JP2003516546A - Receiver for satellite-based position location system - Google Patents

Abstract

  (57) [Summary] SUMMARY OF THE INVENTION The present invention is a method for determining synchronization of a signal received at a receiver of a global positioning system and transmitted by a satellite of the global positioning system, the method comprising transcoding the received signal to convert the received signal into the time domain. And selecting from the frequency domain coefficients frequency domain coefficients that contribute substantially to zero energy, and deriving an in-band noise estimation indication from the selected coefficients. A method is disclosed.

Description

Detailed Description of the Invention

[0001]

【Technical field】

The present invention relates to satellite-based position location systems such as the Global Positioning System (GPS), and more particularly to receivers for use in such systems.

[0002]

[Background technology]

The Global Positioning System (GPS) is an example of a space-based satellite navigation system that has the ability to pinpoint any location on the earth with high accuracy to provide information about receiver position, velocity and time (PVT). . Another example of a space-based satellite navigation system is TIMAT.
ION, Transit and GLONASS. GPS is typically divided into three segments: A space segment essentially containing the satellites and the signals they emit; a control segment for monitoring and maintaining the satellite constellation; and a user segment containing GPS receivers, devices, data acquisition and data processing techniques.

GPS constellations typically consist of 24 satellites orbiting the earth every 24 hours. For the GPS receiver to accurately determine its position, GP
There must be a minimum of 4 GPS satellites in the open field of view of the S receiver. In summary, each satellite broadcasts a signal, which the GPS receiver receives and decodes, thereby calculating the time it takes for the signal to reach the receiver, called the transit time. The receiver then multiplies the transit time by the velocity of the electromagnetic radiation to determine the distance from the satellite to the receiver. From there, the receiver applies trigonometric calculations to calculate the three-dimensional distance, velocity and time of the receiver. Trigonometry involves calculating the intersections between four reference points provided by satellites, from which the positions in three-dimensional space are fixed or searched.

However, it should be noted that range measurements inherently contain errors that are common to measurements caused by the asynchronous operation of satellite and user clocks. For this, GP
S uses four satellites for distance measurement. Measurements from three GPS satellites allow the GPS receiver to calculate three unknown parameters representing its three-dimensional position, while the fourth GPS satellite allows the GPS receiver to generate user clock error. So that more accurate time measurements can be determined. The signals broadcast by the satellite include radio frequency (RF) range finding codes and navigation data messages, which are transmitted using spread spectrum technology. The distance measurement code enables the GPS receiver to measure the transit time of the signal and thereby determine the distance between the satellite and the receiver. The navigation data message is based on certain information about the satellite's orbital path and thus gives an indication of the satellite's position when the signal was transmitted.

The encoded signal produced by a satellite is in the form of a Pseudo Random Noise (PNR) code, which represents a series of random binary chips, each satellite having a unique PNR sequence that repeats itself at regular intervals. Send. GPS has a chip rate of 10.23 MHz and a military-specified precision code (P code), and a chip rate of 1.023 MHz and a course collection code (C / A) assigned for commercial and personal use. Code) and. The chip is 1 or -1. The code consists of two L-band frequencies, namely Link 1 (L
1) and 1227.6 MHz link 2 (L2). Code assignment in L1 is course collection code (C / A code) and precision code (P
Code), and in L2 there is only P code.

The C / A code is composed of a 1023 bit pseudo random (PRN) code,
Then, a different PRN code is designated for each GPS satellite. In addition, a 50 Hz navigation data message is superimposed on the C / A code, which contains the above data. Therefore, the receiver can use the signal from the satellite with the particular C / A code presented to make a pseudorange measurement. Turning to receivers, a wide variety of GPS receivers are available today, typically
The internal architecture of a GPS receiver is a front end that first processes the incoming satellite signal, followed by a signal processing stage that applies an algorithm to determine the position of the receiver,
A signal processing stage for determining speed and time.

