GB2418536A

GB2418536A - Calibration of antenna array elements

Info

Publication number: GB2418536A
Application number: GB0421553A
Authority: GB
Inventors: Philip Wainman
Original assignee: Nortel Networks Ltd
Current assignee: Nortel Networks Ltd
Priority date: 2004-09-27
Filing date: 2004-09-27
Publication date: 2006-03-29
Anticipated expiration: 2024-09-27
Also published as: GB0421553D0; GB2418536B

Abstract

A method or means of calibrating the magnitude and phase of antenna array elements comprises receiving a signal from a positioning satellite at a known position on first and second antenna elements 201, at least partly demodulating 202 and applying a transform 203 to each of the said received signals and comparing the resulting signals to determine the magnitude and phase factors of the second antenna element relative to that of the first antenna element 206. The said method or means may be used in a global positioning system GPS receiver for a vehicle or a telecommunication receiver including a GPS receiver. The partial demodulation of the signal may be the removal of the spreading code associated with the said satellite. The transform may be a Fast Fourier Transform FFT. The calibration process may be repeated to derive calibration factors for each of the antenna elements relative to that of the first element 205 and for each visible positioning satellite 207. Filtering of the signal may be performed prior to the demodulation and the transform procedures.

Description

24 18536 - 1

METHOD OF ANTENNA CALIBRATION

FIELD OF THE INVENTION

This invention relates to methods and apparatus for calbrabon of antenna s arrangements The invention particularly relates to calibration methods for antenna arrangements comprising a plurality of antenna elements, for example an antenna arrangement for a GPS (Global Positioning System) Receiver

BACKGROUND TO THE INVENTION

GPS receivers have antenna arrangements, known as Controlled Reception Pattern Antennas (CRPAs) which include several antenna elements, for example a GPS receiver CRPA may comprise seven antenna elements. In receiving a signal from a satellite, each antenna element receives a slightly different signal and In order to maxmise the overall gain of the system towards the satellite these signals are combined In a beamformer using different weights for each Is received signal, where the weights depend on the location of the satellite which Is sending the signal in question. The beamformer outputs the wanted signal which it has obtained by combination of the seven received signals, each received signal being a combination of the wanted signal, noise and interference.

Although Ideally each antenna element and associated receive chain should be 2' Identical and have denbcal receive performance, and therefore the weights may be calculated from the CRPA geometry, this Is never the reality Therefore it is necessary to calibrate the antenna arrangement to provide calbrabon factors for each antenna element which can then be taken Into consideration In the beamformer.

Known calibration techniques Involve testing the antenna arrangement In an anechoc chamber to produce a table of magnitude and phase calibration factors for each antenna element for each point on the visible hemisphere However, such techniques can only be carried out before the antenna arrangement has been Installed and so the calibration does not take account of the effects of all the cabling or of the structure on/in which the antenna arrangement is mounted - 2

OBJECT TO THE INVENTION

The Invention seeks to provide an Improved method of calibration which mitigates at least one of the problems of known methods.

SUMMARY OF THE INVENTION

s According to a first aspect of the Invention there Is provided a method of calbrabng an antenna arrangement, the antenna arrangement comprising a plurality of antenna elements, and the method composing the steps of receiving a signal from a posibonng satellite at a known position on a first and a second antenna element; at least partially demodulating each of the received signals; 1, performing a transform on each of the at least partially demodulated received signals, and comparing the transforms to determine relative magnitude and relative phase calibration factors for the second antenna element compared to the first antenna element for said known position.

Advantageously, this calbrabon process can be performed with the antenna Is arrangement fully Installed In its carrier vehicle (e 9 a helicopter or ship) which means that the calibration factors take into account the full assembly (all the cables) and the influence of the surrounding structures.

Advantageously, this further enables the calibration process to be carried out periodically whilst the antenna arrangement is In service or alternatively the calibration can be carried out continuously or substantially continuously The step of at least partially demodulating may comprise removing the spreading code associated with the positioning satellite. The satellite may be a GPS satellite and the spreading code may be one of a C/A code, a P code and a P(Y) code.

us The transform may be a fast Founer transform.

The method may be repeated for each of the plurality of antenna elements to determine relative magnitude and relative phase calibration factors for the each antenna element compared to the first antenna element for said known position The method may be repeated for each positioning satellite visible from said :(' antenna arrangement.

