Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-01-19
2002-07-02
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C375S149000, C375S347000, C375S343000
Reexamination Certificate
active
06414987
ABSTRACT:
BACKGROUND
Reference to a Satellite Positioning System or SATPS herein refers to a Global Positioning System (GPS), to a Global Orbiting Navigation System (GLONASS), and to any other compatible satellite-based system that provides information by which an observer's position and the time of observation can be determined.
The Global Positioning System (GPS) is being developed and operated to support military navigation and timing needs at an estimated cost of about $8-10 billion. GPS represents an almost ideal dual-use technology and enjoys increased attention by civilians to explore its suitability for civil applications. The complete GPS system consists of 24 operational satellites and provides 24-hour, all-weather navigation and surveying capability worldwide. A major milestone in the development of GPS was achieved on Dec. 8, 1993, when the Initial Operational Capability (IOC) was declared as 24 satellites were successfully operating.
The implication of IOC is that commercial, national, and international civil users can rely on the availability of the Standard Positioning Service. Current policies quantify SPS as 100-meter, 95% position accuracy for a single user. Authorized (military) users will have access to the Precise Positioning Service (PPS), which provides a greater degree of accuracy. The PPS access is controlled by cryptographic means.
The GPS satellites transmit at frequencies L
1
=1575.42 MHz and L
2
=1227.6 MHz modulated with two types of codes and with a navigation message. The two types of codes are the C/A-code and the P-code. SPS is based on the C/A-code, whereas PPS is provided by the P-code portion of the GPS signal. The current authorized level of SPS follows from an intentional degradation of the full C/A-code capability. This measure is called selective availability (SA) and includes falsification of the satellite clock (SA-dither) and the broadcast satellite ephemeris (SA-epsilon), which is part of the navigation message. Despite selective availability, the C/A-code is fully accessible by civilians. On Jan. 31, 1994 the SA was finally implemented. The purpose of SA is to make the P-codes available only to authorized and military users. Users should be equipped with a decryption device or the “key” in order to lock on to P-codes. SA is implemented through a modification of the mathematical formula of the P-code using a classified rule. The encrypted P-code is referred to as the Y-code.
Two types of observables are of interest to users. One is the pseudo-range, which equals the distance between the satellite and the receiver plus small corrective terms due to clock errors, the ionosphere, the troposphere, and the multipath. Given the geometric positions of the satellites (satellite ephemeris), four pseudo-ranges are sufficient to compute the position of the receiver and its clock error. Pseudo-ranges are a measure of the travel time of the codes (C/A, P, or Y).
The second observable, the carrier phase, is the difference between the received phase and the phase of the receiver oscillator at the epoch of measurement. Receivers are programmed to make phase observations at the same equally spaced epochs. The receivers also keep track of the number of complete cycles received since the beginning of a measurement. Thus, the actual output is the accumulated phase observable at preset epochs.
(The above-referenced discussion is provided in the book “GPS Satellite Surveying”, Second Edition, authored by Alfred Leick, and published by John Wiley & Sons, Inc. in 1995; pp 1-3).
Both the SPS and PPS address “classical” navigation, where just one receiver observes the satellites to determine its geocentric position. Typically, a position is computed for every epoch of observation.
However, in the surveying and geodesy applications the relative or differential positioning is used, wherein the relative location between the receivers is determined. In this case, many of the common mode errors cancel or their impact is significantly reduced. This is particularly important in the presence of selective availability.
The multipath errors originate with contamination of SATPS signals by delayed versions of these signals. For some applications using either pseudo-range carrier phase observables, multipath is the dominant error source. The most direct approach for reducing this error is to select an antenna site distant from reflecting objects, and to design antenna/back plane combinations to further isolate the antenna from its surroundings. In some cases, however, antennas should be located in relatively poor sites, and other techniques for code multipath reduction are required.
In the U.S. Pat. No. 5,414,729, issued to Fenton, a receiver for pseudorandom noise (PRN) encoded signals is disclosed.
The Fenton receiver consists of a sampling circuit, multiple carrier and code synchronizing circuits, and multiple correlators, with each correlator having a selectable code delay spacing. The time delay spacing of the multiple correlators is distributed around an expected correlation peak to produce an estimate of the correlation function parameters which vary with respect to multipath distortion. This information may be used in turn to determine the offset estimates for locally generated PRN reference code and carrier phase tracking signals”, Col. 3, lines 21-36. In another embodiment of the Fenton device, “the majority of the channels in a receiver can be left to operate normally, with one or more of the channels being dedicated to continuously sequencing from channel to channel to determine the multipath parameters for a partial PRN code being tracked”, Col. 3, lines 44-49.
Thus, the Fenton device includes a plurality of tracking satellite channels used to estimate the multipath parameters.
However, the Fenton device '729 includes a number of limitations.
(1) The Fenton device does only tracking. After tracking, there is still a multipath blimp left in the correlation function that has to be removed by some other means.
(2) The Fenton device uses the full correlation. Indeed, the Fenton device correlates through the entire chip period.
(3) The Fenton device employs multiple correlators time delay spacing. This is equivalent to Early minus Late (E−L) response function that is limited to one chip time period. That is, the Fenton device is a “narrow correlator” device. If Fenton's (E−L) response function is not limited to only one chip time period and is extended over two chip periods, the Fenton device would not be beneficial over the prior art at all.
(4) Using the filter function approach, the “narrow correlator” property of the Fenton device can be described as follows: the amplitude of the Fenton's filter function decreases when time increases between zero and two chip time periods.
Another technique for code multipath reduction, that is free of Fenton's limitations, was disclosed by Rayman Pon in the U.S. patent application Ser. No. 08/650,631, entitled “Suppression Of Multipath Signal Effects” (patent application #1), that was assigned to the assignee of the present patent application, and that was filed on May 20, 1996 and now patented U.S. Pat. No. 5,903,597. The patent application #1 is specifically referred to in the present patent application and is incorporated herein by reference. In the patent application #1 the weighted tracking process was used in order to suppress the multipath error signal, wherein the Early and Late signals were non-uniformly weighted in order to suppress the multipath error signal.
One more example of such technique for code multipath reduction was disclosed by Rayman Pon in the U.S. patent application Ser. No. 08/783,616 entitled “Code Multipath Reduction Using Optimized Additional Signals” (patent application #2) filed on Jan. 14, 1997. The patent application #2 assigned to the assignee of the present patent application, is specifically referred to in the present patent application and is incorporated herein by reference. In the patent application #2 the modified track
Chin Stephen
Kim Heechul
Tankhilevich Boris G.
Trimble Navigation Ltd.
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