The front end is similar to the superheterodyne receiver in terms of basic items. The signal is detected by the GPS antenna and fed to a low noise amplifier. Following amplification, the signal is down converted to a lower workable frequency. This is accomplished by mixing or heterodyning the GPS signal with another constant frequency signal. This mixed signal is generated by a local oscillator. When the two signals are mixed, the original frequency, the sum of the two frequencies and the difference of the two frequencies are output.
The filters in the succeeding stages select only the frequencies of difference and reject the other frequencies. The frequency of this difference generated by the down conversion stage is known as the intermediate frequency IF and gives the baseband signal. This signal is then converted from analog to digital with an A / D converter. The output level of the A / D converter is monitored by a voltage comparator to check for levels above or below the threshold level, and an automatic gain controller continuously adjusts the gain of the IF amplifier to a constant level. Try to maintain the output level of. The digital signal from the A / D converter is used as an input to a number of signal processing stages that handle the distance measurement process.

As initially indicated, the distance measurement process uses the incoming PRN code to calculate the distance from the satellite to the receiver, and how much the signal transmitted by the satellite reaches the receiver. The purpose is to keep time or time. To achieve this, each receiver has the ability to generate the exact pattern of codes that each satellite transmits using the PRN signal generator. The incoming signal received from a particular satellite is probably out of phase with the internal one. This is because it is not always known how long it takes to travel from the satellite to the receiver, measured in units of the time period of transmitting one chip. The expected PRN signal internally generated from a particular satellite needs to be appropriately delayed or phase shifted to match it when compared to the received signal. The strength of the match can be measured from the correlation between the two signal parts. It is useful to correlate the portion of the received sequence with the corresponding portion of the expected signal from the satellite. Since the digitized expected signal is periodic,
The internally generated sequence can be delayed by rotating this sequence. The amount of shift or offset required to match two signals is
It gives the receiver a measure of the time delay for the signal to leave the satellite and reach the receiver.
The distance is derived using this measurement.

In principle, if the time it takes for a signal to travel from a satellite to a receiver via a particular path is known, the signal is predicted in proportion to the signal arriving at the receiver via that path. , And the output from the correlator is a quantity that is highly related to the energy in that path. However, the start of this time interval is unknown. It may be useful to collect exact travel times for each important route by using some form of search algorithm. The signal is periodic (at least 20)
Thus, by delaying the portion of the expected signal by m shift positions, a delay of m times the sample period (microseconds) can be simulated. Various delays are tried and the correlator output is monitored. The delays identified as appropriate are those that produce the highest correlation output.
Sub-chip sampling (eg, 4 samples per chip, which is currently common)
If is used, high values are obtained for about 4 out of m revolutions. The rotation that gives the highest correlation from these four consecutive rotations is used to calculate the time it takes for the signal to travel from the satellite to the observer. Once the time is estimated, various tracking algorithms are used to monitor this change in time. Since the baseband signal received from the incoming GPS signal is embedded in noise, the correlation value is often lower than the noise floor of the incoming signal. Under these conditions, it becomes difficult to accurately identify the tip rotation that gives the best match to the correlation.

[0010]

DISCLOSURE OF THE INVENTION

In view of this background, the invention, in one aspect, is a method for determining synchronization of a signal received at a receiver of a global positioning system and transmitted by a satellite of a global positioning system, the received signal being Transform-coding the received signal from the time-domain to frequency-domain coefficients, selecting from the frequency-domain coefficients the frequency-domain coefficients that contribute substantially zero energy, and banding the selected coefficients. A method is provided which comprises the step of deriving an estimation indication of the inner noise. The invention advantageously performs filtering in the frequency domain and then, in the preferred arrangement, again performing correlation in the frequency domain. Therefore, in-band noise is substantially reduced from the incoming GPS signal, which allows the correlation value to be more clearly discriminated and provides improved synchronization. More specifically, the known method uses GPS in places where incoming signals are weak, for example in buildings.
It was often necessary to perform long correlations to obtain the position of. However, the present invention allows for shorter correlation and power savings.

The present invention is based on observing that the spectrum of the GPS baseband signal is a line spectrum, and by observing the line spectrum it is noted that only a portion of the bins is non-zero. . A measure of noise in the received signal can be derived from the bins with zero energy. Therefore, the method of the present invention provides in-band filtering to accurately estimate the in-band characteristics of noise. By removing the estimated noise, the correlation can be compared to the in-band background noise. In-band filtering of GPS signals is conventionally performed using a narrow band filter with a predetermined pass band. Such filters cannot remove noise in the pass band. In contrast, the present invention removes noise over substantially the entire band by estimating the in-band noise. Other aspects and features of the invention are defined in the claims.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred features of the invention and its corresponding advantages will be understood from the following description of various embodiments of the invention. Such embodiments are only provided as examples of some of the specific ways in which the invention may be practiced. As mentioned above, the present invention is related to the problem of the receiver achieving synchronization with an incoming satellite transmitted pseudo-random noise (PRN) signal in a GPS environment, and for the purposes of this description, is in simulation. First, in simulation, a unit step, which is a mathematical device for separating a non-zero function into a specified distance, is defined. UnitSt
ep [x] represents a unit step function equal to 0 if x <0 and equal to 1 if x ≧ 0, which is represented by Equation 1 below.

