EP1240531A1 - A receiver for a satellite based position location system - Google Patents
A receiver for a satellite based position location systemInfo
- Publication number
- EP1240531A1 EP1240531A1 EP00985504A EP00985504A EP1240531A1 EP 1240531 A1 EP1240531 A1 EP 1240531A1 EP 00985504 A EP00985504 A EP 00985504A EP 00985504 A EP00985504 A EP 00985504A EP 1240531 A1 EP1240531 A1 EP 1240531A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- transform
- satellite
- receiver
- signal
- coefficients
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S11/00—Systems for determining distance or velocity not using reflection or reradiation
- G01S11/02—Systems for determining distance or velocity not using reflection or reradiation using radio waves
- G01S11/06—Systems for determining distance or velocity not using reflection or reradiation using radio waves using intensity measurements
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
- G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
- G01S19/13—Receivers
- G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
Definitions
- the present invention relates to a satellite based position location system such as the Global Positioning System (GPS), and in particular to a receiver for use in such a system.
- GPS Global Positioning System
- GPS Global positioning system
- PVT position, velocity and time
- Other examples of spaced based satellite navigation system are TIMATION, transit, and GLONASS.
- GPS is typically divided into three segments: a space segment which essentially comprises the satellites and the signals they emit, a control segment which monitors and maintains the satellite constellation, a user segment which comprises GPS receivers, equipment, data collection and data processing techniques.
- the GPS constellation typically consists of 24 satellites orbiting the earth every 24 hours.
- a minimum of four GPS satellites must be in clear view of a GPS receiver in order for the receiver to determine accurately its location.
- each satellite broadcasts signals that the GPS receiver receives and decodes and from these calculates the time taken for the signals to reach the receiver, this is called the time in transit.
- the receiver then multiplies the time in transit by the speed of electromagnetic radiation to determine the range from the satellite to the receiver. From there, in order to work out the receiver's 3 dimensional distance, velocity and time, the receiver applies the triangulation calculation. Triangulation involves calculating the intersection of points between four reference points given by the satellites and the intersection fixes or locates the position in 3-dimensional space. It should be noted, however, that range measurement inherently contain errors common to measurements created by the unsynchronised operation of the satellite and the user clocks.
- GPS uses four satellites to effect ranging.
- the measurements from three GPS satellites allow the GPS receiver to calculate the three unknown parameters representing its three dimensional position, while the fourth GPS satellite allows the GPS receiver to calculate the user clock error and therefore determine a more precise time measurement.
- the signals broadcast by a satellite comprise radio frequency (RF) ranging codes and navigation data messages which are transmitted using spread spectrum techniques.
- the ranging codes enable the GPS receiver to measure the transit times of the signals and thereby determine the range between the satellite and the receiver.
- the navigation data messages are based on predetermined information regarding the orbital path of the satellite and thus provide an indication of the position of the satellite at the time the signals were transmitted.
- the encoded signal generated by a satellite is in the form of a pseudo random noise (PRN) code which represents a sequence of random binary chips, each satellite transmitting a unique PRN sequence that repeats itself at definite intervals.
- PRN pseudo random noise
- P-Code Precision Code
- C/A Code Course Acquisition code
- a chip is 1 or -1.
- the codes are transmitted on two L-band frequencies: Link 1 (L1 ) at 1575.42 MHz and Link 2 (L2) at 1227.6 MHz.
- the code allocations on L1 are Course Acquisition code (C/A Code) and Precision Code (P-Code), and on L2 is only P-Code
- the C/A Code consists of a 1023 bit pseudo random (PRN) code, and a different PRN code is assigned to each GPS satellite.
- PRN pseudo random
- a 50 Hz navigation data message is superimposed on the C/A Code, and contains the data noted above.
- the receiver can utilise the signal from a satellite by the particular C/A Code being submitted, to make pseudo-range measurements.
- GPS receivers there is a wide range of GPS receivers available today, and typically the internal architecture of a GPS receiver comprises a front end that initially processes the incoming satellite signals, followed by signal processing stages that apply the algorithms to determine the receivers location, speed and time.
- the front end in basic terms is similar to that of a superheterodyne receiver.
- the signal is detected by a GPS antenna and fed to a low noise amplifier. Following amplification, the signal is down converted to a lower workable frequency. This is achieved by mixing or heterodyning the GPS signal with another constant frequency signal. This mixing signal is produced by a local oscillator. When two signals are mixed, the original, the sum of the two and the difference between the two frequencies is output. The filter in the following stages selects only the difference frequency and rejects the others. This difference frequency produced by the down conversion step is known as the intermediate frequency IF.
