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Digital satellite receiver
Sunday, 03 December 2006

Satellite navigation systems are intended for highly accurate self-positioning of users possessing special navigation receivers. Satellite positioning system receivers make precise determinations of latitude, longitude, elevation and time by using time difference of arrival and Doppler measurement techniques on precisely-timed spread spectrum signals transmitted by orbiting satellites. The transmitted signals contain a number of components designed to enable the receivers to extract the requisite information. Global positioning system (GPS) is one example of a space based satellite navigation system that has the ability to pinpoint any location on earth with a high degree of accuracy, providing information on position, velocity and time (PVT) of a receiver. Satellite based positioning systems operate by utilizing constellations of satellites which transmit to earth continuous direct sequence spread spectrum signals. The satellites transmit radio frequency signals containing navigation data and codes which enable each satellite to be identified. These codes phase-modulate (BPSK modulation) a carrier frequency. A satellite signal is usually intended to be received and processed by a plurality of digital satellite receivers. Digital satellite receivers within receiving range of these satellites intercept these signals which carry data (navigation messages) modulated onto a spread spectrum carrier. This data provides the precise time of transmission at certain instants in the signal along with orbital parameters for the satellites themselves. A satellite receiver determines its distance from the satellite by determining the code phase of the transmission from the satellite. The code phase (CP) is the delay that a satellite transmission experiences as it travels the approximately 11,000 mile distance from the satellite to the receiver. GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received pseudo-random (PN, PRN) code signal sequence and internally generated pseudo-random noise signal sequences. The receiver determines the code phase for a particular satellite by correlating shifted versions of the satellite's PN code with the received signal after correction for Doppler shift. Global positioning system receiver systems commonly use differential carrier phase measurements for finding a precise location vector for a geographical location of a GPS user receiver with respect to a geographical location of a GPS reference receiver. A satellite receiver can accurately calculate its distance from each of the satellites, and thereby deduce its precise position in terms of latitude, longitude and altitude, in a terrestrial reference frame. It may also thereby deduce the date and precise time of reception in the temporal reference frame of the system. The use of a global positioning system (GPS) to determine the location of objects is becoming wide spread. Automobiles, wireless phones and other devices are being designed to include global positioning system receivers.

Digital satellite receivers for receiving satellite-based navigation signals have been known for quite some time. Such receivers comprise a receiving unit for receiving navigation signals emitted by satellites, the receiving unit is connected with a processor for processing and forwarding said signals and data for navigation purposes. The satellite receiver is connectable via an interface with a navigation computer, for example a laptop computer or personal digital assistant (PDA). A typical satellite receiver receives a composite signal as well as noise and any interfering signals. A navigation receiver receives and processes radio signals transmitted by satellites located within line-of-sight distance of the receivers. 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. The satellite signals comprise carrier signals that are modulated by pseudo-random binary codes. The receiver measures the time delay of the received signal relative to a local reference clock or oscillator. These measurements enable the receiver to determine the so-called pseudo-ranges between the receiver and the satellites. A satellite receiver converts the code phase for a satellite to a time delay. It determines the distance to the satellite by multiplying the time delay by the velocity of the transmission from the satellite. The receiver also knows the precise orbits of each of the satellites. Updates to the locations of the satellites are transmitted to the receiver by each of the satellites. A global positioning system includes multiple GPS satellites to broadcast location signals, control stations to monitor and control the satellites, and GPS receivers to receive the signals. In a global positioning system (GPS), a receiver monitors signals from a plurality of GPS satellites to determine position, speed or other information about the receiver. The GPS receiver receives a signal from four GPS satellites to determine the receiver's longitude, latitude and altitude as well as an adjustment to a signal generated from the receiver's clock. Global positioning system (GPS) receivers receive GPS signals transmitted from orbiting GPS satellites containing unique pseudo-random noise (PN) codes. The GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences. Each satellite transmits a unique PN code that identifies the particular satellite, and allows signals transmitted simultaneously from several satellites to be received simultaneously by a receiver with very little interference of any one signal by another. The GPS receiver may be in a fixed location, in a mobile location such as an automobile or ship, or incorporated into a handheld device. In some cases, the GPS receiver is integrated with a wireless communication device, such as a cellular phone or personal communications system device. A GPS receiver that is associated with a cell phone or that can communicate over the Internet can be assisted in many ways by network servers connected to other GPS receivers that already have satellite-lock and are tracking. A GPS antenna that is a part of a GPS receiver must have a line of sight to a GPS satellite to receive the GPS signal from that satellite. Commercial GPS receivers now are used to provide accurate location information in many navigation, tracking, and timing applications.

Global positioning system and satellite positioning system receivers use signals received from several earth-orbiting satellites in a constellation to determine user position and velocity, and other navigational data. GPS navigational systems determine positions by timing how long it takes the coded radio GPS signal to reach the receiver from a particular satellite. The satellite receiver generates a set of codes identical to those codes transmitted by the satellites. To calculate the travel time, the receiver determines how far it has to shift its own codes to match the codes transmitted by the satellites. The determined travel times for each satellite are multiplied by the speed of light to determine the distances from the satellites to the receiver. 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 satellite data transmitted from the acquired GPS satellites. The pseudo-random code is a long code, and one of its main functions is to make it possible to extract the satellite's signal from a noise level much higher than the level of the signal. The signal is extracted from the noise using an operation, in the satellite receiver, of correlation between the received signal and a periodic pseudo-random code which is identical to the one expected to be found in the signal. The difference between a time at which the code is transmitted via the satellite and a time at which the code is received by the receiver makes it possible to determine a propagation time of the signals between the satellite and the receiver. Different types of PRN codes are used for different system applications. For example, within the GPS system, C/A code is used for low cost, less accurate commercial applications, and P-code is used for higher accuracy military applications.