JPH05281330A - Method for acquiring radio wave from satellite of gps receiver - Google Patents

Abstract

PURPOSE:To quickly catch a plurality of satellites even when a low-accuracy oscillator is used by resetting the searching center frequencies of the satellites acquired at and after the second time by using an offset value calculated from the frequency actually received from the first-caught satellite. CONSTITUTION:By setting an initial searching band to a wide band, the searching center frequencies fs1-fs3 of satellites to be acquired through channels CH1-CH3 are respectively set by using the Doppler frequency deviations fd1-fd3 of the satellites and a correcting value 4a. Then the satellites are searched by shifting the oscillation frequency of an oscillator upward and downward one step by one step by using the cap challenge of the oscillator as one-step width. In case the first satellite is caught through the channel CH1 at time t1, searching of the other satellites is again continued by calculating the accurate offset value DELTAfosc of a transmitter from the frequency fr1 received from the satellite and resetting the searching center frequencies fs2-fs3 of the channels CH2 and CH3 through which the other satellites are not acquired by using the offset value DELTAfosc as a new correcting value alpha, and then, narrowing the searching band width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a satellite radio wave capturing method for a GPS receiver. GPS (Global Positioning Syst)
em: Global Positioning System) is a positioning system that uses artificial satellites and has an altitude of about 2020.
Set 6 orbits around the earth above 0 km, launch a plurality of satellites in each orbit, and transmit GPS signals including navigation data for positioning from each satellite to the earth by spread spectrum method. By receiving the radio waves of three or four satellites required for positioning with a GPS receiver on the ground (or sea or air), the GP from each satellite is received.
Using the navigation data contained in the S signal, the latitude of the receiving point,
It is designed so that necessary location information such as longitude and altitude can be calculated in real time.

[0002]

2. Description of the Related Art GPS signals open to the private sector are C /
It is a signal called A code and is transmitted at 1575.42 MHz. Therefore, in principle, in order to capture the satellite radio wave, it is sufficient to wait for reception at this 1575.42 MHz. However, since GPS satellites are not geostationary satellites, the reception frequency fluctuates due to the Doppler effect,
Further, the oscillator of the GPS receiver has its oscillation frequency fluctuated due to the offset.

Therefore, in order to actually capture the satellite radio waves, the search center frequency of the satellite radio waves is shifted by the offset of the received frequency due to the Doppler effect (hereinafter referred to as "Doppler frequency excursion") and the offset of the oscillator. Is set, the satellite radio wave is searched while gradually increasing the search bandwidth starting from this search center frequency, and the acquisition of the satellite is completed when the oscillator locks to the satellite radio wave. By repeating such a capturing operation, the number of satellites required for positioning is captured.

In the satellite radio wave capturing operation, the closer the search center frequency is to the actual reception frequency of the satellite radio wave, the faster the time for capturing the satellite. The main factors for setting the search center frequency are the above-mentioned Doppler frequency deviation and the offset value of the oscillator. The Doppler frequency excursion is a phenomenon inevitably caused by the orbital motion of the satellite, and its maximum variation is about ± 6 [KHz] (usually about ± 3 [KHz]). Therefore, in order to set the search center frequency closer to the actual reception frequency, it is desirable to minimize the error in the offset value of the oscillator of the GPS receiver.

Therefore, in the conventional GPS receiver, a crystal oscillator with temperature compensation (hereinafter, referred to as "TCXO") having extremely high accuracy is used as an oscillator. This TCX
O compensates for the temperature characteristics of the crystal unit in an electric circuit manner, so that it is about ± 2 to ± 5 [ppm] (± 3 in terms of frequency) over a wide temperature range of −40 ° C. to + 85 ° C.
It gives an extremely high accuracy of about [KHz] to ± 8 [KHz]. The accuracy of an oscillator used in a normal communication device is usually about ± 50 to ± 100 [ppm], and at best is about ± 20 [ppm].

[0006]

Since the conventional GPS receiver uses the highly accurate TCXO, the value obtained at the previous positioning as the offset value of the oscillator of the GPS receiver is stored in the memory in the receiver. I backed it up and read it out the next time I started capturing satellite radio waves. However, when trying to use a low-precision oscillator,
Since the change in the offset value is large, the backup value obtained in the previous positioning cannot be used as it is, and it is necessary to search a fairly wide frequency range to capture the satellite, which causes a problem that it takes time to capture. It was

Further, in the conventional GPS receiver, a highly accurate T
Since the CXO is used, the change with time of the offset value is ignored, and the search for other satellites is continued without updating the offset value even after the first satellite is acquired. Therefore, if the initially set offset value changes greatly for some reason, even if it is a highly accurate TCXO, the satellite can be captured unless a wide frequency range is searched. Therefore, it takes a considerable time to acquire a plurality of satellites required for positioning.

Further, the TCXO used in the conventional GPS receiver is much more expensive than the low-precision crystal oscillator used in ordinary communication equipment, and this is GP.
This had a great influence on the product cost of the S receiver.

