JP2003139843A - Gps receiving apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a GPS receiving apparatus in which a CN threshold is changed adaptively so as to be matched to the usage environment of the GPS receiving apparatus and in which a signal and disturbance waves can be always discriminated irrespective of the installation environment of a GPS antenna. SOLUTION: The maximum value of the received CN of a signal from each receivable satellite at a time when a satisfactory GPS reception is obtained is stored, and the mean value of the stored maximum value is found so as to be a CN- max at a stage in which the satisfactory GPS reception is continued for a prescribed time. By referring to a table in which an optimum CN threshold is made to correspond in each reception CN, a CN threshold with reference to the CN- max is acquired. The update of the CN- max and the update of the CN threshold are performed continuously, and a satellite to be used for a positioning operation is decided by using the obtained CN threshold.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a GPS receiver, and more particularly, to identification of an interference wave in signal reception by the GPS receiver.

[0002]

2. Description of the Related Art A GPS receiver is a device called L1
Positioning is performed by receiving from the satellite a GPS signal in which a C / A code is mounted as a ranging signal on a 5.42 MHz carrier wave. C /
The A code is a PRN (Pseudo Random Noise) code, and different series of codes are assigned to individual satellites. The GPS receiver can demodulate the GPS signal from the target satellite by using the code unique to each satellite. A GPS receiver generally has a plurality of signal processing units (channels) that demodulate GPS signals, and can simultaneously capture a plurality of satellites and use them for positioning.

In the GPS receiver, multipath and cross-correlation are reception obstacles to be considered in the process from satellite reception to acquisition. The multipath causes an error in the measurement of the distance (pseudorange) between the receiving device and the satellite, and deteriorates the positioning solution accuracy. In addition, SA (Selective Av
availability has been released, the ratio of multipath to the error factors of GPS positioning has increased.

In the PRN code used in GPS,
The cross correlation of different sequences is very low. Ideally, the GPS signal of the satellite cannot be demodulated unless it is the sequence of the target satellite. However, the actual cross-correlation of the C / A code is much smaller than the auto-correlation, but cannot be regarded as zero. Although very rare, when two satellites of different series have Doppler frequencies that are very close to each other, cross-correlation may cause incorrect satellite acquisition, resulting in deterioration of positioning solution accuracy.

In order to eliminate such an influence on positioning solution accuracy due to multipath and cross-correlation, a CN (signal to noise power ratio) threshold is provided. That is,
Since the multipath is a reflected wave, the signal strength is lower than that of the direct wave signal, and therefore the reception CN (CN of the received signal) is low. Therefore, a threshold value is set in the reception CN and used to distinguish it from the direct wave signal. The CN threshold is used in this case. If the received CN is below the CN threshold, the signal is not tracked or the satellite signal is not used for positioning calculation. As a result, it is possible to avoid the influence of multipath on the positioning solution accuracy.

At the time of system design, the CN threshold is determined as a value that is a few tens of dB lower than the reception CN (indicated by reference numeral 81 in FIG. 6) in the best possible condition, and the GPS receiving apparatus has it. , The determined CN threshold is preset. FIG. 6 shows the relationship between the CN threshold and the received CN when GPS reception is good. In FIG. 6, the vertical axis represents the received CN level [d
BHz]. In FIG. 6, a signal refers to a direct wave received from a target satellite, and an interfering wave is a different PRN code that is erroneously demodulated due to multipath reflected by a building or a vehicle or cross-correlation. Is pointing to the signal. By using the CN threshold (reference numeral 91) set as described above, the CN level (reference numeral 61) of the signal becomes higher than the CN threshold, and the CN level of the interference wave (reference numeral 71) becomes equal to or lower than the CN threshold. In the GPS receiver, a fixed value determined at the time of system design is used as the CN threshold.

