The GPS Technology

8/8/2019 The GPS Technology

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Reference manual (LRM). ThisTHE GPS Technology

1 . INTRODUCTION

Throughout time people have developed a variety of ways to figure out their position on earth and to

navigate from one place to another. Early mariners relied on angular measurements to celestial bodies like

sun and stars to calculate their location. The 1920s witnessed the introduction of more advanced

technique-radio navigation-based at first on radios that allowed navigators to locate the direction of shore-

based transmitters when in range. Later development of artificial satellites made possible the transmission


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Reference manual (LRM). Thisof more precise, line of sight radio navigation signals and sparked a new era in navigation technology.

Satellites are first used in position finding in a simple but reliable 2D Navy system called Transit. This laid

the groundwork for a system that would later revolutionize navigation for ever-the Global Positioning

System

The Global Positioning System (GPS) is a satellite based navigation system. The concept of GPS

was introduced by the United States Department of Defense (DoD). It is in the year

1994 that the GPS was completely developed. The GPS is developed to provide

continuous, highly precise positions, velocity and time information to the land, sea, air


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Reference manual (LRM). Thisand space based users. The intent of system is to use a combination of ground stations,

orbiting satellites and special receivers to provide navigation capabilities to virtually

everyone, at any time, anywhere in the world, regardless of weather conditions.


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Reference manual (LRM). This

2. THE GPS SEGMENTS

1. The Space Segment

The space segments, also known as satellite segment, consist of 24 operational satellites

revolving around earth in 6 orbital planes approximately 600 GPS satellites are not

geosynchronous. First satellite was launched in the year 1978. The satellites take approximately


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Reference manual (LRM). Thissatellite contains a Cesium Atomic Clock and all these clocks will be synchronized and are

accurate within a few nanoseconds.

2.2 THE CONTROL SEGMENT

The GPS control segment (CS), called the Operational Control System (OCS), includes all the fixed

locations ground-based monitor stations located throughout the world, a Master Control Station (MCS) and

the up-link transmitters. The monitor stations are simply GPS receivers that track the satellites as they

pass overhead and accumulate ranging and ephemeris data from them. This information is relayed to the


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Reference manual (LRM). ThisMaster Control Station. These ground stations around the world are responsible for monitoring the flight

paths of the GPS satellites and synchronizing the satellites onboard atomic clocks. This information is

relayed to MCS where it is processed and compares the actual satellite position with the GPS computed

position. The MCS receives data from the monitor stations in real time 24hrs a day, and uses that

information to determine if any satellite are experiencing clock or ephemeris change and to detect

malfunctions. Corrections are done and then it is uploaded to the satellites twice per day by the uplink

antennae.


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Reference manual (LRM). This2.3. The User Segment

The GPS user segment consists of all the GPS receivers and the user community.Initially the GPS service was available for military purpose only. But in 1980 the Government of UnitedStates made the GPS service available to civilians also.The Fig 2.3 shows a GPS receiver. GPS receivers convert signals received from space vehicles intoposition, velocity and time estimates .The GPS navigation set contains antennae, receiver, data processorand a display unit. The satellite signals are further processed by data processor of the navigation set todemodulate the data and then decode it to get the users 3D position coordinates. The GPS receivers areused for navigation, positioning, aviation, shipping, geology and other purposes.


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3. WORKING


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Fig 3.1: RANGING CALCULATION


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Reference manual (LRM). Thist = propagation time (seconds)

Time is the most important factor in the working of GPS. Time synchronization between the GPS receiverand the on-board clocks is very important. Then only the Ranging calculations can be done accurately. Thesatellite transmitter and the Earth station receiver produce identical synchronizing (pseudorandom) codesat exactly the same time. This time will be accurate up to a few nanoseconds. Each satellite continuouslytransmits its precise synchronizing code. After a synchronizing code is acquired, the receiver compares thereceived code with its own locally produced code to determine propagation time. The time differencemultiplied by the velocity of radio signal gives the distance to satellite.


