CA1210118A - Receivers for navigation satellite systems - Google Patents
Receivers for navigation satellite systemsInfo
- Publication number
- CA1210118A CA1210118A CA000418551A CA418551A CA1210118A CA 1210118 A CA1210118 A CA 1210118A CA 000418551 A CA000418551 A CA 000418551A CA 418551 A CA418551 A CA 418551A CA 1210118 A CA1210118 A CA 1210118A
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- signal
- components
- signals
- fourier transform
- transformed
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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
- 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
- G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
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- Engineering & Computer Science (AREA)
- Radar, Positioning & Navigation (AREA)
- Remote Sensing (AREA)
- Computer Networks & Wireless Communication (AREA)
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Position Fixing By Use Of Radio Waves (AREA)
Abstract
ABSTRACT
IMPROVEMENTS IN OR RELATING TO RECEIVERS FOR NAVIGATION
SATELLITE SYSTEMS
(See Figure 1 of the drawings) The present invention provides a receiver for a navigation satellite system such as NAVSTAR, which includes an antenna (1) for receiving an incoming coded, time-based, spread spectrum signal which includes navigational data from a plurality, P, of satellites, means (23) for deriving baseband I and Q
components of each of the signals, means (10,11) for digitising the I and Q components, a Fast Fourier Transform (FFT) processor (13) for transforming the digitised I and Q components and their respective codes, and means (15a to 15d) arranged to multiply together the transformed components and codes derived for each of the P satellite signals and to inverse transform the signals and to then determine correlation peaks in the inverse transformed signals.
IMPROVEMENTS IN OR RELATING TO RECEIVERS FOR NAVIGATION
SATELLITE SYSTEMS
(See Figure 1 of the drawings) The present invention provides a receiver for a navigation satellite system such as NAVSTAR, which includes an antenna (1) for receiving an incoming coded, time-based, spread spectrum signal which includes navigational data from a plurality, P, of satellites, means (23) for deriving baseband I and Q
components of each of the signals, means (10,11) for digitising the I and Q components, a Fast Fourier Transform (FFT) processor (13) for transforming the digitised I and Q components and their respective codes, and means (15a to 15d) arranged to multiply together the transformed components and codes derived for each of the P satellite signals and to inverse transform the signals and to then determine correlation peaks in the inverse transformed signals.
Description
~2~
IMPROVEMENTS IN OR RELATING TO RECEIVERS FOR NAVIGATI~N
SATELLITE SYSTEMS
The present invention re]ates to receivers for, for example, a ship or an aircraft, for receiving signals from navigation satellites such as for example NAVSTAR satellites - which form part of a global positioning system.
NAVSTAR is a system which requires a receiver capable of receiving signals simultaneously from at least four NAVSTAR
satellites to obtain a navigational fix. The satellites transmit highly stable time-based, spreàd spectrum signals and navigational data which are received by users. ~he user~s receiver correlates spread spectrum signals from four or ~ore satellites with known spectrum spreading codes to thereby obtain navigational data which is then computed to give the user's position, (see "Navigation", 25, (2), Summer 1978).
The present invention provides a low-cost receiver for a navigation satellite system such as NAVSTAR.
According to the present invention a receiver for a navigation satellite system includes an antenna for receiving coded time-based spread spectrum signals which include navigational data from a plurality, P, of satellites, and means for convolving a segment of the signal with its code by Fourier transforming the segment, multiplying the transform point by point with the segment to give a resultant signal and inverse transforming the resultant signal to produce a correlation peak at a point having a position which gives the relative shift between the incoming signal and the code.
G~
Said means for convolving a aegment of the signal with its code includes means for deriving baseband I and Q components of each of the signals, means for digitising the I and Q compo-nents a Fast Fourier Transform (FFT) proce6sor and for transform-ing the digitised I and Q components and their respective codes, multiplier means arranged to multiply together the transformed components and codes derived for each of the P satellite signals, and an inverse FPT processor arranged to inverse transform the signals from the multiplier means.
The receiver may further include scanning means for deter-mining correlation peaks in the inverse transformed signale.
