Where am I? Where am I going? Where are you? What is the best way to get there? When will I get there? GPS technology can answer all these questions. GPS satellite can show you exact position on the earth at any time, in any weather, no matter where you are! GPS technology has made an impact on navigation and positioning needs with the use of satellites and ground stations the ability to track aircraft, cars, cell phones, boats, and even individuals has become a reality.
A system of satellites, computers, and receivers that is able to determine the latitude and longitude of a receiver on Earth by calculating the time difference for signals from The Global Positioning different satellites to reach the receiver. System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these "Manmade stars" as reference points to calculate positions accurate to a matter of meters.
In fact, with advanced forms of GPS, you can make measurements to better than a centimeter! In a sense, it's like giving every square meter on the planet a unique address. GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone.
Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals. 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.
WHAT IS GPS?
The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defence that continuously transmit coded information, which makes it possible to precisely identify locations on earth by measuring the distance from the satellites.
The satellites transmit very low power specially coded radio signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time thus allowing anyone one with a GPS receiver to determine their location on earth.
Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. The system was designed so that receivers did not require atomic clocks, and so could be made small and inexpensively.
The GPS system consists of three pieces. There are the satellites that transmit the position information, there are the ground stations that are used to control the satellites and update the information, and finally, there is the receiver that you purchased. It is the receiver that collects data from the satellites and computes its location anywhere in the world based on information it gets from the satellites.
GPS receivers only receive data they do not transmit data.
EVOLUTION OF GPS:
The technology evolved from, Mr. Marconi’s transmission of radio waves. This was applied for society during the 1920s by the establishment of radio stations, for which you only needed a receiver. The same applies to GPS- you only need a rather special radio receiver. Significant advances in radio were bolstered by large sums of money during and after the Second World War and were even more advanced by the need for communications with early satellites and rockets and general space exploration. The technology to receive radio signals in a small hand-held, from 20,000kms away, is indeed amazing.
Throughout the 1960s the U.S. Navy and Air Force worked on a number of systems that would provide navigation capability for a variety of applications. In 1973 finally, the U.S. Department of Defence decided that the military had to have a super precise positioning system. In short, development of the GPS satellite navigation system was begun in the 1970s by the US Department of Defence. The basis for the new system was atomic clocks carried on satellites, a concept successfully tested in an earlier Navy program called TIMATION. The Air Force operated the new system, which it called the NAVSTAR Global Positioning System. It has since come to be known simply as GPS.
The first GPS satellite was launched in 1978 and a second-generation set of satellites ("Block II") was launched beginning in 1989. Today's GPS constellation consists of at least 24 Block II satellites. A full constellation of 24 satellites was achieved in 1994. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use.
SEGMENTS OF GPS:
GPS System is comprised of three segments. They are
⦁ Space Segment
⦁ Control Segment
⦁ User Segment
GPS SEGMENTS
⦁ SPACE SEGMENT:
The GPS technology is based on the NAVSTAR (NAVigation Satellite Timing And Ranging) constellation composed of 24 satellites in space, this is the space segment of the GPS system. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. These 24 satellites (21 navigational satellites and 3 active spares) are in 6 circular planar orbits, equally spaced (60 degrees apart), at an inclination angle of 55 degrees to earth relative equator. These satellites weigh 1900 lbs in orbit, travel at speeds of about 14,000 kilometers per hour or 8700 miles per hour with a 12hr period (precisely 11hr 58 min).
Each satellite transmits the following information called NAVIGATION/NAV message as a part of its signal to ground stations and users:
⦁ EPI: Ephemeris Position Information, a message transmitted every 30 seconds by each satellite containing precise information on the status of satellite, current date and time.
⦁ Almanac: It is a file containing information on GPS constellation which includes orbital information for that satellite and every other satellite in the system.
⦁ Atmospheric data: This is necessary to help correct signal interference from satellites to the receiver as GPS signal has to travel through ionosphere where data might get delayed and disturbed
⦁ Clock Correction: A correction parameter to convert GPS time to Universal Coordinated time.
⦁ CONTROL SEGMENT:
It consists of a system of tracking stations located around the world. The control segment is composed of all the ground-based facilities that are used to monitor and control the satellites. This segment is usually unseen by the user but is a vital part of the system. The NAVSTAR control segment, called the operational control system (OCS) consists of 5 monitor stations, a master control station (MCS) and 3 uplink antennas. The satellites send down subsets of the orbital ephemeris data. The monitor stations track GPS satellites in view, collect and send information from the satellites back to the master control station that computes the precise orbits. The master station uploads the data which is necessary for proper operation of the satellite, like ephemeris and clock data to the satellites. Then the information is formatted into updated navigation messages that are transmitted through ground antennas.
