JP2008051567A - Satellite navigation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a noise component intermingled into a pseudo-distance true value, in a satellite navigation device for receiving navigation signals having a plurality of frequencies transmitted from an artificial satellite, and acquiring the pseudo-distance true value based on each pseudo-distance measured with the plurality of frequencies. <P>SOLUTION: An ionosphere delay itself of the L1 frequency is calculated based on each pseudo-distance of L1 and L2 relative to both frequencies of the L1 frequency and L2 frequency from a navigation satellite of GPS, and only the calculated ionosphere delay is smoothed, and then used for calculation of the pseudo-distance true value. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention uses a distance to a navigation satellite (artificial satellite) orbiting the earth and information on the orbit of the navigation satellite to determine the position, velocity, etc. of an object on the earth (hereinafter referred to as “user”). Using the Global Navigation Satellite System (GNSS) represented by the US Global Positioning System (GPS), the European Galileo, the Russian Globalorbiting Navigation Satellite System (GLONASS), etc. The present invention relates to a satellite navigation device capable of positioning.

  In GNSS, information such as transmission timing and orbit information is transmitted as a navigation signal using a spread spectrum modulated carrier wave from a number of navigation satellites launched in orbit around the earth. On the user side, the navigation signal is received by the satellite navigation device. While detecting the reception timing of the navigation signal, the satellite navigation device despreads the received navigation signal and extracts information such as transmission timing and orbit information from the resulting signal.

  The satellite navigation apparatus obtains the difference between the transmission timing included in the extracted information and the detected reception timing, that is, the propagation time of the navigation signal from the navigation satellite to the satellite navigation apparatus. The propagation time obtained in this way represents the distance between the navigation satellite and the satellite navigation device. Since this distance includes an error, that is, a clock offset, which is possessed by a clock mounted on a satellite navigation apparatus, which will be described later, it is particularly called a pseudorange.

  On the other hand, the information extracted from the received signal includes orbit information. Therefore, by using the obtained pseudo distance and the extracted orbit information, a user-existing sphere centered on the position of the navigation satellite can be defined. Under a virtual environment in which the detected pseudo-range does not include an error, if three types of user-existing spheres can be defined, the three-dimensional position of the user can be uniquely determined by calculating the intersection of the user-existing spheres. It can be obtained without error.

  In order to define three types of user-existing spheres, it is necessary to receive navigation signals from at least three navigation satellites. In the actual use environment, the navigation signal is received from one navigation satellite in order to eliminate the error of the clock (receiver clock) installed in the satellite navigation system compared to the GNSS system clock, that is, the clock offset. It must be possible. That is, navigation signals are received from a total of at least four navigation satellites, and based on these, a total of four user-existing spheres are defined, and four unknowns including a three-dimensional position of the user and a clock offset total four elements. It is necessary to perform positioning operation to solve simultaneous equations. In applications where the altitude of the user can be regarded as constant, the position of the user may be obtained two-dimensionally by positioning calculation with three unknowns.

  Furthermore, the following error components also exist under the actual use environment. First, navigation signals are delayed as they pass through the ionosphere surrounding the earth. Next, since the navigation signal passes through the atmosphere, if the speed of light in vacuum is used when converting the propagation time to distance, an error occurs due to the difference between the speed of light in vacuum and the speed of light in the atmosphere.

  In order to suppress the error due to atmospheric propagation among these errors, GPS assumes an approximate model related to atmospheric propagation and uses a technique of converting the propagation time to that during all-path vacuum propagation. In order to suppress errors due to ionospheric delay, GPS uses the following first and second methods.

  The first is a method used by a user who uses only a single frequency. In GPS, civilian signals and military signals are serviced. Conventionally, consumer signals are transmitted only at a frequency of 1575.42 MHz called L1 carrier, so all civilian users have a single It was a user who used only the frequency.

  Such a user assumes an approximate model related to the ionospheric delay, employs a calculation method in which the amount of delay due to the ionosphere is obtained by calculation according to the approximate model, and the amount of delay is subtracted from the pseudorange. In the GPS, parameters necessary for approximate model calculation are further transmitted from the navigation satellite as a part of the navigation signal. This approximate model is shown in Non-Patent Document 1, and the delay amount calculation method is shown in Non-Patent Document 2.