- the signal is next converted from analogue to digital in an AD converter.
- the output level of the AD converter is monitored by a voltage comparator to check levels exceeding or dropping below threshold levels, and an automatic gain control continually adjusts the gain of the IF amplifier to maintain a constant output level.
- the digital signal from the AD converter is used as an input to several stages of signal processing dealing with the ranging process.
- the ranging process aims to calculate the distance from the satellite to the receiver using the incoming PRN codes to time how long it has taken the signals transmitted by the satellite to arrive at the receiver.
- each receiver has the capability to generate an exact pattern of the code that each satellite transmits using a PRN signal generator.
- the incoming signal received from a particular satellite is likely to be out of phase with the internal one since the time it takes to travel from the satellite to the receiver, measured in units of time periods to transmit a chip may not be always known.
- the internally generated expected PRN signal from a particular satellite needs to be suitably delayed or phase shifted so that it matches with the received signal when compared. The strength of match can be measured from correlation between the two signal fragments.
- the internally generated sequence can be delayed by rotating the sequence.
- the amount of shifting or offset that was required to match the two signals provides the receiver with a measurement of the time lag between the signal leaving the satellite and arriving at the receiver. This measurement is then used to derive the range.
- the delay that is identified as being the one that is appropriate is the one that yields the highest correlation output. If sub chip sampling is employed, (say 4 samples per chip which is currently common) then a high value is obtained for about 4 of the m rotations. The rotation that yields the highest correlation from these four contiguous rotations is used to calculate the time the signal has taken to travel from the satellite to the observer. Once the time has been estimated, the changes in this time are monitored using various tracking algorithms.
- the present invention in one aspect provides a method for determining synchronisation of a signal received in a global positioning system receiver and transmitted by a global positioning system satellite, wherein said method comprises the steps of transform coding said received signal so as to transform said received signal from time domain to frequency domain coefficients, dividing respective ones of said frequency domain coefficients by corresponding ones of transform coefficients of the expected transmitted signal associated with said satellite to provide corresponding transform value ratios, and deriving from respective transform value ratios corresponding scaled values of the time delay of the signal between the satellite and the receiver.
- the invention directs the search for synchronisation to a small set of chips thereby providing an indication of the identity of the synchronisation sequence.
- the search for the synchronisation chip is more focused than the prior art method, which results in obtaining synchronisation on average more quickly and more efficiently than in the prior art.
- the present invention is particularly advantageous in that it is a deterministic technique and accordingly assists the prediction in time of when the correlation process should end.
- Figure 1 illustrates in block diagram form a preferred embodiment of the present invention
- Figure 2 shows an Argand diagram of a step in the preferred embodiment of Figure 1 ;
- Figure 3 is a plot of simulation of the present invention.
- the present invention is concerned with the problem of a receiver attaining synchronisation with an incoming satellite transmitted pseudo random noise (PRN) signal in the context of the GPS system.
- Figure 1 shows the functional blocks making up a preferred receiver arrangement of the present invention.
- the incoming signal received by the receiver is represented as R(t) where t is the ideal time of the transmitted signal.
- the receiver initially samples the incoming signal in block 12 by a sampler operating at a sample period T with Nos being the number of samples per chip.
- the sampler typically operates at sampling rates are 4 samples per chip.
- R(t) time is measured with respect to the receiver's clock.
- a sequence of samples of R(t) begins to repeat with a period of M Nos where in GPS M is 1023 chips.
- R(t) is then given by Eqn 1 :
- R[t] R[r + M Nos T]
- the signal transmitted by the satellite follows more than one path, having reflected off natural or man-made obstacles.
- signals arrive along two or more different paths and are separated in time relative to each other.
- the point of maximum correlation power needs to be determined for each path in order to identify the chip where synchronisation/correlation occurs for each path.
- ⁇ (k) is an integer between 0 and M Nos -1 ,
- the output of block 12 is a sampled received signal.
- a Discrete Fourier Transform is applied to the received signal in block 14 in Figure 1.
- the Discrete Fourier Transform maps the signal from the time domain into the frequency domain and this allows for easier processing of the signal.
- the application of the Discrete Fourier Transform to the received signal follows Eqn 4:
- i'th bin of the Discrete Fourier Transform of the product of KMNos consecutive samples is computed to output KMNos DFT coefficients.
- a so-called bin is a sampling point in the frequency domain and is determined by the maximum frequency and the resolution. For efficiency, several bins may be computed together. Eqn 4 yields the value of the i'th bin in a KMNos point DFT.
