air traffic, forestry, and many others.
Types of GPS observations
NAVSTAR satellites (of earlier generations, blocks I, II, IIA and IIR) transmit their information on 2 carrier waves called L1 and L2 at 1.6 GHz to 1.2 GHz (GHz = 109 Hz), whose wavelengths are 19 and 24 cm, respectively. TheL1 carrier wave is modulated by two codes and a message containing, inter alia ephemeris. These codes are: C / A code (Clear / Access or Coarse / Acquisition) and the P code (Precise or Protected). The P code is encrypted and is now known as Y code For its part, the L2 carrier is not modulated by the C / A. Frequency carrier waves and the sequence codes are governed by atomic clocks on board satellites.
The new generation of satellites transmit new signals. The satellites of Block IIR-M broadcast a new code exclusive to the U.S. military and its allies (code M) on carriers L1 and L2. A new civil code (L2C) is also transmitted on L2, thereby correcting the ionospheric effects in combination with measurements of C / A code on L1. Block IIR satellites As they pass over a new L5 carrier wave (whose wavelength is 25 cm) which is modulated by 2 new civil codes (I5 and Q5).
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Because of the frequency carrier waves, GPS signals are attenuated by obstructions such as buildings, mountains, trees. Currently, great efforts are spent on research to obtain a positioning under forest canopies in urban canyons and inside buildings using receivers and antennas and high sensitivity A-GPS services (Assisted GPS). The A-GPS enables a GPS receiver to receive messages from satellites via a cell phone (from a receiver located in a location free) which allows seamless integration of pseudorange measurements over a longer interval thus facilitating receiving signals attenuated by obstructions. The new GPS signals have properties that facilitate their reception in places obstructed.
There are 3 types of GPS observations possible: pseudorange measurements, phase measurements of the carrier wave and Doppler frequency measurements.
The pseudorange measurement is, in simple terms, a measure of the propagation time required for a time stamp transmitted by a satellite reaches the receiver on Earth. These time stamps are encoded on the carrier waves by the technique of phase modulation. So that a receiver can recognize the satellite observed, each satellite transmits a code of its own. A replica of the code sequence is generated by the receiver at the same time as satellite. The gap that must undergo the reply to coincide with the received code corresponds to the propagation time it took the signal to travel from satellite receiver. This time difference multiplied by the speed of light in vacuum (about 300 000 km / s) gives a distance measurement. This measure is distorted by including the wave propagation in the atmosphere and by the synchronization errors between the satellite clocks and receiver. An error of 1 / 1000 second represents an error of 300 km. For these reasons, this distance is called pseudorange.
The magnitude of the resolution of the pseudorange measurement made with the C / A code is about Â± 3 m, the one with the code P (Y) is approximately Â± 0.3 m. However, note that some receptors, recently introduced on the market, allow measurements of the pseudo-C / A code as accurate as those performed on the code P (Y). In addition to these errors, errors of clocks, orbits, and ionospheric and tropospheric refraction. The advantage of the measures with the code P (Y), in addition to being more accurate, is that they can be corrected for ionospheric delay because the code P (Y) is transmitted on two carrier waves of different frequency.
For security purposes, the U.S. military replaced in 1994, the code P (whose sequence is known) by a secret code Y. In fact, a secret code W was added to the code P (Y = P + W). Since the complete sequence of the Y code is unknown (or more precisely the secret code W is not known civilians), this signal can be scrambled by a potential enemy. This device is called the anti-jamming (AS: Anti-Spoofing). However, for various technical code correlation, manufacturers have managed to produce receptors that can still make measurements with the pseudo-code Y. This fact does not compromise the safety device since the interference is still impossible.
The phase measurement of the carrier wave is to compare the phase of the wave received at the receiver with the phase of a wave generated inside the receiver. Theoretically, this phase difference varies between 0 and 2p. This phase can be converted to meters because we know the wavelength of the carrier wave (s). Unfortunately, the integer length of initial wave contained in the receiver-satellite distance is not measured by the receiver. This unknown is called theinitial phase ambiguity. For cons, the receiver is able to count the whole number of cycles (and the fractional part) accumulated since that time (or time) of initial observations, if there is no interruption in the signal reception. Interruptions cause cycle slips and are mainly caused by obstructions (buildings, mountains, trees, ...) between the satellite and receiver. The phase measurement can be interpreted as a precise measure of the variation of the distance receiver-satellite since the early days. If the initial phase ambiguity can be resolved, the phase measurement is corrected and accurate measurement of distance receiver-satellite. The resolution of a phase measurement is a few millimeters.
