GB2418537A

GB2418537A - Carrier to noise ratio used in the adjustment of the gain and phase of antenna array elements

Info

Publication number: GB2418537A
Application number: GB0421555A
Authority: GB
Inventors: Douglas Higgins; Chris Ward
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: GB2418537B; GB0421555D0

Abstract

The invention is directed to a method and apparatus for calibration of antenna arrangements comprising a plurality of antenna elements, for example an antenna arrangement for a GPS (Global Positioning System) Receiver or for a telecommunications receiver with GPS receiving capability. The method comprising the steps of: receiving a signal from a positioning satellite at a known position on a first and a second antenna element; determining a carrier to noise ratio for each received signal; comparing the carrier to noise ratios for each received signal to determine a magnitude calibration factor for said second antenna element compared to said first antenna element; combining said first and second received signals with equal magnitude; adjusting a phase offset of one of said signals; and determining a value of phase offset which results in a peak in carrier to noise ratio of said combined signal, wherein said offset is a phase calibration factor for said second antenna element compared to said first antenna element.

Description

24 1 8537

ANTENNA CALIBRATION METHOD

FIELD OF THE INVENTION

This invention relates to methods and apparatus for calibration of antenna s arrangements The invention particularly relates to calibration methods for antenna arrangements composing 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 lo 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 maxmse the overall gain of the system towards the satellite these signals are combined In a beamformer using different weights for each 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 combnabon of the seven received signals, each received signal being a combnabon of the wanted signal, noise and Interference Although ideally each antenna element and associated receive chain should be Identical and have identical 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 calibration 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 Inventor seeks to provide an improved method of calibration which mitigates at least one of the problems of known methods

SUMMARY OF THE INVENTION

According to a first aspect of the Invention there Is provided a method of calibrating an antenna arrangement, the antenna arrangement comprising a plurality of antenna elements, and the method comprising the steps of. receiving a signal from a positioning satellite at a known position on a first and a second antenna element, determining a carrier to noise ratio for each received signal; to comparing the carrier to noise ratios for each received signal to determine a magnitude calibration factor for said second antenna element compared to said first antenna element; combining said first and second received signals with equal magnitude, adjusting a phase offset of one of said signals, and determining a value of phase offset which results In a peak In carrier to noise ratio of said Is combined signal, wherein said offset Is a phase calibration factor for said second antenna element compared to said first antenna element Advantageously, this calibration process can be performed with the antenna arrangement fully Installed In its carrier vehicle (e g 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 earned out periodically whilst the antenna arrangement Is in service or alternatively the calibration can be carried out continuously or substantially continuously.

The positioning satellite may be a GPS satellite 2s The step of combining comprises may use said magnitude calibration factor such that said first and second signals are combined with equal magnitude.

The step of adjusting may comprise adjusting a phase offset of said second signal The method may further compose the step of repeating said method steps to compare received signals from each of said plurality of antenna elements to a received signal from said first antenna element to obtain a set of magnitude and phase calibrator factors for said posbonng satellite - 3 The method may further comprise the step of repeating said method steps for all visible positioning satellites According to a second aspect of the invention there is provided an apparatus for calibrating an antenna arrangement, the antenna arrangement comprising a s plurality of antenna elements, and the apparatus compnsng. a beamformer having a plurality of inputs, each said input connected to one of said plurality of antenna elements, said antenna elements receiving signals from a positioning satellite at a known position; a receiver, connected to an output from said beamformer and arranged to determine a carrier to noise ratio for each signal lo output from said beamformer, and a processor, the apparatus having a first mode of operation In which the beamformer Is arranged to sequentially pass the signal from each antenna element to said receiver, wherein the processor Is arranged to compare the carrier to noise ratios for each received signal to determine a magnitude calibration factor for said second antenna element compared to said first antenna element, and the apparatus having a second mode of operation in which the beamformer combines first and second received signals with equal magnitude and adjusts a phase offset of one of said signals, wherein the processor Is arranged to determine a value of phase offset which results In a peak In carrier to noise ratio of said combined signal, wherein said offset Is a phase calibration factor for said second antenna element compared to said first antenna element 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 Figures 1 & 2 are schematic diagrams of calibration apparatus, figure 3 shows a flow diagram of the calbrabon process; and figures 4 & 5 are schematic diagrams of calibration apparatus.