[0007]

However, the GPS
The receiving sensitivity of the antenna varies depending on the installation conditions represented below. -Difference in antenna installation position (on the roof / inside the car) -Difference in vehicle type-Distance and angle to the front and rear glass surfaces (when installed inside the car) -Type of antenna and cable length-With other in-vehicle devices (When installed in the passenger compartment) For example, when the antenna is installed on the roof, the signal is attenuated and the reception CN is low when the antenna is installed on the dashboard. In addition, LNA (Low
The received CN may become high depending on the specifications of the Noise Amplifier) and the cable length. Therefore, even with the same GPS receiving device, the relationship between the CN threshold and the receiving CN may be different from that in the case of FIG.

FIG. 7 shows the relationship between the CN threshold and the reception CN in an environment where the GPS receiver has high reception sensitivity. As the reception sensitivity increases, the reception CN of both the direct wave signal and the interfering wave from the target satellite increases. At this time, since the same value (reference numeral 91) as in the case of FIG. 6 is used as the CN threshold, a part of the disturbing wave becomes higher than the CN threshold, and there is a case where the signal and the disturbing wave cannot be distinguished. If the interfering wave is erroneously used for positioning, the positioning solution accuracy will deteriorate.

In order to avoid this, it is necessary to set the CN threshold higher. However, in such a case, when the antenna is installed on the dashboard, the reception sensitivity is lowered, which causes an adverse effect. Low elevation satellite reception,
When the signal reception CN falls below the CN threshold due to attenuation due to a roadside tree or the like, a signal that can originally be used for positioning cannot be used for positioning, and the positioning solution accuracy deteriorates or becomes non-positioning. .

The present invention has been made in view of such circumstances. That is, the present invention adaptively changes the CN threshold according to the usage environment of the GPS receiver,
An object of the present invention is to provide a GPS receiving device capable of always accurately distinguishing a signal from an interfering wave regardless of the installation environment of the S antenna.

[0011]

In order to achieve the above object, a GPS receiver is provided with a threshold value determining means for determining a CN threshold value based on a received CN of a signal being received (claim 1). This makes it possible to adaptively determine the CN threshold according to the installation environment.

The maximum value of the reception CN of the signal being received (hereinafter,
It is appropriate to determine the optimum CN threshold by using CN_max) (claim 2). Acquire CN_max of the signal being received multiple times, find the average value, and calculate it.
It can be CN_max (claim 3).

More preferably, the CN_max is acquired only when a predetermined condition corresponding to good GPS reception is satisfied (claim 4). Good GPS reception can be determined, for example, on the condition that the number of satellites that can receive GPS signals is a predetermined number or more (claim 5).

CN_max is continuously calculated to obtain CN_max.
By continuously updating the threshold, the positioning solution accuracy can be always kept high (claim 6).

The CN threshold can be determined by subtracting a predetermined value from CN_max (claim 7). For example, a fixed value (for example, a dozen dB) may be subtracted from CN_max.

Alternatively, the CN threshold may be obtained by preparing a CN table in which an optimum CN threshold is assigned for each CN_max and referring to this CN table (claim 8).

It should be noted that CN_max can be used as a default value until the CN threshold is determined by initializing it to a default value when the GPS receiver is manufactured or in a program area. 9).

According to a tenth aspect of the present invention, there is provided a GPS receiving device which receives GPS signals from a plurality of satellites and performs positioning, and when the number of satellites that can be captured is a predetermined number or more, the satellites that can be captured are transmitted. First storage means for storing the maximum value among the received CNs of GPS signals, calculation means for calculating an average CN value as an average value among the maximum values stored in the first storage means, and an assumed average CN value Second storage means for storing a table in which optimum threshold values are associated with each other, and a threshold value corresponding to the average CN value calculated by the calculation means are obtained by referring to the table stored in the second storage means. And a control means for updating the threshold value. The control means can always update the threshold value to an optimum value. In addition, this GPS receiver has updated thresholds and received G
A satellite determination means for determining the satellite used for positioning is provided by comparing the received CN of the PS signal. Since the satellite determination unit can determine the satellite by using the threshold value that is optimally maintained according to the usage environment, it is possible to always perform the positioning by eliminating the interference wave regardless of the usage environment.