The method may further comprise the step of filtering said received signals prior to the step of at least partially demodulating each of the received signals.

Advantageously, this can reduce the processing power required to perform the transform whilst still providing sufficient resolution.

The method may further comprise the step of filtering said at least partially demodulated signals prior to performing the fast Fourier transforms Advantageously, this can reduce the processing power required to perform the transform whilst still providing sufficient resolution.

According to a second aspect of the invention there is provided an apparatus for lo calibrating an antenna arrangement, the antenna arrangement comprising a plurality of antenna elements, and the apparatus comprising. an input for receiving signals from a first and a second antenna element, the signals comprising a signal from a positioning satellite at a known position; means for at least partially demodulating the signals, means for performing transforms on the at least partially demodulated signals; and means for comparing the transforms to determine relative magnitude and relative phase calibration factors for the second antenna element compared to the first antenna element for said known position.

The input may comprise a plurality of inputs and wherein there is one input for go each of the plurality of antenna elements The means for at least partially demodulating the signals may comprise a code tracking loop.

The means for performing transforms may comprise a processor The apparatus may further comprise addbonal means for at least partially demodulating, wherein there Is one said means for each of the plurality of antenna elements.

The apparatus may further comprise additional means for performing transforms, wherein there is one said means for each of the plurality of antenna elements.

The apparatus may further comprise a filter on each input.

The apparatus may further comprise a filter prior to said means for performing transforms According to a third aspect of the Inventor there is provided a GPS receiver comprising a plurality of antenna elements, and calibration apparatus comprising. an Input for receiving signals from a first and a second antenna element, the signals comprising a signal from a GPS satellite at a known position; means for at least partially demodulating the signals; means for performing transforms on the partially demodulated signals; and means for comparing the transforms to determine relative magnitude and relative phase calbrahon factors for the second antenna element compared to the first antenna element for said known position.

o According to a fourth aspect of the invention there Is provided a vehicle comprising a GPS receiver as described above The vehicle may be for use In the air, on the sea or on land.

According to a fifth aspect of the invention there Is provided a telecommunications receiver comprising. a plurality of antenna elements, a GPS is receiver, and calibration apparatus composing. an Input for receiving signals from a first and a second antenna element, the signals comprising a signal from a GPS satellite at a known position; means for at least partially demodulating the signals; means for performing transforms on the partially demodulated signals, and means for comparing the transforms to determine relative magnitude and so relative phase calibration factors for the second antenna element compared to the first antenna element for said known position.

The method may be performed by software In machine readable form on a storage medium.

The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the Invention

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference to the accompanying drawings in which.

figure 1 is a schematic diagram of calibration apparatus; figure 2 Is a flow diagram of the calibration process; - 5 Figure 3 Is a graphical representation of the output of the code-tracking loop, and figure 4 Is a graphical representation of an antenna arrangement with two visible satellites and the corresponding measurements

DETAILED DESCRIPTION OF INVENTION

Embodiments of the present Invention are described below by way of example only. These examples represent the best ways of putting the nvenbon into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved.

Jo Typically antenna arrangements containing more than one antenna element, such as those used on GPS receivers, are calibrated in an anechoic chamber prior to deploymenVinstallaton. This provides magnitude and phase calibration factors for each antenna element for each point on the visible hemisphere However, these calibration factors do not take Into account the full assembly, e.g. all the cables and the effects of the structure n/on which the antenna arrangement is mounted (e.g. ship, plane, helicopter, land vehicle). It is therefore beneficial to have a calibration process that can be used with the antenna arrangement in its final position Additionally, a calibration process that can be used with the arrangement in its ho final position enables the arrangement to be recalibrated penodcally, or for calibration to be carried out continually.

Ideally, calibration is done using a single tone signal received on the antenna elements. This is feasible in an anechoic chamber where a test signal generator can be used. Using such a signal generator to generate the calibration signal Is :5 however Impractical when calibrating a device once Installed, particularly if period re-calbrabon Is required It Is Impossible to use such a signal generator if the calibration is to be carried out as a continual process throughout the operating life of the antenna arrangement.