  The problem with this first method is that the accuracy of ionospheric delay estimation using an approximate model is low. First, the update period of the parameter transmitted from the satellite navigation for calculating the approximate model is a long period of one day, and does not sufficiently follow the state change in the short period of the ionosphere (<< daily period). Furthermore, the accuracy of the approximate model itself is not high. For this reason, even if the received parameter is used, the ionospheric delay cannot be obtained with sufficiently high accuracy, and the accuracy of about ± 50% of the true value is unavoidable at present.

In general, considering that the maximum value of the ionospheric delay is about 50 ns, the calculation using the above approximate model has a delay detection error of about 25 ns, that is, a pseudo distance of 25 ns × 3 × 10 ⁸ m / s≈8 m. An error will occur. In a standard situation where the number of visible satellites, that is, the number of navigation satellites that can be seen from the current position of the user is 6-8, the HDOP (Horizontal Dilution of Precision) value is 1-2. Therefore, a value obtained by multiplying the error of the pseudo distance by the HDOP, that is, a horizontal error becomes a large value of 8 to 16 m.

  The horizontal direction error here refers to a plane in which the user's current position is in contact with the spherical surface centered on the earth and the user's current position on the surface of the error in the user position measurement result. The component on the tangent plane at the person's position.

  The second is a method used by a user who can use a plurality of frequencies. Such a user can remove the ionospheric delay by a technique called ionosphere-free linear combination described below.

The ionospheric delay d (t) of the GPS signal is expressed by the following formula 1. Here, the ionospheric delay d (t) is a physical quantity converted into a dimension of distance by multiplying the delay time by the speed of light.
d (t) = k (t) / f ² −−− (1)
Here, t: measurement time, k (t): proportional constant, f: carrier frequency.
From Equation 1, it can be seen that the ionospheric delay d (t) is inversely proportional to the square of the frequency f.

Since the pseudorange true value r (t) is the same value at any frequency, for example, the first and second values measured at two frequencies of the GPS L1 frequency (1575.42 MHz) and the L2 frequency (1222.70 MHz). The pseudo distances r1 (t) and r2 (t) are expressed by the following formulas 2 and 3. Here, for the sake of simplicity, it is assumed that errors other than ionospheric delay are completely corrected.
r1 (t) = r (t) + d1 (t) = r (t) + k (t) / (f1) ^2- (2)
r2 (t) = r (t) + d2 (t) = r (t) + k (t) / (f2) ^2- (3)
Here, r1 (t): pseudo distance of L1, r2 (t): pseudo distance of L2, d1 (t): ionospheric delay of L1, d2 (t): ionospheric delay of L2, f1: L1 frequency (1575. 42 MHz), f2: L2 frequency (122.60 MHz)

The proportionality constant k (t) varies depending on the ionospheric location and time, but if the two frequencies f1 and f2 transmitted from the same navigation satellite are measured simultaneously, the proportionality constant k (t) is common and can be canceled. . Therefore, the pseudorange true value r (t) from which the ionospheric delay is removed can be obtained by using the first and second pseudoranges r1 (t) and r2 (t) as shown in the following Expression 4.
r (t) = {r2 (t) −γ · r1 (t)} / (1-γ) −−− (4)
Here, γ: (f1 / f2) ² .

  The above is the outline of the ionosphere-free linear combination. Non-Patent Document 3 shows the ionosphere-free linear combination.

Conventionally, in GPS, only military signals are transmitted at two frequencies, the L1 frequency and the L2 frequency. Therefore, users who can use a plurality of frequencies are military personnel. However, since the block IIR-M satellite launched in September 2005, consumer signals will be transmitted at two frequencies, L1 frequency and L2 frequency in the future, so that even civilian users will be ionosphere-free linear It will be possible to perform the binding.
Bradford W. Parkinson, James J .; Spilker Jr. , Global Positioning System: Theory and Applications, Vol. 1, American Institute of Aeronautics and Astronautics, Inc. , 1996, pp. 485-515 Space and Missile Systems Center (SMC), Navstar GPS Joint Program Office (SMC / GP), Navstar G1oba1 Positioning System ID in SCI-DAR200. 122-125 Prapap Misra, Per Eng, Global Positioning System Signals, Measurements, and Performance, Ganga-Jamuna Press, 2001, pp. 141-142

  The problem with this second method is that the noise components contained in the pseudorange true value r (t) obtained by ionosphere-free linear combination are originally pseudoranges r1 (t) and r2 (t) of the L1 frequency and L2 frequency. The noise component included in the signal is increased to about 3 times. The reason is as follows.

