GPS Satellite Navigation System Explained
Rapid development in communication technology, especially mobile communication resulted in the development of modern communication equipments with ultra compactness and variety of services. This has also paid lot of attention in to the utilisation of satellite navigation services in day to day life. U.S. designed Global positioning system (GPS), Russian GLONASS are the popular satellite navigation systems available today. Indian Space research Organisation is also developing a regional navigation satellite system, which will be operational in near future. Global positioning system as its name implies, can offer accurate information about the position and time of a user at any time and any weather conditions. There area different types of GPS receivers available in the market depending upon the application.For a very high accuracy requirement Differential GPS are being used. Now a days even mobile phones are equipped with GPS services. Even though GPS provides accurate coordinates, its accuracy depends on various parameters, like receiver errors, ionospheric effects, multipath effect etc.
BASICS OF GLOBAL POSITIONING SYSTEM
GPS acronym for Global Positioning System is a USA owned satellite based navigation system providing users with position, navigation and time services. It is developed maintained and operated by the US air force. It consists of three segments namely Space segment, Control segment and the User segment.
• The Space Segment: This segment consists of a constellation of satellites. It is ensured that 24 GPS satellites are available for use 95% of the time rest satellites serve as a backup in case of malfunctioning of any of the working satellites. The 24 satellite arrangement ensures the availability of at least four satellites are in view from any point on earth. As of June 2011 The 24 operational satellites have been expanded upto 27 by expanding three of the slots out of 24 and repositioning six satellite making the number go up at 27. Fig 1.1 shows the constellation of GPS satellites.
• The Control Segment: This segment consists of a global network of monitoring and control stations which tracks the GPS satellites, also maintaining and correcting the satellite constellation orbit, satellite clock and updating navigational data. Fig 1.2 shows the global network of ground control stations.
• The User Segment: The User segment consists of a receiver device which receives information signal from satellites for calculation of user’s 3-D position and time. It can be any receiver, handheld or DGPS.
Working Of GPS
The GPS Signal:
The GPS signal is broadcasted on two frequencies L1 and L2. The values of L1 is s 1,575.42 MHz and L2 is 1,227.60 MHz. L1 frequency is modulated using two codes namely C/A(Coarse and Acquisition code) and P ( Precise code) code also a copy of P code is send along with L2 frequency.
The Triangulation Method: Triangulation means the determination of a distance from three points. The GPS receiver receive the time and position information from the satellite And it calculate the distance to the satellite by using standard distance formula assuming the signal speed close to speed of light. But the problem here is that only the distance is known so what about the direction. Here the triangulation method comes into use. The method can be described as follows:
• When the receiver detects the first satellite say S1, it can be assumed that the position of receiver is somewhere on the virtual sphere with S1 as its centre.
• Similarly for the second satellite S2 the receiver calculates the distance. Now it a second sphere with centre S2 can be predicted. So the position of the receiver narrows down to be located on the perimeter of the circle made from the intersection of two spheres S1 and S2.
• Now the exact location can be determined by intersection of sphere of third satellite S3. The sphere S3 intersects the circle (S1, S2) at two points. One of the two points is actually in space and one is on the earth. The point in space is neglected and hence the receiver position is calculated.
Need of Forth Satellite: It may be argued if three satellites can determine user’s position then why it’s advised to have four satellites. The reason behind it is that position determined from three satellites is a 2D-position fix, as it is assumed that the user is located on earth’s surface. There may be errors if the user is on mountains. The fourth satellite gives the absolute position in 3D-space and also gives heights above earth’s surface.
Sources of error in GPS
There are many factors which may lead to error in position calculation using GPS. They can be broadly divided into following categories:
- Multipath Reflection: Sometimes the satellite signals may experience reflection from various earth’s features such as mountains before reaching the receiver. These signals may interfere or could be mistaken for the original signal. The receiver is able to distinguish between the two signals as the reflected signal arrive later than direct signal and can be corrected by receiver. The error caused is less than one meter.
- Satellite Orbit errors: The orbital position of the satellites is constantly monitored by the ground stations. The ground station detects and updates the orbit data if the satellite drifts from its intended position.
- Malfunctioning Satellites: Sometimes a satellite may malfunction and transmit data which may cause high position inaccuracy. The ground stations usually detect the malfunctioning satellite and inform the receiver to disregard the satellite.
- Satellite Clock errors: Although the satellites have atomic clocks which are extremely accurate, they are not perfect and may lead to position error of upto one meter.
- Deliberate signal degradation: In selected parts of the world the GPS operating body deliberately degrade the signal signature causing high inaccuracy.
- Satellite distribution: Ideally the visible satellites should be evenly distributed in space but due to geometric constraints it is not possible. So the position accuracy depends on the spacing between the cluster of visible satellites.
- Signal Obstruction: Mostly caused by the obstruction due to trees or other earth features which block the satellite signals and cause position inaccuracy.
- Atmospheric Conditions: Before signal reaches the receiver/satellite it has to travel through the earth’s atmosphere. The atmosphere contains charged particles(mainly ionosphere) which slows down the signal. The time delay is proportional to the number of charged particle encountered by the signal. Also Air molecules and water vapour may reduce the signal speed. This error is usually reduced by considering an atmospheric model and applying correction accordingly. The error correction will be dependent on how closely the predicted model resembles the real atmospheric conditions.
Applications of GPS:
GPS the Global positioning System was initially developed for Defense purposes but soon found applications in general day to day life. GPS along with GIS find application in road navigation. Today we can see many mobile devices are equipped with GPS navigation system which can give the user turn by turn directions towards his/her location. GPS timing precision is very accurate and finds applications in aviation industry for safe landing of airplanes and air traffic control.