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The Global Positioning System is a space-based radio navigation system managed for the Government of the United States by the U.S. Department of Defense (DoD), the system operator. The GPS provides highly accurate position, velocity and time information to properly equipped users in the air, at sea, on the ground and in space. Although GPS was originally developed for military users, it also has significant potential for civilian users, i.e., flight navigation and landing, marine navigation and surveying, vehicle navigation and
tracking, outdoor leisure (i.e., camping and hiking), and so on.
Figure Major Segments of GPS
GPS system consists of three major segments which are known as space, control and user segments.
The space segment includes 24 GPS satellites, each with an orbit altitude of 20,200 Km and orbit inclination of 55 degrees with respect to the equator .The 24 satellites are positioned in six orbital planes. The spacing of the satellites in their orbital planes is arranged such that a minimum of four satellites are in view over each location on the earth's surface.
Figure GPS Satellite constellation.[Book]
The period of a GPS satellite is 11 hours 58 minutes, and each satellite thus will appear in the same place over the earth 4 minutes earlier than the previous day .Each GPS satellite broadcasts its orbital information and time reference to allow the user to determine the position of the satellite at any arbitrary instant. [*1]
The user segment consists of receivers, which could be a hand held device or device mounted in a vehical. [*6]
Table Functions of each segment in GPS.
FUNDAMENTALS OF GPS
Indian Institute of Remote Sensing, Dehra Dun
Each GPS satellite is transmitting its own position (in three dimensions) and time. In order for the receiver to determine its own position it will need data from four different satellites. All GPS receivers are passive, i.e. they do not transmit any signal and depend solely on the received signal for the necessary processing. The receivers make use of one way Time Of Arrival (TOA), which means that they measure the transmission time from satellites to the receivers. The condition of the satellites are being monitored by a ground control network which has its five control stations strategically placed around the earth, (on Hawaii, Ascension Island, Diego Garcia, Kwajalein and Colorado Springs). The master control station at Schriever Air Force Base, near Colorado Springs, Colorado, runs the system.
Position and time data are being updated from the network to the satellites several time every orbit. Only two L band frequencies, L1 (1575.42 MHz) and L2 (1227.6 MHz), are used for broadcast of navigation data and ranging codes from the satellites, by the use a code division multiple access (CDMA) and each satellite uses a different ranging code. The navigation data provides the receiver the satellite location at the time of transmit. The GPS signal is produced with BPSK modulation and Gold code spread spectrum modulation to the navigation data whose code velocity is 50 bit/s, and the whole satellite constellation whose satellites adopt different Gold sequences to distinguish has the same frequency.
Figure Basic GPS Signal Simulation Modal.
The function of high dynamic GPS receiver is gaining the message the timing, three-dimensional position and velocity before calculating out the pseudo-range of each visual satellite and demodulating navigation data upon the foundation of two-dimension acquisition, carrier tracking, code tracking, digital synchronization and frame synchronization to GPS signal under the condition of high dynamic and low Signal to Noise Ratio (SNR) while receiving the GPS signal from satellites.[*5]
The ranging code may be used by the receiver to determine the propagation delay of the satellite signal. Only the L1 carrier frequency is used by the standard positioning service (SPS), which is available to all users and it provides the users with navigation accuracies, in the horizontal plane, of 100 m and in the vertical plane with accuracies of 156 m. The precise positioning service (PPS) is only available to U.S. government and military users. The PPS is using both the L band frequencies and is providing accuracies, in the horizontal plant of 22 m and in the vertical plane of 27.7 m. To prevent public access to the GPS PPS two cryptographic features are used, Antispoofing (AS) and Selective Availability (SA). AS is utilized to prevent deception jamming, which is a technique for the enemy to replicate satellites PRN codes, navigation data and carrier frequencies to confuse one's own receivers SA is utilized to diminish accuracy available to SPS users. SA decreases TOA accuracy by distorting the satellite's time. [*4]
Binary phase shift keying (BPSK) is a simple digital signaling scheme in which an RF carrier is either transmitted "as is" or with a 180° phase shift over successive intervals in time depending on whether a digital 0 or 1 is being conveyed .A BPSK signal, as illustrated in Figure ( ), can be thought of as the product of two time waveforms-the unmodulated RF carrier and a data waveform that takes on a value of either +1 or âˆ’1 for each successive interval of Tb = 1/ Rb seconds, where Rb is the data rate in bits per second. The data waveform amplitude for the kth interval of Tb seconds can be generated from the kth data bit to be transmitted using either the mapping [0, 1]â†’[âˆ’1, +1] or [0, 1]â†’[+1, âˆ’1].