[Equation 1] Where T is the chip duration and M is the number of chips in the spreading code.

[0013]   The satellite signal s [t] is represented by the following Expression 2.

[Equation 2] The contribution of the received signal from a single satellite to the baseband signal is proportional to s [t]. The signal r [t] received in the base band from a single satellite can be expressed by Equation 3 below.

[Equation 3] Where α represents the strength of each path and Δt represents the corresponding delay.

S [f] is a spectrum of s [t] and represents a frequency variable Fourier spectrum of s [t]. Equation 4 represents S [f].

[Equation 4] The corresponding spectrum for r [f] is then given by Equation 5 below.

[Equation 5] From the formula of S [f], in the present invention, the spectrum is a line spectrum and the line is 1
Note that they are separated by a distance of / (MT). g [f] is a filter that is a function of frequency f. The amplitude of each line can be obtained by applying g [f] to S [f]. g [f] is given by the following Expression 6.

[Equation 6]

As mentioned above, the received signal is embedded in noise, and the present invention, in its preferred form, provides a process for performing in-band filtering using a discrete Fourier transform. The receiver samples the incoming signal with a sampling period of T / samp, where samp is an integer, typically 4. The number of consecutive samples is Nsam =
sampxMxK, where K is selected as a prime number {3,11,31}, which is a factor of 1023, which is the number of chips in the spreading code. As mentioned above, the receiver produces a local copy of the satellite PRN code. This is precalculated and stored for I ′ = 0, 1, 2, ... K M−1, and is represented by Equation 7 below.

[Equation 7]

Next calculated is the dot product of the pre-calculated inverse Fourier transform of the spreading code (satellite PRN code) and the Fourier transform of the received signal, which is
Is represented by

[Equation 8] This equation can be simplified as follows. In Equation 9 below, the multiplication is distributed with respect to the addition.

[Equation 9] This is simplified in Equation 10 below.

[Equation 10]

[0017]   In the following Expression 11, first, i'is added.

[Equation 11] This is simplified in Equation 12 below.

[Equation 12] The result is maximum when Δ is zero and indicates synchronization. However,
This is generally not zero. To obtain the Fourier transform of the code shifted by Δ, the calculation begins with the pre-calculated inverse Fourier transform of the spreading code already shown in equation 7.

[Equation 13] The inverse Fourier transform of the spreading code shifted by Δ is generated as follows.
First, this is expressed in Equation 13 below.

[Equation 14]

[0018]   This formula is equivalent to the following formula 14.

[Equation 15] When bracketed and factored, the following equation 15 is obtained.

[Equation 16] This can be expressed as in Equation 16 below.

[Equation 17] After changing the variables, this becomes:

[Equation 18]

Examining each term, it is clear that the Fourier transform of the code shifted by Δ bins can be efficiently generated. This equation is the i'term of the inverse Fourier transform of the code that is repeatedly shifted by Δ bins. From Equation 17 above, the i'bin of the Fourier transform of the code shifted by Δ is the product of the inverse Fourier transform of the unshifted code and the next term.

[Formula 19] Also note that only the following need be remembered:

[Equation 20] Therefore, the value

[Equation 21] Can be calculated from Equation 18,

[Equation 22] That is, look at the position Mod [i ′ Δ, KM] +1 and use the pre-calculated value.

To perform the convolution in the Fourier domain, first, Δ ′
Consider the conjugate of the Fourier transform of the pure code Rc [j] repeated in.
This is expressed by Equation 19 below.

[Equation 23] Next, consider the following Expression 20.

[Equation 24] This is distributed in Equation 21 as follows.

[Equation 25] Following simplification, the following equation 22 is obtained.

[Equation 26]

[0021]   When i'addition is performed, the following Expression 23 is first obtained.

[Equation 27] Simplification yields the following equation 24:

[Equation 28] This is the maximum when Δ = Δ ′. Therefore, the following Expression 25 is obtained.

