JPH05256925A - Initial-offset setting method for oscillator in gps receiver - Google Patents

Abstract

PURPOSE:To shorten the capturing time of a satellite by regarding the change in offset value of an oscillator as the function of the elapsed time from the OFF time of the power supply of a GPS receiver, and utilizing the offset value at the previous position measuring time. CONSTITUTION:When the power supply of a GPS receiver is turned OFF, the offset value of the oscillator is stored in a memory, and an internal clock starts the counting of time. When the power supply is turned ON, the OFF time from the previous position-measuring time is detected. For example, whether 30 minutes are exceeded or not is judged. When the OFF time is less than 30 minutes, the width is set narrowly with an initial-search band width Wo as the function of the OFF time. A central frequency for searching is obtained as the sum of a Doppler-frequency shift DELTAfd and a stored offset value fosc. With the fs as the center, the searching is performed by the up and down scanning at the width of Wo. When the satellite cannot be captured, the searching width is gradually expanded. When the OFF time exceeds 30 minutes, the initial searching width Wo is expanded to the entire range of the offset fluctuation, and the previous offset value is not used for the computation of the central frequency fs.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for setting an initial offset of an oscillator in a GPS receiver. GPS (Glo
bal Positioning System (Global Positioning System) is a positioning system using artificial satellites that orbits the earth above an altitude of approximately 20,200 km6.
Set one orbit, launch multiple satellites in each orbit,
A GPS signal containing navigation data for positioning is transmitted from each satellite to the earth using a spread spectrum method. Three or four GPS receivers on the ground (or sea or aviation) required for positioning. By receiving the radio waves from the satellites, necessary navigation information such as latitude, longitude, and altitude of the receiving point can be calculated in real time by using the navigation data included in the GPS signal from each satellite.

[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 order to capture the satellite, in principle, it is sufficient to wait for reception at this 1575.42 MHz. However, since the GPS satellite is not a geostationary satellite, its reception frequency fluctuates due to the Doppler effect, and the oscillator of the GPS receiver also fluctuates its oscillation frequency due to the offset.

Therefore, in order to actually acquire a satellite,
The deviation of the reception frequency due to the Doppler effect (hereinafter,
"Doppler frequency excursion") and the offset of the oscillator are shifted to set the search center frequency of the satellite radio wave, and the search bandwidth is gradually widened starting from this search center frequency to search for the satellite. The acquisition of the satellite is completed when locked. By repeating such a capturing operation, the number of satellites required for positioning is captured.

In the satellite capturing operation, the closer the search center frequency is to the actual reception frequency of the satellite radio wave, the faster the satellite capturing time becomes. 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 deviation is a phenomenon inevitably caused by the orbital motion of the satellite, and its value (up to ± 6 KHz) can be calculated in advance from the satellite orbital data and the approximate current position data. Therefore, to set the search center frequency as close as possible to the actual received frequency, use the GPS
It is desirable to minimize the error in the offset value of the receiver oscillator.

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]

The offset value of the oscillator can be calculated accurately after the accurate position of the receiver is known by positioning, but before that, the current actual offset value is unknown. Is. Therefore, in the conventional GPS receiver, the high-precision TCXO is used as described above, the offset value obtained at the end of the previous positioning is stored in the memory for backup, and the receiver is used for the next positioning. When the power supply of is turned on, the offset value that has been packed up is read and used as the offset value of the oscillator.

However, when a low-precision oscillator is used, the actual offset value is the same as the previous value when the receiver is powered on after the previous positioning is completed and the receiver is powered off. It is highly likely that the value at the time of positioning greatly changes, and the offset value backed up in the memory cannot be used as it is as the offset value of the oscillator for the next positioning. Therefore, when a low-precision oscillator is used, it is necessary to search the satellite radio wave in a wide band over the entire range of the offset change of the oscillator, and it takes time to capture the satellite.

The present invention has been made in view of the above circumstances. An object of the present invention is to effectively use the offset value at the time of the previous positioning backed up in the memory even when a low-precision oscillator is used. (EN) A method for setting an initial offset of an oscillator in a GPS receiver capable of promptly capturing satellites.