Direct sequence spread spectrum (DSSS)
Direct sequence spread spectrum (DSSS) is an extension of BPSK or other phase shift keyed modulation used by GPS and other satellite navigation systems .As shown in Figure ( ), DSSS signaling adds a third component, referred to as a spreading or PRN waveform, which is similar to the data waveform but at a much higher symbol rate. This PRN waveform is completely known, at least to the intended receivers. The PRN waveform is often periodic, and the finite sequence of bits used to generate the PRN waveform over one period is referred to as a PRN sequence or PRN code. The minimum interval of time between transitions in the PRN waveform is commonly referred to as the chip period, Tc; the portion of the PRN waveform over one chip period is known as a chip or spreading symbol; and the reciprocal of the chip period is known as the chipping rate, Rc. The independent time parameter for the PRN waveform is often expressed in units of chips and referred to as codephase.
The signal just described is called spread spectrum, because of the wider bandwidth occupied by the signal after modulation by the high-rate PRN waveform. In general, the bandwidth is proportional to the chipping rate.
There are three primary reasons why DSSS waveforms are employed for satellite navigation.
Those are as follows :-
The frequent phase inversions in the signal introduced by the PRN waveform enable precise ranging by the receiver.
The use of different PRN sequences from a well-designed set enables multiple satellites to transmit signals simultaneously and at the same frequency. A receiver can distinguish among these signals, based on their different codes. For this reason, the transmission of multiple DSSS signals having different spreading sequences on a common carrier frequency is referred to as code division multiple access (CDMA).
DSSS provides significant rejection of narrowband interference. The chip waveform in a DSSS signal does not need to be rectangular. So that, different shapes can be used for different chips.
In navigation applications, it is frequently required to broadcast multiple signals from a satellite constellation, from a single satellite, and even upon a single carrier frequency. There are a number of techniques to facilitate this sharing of a common transmission channel without the broadcast signals interfering with each other. The use of different carrier frequencies to transmit multiple signals is referred to as frequency division multiple access (FDMA) or frequency division multiplexing (FDM).
Sharing a transmitter over time among two or more signals is referred to as time division multiple access (TDMA) or time division multiplexing (TDM). CDMA, or the use of different spreading codes to allow the sharing of a common carrier frequency, as it was described in the previous section.
GPS satellites broadcast the PRN codes modulated with the other GPS information such as orbital (ephemeris) and clock parameters. By combining the PRN code with the 50 Hz data, as shown in the figure ( ), the result signal is spread out over a broad part of the spectrum. (Spread Spectrum). The result is so that the signal power is very low, even beneath the noise floor. In other words: there will be no difference between noise and the signal. Therefore it is very hard to distinguish the signal from noise. These PRN codes characterized with minimal Cross Correlation to other PRN codes, noise and interferers, high autocorrelation value only at a phase shift of zero which allows all satellites signals to be transmited at the same frequency. Three types of Golden codes are being used by GPS: C/A, P (Y) . C/A code is used for civilian positioning purposes and Precision or Y code is used for high security military purposes. In this project the attention was mainly focused on the C/A code which is used for civilian positioning and navigation purposes.[*3]
Figure C/A code and navigation message and combination of them with XOR (module 2 adding)
Even though these Pseudo Random Noise Codes have random noise characteristics those are precisely defined.
