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Introduction and Background
GPS transmission frequencies are divided into two major categories namely: the military and civilian frequencies. This paper talks about the navigation techniques, allowable civilian frequency range and its control techniques which are studied in detail to understand the basic principle of GPS system. To conclude the paper, a further insight into implementation and future possible applications and positioning techniques will also be discussed in this paper with focus on GPS receiver technology capable of tracking civilian frequency only.
GPS provides reliable positioning, navigation and timing services to all users in all weather conditions anywhere on the earth with unobstructed signals. GPS satellites broadcast signals from space and identified by GPS receivers. GPS receiver provides information to the user to determine the position, velocity and time with a high degree of accuracy and reliability. Now a day's GPS receiver handset are readily available in the market which is commonly used by all the users in various civilian applications such as marine, aircraft, on-road navigation and military applications such as mapping.
The aim of the paper is to investigate the Global Positioning System, application and also the future improvement to this system.
To investigate the concept of Global Positioning System
To investigate the current Global Positioning System signal and application
To investigate the civilian frequency and further development
The Global Positioning System is a radio navigation system which helps to find the location of the user. It finds its use in navigation and surveys provided by the Defence department of the United States .
The constellation of 24 satellites is placed in six orbital planes, with each plane containing 4 satellites. The orbital planes are equally spaced by 60 degrees, and the planes are inclined by 55 degree. The semi major axis of the orbit is about 26,578 km and the orbit is almost circular. The satellites alter their position in such a way that the solar panels in the satellites face the sun and the antennas face the earth. Each satellite posses 4 atomic clocks with each clock weighing 1000 kg .
The GPS makes use a group of 24 satellites in order to determine the position of the user on the earth. The GPS satellites send signals to the earth which are received by the GPS receivers on the earth, and the position of the user is calculated using the signals. However, the accuracy in determining the position of the user differs depending on the location of the user. For example, if the user is in a desert then the position of the user can be determined with an accuracy of +/- 15m, where as it is 5m if the person is in water and it is less than 1cm if the user is in land .
VARIOUS COMPONENTS OF THE GPS
The Global Positioning System (GPS) find its application in PNT (Position, Navigation, & Timing) services, which is owned by the United States.
There are three segments in the Global Positioning system. They are,
The space segment
Control segment, and
The User segment.
The three segments in the GPS are shown in the figure 2.2. The space segment provides information about the current time and the position of the GPS satellite, by sending one way signals using twenty four satellites . The satellites are maintained in their respective orbits, and the satellite clocks are adjusted using the control stations and a worldwide monitor by the control segment. It also helps in tracking the satellites and recording the navigational data thus maintaining the status and the health of the satellite constellation .
Figure 2.2 Different Segments in GPS
Source: Jones, P. W. (2010) 
DETERMINING THE POSITION AND DISTANCE
The signals are received from the GPS satellites by the receiver contained in the user segment and are transmitted in order to calculate the three dimensional position and time of the user .
Signals from at least three different satellites are required by the GPS receiver in order to determine the 2D position of the user, where as the 3D position is determined if the signals are received from four or more number of satellites. Information such as speed, distance to the destination etc can be calculated once the position of the user if determined .
The GPS satellites consist of many atomic clocks. A random sequence of signal named as pseudo-random code is sent to the receiver and the sequence is repeated continuously. The GPS receiver also repeats the sequence internally so that the Satellites and the receivers are in synch. The GPS receiver compares the incoming signal from the satellites and compares it with its own integral, and makes use of the amount of lagging in the satellite signal to determine the travel time .
DESCRIPTION OF THE SATELLITES
The US Department of Defence which operates all GPS satellites travel at a height of 11,500 miles from the earth at a speed of 2000 mph. The GPS satellite signals are transmitted using 50 watt power, and these satellites travel at a speed of around 7,000 miles an hour. The GPS satellites are powered by solar energy, and each one has its own backup batteries. A GPS satellite weighs roughly 2,000 pounds and is about 17 feet across with the solar panels extended .
It is quite difficult to receive signals in place of bad signals where the signal interruptions are high like forest, rooms inside tall buildings etc. Now a day's highly sensitive GPS receivers like SiRFStarIII, MTK etc, are available, which can receive signals even in places which has got more signal interruptions .
The type of receiver is important when it comes in determining the accuracy of the user position. Some receivers have +/- 10m accuracy and some may have different limits. The differential GPS (DGPS) have a high accuracy. DGPS requires an additional receiver to be fixed in a nearby location. The position is recorded by the roving units, and the correct position is determined by using the observations made by the stationary receiver with an accuracy of greater than a meter .
Level 1(L1) and level 2(L2) are the two different types of signals transmitted by GPS satellite. These two L1 and L2 radio signals are transmitted at low power. The frequency of the L1 signal is 1575.42 MHz in the UHF band. It's called the civilian frequency. This civilian signal will pass through clouds, glass and plastic, but finds difficult to pass through heavy solids like mountains and buildings .