⦁ USER SEGMENT:
The user segment is composed of GPS receivers composed of processors and antennas that allow for sea, land and airborne operators to receive the broadcast. The receivers convert space vehicle signals into position, velocity and time. A total of 4 satellites are required to compute these calculations. In order to make this simple calculation, then, the GPS receiver has to know two things:
⦁ The location of at least three satellites above you.
⦁ The distance between you and each of those satellites.
The GPS receiver figures both of these things out by analyzing the high frequency, low-power radio signals from the GPS satellites. Better units have multiple receivers so they can pick up signals from several satellites simultaneously.
A typical GPS receiver consists of
⦁ L-band receiver
⦁ Antennas which receives GPS signals
⦁ Determines pseudo ranges to determine from which satellite the signal came from.
PRINCIPLE OF OPERATION OF GPS: TRILATERATION
The principle behind GPS is the measurement of distance (or "range") between the receiver and the satellites. The satellites also tell us exactly where they are in their orbits above the Earth. It works something like this-If we know our exact distance from a satellite in space, we know we are somewhere on the surface of an imaginary sphere with radius equal to the distance to the satellite radius. By measuring its distance from a second satellite, the receiver knows it is also somewhere on the surface of a second sphere with radius equal to its distance from the second satellite. Therefore, the receiver must be somewhere along a circle which is formed from the intersection of the two spheres. Measurement from a third satellite introduces a third sphere. Now there are only two points which are consistent with being at the intersection of all three spheres. One of these is usually impossible, and the GPS receivers have mathematical methods of eliminating the impossible location. Measurement from a fourth satellite now resolves the ambiguity as to which of the two points is the location of the receiver. The fourth satellite point also helps eliminate certain errors in the measured distance due to uncertainties in the GPS receiver's timing as well.
This Principle we are using now call it “TRILATERATION”.
Two distances of two satellite signals from corresponding satellites to GPS receiver places the GPS receiver somewhere along the intersection of two spheres
Three distances of three satellite signals from corresponding satellites to GPS receiver places the GPS receiver at the intersection of three spheres(one of the two possible points).
Distance is calculated by d=c*t;
Where d is the distance between satellite and GPS receiver;
c=speed of GPS radio-signals @180,000miles/sec
t=time taken by a GPS signal to travel from space to receiver.
Here’s how GPS works in five logical steps:
⦁ The basis of GPS is "TRILATERATION" from satellites.
⦁ To trilaterate, a GPS receiver measures distance using the travel time of radio signals.
⦁ To measure travel time, GPS needs very accurate timing, which it achieves with some tricks.
⦁ Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secrets.
⦁ Finally, you must correct for any delays the signal experiences as it travels through the atmosphere.
Transmission channels of Satellite vehicle’s signal:
Each GPS satellite currently uses two frequencies to accomplish data transmission, L1 and L2. These carry carrier frequencies of L1 @1575 MHz and L2 @1227 MHz.
The signals that are superimposed on these two carrier waves are
⦁ C/A code
⦁ NAVIGATION message (NAV message)
⦁ P-code
⦁ C/A PRN code:@1.023 MHz
-Coarse Acquisition code is a pseudo-random code with which receiver will come to understand from which satellite a signal came from.
-Each satellite has its own C/A PRN code to get identified by the GPS receiver.
-This code is used by the civilians so, this code is transmitted through L1 carrier wave.
⦁ P-code:@10.23 MHz
-It is also a pseudo-random code to identify from which satellite a signal came from but it is a private and precise code enabling the receiver to get more accurate results, only used for military purpose. But there may be cases where civilians also use this code. So this code is transmitted on L1 and L2 carrier waves.
⦁ NAV message:
This is the real information of the satellite signal that is carried on both L1 and L2 carrier waves.
C/A code and NAV message is added i.e., modulo 2 sum (if the sum of the two signals is odd then signal stays at 1 and if even stays at 1) and mixed with the L1 carrier wave and its result is mixed with the P-code and sends output as the L1 signal.
P-code and NAV message is added and mixed with the L2 carrier wave and sends output as the L2 signal.
These C/A and P-codes are called Pseudo-random codes because each satellite produces a random code which looks random but deterministic just like the numbers after the decimal in pi value which looks random but very deterministic.
Signal Structure of Satellite Vehicle
C/A PRN CODE GENERATION:
Coarse-Acquisition code frequency is 1.023MHz which is much lesser frequency than P-code and that is why it is only used by civilians not militaries
C/A PRN code transmission rate is 1023Mbps i.e., for one message 1023 bits are sent. All 1023 bits are generated and sent by each satellite for every millisecond. The number of bits in a message depends on the number of bits you want to use to develop the C/A code. Here we use 10 bits to do so.
Each C/A code from each satellite is produced by combining two10-bit streams and those are combined based on “10-state linear feedback system”. These two 10-bit streams bits go through manipulation under two generator polynomials.
We use the following polynomials to manipulate the two 10 bit streams in such a way that they’ll produce a unique code to each satellite.
