JP3115084B2 - Initial offset setting method for oscillator in GPS receiver - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for setting an initial offset of an oscillator in a GPS receiver. GPS (Glo
The bal Positioning System (Global Positioning System) is a positioning system using satellites, which orbits the earth above an altitude of about 20,200 km.
One orbit, launch multiple satellites in each orbit,
A GPS signal containing navigation data for positioning is transmitted from each satellite to the earth in a spread spectrum system. Three or four GPS signals required for positioning by a terrestrial (or sea or air) GPS receiver are used. By receiving the radio waves of the satellites, necessary position information such as the latitude, longitude and altitude of the receiving point can be calculated in real time using the navigation data included in the GPS signal from each satellite.

[0002]

2. Description of the Related Art GPS signals that are open to the public are C /
This signal is called an A code, and is transmitted at 1575.42 MHz. Therefore, in order to acquire a satellite, in principle, it is sufficient to wait for reception at 15575.42 MHz. However, since the GPS satellite is not a geostationary satellite, its reception frequency fluctuates due to the Doppler effect, and the oscillation frequency of the oscillator of the GPS receiver also fluctuates due to offset.

Therefore, in order to actually acquire a satellite,
The deviation of the reception frequency due to the Doppler effect (hereinafter, referred to as
"Doppler frequency deviation") and the offset of the oscillator to set the search center frequency of the satellite radio wave, search for the satellite while gradually widening the search bandwidth starting from this search center frequency, the oscillator to the satellite radio At the time of locking, the acquisition of the satellite is completed. By repeating such a capturing operation, the number of satellites required for positioning is captured.

At the time of the satellite capturing operation, the closer the search center frequency is to the actual reception frequency of the satellite radio wave, the earlier the time of capturing the satellite 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 movement of the satellite, and its value (up to about ± 6 KHz) can be calculated in advance from the orbit data of the satellite and the approximate current position data. Therefore, to set the search center frequency to a value as close as possible to the actual reception frequency, the GPS
It is desirable to minimize the error in the offset value of the oscillator of the receiver.

[0005] Therefore, in the conventional GPS receiver, a crystal oscillator with extremely high accuracy (hereinafter, referred to as "TCXO") is used as an oscillator. This TCX
O is about ± 2 to ± 5 [ppm] (± 3 in frequency conversion) over a wide temperature range of −40 ° C. to + 85 ° C. by compensating the temperature characteristics of the crystal unit in an electric circuit.
[KHz] to about ± 8 [KHz]). Note that the accuracy of an oscillator used in ordinary communication equipment is generally about ± 50 to ± 100 [ppm], and at best about ± 20 [ppm].

[0006]

The offset value of the oscillator can be calculated accurately after the precise position of the receiver is known by positioning, but before that the actual actual offset value is unknown. It is. For this reason, the conventional GPS receiver uses the high-precision TCXO as described above, stores the offset value obtained at the last time of the previous positioning in the memory and backs up it, and uses the receiver at the next positioning. When the power supply is turned on, the offset value that has been backed up is read and used as the offset value of the oscillator.

However, when a low-precision oscillator is used, after the previous positioning is completed and the power of the receiver is once turned off, the next time the power of the receiver is turned on, the actual offset value becomes equal to the previous offset value. There is a high possibility that the value has greatly changed from the value at the time of positioning, 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 much time to acquire the satellite.

The present invention has been made in view of the above circumstances. It is an object of the present invention to effectively utilize the offset value at the time of the previous positioning backed up in the memory even when a low-precision oscillator is used. It is an object of the present invention to provide a method for setting an initial offset of an oscillator in a GPS receiver, which can quickly acquire a satellite by using the GPS receiver.

[0009]

In order to achieve the above object, an initial offset setting method according to the present invention sets a search center frequency using an offset value of an oscillator and a Doppler frequency deviation of a target satellite. GPS
In the receiver, the power of the GPS receiver in the last positioning is changed from the power OFF of the GPS receiver in the previous positioning to the power of the GPS receiver in the current positioning.
The elapsed time up to N is measured, and whether the offset value of the oscillator at the previous positioning stored in the memory in the GPS receiver is used as the offset value of the oscillator in the current positioning is determined based on the length of the elapsed time. It is like that.

[0010]

[Operation] The offset value of a low-precision oscillator greatly changes depending on the ambient temperature. For example, when the power of the GPS receiver is turned on, the temperature of the entire receiver including the oscillator is usually higher by the heat of the circuit elements and the like, and the power of the receiver is turned off. Then, the temperature also decreases over time. Therefore, if the change in the offset value of the oscillator is regarded as a function of the time elapsed since the power of 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 due to the length of the elapsed time, is used as it is in this time. It can be estimated whether or not it can be used as an offset value of an oscillator for positioning.