According to this invention, the GPS signals from the satellites themselves are so used to calibrate the antenna arrangements. However a GPS signal is not a single tone but a spread spectrum signal. A GPS signal typically includes data at 50Hz and the signal Is then spread using a pseudo-random code, with each of the GPS satellites in orbit using a different pseudo-random code. If C/A code - 6 (Course/Acqusbon code) Is used the resultant bandwidth Is about 1 MHz and if P code (Precise code) Is used the resultant bandwidth is about 10MHz The position of each satellite is known at any particular bme, and therefore if the antenna arrangement is calibrated using a signal from a satellite X at bme T and s hence a position of a elevation and b azimuth, then the resultant calibration factors can be used for any satellite at the same position of a elevation and b azimuth (the differences in antenna element performance within the antenna arrangement relate to the angle of arrival of the signal and not to which satellite transmitted the signal) lo Although the description herein refers to the calibration for satellite position, it will be apparent to a person skilled in the art that satellite direction relative to the antenna onentaton Is required. This can be found from the satellite azimuth and elevation (determined from the GPS receiver) and the antenna orientation from associated Inertial Navigation System (INS).

Figure 1 shows a schematic diagram of calibration apparatus which can be used to calibrate an antenna arrangement composing more than one antenna element In situ. Figure 2 is a flow diagram of the calibration process 200.

A signal Is simultaneously captured on two or more antenna elements In an antenna arrangement (step 201). The signal will contain signals from a number of satellites plus noise and interference The captured signals are then processed and this processing Is described below for a single signal. Each captured signal Is however processed in the same way, either sequentially (i.e. comparison of 1 and 2, followed by comparison of 1 and 3, then 1 and 4 etc) or in parallel 2s In known GPS receivers, the captured signal from each antenna element is fed to a Digital Downconverter 10 (DDC) which shifts the signal to a lower IF (or baseband) and performs filtering and rate conversion. There is typically one DDC per Input channel. The output of the DDC 10 Is fed to the beamformer and part of the output Is split off and fed to the calbrabon apparatus 14 The first stage in the calibration process Is to remove the pseudo-random code modulation (step 202) for a selected satellite e.g. Code1 used by satellite SV1 (Space Vehicle 1). The input 11 to the calbrabon apparatus 14 (which is the output of the DDC 10) Is Split and fed to both the correlation loop 12 (also referred to as a code-tracking loop) and the mixer 13 The correlation loop 12 selects the particular pseudo-random code to be removed (e.g. Code1) and the output of the correlation loop Is mixed in the mixer 13 with the output of the DDC to remove the selected pseudorandom code (e g Code1). The removal of the code collapses the spectrum of the signal from the broad spectrum with a bandwidth of about 1 MHz (for the C/A code case), to almost a CW (Continuous Wave) tone. It Is in fact not CW but a very slow modulation for carrying data, at 50Hz. This tone Is offset from the centre frequency by the Doppler shift of the satellite (+6kHz) as shown In figure 3. This stage is also referred to as "partial demodulation" because the code removal demodulates the signal but does not to reduce the signal completely to a CW signal The second stage In the calibration process is to perform a fast Fourier transform (FFT) on the output of the mixer 13 (step 203) and this is performed In a processor 15. From the FFT, a phase and amplitude of the captured signal is obtained (step 204) Although this description refers to use of a FFT, other transforms may alternatively be used to extract the phase and amplitude information As described above, a signals simultaneously captured on two or more antenna elements, and each captured signal (one for each antenna element) Is processed In the same way (steps 202-205), as described above Having performed this processing, phase and amplitude is obtained for each antenna element corresponding to the signal from SV1. The third stage In the calibration process Is then to compare the phases and amplitudes (step 206) The phase and amplitude (also referred to as magnitude) of one antenna element is denoted the reference phase and reference amplitude and then the differences between these reference values and the values for the other antenna elements are calculated In order to obtain relative calibration factors for each antenna element.

This is shown below Example values obtained from the FFTs for SV1: Element Number Phase Amplitude 1 45 1 2 2 48 1 2 _.40 1 1 - 8

_

4 42 115 __ _ _. . . _.