[0019]

1 is a block diagram showing the configuration of a GPS receiving apparatus 1 as an embodiment of the present invention. G
The PS receiver 1 includes an RF unit 2, a digital correlation processing unit 3,
It is composed of the positioning calculation unit 4. The RF unit 2 includes a reference clock 24, a synthesizer 23, a down converter 21, and an A / D converter 22. The RF unit 2 converts the signal received by the GPS antenna 5 into an intermediate frequency signal and then digitizes it to obtain a digital correlation processing unit 3 Output to. The digital correlation processing unit 3 is a functional block for despreading the spread spectrum GPS signal (carrier correlation unit 3
1, PRN code correlation unit 32, carrier NCO 33, P
A plurality of RN code generators 34 and codes NCO 35) are provided corresponding to the number of channels. The digital correlation processing unit 3 performs a correlation process with a carrier wave and a correlation process with a PRN code. The carrier tracking loop 42 and the code tracking loop 41 allow a carrier NCO (NCO; Numerically Controlled Oscillator) 33 and a code NCO 35 in acquisition (tracking) and tracking (tracking) of GPS signals.
Are controlled respectively.

The positioning calculation unit 4 calculates a positioning solution such as a position, a speed, and an azimuth based on the code generation timing of the PRN code generation unit 34 and the phase change amount of the carrier NCO 33 in each channel. It is assumed that the positioning calculation for obtaining the positioning solution is executed at fixed time intervals, here, every 1 second. The obtained positioning solution is the input / output data 43.
Can be passed to the host system as. The positioning calculation unit 4 is composed of a processor, and is connected to a ROM, a RAM, a non-volatile memory and the like (not shown) necessary for executing the program.

The positioning calculation unit 4 executes the following processing in parallel with the positioning calculation processing.・ Sequence for obtaining the maximum value CN_max of the received CN (Fig. 2) ・ Sequence for obtaining CN threshold from CN_max (Fig. 3) Then, the CN threshold determined by the above processing is within the satellite search sequence in the process of positioning calculation. , Used in the GPS signal selection process (FIG. 5).

Referring to the flowchart of FIG. 2, CN_max
The sequence for obtaining is described. In FIG.
CNbuf [i] is a buffer that stores the maximum value of the received CN in the positioning calculation process every second. Counter i is
It is a counter that measures the duration of good GPS reception, and is initialized to zero immediately after the system starts operating, such as when the power is turned on. This sequence is executed as a subroutine each time the positioning calculation is executed, that is, every one second.

First, it is determined whether good GPS reception is obtained (S11). Here, the criterion that the number of satellites used for positioning is five or more is used as a criterion for determining whether or not good GPS reception is performed. If this criterion is not satisfied (S11: NO), the counter i is initialized to 0 and the sequence is exited (S13, S1).
8). When the criterion is satisfied (S11: YES), 1 is added to the counter i (S12). In addition, the highest value among the received CNs of the satellites used for positioning is stored in the buffer Cnbu.
It is recorded in f [i] (S14).

Next, the elapsed time of good GPS reception is cut off (S15). Here, it is determined whether 10 seconds (i = 10) have been reached. If it has reached 10 seconds (S15: Y
ES), a buffer Cnbuf [10: 1] that records the maximum received CN
The average value is taken as CN_max. Then, the sequence is ended by initializing i = 0. If the condition of step S15 is not satisfied (S15: NO), the sequence ends without doing anything (S18). By the above operation, GP
The maximum value of the reception CN adapted to the S reception environment can be obtained and updated.

The CN threshold is determined from the CN_max obtained by the above operation. A flowchart of this operation is shown in FIG. That is, here, each CN_max stored in the non-volatile memory in advance is stored.
And the optimal CN threshold for it are referred to to determine the CN threshold (S21). In addition, in step S21, instead of referring to the table to determine the CN threshold,
For example, the CN threshold may be calculated by, for example, setting the CN threshold lower than CN_max by a predetermined numerical value.