The pseudorange true value r (t) obtained by ionosphere-free linear combination is expressed by the above-described equation 4. From the frequency of L1 (1575.42 MHz) and the frequency of L2 (1222.70 MHz), γ = (f1 / f2) ² = (1575.42 / 12227.60) ² ≈1.647.

Therefore, the pseudo-range true value r (t) is expressed by the following Expression 5.
r (t) = {r2 (t) −γ · r1 (t)} / (1-γ)
= 2.546r1 (t) -1.546r2 (t) --- (5)

Assuming that the noise included in the pseudorange r1 (t) of L1 and the pseudorange r2 (t) of L2 is white noise, and the standard deviation thereof is 1, the noise included in the pseudorange true value r (t) Is {(2.546) ² + (1.546) ² } ^1/2 ≈3. The noise included in the pseudorange true value r (t) is about three times the noise component originally included in the pseudoranges r1 (t) and r2 (t) of the L1 frequency and the L2 frequency.

  The present invention has been made in view of the above points, receives a navigation signal of a plurality of frequencies transmitted from an artificial satellite, and obtains a pseudorange true value based on each pseudorange measured at a plurality of frequencies. An object of the satellite navigation apparatus is to reduce a noise component mixed in a pseudo range true value.

The satellite navigation apparatus according to claim 1 is a satellite navigation apparatus that receives navigation signals from a plurality of carriers having different frequencies transmitted from an artificial satellite,
First code pseudorange calculating means for calculating a first code pseudorange by measuring a phase of a first modulation code superimposed on a carrier having a first frequency from the artificial satellite;
Second code pseudorange calculation means for calculating a second code pseudorange by measuring a phase of a second modulation code superimposed on a second frequency carrier wave from the artificial satellite;
First ionospheric delay calculating means for calculating an ionospheric delay at a first frequency from the first code pseudorange and the second code pseudorange;
Ionospheric delay smoothing means for smoothing the ionospheric delay at the first frequency and calculating a smoothed first ionospheric delay;
Code pseudorange correction means for correcting the first code pseudorange with a smoothed first ionosphere delay and calculating a code pseudorange true value obtained by removing the ionosphere delay;
It is provided with.

A satellite navigation device according to claim 2 is the satellite navigation device according to claim 1,
The first ionospheric delay calculating means calculates an ionospheric delay at the first frequency by the following equation (6):
d1 (t) = {r2 (t) −r1 (t)} / (γ−1) −−− (6)
Where d1 (t): ionospheric delay at the first frequency, r1 (t): first code pseudorange, r2 (t): second code pseudorange, γ: (first frequency / second frequency) ²
The code pseudo distance correcting means calculates the code pseudo distance true value by the following equation (7): r (t) = r1 (t) −sd1 (t) −−− (7)
However, r (t): code pseudorange true value, sd1 (t): smoothed first ionospheric delay

  According to the satellite navigation apparatus of the present invention, the ionospheric delay itself of the L1 frequency or the L2 frequency is calculated based on the pseudoranges measured for both the L1 frequency and the L2 frequency, and only the calculated ionospheric delay is smoothed. After that, it is used to calculate the pseudorange true value. Since noise in the ionospheric delay term is reduced by this smoothing, the noise component mixed in the pseudorange true value is slightly increased from the noise component originally included in the pseudorange of the L1 frequency (or L2 frequency). do not do. Furthermore, the first code pseudorange (or second code pseudorange) used for calculating the pseudorange true value together with the ionospheric delay is not smoothed, so that even if the user is a moving object, the pseudorange changes with the movement. Does not occur.

  In the above description, the GPS L1 frequency and L2 frequency are used. However, since the two frequencies used in the present invention are not limited to the GPS L1 frequency and L2 frequency, broadcasting is planned from a satellite to be launched in the future. By using the L5 frequency (1176.45 MHz), the same effect can be obtained by combining the L1 frequency and the L5 frequency or the combination of the L2 frequency and the L5 frequency.

  Furthermore, E1, E5, and E6 frequencies planned for broadcasting from GALILEO, a satellite navigation system planned in Europe, L1, L2, and L5 frequencies planned for broadcasting from a quasi-zenith satellite system planned in Japan, The same effect can be obtained when applied to the L1 and L2 frequencies broadcast from GLONASS, a satellite navigation system operated by Russia.