In the other word PRN code is a sequence of zeros and ones (a long series of bits 0's and 1's), each zero or one referred to as a "chip" because they doesn't seem to be a regular pattern in the bits and they carry no data. But in real it has regular arrangement and the arrangement is selected from a set of Golden Codes. The codes-patterns repeat after the 1023rd bit. For the 1023 bit pattern 10 shifting registers and some digital adders are needed. In general with "n'' shifting registers a series of 2n -1 bits can be generated. For n = 10 this will become 1024 (= 210) - 1 = 1023 bits. The codes are generated with a speed of 1.023 MHz (or 1023000 bits per second).
Gold codes use 2 generator polynomials for generating PRN code.
Two "10-bit generator polynomials"(G1 and G2):
G1 = 1 + x^3 + x^10
G2 = 1 + x^2 + x^3 + x^6 + x^8 + x^9 + x^10
Table GPS code generator polynomials and initial states.
Figure ( ) illustrates the two 10-bits for each of two polynomials G1 and G2. At first step 10 initial bits for each polynomial containing the coefficient of each nominal have been considered then the first step bits shift to the right and substitute the left bit with output of XOR of bits (in G1: bits 3 and 10 and in G2: bits 2, 3, 6, 8, 10) C/A cod generates by XOR the first bit of G1 with the XOR result from two bits of G2 that called phase tap. In the following figure tap 3 & 8 has been used for making PRN31.
Selecting two bits (Phase Taps) from G2 polynomial has individually done for each SV according to the following table. In this way different C/A codes with unique composition will be prepared for each of the GPS satellites.
Figure C/ A generate by two 10 bits generator polynomials
In the receiver, the PRN Code and GPS data have to be separated. This is done by again mixing the received signal from GPS satellite with a locally generated PRN Code by GPS receiver. This code must be the same as PRN Code which has been generated in the satellite. As these two codes are not synchronised with each other and the equal parts of the code should be mixed with each other, so the code generated in the receiver must be shifted in time until the two codes be exactly synchronised with each other. In this case the receiver 'locks' (full correlates) and these two codes can block each other out. This method is called dispreading. Every satellite has its own unique PRN-code so that the GPS receiver can distinguish the signals from various satellites. GPS receivers are able to generate 32 PRN-codes.
At a time the GPS receiver is started up it couldn't recognize which GPS signal is from which satellite. Therefore receiver makes itself to be lock with the 32 known PRN codes one by one. If one code locks then the information of one satellite can be decoded. This information contains data which is used as positioning information.
The main reason for using PRN codes in the GPS system is that the PRN code enlarges the unambiguous measurement range. After 1023 bits the code is repeated. In this case the GPS receiver is aware it is 'looking' at the right code and not at its predecessor or successor.
Looking at the wrong code gives a navigation error of 300 km (corresponding to the code length of 1 millisecond).
Code Phase Assignments and Initial Code Sequences for C/A Code
As described in Section ( above), both the C/A code and P(Y) code signals are modulated with 50-bps data. This data provides the user with the information necessary to compute the precise locations of each visible satellite and time of transmission for each navigation signal. The data also includes a significant set of auxiliary information that may be used, for example, to assist the equipment in acquiring new satellites, to translate from GPS system time to UTC (see Section 2.6), and to correct for a number of errors that affect the range measurements.
The GPS navigation message is transmitted in five 300-bit sub frames, as shown in Figure ( ). Each sub frame is itself composed of ten 30-bit words.
Although there are provisions for a loss of ground contact, normally the control segment uploads critical navigation data elements once or twice per day per satellite. In this nominal mode of operation, the same critical navigation data elements (e.g., satellite ephemeris and clock correction data) are broadcast repeatedly over 2-hour time spans (except if an upload occurs during this interval). On 2-hour boundaries, each satellite switches to broadcasting a different set of these critical elements, which are stored in tables in the satellite's RAM. The control segment generates these message elements based upon its current estimates of each satellite's position and clock error and prediction algorithms on how these parameters will change over time.
User equipment must have precise knowledge of the satellite position and signal transmission time. This information is encapsulated in data messages modulated onto the satellite signal transmissions. A low data rate of 50 bps is used to make signal reception more robust.