[Equation 29]

[0022]   In the following equation 26,

[Equation 30] FRc is shown as the inverse Fourier transform of the spreading code and i '= 0,1,2
, ... KM−1, it is represented by the following Equation 27.

[Equation 31] In the case of i '= 0, 1, ... KM-1, consider as follows.

[Equation 32] Therefore, the following equation 28

[Expression 33] Can be calculated by the following equation 28.

[Equation 34]

[0023]   Therefore, the following Expression 30 is obtained.

[Equation 35] Since SF [i '] may be zero, it is not necessary to perform each multiplication. In particular, if the spectrum is a line spectrum, then only a small part of the multiplication need be performed, for example as shown in equation 31 below.

[Equation 36] For K = 10, two complex multiplications per term are required, and the multiplications are multi-bit precision rather than single-bit multiplication, but savings are obtained.

In summary, the algorithm steps for implementing the preferred form of the invention are as follows. 1) Nsan samples are {s [0], s [1], s [2], s [3], s [4], s [
5], s [6], s [7], s [8], ... s [Nsamp-1]}. Perform a discrete Fourier transform of these samples. 2) {sf [0], sf [1], sf [2], sf [3], sf [4], sf [5], sf
[6], sf [7], sf [8], ... sf [Nsamp-1]} are {s [0], s [1]
, S [2], s [3], s [4], s [5], s [6], s [7], s [8], ... s [Nsa
mp-1]}.

Generate the following: {Sf [0] g [0], sf [1] g [1], sf [2] g [2 / Nsam], sf [3] g
[3 / Nsam], sf [4] g [4 / Nsam], sf [5] g [5 / Nsam], sf
[6] g [6 / Nsam], sf [7] g [7 / Nsam], sf [8] g [8 / Nsam]
, Sf [9] g [9 / Nsam], ... sf [N / samg-1] g [Nsam-1 /
Nsam] Here, g [n] is a function that executes filtering. Choosing g [n] = 1 is equivalent without filtering, g [n] is zero for many values of n, and under these conditions the improved computational speed is particularly It will be remarkable.

[0026]   Knowing the zero bins allows us to estimate the noise in the non-zero bins.   nf (0) is a conversion coefficient for n.   {(Sf (0) -nf (0)), sf (1) -nf (1), ... sf (N) -nf (N)}   nf (m) = sf (m) m is known to be one of the zero coefficients   = Est f (sf (m-1), sf (m-2), ...               (Sf (m + 1), sf (m + 2), ...) However, the number of coefficients to be used depends on the accuracy of the estimation with respect to the calculation load.

[Equation 37]

3) DelFac is a delay factor. {DelFac [0], DelFac [1], DelFac [2], DelFac [
3], ..., DelFac [Nsamp-1]} is a set of values defined above.

[Equation 38] 4) Recall the pre-calculated and stored values. {FRc [0], FRc [2], FRc [3], FRc [4], ... FRc [Nsam
−1]} is a value calculated and stored in advance. However, FRc [I '] is represented by the following term.

[Formula 39]

5) Then, the correlation of the input is a rotational correlation in the Fourier domain. {S [0], s [1], s [2], s [3] with code Rc (j) repeated at Δ '
, S [4], s [5], s [6], s [7], s [8], ... s [Nsamp-1] are represented by the following equations.

[Formula 40] The present invention may be embodied in other specific forms without departing from its essential attributes. Therefore, reference should be made to the appended claims and other general statements as to the scope of the invention rather than the above description.

Furthermore, each feature disclosed in this specification (including the claims) and / or shown in the accompanying drawings is incorporated independently of the other disclosed and / or illustrated features. Good. In this regard, the present invention, regardless of whether it relates to the invention described in the claims or alleviates any or all of the problems to be addressed,
A novel feature or combination of features disclosed herein is expressly included or a generalization thereof. For reference, a summary is also attached to this specification.