  Hereinafter, an embodiment for carrying out the present invention will be described using GPS as an example. The satellite navigation apparatus of the present invention includes means for measuring the pseudoranges r1 (t) and r2 (t) at two frequencies, the L1 frequency and the L2 frequency, and the pseudoranges r1 (t) and r2 (t ) Is used to set the ionospheric delay d1 (t) at the L1 frequency to “d1 (t) = {r2 (t) −r1 (t)} / (γ−1)” (where γ is “the frequency of L1 / L2 Means for calculating the square of the frequency of), means for smoothing the ionospheric delay d1 (t) and obtaining the ionospheric delay sd1 (t) with reduced noise component, pseudorange r1 (t) of the L1 frequency and noise component And means for calculating the pseudorange true value r (t) by “r (t) = r1 (t) −sd1 (t)” using the ionospheric delay sd1 (t) with reduced.

  The satellite navigation apparatus of the present invention will be described with reference to the block diagram of the satellite navigation apparatus of FIG.

  First, the receiving antenna 1 receives GPS signals from a plurality of navigation satellites. The L1 frequency receiver 2 is a down converter that converts a GPS L1 frequency signal of 1575.42 MHz into an intermediate frequency of several MHz.

  Next, the transmission time measuring unit 3 despreads the PN code of the received signal, and tracks the PN code phase that provides the maximum correlation. A daily delay locked loop is usually used for this tracking. Since the GPS signal is modulated and transmitted with the PN code synchronized with the GPS system time, the PN code phase measured by the receiver represents the time when the signal is transmitted from the satellite.

  Therefore, the code phase measured at the reception time t of the receiver clock 4, that is, the difference between the transmission time and the reception time t is the propagation time from the satellite to the receiver. The pseudo distance calculation unit 5 calculates the propagation time and multiplies the speed of light to determine the propagation distance. Since this propagation distance includes an error that the receiver clock 4 has, that is, a clock offset, it is particularly called a pseudo distance. In this way, the pseudo distance r1 (t) of the L1 frequency signal is obtained.

  On the other hand, the GPS signal received by the receiving antenna 1 is also input to the L2 frequency receiving unit 6. The L2 frequency receiver 6 is a down converter that converts a GPS L2 frequency signal of 1227.60 MHz into an intermediate frequency of several MHz.

  Next, the transmission time measuring unit 7 despreads the PN code of the received signal, and tracks the PN code phase that provides the maximum correlation. Since the GPS signal is modulated and transmitted with a PN code synchronized with the GPS system time, the PN code phase measured by the receiver represents the time when the signal is transmitted from the satellite.

  Therefore, the code phase measured at the reception time t of the receiver clock 4, that is, the difference between the transmission time and the reception time t is the propagation time from the satellite to the receiver. The pseudo distance calculation unit 8 calculates the propagation time and multiplies the speed of light to determine the propagation distance. In this way, the pseudo distance r2 (t) of the L2 frequency signal is obtained.

  Next, the L1 ionosphere delay calculation unit 9 serving as the first ionosphere delay calculation means uses the pseudoranges r1 (t) and r2 (t) as follows and uses the proportional constant k (t) and the ionosphere delay at the L1 frequency as follows. d1 (t) is obtained.

The proportionality constant k (t) is obtained by the following equation 8 based on the equations 2 and 3 regarding the pseudo distances r1 (t) and r2 (t).
k (t) = (f1) ² · (f2) ² {r2 (t) −r1 (t)} / {(f1) ² − (f2) ² } ---- (8)

Substituting this proportionality constant k (t) into Equation 2 of the pseudorange r1 (t), the ionospheric delay d1 (t) at the L1 frequency is obtained by the following Equation 9.
d1 (t) = {r2 (t) −r1 (t)} / (γ−1) −−− (9)
Here, γ: (f1 / f2) ² .

Next, the smoothing unit 10 serving as ionospheric delay smoothing means smoothes the ionospheric delay d1 (t) at the L1 frequency to obtain a smoothed L1 ionospheric delay sd1 (t). The smoothing method in the smoothing unit 10 can be realized by the following formula 10 when, for example, parameters such as formula 8 and formula 9 are measured discretely.
sd1 (k) = {1-1 / (1 + T)} sd1 (k−1) + {1 / (1 + T)} d1 (k)
--- (10)
Here, d1 (k): kth ionospheric delay, sd1 (k): kth smoothed ionospheric delay, and T: smoothing time constant.

Next, the pseudo distance true value calculation unit 11 which is a code pseudo distance correction means obtains a pseudo distance true value r (t) in which an error due to noise is suppressed as in the following Expression 11.
r (t) = r1 (t) -sd1 (t) --- (11)

  Thus, in order to obtain the pseudorange true value r (t), the pseudorange r1 (t) of the L1 frequency signal is used without being smoothed, and only the ionospheric delay d1 (t) at the L1 frequency is smoothed. Used.