Interference Mitigation Approaches for the Global Positioning System
Jay R. Sklar
LINCOLN LABORATORY JOURNAL VOLUME 14, NUMBER 2, 2003
GPS Signal Generation
GPS Channel Impairments
The propagation of the GPS signal through ionosphere undergoes three major effects: attenuation, scintillation and delay. The scintillation and attenuation effects are associated with the time-variant character of the ionospheric electronic content (TEC), leading to fluctuations in the GPS signal amplitude. They depend on the user position, hour, season and on the solar cycle. The ionosphere also introduces a delay in the received GPS signal, which depends on the relative
[International Telecommunications Symposium - ITS2002, Natal, Brazil
A GPS Simulator for Analysis of Channel
Impairments in Practical Scenarios
Cynthia Junqueira1,2, Danilo Zanatta Filho1
João Batista Destro Filho1 , Murilo B. Loiola1 and João Marcos T. Romano1
1State University of Campinas (Unicamp), Campinas, Brazil
2AerospaceTechnical Center - Aeronautic and Space Institute, São José dos Campos, Brazil]
Irregularities in the ionospheric layer of the Earth's atmosphere can at times lead to rapid fading in received signal power levels [26-28]. This phenomenon, referred to as ionospheric scintillation, can lead to a receiver being unable to track one or more visible satellites for short periods of time. This section describes the causes of ionospheric scintillation, characterizes the fading associated with scintillation, and details the effects of scintillation upon the performance of a receiver. The ionosphere is a region of the Earth's atmosphere from roughly 50 km up to several Earth radii where incident solar radiation separates a small fraction of the normally neutral constituents into positively charged ions and free electrons. The maximum density of free electrons occurs at an altitude of around 350 km above the surface of the Earth in the daytime. Most of the time, the principal effect of the presence of free electrons in the ionosphere is to impart a delay on the signals (see Section 22.214.171.124). However, irregularities in the electron density occasionally arise that cause constructive and destructive interference among each signal. Such irregularities are most common and severe after sunset in the equatorial region (within €«/âˆ’20° from the geomagnetic equator). High-latitude regions also experience scintillation which is generally less severe than in the equatorial region, but may persist for long periods of time. Scintillation is also more common and severe during the peak of the 11-year solar cycle. [Book]
The troposphere also imposes attenuation and delay on the GPS signal. The attenuation may be neglected in view of the high amplitude of fading in the ground.
Doppler shift is caused by the relative speed difference between a transmitter and a receiver. The change in the frequency depends on several parameters, such as the distance between the transmitter and the receiver, the speed of the electromagnetic waves, and their relative velocity.
In a GPS context, it is also necessary to consider that the Doppler shift depends on several other parameters, such as the altitude of the satellite, rotational speed of the Earth, and the elevation angle (from which the station is seen by the satellite).
Three factors contribute to carrier frequency offset when a GPS receiver receives a satellite signal. As mentioned in the previous chapter, these factors are satellite motion, user motion and oscillator frequency error. Satellite motion causes a Doppler effect when transmitting the signal. Hence, the received carrier frequency will change because of the Doppler effect. The user motion is similar to the satellite's motion in that it also changes the line-of-sight (LOS) velocity and will contribute to the frequency offset. Figure 4-1 illustrates the Doppler shift contributed by satellite and user motion.
The last factor is oscillator frequency error. The oscillator frequency error appears as a Doppler effect when down-converting the carrier frequency in processing the GPS signal. This frequency offset range depends on the accuracy of the oscillator. This chapter will discuss the Doppler shift effect of each factor and derive the overall Doppler shift PDF.
A major goal of the thesis work is to find the frequency offset PDP of each factor and derive the overall frequency offset PDF from the individual PDF's. This will serve as the background for the design of a fast GPS signal search algorithm. The first PDP model is the satellite motion. In the cold start situation, there is no a priori time or position information. Hence, it is not known what time it is or the position of the user relative to the visible satellites. The user can be any place on the surface of the earth at any time. How the sample sites and sample period are chosen is very important in building the frequency offset PDP model.