[0009]

In order to achieve the above object, an initial offset setting method of the present invention sets a search center frequency by using an offset value of an oscillator and a target Doppler frequency deviation of a satellite. Did the GPS
In the receiver, from the power-off of the GPS receiver in the previous positioning to the power-supply of the GPS receiver in the current positioning O
The elapsed time up to N is measured, and it is determined whether the offset value of the oscillator at the time of the previous positioning stored in the memory in the GPS receiver is used as the offset value of the oscillator at the time of the current positioning, depending on the length of the elapsed time. It was done like this.

[0010]

[Operation] The offset value of a low-precision oscillator changes greatly depending on the ambient temperature. For example, when the GPS receiver is powered on, the temperature of the entire receiver including the oscillator is usually high due to the heat generated by the circuit elements, etc., and the receiver is powered off. If this is done, the temperature will also drop over time. Therefore, if we consider the change in the offset value of the oscillator as a function of the elapsed time since the power to the GPS receiver was turned off, the offset value of the oscillator at the time of the previous positioning, which was backed up in the memory depending on the length of this elapsed time, is used as is. It is possible to estimate whether or not it can be used as an offset value of the oscillator for the positioning.

The present invention has been made paying attention to such a point, and is obtained at the time of the previous positioning by the elapsed time from the power-off of the GPS receiver at the time of the previous positioning to the power-on at the time of this positioning. It is determined whether or not to use the offset value of the oscillator as the offset value for this positioning. Therefore, even when a low-precision oscillator is used, the search center frequency and initial search bandwidth can be set by effectively using the offset value at the time of the previous positioning backed up in the memory. Can be shortened.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a flow chart of the operation of an embodiment of the method of the present invention, FIG. 2 is an explanatory view of switching search frequencies when the OFF time is short in the embodiment, and FIG. 3 is a search frequency when the OFF time is long in the embodiment. It is an explanatory view of switching of.

In the case of the present invention, when positioning is completed and the GPS receiver is turned off, the GPS receiver stores the offset value Δf _OSC of the oscillator obtained at the end of positioning in the memory and backs it up. Elapsed time (hereinafter referred to as "OFF time") starting from the power OFF using a clock
That is) to start timing.

When the power of the GPS receiver is turned on for positioning while the GPS receiver is counting the OFF time, the GPS receiver is powered off for the current positioning from the power off for the previous positioning. Detects the OFF time until ON,
It is determined whether or not this OFF time exceeds a predetermined time, for example, 30 minutes (step S1 in FIG. 1).

If the OFF time does not exceed 30 minutes, it is considered that the current offset value of the oscillator has not changed so much from the previous positioning, and the process proceeds to step S2. If the OFF time exceeds 30 minutes, the current offset value of the oscillator is considered to have changed significantly from the previous positioning, and the process proceeds to step S4.

If the OFF time is within 30 minutes,
First, in step S2, the initial search bandwidth W ₀ is set to a narrow bandwidth. That is, in the embodiment of FIG. 1, the initial search bandwidth W ₀ is variably set according to the length of the OFF time, and the basic bandwidth is set to 3000 [Hz], and (OFF time × 400 Hz) ) Is set as the initial search bandwidth W ₀ . The setting state of the initial search bandwidth W ₀ is shown in FIGS. 2 and 3. As described above, FIG. 2 corresponds to the case where the OFF time is short, and FIG. 3 corresponds to the case where the OFF time is long.

Next, in step S3, satellite radio waves
Search center frequency f for capturing _STo set. sand
That is, the search center frequency f_SIs the orbit of the desired satellite
Guards obtained by calculation from data and rough current position data
Star Doppler frequency deviation Δf_dBack to memory
Offset value Δf at the time of the last positioning_OSCWhen
Using f_S= Δf_d+ Δf_OSCIs set as. Na
As for the satellite orbit data and the approximate current position data,
Almanac information obtained during the previous positioning and the previous positioning
It is sufficient to use the current location information obtained in. This sir
H center frequency f _S2 and 3 show the setting status of
You

After setting the initial search band W ₀ and the search center frequency f _S as described above, the search for the target satellite is started (step S7). This search is shown in Figure 2.
And as shown in FIG. 3, the oscillation frequency of the oscillator is set to the initial search bandwidth W with the capture range as one step width.
Shake up and down step by step within ₀ .