⦁ Polynomial 1 :
3rd and 10th-bit positions are added/modulo 2 sum i.e. if the sum of those two bits is odd it stays at 1 and if even it stays at 0.
⦁ Polynomial 2: 1+
In this 2nd, 3rd, 6th,8th,9th, 10th-bit positions are added/modulo 2 sum.
Here we are combining the two 10 bit streams and delaying one of the two-bit streams by an integer number of periods and this delay period is unique to each satellite.
we also use a unique phase tap to each satellite to produce a unique C/A code.
Phase taps for some satellites:
EG:
C/A PRN CODE GENERATION FOR SV-1:
One 10-bit stream is manipulated by the polynomial 1 and other 10-bit stream is manipulated by polynomial 2. Initially, the two 10 bit streams are reset to 1’s at the start of a millisecond. According to polynomial 1, the 3rd and 10th bit positions are added 1+1=2(even) stays at 0 and the result 0 of this is again stored in the initial bit of the 1st 10 bitstream and then all bits are shifted 1 bit right and whatever the bit in 10th bit position that bit is the output to the 2nd polynomial.(here it is bit ‘1’)
In the 2nd polynomial, 2nd,3rd,6th,8th,9th,10th position bits are added 1+1+1+1+1+1=6(even) stays at 0 and the result 0 is stored in the initial bit of 2nd 10-bit stream. The phase tap positions of the satellite vehicle-1 are 2 and 6, so the 2nd and 6th-bit positions of the 2nd polynomial is added with the output of the first polynomial 1+1+1=3(odd) stays at ‘1’ and all bits are shifted one bit to right in polynomial 2. Whatever the output generated by adding the phase tap position bits and output of the first polynomial is the 1st bit of the C/A PRN code, the same process is done for the generation of 2nd , 3rd, 4th ….1023rd bits of C/A PRN code and this process is repeated for 1023 times and sends the message to receiver. For 1024th time again it resets all bits to 1’s in both polynomial-1 and 2 and repeats the entire process for 1023 times in a millisecond.
When the receiver receives this code bit stream then it starts shifting back one unit at a time until the two matches (GPS receivers will store this C/A code in memory) and then observe how many units are apart and what delay was on the second bitstream compared to first bit stream and knows from which satellite it came from because each satellite will delay a specific number of integer periods as we using phase taps.
The last column in the diagram will be the C/A PRN code for satellite-1 with 1023 bits long
11001000001……….
ADVANTAGES OF GPS:
GPS FOR PRIVATE AND COMMERCIAL USE:
The GPS system is free for everyone to use, all that is needed is a GPS receiver, which costs about $90 and up (March 2005). This has led to widespread private and commercial use. An example of private use is the popular activity Geocaching where a GPS unit is used to search for objects hidden in nature by traveling to the GPS coordinates. Commercial use can be land measurement, navigation and road construction.
GPS ON AIRPLANES
Most airline companies allow private use of ordinary GPS units on their flights, except during landing and take-off, like all other electronic devices. The unit does not transmit radio signals like mobile phones, it can only receive. Note, however, that some airline companies might disallow it for security reasons, such as unwillingness to let ordinary passengers track the flight route.
GPS FOR HORTICULTURE:
In orchards, GPS is used mainly for orchard mapping or electrical mapping. The GPS system allows orchardists to accurately keep records of chemical applications, which is extremely important where the government is concerned. It can keep track of orchard costs, record and track yields. GPS also allows for the fine-tuning of orchard management techniques for the grower.
GPS ON HORTICULTURE:
Marine GPS receivers feature waterproof casings, marine chart plotter maps, and even fishing tables and celestial schedules. Most can also store highway map information so you can use your marine GPS to get you to the marina and then out to the fish.
DRAWBACKS OF GPS:
Sometimes your GPS may fail in the middle of your way to some destination then you need to have a backup plan like maps.
GPS may not be accurate sometimes due to obstacles like buildings, extreme atmospheric conditions.
Using a GPS in a battery operated device, then a battery will be dead at some time, then you need to have an external power supply where it is not always possible.
There must be always a clear line of sight between satellites and antennas otherwise GPS will not work properly.
CONCLUSION:
There will probably be a time soon when every car on the road can be equipped with a GPS receiver, including a video screen installed in the dashboard. The in-dash monitor will be a full-color display showing your location and a map of the roads around you. It will probably monitor your car's performance and your car phone as well. Systems as amazing as this one are already being tested on highways in the United States.
GPS is rapidly changing the way people are finding their way around the earth. Whether it is for fun, saving lives, getting there faster or whatever use you can dream of, GPS navigation is becoming more common every day.
GPS will figure in history alongside the development of the sea-going chronometer. This device enabled seafarers to plot their course to an accuracy that greatly encouraged maritime activity, and led to the migration explosion of the nineteenth century. GPS will affect mankind in the same way. There are myriad applications that will benefit us individually and collectively
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