The present invention has been made based on the above circumstances, and it is an object of the present invention to shorten the acquisition time of a plurality of satellites necessary for positioning as a whole even when a low-precision oscillator is used. It is to provide a satellite radio wave capturing method for a GPS receiver.

[0010]

In order to achieve the above object, the present invention sets a search center frequency for each acquisition satellite and swings the oscillation frequency of an oscillator up and down with the set search center frequency as a starting point. However, in the satellite radio wave capturing method of the GPS receiver that captures the radio wave of the target satellite, the oscillator of the GPS receiver is obtained by subtracting the Doppler frequency deviation of the satellite from the actual reception frequency of the first captured satellite. Calculate the offset value of
The second and subsequent acquisition satellites are characterized in that the search center frequency is reset by using the calculated offset value.

[0011]

[Operation] FIG. 3 shows a satellite radio wave capturing method of the present invention.
It is explanatory drawing of the calculation method of the offset value of an oscillator. I
The reference reception frequency of the GPS signal is f₀(1575.4
2MHz), the actual received frequency of the first captured satellite
f_r, The Doppler frequency deviation of the first satellite captured
Δf_d, The offset value of the GPS receiver oscillator is Δf
_OSCThen, the relationship between these three is as shown in the figure.
Becomes

[0012] FIG 3 As is apparent from, accurate offset value Delta] f _OSC of the oscillator, the actual reception frequency f _r = Δf _OSC by subtracting the Doppler frequency deviation Delta] f _d of the captured satellite from f _r -.DELTA.f It can be obtained as _d −f ₀ .

In the above equation, the Doppler frequency deviation Δf _d is
It can be calculated from the satellite orbit data and the current value data of the GPS receiver. As the satellite orbit data, it is possible to use the almanac information obtained at the time of the previous coronation, and since the GPS receiver current position data can be an extremely rough value, this can also be obtained at the previous positioning. The determined positioning result can be used.

Therefore, when the accurate offset value Δf _{OSC of the} oscillator is calculated from the above equation, the search center frequency f _S of each satellite thereafter may be reset by the following equation. f _S = f ₀ + Δf _d + Δf _OSC where f ₀ : Reference reception frequency (1575.42 MHz) Δf _d : Doppler frequency deviation of the satellite to be captured

In this way, when the first satellite is acquired, the search center frequency f _s of each satellite is reset by using the obtained offset value Δf _OSC of the oscillator. and for that, so that the search center frequency f _s of each satellite is set to the actual value very close to the reception frequency f _r of the respective satellite. Therefore, each satellite can be captured in a short time, and the number of satellites required for positioning can be captured at high speed even when a conventional high-precision oscillator is not used.

Since an accurate offset value Δf _OSC has not yet been calculated until the first satellite is captured, the offset value obtained at the time of the last positioning, for example, is acquired until the first satellite is captured. Can be used as it is, or a wideband search can be performed with the offset value of the oscillator set to 0.

The error factors of the offset value Δf _OSC of the oscillator calculated as described above include the error of the current position of the GPS receiver and the error of the satellite orbit data. For GPS, even if the GPS receiver moves from Hokkaido to Kyushu, the calculation error is about 2 [KHz] and the accuracy is low (± 50 to ± 100).
It is sufficiently smaller than the offset error of the crystal oscillator (about [ppm]) (about ± 80 to ± 160 [KHz] in frequency conversion), and the error can be ignored as long as it is used in Japan.

Regarding errors caused by the satellite orbit data, if it is dealt with by not using the orbit data which is a little old, there will be no problem in most cases. Also,
Even if a large error occurs due to some reason, once you calculate with that data, if you repeat the calculation using the latest orbit data obtained from the acquisition satellite, when new orbit data is obtained The error can be corrected.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an explanatory diagram for switching search frequencies in an embodiment of the satellite radio wave capturing method of the present invention, and FIG. 2 is a flowchart of the operation. Note that this embodiment is shown in FIG.
As shown in (B), three channels CH1 to CH3
Is used as an example to search three satellites simultaneously in parallel. FIG. 1A shows a satellite search state for the channel CH2 of the three channels.

When the GPS receiver is turned on, satellites to be captured are assigned to each channel,
An initial search bandwidth for each satellite is set (step S1 in FIG. 2). As for this initial search bandwidth, the exact reception frequency position of the satellite is still unknown.
As shown in (A), it is set to a wide band.

Next, for each channel,
Search center frequencies f _{S1 to} f of each satellite to be captured
_S3 is set using the calculated Doppler frequency displacements Δf _{d1 to} Δf _d3 for each satellite and the correction value α prepared in advance (step S2). In this embodiment, the correction value α is 0
For example, in the case of channel CH2, as shown in FIG.
As shown in (A), the reference reception frequency f ₀ (1575.42MH
The initial search center frequency f _S2 is set at a position deviated from z) by the Doppler frequency deviation Δf _d2 of the capture satellite of the channel.