Fast time-domain-based GPS acquisition
Ohio University, Electrical Engineering (Engineering), 1996. [*8]
Because of the Doppler effect due to satellite and user motion, and the user's oscillator drift, there will be a frequency offset between the received signal frequency and the nominal signal frequency. The process of signal frequency acquisition is simply the matching of the receiver frequency setting to the actual signal frequency. The search operation essentially scans all frequency cells in the uncertainty region, measures the output power in the post correlation bandwidth, and integrates the power and compares with a fixed or variable threshold. When the power of a certain cell passes the threshold, the incoming signal frequency has been determined within 500 Hz, and the GPS receiver tracking loops can "pull-in" the exact frequency. [*3]
Other Satellite Based Navigation Systems.
There are three main global navigation satellite systems. Those are the Navigation Satellite Timing and Ranging system (NAVSTAR), commonly referred to as the Global Positioning System (GPS) and owned by the United States of America, and GLONASS (Global'naya Navigatsivannaya Sputnikovaya Sistema) of the Russian Federation. A third system called GALILEO is under development by the European Community (EC) countries. The characteristics of those syatems are compared on the table below(Table 1).
Global navigation satellite systems (GNSS), such as the Global Positioning System (GPS), its Russian counter- part (GLONAS), and the upcoming European GALILEO system, are the most widely used positioning technology. GNSS transmit signals bearing reference information from a constellation of satellites; computing platforms nodes), equipped with the appropriate receiver, can decode them and determine their own location.
GNSS-based Positioning: Attacks and Countermeasures
Panos Papadimitratos and Aleksandar Jovanovic
Email: [email protected]
Modern aircraft navigation/landing systems typically consist of a combination of inertial navigation or reference systems and radio navigation systems, utilising navigation aids such as the Global Positioning System (GPS), VHF Omni-directional Radio-range (VOR), Distance Measuring Equipment (DME), Tactical Air Navigation (TACAN), Instrument Landing System (ILS), and Microwave Landing System (MLS).
There are also other RF based positioning systems. LOng Range Navigation (LORAN), Omega, Standard inertial navigation systems, TACtical Air Navigation (Tacan) and Transit. But GPS reigns above all those with 24 hour all weather coverage around the globe.
Today, aircraft are equipped with a variety of navigation systems depending on the application (Table 2-1).
For long range navigation aircraft are normally equipped with INS, and/or Omega, and/or LORAN C, where LORAN C is only available in certain areas like the continental USA. More and more aircraft already use GPS for the same purpose as well as for medium range navigation. This navigational task is traditionally performed with VOR and DME or, for military aircraft, with TACAN. Instrument (ILS) and microwave landing systems (MLS) provide the guidance signals for landing. The highest horizontal accuracy is required for ILS and MLS. These systems also yield a very accurate vertical position reference. For less demanding vertical positioning, barometric and radar altimeters can be used.
The NATO Research & Technology Organisation
FLIGHT TEST INSTRUMENTATION
Number of satellites in the constellation
Carrier frequency/frequency range
L1-1575.420 MHz, L2-1227.600 MHz
E5a-1176.45 MHz, E5b-1207.14MHz, E6-1278.75,L1-1575.42MHz
BPSK and BOC (Binary off-set carrier)
Multiple access technique
Table 1. Comparison of characteristics of each Satellite navigation system.
Tampere University of Technology. Publication 906
Weak Signal Acquisition in Satellite Positioning.
September , 2010
The Global Navigation Satellite System (GLONASS) is based on a constellation of active satellites which continuously transmit coded signals in two frequency bands, which can be received by users anywhere on the Earth's surface to identify their position and velocity in real time based on ranging measurements. The system is a counterpart to the United States Global Positioning System (GPS) and both systems share the same principles in the data transmission and positioning methods. GLONASS is managed for the Russian Federation Government by the Russian Space Forces and the system is operated by the Coordination Scientific Information Center (KNITs) of the Ministry of Defense of the Russian Federation.
The operational space segment of GLONASS consists of 21 satellites in 3 orbital planes, with 3 on-orbit spares. The three orbital planes are separated 120 degrees, and the satellites within the same orbit plane by 45 degrees. Each satellite operates in circular 19,100 km orbits at an inclination angle of 64.8 degrees and each satellite completes an orbit in approximately 11 hours 15 minutes.