Thus, the present invention provides a navigation satel-- lite system receiver comprising: an antenna for .receiving an incoming coded, time-based, spread-spectrum, continuous signal which includes navigational data from a plurality, P, of satel-lites; and convolver means, connected to receive signals from said antenna for (a) convolving segments of said signal with codes therein by Fourier transforming said segments, (b) cycli-cally shifting points of the Fourier transform to produce a transform of a near baseband signal, (c) multiplying said shifted points with corre6ponding points of a pre-computed transform of a segment of spread spectrum code to give a resultant signal, and (d) inverse transforming said resultant signal to produce a cor-relation peak at a point having a position which gives the rela-tive shift between said incoming slgnal and said code.
The present invention further provides a receiver for a navigation satellite system, comprising: an antenna adapted to receive coded, time-based, spread-spectrum, continuous signals whlch include navigational data from a plurality, P, of satel-litea; means connected to said antenna for deriving baseband I
and Q components from said signals; means for digitizing said I
and Q components; a Fast Fourier Tranaform processor connected to recei.ve said digitized I and Q components and to transform said components and their respective codes and to provide a multiplied signal; mul'iplier means connected to said processor, ~or receiv ing and multiplying together said transformed components and their 2a codes; an inverse Fast Fourier Transform processor connected to said multiplier means, for inverse trans-forming said multiplied signal and providing a trans-formed signal; and amplitude scanning means connected to said inverse Fast Fourier Transform processor, for determining correlation peaks in said transformed signal.
An advantage of the system is that it does not require the generation of a local oscillator frequency for each satellite with Doppler shift compensation.
If a sequence of measurements x(tn) taken at intervals equally spaced by time T is represented by the discrete frequency spectrum N-l ( n) ~ a(~k) exp i~k n (l) k=0 N-l then a(~k) = ] ~ x(tn) exp(-j~ktn) (2) n=0 where t =nT, ~k = 2rrkéNT and N is the total number of measuraments in the sequence. This is the Discrete Fourier Transform (DFT) pair. The interval between frequency components is leNT, ie the reciprocal of the sequence length. The Fast Fourier Transform tFFT) is a way of calculating the DFT quickly, and is most convenient if N is a power of 2.
, ~
~2~
A digitised ~avstar signal s(t ) can be expressed as a series using (1):
s(tn) = aO exp jwOtn ~ a1 exp i~1 n and so may the code: .
c(tn) = bo exp j~otn ~ b1 exp i~1 n ; q'hen the cro~_oorrelation RSC(~) is given by:
I ~sc(~) = <~(t~ ~ ~)c (tn)>
where ~ denotes averaging over a long series of tn values, and can only have values which are integer multiples of T O ~hen from (3) and (4):
sc( ) ~ aO exp(j~O(tn ~)J ~ a1 exp~ (t ~ ~ ..3 * *
x ~bo e~p~ Otn)~ b1 exp~ i~1 n~
* *
= aObO exp j~O~ ~ a1b1 exp i~1 since all terms of the form exp`j(w~ )tn~average to zero if i ~ j. Comparing (5) with (3)~ and since the coefficients aO, a1 etc are the transform of s~ it follows that the cross-correlation may be performed by multiplying corresponding elements of the transforms (with conjugation since s and c are complex) and then performing an inverse transform.
An embodiment of the invention will now be described by way of example only with reference to the drawings of which:
Figure 1 is a schematic circuit diagram of a receiver for receiving signals from ~VS~AR navigation atellites and compu-ting position data therefrom.
Figure 2 is a schematic diagram of a sub-circuit of the cirouit shown in Figure 1.
The receiver of Figure 1 has an antenna 1 for simultaneously receiving signals from four ~AVSTAR satellites.
~he incoming signals -include a Coar~e/Acquisition (C/A) signal co~prising a 1.023 M~z olock rate code bi-phase modula-ted on a 1575.4~ ~Hz carrier which is also bi~phase modulated by 50 bit/sec navigation data, the whole being su~jec-t to a Doppler shift of up to 5 parts in 10 due to the motion of the satellites and the reoeiver. ~he signals received by -the antenna are fed to an input circuit 23 which employs fixed tuning to bring a 1575.42 MHz carrier to zero frequency and produces in-phase and quadra-tur0 componen-ts~ the outpu-t bandwidth being about 1 MHz with a , noise fig~re of about 5 dB to give a typical signallnoise ratio of the code at output, indicated as A and B in Figure 1, of ~20 dB.