52 12.0 11 3 _._ 1 14 Relative calibration factors for the position of SV1 at the bme the signals were captured (note that the amplitude calibration factor Is shown here, by way of example as a multiplication factor): Element Number Phase Amplitude 1 O 1 3 -5 0 92 4 -3 1 25 +7 1 67 6 0 1 08

_ 1.17

The position of SV1 (elevation and azimuth) at the time the signals were captured can be determined from the GPS receiver As detailed above, each satellite uses a different code and each received signal comprises a combnabon of signals from different satellites which are visible to JO the antenna element ("visible satellites") If for example, the antenna arrangement has 4 visible satellites, SV1 (Space Vehicle 1), SV2, SV3, and SV4, by sequentially performing the calibration process using the codes for each visible satellite (steps 202-207) on the same captured signals, relative calibration factors can be obtained for each antenna element for the positions of each of the satellites at the time the signals were captured - 9 - The flow chart of figure 2 Is shown by way of example only and is not Intended to indicate the only possible order of steps In the calbrabon method It will be apparent to those skilled In the art that the orders of some steps can be changed, for example the signal from a single antenna element could be analysed for all s the visible satellites prior to comparison of the results with any other antenna elements.

Figure 4 shows a seven element antenna arrangement 401 and two of its visible satellites, SV23 and SV5 The phase and magnitudes for each satellite are shown in the graph 402 for antenna 1 and graph 403 for antenna 2 If antenna 1 lo is used as the reference, the following calibration factors would be required for antenna 2 _. _ Elevation Azimuth Phase Amplitude 330 +145 1.18 33 6D +134 1 5 By repeating the calibration process 200 at different bmes, calibration factors can be obtained for different positions as the satellites will have moved, and it is therefore possible to obtain calibration factors for all possible satellite positions The processing power required in the processor 15 (figure 1) which performs the FFT, can be reduced by filtering the output of the DDC and/or filtering the de spread signal output from the code-trackng loop 11. For example, the output of the DDC may be filtered to about 1 MHz bandwidth (for the C/A code case) and the de-spread signal could be filtered to about 6kHz bandwidth. In order to achieve 50Hz resolution only about 300 points are required so a 1024 point FFT Is ample.

As described above, in determining the calibration factors, the pseudorandom code is removed from the signal (step 202, figure 2). There are two types of codes which are used in current GPS systems: the C/A (Course/Acquisition) code which Is a sequence of 1023 pseudo- random, binary, bphase modulations on the GPS carrier at a chip rate of 1 023MHz and Is the civilian code. There is one code per satellite. -

the P (Precise) code which Is a very long sequence of pseudo-random binary biphase modulations on the GPS carrier at a chip rate of 1 0.23MHz which repeats about every 267 days Each one week segment of this code Is unique to one GPS satellite and is reset each week. Once s encoded, this is referred to as the P(Y) code and is the military code Although the above embodiment described the invention with reference to the C/A code, the Inventor Is also applicable to the P or P(Y) code A person skilled In the art will appreciate that references in the above description relating to a 1 MHz bandwidth relates to the use of the C/A code and these values will change appropriately If P code Is used GPS signals operate on two carrier frequencies, 1575 42MHz and 1227 6MHz, referred to as L1 and L2. Currently only L1 has the C/A modulation although both have P(Y) modulation Consequently, currently If the calibration is performed using C/A code, calbrabon factors could only be obtained for L1 is However, calibration factors for L2 can be determined with reference to the L1 calibration factors by performing some selective calibration on both L1 and L2 in an anechoc chamber before installation to determine the relationship (or correction factors) between L1 and L2 calbrabon factors it Is expected that future satellites will also transmit a C/A code on L2 and therefore calibration factors for L2 could be determined directly In addbon to the L2 calibration factors. As P(Y) modulation Is present on both L1 and L2, calibration factors for both L1 and L2 could be determined directly from the GPS signals using the method 200 described above.