Common reference numerals have been used throughout the figures where appropriate

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 Invention Into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved Typically antenna arrangements containing more than one antenna element, such as those used on GPS receivers, are calibrated in an anechoic chamber lo 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 Anion 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.

Addbonally, a calbrabon process that can be used with the arrangement in its final position enables the arrangement to be re-calibrated periodically, or for calibration to be camed out continually go Ideally, calibration Is done using a single tone signal received on the antenna elements. This is feasible In an anechoc chamber where a test signal generator can be used Using such a signal generator to generate the calibration signal is however impractical when calibrating a device once installed, particularly if period re-calibration 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 used to calibrate the antenna arrangements However a GPS signal is not a single tone but a spread spectrum signal The position of each satellite is known so at any particular bme, and therefore If the antenna arrangement is calibrated using a signal from a satellite X at time T and 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 - 5 element performance within the antenna arrangement relate to the angle of arrival of the signal and not to which satellite transmitted the signal) 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 s antenna orientation 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) Figures 1 and 2 show schematic diagrams of a first embodiment of the calibration apparatus which can be used to calibrate an antenna arrangement composing lo more than one antenna element in situ. figure 3 is a flow diagram of the calibration process 300 When operating with a multiple element CRPA, a beamformer 102 is required to combine the antenna element signals onto a single output 104 as shown In figure 1. The GPS receiver 106 will contain one or more number of tracking loops 108 The GPS receiver may be configured such that each tracking loop will track a unique satellite signal or alternatively the satellite to be tracked by each tracking loop may be selectable The GPS receiver 106 outputs the carrier to noise ratio (C/No) for each of the tracking loops 108 The first stage of the calibration process obtains an estimate of the relative amplitudes on each of the antenna elements by setting beamformer weights 110 so that each antenna channel in turn Is connected to the tracking loops within the GPS receiver (steps 301-304), and such a configuration is shown in figure 1.

In this first stage, the first beamformer channel is set in turn to use a weight of 1 and all the other weights set to zero (step 301) and the value of C/No output from 2s the GPS receiver recorded (step 302). This Is then repeated for each antenna element In turn (step 303) by sequentially setting each weight to 1, whilst all other weights are set at zero The set of values of C/No recorded represent one per antenna element for a particular satellite. These values are then compared by denoting one antenna element (e.g antenna element 1) as the reference and so determining how the values of C/No for each antenna element compare to the reference figure (step 304). This comparison results in amplitude (also referred to as magnitude) calibration factors for each antenna element for the given position In space of the selected satellite at the time the measurement was made - 6 (312) The position information relating to the satellites Is stored within the GPS receiver.

This process Is demonstrated below for a given satellite, SV1 (Space Vehicle 1) at a position of a elevation and b azimuth The amplitude calibration factor In this example Is a multplicabon factor Element Number C/No (dB) . Amplitude calibration factor 1 = 32 == 1 2 31 0 89 3 33 1.12 4 30 0 79 34 1 26 6 33 1.12 7 32 1 The second stage of the calibration process obtains an estimate of the relative phases of each antenna channel for a given satellite direction, again with respect to a reference channel, e.g channel 1. The beamformer 102 combines the lo reference channel and one other channel, referred to here as an auxiliary channel, with amplitudes equalised (step 305), as shown in figure 2 for antenna element 2. This combination and equalization of channels 1 and 2 is achieved by setting weights W. and W2 appropriately whilst setting all the other weights to zero. Information on the relative amplitudes of the two channels, In order to Is perform the equalization may be determined from the first stage of the calibration process The phase on each auxiliary channel is then rotated to maximise the C/No output from the GPS receiver for each tracking loop in turn (steps 306-308) At the point where the C/No is maximsed, the two channels, the reference and the auxiliary, are phase aligned and therefore the relative phase calibration factor go for the auxiliary channel Is equal to the required phase offset to reach this point (steps 309, 313) An increase in C/No of 3dB is expected compared to that obtained from amplitude calibration This process is then repeated (step 310) comparing each antenna channel in turn to the reference channel to determine a set of relative phase calibration factors for the given satellite It will apparent to a person skilled in the art, that instead of rotating the phase of the auxiliary channel, the phase calibration factor could alternatively be obtained s by rotating the phase of the reference channel Furthermore it will be apparent that Instead of rotating the phase to achieve the maximum value of C/No it would alternatively be possible to rotate the phase the achieve the minimum value of C/No, which would correspond to the signals being out of phase However, detecting a minimum value is unlikely to be as accurate as detecting the lo maximum value because there Is likely to be a measurement floor below which the value of C/No cannot be measured.

