Processes performed by a GPS receiver generally include an acquisition process for acquiring a desired satellite among a plurality of satellites and a tracking process for demodulating the transmission signal transmitted from the satellite every predetermined cycle after the acquisition of the desired satellite while tracking the satellite. Global positioning system receivers determine location, velocity, and time by receiving and processing information in GPS signals received from GPS satellites. The GPS signal from each satellite carries data for the location-in-space of the satellite and time-of-transmission on carrier frequencies that are the same for all the satellites. The data from each satellite is spread with a pseudo-random noise (PRN) code that is distinct for that satellite. In a typical global positioning system, a receiver monitors signals from a plurality of GPS satellites to determine position, speed or other information about the receiver. In GPS systems for obtaining positions of mobile bodies by use of GPS satellites, the basic functions of each GPS receiver are that the receiver receives signals from four or more GPS satellites, computes the current position of its position from the received signals, and let the user know the computed position. The GPS receiver demodulates the signals received from GPS satellites to obtain the orbit data of the GPS satellites and, from the each GPS satellite's orbit, time information and delay time, derives the three-dimensional position of the own receiver on the basis of simultaneous equations. The signals from four GPS satellites are necessary for positioning computation because the effects of the error between the time in the GPS receiver and the time of each satellite must be eliminated. From any location on or near the earth, a GPS receiver with an unobstructed view of the sky should be able to track at least four satellite vehicles thereby being able to calculate the receiver's precise latitude, longitude, and elevation. Specifically, a GPS satellite in the air is searched for from the present time and the present position of a GPS receiver mounted on an automotive vehicle. The signal reception frequency is estimated from the orbit information of the GPS satellite in consideration of influence of doppler effect. If three or more GPS satellites thus searched for are acquired, it becomes possible to execute a positioning processing, that is, to calculate the position of the automotive vehicle by using satellite orbit data included in GPS signals (satellite data) transmitted from the acquired GPS satellites.

The GPS signal emitted by the satellites contains an L-band carrier component (L1) transmitted at a frequency of 1575.42 MHz. The L1 carrier component is modulated by a coarse acquisition (C/A) pseudorandom (PRN), also called pseudo noise (PN), code component unique to the satellite and a data component. Use of the PRN codes allows use of a plurality of GPS satellite signals for determining an observer's position and for providing the navigation information. The PRN code provides timing information for determining when the GPS signal was broadcast and identifying which satellite emitted the signal. The data component provides information defining the satellite ephemeris, satellite clock corrections and other GPS information. The carrier component allows a receiver to more easily acquire the GPS signal. The GPS signal receiver can generate local PN codes corresponding respectively to multiple kinds of GPS satellites, and secures synchronism by matching the phases of the local PN codes generated with the phase of the PN code of the satellite signal. A signal transmitted by a particular GPS satellite is selected by generating and matching, or correlating, the PRN code for that particular satellite. A primary goal of a GPS receiver is to determine the time-of-arrival of the PN codes, which is accomplished by comparing a locally generated PN reference against the received signal and sliding the local reference in time until it is time-aligned with the received signal. The two signals are compared with one another by a multiplication and integration process known as a correlation process. The L1 signal carrier is modulated in quadrature with both a clear acquisition code (CA code) and a precise code (P code). Dual frequency receivers that utilize both L1 and L2 frequency signals can determine the position much faster than a single frequency receiver can. A technique that uses both L1 and L2 carrier phase measurements is faster than the one using just L1 carrier phase measurements. In order to obtain the precise location of the receiver, a received GPS signal must be converted from an RF signal to a baseband signal before the GPS information can be extracted from the received GPS signal.