The ground control segment of GLONASS is entirely located within former Soviet Union territory. The Ground Control Center and Time Standards is located in Moscow and the telemetry and tracking stations are in St. Petersburg, Ternopol, Eniseisk, Komsomolsk-na-Amure.
The first GLONASS satellites were launched into orbit in 1982. Two Etalon geodetic satellites were also flown in the 19,100 km GLONASS orbit to fully characterise the gravitational field at the planned altitude and inclination. The original plans called for a complete operational system by 1991, but the deployment of the full constellation of satellites was not completed until late 1995 / early 1996. GLONASS was officially declared operational on September 24, 1993 by a decree of the President of the Russian Federation.
Galileo will be Europe's own global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control. It will be inter-operable with GPS and GLONASS, the two other global satellite navigation systems.
A user will be able to take a position with the same receiver from any of the satellites in any combination. By offering dual frequencies as standard, however, Galileo will deliver real-time positioning accuracy down to the metre range.
What is Galileo?
11 May 2010
European Space Agency
Compass/BeiDou Navigation Satellite System (CNSS )
The space segment of CNSS consists of 5 geostationary earth orbit (GEO) and 30 medium earth orbit (MEO) satellites. The carrier frequency of CNSS is 1195.14½ž1219.14MHz, 1256.52½ž 1280.52MHz, 1559.05½ž1563.15MHz and 1587.69½ž1591.79MHz.
Two kinds of service will be provided. One is the Open Service, which is designed to provide users with positioning accuracy within 10 meters, velocity accuracy within 0.2 meter per second and timing accuracy within 50 nanoseconds. The other is the Authorized Service, which will offer "safer" positioning, velocity, timing, communication services and integrity information for authorized users.
Satellite System (CNSS)
China Satellite Navigation Project Center
Japanese QZSS Programme. Quasi-zenith Satellite System (QZSS)
The QZSS consists of a multiple number of satellites that fly in the orbit passing through the near zenith over Japan. By sharing almost the same positioning signals for transmission with the currently operated GPS as well as the new GPS, which is under development in the U.S., the system enables us to expand the areas and time duration of the positioning service provision in mountainous and urban regions in Japan.
Furthermore, the QZSS aims at improving positioning accuracy of one meter to the centimeter level compared to the conventional GPS error of tens of meters by transmitting support signals and through other means.
In order to have at least one quasi-zenith satellite always flying near Japan's zenith, at least three satellites are necessary. The first quasi-zenith satellite "MICHIBIKI" carries out technical and application verification of the satellite as the first phase, then the verification results will be evaluated for moving to the second phase in which the QZ system verification will be performed with three QZ satellites.
Satellite and Space Craft - Quasi-zenith Satellite -1 "MICHIBIKI"
Applications of GPS.
The free, open, and dependable nature of the Global Positioning System (GPS) has enabled users around the world to develop hundreds of applications affecting nearly every facet of modern life. The applications described on this website represent just a few examples. New uses of GPS are invented every day and are limited only by the creativity of the human imagination.
To sustain the Earth's environment while balancing human needs requires better decision making with more up-to-date information. Gathering accurate and timely information has been one of the greatest challenges facing both government and private organizations that must make these decisions. The Global Positioning System (GPS) helps to address that need.
The Global Positioning System (GPS) has changed the way the world operates. This is especially true for marine operations, including search and rescue. GPS provides the fastest and most accurate method for mariners to navigate, measure speed, and determine location. This enables increased levels of safety and efficiency for mariners worldwide.
GPS-based applications in precision farming are being used for farm planning, field mapping, soil sampling, tractor guidance, crop scouting, variable rate applications, and yield mapping. GPS allows farmers to work during low visibility field conditions such as rain, dust, fog, and darkness.
Aviators throughout the world use the Global Positioning System (GPS) to increase the safety and efficiency of flight. With its accurate, continuous, and global capabilities, GPS offers seamless satellite navigation services that satisfy many of the requirements for aviation users.
Public safety and disaster Relief
A critical component of any successful rescue operation is time. Knowing the precise location of landmarks, streets, buildings, emergency service resources, and disaster relief sites reduces that time -- and saves lives. This information is critical to disaster relief teams and public safety personnel in order to protect life and reduce property loss. The Global Positioning System (GPS) serves as a facilitating technology in addressing these needs.