  This is because, when the user is a moving body, the pseudo distance r1 (t) to each satellite changes as the user moves. When the pseudo distance r1 (t) is smoothed, a delay occurs in the pseudo distance change accompanying the movement, and if the positioning calculation is performed using the delayed pseudo distance, a delay also occurs in the positioning position of the user. On the other hand, since the ionospheric delay d1 (t) changes very slowly as compared with the change in the pseudo distance r1 (t) due to the movement of the user, the delay due to smoothing hardly becomes a problem. For example, even when the user is moving violently, it is the pseudorange r1 (t) that changes drastically and the ionospheric delay d1 (t) changes very slowly, so the ionospheric delay d1 (t) is smoothed. The pseudorange r1 (t) due to the movement of the user can be correctly observed even if the change is made.

  In addition, if it is an application for observing very slow movement, such as a crustal deformation monitoring post, the pseudorange r1 (t) may be smoothed together with the ionospheric delay d1 (t). Noise components can be reduced.

  The above processing in the transmission time measurement unit 3 to the pseudorange true value calculation unit 11 is performed on a plurality of satellite signals in a plurality of signal processing channels, and the obtained pseudorange true value r (t) of the plurality of channels is obtained. The positioning calculation unit 12 is used to determine the position of the user.

  In the embodiment described above, the pseudorange true value r (t) is obtained using the pseudorange r1 (t) and the ionospheric delay d1 (t) at the L1 frequency. Instead, the pseudorange at the L2 frequency is obtained. The pseudorange true value r (t) may be obtained using r2 (t) and the ionospheric delay d2 (t).

  Further, although the GPS L1 frequency and the L2 frequency are used, the two frequencies used in the present invention are not limited to the GPS L1 frequency and the L2 frequency. Therefore, the L5 frequency (1176) planned to be broadcast from a satellite to be launched in the future. .45 MHz), the intended effect of the present invention can be obtained even with a combination of the L1 frequency and the L5 frequency or the L2 frequency and the L5 frequency.

  In addition, the E1, E5, and E6 frequencies planned for broadcasting from the GALILEO system, which is a satellite navigation system planned in Europe, and the L1, L2, and L5 frequencies planned for broadcasting from the quasi-zenith satellite system planned in Japan. It goes without saying that the intended effect of the present invention can be obtained even when applied to the L1 and L2 frequencies broadcast from GLONASS, which is a satellite navigation system operated by Russia.

1 is an overall configuration diagram of a satellite navigation apparatus according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Antenna 2 L1 frequency receiving part 3 Transmission time measuring part 4 Receiver clock 5 Pseudo distance calculating part 6 L2 frequency receiving part 7 Transmission time measuring part 8 Pseudo distance calculating part 9 L1 ionospheric delay calculating part 10 Smoothing part 11 Pseudo distance true Value calculation unit 12 Position calculation unit

Claims (2)

A satellite navigation device for receiving navigation signals from a plurality of carriers having different frequencies transmitted from an artificial satellite,
First code pseudorange calculating means for calculating a first code pseudorange by measuring a phase of a first modulation code superimposed on a carrier having a first frequency from the artificial satellite;
Second code pseudorange calculation means for calculating a second code pseudorange by measuring a phase of a second modulation code superimposed on a second frequency carrier wave from the artificial satellite;
First ionospheric delay calculating means for calculating an ionospheric delay at a first frequency from the first code pseudorange and the second code pseudorange;
Ionospheric delay smoothing means for smoothing the ionospheric delay at the first frequency and calculating a smoothed first ionospheric delay;
Code pseudorange correction means for correcting the first code pseudorange with a smoothed first ionosphere delay and calculating a code pseudorange true value obtained by removing the ionosphere delay;
A satellite navigation apparatus characterized by comprising: The first ionospheric delay calculating means calculates the ionospheric delay at the first frequency by the following equation:
d1 (t) = {r2 (t) −r1 (t)} / (γ−1)
Where d1 (t): ionospheric delay at the first frequency, r1 (t): first code pseudorange, r2 (t): second code pseudorange, γ: (first frequency / second frequency) ²
The code pseudo distance correcting means calculates a code pseudo distance true value by the following equation: r (t) = r1 (t) −sd1 (t)
However, r (t): Code pseudorange true value, sd1 (t): Smoothed first ionospheric delay The satellite navigation device according to claim 1, wherein:

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