After setting the search bandwidth and the initial search center frequency as described above, the search for the satellites to be captured for each channel is simultaneously started in parallel (step S3). That is, as shown in FIG.
The S receiver has an oscillator capture range (eg 300
[Hz]) is set as one step width, and a satellite search is performed while oscillating the oscillation frequency up and down by one step.

At time t ₁ , any channel,
For example, it is assumed that the first satellite is captured on the channel CH1 (step S4). Now, let _{us say} that the actual reception frequency of the first satellite thus captured is _fr1 (see FIG. 1B).

When the first satellite is acquired, step S5
At, the GPS receiver checks whether or not the satellite orbit data is new, and if new, proceeds to step S6, and if old, proceeds to step S9. Further, in step S6, it is checked whether or not the user has the approximate current position data. If yes, the process proceeds to step S7. If not, the process proceeds to step S9.

If the orbit data of the satellite is new and has approximate current position data, in step S7, the accurate offset value Δf _OSC of the oscillator from the reception frequency f _{r1 of the} first captured satellite at that time point. Is calculated, and the calculated offset value Δf _OSC is set as a new correction value α.

Then, in step S8, the search center frequency f of the satellites of the other channels CH2 and CH3 that have not been acquired yet is calculated by using the new correction value α = Δf _{OSC.
Reset S3 and} f _S3 . For example, in the case of channel CH2, as shown in FIG. 1 (A), f ₀ + Δf _d2 + Δf _OSC
The new search center frequency f _S2 is reset at the position of.

After resetting the search center frequency,
In step S10, the search bandwidth is changed from the wide band to the narrow band, and the satellite search is continued again. For example, taking the case of the channel CH2 as an example, as shown in FIG. 1A, the search is continued with a predetermined narrow bandwidth centered on the newly set search center frequency f _S2 . In addition,
The search bandwidth is changed from a wide band to a narrow band because the search center frequency f _S2 reset as described above is a value very close to the actual reception frequency f _r2 (see FIG. 1B). Therefore, it is not necessary to search over a wide frequency range.

As described above, when the search center frequencies of the second and subsequent acquisition satellites are reset using the offset values of the oscillator calculated from the first acquisition satellite, the newly set respective search center frequencies are set. The satellite search center frequency is extremely close to the actual reception frequency, and each satellite can be acquired in a short time.

On the other hand, if the satellite orbit data is old in step S5, or if the current position data is not roughly present in step S6, the accurate offset value Δf _OSC of the oscillator cannot be calculated. Advances to step S9, the reception frequency f _r1 of the first captured satellite is set as new search center frequencies f _S2, f _S3 of other satellites, and the search bandwidth is changed to a narrow band in step S10. The satellite search is continued again.

When the offset error of the oscillator used is larger than the Doppler frequency displacement of the satellite in step S9, the Doppler frequency displacement Δf _d1 of the first captured satellite is set to the offset value Δ of the oscillator.
_Instead of f _OSC , f _r1 + Δf _d1 may be set as the new search center frequencies f _S2 and f _S3 of the respective satellites.

In the above-mentioned embodiment, the case where a plurality of satellites are simultaneously searched in parallel by using a plurality of channels has been described, but a so-called sequential search in which a plurality of satellites are sequentially searched by using one channel. It goes without saying that the same can be applied to the case.

[0032]

As is apparent from the above, according to the method of the present invention, the offset of the oscillator of the GPS receiver is subtracted by subtracting the Doppler frequency deviation of the satellite that was first captured from the actual reception frequency of the satellite. Values are calculated, and for the second and subsequent acquisition satellites, the search center frequencies are reset using the calculated offset values. Therefore, the search center frequencies of the second and subsequent acquisition satellites are set to actual reception frequencies. The positions can be set extremely close to each other, and the acquisition time of a plurality of satellites required for positioning can be shortened as a whole even when a low-precision oscillator is used. Moreover, since a low-precision oscillator can be used, the cost of the GPS receiver can be reduced.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of search frequency switching according to an embodiment of the present invention.

FIG. 2 is a flowchart of the operation of the above embodiment.

FIG. 3 is an explanatory diagram of a method of calculating an offset value of an oscillator in the method of the present invention.

[Explanation of symbols]

f ₀ Reference reception frequency (1575.42 MHz) f _S Acquisition center frequency of acquisition satellite f _r Actual reception frequency of acquisition satellite Δf _d Doppler frequency deviation of acquisition satellite Δf _OSC oscillator offset value

Claims (1)

[Claims] 1. GPS reception in which a search center frequency is set for each captured satellite, and the radio wave of the target satellite is captured while swinging the oscillation frequency of the oscillator up and down starting from the set search center frequency. In the satellite radio wave capturing method of the satellite, the offset value of the oscillator of the GPS receiver is calculated by subtracting the Doppler frequency deviation of the satellite that was first captured from the actual reception frequency of the satellite that was captured. A satellite radio wave capturing method for a GPS receiver, wherein the search center frequency is reset using the calculated offset value.