The signals from the antenna 1 are fed via filter 2 and amplifier 3 to a mixer 4 which ~as a second input from a local oscillator 24. After passing filter and amplification staees 5~ 6 respectively the ou-tput eig~al is then divided into in-plane and quadrature components in mixers~ 8~ 9.
The output signals A~ B are digitised in analogue-to-digital ¢onverters 10~ 11 respec-tively~ which produce 1024 sampleS of the I and Q signals at 977.5 ns intervals every 10 ms, the sample sequence lasting 1 ms and hence corresponding to 1 kHz bandwidth, the resolution being 4 to 5 bi-ts. A
store 12 holds the 1024 samples which are then processed by a Fast Fourier Transform (FFT) processor 13 which produces the required transform. The output signals ~rom the ~
processor 13 are fed in parallel on four lines 14 to four identical signal processors 15 a to d~ each corresponding to a particular satellite. ~he processor 13 includes ISI F$~
circuits ïn the form of simple card sub-systems as described in "3lectronic Design", 9, pp 78-85, 26 April 1979, b~
L. Schirm.
One of the processors 15 a is shown in Figure 2 and includes a code transform store 17, which contains -the -transformed C/A code for a particular satelli-te with an adjustmen-t for Doppler shift~ The other processors 15 b to d hold oorresponding transformed C/A codes for the other three satellites. In processor 15 a~ each point in the transform of the signal input to the processor is m~tiplied in a multiplier 18, by the corresponding point in the transform of the code. Tne product signals from the multiplier 18 are then inverse transformed in an inverse h'~ processor 20. The re-ordering of data which is usually performed ~n a~ ~FT i~ not required in the PFT processors 13 and 20, since processor 20 reRtores the na-tural order los-t in processor 13. The output signal from the inverse ~$T processor 20 is then scanned to find a correlation peak in an amplitude scanner 21 whlch determines the correlation point of greatest amplitude and o~tputs the position of the peak~ which gives range information. The speed of operation of the ~FT processors , and multipliers mus-t be such that a batch of 1024 samples ¦ 20 can be processed in 10 ms or lessD ~atches of samples are ta~en at 10 ms in-tervals rather than 20 ms since if one batch ,coincides with a data edge which suppresses correlation the next batch will not do so. A data extraction processor 22 accepts from the ~canner 21~ complex values of the correlation pea~ a-t 20 ms intervals and fits them to a phase curve and ex-tracts -the data. The peaks have a residual Doppler shift of up -to ~00 Hz but are sampled at 20 m~ interval~ which corresponds to 50 Hz. It i3 highly lIkely that the data extrao-tion processor 22 will fi-t the correlation phases to a curve corresponding to an alias of the Doppler residual! but this is unimportant since all such aliased frequencies also carry the data.
- ~2~
The outputs from all four processors 15 a to d are fed to a standard ~AVSTAR computer 16 which calculates position from the range information from the oorrelators ; and data from -the da-ta extraotors in a known manner.
The embodimen-t described abo~e has been given by way of example only and other embodiments incorporating variations or modifications to the desoribed reoeiver will be apparen-t to -those skilled in -the art. For example, in the described embodiment -the code transform store 17 in each of the processore 15 a to d contain the transformed C/A
code for its satellite. Rather than store the transformed C/A oode it may be convenient to generate the code and transform and store it using the ~FT prooessor 13 before acquisition is attempted.
Fhrther, in the described embodiment 1 ms of signal is convolved at a time and has a bandwidth of about 1 kHz.
For further noise reduotion the sample sequenoe may be increased to give a corresponding reduction in bandwidth~
_ ~hu~, for example, 16384 samples span 16 ms of signal? and occupy most of a data bit with 4 ms left ~or errors in synchronisa-tion with the data~ ~rans~orming the oode is not so diffioult since it is rep~titive at 1 ms intervals~ unlike noise~ and so has sidebands at 1 kHz intervals. If appropria-te samples of the code are chosen, only one in every sixteen of the 16384 transform points will be non-zero7 and these points can be calculated using a 1024-point -transform.
~imilarly~ only 1024 multiplica-tions are needed~ bu-t -the inverse transform mus-t s-tart from -the 16384 values which clude all the zeros.
