GPS - Global Positioning System
It is generally used for navigational purposes.So before going to the details let me tell you what it is,
GPS is Satellite based navigational system which is used to find the accurate and precise position of the receiver system which is situated on the earth(basically receiver means the user equipment/GPS module), the receiving data consists the positional coordinates of the user.GPS is designed and operated by the U.S. Department of Defense
So i will give a brief over-view of how this system works , it might help you when we are discussing it in detail
1.Satellite (artificial)-terrestrial body which rotates around the planet(earth) which is human made and intended to carry out specific purposes,like communication , weather forecasting etc.
2.Orbital plane-the path in space through which the satellites rotates.
3.Period of revolution-time taken by the satellite to complete one revolution
the basic working principle is as follows
There are 24 GPS satellites including 3 spares which revolve around the earth with a period of revolution 12 hour orbiting at an altitude about 20000 km in 6 orbital planes which are inclined at 55 degree.this ensures that , at any time at least 3 or more satellites will be visible from the earth for the determination of position of user.The satellites transmits timing signal in L band(1-2 GHz ),using spread spectrum technique, thus the received timing signal is compared with a locally generated timing signal within the GPS receiver and the delay is reception is found out.Thus from 3 satellites the delays are found out and the distance or range from the satellites are calculated as
R1=C*Δ t1
R2=C*Δ t2
R3=C*Δ t3 where C is the speed of light= 3*10^8 m/s
In a GPS navigation system, the information of time is also important for which the signal from a fourth GPS satellite at a range R4 has also to be utilized to get four equation in relation to R1, R2, R3, R4. Three of the equations will help us to find the positional co-ordinates while the fourth is used to get time information.
now we will discuss about the GPS segments
The GPS system comprises of three segments
1.Space segment
2.Control segment
3.User segment
Space segment
Space segment consists of 24 satellites orbiting in circular orbits at an altitude about 20000 Km at an inclination of 55 degree with a periods of 12 hour.Their relative positions has been arranged in such away that at a time for any user minimum 4 satellites will be visible from the ground, thus ensuring global coverage.
A typical GPS satellite weighs about 816 Kg consuming an average electric power of 700 Watts derived from solar cell arrays , which in turn, are backed by Ni-Cd (105AH) batteries , the life span of a GPS satellite is typically 7-10 years.GPS sateliites have four atomic clocks on board ,but only one is used at a time.These highly accurate clocks provides GPS system with highly precise timing signals
GPS provides 2 types of services
1. Standard positioning service (SPS)
2. Precise positioning service (PPS)
1. Standard positioning service (SPS)- Positioning and timing service available to all users (military, private & commercial).this service is continuous and free of charge and provides a predictable positioning accuracy of 95% of the time is within in 100m horizontally, 156 m vertically and 185m in 3D with time transfer accuracy of 340nsec. Accuracy is downgraded during time of emergency services ,the accuracy is intentionally downgraded by US defense using Selective Availability (SA) Technique
2. Precise positioning service (PPS)- Highly accurate military positioning It is available continuous to worldwide users authorized by US DoD. Predictable positioning accuracy of 95% of the time is within in 22m horizontally, 27.7 m vertically and 35.4m in 3D with time transfer accuracy of 200nsec. Authorized users with cryptographic equipment and keys with specially designed GPS receivers can user PPS.Designed primarily to be used with US Govt Agencies and US Govt approved civil users
Each satellite is designed to transmit radio signal at two L band frequencies , L1 at 1575.42 MHz for commercial services (C/A code P code) and L2 at 1277.6 MHz for military services only.C/A (coarse acquisition) code is available to only civilian users and provides SPS service , P (precise) code is broadcast on both L1 and L2 frequencies and is used only by military services.Using P code on both frequencies military receivers can achieve better accuracy than civilian users.
GPS signals include pseudo random code which identifies which satellite is transmitting-in other words, an ID code. The navigation messages include ephemeris data,contains important information such as status of the satellite(healthy or unhealthy), current date and time , without this part of the message the GPS receiver would have no idea what the current time and date are .This part of the signal is essential to determine a position.Then the almanac data, it tells the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system ,ie; each satellite
transmits a navigation message containing its orbital elements, clock behavior,
system time and status messages. This allows the user set to
find all satellites once the first has been acquired.
Besides these for allowing control and telemetry function by ground(for correcting errors and getting back satellite into actual orbital position), the satellite has an S -band antenna operating on a down-link frequency of 2227.5MHz together with an up-link frequency of 1783.74MHz.