- Optimal accuracy of the preferred method is obtained when the bit sequence being transformed in the receiver lies within one duration of the navigational data, although if the expected signal is longer this can be extended.
- Eqn 10 is a known parameter since it is the Fourier Transform of the ideal transmitted signal, more specifically it is the i'th bin of the Fourier Transform of the ideal transmitted signal. This can be found for example from a look-up table.
- the division step results in the output of a series of corresponding transform value ratios given by Eqn 11 :
- the transform value ratios represent the effective amplification of the system at the frequency given by the i'th bin of the Fourier Transform when regarded as a low pass filter and is indicative of each amplified value and relative phase value of the i'th bin.
- a set of bins is selected for example a 256 FT in taken in which there are 256 bins. A fraction of these bins is selected to which the algorithm is applied. For each C/A code, there may be certain bins with greater energy and these are selected preferentially.
- Each of these transform value ratios discretely and independently includes the delay required to obtain synchronisation. It should be noted that the delay spread due to multipath propagation between the different bins is much smaller than the delay itself, and this may be expressed in terms of the mean delay and the incremental delay due to multipath propagation as follows in Eqn 12:
- the result of the calculation is expected to be noisy and have a large variance, i.e. diverge significantly in the range of values it can assume. From here it is possible to obtain a term that is independent of i'th bin by considering the i' + KNos bin and dividing one expression by the other. This allows the transform value ratios to be treated in a uniform manner so as to drive towards obtaining a series of scaled values.
- the output of block 20 is a series of corresponding transform value ratio coefficients that are independent of i thus enabling all terms to be treated in a uniform way.
- the delay can be estimated from these ratios by taking a weighted average after some processing of these ratios.
- each processed ratio may be given equal weight.
- the processing can be designed so as to emphasise the result delivered by a particular bin that may be known to be more accurate by weighting that particular bin in accordance with predetermined criteria.
- Eqn 16 the term (herein denoted Eqn 16)
- Equation 14 is an expression involving complex numbers.
- the delay can be derived from the phase of the complex number.
- the determination of the phase is achieved by taking the Argument (Arg) of the result obtained by calculating equation 14.
- An expression is thereby obtained that can be averaged. This is expressed in a form such that the terms dependent on the delay are separated from the remaining terms.
- Equation 15 may be re-expressed as Eqn 17
- the Arg of the transform value ratio coefficients yields noisy scaled values of the time delay and a noise factor.
- Other operations could be used to obtain the delay from the transform value ratios, such as a non-linear estimation of the delay directly from the transform value ratios. This operation is such that the output is able to be averaged in a manner so as to isolate the time delay so that it can be extracted.
- Eqn 18 is dependent on the delay and not the bin number of the Fourier Transform used, and so the operation performed in Eqn 18 leads to an expression that can be regarded as being composed of the sum of two parts: one of which is not random, while the other is random. Because the random number has with a zero mean, this leaves
- Eqn 20 removes the mean.
- the term below is a zero mean random number (herein denoted Eqn 21 ):
- both the satellite and receiver contain a C/A generator that generate the C/A code sequence.
- the generation of the C/A code in the receiver is set up as follows:
- 1023 G1 register is the state register of the linear feedback shift machine that generates the C/A code
- the output from this subroutine is a sequence of 1023 chips in which the bipolar notation -1 and +1 represents the binary digits 0 and 1 respectively. This is the required C/A code.
- the C/A sequence modulates the carrier signal using the binary phase shift key (BPSK) scheme, as depicted in Figure 3.
- BPSK binary phase shift key
- initialisation begins with generating a sequence that is used for correlation with the received satellite signal.
- the generated sequence is defined by a set of parameters given by:
- M is the number of chips in the sequence
- K is the number of repeat sequences of the 1023 chips that are used for correlation of the generated signal with the received signal
- Nos is the oversampling rate
- the Binary Phase Shifted signal modulated for any time in the C/A code given any particular carrier frequency of fO cycles per chip is given by:
- the sequence that results is the sequence that is to be transformed in accordance with Eqn 10.
- the Discrete Fourier Transform (DFT) of the sequence is pre-calculated and stored for recall, with the resulting coefficients derived from the spectrum being resolved into frequency bins and provided in a look-up table.
- the receiver calculates the DFT of this sequence by sampling the data using the initialisation data defined by the sample sequence above.
- Iset a set of values (Iset) must be defined and are chosen arbitrarily as multiples of KNos.
