Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-01-06
2002-12-10
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S373000
Reexamination Certificate
active
06493378
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to receivers employed for range measurements using radio-signals encoded by a pseudo-random code, and more particularly to such receivers which reduce the reception errors caused by reflected versions of the directly received signal (i.e., multipath error reduction).
BACKGROUND OF THE INVENTION
Multichannel receivers for receiving broadcasted radio-signals encoded by repeating pseudo-random (PR) codes are employed by consumers using the global satellite navigation systems GPS (“NAVSTAR”) and GLN (“GLONASS”). These systems enable one to determine the receiver's location and its velocity vector, and also to determine the time of day. The present invention is applicable to these types of receivers, and to other types of receivers and demodulators which receive signals which are encoded by repeating PR-codes. The signals to be demodulated may be broadcast by antenna, may be transmitted through wire or optical fiber, or may otherwise be transmitted by known transmission means.
In the GPS and GLN systems, the receiver simultaneously receives several broadcasted signals from several corresponding satellites which are “visible” to the receiver (as used herein, the term “visible” means that the satellite broadcast signal is not blocked from the receiver by a physical object, such as a building or the Earth). At any one time, 5 to 11 of the 24 GPS satellites are visible to the receiver. Every GPS satellite broadcasts signals in two frequency bands: the L
1
band and the L
2
band. In the L
1
band, two radio-signal carriers are simultaneously broadcasted, each carrier having the same frequency but being phase shifted in time with respect to one another by &pgr;/2 (90°), which is one-quarter (¼) of the carrier wavelength. One of these carriers is modulated by a clear acquisition code (C/A-code) and the other carrier is modulated by a precision code (P-code). Both of these codes are pseudo-random sequences of +1 and −1 values, each sequence having a predetermined length and being continuously repeated. In the L
2
band, a single radio-signal carrier is modulated by the above-described P-code, but the frequency of the L
2
-carrier is substantially lower than that of the L
1
-carrier. This frequency difference enables one to account for the changing transmission characteristics of the ionosphere. The present invention is applicable to the reception of either or both of the C/A-code and the P-code (in either band). The reception of the C/A-code will be used herein to illustrate the present invention.
In the GPS system, each satellite is assigned its own unique C/A-code, which enables the receiver to distinguish between the satellites even though all the C/A-codes are being modulated onto carriers having substantially the same frequency. Each repeating sequence of a C/A-code is quantized into 1,023 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 sequence repeats itself every millisecond. When the C/A code is modulated onto the carrier, it is often called the C/A-signal. The particular set of chip values for each sequence is unique to the satellite, and the cross-correlation of each C/A-code sequence with those of the other satellites is substantially zero in comparison with the cross-correlation of the signal with itself (auto-correlation). This last feature applies as well to the corresponding C/A-signals and it enables a GPS receiver to isolate any one of the C/A-signals from the others being received by correlating the input signal (after appropriate down-conversion) with the C/A-signal to be isolated.
In order to provide useful timing and positional information, the C/A-signals in some of the satellite-broadcast bands are modulated by a low-frequency (50 Hz), binary-coded information signal A(t) which is transmitted as a repeating sequence of 1,500 bits. Each bit of A(t) has a value of −1 or +1, is synchronized to start with the beginning of a C/A-code sequence in the C/A-signal, and spans 20 sequences of the C/A-code. The C/A-signal is so modulated by multiplying its C/A-code sequence values by either −1 or +1, depending upon the current bit value of the information signal A(t). In the receiver, the information signal A(t) can be demodulated by correlating the broadcasted C/A-signal with an internally generated version of the C/A-code to provide a correlation signal I(t), and by noting the sign changes in the correlation signal I(t) (assuming that both C/A-signals are substantially synchronized). As a fine technical point, we note that correlation signals in the GPS art, such as I(t), are usually not continuous functions of time since they are often generated by accumulating multiplication products over fixed time durations (T
A
), with these time durations following one another consecutively. As a result, typical correlation functions are discrete time signals (with update frequency of f
A
=1/T
A
). To denote such a discrete time signal, we will use the time indicator symbol “t
K
” as the signal's time argument, where the subscript “K” indicate the discrete nature of the time, such as in the form: I(t
K
).
For the general benefit of the reader, the following differences between the GPS and GLN systems are noted:
1) In the GPS system, the signals from all the GPS satellites are transmitted on the same carrier frequency (1575.42 MHz in the L
1
band, 1227.60 MHz in the L
2
band), but each has its own unique PR-code. In the GLN system, the signals from different satellites have different carrier frequencies, but have the same C/A-codes and P-codes.
2) In the GPS system, the “chips” in the C/A-codes are clocked out at a rate of 1.023 MHz (for a chip period &Dgr;=977.52 ns), whereas the chips for the C/A-code in the GLN system are clocked out at one-half that rate, which is 0.511 MHz (for a chip period &Dgr;=1956.95 ns).
3) In both systems, the chips for the P-codes are clocked out at a rate which is ten-times that for the clocking rate of their C/A-codes. Specifically, the P-code clocking rate is 10.23 MHz (&Dgr;=97.75 ns) in the GPS system and 5.110 MHz (&Dgr;=195.69 ns) in the GLN system.
4) In the GPS system, the P-code is additionally modulated by W-code whose sequence structure is unknown to civilian users. The GLN system has no similar modulation.
In conventional receivers, the broadcast signal from the satellites are received, and down-converted by an appropriate means to provide the desired C/A-codes (or P-codes), usually in a digital form. The down-converted signal bearing all of the received C/A-codes is then provided to one or more individual tracking channels, each of which is intended to isolate one of the satellite C/A-codes from the others and track it. The C/A-code to be tracked by a specific tracking channel is not only obscured by the C/A-codes of other satellites, but by noise signals received by the receiver and noise generated within the electronics of the receiver. In order to isolate the desired C/A-code for the others and from the noise, the down-converted signal is correlated against a replica of the desired C/A-code as indicated above. The reference replica is phase-shifted in time to a point where the correlation of the two signals is at a maximum. The correlation may be performed by multiplying the values of the two signals together at periodic intervals, and then accumulating multiplication products to form the correlation signal I(t
K
). Devices which perform this operation are called correlators, and may be implemented by hardware, software (i.e., DSP processor) or a combination of both. In typical implementations, the correlator accumulates the multiplication products for a fixed period of time, which is sometimes referred to as the accumulation period or accumulation window. After each accumulation period, the correlator is reset to a value of zero. Typical accumulation periods for GPS/GLN applications are 1-ms, 5-ms, and
Ashjaee Javad
Veitsel Victor Abramovich
Zhodzishsky Mark Isaakovich
Chin Stephen
Coudert Brothers LLP
Kim Kevin
Topcon GPS LLC
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