Although the above description relates to an antenna arrangement for a GPS us receiver, the invention is also applicable to other antenna arrangements comprising a plurality of antenna elements where the receiver has GPS receiving capabilities, e.g. for telecommunications applications where the telecommunications device has assisted-GPS capabilities. In this case, the antenna may be calibrated at a GPS frequency, e.g. L1 and then appropriate correction factors may be used to determine the calibration factors required for use at the telecommunications frequency This is an analogous process to that described above for determining the L2 calibration factors when using C/A code Although the above description refers to GPS posbonng satellites, the Invention is also applicable to other positioning satellite systems. - 11

It will be understood that the above descrpbon of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art without departing from the spirit and scope of the invention. s - 12

Claims

1 A method of calbrabng an antenna arrangement, the antenna arrangement comprising a plurality of antenna elements, and the method compnsng the steps of receiving a signal from a positioning satellite at a known position on a first and a second antenna element; at least partially demodulating each of the received signals, performing a transform on each of the at least partially demodulated received signals; and lo comparing the transforms to determine relative magnitude and relative phase calibration factors for the second antenna element compared to the first antenna element for said known position 2. A method of calbrabng an antenna arrangement as claimed in claim 1, wherein said step of at least partially demodulating comprises removing the l5 spreading code associated with the positioning satellite.

3. A method of calibrating an antenna arrangement as claimed in claim 2, wherein the positioning satellite is a GPS satellite and the spreading code is one of a C/A code, a P code and a P(Y) code.

4. A method of calibrating an antenna arrangement as claimed in any of the go preceding claims, wherein said transform is a fast Fourier transform A method of calibrating an antenna arrangement as claimed in any of the preceding claims, wherein said method is repeated for each of the plurality of antenna elements to determine relative magnitude and relative phase calibration factors for the each antenna element compared to the first antenna element for said known position 6 A method of calbrabng an antenna arrangement as claimed in any of the preceding claims, wherein said method is repeated for each positioning satellite visible from said antenna arrangement.

7 A method of calbrabng an antenna arrangement as claimed in any of the preceding claims, further comprising the step of: - 13 filtenng said received signals prior to the step of at least partially demodulating each of the received signals 8. A method of calibrating an antenna arrangement as claimed In any of the preceding claims, further composing the step of: filtering said at least partially demodulated signals poor to performing the transforms.

9 Apparatus for calibrating an antenna arrangement, the antenna arrangement composing a plurality of antenna elements, and the apparatus comprising: an Input for receiving signals from a first and a second antenna element, the t' signals composing a signal from a positioning satellite at a known position, means for at least partially demodulating the signals; means for performing transforms on the at least partially demodulated signals; and means for comparing the transforms to determine relative magnitude and relative phase calibration factors for the second antenna element compared to the first antenna element for said known position.

10. Apparatus for calbrabng an antenna arrangement as claimed in claim 9, wherein said input comprises a plurality of inputs and wherein there is one Input for each of the plurality of antenna elements 11. Apparatus for calibrating an antenna arrangement as claimed in claim 9 or 10, wherein said means for at least partially demodulating the signals comprises a code tracking loop.

12. Apparatus for calibrating an antenna arrangement as claimed in any of claims 9-11, wherein said means for performing transforms comprises a processor.

13. Apparatus for calibrating an antenna arrangement as claimed In any of claims 9-12, further compnsng additional means for at least partially demodulating, wherein there is one said means for each of the plurality of antenna elements. - 14

14. Apparatus for calibrating an antenna arrangement as claimed in any of claims 9-13, further composing additional means for performing transforms, wherein there is one said means for each of the plurality of antenna elements 15. Apparatus for calibrating an antenna arrangement as claimed in any of claims 9-14, further composing a filter on each input 16 Apparatus for calibrating an antenna arrangement as claimed in any of claims 9-15, further composing a filter prior to said means for performing transforms.

17. A GPS receiver comprising lo a plurality of antenna elements; and calbrabon apparatus comprising: an input for receiving signals from a first and a second antenna element, the signals composing a signal from a GPS satellite at a known position, means for at least partially demodulating the signals; means for performing transforms on the partially demodulated signals; and means for comparing the transforms to determine relative magnitude and relative phase calbrabon factors for the second antenna element compared to the first antenna element for said known position 18. A vehicle comprising a GPS receiver as claimed in claim 17.

19. A vehicle as claimed in claim 18, wherein the vehicle is for use either in the air, on the sea or on land 20. A telecommunications receiver comprising.

a plurality of antenna elements; a GPS receiver; and :5 calibration apparatus comprising.

an input for receiving signals from a first and a second antenna element, the signals comprising a signal from a GPS satellite at a known position; - 15 means for at least partially demodulating the signals; means for performing transforms on the partially demodulated signals; and means for comparing the transforms to determine relative magnitude and relative phase calibration factors for the second antenna element compared to the first antenna element for said known position