Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1998-07-02
2001-05-22
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C342S357490, C701S213000
Reexamination Certificate
active
06236354
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the field of satellite positioning system (SPS) receivers, and more particularly to reducing satellite signal interference in an SPS receiver.
BACKGROUND OF THE INVENTION
Global Positioning System (GPS) receivers normally determine their position by computing relative times of arrival of signals transmitted simultaneously from a multiplicity of GPS (or NAVSTAR) satellites. These satellites transmit, as part of their message, both satellite positioning data as well as data on clock timing, so-called “ephemeris” data.
GPS receivers determine pseudoranges to the various GPS satellites, and compute the position of the receiver using these pseudoranges and satellite timing and ephemeris data. The pseudoranges are time delay values measured between the received signal from each satellite and a local clock signal. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked. Acquiring GPS signals can take up to several minutes and must be accomplished with a sufficiently strong received signal in order to achieve low error rates.
Most GPS receivers use correlation methods to compute pseudoranges. GPS signals contain high rate repetitive signals called pseudorandom (PN) sequences. The codes available for civilian applications are called C/A (coarse/acquisition) codes, and have a binary phase-reversal rate, or “chipping” rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 millisecond. The code sequences belong to a family known as Gold codes, and each GPS satellite broadcasts a signal with a unique Gold code.
For a signal received from a given GPS satellite, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates 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.
Global Position Satellite Systems utilize a multiplicity of satellites to simultaneously transmit signals to a receiver to permit position location of the receiver by measurement of time-differences of arrival between these multiple signals. In general, the signals from the different satellites do not significantly interfere with one another, since they utilize different pseudorandom spreading codes that are nearly orthogonal to one another. This low interference condition depends upon the power levels (amplitudes) of the received signals being similar to one another.
In certain circumstances, however, it may be the case that one or more GPS signals are highly attenuated relative to the other satellite signals. Such a condition may arise from blockage of certain satellite signals, as may occur in urban canyon environments. Under these conditions, the presence of the strong GPS signals produces interference that can reduce the ability to detect the weaker signals.
SUMMARY OF THE INVENTION
A method and apparatus for reducing crosstalk interference between a plurality of received satellite signals in a Global Positioning System (GPS) receiver is disclosed. A GPS receiver receives as an input waveform a first signal and a second signal from respective satellites of a plurality of GPS satellites. The first signal is presumed to be sufficiently stronger than the second signal, so that detection of the second signal is hindered due to interference from the first signal.
In one embodiment of the present invention, the time-of-arrival, amplitude, and carrier frequency of the stronger signal is estimated. An interference waveform is constructed using the estimated data, as well as a hypothesized carrier frequency of the weaker signal and pseudorandom codes corresponding to the stronger and weaker signals. Hypothesized times of arrival of the weaker signal are used to produce an output waveform using correlation or matched filter methods. A portion of the interference waveform is subtracted from the output waveform to remove interference effects of the stronger signal on the weaker signal. Alternatively, portions of the output waveform are ignored where the interference waveform produces strong spurious signals.
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Blakely & Sokoloff, Taylor & Zafman
Blum Theodore M.
SnapTrack, Inc.
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