The present invention has been made in view of such a point, and is obtained at the time of the previous positioning based on 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 the current positioning. It is determined whether or not the offset value of the generated oscillator is used as the offset value of the current positioning. Therefore, even when a low-precision oscillator is used, the search center frequency and the initial search bandwidth can be set by effectively using the offset value at the time of the last positioning backed up in the memory, and the satellite acquisition time can be set. Can be shortened.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a flowchart of the operation of one embodiment of the method of the present invention, FIG. 2 is an explanatory diagram of the switching of the search frequency when the OFF time is short in the embodiment, and FIG. 3 is the search frequency when the OFF time is long in the embodiment. FIG. 4 is an explanatory diagram of switching.

In the case of the present invention, when the positioning is completed and the power of the GPS receiver is turned off, the GPS receiver stores the offset value Δf _OSC of the oscillator obtained at the end of the positioning in the memory to back up the data, Using a clock, the elapsed time (hereinafter referred to as “OFF time”) starting from the power OFF
Start).

When the power of the GPS receiver is turned on for positioning while the GPS receiver is measuring the OFF time, the power of the GPS receiver is changed from the power OFF at the previous positioning to the power supply for the current positioning. Detects the OFF time until ON,
It is determined whether or not the 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 assumed that the current offset value of the oscillator has not changed so much since the last positioning, and the process proceeds to step S2. If the OFF time exceeds 30 minutes, it is considered that the current offset value of the oscillator has greatly changed from the time of 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], which is (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. 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, the satellite radio wave
Search center frequency f for acquisition of _SSet. sand
That is, the search center frequency f_SIs the orbit of the target satellite
Data obtained from data and approximate current location data
Star Doppler frequency deviation Δf_dAnd back up to memory
Offset value Δf at the previous positioning_OSCWhen
And f_S= Δf_d+ Δf_OSCIs set as What
As satellite orbit data and approximate current location data,
Almanac information obtained during previous positioning and previous positioning
It is sufficient to use the current location information obtained in. This service
H center frequency f _SFIG. 2 and FIG.
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 FIG.
As shown in FIG. 3, the capture range is set to one step width, and the oscillation frequency of the oscillator is set to the initial search bandwidth W.
_{This is} done while swinging up and down one step at a time within ₀ .

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

[0020] Incidentally, if it can not capture the object of the satellites in the initial search band width W in _0, since the radio wave of interest of the satellite is estimated to be present on the outside of the initial search band width W _0, in the following the next search iteration as shown respectively in FIGS. 2 and 3, to search while expanding gradually as the search bandwidth W _1, W _2, W _3.

[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 the search only when the satellite cannot be acquired by the above, the target satellite can be acquired 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 has exceeded 30 minutes, the process proceeds to step S4. In this case, it is assumed that a considerable amount of time has passed since the previous positioning, and the offset value of the oscillator is considered to have changed significantly from the time of the previous positioning, and the offset value of the previous positioning backed up in the memory is used. Abort.

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

In the above embodiment, 30 minutes is used as the determination time of the OFF time. However, this value is arbitrarily adopted according to the specifications of the apparatus, and is merely an example. In the above-described 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. However, the initial search bandwidth W ₀ may be set to a predetermined narrow bandwidth.

[0025]

As is apparent from the above description,
When the initial offset setting method of the oscillator according to the present invention is used, 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. Since it is determined whether or not to use the offset value of the oscillator at the time of the previous positioning stored in the memory in the aircraft as the offset value of the oscillator for the current positioning, even if a low-precision oscillator is used, The backed-up offset value at the time of the last positioning can be effectively used, and the satellite can be quickly captured.

[Brief description of the drawings]

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

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

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

[Explanation of symbols]

f _S search center frequency W ₀ initial search bandwidth Δf Doppler frequency deviation of satellite obtained by _d Δd calculation Δf _OSC Offset value of oscillator obtained at previous positioning α Fixed offset value

Continuation of front page (58) Fields investigated (Int. Cl. ⁷ , DB name) G01C 21/00-21/36 G01C 23/00-25/00 G01S 5/00-5/14 WPI (DIALOG)

Claims (1)

(57) [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 elapsed time until the power of the GPS receiver is turned on in the positioning of the GPS receiver 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. A method for setting an initial offset of an oscillator in a GPS receiver, characterized in that it is determined whether or not to use as an offset.