.r~
'rhe bandwidth of -the 1~384 sample convolution is 1/16 kHz, about the narrowest in which single data bits may still be recovered. If the Doppler uncertainty9 hitherto ignored, is larger than this it is possible to search several Doppler cells by shifting the transform of the signal a suitable number of points in ei-ther direction before multiplying by the transform of the code. 'rhis is because the signal transform is in fact the frequency spectrum, so a shift of 1 point corresponds to a baseband frequency shift of 1/16 kXz. In this way the signal can always be moved to within 1/32 kHz of the true baseband, and the correlation found. Parallel inverse transform processors could be used -to search several Doppler oells at once, starting with the same signal samples.
With respect to the C/A code an alternative to a longer transform is possible if the Doppler shi~t is sufficiently well-kno~Jn. 'rhe signal samples are accumulated cyslically in 1023 memory locations, so that the signal enhances and the noise tends to cancel. '~his process can be continued for 1/2 of a Doppler error cycle before the enhancement is lost. If -this is longer than 20 ms, phase corrections for tha data must be applied.
Regarding data recovery once correlation has been identified~ the complex da-ta poin-t at the correlation point is an estima-te of the phase difference between the slgnal and . _ . .. ... . _ ,,,, . _,,,, ... _ ... ... _ ", .. " .. , _ . _ _ .. . _ . , local oscillator durin2 the sampled period~ If this period is the 20 ms of a data bit the bes-t estimate of the data phase will be obtained. A worse S/N ratio is -tolerable for data recovery than for initial correlation, since false correlations are not a problem and a fairly high bit error rate can be corrected by the pari-ty coding.
- If it is required to measure -the carrier frequency accurately, it will be necessary to inolude the processor inside a phase locked loop. ~or this purpose, use can be made of the fact tha-t the I and Q values of tha correlation peak are equal to the I and Q outputs of the arm filters .
of a Costas carrier loop in a conventional receiver, whers these ~ilters are of the integra-te-and-dump type with an integrated time equ~l to the correlator~s sample sequence length. This is the optimal detector for biphase modulation~ The loop can there~ore be closed by multiplying the I and Q values together to generate the local oscillator control signalO This, of course, requires a separate local osoillator and signal digitiser for each satellite~
To avoid the problem of the samples coinciding with the code bit edges, it may not be disadvantageous to sample at a different frequency from the code bit rate, eg 1024 samples during 1 ms (1023 bits). The comparison code should be similarly sampled.
The number o~ signal processors corresponding to the processors 15 of ~ig~re 1 employed in a receiver according to the invention will depend on the maæimum number of satellites from which signals can be simultaneously received.
~lthough a multiple element receiver has been described it ~5 will be appreciated that by using time sharing techniques the number of ohannels can be reduced.
IMPROVEMENTS IN OR RELATING TO RECEIVERS FOR NAVIGATI~N
SATELLITE SYSTEMS
The present invention re]ates to receivers for, for example, a ship or an aircraft, for receiving signals from navigation satellites such as for example NAVSTAR satellites - which form part of a global positioning system.
NAVSTAR is a system which requires a receiver capable of receiving signals simultaneously from at least four NAVSTAR
satellites to obtain a navigational fix. The satellites transmit highly stable time-based, spreàd spectrum signals and navigational data which are received by users. ~he user~s receiver correlates spread spectrum signals from four or ~ore satellites with known spectrum spreading codes to thereby obtain navigational data which is then computed to give the user's position, (see "Navigation", 25, (2), Summer 1978).
The present invention provides a low-cost receiver for a navigation satellite system such as NAVSTAR.
According to the present invention a receiver for a navigation satellite system includes an antenna for receiving coded time-based spread spectrum signals which include navigational data from a plurality, P, of satellites, and means for convolving a segment of the signal with its code by Fourier transforming the segment, multiplying the transform point by point with the segment to give a resultant signal and inverse transforming the resultant signal to produce a correlation peak at a point having a position which gives the relative shift between the incoming signal and the code.