The correspondence between CN_max and the CN threshold can be stored as a table in the non-volatile memory. At that time, it is efficient to obtain a representative value CN_max_cent of CN_max divided into an appropriate range and store the correspondence between CN_max_cent and the CN threshold as a table. FIG. 4A is a graph for obtaining a representative value of CN_max. The representative value of quantized CN_max is CN_max_cen
required as t. FIG. 4B is an example of an optimum CN threshold table corresponding to CN_max_cent.

According to the sequence of FIG. 3 explained above,
An optimal CN threshold corresponding to CN_max can be obtained. Then, the CN threshold is used in the process of positioning to select the received GPS signal. This selection is made as part of the satellite search sequence. Figure 5
Represents a satellite search sequence that includes selection of GPS signals using the CN threshold.

First, the satellite to be searched, that is, PRN is determined and listed from 1 to N (S31). GP
If you have a rough position and time of the S receiver itself,
Since the navigation data that is saved shows which direction each satellite is in, we will list them in descending order of elevation angle. When the position and time of the GPS receiver are not available, the PRNs are searched in numerical order. Step S3
The search is performed in the loop starting from 2. First, the frequency stability of the oscillator circuit used for the reference clock 24 is estimated from the temperature and the frequency shift at the time of the previous positioning (S3).
3). Next, the Doppler frequency of the j-th satellite to be searched is estimated from the moving speed of the GPS receiver, navigation data, the search situation, etc. (S34), and the estimated Doppler frequency is searched (S35).

As a result of the search, if the acquisition is successful (S36: YES), the reception CN of the captured signal is obtained (S37). If acquisition failed, j
Is incremented by 1 (S41), the process returns to the beginning of the loop and the next P
Perform a search for RN. Received CN in step S37
Then, the calculated reception CN and the CN threshold are compared (S38). As a result, if the received CN of the received satellite exceeds the CN threshold (S3
8: YES), the signal of the satellite is tracked and used for positioning (S39). On the other hand, if the received CN is equal to or lower than the CN threshold (S38: NO), the satellite signal is not tracked, that is, not used for positioning (S40), 1 is added to j (S41), and the loop Returning to the head, the next PRN is searched.

After adding 1 to j in step S41, it is determined whether or not all satellites listed are processed (S42). If j does not exceed N, the process has not been completed for all the listed satellites (S42: NO), and the process returns to the beginning of the loop to perform the process for the next PRN. When the processing is completed for all the listed satellites (S42: YES), the loop ends. Then, if tracking is performed for all satellites in the list (S43: YES), this sequence ends (S44). If there is a satellite for which the acquisition is not successful (S43: NO), the process returns to step S32 and the loop is executed again.

The satellite search sequence described here is CN
_max is obtained in sequence (FIG. 2) or CN threshold is obtained in sequence (FIG. 3), and the positioning calculation process is performed in parallel, and the data of the tracked satellites are used for sequential positioning. Therefore, during the period when the CN threshold is not determined, the default CN threshold stored in advance in the nonvolatile memory is used.
This default value is decided at the time of design or production,
It is used when the GPS receiver is initialized under predetermined conditions.

Various modifications can be made to the embodiment of the present invention described above. For example, with respect to the sequence (FIG. 2) for obtaining the maximum value of the reception CN, various modified examples for obtaining the maximum value of the reception CN can be considered. The criterion of 5 or more satellites used for positioning, which is a criterion for determining whether the GPS reception is good in step S11, uses another number or another parameter (DOP or a parameter related thereto). Etc. can be replaced by other criteria.

[0033]

As described above, the GPS receiver of the present invention adaptively changes the CN threshold according to the installation environment, and always accurately distinguishes the signal and the interference wave regardless of the installation environment. It is possible to always select the most suitable satellite for positioning. Positioning solution accuracy can be improved regardless of the installation environment.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a GPS receiving device as an embodiment of the present invention.

