GPS City Canada
GPS City Canada
How GPS Works and it’s History
Global Positioning System , usually called GPS (the U.S. military refers to it as NAVSTAR GPS – NAV igation S ignal for TIMing A nd R anging) is a satellite navigation system used to determine the exact position and providing very accurate time reference almost anywhere on earth or the earth orbit . Uses a set of at least 24 satellites in medium earth orbit.
It is able to provide location data regardless of the weather 24 hours a day. It is a passive satellite system. The aim of the operator of this system , the Ministry of Defence of USA , originally was that the troops could accurately determine the location, speed and time in a single reference system. It follows that the system was developed mainly for military purposes, but the U.S. Congress later approved its use to certain limitations for the civil sector.
History of GPS
History of satellite systems, we started dating from the early 60′s, when the Memorandum of the Ministry of Defense was responsible for merging the Air Force test program timation and 621B to the NAVSTAR GPS as indicated. The first electronic navigation systems were radiobeacons. After the invention of artificial satellites began to consider whether it would not be using them to develop more precise navigation systems. The first such navigation system put into operation the U.S. in 60′s and this system was called Transit . Late 60-ies and the former Soviet Union put into operation navigation system called Cyclone and name are still used two other similar systems – six military satellite called Parus and 4 civilian satellite called cicada. Both systems have the same disadvantages. Provide only two-dimensional coordinates, the positioning accuracy of 500 m when receiving a signal from only one satellite and imprecise timing signal . These navigation systems are also called Doppler .
After the experience with Doppler systems in the early seventies the U.S. decided to build a new satellite navigation system that would allow positioning in three dimensions with the exact time and made available by the satellite navigation and aviation. From17th December 1973 governing the development of the GPS Joint Program Group ( Joint Program Office ) Space Systems Division Headquarters U.S. Air Force. JPO representatives from the Air Force , Navy , Army , Marines , Coast Guard, defense mapping agencies, representatives of States , NATO and Australia .
The first of 28 satellites block II was launched in 1989 . Block I satellites were gradually supplemented and replaced completely block II satellites. This will gradually expand system capabilities. Three-dimensional positioning anywhere on Earth 24 hours a day is possible since the beginning of 1993 . Tenth to 28 satellites are known as Block IIA satellites. Nineteen of these satellites has modified the memory unit which allows them to work 108 days without contact with the control center. Even in 1989, with General Electrics awarded a contract for reconstruction and production of 20 Block IIR satellites improved. These satellites can be no contact with the control center to 180 days and can communicate with each other and establish their position. This system ensures faster detection of errors on some of the satellites and send the message without contact with the control center. Deleting the 35th satellite 8th December 1993 was achieved initial operating state of the system (IOC – I thread about perational Capability). This meant that the navigation system works 21 and 3 active spares satellites that provide service and SPS is able to notify changes the operational status of users (civil) 48 hours in advance. Condition for the FOC (FOC – F ull O perational C apability) activity was 24 Block II satellites. This condition was fulfilled third March 1994 and the system was necessary after the completion of all tests specified in the FOC. In this state it is up to our time.
Originally it was planned that the satellite will be launched on three orbits with an inclination of 63 ° polar plane. Three orbits were chosen in order to ensure the continued operation of the system were able to three spare satellites are able to position itself whenever the damaged spacecraft. This selection of orbits should be guaranteed a fixed minimum visibility of the sixth and a maximum of eleven satellites in the allowable elevation angle of 5 °. This should be ensure maximum robustness of the system. If necessary, it is possible to change the path of circulation and other satellites. Backup satellites are on Earth and they can send into orbit within two days. Orbital period should be around 11 h 58 min , which is almost two circuits in one sidereal day . This should ensure a periodic shift of the control station located on U.S. territory, which communicates with satellites and possibly corrects their orbits. It was later adopted a new concept, orbital inclination was reduced to 55 ° and the number of orbits was increased to six with four satellites in each of them. Thus the number of satellites remained only 24 were moved to new orbits .
How GPS works
What is happening in any GPS receiver, we could describe as measured by positioning point of intersection of the spherical surface whose radius is the measured distances. The measurand is the time spread of the radio signal from the satellite antenna to the GPS antenna TDI. Propagation speed is equal to the speed of light . Each satellite in the navigation message, among other data, sends the parameters of its orbit (ephemeris) from which we can calculate the current position of satellites (XS, YS, ZS). Knowing the coordinates of the satellites, can the user’s location (X, Y, Z) determined by calculating the system of three equations with three unknowns. The problem of measuring position is simple, if time base (clock) and satellite user are synchronous. The main problem is the time that elapses between sending a signal from diaľkomerného GPS satellites and its acceptance by the user GPS receiver. Timebase user equipment is shifted by an unknown time intervalDt , which can be converted to a distance Dt b = c (where c is the speed of light). The unknown coordinates of the user’s approach is thus unknown b to calculate the position and need a total of four equations
(X i – x) 2 + (y i – y) 2 + (z i – z) 2 = (D i + b) 2
D i = c TMI
i = 1, 2, 3, 4
GPS user equipment generates a copy of the signal emitted by the selected satellite, this copy in sync and reception of signals measured shift TMI beginning of the copy relative to the beginning of its time base. Measured time TMI can calculate the distance Di called (pseudorange). If the measurement is at least four satellites, we have all the variables necessary for solving systems of linear equations whose unknowns are the coordinates (X, Y, Z) and a shift Dt user’s time base relative to the satellite time base.
Positioning using the GPS is very precise, using one channel code C / A (Coarse / Acquistions). American vláda regard to the strategic interests and security of the country decided to introduce a special regime of operation GPS – sampling approach (SA – Selective Availability). The sampling approach is deliberate deterioration of accuracy of measurement of distance that varied hourly frequency signal and broadcast ephemeris in the navigation message. The result did not correspond to distances measured user from satellites and does not even shift his hours due to system clock. This random function, which deteriorated the accuracy of measurement was introduced in the 25th 3rd 1990 block of all satellites II. Following protests from the public, professional U.S. government to reconsider its decision on the introduction of SA and from 1 5th 2000 was a special mode selection approach (Selective Availability) canceled! Achievable accuracy of GPS receivers determine position is now 10 to 15 feet .Since the beginning of building the system was intended to use the system not only military but also civilian users. Are you seeking a male massage in houston, then visit our spons. Therefore, looking for a reasonable degree of access restrictions civilian users, which would ensure an acceptably low risk of abuse of the system, whilst substantially restrict civilian use. Finally decided on the distribution of access to authorized and unauthorized users. Authorized user of the Army U.S. Army some other countries of NATO and, if selected civilian users. Authorized users use the service marked the precise position service (PPS – Precise Positioning Service). This service consists of access to C / A code on L1 carrier frequency if P – code (Precision code), respectively. Y – code (encrypted Precision code) on L1 and L2 frequencies without restrictions. To use the service must be authorized PPS user equipped with a special GPS receiver that a password can eliminate artificial errors and process the encrypted Y – code . Unauthorized user is granted a standard position service (SPS – Standard Positioning Service). This service consists of access to C / A code and navigation data messages on the L1 carrier frequency and is continuously available to all GPS users worldwide.