G~
Said means for convolving a aegment of the signal with its code includes means for deriving baseband I and Q components of each of the signals, means for digitising the I and Q compo-nents a Fast Fourier Transform (FFT) proce6sor and for transform-ing the digitised I and Q components and their respective codes, multiplier means arranged to multiply together the transformed components and codes derived for each of the P satellite signals, and an inverse FPT processor arranged to inverse transform the signals from the multiplier means.
The receiver may further include scanning means for deter-mining correlation peaks in the inverse transformed signale.
Thus, the present invention provides a navigation satel-- lite system receiver comprising: an antenna for .receiving an incoming coded, time-based, spread-spectrum, continuous signal which includes navigational data from a plurality, P, of satel-lites; and convolver means, connected to receive signals from said antenna for (a) convolving segments of said signal with codes therein by Fourier transforming said segments, (b) cycli-cally shifting points of the Fourier transform to produce a transform of a near baseband signal, (c) multiplying said shifted points with corre6ponding points of a pre-computed transform of a segment of spread spectrum code to give a resultant signal, and (d) inverse transforming said resultant signal to produce a cor-relation peak at a point having a position which gives the rela-tive shift between said incoming slgnal and said code.
The present invention further provides a receiver for a navigation satellite system, comprising: an antenna adapted to receive coded, time-based, spread-spectrum, continuous signals whlch include navigational data from a plurality, P, of satel-litea; means connected to said antenna for deriving baseband I
and Q components from said signals; means for digitizing said I
and Q components; a Fast Fourier Tranaform processor connected to recei.ve said digitized I and Q components and to transform said components and their respective codes and to provide a multiplied signal; mul'iplier means connected to said processor, ~or receiv ing and multiplying together said transformed components and their 2a codes; an inverse Fast Fourier Transform processor connected to said multiplier means, for inverse trans-forming said multiplied signal and providing a trans-formed signal; and amplitude scanning means connected to said inverse Fast Fourier Transform processor, for determining correlation peaks in said transformed signal.
An advantage of the system is that it does not require the generation of a local oscillator frequency for each satellite with Doppler shift compensation.
If a sequence of measurements x(tn) taken at intervals equally spaced by time T is represented by the discrete frequency spectrum N-l ( n) ~ a(~k) exp i~k n (l) k=0 N-l then a(~k) = ] ~ x(tn) exp(-j~ktn) (2) n=0 where t =nT, ~k = 2rrkéNT and N is the total number of measuraments in the sequence. This is the Discrete Fourier Transform (DFT) pair. The interval between frequency components is leNT, ie the reciprocal of the sequence length. The Fast Fourier Transform tFFT) is a way of calculating the DFT quickly, and is most convenient if N is a power of 2.
, ~
~2~
A digitised ~avstar signal s(t ) can be expressed as a series using (1):
s(tn) = aO exp jwOtn ~ a1 exp i~1 n and so may the code: .
c(tn) = bo exp j~otn ~ b1 exp i~1 n ; q'hen the cro~_oorrelation RSC(~) is given by:
I ~sc(~) = <~(t~ ~ ~)c (tn)>
where ~ denotes averaging over a long series of tn values, and can only have values which are integer multiples of T O ~hen from (3) and (4):
sc( ) ~ aO exp(j~O(tn ~)J ~ a1 exp~ (t ~ ~ ..3 * *
x ~bo e~p~ Otn)~ b1 exp~ i~1 n~
* *
= aObO exp j~O~ ~ a1b1 exp i~1 since all terms of the form exp`j(w~ )tn~average to zero if i ~ j. Comparing (5) with (3)~ and since the coefficients aO, a1 etc are the transform of s~ it follows that the cross-correlation may be performed by multiplying corresponding elements of the transforms (with conjugation since s and c are complex) and then performing an inverse transform.
An embodiment of the invention will now be described by way of example only with reference to the drawings of which:
Figure 1 is a schematic circuit diagram of a receiver for receiving signals from ~VS~AR navigation atellites and compu-ting position data therefrom.
Figure 2 is a schematic diagram of a sub-circuit of the cirouit shown in Figure 1.
The receiver of Figure 1 has an antenna 1 for simultaneously receiving signals from four ~AVSTAR satellites.