Note that the combination of phase measurements between L2 and L5 carrier will create a hybrid signal with the wavelength of 5.9 m will result (extra-wide band), thus facilitating the resolution of phase ambiguities on long vectors . The current combination of phase measurements between the carriers L1 and L2 creates a hybrid signal whose wavelength is 86 cm (broadband).
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The measurement of Doppler frequency is the difference between the received frequency and the nominal frequency of transmission caused by relative motion between the satellite and receiver. This measure is primarily used to determine the instantaneous speed of mobile receivers and for detecting and correcting cycle slips potentially present in the phase measurements.
Geometry of GPS positioning
GPS positioning is based on the principle of trilateration space. Take the example of a planimetric survey (in 2 dimensions) as used in surveying. The measurement of distance traveled from an unknown point to 2 points with known coordinates to calculate the coordinates of unknown point since it lies at the intersection of two circles centered on the known points. The radii of the circles are given by the measurement of two distances. Only 2 points satisfy the equations of two circles, one of these two points can be rejected as too remote from the approximate coordinates of the point to be determined.
In positioning, our three-dimensional space requires us to make a measurement of distance 3-point whose coordinates are known. The position sought is at the intersection of 3 spheres. Each sphere is centered at the known position of the satellite (computed with the ephemeris) when the distance measurement. The radii of the spheres correspond to the distance measurements. In practice, since our distance measurements are affected by clock errors, a distance measure simultaneously on a fourth satellite is used to resolve four unknowns which are three-dimensional coordinates and clock error of receiver. The satellite clock error is corrected using the correction terms sent in the message broadcast by the satellites themselves. If more than 4 satellites are observed, the accuracy and reliability of positioning are higher. This position is referenced to a geocentric coordinate system.
The three-dimensional coordinates obtained are expressed in the coordinate system used for the calculation of satellite positions. This coordinate system is WGS84 (World Geodetic System 1984). Note that the altitude obtained from the GPS system is measured above the reference ellipsoid (geodetic elevation) and not relative to the geoid. In Quebec, the difference between the geoid and reference ellipsoid (also called the geoid height) can reach forty meters.
Geometrically speaking, if the intersections of the spheres are at angles too obtuse or too acute, the positioning quality will be compromised. In simple terms, it is not enough just to measure distances on a minimum of 4 satellites. Moreover, the distribution of satellites relative to the observation site must be favorable. Satellites evenly distributed in the sky (good geometry) is preferable to a situation where the satellites are all in the same area of sky (poor geometry). The constellation of GPS satellites has been designed in order to meet this criterion. However, if obstructions above the site observations to allow reception of satellite signals in certain directions of the sky, the geometry of trilateration can cause problems. The degradation of geometric precision GDOP (Geometrical Dilution Of Precision) is a parameter that quantifies the impact of the satellite configuration. This parameter indicates how much the errors of distance measurements propagate through to solve the unknowns (coordinates and setting the clock). The GDOP factor indicates the effect of the satellite configuration on the accuracy of instantaneous positioning.
The GDOP factor can be segmented, for example, by three-dimensional position (PDOP), horizontal component (HDOP) and vertical component (VDOP). The GPS constellation has been designed so that the PDOP rarely exceed a value of 6, when there are no obstructions to hide the satellite signals. Using the approximate positions of the satellites calculated with almanacs, it is possible to predict the temporal factors DOP values for a given observation site. It is important to note that predictions of DOP factors must take into account obstructions to observation sites and the number of satellites can be received simultaneously by receivers used. In general, a PDOP of 6 or less is considered good. Conversely, a PDOP greater than 6 deteriorates the quality of positioning.