Road and Highway Applications
The availability and accuracy of the Global Positioning System (GPS) offers increased efficiencies and safety for vehicles using highways, streets, and mass transit systems. Many of the problems associated with the routing and dispatch of commercial vehicles is significantly reduced or eliminated with the help of GPS.
The Global Positioning System (GPS) is revolutionizing and revitalizing the way nations operate in space, from guidance systems for crewed vehicles to the management, tracking, and control of communication satellite constellations, to monitoring the Earth from space.
Surveying and Mapping Applications
Using the near pinpoint accuracy provided by the Global Positioning System (GPS) with ground augmentations, highly accurate surveying and mapping results can be rapidly obtained, thereby significantly reducing the amount of equipment and labor hours that are normally required of other conventional surveying and mapping techniques.
Simulink Model Development for GPS Transmitter and Receiver.
GPS with single transmitter and receiver.
C/A code generation.
Figure 4.9 depicts a high-level block diagram of the direct sequence PRN code generation used for GPS C/A code and P code generation to implement the CDMA technique. Each synthesized PRN code is derived from two other code generators. In each case, the second code generator output is delayed with respect to the first before their outputs are combined by an exclusive-or circuit. The amount of delay is different for each SV. In the case of P code, the integer delay in P-chips is identical to the PRN number. For C/A code, the delay is unique to each SV, so there is only a table lookup relationship to the PRN number. These delays are summarized in Table 4.3. The C/A code delay can be implemented by a simple but equivalent technique that eliminates the need for a delay register. This technique is explained in the following paragraphs.
The GPS C/A code is a Gold code  with a sequence length of 1,023 bits (chips). Since the chipping rate of the C/A code is 1.023 MHz, the repetition period of the pseudorandom sequence is 1,023/(1.023 ƒ-€ 106 Hz) or 1 ms. Figure 4.10 illustrates the design architecture of the GPS C/A code generator. Not included in this diagram are the controls necessary to set or read the phase states of the registers or the counters. There are two 10-bit shift registers, G1 and G2, which generate maximum length PRN codes with a length of 210 âˆ’€ 1 €½€ 1,023 bits. (The only state not used is the all-zero state). It is common to describe the design of linear code generators by means of polynomials of the form 1 €«€ ƒ“€ Xi, where Xi means that the output of the ith cell of the shift register is used as the input to the modulo-2 adder (exclusive-or), and the 1 means that the output of the adder is fed to the first cell . The design specification for C/A code calls for the feedback taps of the G1 shift register to be connected to stages 3 and 10. These register states are combined with each other by an exclusive-or circuit and fed back to stage 1. The polynomial that describes this shift register architecture is: G1 €½€ 1 X3 X10. The polynomials and initial states for both the C/A-code and P-code generator shift registers are summarized in Table 4.4. The unique C/A code for each SV is the result of the exclusive-or of the G1 direct output sequence and a delayed version of the G2 direct output sequence. The equivalent delay effect in the G2 PRN code is obtained by the exclusive-or of the selected positions of the two taps whose output is called G21. This is because a maximum-length PRN code sequence has the property that adding a phase-shifted version of itself produces the same sequence but at a different phase. The function of the two taps on the G2 shift register in Figure 4.10 is to shift the code phase in G2 with respect to the code phase in G1 without the need for an additional shift register to perform this delay. Each C/A code PRN number is associated with the two tap positions on G2.
Table 4.3 describes these tap combinations for all defined GPS PRN numbers and specifies the equivalent direct sequence delay in C/A code chips. The first 32 of these PRN numbers are reserved for the space segment. Five additional PRN numbers, PRN 33 to PRN 37, are reserved for other uses, such as ground transmitters (also referred to as pseudosatellites or pseudolites). Pseudolites were used during Phase I (concept demonstration phase) of GPS to validate the operation and accuracy of the system before any satellites were launched and in combination with the earliest satellites. C/A codes 34 and 37 are identical.
GPS with multiple transmitters and receivers.