Global Positioning System(GPS)

Introduction to Global Positioning System

BEFORE GPS

For thousands of years, speed was limited to a walking pace and landmarks were used to find location. At sea, early navigators limited their voyages to coastal routes to avoid becoming lost. New methods for determining position arose as trade between distant ports increased. Polaris, the North Star, was used to determine north-south distance (latitude) in the northern hemisphere. But mariners also had to find latitude when sailing in the southern hemisphere, and they lacked a method for determining east-west position (longitude). The solution, celestial navigation, required accurate time. In the late 18th century this led to the development of the marine chronometer, an accurate sea-going timepiece. Beginning in the 19th century the U.S. Naval Observatory, the nation's official timekeeper, provided accurate time for navigators from an array of chronometers.

Sextant

A sextant is used to make precise celestial observations of the Sun, stars, and planets. It measures height, in degrees, above the horizon, which is used with the exact time to calculate position. The sextant shown here was made in the first half of the 19th century. "Shooting the stars" remains a basic skill for the sea-going navigator.

HISTORY OF GPS

Attempting to locate where you are or to determine the best way to reach a destination can be a particularly challenging task - especially if you are unfamiliar with the area and the terrain is rough. A good map can help, but it can even be difficult to determine where you currently are on the map. GPS is a technology that is ideally suited to both navigation and positioning.

GPS technology originated in the US military. The Defense Department recognized the need to have a precise positioning technology to locate soldiers, vehicles, enemies, and supplies on a battlefield and monitor their movements. The Global Positioning System was developed in response to fulfill this important military need and included a satellite network, ground communications stations, and receivers that cost over $12 billion to build.

On March 29, 1996, President Clinton, a Presidential Decision Directive (PDD) changed the categorization of GPS into an international information utility. The Presidential directive included the following relevant points:

1. The U.S. government will continue to operate, maintain and provide basic GPS signals worldwide, free of direct user fees.

2. The U.S. will advocate the acceptance of GPS and it's augmentations as a standard for use by initiating international discussions in agreement with Japan and Europe.

Currently the Global Positioning system is operated by the military, but is used by both military and commercial users.

WHAT GPS REQUIRES?

· SATELLITES: The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. Ground stations are used to precisely track each satellite's orbit.

· RECEIVERS: GPS receivers passively receive satellite signals; they do not transmit. GPS receivers require an unobstructed view of the sky, so they are used only outdoors and they often do not perform well within forested areas or near tall buildings. GPS operations depend on a very accurate time reference, which is provided by atomic clocks at the U.S. Naval Observatory. Each GPS satellite has atomic clocks on board.

HOW GPS WORKS?

THE WORKING OF GPS IS DIVIDED INTO THREE SEGMENTS:

· SPACE SEGMENT(includes SATELLITES)

· CONTROL SEGMENT

· USER SEGMENT(includes GPS receivers)

SPACE SEGMENT:

There are at least 24 operational GPS satellites at all times. The satellites, operated by the U.S. Air Force, orbit with a period of 12 hours. Ground stations are used to precisely track each satellite's orbit. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.

GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.

CONTROL SEGMENT

The Control Segment consists of a system of tracking stations located around the world.

This portion of the network is composed of a system of communications or monitor stations around the world. In addition to each of the stations around the world, there is one "Master Control" facility located at Schriever Air Force Base in Colorado. Each station is equipped with sophisticated computer equipment that allows the station to measure signals from each satellite. These measurements are then used compute the exact orbital data and clock functions of the satellites. The "Master Control" facility uploads the orbital and clock data to each of the 24 satellites to ensure that they are all synchronized. The satellites then send this data to the GPS receivers as radio signals.

USER SEGMENT

The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time.

Uses of GPS

GPS receivers are used for navigation, positioning, time dissemination, and other research.

Precise positioning is possible using GPS receivers at reference locations providing corrections and relative positioning data for remote receivers. Surveying, geodetic control, and plate tectonic studies are examples.

Time and frequency dissemination, based on the precise clocks on board the SVs and controlled by the monitor stations, is another use for GPS. Astronomical observatories, telecommunications facilities, and laboratory standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS receivers.