Control Segment
Control segment of GPS is used for monitoring and maintaining the performance of gps satellites, it involves 4 monitoring ground stations distributed throughout the globe at locations 1. Hawaii, 2. Kwaje-lein, 3.Diego Garcia and 4. Accension Island.thus the collected data's are processed at the Master control station at Colorado U.S.A.To derive the orbit of GPS satellite and update the navigation message of each satellite for onward transmission to the respective satellite through the ground station.So the uplink data includes satellite ephemeris,position constants,clock correction factors ,atomic data, almanac.
GPS User Segment
The user segment of GPS is meant for providing the user with their position information by the GPS signals using GPS receivers.this involves
1.selection of one or ore satellites in view
2. acquiring GPS signals by code division de-multiplexing
3. measuring and tracking of GPS satellite
4. recovering navigation data
Position determination
The GPS receiver uses the following information to determine
a position.
The receiver thus calculates the distance from the satellites in view by the range equation R=CΔt and it uses a spatial coordinate system having origin at the center of the earth,it then constructs spheres using this distances calculated from the satellites so if 3 satellites are in view 3 spheres whose individual radius are equal to the distance between that particular satellite and surface of earth.So we get three intersecting spheres and earth itself act as a fourth sphere ,so the each sphere increases the probability of finding out area where the user is located and finally by calculating the intersecting common area of all spheres.Since all the calculations are done in same coordinate system we get the exact location of the user
- Precise location of satellites
- Distance from each satellite
- Triangulation to determine position
The receiver thus calculates the distance from the satellites in view by the range equation R=CΔt and it uses a spatial coordinate system having origin at the center of the earth,it then constructs spheres using this distances calculated from the satellites so if 3 satellites are in view 3 spheres whose individual radius are equal to the distance between that particular satellite and surface of earth.So we get three intersecting spheres and earth itself act as a fourth sphere ,so the each sphere increases the probability of finding out area where the user is located and finally by calculating the intersecting common area of all spheres.Since all the calculations are done in same coordinate system we get the exact location of the user
Sources of Errors
a GPS receiver
- User mistakes- Unknowingly relying on a 2D position instead of a 3D position for determining position coordinate.The human body can cause signal interference. Holding a GPS receiver close to the body can block some satellite signals and hinder accurate positioning
- Multipath interference-Multi path interference is caused by the satellite signal reflecting off of vehicles, buildings, power lines, water and other interfering objects
- Satellite and receiver clock errors-These can be slight discrepancies in the satellite’s atomic clocks which may cause slight position errors in the GPS receiver. Errors are monitored and corrected by the Master Control Station
- Orbit errors-Satellite orbit pertains to the altitude, position, and speed of the satellite. Satellite orbits vary due to gravitational pull and solar pressure fluctuations. Orbit errors are also monitored and corrected by the Master Control Station.
- Atmospheric interference-The atmosphere can slow or speed up the satellite signal. Fortunately, error caused by atmospheric conditions (ionized air, humidity, temperature, pressure)
Correction techniques for greater accuracy(DGPS)
Differential GPS basically involves two GPS receivers.One of these receivers is stationary (base or reference station) and the other is roving and making position measurements.As the base station knows its location exactly , it can determine satellite signal errors . This is done by measuring the ranges to each satellite using the received signals which are compared to actual ranges calculated from its known location.These differential corrections for each satellite are transmitted to the moving GPS receiver and applied to its calculations.Transmissions for real-time use can be over FM radio frequencies,by satellite or beacon transmitter that are used by the U.S coast guard. Calculations can also be recorded for post processing.
NEW REVOLUTION IN THE WORLD OF GPS
Since the GPS is controlled and operated by US, these technology can be withdrawn in times of hostility.So the different nations begun to develop their own navigational satellite systems some of them are given below
GLONASS
The Russian government has developed a system,similar to GPS, called GLONASS. The first GLONASS satellite launch was in October 1982. The full constellation consists of 24 satellites in 3 orbit planes, which have a 64.8 degree inclination to the earth's equator. The GLONASS system now consists of 12 healthy satellites. GLONASS uses the same code for each satellite and many frequencies, whereas GPS which uses two frequencies and a different code for each satellite. Some GPS receiver manufacturers have incorporated the capability to receive both GPS and GLONASS signals. This increases the availability of satellites and the integrity of combined system.GALILEO
Galileo is Europe's contribution to the next generation Global Navigation Satellite System
(GNSS). Unlike GPS, which is funded by the public sector and operated by the U.S. Air Force, Galileo will be a civil‑controlled system that draws on both public and private sectors for funding. The service will be free at the point of use, but a range of chargeable services with additional features will also be offered. These additional features would include improved reception, accuracy and availability. Design of the Galileo system is being finalized and the delivery of initial services is targeted for 2008.
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