1. Field of the Invention
The invention relates to a GNSS receiver and positioning method that receive a signal from an orbit satellite for a global navigation satellite system (GNSS) for positioning.
2. Description of the Related Art
Satellite navigation (GNSS) is a navigation system in which three navigation satellites (GNSS orbit satellites) (hereinafter, referred to as “GNSS satellites”) are captured from an airplane to acquire distances from the respective GNSS satellites and then time is set using a signal from the fourth navigation satellite to thereby make it possible to obtain a three-dimensional flying position of the airplane. The satellite navigation includes a global positioning system (GPS), a GALILEO, and the like.
For example, a GNSS receiver is equipped for a mobile unit to measure the location and speed of the mobile unit. For example, the GNSS receiver receives radio waves from a plurality of GNSS satellites to calculate respective distances (pseudo-ranges) from the plurality of GNSS satellites to the GNSS receiver to thereby position the mobile unit equipped with the GNSS receiver on the basis of the calculated pseudo-ranges. Signals emitted from the GNSS satellites reach the GNSS receiver with a delay of a period of time during which a radio wave propagates the distance between each of the GNSS satellites and the GNSS receiver. Thus, when a period of time required for radio wave propagation is obtained for the plurality of GNSS satellites, the location of the GNSS receiver may be obtained through positioning calculation. For example, a pseudo-range calculation unit of the GNSS receiver uses radio waves emitted from the plurality of GNSS satellites to obtain a pseudo-range from each GNSS satellite to the GNSS receiver. Then, a positioning calculation unit obtains the location of the GNSS receiver on the basis of the pseudo-ranges obtained by the pseudo-range calculation unit.
The GNSS receiver captures a GNSS satellite and then examines the correlation between the signal received from the GNSS satellite and a C/A code replica signal to thereby detect a correlation peak. For example, the correlation peak between the C/A code replica signal and the signal received from the GNSS satellite is obtained by adjusting the phase of the C/A code replica signal. The GNSS receiver obtains a pseudo-range between the GNSS satellite and the GNSS receiver from a phase delay of the correlation peak. The location of the GNSS receiver is obtained on the basis of the pseudo-range.
However, even when the GNSS receiver has succeeded in capturing a GNSS satellite, the GNSS receiver may receive not only a direct wave from the GNSS satellite but also a radio wave reflected or diffracted from an architecture, such as a tall building. A phenomenon that a radio wave transmitted from a GNSS satellite is reflected or diffracted and is received through a plurality of propagation paths is called multipath. The influence of multipath causes an error in pseudo-range between the GNSS receiver and the GNSS satellite. An error in pseudo-range causes a positioning error.
One of causes of a positioning error in the GNSS receiver is the influence of multipath. One of methods for reducing the influence of multipath may be a multipath error reduction technique, such as narrow-correlator. An error in pseudo-range may be reduced using the multipath error reduction technique. Because an error in pseudo-range may be reduced, a positioning error may reduced.
FIG. 1 shows an example of the GNSS receiver.
A radio wave from a GNSS satellite is input to a high-frequency processing unit 2 through an antenna. The high-frequency processing unit 2 processes a high-frequency analog signal input through the antenna. A satellite capturing unit 4 captures the GNSS satellite on the basis of the signal processed by the high-frequency processing unit 2. A correlator unit 6 examines the correlation between the signal received from the GNSS satellite captured by the satellite capturing unit 4 and the C/A code replica signal to thereby detect a correlation peak.
FIG. 2 shows an example of processing performed by the correlator unit 6. In FIG. 2, the abscissa axis represents a chip, and the ordinate axis represents a signal intensity level of a correlation value.
For example, in the correlator, tracking is performed so that the width between an early (E) and a late (L) (hereinafter, referred to as “spacing”) is 1 chip and the difference in signal intensity level between the correlation values is zero. In FIG. 2, phase control is performed so that the signal intensity levels (indicated by 0.5E and 0.5L) of the correlation values at both ends of the 1-chip spacing are equal to each other. The middle of the spacing is a tracking point. The phase of the C/A code replica signal is adjusted to maximize the tracking point to thereby obtain a maximum value P (referred to as “correlation peak”) of the tracking point. Because of the influence of multipath, the maximum value of the tracking point may deviate to the early side or the late side.
In order to reduce the influence of multipath, after the tracking, the spacing is narrowed, and the signal intensity levels (indicated by NE and NL) of correlation values at chips that are further close to the chip corresponding to the correlation peak obtained by the one-ship spacing are obtained. Phase control is performed so that NE is equal to NL. The middle of the spacing is a tracking point. The phase of the CIA code replica signal is adjusted to maximize the tracking point to thereby obtain a correlation peak.
A pseudo-range calculation unit 8 obtains a pseudo-range between the GNSS receiver and the GNSS satellite on the basis of a phase delay of the correlation peak detected when the spacing is narrowed by the correlator unit 6.
When the spacing is narrowed and then the signal intensity levels of correlation values at chips that are further close to the chip corresponding to the correlation peak obtained by the 1-chip spacing, variations in correlation values through phase control are small because NE and NL are close to the correlation peak When the level of a signal of a radio wave received from the GNSS satellite decreases and, therefore, the influence of noise increases, the received signal level of noise may possibly be higher than that of NE or NL because of small variations in correlation values through phase control. When the received signal level of noise is higher than that of NE or NL, it is impossible to obtain correlation values at chips that are further close to the chip corresponding to the correlation peak obtained by the 1-chip spacing although the spacing is narrowed. When correlation values cannot be obtained, the tracking GNSS satellite may be lost. When satellite tracking is lost, it is necessary to perform processing from satellite capturing again, and there is a possibility that positioning cannot be performed during the satellite capturing. Because positioning cannot be performed during then, the rate of positioning decreases.