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Claims (18)

[Claims] 1. A method for determining synchronization of a signal received at a Global Positioning System receiver and transmitted by a Global Positioning System satellite, wherein the received signal is transform coded to time-domain the received signal. To a frequency domain coefficient, select a frequency domain coefficient that contributes substantially zero energy from the frequency domain coefficient, and derive an in-band noise estimation instruction from the selected coefficient. How to prepare. 2. A method for determining synchronization as claimed in claim 1, comprising the step of determining, from the estimation of the in-band noise, the noise present in the frequency domain coefficients contributing to the non-zero energy of the received signal. 3. The synchronization according to claim 1 or 2, comprising the step of using the estimation indication of the in-band noise to determine the energy contribution of frequency domain coefficients contributing to the non-zero energy of the received signal. How to decide. 4. A method for determining synchronization according to claim 1, comprising the step of generating a signal expected to arrive at a receiver from a satellite. 5. The method of claim 4, wherein the estimated indication of the in-band noise is subtracted from the frequency domain coefficients contributing to the non-zero energy of the received signal to give an indication of the signal expected to reach the receiver from the satellite. How to determine the described synchronization. 6. A method for determining synchronization according to claim 1, comprising the step of generating a line spectrum of the frequency domain transform coefficients. 7. A method for determining synchronization according to claim 6, comprising the step of using the line spectral frequency domain to distinguish between transform coefficients having zero and non-zero energies of a received signal. 8. The method according to claim 7, further comprising: extracting from the line spectrum of the frequency domain coefficient, an average value of in-band noise estimation instructions from adjacent coefficients of the frequency domain coefficient contributing to non-zero energy of the received signal. How to determine synchronization as described in. 9. A method for determining synchronization according to claim 1, wherein the number of times the received signal is oversampled is proportional to the number of frequency domain coefficients contributing to substantially zero energy of the received signal. 10. A method for determining synchronization as claimed in any one of claims 1 to 9, wherein the transform coding step comprises Fourier transform coding. 11. A method for determining synchronization according to claim 1, comprising the step of performing correlation in the frequency domain. 12. Converting the coefficients to the time domain by applying inverse transform coding to the frequency domain coefficients when a substantial number of frequency domain coefficients are identified as contributing to substantially zero energy. A method for determining synchronization according to any one of the preceding claims. 13. A method of determining synchronization as claimed in any one of claims 1 to 12, wherein the identification comprises adjacent spectra of a line spectrum of frequency domain transform coefficients. 14. A receiver for a satellite-based positioning system operable for synchronizing to a received signal transmitted from a satellite, said receiver signal being transcoded to convert said received signal. Transform coding means for transforming time domain to frequency domain coefficients, selecting means for selecting frequency domain coefficients contributing substantially zero energy from the frequency domain coefficients, and selecting from the selected coefficients And a processing unit for deriving an in-band noise estimation instruction. 15. A satellite-based positioning system in which a plurality of satellites transmit range finding signals and a receiver receives the transmitted range finding signals and synchronizes to the range finding signals. , Transform coding the received signal to transform the received signal from a time domain to a frequency domain coefficient, and from the frequency domain coefficient, a frequency domain coefficient that contributes to substantially zero energy, A satellite-based positioning system comprising selection means for selecting and processing means for deriving an in-band noise estimation instruction from the selected coefficient. 16. A computer program on a carrier for synchronizing a receiver operating in a global positioning system comprising a plurality of satellites transmitting range finding signals, said synchronization being the transmitted range finding. Conversion coding means for transform-coding the received signal to transform the received signal into a coefficient in the frequency domain from the time domain, and substantially from the frequency domain coefficient. A computer program comprising: selecting means for selecting a frequency domain coefficient that contributes to zero energy, and processing means for deriving an in-band noise estimation instruction from the selected coefficient. 17. A computer program product operable to synchronize a receiver operating in a global positioning system comprising a plurality of satellites transmitting range finding signals, said synchronization being performed by said transmitting. Is performed on the distance measurement signal, and by transform-coding the received signal, transform coding means for transforming the received signal from the time domain to the frequency domain coefficient, and from the frequency domain coefficient, A computer program product comprising selection means for selecting a frequency domain coefficient that contributes substantially zero energy, and processing means for deriving an in-band noise estimation instruction from the selected coefficient. 18. A portable radio including a transceiver for cellular communication and a receiver for a satellite-based positioning system operable for synchronizing to a received signal transmitted from the satellite. In the communication device, transform coding the received signal, transform coding means for transforming the received signal from a time domain to a frequency domain coefficient, and a frequency contributing substantially zero energy from the frequency domain coefficient. A portable wireless communication device comprising: selecting means for selecting a domain coefficient; and processing means for deriving an in-band noise estimation instruction from the selected coefficient.