Research projects have used GPS signals to measure atmospheric parameter.

Let us talk more about GPS receivers.

GPS RECEIVERS

HOW GPS RECEIVERS WORK?

In order to understand the working of GPS, it is necessary to understand the concept of trilateration.

To understand trilateration let’s take an example:-

Let's say that you are somewhere in the India and you are TOTALLY lost -- you don't have a clue where you are. You find a friendly-looking person and ask, "Where am I?" and the person says to you, "You are 300 Km from Mumbai." This is a piece of information, but it is not really that useful by itself. You could be anywhere on a circle around Mumbai that has a radius of 300 Km.

So you ask another person, and he says, "You are 500 Km away from Delhi." This is helpful -- if you combine this information with the Mumbai information, you have two circles that intersect. You now know that you are at one of two points, but you don't know which one.

If a third person tells you that you are from 700Km Kolkatta, you can figure out which of the two points you are at!

With three known points, you can see that you are near Ahmedabad!

Trilateration is a basic geometric principle that allows you to find one location if you know its distance from other, already known locations. The geometry behind this is very easy to understand in two dimensional space.

This same concept works in three dimensional space as well, but you're dealing with spheres instead of circles. You also need four spheres instead of three circles to find your exact location. The heart of a GPS receiver is the ability to find the receiver's distance from four (or more) GPS satellites. Once it determines its distance from the four satellites, the receiver can calculate its exact location and altitude on Earth! If the receiver can only find three satellites, then it can use an imaginary sphere to represent the Earth and can give you location information but no altitude information.

FINDING THE TARGET:-

For a GPS receiver to find your location, it has to determine two things:

· The location of at least three satellites above you

· The distance between you and each of those satellites

MEASURING THE DISTANCE:-

GPS satellites send out radio signals that your GPS receiver can detect. But how does the signal let the receiver know how far away the satellite is? The simple answer is: A GPS receiver measures the amount of time it takes for the signal to travel from the satellite to the receiver. Since we know how fast radio signals travel -- they are electromagnetic waves and so (in a vacuum) travel at the speed of light, about 186,000 miles per second -- we can figure out how far they've traveled by figuring out how long it took for them to arrive.

Measuring the time would be easy if you knew exactly what time the signal left the satellite and exactly what time it arrived at your receiver, and solving this problem is key to the Global Positioning System. One way to solve the problem would be to put extremely accurate and synchronized clocks in the satellites and the receivers. The satellite begins transmitting a long digital pattern, called a pseudo-random code, as part of its signal at a certain time, let's say midnight. The receiver begins running the same digital pattern, also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern. The length of the delay is equal to the time of the signal's travel. The receiver multiplies this time by the speed of light to determine how far the signal traveled. If the signal traveled in a straight line, this distance would be the distance to the satellite.

The only way to implement a system like this would require a level of accuracy only found in atomic clocks. This is because the time measured in these calculations amounts to nanoseconds. To make a GPS using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself.

Drawbacks of Atomic Clock:-

Atomic clocks usually cost somewhere between $50,000 and $100,000, which makes them a little too expensive for everyday consumer use!

Solution to problem:-

The Global Positioning System has a very effective solution to this problem -- a GPS receiver contains no atomic clock at all. It has a normal quartz clock. The receiver looks at all the signals it is receiving and uses calculations to find both the exact time and the exact location simultaneously. When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point, as illustrated above. Four spheres of this sort will not intersect at one point if you've measured incorrectly. Since the receiver makes all of its time measurements, and therefore its distance measurements, using the clock it is equipped with, the distances will all be proportionally incorrect. The receiver can therefore easily calculate exactly what distance adjustment will cause the four spheres to intersect at one point. This allows it to adjust its clock to adjust its measure of distance. For this reason, a GPS receiver actually keeps extremely accurate time, on the order of the actual atomic clocks in the satellites!