FIG. 2 is a reception CN executed by the GPS reception device of FIG.
3 is a flowchart of a process for obtaining the maximum value of

FIG. 3 is a reception CN executed by the GPS reception device of FIG.
7 is a flowchart of a process for obtaining a CN threshold from the maximum value of

FIG. 4 is a diagram illustrating an example of a table that associates a maximum value of a received CN with a CN threshold.

FIG. 5 depicts a satellite search sequence that includes selection of GPS signals using CN threshold.

FIG. 6 is a diagram showing a relationship between a CN threshold and a reception CN when GPS reception is good.

FIG. 7 shows C in an environment where the GPS receiver has high reception sensitivity.
It is a figure which shows the relationship between N threshold and reception CN.

[Explanation of symbols] 1 GPS receiver 5 GPS antenna 2 RF section 3 Digital correlation processor 4 Positioning calculation section 21 Down Converter 22 A / D converter 31 Carrier correlation unit 32 PRN code correlation unit

Claims (10)

[Claims] 1. A GPS receiver that receives GPS signals from a plurality of satellites and performs positioning, and determines whether or not to use for positioning calculation based on the received CN of GPS signals from each satellite. Receive C to do
Threshold value determining means for determining a threshold value for N, the determined threshold value, and reception C of the received GPS signal
A GPS receiving device comprising: a satellite determining unit that determines a satellite to be used for positioning by comparing with N. 2. The threshold value determining means obtains the maximum value of the received CNs of GPS signals from the respective satellites received,
The GPS receiver according to claim 1, wherein the threshold value is determined based on the acquired maximum value. 3. The threshold value determining means performs the operation of acquiring the maximum value of the GPS signals from each satellite being received a plurality of times, and calculates the average value of the acquired maximum values. Determining the threshold value based on the average value,
The GPS receiver according to claim 2, characterized in that. 4. The method according to claim 2, wherein the threshold value determining unit acquires the maximum value only when a predetermined condition corresponding to good GPS reception is satisfied. GPS receiver. 5. The predetermined condition is that the number of satellites that can receive GPS signals is a predetermined number or more.
The GPS receiver according to claim 4, characterized in that. 6. The threshold value determining means obtains the maximum value or an average value of the plurality of maximum values, and determines the threshold value based on the maximum value or the average value of the plurality of maximum values. The GPS receiver according to any one of claims 2 to 5, wherein the threshold value is continuously updated by continuously executing. 7. The threshold value determining means determines the threshold value by subtracting a predetermined value from the maximum value or an average value of the plurality of maximum values. The GPS receiver according to the item. 8. The storage device further includes a storage unit that stores in advance a table in which an optimum threshold value is associated with each reception CN, and the threshold value determination unit refers to the table stored in the storage unit to determine the threshold value. The GPS receiver according to any one of claims 1 to 6, wherein 9. A default value as the threshold is further stored in advance in the storage means, and the satellite determination means sets the threshold as the threshold if the threshold has not been determined by the threshold determination means.
The GPS receiving device according to any one of claims 1 to 8, wherein the default value stored in the storage means is used. 10. A GPS receiving apparatus for receiving GPS signals from a plurality of satellites and performing positioning, wherein when there are a predetermined number or more of the satellites that can be captured, a CN for receiving GPS signals from the satellites that can be captured is provided. First storage means for storing the maximum value, calculation means for calculating an average CN value as an average value among the maximum values stored in the first storage means, and an optimum threshold value for each assumed average CN value And a threshold value corresponding to the average CN value calculated by the calculation section, obtained by referring to the table stored in the second storage section. Control means for updating the received GPS signal, the updated threshold value, and reception C of the received GPS signal
A GPS receiving device comprising: a satellite determining unit that determines a satellite to be used for positioning by comparing with N.