Accuracy of GPS positioning
The type of position which has been discussed so far was performed using a single receiver. Such positioning is called absolute positioning, since only observations collected by a receiver contribute to determining its position. The theoretical precision of absolute positioning is now about 20 m (May 2000), since the U.S. military deliberately introduces more errors in the ephemeris and no change in the nominal frequency of the clocks of the satellites. That other safety device is called the Selective Availability (SA: Selective Availability). It was intended to restrict access to the full potential of GPS. With this device, the horizontal positioning accuracy was Â± 100 m, 95 times out of 100. Selective Availability was in continuous operation since 1991. This restriction was lifted at the beginning of May 2000. With the use of ephemeris and clock corrections precise satellite, as calculated by the Service IGS (International GNSS Service), the absolute positioning accuracy is of the order of decimeters. Some online services are offered via the Internet that can be treated (delayed) data PPP (Precise Point Positioning and Precise Point Positioning).
An effective way to reduce the effect of errors inherent to GPS is the relative positioning. The principle is to simultaneously collect observations to a receiver located at a reference station whose coordinates are known. The distance measurements are compared with theoretical distances calculated from the known coordinates of the station and satellites. These differences represent the distance measurement errors and are calculated for each satellite at each epoch. Subsequently, these differences become distance correction terms (also called differential corrections) that are applied to distance measurements collected by the rover. In this way, errors common observations of the reference station and rover are eliminated. Errors are more similar when the 2 receptors are more frequent.
The positioning accuracy relative (with pseudorange measurements) is of the order of 2 to 10 m. This accuracy depends on the accuracy of pseudorange measurements, the geometry of the satellite configuration and spacing between the receivers that can easily be several hundred kilometers. It is important to note that differential corrections should not be applied at the contact, unless the same satellites are observed by two receivers. Differential corrections can be applied to more than one rover and intervisibility between the receptors is not required.
The relative positioning reduces number of errors inherent to GPS. Unfortunately, the relative positioning does not eliminate multipath, since the conditions conducive to signal reflections, reflective surfaces near the antennas, are not the same site to another. The interference on the air between the direct wave coming from the satellite and the wave of the same satellite that is reflected, because an error in the measurement of the satellite-receiver distance. This error can reach several meters in pseudorange measurements and a few centimeters for phase measurements.
When the precision of relative positioning is necessary real time (eg for navigation) a communication link radio-frequency (VHF, UHF, cellular, ...) should be established to ensure the transmission of correction terms. An organization called "Radio Technical Commission for Maritime Services(RTCM) established a communication protocol of differential corrections in real time. Most services of DGPS (Differential GPS) and RTK systems (Real-Time Kinematic) and network RTK using RTCM standard.
Other services that provide improved positioning accuracy in real time have also been developed in recent years. These services WAAS (Wide-Area Augmentaiotn System) and CDGPS (Canada-Wide DGPS Service). They offer typical accuracies of 3 m and 0.5 m, respectively.
When corrections can be applied post-treatment, but as long as the raw observations have been previously saved, users can opt to treat their comments in deferred time. A format for data exchange between receivers of different manufacturers was established by geodetic. This format called RINEX (Receiver Independent Exchange format) and can combine observations of receivers from different manufacturers and use a single software post-processing.
When the receiver is at rest, said positioning is static and when the receiver is in motion it comes to kinematic positioning. The advantage of static positioning is that the number of measurements collected on the same station becomes much greater than the number of unknowns to be solved, resulting in greater precision positioning. That is what is called a cumulative solution because the observations are combined to calculate a single position. In kinematic mode, 3 new coordinates to be estimated at each epoch. In the latter case, a solution must be calculated at each time instant or comments, hence the term quick fix. Note that, in general, the accuracy of the altitude is about 2 times less than the accuracy of horizontal coordinates.
The operation of GPS receivers is simple and does not cause significant difficulties. However, the main difficulty lies in the selection of equipment and procedure that will achieve the desired clarifications to lower costs both in terms of the lease or purchase equipment at the level of execution time of the survey and data processing. In other words, it is important to identify needs and select the best methodology to address them.
In closing, we note that the Europeans are in the process of building their civil system of navigation satellites called Galileo. It should be operational in 2014. Without forgetting the Russian system GLONASS to it should be upgraded with a new full constellation of 24 satellites in late 2010. Note also that the Chinese are developing their own satellite navigation system Beidou-Compass. Theintegration of these positioning systems and satellite navigation (also called GNSS: Global Navigation Satellite Systems) provide greater coverage, greater integrity and greater accuracy.
The revolution continues and the number of applications in the field of positioning and navigation will be limited only by our imagination ...