One problem with this method is the measure of speed. As we saw earlier, electromagnetic signals travel through a vacuum at the speed of light. The Earth, of course, is not a vacuum, and its atmosphere slows the transmission of the signal according to the particular conditions at that atmospheric location, the angle at which the signal enters it, and so on. A GPS receiver guesses the actual speed of the signal using complex mathematical models of a wide range of atmospheric conditions. The satellites can also transmit additional information to the receiver

FINDING THE SATELLITES:-

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The other crucial component of GPS calculations is the knowledge of where the satellites are. This isn't difficult because the satellites travel in a very high, and predictable orbits. The satellites are far enough from the Earth (12,660 miles) that they are not affected by our atmosphere. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellite signals.

The most essential function of a GPS receiver is to pick up the transmissions of at least four satellites and combine the information in those transmissions with information in an electronic almanac, so that it can mathematically determine the receiver's position on Earth. The basic information a receiver provides, then, is the latitude, longitude and altitude (or some similar measurement) of its current position. Most receivers then combine this data with other information, such as maps, to make the receiver more user friendly.

VARIOUS GPS RECEIVERS:-

· GARMIN’S STREET PILOT III

Garmin's ColorMap StreetPilot III is Garmin's first receiver with automatic address to address routing. It also has voice command output. StreetPilot III coupled with Garmin's CityNavigator mapping program delivers one of the most detailed street maps available on the market. Users can load detailed maps for one or more of 120 US cities. These are not included with StreetPilot II. These detailed maps are also supplemented with a feature called "base maps" that are included in the system. These two types of maps let a user plot routes from one city to another without loading all of the maps in between. The price for StreetPilot III is $850, on of the lowest priced Automatic Car Navigators available.

Colormap StreetPilot III is designed to combine a GPS receiver, a laptop, and map software into one system. For additional memory, you can add 8, 16, 32, 64, and 128 megabyte memory cartridges. Most city maps are between 500k and 2mb of memory. The auctomaitc address to address routing requires the CityNavigator map offering in order to work effectively. The routes that the StreetPilot III chooses are usually the best routes, but again it is difficult for the machine to take into consideration other factors that might affect travel. Address to Address route generation is really fast. It can take anywhere from 30 seconds for a local route to 2 minutes for a national route. Since this unit is larger and designed for automobile or street use, the battery life is somewhat low. It lasts around two hours before it needs to be recharged.

All and all, for a price of $850, StreetPilot III is an excellent value and provides cutting edge features at a reasonable price.

· MAGELLAN GPS COMPANION FOR HANDSPRING VISOR

The GPS Companion for Handspring Advisor represents Magellan Corporation's first forray into that handheld ad on market. The GPS companion is competitively priced and can enhance the power of your Handspring Visor be adding navigation and street level mapping capability. The add-on doesn't add too much weight. The GPS Companion weighs 3.7 ounces and has dimensions of 1.13 by 2.06 by 4.75 inches. These dimensions give the GPS companion an added battery life. The package includes the GPS module composed of a 12 channel receiver, A CD-ROM containing MAP Companion and NAV Companion software, along with two AAA batteries. The GPS companion fits nicely into Handspring Visor's expansion slot and initializes rapidly.

The maps available through the companion make full use of Visor's color display. Colorful icons and rich graphics make it quick and easy to locate points of interest on the map. The MAP Companion, GPS mode, and the NAV Companion give you the ability to:

• Search for destinations
• Enable scrolling and positioning
• Switch between Map and GPS mode
• View current logitude and latitude information
• View altitude
• Display bearing
• Display speed of travel
• Display the satelites that you are locked on to
• Create waypoints
• Create routes
• Beam these into palms or other handhelds
• Create logs to show distance, speed and bearing
• Convert the logs to a route

With all of these features, the Magellan GPS Companion for the Handsrping Visor is one of the better GPS add-ons for a Palm OS device. The only drawback to the GPS Companion is that it doesn't give you the ability to locate street addresses. Also, It took about 10 minutes to lock onto a satelite during the first use. Subsequent locks were easier. All in all, though, the Handspring advisor companion is a good value.

GPS EVERYWHERE:-

GPS NAVIGATION IN THE AIR:-

Pilots on long distance flights without GPS rely on navigational beacons located across the country. Using GPS, aircraft can fly the most direct routes between airports.

