Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
2000-01-06
2002-05-21
Arthur, Gertrude (Department: 3661)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C342S357490, C342S357490, C701S207000, C701S213000, C701S214000
Reexamination Certificate
active
06392590
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to positioning devices used to determine the position of a user depending on a signal from a satellite.
2. Description of the Background Art
In recent years, a global positioning system or GPS which can receive a signal from an orbital satellite to determine the position of a user on the terrestrial ground with high precision, has been put to practical use. This GPS employs a GPS receiver simultaneously receiving signals transmitted from multiple GPS satellites, to detect a position.
A GPS satellite multiplies data to be transmitted (of 50 bps), referred to as a navigation message, by a pseudo random code (of 1.023 MHz, having a period of 1 ms) allotted uniquely to the GPS satellite and sends it via a carrier (a carrier wave of approximately 1.5 GHz) to transmit a signal. While the carrier's frequency is common to all GPS satellites, data to be transmitted is each spectrum-spread by a unique pseudo random code and thus do not interfere with each other.
A GPS receiver simultaneously receives signals transmitted from multiple GPS satellites and spectrum-despreads a signal from each GPS satellite to extract a navigation message. “Despread” herein means demodulation in a spread spectrum system. In general, up to 8 to 16 satellites can be simultaneously subjected to such signal processing. In spectrum-despreading, a received signal is multiplied by a carrier wave and a pseudo random code identical to those used in the multiplication for the transmission in the GPS generation and the resultant multiplication is integrated over a predetermined period of time to derive a correlation value. It should be noted that the integration time above is approximately one period of the pseudo random code (1 ms).
A pseudo random code and a carrier used in the multiplication to despread a spectrum each have an uncertainty, as described below:
(1) the phase of the pseudo random code
(2) the frequency of the carrier
if a pseudo random code generated in a receiver is offset in phase from that multiplied in the received signal, the receiver cannot extract a navigation message from the carrier. Since it is difficult to predict a phase of a pseudo random code, in general an uncertain region corresponds to the entire phase of the pseudo random code.
The uncertainty of a carrier frequency is attributed to two factors, i.e., the Doppler effect resulting from the movement of a GPS satellite and a frequency error of an oscillator internal to a receiver. The effect of the Doppler effect on a carrier frequency can reach as high as 5 kHz. However, the magnitude thereof can be predicted to reduce any uncertain region to less than 5 kHz. The error of the internal oscillator significantly depends on the characteristics of the oscillator. If an oscillator with temperature compensation is used, the error is on the order of at most 1 kHz. Otherwise, the effect on a carrier frequency can reach as high as approximately 100 kHz.
Accordingly, in despreading a spectrum in a GPS receiver a two-dimensional, uncertain region resulting from the above two uncertainties must be entirely searched until a navigation message is obtained. The region is searched by obtaining a correlation value for each searching point determined depending on a specific pseudo random code phase and a specific carrier frequency, and comparing the obtained correlation value with a preset threshold value.
The interval between searching points must be no more than twice a maximal offset acceptable for capturing a signal, and it is approximately 0.5 &mgr;sec for a pseudo random code phase and approximately 1 kHz for a carrier frequency. Thus if an uncertain region of a carrier frequency is 10 kHz, the number of searching points is represented by the following expression:
((1 ms/0.5 &mgr;sec))×(10 kHz/1 kHz))=20,000
Thus when an integration time of 1 msec is provided at one searching point searching the entirety of the uncertain region requires approximately 20 seconds.
It should be noted, however, that if there is a significant error in an oscillation frequency (a local oscillation frequency) of an internal oscillator and an uncertain region of a carrier frequency expands, then the time required for searching the region is accordingly increased.
A method employing Fast Foulier Transform (FFT) allows rapid search when an inexpensive oscillator without temperature compensation is used and a significant error is introduced in a local oscillation frequency. FFT allows a true carrier frequency to be estimated among a range of frequencies.
More specifically, correlation values obtained through the integration are stored and used as an input to perform a FFT. It should be noted that the number of the correlation values stored is equal to the point count of the FFT performed. If a pseudo random code is in phase and the difference between a true carrier frequency and an internal carrier's frequency (a local oscillation frequency) falls within a range searched via the FFT, the frequency corresponding to the difference has a peak and the true carrier frequency can thus be estimated.
Important factors in a method employing such FFT are the frequency range which the FFT can search, and the precision with which the FFT can search the frequency. A range of frequencies which can be searched is the same as a sampling frequency and is widen as an integration time required to obtain one correlation value is reduced.
A searching precision is obtained by dividing a sample frequency by a point count and is thus enhanced as the point count increases. As has been described above, carrier frequency searching requires a precision of approximately 1 kHz. Accordingly the time corresponding to a single period thereof or 1 msec divided by a point count may be adopted as an integration time to obtain one correlation value.
For example, a point count of 32 provides an integration time of approximately 31 &mgr;sec and FFT can search a frequency range of 32 kHz at one time.
The range that can be searched is widen if the point count is further increased. Because of a characteristic of a pseudo random code, however, with too short an integration time a correct correlation cannot be obtained and the sensitivity can thus be deteriorated.
FIG. 6
is a schematic block diagram showing a configuration of a conventional GPS receiver, particularly of a carrier search unit
2000
disposed to search a carrier.
In the figure, a downconversion unit (not shown) downconverts a received signal to a signal of approximately several MHz. The downconverted signal is then sampled by an A-D converter (not shown) to provide a 2-bit digital signal which is input to a signal input port
10
.
A local carrier oscillator
20
generates an in-phase carrier and an quadrate carrier which have a designated frequency and are offset in-phase from each other by 90°. The carriers are generated with a precision of two bits. Carrier multipliers
31
and
32
multiply an input signal by the in-phase and quadrate carriers, respectively. Carrier multipliers
31
and
32
multiply the 2-bit input signal by the in-phase or quadrate carrier and output a 4-bit signal.
A code generator
40
generates a pseudo random code corresponding to a GPS satellite. Code multipliers
51
and
52
multiply the outputs from carrier multipliers
31
and
32
, respectively, by the pseudo random code from code generator
40
.
Integrators
61
and
62
receive outputs from code multipliers
51
and
52
, respectively, and integrate them, respectively, over a predetermined period of time.
A memory
70
stores integrals obtained via integrators
61
and
62
.
A frequency difference calculation unit
80
follows the procedure described below to perform an FFT depending on the integrals stored in memory
70
to obtain a frequency difference between a carrier included in an input signal and a local carrier generated by local carrier generator
20
.
Carrier search unit
90
uses the frequency difference from frequency difference calculation unit
Arthur Gertrude
Mitsubishi Denki & Kabushiki Kaisha
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