A GPS receiver measures the time offset between a pseudo random noise (PRN) code received from a GPS satellite and a locally generated PRN code. Two types of orbital parameters are transmitted for determining locations-in-space for the satellites: almanac data and ephemeris data. The almanac data includes relatively few parameters and is generally sufficient for determining locations-in-space to a few kilometers. The ephemeris data provides relatively more parameters and is much more accurate. Typically, current ephemeris data is sufficient for determining locations-in-space to a few meters without selective availability or a few tens of meters at current levels of selective availability. Upon detecting and synchronizing with a PRN code, a receiver decodes the PRN encoded signal to recover the navigation data, including ephemeris data. Each GPS satellite broadcasts its own ephemeris data on a thirty second cycle. Ephemeris data is updated each hour. However, after about two hours the accuracy of the ephemeris data begins degrading. Typically, ephemeris data that is more than about four hours old is avoided for determining the pseudoranges for a user location. The ephemeris data is used in conjunction with a set of Keplerian equations to precisely determine the location of each satellite. The receiver measures a phase difference of signals from at least four satellites. The time differences are then used to solve a matrix of four equations. The GPS receiver is previously configured to be able to detect which GPS satellite uses which PN sequence code. A navigation message is used to indicate from which GPS satellite the GPS receiver can receive signals at a given point and time. In general, a GPS receiver provides data indicative of a position (a GPS position) and a velocity (a GPS velocity), in real time, by performing the GPS measurement. Velocity of the receiver may be determined by a precise measurement of the L1 and L2 frequencies. The measured frequencies are used to determine Doppler frequency shifts caused by differences in velocity. The data (the GPS position and/or the GPS velocity) obtained by performing the GPS measurement is also referred to as a GPS solution. In the GPS measurement, the GPS position is calculated using a range, or a distance between the GPS receiver and GPS satellite, which is measured using a satellite signal.

Global positioning system receivers are primarily used for navigation purposes. Global positioning system (GPS) receivers provide accurate location, velocity and time information based on specially coded data broadcast from orbiting satellites. The system generates a position fix by way of outputs, position, speed and other vital navigation information. The GPS receiver demodulates a signal from the GPS satellite to obtain the GPS satellite's orbital data. The GPS receiver then derives the receiver's three-dimensional position using simultaneous equations from the GPS satellite's orbit and time information and the received signal's delay time. The position finding is based on the principle of a transit time measurement of signals, which are modulated upon electromagnetic carrier waves with a carrier frequency of 1575.42 MHz.  The GPS receiver calculates the latitude, longitude and altitude of the GPS receiver's location (i.e., the co-ordinates of the receiver) upon receiving a number of GPS signals from a network of GPS satellites that orbit the earth. The calculation of the co-ordinates of the GPS receiver typically begins by comparing the timing associated with a select number of received GPS signals. After the initial comparison of the received GPS signals, values for timing corrections associated with the select group of received GPS signals are established. GPS receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS satellites. These satellites transmit both satellite positioning data as well as data on clock timing, so-called ephemeris data. Once the GPS receiver has acquired and decoded signals from a minimum of three satellites, the GPS receiver can calculate the user's position by geometric triangulation. Upon acquiring signals from a minimum of four satellites, the GPS receiver can also calculate the user's altitude. In addition, GPS receivers are able to calculate the user's speed and direction of travel by continuously updating the user's position.

GPS receivers typically derive two types of measurements from the received GPS signals, referred to as code measurements and carrier measurements. A GPS receiver uses the distinct PRN code for distinguishing the global positioning system signals from typically at least four satellites and then finds its own location, velocity, and time by solving simultaneous equations using the relative times that the signal from each of the satellites arrives at the receiver and the locations-in-space and times-of-transmission from the satellites. Because the signal transmitted by each satellite uses a PRN code or a carrier frequency unique to that satellite, the receiver may separate the signals from different satellites using code division multiple access (e.g., each GPS satellite has a unique PRN code) or frequency division multiple access techniques. The composite signal is first fed to a down-converter which amplifies and filters the incoming composite signal, mixes it with a locally generated carrier reference signal, and thus produces a composite intermediate frequency (IF) signal. For a global positioning system receiver, a decoder or channel circuit then correlates the composite signal by multiplying it by a locally generated version of the PRN code signal assigned to a particular satellite of interest. If the locally generated PRN code signal is properly timed, the digital data from that particular satellite is then properly detected. The PRN codes also provide a mechanism to precisely determine the signal transmission time from each satellite. By determining the transmission time from at least four satellites, and knowing each satellite's ephemeris and approximate time of day information, the receiver's three dimensional position, velocity and precise time of day may be calculated.

A GPS signal contains timing information that allows a user to determine the time elapsed for the GPS signal to transverse the distance between the GPS satellite and a receiver receiving the signal. The accuracy of a GPS receiver depends on the accuracy with which the receiver is capable of measuring the time that has elapsed between the broadcast of the range information by a satellite vehicle and the reception of the information by the receiver. By knowing the time the GPS signal left the GPS satellite, the time the GPS signal arrived at the user, and the speed of the GPS signal, the receiver can determine the distance from itself to the GPS satellite. The pseudo range and Doppler measurements (and navigation data) from four satellites are used to compute a three dimensional position and velocity fix, and to calibrate the receiver's clock offset and provide an indication of GPS time. The measured range is referred to as "pseudorange" because there is generally a time difference or offset between timing clocks on the satellites and the GPS receiver clock. The pseudorange includes both the true range to the satellite and the affected offset of the receiver clock from the GPS master time reference. The GPS satellite pseudoranges are measured by determining phase offsets between GPS pseudorandom (PRN) codes received in the GPS signals and internal GPS replica PRN codes referenced to the internal clock. The GPS-based time is used to determine the times that the phase offsets were measured. The measurement times are then used with ephemeris data that is received in the GPS signals for calculating the instantaneous locations-in-space of several GPS satellites and for linearizing location equations relating the calculated locations-in-space to the measured pseudoranges. There are needed four GPS satellites to obtain received signals because an error occurs between the GPS receiver's inside time and the satellite time and it is necessary to remove an effect of the error.