~he incoming signals -include a Coar~e/Acquisition (C/A) signal co~prising a 1.023 M~z olock rate code bi-phase modula-ted on a 1575.4~ ~Hz carrier which is also bi~phase modulated by 50 bit/sec navigation data, the whole being su~jec-t to a Doppler shift of up to 5 parts in 10 due to the motion of the satellites and the reoeiver. ~he signals received by -the antenna are fed to an input circuit 23 which employs fixed tuning to bring a 1575.42 MHz carrier to zero frequency and produces in-phase and quadra-tur0 componen-ts~ the outpu-t bandwidth being about 1 MHz with a , noise fig~re of about 5 dB to give a typical signallnoise ratio of the code at output, indicated as A and B in Figure 1, of ~20 dB.
The signals from the antenna 1 are fed via filter 2 and amplifier 3 to a mixer 4 which ~as a second input from a local oscillator 24. After passing filter and amplification staees 5~ 6 respectively the ou-tput eig~al is then divided into in-plane and quadrature components in mixers~ 8~ 9.
The output signals A~ B are digitised in analogue-to-digital ¢onverters 10~ 11 respec-tively~ which produce 1024 sampleS of the I and Q signals at 977.5 ns intervals every 10 ms, the sample sequence lasting 1 ms and hence corresponding to 1 kHz bandwidth, the resolution being 4 to 5 bi-ts. A
store 12 holds the 1024 samples which are then processed by a Fast Fourier Transform (FFT) processor 13 which produces the required transform. The output signals ~rom the ~
processor 13 are fed in parallel on four lines 14 to four identical signal processors 15 a to d~ each corresponding to a particular satellite. ~he processor 13 includes ISI F$~
circuits ïn the form of simple card sub-systems as described in "3lectronic Design", 9, pp 78-85, 26 April 1979, b~
L. Schirm.
One of the processors 15 a is shown in Figure 2 and includes a code transform store 17, which contains -the -transformed C/A code for a particular satelli-te with an adjustmen-t for Doppler shift~ The other processors 15 b to d hold oorresponding transformed C/A codes for the other three satellites. In processor 15 a~ each point in the transform of the signal input to the processor is m~tiplied in a multiplier 18, by the corresponding point in the transform of the code. Tne product signals from the multiplier 18 are then inverse transformed in an inverse h'~ processor 20. The re-ordering of data which is usually performed ~n a~ ~FT i~ not required in the PFT processors 13 and 20, since processor 20 reRtores the na-tural order los-t in processor 13. The output signal from the inverse ~$T processor 20 is then scanned to find a correlation peak in an amplitude scanner 21 whlch determines the correlation point of greatest amplitude and o~tputs the position of the peak~ which gives range information. The speed of operation of the ~FT processors , and multipliers mus-t be such that a batch of 1024 samples ¦ 20 can be processed in 10 ms or lessD ~atches of samples are ta~en at 10 ms in-tervals rather than 20 ms since if one batch ,coincides with a data edge which suppresses correlation the next batch will not do so. A data extraction processor 22 accepts from the ~canner 21~ complex values of the correlation pea~ a-t 20 ms intervals and fits them to a phase curve and ex-tracts -the data. The peaks have a residual Doppler shift of up -to ~00 Hz but are sampled at 20 m~ interval~ which corresponds to 50 Hz. It i3 highly lIkely that the data extrao-tion processor 22 will fi-t the correlation phases to a curve corresponding to an alias of the Doppler residual! but this is unimportant since all such aliased frequencies also carry the data.
- ~2~
The outputs from all four processors 15 a to d are fed to a standard ~AVSTAR computer 16 which calculates position from the range information from the oorrelators ; and data from -the da-ta extraotors in a known manner.
The embodimen-t described abo~e has been given by way of example only and other embodiments incorporating variations or modifications to the desoribed reoeiver will be apparen-t to -those skilled in -the art. For example, in the described embodiment -the code transform store 17 in each of the processore 15 a to d contain the transformed C/A
code for its satellite. Rather than store the transformed C/A oode it may be convenient to generate the code and transform and store it using the ~FT prooessor 13 before acquisition is attempted.
Fhrther, in the described embodiment 1 ms of signal is convolved at a time and has a bandwidth of about 1 kHz.
