Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-06-10
2001-11-06
Pihulic, Daniel T. (Department: 3642)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
Reexamination Certificate
active
06313789
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to receivers which are employed for the determination of the user location with the use of the signals received from many navigation system satellites.
BACKGROUND OF THE INVENTION
Multichannel receivers for radio signals which are encoded by pseudo-random (PR) code signals are employed by consumers using global satellite navigation systems—for example, GPS (“NAVSTAR”) and GLN (“GLONASS”) systems. These systems enable one to determine the receiver's location, its velocity vector, and to also correct errors in the receiver's time clock (in other words, to make the time scale of the receiver more precisely synchronized to the GPS time).
Such a receiver receives signals simultaneously from many satellites. Each of these satellite radiates signals in two bands: the L
1
band and the L
2
band. Two carrier signals are simultaneously transmitted in the L
1
-band; both carrier signals have the same frequency, but are shifted in phase by &pgr;/2 (90°). The first L
1
carrier signal is modulated by the clear acquisition C/A-code signal and the second L
1
carrier signal is modulated by the precision P-code signal. One carrier signal is transmitted in the L
2
band, but using a different frequency than the L
1
carrier signals. The L
2
carrier signal is modulated by the same P-code signal used to modulate the second L
1
carrier signal. Each C/A-code signal and P-code signal comprises a repeating sequence of segments, or “chips”, where each chip is of a predetermined time period (&Dgr;) and has pre-selected value, which is either +1 or −1. The segment values follow a pseudo-random pattern, and thus the C/A-codes and the P-codes are called pseudo-random code signals, or PR-code signals. Additionally, before each C/A-code signal and P-code signal is modulated onto its respective carrier signal, each code signal is modulated by a low frequency (50 Hz) information signal (so-called information symbols).
In the receiver, the time delay between transmission from the satellite and reception by the receiver is measured for each of the incoming satellite signals. The delays are measured relative to a time clock within the receiver, which is based upon a main reference oscillator. Typically, the time base of all of the clock signals, reference carrier signals, and reference code signals generated by the receiver is derived from this main reference oscillator, and each of these signals drifts with any drift in the main reference oscillator. While the receiver's main reference oscillator and time clock are relatively precise, they are usually not as precise as the satellite's clock and reference oscillator and, accordingly, they drift in time with respect to the satellite's clock and oscillator.
For each satellite, the delay is measured with the aid of several time scales present within the received satellite signal. There is the low-frequency information signal, which provides the least precise timing information, and then there is the more precise time scale in the delay measurement of the C/A-code signal, and the even more precise time scale in the delay measurement of the P-code signal. Additionally, the delay can be measured by using the phase of the satellite's carrier signal, which can provide more precise measurements than the P-code signal. Carrier-phase measurements have a small ambiguity interval (the carrier frequency period), but provide higher precision, and are of importance in differential navigation applications. With the exception of the low-frequency information signal, the accuracy of each of the time scales is affected by the drift in the oscillation frequency of the main reference oscillator.
A typical receiver has several individual tracking channels, each channel for tracking one satellite signal, as received by one antenna. Each tracking channel measures the delay of one PR-code signal within the satellite signal (e.g., C/A-code or P-code signal) and usually also the phase of the satellite's carrier signal. A typical tracking channel may comprise a Delay-Lock Loop (DLL) circuit for tracking the delay of the PR-code, and a Phase-Lock Loop (PLL) circuit for tracking the phase of the satellite's carrier signal.
The processing of the received signal in each individual tracking channel of the receiver comprises the steps of sequential multiplying the received signal (as already passed through the input and filter circuits and down converter) with a reference carrier signal and a reference code signal, both of which are generated within the tracking channel, and then accumulating (or integrating) the resulting multiplication products. The reference carrier signal corresponds to the received carrier signal for the given satellite, and the reference code signal corresponds to the corresponding PR-code signal of the same satellite. Devices that carry out the two multiplication steps and the subsequent accumulation step are called correlators, and the corresponding process is called the correlation of the input signal with the reference signals. Each tracking channel generates three or more correlation signals, each such signal being generated in a respective correlator, which correlates the input signal with a respective reference code signal and the reference carrier signal for the channel. At least two of the reference code signals used in a channel are different from one another (in form and/or by phase shift). However, each of the channel's reference code signals has a range of high correlation with the satellite code signal being tracked. Also, for GPS satellite signals, each of channel's reference code signals has substantially no correlation with the code signals of other satellites; this enables a receiver channel to distinguish the satellites from one another. In the GLN system the same C/A-code is used, but each satellite has a different carrier frequency, which enables a receiver channel to distinguish the satellites from one another.
There are many different structures for the individual tracking channel known in the prior art. Most typical is a structure which uses a coherently-generated reference carrier signal, three correlators, a Delay-Lock Loop (DLL) circuit, and a Phase-Lock Loop (PLL) circuit. The reference carrier signal may be a sinusoid, a square wave, a triangular wave, or other periodic waveform. The PLL circuit relies on the output of one or more of the correlators and controls the generation of the reference carrier signal so that the carrier signal is synchronized to the satellite's carrier signal (i.e., coherent). This action of synchronizing the reference carrier signal to the satellite's carrier signal by controlling the generation of the reference signal is referred to as the tracking action of the PLL circuit. Typically, the PLL circuit adjusts the phase of the reference carrier signal in order to reach synchronization. The phases of the satellite carrier signal and the reference carrier signal are the same when the two are synchronized, and are approximately the same when the phase of reference carrier signal is being adjusted to reach synchronization. (By approximately, we mean that the phases are within &pgr; of one another when the low-frequency information signal is not present, and within &pgr;/2 of one another when the low-frequency information signal is present; typically, the phases are within 10% of a cycle.) The phase difference between the signals may be measured and defined by the distance between corresponding zero-crossings.
As a result of synchronizing the phases of the two signals, their average frequencies coincide. When the phases are being adjusted to reach synchronization, the average frequencies are approximately the same, typically being within 10% of one another. When we speak of the frequency of the satellite's carrier signal in the context of PLL synchronization and tracking, we mean the frequency that the carrier signal has when it is presented to the PLL circuit, which is typically (but not nece
Ashjaee Javad
Zhodzishsky Mark
Coudert Brothers
Pihulic Daniel T.
Topcon Positioning Systems, Inc.
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