Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-11-09
2002-02-12
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000
Reexamination Certificate
active
06347113
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a Global Positioning System (GPS) receiver for satellite-based navigation systems such as the U.S. Global Positioning System, the Russian Global Navigation System or the like. More specifically, the present invention is directed to a GPS receiver configured to decode pseudo random noise (PRN) codes from broadcasted signals of the navigation systems.
BACKGROUND OF THE INVENTION
Radio navigation systems such as the United States' Global Positioning System (GPS), the Russian Global Navigation System (GLONASS) or the like are configured to broadcast signals that include navigation data encoded therein. The navigation data include each satellite's ephemeris data, e.g., their positions and time indicated by their onboard navigational devices and clocks, respectively.
A conventional GPS receiver determines positions of the GPS satellites and transmission time of broadcasted signals therefrom by passively receiving and decoding the broadcasted signals. More specifically, the GPS receiver, by obtaining the arrival times of the broadcasted signals transmitted by the satellites in the receiver's field of view, which are precisely measured relative to the receiver's own clock, obtains its distances (ranges) to the respective satellites within a constant bias. The value of the constant bias is substantially equal to the difference between the satellites' and the receiver's clock times. Since the satellites' clocks are synchronized closely to a system time, this constant bias is, within some very small error, substantially the same for all satellites. Distances between the GPS receiver and the satellites from which the received broadcasted signals sent are called pseudoranges, and they are offset from the actual distances by a constant value proportional to the constant bias. With four or more satellites in view, the four unknowns consisting of the receiver's position (longitude, latitude, and elevation) and the receiver's clock offset with respect to the system time can be solved using the measured receiver-to-satellite pseudoranges, and the satellites' ephemeris data in the navigation data broadcasted by the satellites.
In order to allow the receiver to measure the pseudoranges, the satellites' broadcasted signals are wide band pseudo random noise (PRN) coded signals. These PRN coded signals are radio frequency (RF) carriers modulated by a wide band PRN code, e.g., modulo-2 added to the navigation data. A unique PRN code is assigned to each satellite, and different types of PRN codes with different chip rates are used for different system applications. For instance, in the U.S. GPS, a 1 MHZ chip rate C/A code is used for initial acquisition and less accurate civilian applications, while a 10-MHZ chip rate P-code is used for higher accuracy military applications. The PRN codes are designed such that cross-correlation between different PRN codes from different satellites is minimized. For example, the GPS C/A codes are 1023 chip length Gold codes which have a maximum periodic cross-correlation value of only 65. This design feature allows the receiver to separate the ranging signals received from a number of GPS satellites by acquiring and tracking the unique PRN codes of the GPS satellites even though they are transmitted on the same RF frequency (1575.42 MHZ for GPS LI and 1227.6 MHZ for GPS L2).
A conventional circuit used to track the PRN signals is a delay lock loop that correlates early, punctual, and late versions of locally generated PRN codes against the received signal. Further, the delay lock loop obtains an estimate of the time difference between the locally generated code and the received code from the difference between the correlation of the early version of the locally generated code against the received signal and the correlation of the late version of the locally generated code against the received signal.
The accuracy of the receiver is affected by several factors: the accuracy of the satellite ephemeris data and clock time given in the satellite's navigation data, the propagation delays introduced by the ionosphere and the troposphere, receiver noise and quantization effects, radio frequency interferences, multipath effects, and the relative geometry of the satellites and the GPS receiver, which is measured in terms of geometric dilution of precision. Some of these error sources, in particular, the satellite ephemeris data and clock errors, the ionospheric and tropospheric delays, can be substantially eliminated in a differential GPS (DGPS) system, which can be either local area differential or wide area differential. (The Federal Aviation Agency (FAA) is developing both types of differential systems. Specifically, a Local Area Augmentation System (LAAS) is being developed for installation at airports for use in aircraft landing, and a Wide Area Augmentation System (WAAS) is being developed for en route navigation and lower accuracy landing requirements.)
In local area differential systems, the error in the determined position of a reference receiver, which is generally located at a precisely surveyed site, are subtracted from the position obtained by of the GPS receiver or, alternatively, the measured errors in the pseudoranges at the reference site are subtracted from the measured pseudoranges of the GPS receiver. For such local DGPS applications the GPS receiver is located usually within 10 to 50 kilometers of the reference receiver. Under these circumstances the error caused by the satellite ephemeris data, clock and by ionospheric and tropospheric delays are almost identical at both the user receiver and reference receiver, and are thus practically canceled in the correction process. The major error sources remaining are then receiver noise, receiver quantization and multipath effects. These effects are uncorrelated between the reference and user receivers, and are not canceled from each other in the correction process.
In wide area differential systems, the correlation of ionospheric, tropospheric and satellite orbital errors are decreased because of the increased distance between the reference receivers and the user receivers. To alleviate this decreased correlation, a wide area DGPS system, e.g., the FAA's WAAS, will typically send separate corrections for the satellite clock errors, the satellite orbit errors and for the ionospheric refraction effects. The correction signals are derived from a network of ground reference stations that estimate the satellite clock and ephemeris data errors and the ionospheric refraction effects at specific grid points from which the user receiver can compute its specific expected errors. Similar to the local area DGPS systems, most of the error sources are mitigated in the correction process except for receiver noise, quantization noise, interference, and multipath effects. The effects of the receiver's thermal and quantization noise can be reduced through averaging. The interference can be reduced with frequency management and regulation. The multipath effects thus become the most detrimental error source in DGPS systems. Sub-meter accuracy can generally be obtained by DGPS if the multipath error can be reduced sufficiently. However, multipath error in C/A code PRN tracking with early minus late discriminators can be as large as one chip (300 meters) and is usually several meters in magnitude. Reduction of multipath effects is thus an important design consideration in high quality GPS receivers.
The number of multipath signals, their relative delays and RF phase offsets with respect to the direct path signal are all functions of the satellite to the receiver's antenna geometry relative to reflecting objects around the receiver antenna. Since multipath signals always travel a longer distance than the direct path signal, they are invariably delayed with respect to the direct path signal and will suffer a loss in power in the reflection process. If the multipath signal has a delay in excess of
Bocure Tesfaldet
NavCom Technology, Inc.
Pennie & Edmonds LLP
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