For further noise reduotion the sample sequenoe may be increased to give a corresponding reduction in bandwidth~
_ ~hu~, for example, 16384 samples span 16 ms of signal? and occupy most of a data bit with 4 ms left ~or errors in synchronisa-tion with the data~ ~rans~orming the oode is not so diffioult since it is rep~titive at 1 ms intervals~ unlike noise~ and so has sidebands at 1 kHz intervals. If appropria-te samples of the code are chosen, only one in every sixteen of the 16384 transform points will be non-zero7 and these points can be calculated using a 1024-point -transform.
~imilarly~ only 1024 multiplica-tions are needed~ bu-t -the inverse transform mus-t s-tart from -the 16384 values which clude all the zeros.
.r~
'rhe bandwidth of -the 1~384 sample convolution is 1/16 kHz, about the narrowest in which single data bits may still be recovered. If the Doppler uncertainty9 hitherto ignored, is larger than this it is possible to search several Doppler cells by shifting the transform of the signal a suitable number of points in ei-ther direction before multiplying by the transform of the code. 'rhis is because the signal transform is in fact the frequency spectrum, so a shift of 1 point corresponds to a baseband frequency shift of 1/16 kXz. In this way the signal can always be moved to within 1/32 kHz of the true baseband, and the correlation found. Parallel inverse transform processors could be used -to search several Doppler oells at once, starting with the same signal samples.
With respect to the C/A code an alternative to a longer transform is possible if the Doppler shi~t is sufficiently well-kno~Jn. 'rhe signal samples are accumulated cyslically in 1023 memory locations, so that the signal enhances and the noise tends to cancel. '~his process can be continued for 1/2 of a Doppler error cycle before the enhancement is lost. If -this is longer than 20 ms, phase corrections for tha data must be applied.
Regarding data recovery once correlation has been identified~ the complex da-ta poin-t at the correlation point is an estima-te of the phase difference between the slgnal and . _ . .. ... . _ ,,,, . _,,,, ... _ ... ... _ ", .. " .. , _ . _ _ .. . _ . , local oscillator durin2 the sampled period~ If this period is the 20 ms of a data bit the bes-t estimate of the data phase will be obtained. A worse S/N ratio is -tolerable for data recovery than for initial correlation, since false correlations are not a problem and a fairly high bit error rate can be corrected by the pari-ty coding.
- If it is required to measure -the carrier frequency accurately, it will be necessary to inolude the processor inside a phase locked loop. ~or this purpose, use can be made of the fact tha-t the I and Q values of tha correlation peak are equal to the I and Q outputs of the arm filters .
of a Costas carrier loop in a conventional receiver, whers these ~ilters are of the integra-te-and-dump type with an integrated time equ~l to the correlator~s sample sequence length. This is the optimal detector for biphase modulation~ The loop can there~ore be closed by multiplying the I and Q values together to generate the local oscillator control signalO This, of course, requires a separate local osoillator and signal digitiser for each satellite~
To avoid the problem of the samples coinciding with the code bit edges, it may not be disadvantageous to sample at a different frequency from the code bit rate, eg 1024 samples during 1 ms (1023 bits). The comparison code should be similarly sampled.
The number o~ signal processors corresponding to the processors 15 of ~ig~re 1 employed in a receiver according to the invention will depend on the maæimum number of satellites from which signals can be simultaneously received.
~lthough a multiple element receiver has been described it ~5 will be appreciated that by using time sharing techniques the number of ohannels can be reduced.
Claims (3)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A navigation satellite system receiver comprising:
an antenna for receiving an incoming coded, time-based, spread-spectrum, continuous signal which includes navigational data from a plurality, P, of satellites; and convolver means, connected to receive signals from said antenna for (a) convolving segments of said signal with codes therein by Fourier transforming said segments, (b) cyclically shifting points of the Fourier transform to produce a transform of a near baseband signal, (c) multiplying said shifted points with corresponding points of a pre-computer transform of a seg-ment of spread spectrum code to give a resultant signal, and (d) inverse transforming said resultant signal to produce a correlation peak at a point having a position which gives the relative shift between said incoming signal and said code.
an antenna for receiving an incoming coded, time-based, spread-spectrum, continuous signal which includes navigational data from a plurality, P, of satellites; and convolver means, connected to receive signals from said antenna for (a) convolving segments of said signal with codes therein by Fourier transforming said segments, (b) cyclically shifting points of the Fourier transform to produce a transform of a near baseband signal, (c) multiplying said shifted points with corresponding points of a pre-computer transform of a seg-ment of spread spectrum code to give a resultant signal, and (d) inverse transforming said resultant signal to produce a correlation peak at a point having a position which gives the relative shift between said incoming signal and said code.