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If the Earth station receiver knows the location of the single satellite and the distance the satellite is fromthe receiver, it knows that it must be located somewhere on an imaginary sphere centered on the satellitewith a radius equal to the distance the satellite is from the receiver. If the receiver knows the location of

the two satellites and their distances from the receiver, it can narrow its location to somewhere on thecircle formed where the two spheres intersect as shown in Fig 3.2.


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Reference manual (LRM). ThisIf the location and distance to a third satellite is known, a receiver can pinpoint its location to one of thetwo possible locations in space as shown in Fig 3.3.If the location and distance from a fourth satellite is known, the altitude or the 3D position of the Earthstation can also be determined.

4. LEVELS OF SERVICES

1. STANDARD POSITIONING SERVICE (SPS).


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Reference manual (LRM). Thishorizontal accuracy of 20m, vertical accuracy of 27m and a 3D accuracy of 35m. Cryptographicequipments are used to prevent the unauthorized use of these PPS. This is more precise than the SPS.


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5. APPLICATIONS OF GPS

5.1 POSITIONING


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The GPS can be used to find the exact position of a person or a vehicle etc very easily. Whatever may bethe whether conditions we can easily locate a person or a vehicle having the GPS receiver. The Fig 5.1shows the positioning of a receiver in a smoky weather condition. Precise location data for any point onplanet is possible using this GPS. This system is used to locate persons and vehicles when they are lost.


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5.2 NAVIGATION SYSTEM

For the Navigation purposes GPS receivers are used in cars, aeroplanes, ships and even space vehicles.The Fig 5.2 shows a GPS system used in cars. The current location of the vehicle and the road maps willbe displayed on an LCD screen. Thus the shortest path to the destination can be selected. This is used inAviation purposes to know the exact location of the plane and its distance from the different ports.Automatic pilot system is based on this technology. This is also used by sailors and other cruise ships toknow their position in sea. The space vehicles (SV) also use this GPS to know their current locations.


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5.3 TRACKING


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5.4 MILIATRY PURPOSE

GPS is used in military for setting the targets and guiding the missiles. During the war period, military


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Reference manual (LRM). Thisuses this technology to know the positions of their forces and their movements in the war region.Aeroplanes like F-16, B2 Bomber, KC-135 Arial refuelers make use of this technology. Missiles like Toma-Hawk are guided using GPS to destroy the targets. When a country uses the GPS guided missiles againstan enemy, it is possible for the enemy country to locate some of the points in its trajectory by making useof its own GPS. Using these points the actual trajectory can be manipulated and location from which themissile was fired could be predicted. There is a good chance that this location would be a military base. Fig5.4 shows GPS guided missile.

5.5 PUBLIC SAFETY


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Reference manual (LRM). ThisThe GPS technology is used to help the people in case of emergencies. When an emergency call is madeby a person, the call is automatically forwarded to a public-safety answering point (PSAP), also called anemergency call center. When the call is answered, the call center operator is provided with automaticlocation information (ALI), pinpointing the exact position of the call. The PSAP will give information to thenearby rescue operation team and thus makes the rescue operations fast.

5.6 TIME SYNCHRONIZATION Many synchronization systems use GPS as a source of accurate time;hence one of the commonest applications of this use is that of GPS as a reference clock for time codegenerators. For instance, when deploying sensors (for seismology or other monitoring application), GPSmay be used to provide each recording apparatus with some precise time source, so that the time of events may be recorded accurately. For geographically dispersed stations, the time synchronization isdone using the GPS.


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6. CONCLUSION

Though originally designed to help US Forces around the world to locate targets and move quickly, it isnow being used across the world, from mountaineers climbing up Mount Everest to sail boats journeyingin to oceans. Its functions have been extended to over positioning, tracking, mapping etc.GPSs future seems secure. Its biggest push now is the Federal Communications Commission (FCC)enhanced 911 mandates. The new cell phones will be enabled with GPS. GPS receivers in future will beable to give accuracy up to 5mm. there is still room for improvement for GPS, as it does so, we will find itbeing used more and more in our daily lives, to the point where it would be hard to perform many traveland industry tasks without it.


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Documents

The GPS Technology