Print Email Download Reference This Send to Kindle Reddit This
submit to reddit

A Study Of Global Positioning System Information Technology Essay

The Global Positioning System was initially developed by the U.S. Department of Defense. It provides precise estimates of position, velocity, and time to users worldwide. The GPS Satellite was launched in 1978, and the system was declared operational in 1995. Today, GPS serves over twenty million users with a breath-taking variety of applications built on these basic capabilities. Currently, GPS is undergoing substantive changes that will enhance both military and civil capabilities [1].GPS is being modernized. In fact, modernization is a fabric of change with the following elements.

A. Integrity Machines provide error bounds to safety-critical users in real time. The Wide Area Augmentation System or WAAS began operation on July 10, 2003.

B. Navigation Signals of Opportunity refer to all the other [12] signals (inertial and radio).

C. Frequency Diversity the new signals L1 and L2 will be accurate to positioning, or finding the user’s location, with gps requires some understanding of the gps signal structure and how the measurements can be made [1].

II. GPS SIGNAL STRUCTURE:

Gps satellite transmits a microwave radio signal. It is composed of two carrier frequencies modulated by two digital codes and a navigation message. The two carrier frequencies are generated at 1,575.42 MHz L1 carrier and 1,227.60 MHz L2 carrier. The corresponding carrier wavelengths are approximately 19 cm and 24.4 cm, respectively, which result from the relation between the carrier frequency and the speed of light in space. The availability of the two carrier frequencies, allows correcting a major gps error, known as the ionosperic delay. All of the gps satellites transmit the same L1 and L2 carrier frequencies. The code modulations are different for each satellite, which significantly minimizes the signal interference.

GPS-Satellite.png

Figure1: Gps signal structure

The two gps codes are coarse acquisition code or C/A code and precision code or p-code. Each code consists of a stream of binary digits, zeros and ones known as bits and chips. The codes are commonly known as PRN codes which are similar to random signals. The C/A-code is modulated onto the L1 carrier only, while the P-code is modulated onto both L1 and the L2 carriers. This modulation is biphase modulation, when the carrier phase is shifted by 180 degree and the code value changes from zero to one or from one to zero. The C/A code is a stream of 1,023 binary digits (1,023 zeros and ones) that repeats itself every millisecond. Chipping rate of the C/A code is 1,023 mbps. Each satellite is assigned a unique C/A code, which enables the gps receivers to identify which satellite is transmitting a particular code. The C/A -code range measurement is relatively less precise compared with that of the P-code, which is less complex and available to all users [2].

III. CIVIL SIGNALS:

Each satellite sends three rather similar signals. Any of these can be described as the product of four terms

Amplitude 2P

Navigation data, D (t)

Spread spectrum code, x (t) or y (t)

Radio frequency (RF) carrier cos (2+) or sin (2+).

The first signal, with amplitude 2PC, is the basis for the vast majority of civil applications. It refers to as the civil signal, even though the military also uses this signal.

The codes, {x (k) (t) k =1 K} are unique to each satellite. These codes are C/A codes and they spread the spectrum of the transmitted signal. GPS is a direct sequence, spread-spectrum system. The C/A codes are sequences of chips each having a polarity of either +1 or –1, and these chips modulate the transmitted signal. The chipping rate for the C/A codes is 1.023×106 chips/second, and so the duration of a single chip is approximately 1 microsecond or 300 meters.

The C/A chipping rate also means that the null-to-null bandwidth is 2.046 MHz this bandwidth is much greater than the bandwidth normally required to send the 50 bps data rate required for the navigation message, D (t). The codes enable precise ranging measurements and allow the receiver to distinguish the signals from the different satellites.

The second and third signals have amplitudes 2PY 1 and 2PY 2. The military is the main beneficiary of these latter signals and so we refer to them as the military signals. One of the military signals is broadcast in phase quadrature with the civil signal at fL1=1575.42 MHz [2].

IV. ADDITION OF NEW CIVIL SIGNALS:

More significant benefits will occur when a second civil frequency is made available to civil users as significant numbers of GPS Block IIR-M (supporting L2C) and IIF [12] (supporting L5C) satellites are fielded.

A. New L2 Civil Signal: A major milestone for civil users of GPS is to broadcast of civil signals on a second frequency. When combined with the existing L1 signal, this will allow users to track both signals to estimate and remove the impact of ionosphere delay on their range measurements. The Block IIR-M satellites are equipped to broadcast coded civil signals on the L2 frequency now used by the military and used (but in codeless or semi-codeless form) by civil survey receivers and SBAS reference receivers. The IIR-M satellites may initially broadcast the existing C/A code definition on L2.The goal is to eventually switch to a new L2C signal. The L2C signal has the same chipping rate as L1 C/A (1.023 MCPc). But this is split between a moderate-length 10,230- chip “CM” code with data and much longer 767,250-chip “CL” code without data [12] modulation.