2. A receiver for a navigation satellite system, com-prising:
an antenna adapted to receive coded, time-based, spread-spectrum, continuous signals which include navigational data from a plurality, P, of satellites;
means connected to said antenna for deriving baseband I and Q components from said signals;
means for digitizing said I and Q components;
a Fast Fourier Transform processor connected to receive said digitized I and Q components and to transform said compo-nents and their respective codes and to provide a multiplied signal;
multiplier means connected to said processor, for receiv-ing and multiplying together said transformed components and their codes;
an inverse Fast Fourier Transform processor connected to said multiplier means, for inverse transforming said multiplied signal and providing a transformed signal; and amplitude scanning means connected to said inverse Fast Fourier Transform processor, for determining correlation peaks in said transformed signal.
an antenna adapted to receive coded, time-based, spread-spectrum, continuous signals which include navigational data from a plurality, P, of satellites;
means connected to said antenna for deriving baseband I and Q components from said signals;
means for digitizing said I and Q components;
a Fast Fourier Transform processor connected to receive said digitized I and Q components and to transform said compo-nents and their respective codes and to provide a multiplied signal;
multiplier means connected to said processor, for receiv-ing and multiplying together said transformed components and their codes;
an inverse Fast Fourier Transform processor connected to said multiplier means, for inverse transforming said multiplied signal and providing a transformed signal; and amplitude scanning means connected to said inverse Fast Fourier Transform processor, for determining correlation peaks in said transformed signal.
3. A receiver for a navigation satellite system, having an antenna adapted to receive coded, time-based, spread-spectrum, continuous signals which include navigational data from a plural-ity, P, of satellites, comprising:
means connected to said antenna for deriving baseband I and Q components from said signals;
means for digitizing said I and Q components;
a Fast Fourier Transform processor connected to receive said digitized I and Q components and to transform said components and their respective codes and to provide a multiplied signal;
multiplier means connected to said processor, for receiving and multiplying together said transformed components and their codes;
an inverse Fast Fourier Transform processor connected to said multiplier means, for inverse trnasforming said multiplied signal and providing a transformed signal; and amplitude scanning means connected to said inverse Fast Fourier Transform processor, for determining correlation peaks in said transformed signal.
means connected to said antenna for deriving baseband I and Q components from said signals;
means for digitizing said I and Q components;
a Fast Fourier Transform processor connected to receive said digitized I and Q components and to transform said components and their respective codes and to provide a multiplied signal;
multiplier means connected to said processor, for receiving and multiplying together said transformed components and their codes;
an inverse Fast Fourier Transform processor connected to said multiplier means, for inverse trnasforming said multiplied signal and providing a transformed signal; and amplitude scanning means connected to said inverse Fast Fourier Transform processor, for determining correlation peaks in said transformed signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8139168 | 1981-12-31 | ||
| GB8139168 | 1981-12-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1210118A true CA1210118A (en) | 1986-08-19 |
Family
ID=10526901
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000418551A Expired CA1210118A (en) | 1981-12-31 | 1982-12-23 | Receivers for navigation satellite systems |
Country Status (4)
| Country | Link |
|---|---|
| US (1) | US4601005A (en) |
| EP (1) | EP0083480B1 (en) |
| CA (1) | CA1210118A (en) |
| DE (1) | DE3278915D1 (en) |
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-
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- 1982-12-10 EP EP82306609A patent/EP0083480B1/en not_active Expired
- 1982-12-10 DE DE8282306609T patent/DE3278915D1/en not_active Expired
- 1982-12-14 US US06/449,789 patent/US4601005A/en not_active Expired - Lifetime
- 1982-12-23 CA CA000418551A patent/CA1210118A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| EP0083480A1 (en) | 1983-07-13 |
| EP0083480B1 (en) | 1988-08-17 |
| DE3278915D1 (en) | 1988-09-22 |
| US4601005A (en) | 1986-07-15 |
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