GPS receiving systems have two principal functions. The first is the computation of the pseudoranges to the various GPS satellites, and the second is the computation of the position of the receiver using these pseudoranges and satellite timing and ephemeris data. Most GPS receivers utilize correlation methods to compute pseudoranges. These correlation methods are performed in real time, often with hardware correlators. Generally, in the satellite acquisition process, the GPS receiver calculates a correlation value of a reception signal and a pseudo noise signal corresponding to a number of the satellite, while changing a code phase and a carrier frequency. The GPS receiver determines whether the satellite has been acquired or not based on an amount of energy of the correlation value. For a signal received from a given GPS satellite, following a downconversion process to baseband, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates, or lowpass filters, the product in order to obtain an indication of the presence of the signal. This process is termed a correlation operation. By sequentially adjusting the relative timing of this stored replica relative to the received signal, and observing the correlation output, the receiver can determine the time delay between the received signal and a local clock. The initial determination of the presence of such an output is termed acquisition. Once acquisition occurs, the process enters the tracking phase in which the timing of the local reference is adjusted in small amounts in order to maintain a high correlation output. In the satellite tracking process after the acquisition of the desired satellite in the above described manner, the GPS receiver generally calculates a correlation value every predetermined cycle in the same manner as in the acquisition process, and performs a data demodulating process based on the magnitude and the sign of the correlation value.

A GPS receiver is basically composed of an antenna unit for receiving high frequency satellite signals and a processing unit for processing the signals to compute positional coordinates of the GPS receiver. The processing unit comprises a radio frequency (RF) down converter, correlator and a navigation processor. The receiver receives satellite signals from the antenna, down converts the signal in the RF down converter and processes the signal in the correlator. The code correlation and carrier tracking functions can be performed using either analog or digital signal processing. This signal is then fed to the GPS processor assembly where the PRN (pseudo random) code modulating the L-band signal is tracked through code-correlation at the receiver. The antenna unit is required to have increased area of ground plane for improving the antenna gain. The position and other navigation information is computed in the navigation processor and transmitted in a standard format which can be used by the system integrators to develop various applications around these GPS receivers. The digital circuit is a source of developing a noise which interferes with the antenna and impedes the antenna gain. In order to avoid the interference, the digital circuit has to be surrounded by an EMI (electromagnetic interference) shield. GPS receivers have local oscillators or oscillators that are used in a heterodyne or superheterodyne configuration for acquiring GPS satellite signals. GPS receivers require a highly accurate internal frequency reference in order to acquire the spread spectrum GPS satellite signals. Acquiring spread spectrum satellite signals from a sufficient number of satellites to perform calculations requires determining the frequency of oscillation of the crystal oscillator utilized in the GPS receiver. A GPS receiver is constructed typically in a block diagram form with a clock unit which generates a reference clock signal at a fixed frequency. Such clocks are synchronized with clocks in each of the satellites to determine how long it takes the signals to travel from the satellites to the receiver.

The global positioning system (GPS) has become extremely popular for a number of applications. Global position system receivers and related apparatus have become widely used for determining geographical location and/or time in commercial applications including navigation, timing, mapping, surveying, machine and agricultural control, vehicle tracking, and marking locations and time of events. For example, GPS receivers used in search and rescue operations, humanitarian missions, or stored in warehouses or on store shelves can go months or even years before use. In navigation applications, a user uses a GPS receiver to determine her instantaneous position as well as her position over time. GPS receivers are now incorporated into a variety of consumer electronic systems in which the location information or time information provided by GPS supplements the other information provided by the system. Recent technological developments have allowed the combination of SPS receivers and communication systems in integrated units, such as a combination GPS receiver and cellular phone unit. Such combined devices have many applications such as personal security, emergency response, vehicle tracking, and inventory control. Numerous additional and new services are being deployed and developed for using the position-determining capabilities of GPS receivers.