There are three different kinds of data in a GPS signal, they are
Pseudo random code
Pseudo random code:
The pseudo random code which is usually called as I.D code is useful in finding out the right satellite which transmits the data.
The satellites in the sky transmit almanac data to all other satellites which is used by the GPS receiver, and the data contains information about which satellite has to be tracked in the local sky. The GPS receiver makes use of the data to trace the required satellite in the sky and ignores the rest.
The GPS receiver uses this data to determine the position of the satellite at any time for the entire day. Each satellite broadcasts an ephemeris data with orbital information about that particular satellite. The validity of this data is very shorter which is up to just four hours and the data is very specific. Some manufacturers use different validity for this data .
Several different techniques have been developed for using the GPS to pinpoint a user's position. Some of the popular techniques are autonomous positioning, differential positioning and server-assisted positioning.
A. Autonomous GPS positioning.
This positioning, also known as single-point positioning, is the popular positioning technique used today. Autonomous positioning is the practice of using a single GPS receiver to acquire and track all visible gps satellites, and calculate a PVT solution. Depending upon the capabilities of the system being used and the number of satellites in view, a user's latitude, longitude, altitude and velocity may be determined .
B. Differential GPS Positioning.
DGPS effectively eliminated the intentional errors of S/A, the errors introduced as the satellite broadcasts passes through the ionosphere and troposphere. DGPS uses two receivers to calculate PVT, one placed at a fixed point (known as the master site), and second can be located anywhere in the vicinity of the master site. The mater site tracks as many visible satellites as possible, and processes that data to derive the difference between the positions calculated based on the SV broadcasts and the known position of the master site.
Fig. : Positioning
US coast guard, which operates a series of DGPS master sites that broadcast DGPS corrections across approximately 70 percent of the continental US, including all coastal areas .
C. Inverse Differential GPS Positioning.
IDGPS is a variant of DGPS in which a central location collects the standard GPS positioning information from one or more mobile units, and then refines that positioning data locally using DGPS techniques .
D. Server- Assisted GPS Positioning.
Server-assisted GPS is a positioning technique that can be used to achieve highly accurate positioning in obstructed environments. This technique requires a special infrastructure that includes a location server, a reference receiver in the mobile unit, and a two-way communication link between the two, and is best suited for applications where location information needs to be available in the mobile unit for calculating position is minimal.
E. Enhanced Client-assisted GPS Positioning. The enhanced client-assisted GPS positioning techniques is a hybrid between autonomous GPS and server-assisted GPS. This type of solution is similar to the server-assisted GPS. This technique essentially requires processing power and capabilities as an autonomous GPS solution, in addition to a communication link between the mobile unit and the location server .
THE NEXT GENERATIONS
U.S. Department of Defence have begun planning the next generation of satellite navigation technology, known  as GPS III (the current system is the second generation). GPS III satellites will start to be launched in 2010, in what will be a multibillion-dollar market eyed by Boeing or Lockheed Martin. Per Enge, director of Stanford University's gps laboratory thinks that the evolution of the technology will be  driven by three factors.
The first is frequency diversity, which in fact is already being addressed as aging GPS II satellites are replaced periodically. When completed, the constellation of modernized orbiters will furnish civilian users with three new positioning signals.
The second big trend concerns overcoming radio-frequency interference   (RFI). "GPS broadcasts are extremely low power equivalent to that of five light bulbs," Enge explains. "With received power levels of 10-16 watt, the signal can be easily accessible nearby radio emitters. The third one revolves around the installation of "integrity machines -- systems that guarantee that the positioning error   is smaller than a stated size ."
FUTURE OF GLOBAL POSITIONING SYSTEM
The future of global positioning system is bright as predictions range from its' increased usage to expansion into new areas of application. It is estimated that there will be 50 million users of the global positioning system by  2010 that perform applications in the following fields :
A. Technology. Additional advances in GPS technology increased positional accuracy and more reliable calculations. The addition of civilian codes and civilian frequencies will be developed to solely meet   the needs of civilian users with no military application.
B. GPS Satellite System Interoperability. With the advent of the European GALILEO system, GPS developers and users have increasingly pondered the benefits of interoperating the NAVSTAR and GALILEO systems. The benefits are
More available signals
Additional signal power and spectrum diversity lessen the impact of expected signal noise interference
Improved signal redundancy
C. Global Navigation Satellite System. Many experts expect a GNSS (Global Navigation Satellite System) to be developed that capitalizes on the compatibility of technology from the NAVSTAR GPS system and the GALILEO GPS system.  This system will support navigation information with higher accuracy data  .
SUMMARY AND CONCLUSIONS
GPS modernization includes frequency diversity for the satellite signals. The new signals will use longer codes, and in the case of L5, faster chipping rates. Both of the new frequencies will also carry data-free signal components. These measures will significantly improve the signal acquisition and tracking performance of the basic GPS engine. The multiplicity of signals will also allow a new level of accuracy and robustness by offering new techniques to remove the ionosperic delay from the measurements. Thus the largest current error source will be eliminated. So, the future of civil user navigation using GNSS looks very bright indeed  .