GPS ON THE OCEANS:-

GPS is a powerful tool that can save a ship's navigator hours of celestial observation and calculation. GPS has improved efficient routing of vessels and enhanced safety at sea by making it possible to report a precise position to rescuers when disaster strikes.

GPS ON THE LAND:-

GPS improves efficiency on land as well. Delivery trucks can receive GPS signals and instantly transmit their position to a central dispatcher. Police and fire departments can use GPS to dispatch their vehicles efficiently, reducing response time. GPS helps motorists find their way by showing their position and intended route on dashboard displays. Railroads are using GPS technology to replace older, maintenance-intensive mechanical signals.

PORTABLE GPS RECEIVERS:-

This type of GPS receiver can be used by hikers for moderate accuracy in a hand-held unit. This GPS receiver incorporates such capabilities as navigation tools, internet access, and a digital camera.

ONLY DIFFICULTY:-

• Receiver costs vary depending on capabilities. Small civil SPS receivers can be purchased for under $200, some can accept differential corrections. Receivers that can store files for post-processing with base station files cost more ($2000-5000). Receivers that can act as DGPS reference receivers (computing and providing correction data) and carrier phase tracking receivers (and two are often required) can cost many thousands of dollars ($5,000 to $40,000). Military PPS receivers may cost more or be difficult to obtain.
• Other costs include the cost of multiple receivers when needed, post-processing software, and the cost of specially trained personnel.

GPS Positioning Services Specified In The Federal Radionavigation Plan

Precise Positioning Service (PPS)

• Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System. U. S. and Allied military, certain U. S. Government agencies, and selected civil users specifically approved by the U. S. Government, can use the PPS.
• PPS Predictable Accuracy
• 22 meter Horizontal accuracy
• 27.7 meter vertical accuracy
• 200 nanosecond time (UTC) accuracy

Standard Positioning Service (SPS)

• Civil users worldwide use the SPS without charge or restrictions. Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the DOD by the use of Selective Availability.
• SPS Predictable Accuracy
• 100 meter horizontal accuracy
• 156 meter vertical accuracy
• 340 nanoseconds time accuracy
• These GPS accuracy figures are from the 1999 Federal Radionavigation Plan. The figures are 95% accuracies, and express the value of two standard deviations of radial error from the actual antenna position to an ensemble of position estimates made under specified satellite elevation angle (five degrees) and PDOP (less than six) conditions.
• For horizontal accuracy figures 95% is the equivalent of 2drms (two-distance root-mean-squared), or twice the radial error standard deviation. For vertical and time errors 95% is the value of two-standard deviations of vertical error or time error.
• Receiver manufacturers may use other accuracy measures. Root-mean-square (RMS) error is the value of one standard deviation (68%) of the error in one, two or three dimensions. Circular Error Probable (CEP) is the value of the radius of a circle, centered at the actual position that contains 50% of the position estimates. Spherical Error Probable (SEP) is the spherical equivalent of CEP, that is the radius of a sphere, centered at the actual position, that contains 50% of the three dimension position estimates. As opposed to 2drms, drms, or RMS figures, CEP and SEP are not affected by large blunder errors making them an overly optimistic accuracy measure
• Some receiver specification sheets list horizontal accuracy in RMS or CEP and without Selective Availability, making those receivers appear more accurate than those specified by more responsible vendors using more conservative error measures.

GPS Satellite Signals

• The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers.
• Three binary codes shift the L1 and/or L2 carrier phase.
• The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This noise-like code modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits (one millisecond). There is a different C/A code PRN for each SV. GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS.
• The P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P (Y)-Code is the basis for the PPS.
• The Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters

GPS Data

• The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. SV Clock corrections are sent in subframe one and precise SV orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period.
• Data frames (1500 bits) are sent every thirty seconds. Each frame consists of five subframes.
• Data bit subframes (300 bits transmitted over six seconds) contain parity bits that allow for data checking and limited error correction.
• Clock data parameters describe the SV clock and its relationship to GPS time.
• Ephemeris data parameters describe SV orbits for short sections of the satellite orbits. Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours without much error. The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set.
• Almanacs are approximate orbital data parameters for all SVs. The ten-parameter almanacs describe SV orbits over extended periods of time (useful for months in some cases) and a set for all SVs is sent by each SV over a period of 12.5 minutes (at least). Signal acquisition time on receiver start-up can be significantly aided by the availability of current almanacs. The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency (the frequency shift caused by the rate of change in range to the moving SV) of each SV in the constellate