- a set of bins to be used is selected, the selected set being given by:
- Iset Table [i Nos , ⁇ i , 500 , 520 ⁇ ]
- the next Iset consists of the set of bins that are l+K * Nos for every i in the Iset. This is the set of bins which will be necessary to be used in Eqn 14 above for the division to obtain transform value ratio coefficients.
- x3Pre Map [FourierSampR [ [ # + 1 ] ] & , I set] ;
- x4Pre Map [FourierSampR [ [ # + 1 ] ] £ , Nextl set] ;
- X4Pr Map.... are the values of the DFT bins selected in the Nextlset of the expected transmitted signal.
- the transmitted signal follows more than one path, as a result of reflections.
- the received signal may contain two or more different contributions separated in time relative to each other.
- the received signal is considered as receiving contributions from three paths with delays: 81/Nos T, 85/Nos T and 87/Nos T, which leads to a composite BPS signal given by:
- Step 1 Collect a sequence of consecutive samples of S[t] taken with sampling period of T/Nos in accordance with:
- Step 2 Take the Discrete Fourier Transform of the sequence:
- Step 3 In order to carry out successive correlations, frequency bins, denoted by x1 , are selected from FourierSamp specified by Iset:
- Step 4 frequency bins are selected from FourierSamp, denoted by x2, specified by Nextlset:
- x2 Map [FourierSamp [ [# +1]] &, Nextlset];
- Step 5 By term-wise division of the x2 term by x4Pre, denoted by x5, the output of the transformed values is a vector/matrix of complex vectors:
- Step 7 The x5 term is divided by the x6 term to give a complex vector x7 from which the arguments can be calculated:
- Step II Round to find the time delay in units of T/Nos.
- a bank of filters may be used and the phase between the received filtered signal and ideal signal may be computed.
- the signal is passed through a bank of digital filters which serves to separate the different energies present in the different bands so that the received signal can be split into individual component energies. Ratios may be obtained by combining selected ones of the bands in an appropriate manner. Then the band ratios may be processed in an appropriate manner to acquire a delay figure.
Abstract
Description
Claims
Applications Claiming Priority (5)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GBGB9929327.6A GB9929327D0 (en) | 1999-12-10 | 1999-12-10 | Data processing |
GB9929327 | 1999-12-10 | ||
GB0016246A GB2357208A (en) | 1999-12-10 | 2000-06-30 | Synchronising GPS receivers and transmitters |
GB0016246 | 2000-06-30 | ||
PCT/GB2000/004706 WO2001042811A1 (en) | 1999-12-10 | 2000-12-08 | A receiver for a satellite based position location system |
Publications (1)
Publication Number | Publication Date |
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EP1240531A1 true EP1240531A1 (en) | 2002-09-18 |
Family
ID=26244583
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP00985504A Pending EP1240531A1 (en) | 1999-12-10 | 2000-12-08 | A receiver for a satellite based position location system |
Country Status (4)
Country | Link |
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EP (1) | EP1240531A1 (en) |
JP (1) | JP2003516547A (en) |
AU (1) | AU2192001A (en) |
WO (1) | WO2001042811A1 (en) |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
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CN108369264B (en) * | 2015-12-16 | 2021-08-10 | 皇家飞利浦有限公司 | System and method for wireless communication synchronization for Magnetic Resonance Imaging (MRI) systems |
Family Cites Families (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5420592A (en) * | 1993-04-05 | 1995-05-30 | Radix Technologies, Inc. | Separated GPS sensor and processing system for remote GPS sensing and centralized ground station processing for remote mobile position and velocity determinations |
CA2667775C (en) * | 1995-10-09 | 2013-12-10 | Norman F. Krasner | Gps receiver and method for processing gps signals |
US6133871A (en) * | 1995-10-09 | 2000-10-17 | Snaptrack, Inc. | GPS receiver having power management |
GB2333674B (en) * | 1998-01-21 | 2003-08-27 | Nokia Mobile Phones Ltd | A radio telephone |
-
2000
- 2000-12-08 EP EP00985504A patent/EP1240531A1/en active Pending
- 2000-12-08 WO PCT/GB2000/004706 patent/WO2001042811A1/en active Application Filing
- 2000-12-08 JP JP2001544049A patent/JP2003516547A/en active Pending
- 2000-12-08 AU AU21920/01A patent/AU2192001A/en not_active Abandoned
Non-Patent Citations (1)
Title |
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See references of WO0142811A1 * |
Also Published As
Publication number | Publication date |
---|---|
AU2192001A (en) | 2001-06-18 |
WO2001042811A1 (en) | 2001-06-14 |
JP2003516547A (en) | 2003-05-13 |
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