Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1997-09-12
2001-03-06
Le, Amanda T. (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S148000, C375S149000, C375S150000, C370S335000, C370S350000
Reexamination Certificate
active
06198765
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates in general to radio frequency receivers subject to the reception of multipath signals and in particular to receivers measuring the time of arrival of signals received, such as those used in GPS navigation systems, which are adversely affected by multipath signals.
2. Description of the Prior Art
GPS systems, as well as many other radio frequency (RF) communication systems utilizing frequencies high enough to be considered line of sight systems in which there must be a substantially direct line of sight between the transmitter(s) and receivers(s) for optimum operation, often suffer from multipath effects in which the receiver(s) must process signals received over a multiplicity of different paths. A common example is a simple broadcast TV system in which a TV receiver with an antenna receives multiple copies of the signal being transmitted.
The multiplicity of signals being received results from additional, typically unwanted, signal paths including one or more reflections. When the signal path from the transmitter to receiver includes a reflection, this signal path must by definition be longer than the direct path. Multipath signals present a problem in systems, such as GPS systems, in which the time of arrival of the signal is to be measured or used because the time of arrival of the multipath signals depends on the length of the path(s) taken.
The straightforward processing of all signals, including multipath or reflected signals, often degrades the processing performed by the receiver. The direct path is the shortest and therefore requires the least travel time from transmitter to receiver while the various unwanted multipath signals have various greater lengths, and therefore various longer travel times, than the direct path signals.
GPS transmitters are positioned on satellites with complex orbital paths so that the locations of the multiple transmitters are constantly changing. This makes a highly directional antenna system almost completely unusable. Similarly, digital receivers, including those used in a GPS receiver, often do not rely solely on the amplitudes of the signals received, but rather rely on other signal characteristics, such as time of arrival.
Multipath processing techniques currently used for complex receivers, such as GPS receivers, are often quite complex and subject to inaccuracies. An example of one such conventional technique is described in U.S. Pat. No. 5,414,729 issued on May 9, 1995 to Patrick Fenton and assigned as issued to NovAtel Communications Ltd., Canada. In this technique, an autocorrelation function of a partially processed received signal, including multipath components, is compared to an estimated autocorrelation function of an estimated direct path signal to attempt to discern direct path signals from multipath signals for further processing. This technique of comparing processed and estimated correlation power, is complex and may be subject to error in that the partially processed signals relied on are themselves subject to degradation from many effects in addition to multipath effects including receiver limitations, which may reduce the accuracy or effectiveness of the multipath processing techniques.
For example, in tracking a GPS C/A signal to determine position information from GPS satellite transmitters, it is typically important to derive an accurate estimate of the time of arrival, known as code phase, of the PRN modulation of the direct path component of the C/A signals received from each of the various GPS satellites. It is also important to derive an accurate estimate of the phase of the underlying carrier signals transmitted from the satellites on which the modulation is applied, known as the carrier phase. However, as apparently shown for example in
FIGS. 6
,
7
and
8
of the above referenced Fenton patent, the delayed multipath components degrade the tracking of the code and carrier phase estimates by distorting the correlation functions used is such tracking.
What is needed is an improved technique for multipath signal processing which is less complex, less subject to error than conventional techniques and is applicable to a wide range of communication systems, signal encoding approach and conditions.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides an improved terrestrial navigation system using a GPS receiver in which the residual code phase tracking, or pseudorange, error due to simultaneous reception of multipath signals is detected, estimated and corrected. In particular, the distortion of the correlation function of the multi- and direct path signal composite as received with the internally generated code is detected by comparison of an aspect of resultant correlation function with a model of the correlation function expected in the absence of multipath distortion. The comparison provides an indication of the sign of the residual error.
It has been determined that the composite of a direct path and one or more multipath signals distorts the correlation function. If, as in most common cases, the multipath signal(s) are weaker than the direct path signals, the interference between such signals as received results in a predictable distortion of the correlation function. If the carrier phase(s) of the multipath signal(s) are shifted from between about 0° to 90° from the carrier phase of the direct path signal, the signals tend to reinforce each other resulting in a widening of the correlation function. Similarly, if the carrier phase(s) of the multipath signal(s) are shifted from between 90° to about 180° from the carrier phase of the direct path signal, the signals tend to cancel each other resulting in a narrowing of the correlation function.
The correlation products are used in a code tracking loop to track and determine code phase. The most common scheme is to track points of equal magnitude (or power) separated by one C/A code chip width and estimate the time of arrival of the direct path signal as the mid-point between these points of equal magnitude. The points of equal magnitude on either side of the direct path arrival time are known as the early and late correlation time and the estimated arrival time of the direct path is called the punctual correlation time. In the presence of multipath signals, the correlation function has been found to be distorted so that the mid-point between the early and late correlations, that is the prompt correlation, is not an accurate estimation of the arrival time of the direct path signal.
When the correlation function of the composite is distorted to be wider than the correlation function expected for a direct path only signal, the distortion results in a lag error in which the prompt correlation lags the actual direct path signal received. Similarly, when the correlation function of the composite is narrower than expected, the distortion results in a lead error in which the prompt correlation leads the time of arrival of the direct path signal.
The prompt correlation therefore leads or lags the actual time of arrival of the direct path signal by an amount designated herein as the residual code tracking error. The magnitude of the error can be approximated by the degree of narrowing or widening of the correlation function. Reduction of this error, or detection and correction of the error, enhances the accuracy of the resultant position determination.
In another aspect, the present invention provides a spread spectrum receiver including means for correlating a received, code modulated spread spectrum signal with a code modulated signal replica at a first selected code phase delay and at a second selected code phase delay and comparison means for comparing characteristics of said correlations to derive information related to any residual code phase error in said second code phase delay.
In another aspect, the present invention provides a method of processing coded, spread spectrum signals with potential multipath interference by generating a local replica of the code, correla
Cahn Charles R.
Chansarkar Mangesh
Kohli Sanjai
Fulwider Patton Lee & Utecht LLP
Ha Dac V.
Le Amanda T.
SiRF Technologies, Inc.
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