• Each complete SV data set includes an ionospheric model that is used in the receiver to approximate the phase delay through the ionosphere at any location and time.
• Each SV sends the amount to which GPS Time is offset from Universal Coordinated Time. This correction can be used by the receiver to set UTC to within 100 ns.
• Other system parameters and flags are sent that characterize details of the system.

Position, and Time from GPS

• Code Phase Tracking (Navigation)
• The GPS receiver produces replicas of the C/A and/or P (Y)-Code. Each PRN code is a noise-like, but pre-determined, unique series of bits.
• The receiver produces the C/A code sequence for a specific SV with some form of a C/A code generator. Modern receivers usually store a complete set of precomputed C/A code chips in memory, but a hardware, shift register, implementation can also be used.
• The C/A code generator produces a different 1023 chip sequence for each phase tap setting. In a shift register implementation the code chips are shifted in time by slewing the clock that controls the shift registers. In a memory lookup scheme the required code chips are retrieved from memory.
• The C/A code generator repeats the same 1023-chip PRN-code sequence every millisecond. PRN codes are defined for 32 satellite identification numbers.
• The receiver slides a replica of the code in time until there is correlation with the SV code.
• If the receiver applies a different PRN code to an SV signal there is no correlation.
• When the receiver uses the same code as the SV and the codes begin to line up, some signal power is detected.
• As the SV and receiver codes line up completely, the spread-spectrum carrier signal is de-spread and full signal power is detected.
• A GPS receiver uses the detected signal power in the correlated signal to align the C/A code in the receiver with the code in the SV signal. Usually a late version of the code is compared with an early version to insure that the correlation peak is tracked.
• A phase locked loop that can lock to either a positive or negative half-cycle (a bi-phase lock loop) is used to demodulate the 50 HZ navigation message from the GPS carrier signal. The same loop can be used to measure and track the carrier frequency (Doppler shift) and by keeping track of the changes to the numerically controlled oscillator, carrier frequency phase can be tracked and measured.
• The receiver PRN code start position at the time of full correlation is the time of arrival (TOA) of the SV PRN at receiver. This TOA is a measure of the range to SV offset by the amount to which the receiver clock is offset from GPS time. This TOA is called the pseudo-range.

• Pseudo-Range Navigation
• The position of the receiver is where the pseudo-ranges from a set of SVs intersect.
• Intersection of Range Spheres
• Position is determined from multiple pseudo-range measurements at a single measurement epoch. The pseudo range measurements are used together with SV position estimates based on the precise orbital elements (the ephemeris data) sent by each SV. This orbital data allows the receiver to compute the SV positions in three dimensions at the instant that they sent their respective signals.

• Four satellites (normal navigation) can be used to determine three position dimensionsand time. Position dimensions are computed by the receiver in Earth-Centered, Earth-Fixed X, Y, Z (ECEF XYZ) coordinates.
• Time is used to correct the offset in the receiver clock, allowing the use of an inexpensive receiver clock.
• SV Position in XYZ is computed from four SV pseudo-ranges and the clock correction and ephemeris data.

