Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2001-01-18
2002-07-16
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490
Reexamination Certificate
active
06421002
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to receivers capable of determining position information of satellites and, in particular, relates to such receivers which find application in satellite positioning systems (SPS) such as the U.S. global positioning satellite (GPS) systems.
2. Background Art
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. The process of searching for and acquiring GPS signals, reading the ephemeris data for a multiplicity of satellites and computing the location of the receiver from this data is time consuming, often requiring several minutes. In many cases, this lengthy processing time is unacceptable and, furthermore, greatly limits battery life in micro-miniaturized portable applications.
Another limitation of current GPS receivers is that their operation is limited to situations in which multiple satellites are clearly in view, without obstructions, and where a good quality antenna is properly positioned to receive such signals. As such, they normally are unusable in portable, body mounted applications; in areas where there is significant foliage or building blockage; and in in-building applications.
There are two principal functions of GPS receiving systems: (1) computation of the pseudoranges to the various GPS satellites, and (2) computation of the position of the receiving platform using these pseudoranges and satellite timing and ephemeris data. The pseudoranges are simply the time delays measured between the received signal from each satellite and a local clock. The satellite ephemeris and timing data is extracted from the GPS signal once it is acquired and tracked. As stated above, collecting this information normally takes a relatively long time (30 seconds to several minutes) and must be accomplished with a good received signal level in order to achieve low error rates.
Virtually all known GPS receivers utilize correlation methods to compute pseudoranges. These correlation methods are performed in real time, often with hardware correlators. GPS signals contain high rate repetitive signals called pseudorandom (PN) sequences. The codes available for civilian applications are called C/A 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 msec. The code sequences belong to a family known as Gold codes. Each GPS satellite broadcasts a signal with a unique Gold code.
For a signal received from a given GPS satellite, following a downconversion process to baseband, a correlation receiver multiplies the received signal by a stored replica of the appropriate Gold code contained within its local memory, and then integrates, or lowpass filters, 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 The correlation output during the tracking phase may be viewed as the GPS signal with the pseudorandom code removed, or, in common terminology, “despread.” This signal is narrow band, with bandwidth commensurate with a 50 bit per second binary phase shift keyed data signal which is superimposed on the GPS waveform.
The correlation acquisition process is very time consuming, especially if received signals are weak. To improve acquisition time, many GPS receivers utilize a multiplicity of correlators (up to 12 typically) which allows a parallel search for correlation peaks.
Another approach to improve acquisition time is described in U.S. Pat. No. 4,445,118, referred to as the “Taylor patent.” This approach uses the transmission of Doppler information from a control basestation to a remote GPS receiver unit in order to aid in GPS signal acquisition. While this approach does improve acquisition time, the Doppler information is transmitted from a basestation to a mobile GPS receiver by a point to point transmission system, and there is no indication of how this Doppler information is obtained.
An approach for improving the accuracy of the position determination by a remote GPS receiver unit is also described in the Taylor patent. In the Taylor patent, a stable frequency reference is transmitted to a remote GPS receiver unit from a basestation in order to eliminate a source of error due to a poor quality local oscillator at the remote GPS receiver unit. This method uses a special frequency shift keyed (FSK) signal that must be situated in frequency very close to the GPS signal frequency. As shown in
FIG. 4
of the Taylor patent, the special FSK signal is about 20 MHz below the 1575 MHz GPS signal which is also received by the receiver in order to demodulate the GPS satellite signals from the GPS satellites so as to extract satellite position data Moreover, the approach described in the Taylor patent uses a common mode rejection mechanism in which any error in the local oscillator (shown as L.O.
52
) of the receiver will appear in both the GPS channel and the reference channel and hence be canceled out. There is no attempt to detect or measure this error. This approach is sometimes referred to as a homodyne operation. While this approach provides some advantages, it requires that the two channels be closely matched, including closely matched in frequency. Moreover, this approach requires that both frequencies remain fixed, so frequency hopping or frequency tuning (channelization) techniques are not compatible with this approach.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a method is described for reducing processing time due to Doppler error in a satellite positioning system (SPS) receiver having a cell based communication receiver. The method includes determining an approximate location of the SPS receiver from a cell based information source. This approximate location is determined by using at least one of a location of a cellular service area which includes a cell site which is capable of communicating with the cell based communication receiver or a location of the cell site itself. The method further includes determining an approximate Doppler for at least one SPS satellite relative to the SPS receiver, where the approximate Doppler is based upon the approximate location. This approximate Doppler is used in the SPS receiver to reduce processing time in either determining at least one pseudorange to the at least one SPS satellite, or in acquiring signals from the at least one SPS satellite.
An exemplary embodiment of this method is a cellular telephone which includes a GPS receiver. The cellular telephone operates by communicating with cell sites, each of which are connected to a cellular switching center. A database, which represents a cellular based information source, may be maintained at the cellular switching center or at the cell site or at a remote processing station, which may be termed a “server,” may be used to determine an approximate location of the cellular telephone based upon the cell site (or cellular service area) with which the cellular telephone is communicating. This approximate location may then be used to derive an approximate Doppler relative to the various SPS satellites which are transmitting SPS signals to the GPS receiver in the cellular telephone. This approximate Doppler is then transmitted in one embodiment from the cell site to the cellular telephone,
Blakely & Sokoloff, Taylor & Zafman
Blum Theodore M.
SnapTrack, Inc.
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