The search operation is repeated a predetermined number of times (three times in FIG. 2, two times in FIG. 3) or a predetermined time within the range of the initial search bandwidth W ₀ . This is because, for example, when a vehicle-mounted GPS receiver is taken as an example, the case where the vehicle happens to be in the shadow of a building during the first search and the satellite cannot be captured, and the like is relieved.

If the target satellite cannot be captured within this initial search bandwidth W ₀ , the radio wave of the target satellite is presumed to exist outside the initial search bandwidth W ₀ . After the next search is repeated, as shown in FIGS. 2 and 3, the search bandwidth is gradually widened to W ₁ , W ₂ , and W ₃ , and the search is performed.

[0021] As in this, first, to the highest narrowband search for satellites in the initial search band width W ₀ around the search center frequency f _S of the probability that the presence of satellites, the initial search band width W within ₀ By expanding the search band and performing a search only when the satellite could not be captured by, the target satellite can be captured in a short time even when an oscillator with low accuracy is used.

On the other hand, in step S1 described above, O
If the FF time exceeds 30 minutes, the process proceeds to step S4. In this case, it is considered that the offset value of the oscillator has changed significantly since the last positioning because a considerable amount of time has passed since the last positioning, and the offset value at the last positioning backed up in the memory should be used. Abort.

In this case, the offset of the oscillator
Initial search bandwidth W over the entire range of fluctuation ₀When set as
Both (step S5), Doppler obtained by calculation
-Frequency deviation Δf_dAnd a fixed offset prepared in advance
Use the value α (for example, the intermediate value of the maximum offset change)
Search center frequency f_SAfter setting (Step S
6), start the search for the target satellite in the same way as above
(Step S7).

In the above embodiment, 30 minutes is adopted as the determination time of the OFF time, but this value is arbitrarily adopted according to the specifications of the apparatus and is merely an example. Further, in the above embodiment, the initial search bandwidth W ₀ at the start of the search is variably set in proportion to the length of the OFF time, but it may be configured to set a predetermined narrow bandwidth.

[0025]

As is clear from the above description,
According to the oscillator initial offset setting method of the present invention, the elapsed time from the power-off of the GPS receiver in the previous positioning to the power-on of the GPS receiver in the current positioning is measured, and the GPS reception is performed depending on the length of the elapsed time. Since it was decided whether to use the offset value of the oscillator at the time of the last positioning stored in the onboard memory as the offset value of the oscillator at the time of this positioning, even if a low precision oscillator is used, The backed up offset value at the time of the previous positioning can be effectively used, and the satellite can be captured promptly.

[Brief description of drawings]

FIG. 1 is a flow chart of the operation of one embodiment of the present invention.

FIG. 2 is an explanatory diagram of switching the search frequency when the OFF time is short in the embodiment.

FIG. 3 is an explanatory diagram of search frequency switching when the OFF time is long in the embodiment.

[Explanation of symbols]

f _S Search center frequency W ₀ Initial search bandwidth Δf _d Doppler frequency deviation of satellite obtained by calculation Δf _OSC Oscillator offset value obtained at the previous positioning α Fixed offset value

Claims (1)

[Claims] 1. A GPS receiver in which a search center frequency is set using an offset value of an oscillator and a Doppler frequency deviation of a target satellite. The time elapsed until the power of the GPS receiver is turned on in the positioning is measured, and the offset value of the oscillator at the time of the previous positioning stored in the memory in the GPS receiver is determined by the length of the elapsed time, and the offset value of the oscillator at the time of the positioning is measured. A method for setting an initial offset of an oscillator in a GPS receiver, characterized in that it is determined whether or not to be used as