• Receiver Position, Velocity, and Time
• Position in XYZ is converted within the receiver to geodetic latitude, longitude and height above the ellipsoid.
• Latitude and longitude are usually provided in the geodetic datum on which GPS is based (WGS-84). Receivers can often be set to convert to other user-required datums. Position offsets of hundreds of meters can result from using the wrong datum.
• Velocity is computed from change in position over time, the SV Doppler frequencies, or both.
• Time is computed in SV Time, GPS Time, and UTC.
• SV Time is the time maintained by each satellite. Each SV contains four atomic clocks (two cesium and two rubidium). SV clocks are monitored by ground control stations and occasionally reset to maintain time to within one-millisecond of GPS time. Clock correction data bits reflect the offset of each SV from GPS time.
• SV Time is set in the receiver from the GPS signals. Data bit subframes occur every six seconds and contain bits that resolve the Time of Week to within six seconds. The 50 Hz data bit stream is aligned with the C/A code transitions so that the arrival time of a data bit edge (on a 20 millisecond interval) resolves the pseudo-range to the nearest millisecond. Approximate range to the SV resolves the twenty millisecond ambiguity, and the C/A code measurement represents time to fractional milliseconds. Multiple SVs and a navigation solution (or a known position for a timing receiver) permit SV Time to be set to an accuracy limited by the position error and the pseudo-range error for each SV.
• SV Time is converted to GPS Time in the receiver
• GPS Time is a "paper clock" ensemble of the Master Control Clock and the SV clocks. GPS Time is measured in weeks and seconds from 24:00:00, January 5, 1980 and is steered to within one microsecond of UTC. GPS Time has no leap seconds and is ahead of UTC by several seconds.
• Time in Universal Coordinated Time (UTC) is computed from GPS Time using the UTC correction parameters sent as part of the navigation data bits.
• At the transition between 23:59:59 UTC on December 31, 1998 and 00:00:00 UTC on January 1, 1999, UTC was retarded by one-second. GPS Time is now ahead of UTC by 13 seconds.

• Carrier Phase Tracking (Surveying)
• Carrier-phase tracking of GPS signals has resulted in a revolution in land surveying. A line of sight along the ground is no longer necessary for precise positioning. Positions can be measured up to 30 km from reference point without intermediate points. This use of GPS requires specially equipped carrier tracking receivers.
• The L1 and/or L2 carrier signals are used in carrier phase surveying. L1 carrier cycles have a wavelength of 19 centimeters. If tracked and measured these carrier signals can provide ranging measurements with relative accuracies of millimeters under special circumstances.
• Tracking carrier phase signals provides no time of transmission information. The carrier signals, while modulated with time tagged binary codes, carry no time-tags that distinguish one cycle from another. The measurements used in carrier phase tracking are differences in carrier phase cycles and fractions of cycles over time. At least two receivers track carrier signals at the same time. Ionospheric delay differences at the two receivers must be small enough to insure that carrier phase cycles are properly accounted for. This usually requires that the two receivers be within about 30 km of each other.
• Carrier phase is tracked at both receivers and the changes in tracked phase are recorded over time in both receivers.
• All carrier-phase tracking is differential, requiring both a reference and remote receiver tracking carrier phases at the same time.
• Unless the reference and remote receivers use L1-L2 differences to measure the ionospheric delay, they must be close enough to insure that the ionospheric delay difference is less than a carrier wavelength.
• Using L1-L2 ionospheric measurements and long measurement averaging periods, relative positions of fixed sites can be determined over baselines of hundreds of kilometers.
• Phase difference changes in the two receivers are reduced using software to differences in three position dimensions between the reference station and the remote receiver. High accuracy range difference measurements with sub-centimeter accuracy are possible. Problems result from the difficulty of tracking carrier signals in noise or while the receiver moves.
• Two receivers and one SV over time result in single differences.
• Two receivers and two SVs over time provide double differences.
• Post processed static carrier-phase surveying can provide 1-5 cm relative positioning within 30 km of the reference receiver with measurement time of 15 minutes for short baselines (10 km) and one hour for long baselines (30 km).
• Rapid static or fast static surveying can provide 4-10 cm accuracies with 1 kilometer baselines and 15 minutes of recording time.
• Real-Time-Kinematic (RTK) surveying techniques can provide centimeter measurements in real time over 10 km baselines tracking five or more satellites and real-time radio links between the reference and remote receivers.

Ø Application of GPS

(1) Location based services

- Vehicle Tracking System

- Creating Data base for Transport Engineering

(2) Disaster Monitoring System

- Earthquake monitoring

(3) Surveying

(4) Resource Utilization

Ø CONCLUSION

GPS is used to make precise celestial observation of the sun stars and planets.Gps receiver are used for nevigation,positioning time dissemination and other research.it locate the correction and relative positioning data for remote receiver. It is based vehical tracking system & monitoring earthquake.

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