The primary advantage of the “data less” code is that integration times for resolving the code can be much longer than the 20 ms limit that applies to the 50 bps L1 C/A code data (this integration time must be shorter than the interval between bit transitions triggered by data bit flips). This advantage of the navigation data must be obtained in some other manner: either from L1 C/A or from an external source (e.g. over the Internet or another data network). The navigation data broadcast on the CM code will either be the existing 50 bps framework or a “CNAV” data structure at 25 bps like that adopted for L5. Another advantage to the longer codes is much better rejection of code cross-correlation and narrowband interference (a 24-dB improvement Compared to C/A code).

B. New L5 Civil Signal: In 2006, the Block IIF generation of satellites are launched in addition to Block IIR satellites. In addition to broadcasting L2 civil signals, these satellites will be capable of broadcasting an all-new civil-only signal on the L5 frequency (1176.45 MHz). This frequency is in an ARNS band, it will be fully usable by civil aviation users, and the resulting dual-frequency capability will provide a major improvement to their performance and availability.

Figure2:Frequency graph

The new L5 civil signal will consist of one PRN code with data encoded on it and another one without data because their implementation is different. The data and data less bit trains will be modulated by two similar PRN codes in phase quadrature. Thus, the code with data modulation will be the “in-phase component (I5-code), and the code without data modulation will be the quadrature component (Q5-code).

Also, fewer constraints exist on L5C because no military signals will be transmitted on L5. Thus, the L5C signal will have a chipping rate of 10.23 MCPc (equivalent to military P/Y code) while retaining the 1 msec period of C/A code, and it will occupy 24 MHz of bandwidth around L5 without overlapping military signals[3].

C. Modernization of Existing L1 Civil Signal: Due to the availability of the L2 and L5 limitation on the existing L1C/A code will be less tolerable to high performance users. In order to use the previous L1 civil signal that will provide improvements similar to those of L2C and L5C for users with modernized

V. THE NEXT GENERATIONS:

U.S. Department of Defence have begun planning the next generation of satellite navigation technology, known [7] 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 [7] 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 [7] [11] (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 [11] [7] is smaller than a stated size [3]."

VI. GPS RECEIVERS:

In 1980, only one commercial gps receiver was available on the market. The price was several hundred thousand U.S dollars. Now a day’s more than 500 different gps receiver are available in the market. GPS (1).jpg

Figure3: GPS receiver

Gps receiver may be divided into four types, according to their receiving capabilities.

Single frequency code receiver

Single frequency carrier smoothed code receiver

Single frequency code and carrier receiver

Dual frequency receiver

Single frequency receivers access the L1 frequency only, while dual frequency receivers access both L1 and L2 frequencies. Single frequency carrier smoothed code receiver, also measures the C/A code only. Single frequency code and carrier receivers output the raw C/A code pseudo ranges, the L1 carrier-phase measurements, and the navigation message. GPS receiver can also be categorized according to their number of tracking channels, which varies from 1 to 12 channels. A good gps receiver would be multichannel, with each channel dedicated to continuously tracking a particular satellite. Presently, most gps receivers have 9 to 12 independent channels [2] [6].

VII. OVERVIEW OF GPS:

GPS satellite are arranged so that four satellite are placed in each of six orbital planes .with this constellation geometry, four to ten gps satellites will be visible anywhere in the world, if an elevation angle of 10 degree is considered. The gps system was officially declared to have achieved full operational capability on July 17, 1995, ensuring the availability of at least 24 operational, nonexperimental, gps satellites.

Gps segments: Gps technology requires the following three segments:

Space segment

Control segment

User segment

A. Space segment: At least 24 gps satellite orbits the earth twice a day in a specific pattern. They travel at approximately 7,000 miles per hour about 12,000 miles above the earth’s surface. Each gps satellite constantly sends coded radio signals to the earth and contains the following information.

The particular satellite that is sending the information

Where that satellite should be at given time

Whether or not the satellite is working properly

The data and time that the satellite sent the signal

B. Control segment: The control segment is responsible for constantly monitoring satellite health, signal integrity and orbital configuration from the ground. The control segment includes the following sections.

Master control station

Monitor stations

Ground antennas

C. User Segment: The gps user segment consists of gps receiver. The receiver collects and processes signals from the gps satellites, and display your location, speed, time, and the gps receiver does not transmit any information back to the satellites [4].

FAQ_gps_comp.gifFigure4: GPS basic components

VIII. POSITIONING TECHNIQUES:

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 [5].

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.

0.jpg

Figure5: 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 [5].

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 [5].

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[5].

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 [5].

XI. 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 [10] 2010 that perform applications in the following fields [8]:

Ships

Aircrafts

Military systems

Farm vehicles

Automobiles

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 [8] [9] 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. [8] This system will support navigation information with higher accuracy data [7] [8].

X. 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 [1] [3].

Print Email Download Reference This Send to Kindle Reddit This

Share This Essay

To share this essay on Reddit, Facebook, Twitter, or Google+ just click on the buttons below:

Request Removal

If you are the original writer of this essay and no longer wish to have the essay published on the UK Essays website then please click on the link below to request removal:

Request the removal of this essay.


More from UK Essays

Doing your resits? We can help!