The above process determines a relative magnitude and a relative phase calibration factor for each antenna element for the particular position of the selected satellite at the time the measurements were made. In order to obtain i, further measurements, the process Is then repeated for each of the visible satellites in turn (step 311).

The flow chart of figure 3 Is shown by way of example only and Is not intended to Indicate the only possible order of steps In the calibration method It Will be apparent to those skilled in the art that the orders of some steps can be changed, 3 for example the magnitude calbrabon factors could be determined for all antenna elements for all visible satellites before the phase calibration factors are determined Where multiple Inputs to the GPS receiver are available, providing direct Input to individual tracking loops, multiple beamformers may be employed to accelerate us the process, and an example of such a situation Is shown in figures 4 and 5.

Here a Digital Adaptive Antenna Control Unit (DACU) 401 comprises a plurality of beamformers 402 and the GPS receiver or Digital Receiver Module (DRM) 403 comprises a plurality of tracking loops 404 The configuration for the first stage of the calibration (steps 301-304) Is shown In so figure 4, where each of the beamformers is connected to one of the antenna elements 405. Each tracking loop 404 In the DRM 403 receives an output from one of the beamformers 402 In the DACU 401. Each tracking loop 404 is set to track the same satellite In this manner, all the magnitude calibration factors for the given satellite can be obtained in parallel - 8 The confgurabon for the second stage of the calibration (steps 305-310) Is shown In figure 5 This second stage uses N-1 beamformers 402 and N-1 tracking loops 404, where N Is the number of antenna elements 405 Each of the beamformers 402 combines the reference channel and one of the auxiliary channels with amplitudes equalsed (as described above In relation to figure 2) as shown in the expanded section 502, and the phases of the auxiliary channels are rotated independently by each of the beamformers. Again the output of each beamformer 402 Is input to one of the tracking loops 404 in the DRM 403. As before, each tracking loop 404 is set to track the same satellite. In this manner, o all the phase calibration factors for the given satellite can be obtained In parallel.

To build a complete calibration table for the hemisphere use may be made of all of the visible satellites (noting that which satellites are visible will change with time and location) as they move across the sky over a significant period of time (the satellites have a 12 hour orbit time) 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 10.23MHz 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 encoded, this is referred to as the P(Y) code and is the military code The GPS receiver may use either code In order to receive signals from satellites and the invention is not dependent on a particular code type. However, as GPS signals operate on two comer frequencies, 1575.42MHz and 1227.6MHz, referred to as L1 and L2, and currently only L1 has the C/A modulation although both have P(Y) modulation, it may be preferable to use the P(Y) code in order to obtain separate calibration factors for both L1 and L2 It is expected that future satellites will also transmit a C/A code on L2 and therefore there may not be a benefit in selecting either code type in preference to the other.

Although the above description relates to an antenna arrangement for a GPS receiver, the invention Is also applicable to other antenna arrangements comprising a plurality of antenna elements where the receiver has GPS receiving - 9 - capablties, e g. for telecommunications applications where the telecommunications device has asssted-GPS capabilities In this case, the antenna may be calibrated at a GPS frequency, eg L1 and then appropriate correction factors may be used to determine the calbrabon 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 positioning satellites, the Invention is also applicable to other positioning satellite systems.

It will be understood that the above description of a preferred embodiment is lo 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. - 10

Claims

1 A method of calibrating an antenna arrangement, the antenna arrangement comprising a plurality of antenna elements, and the method composing the steps of receiving a signal from a positioning satellite at a known position on a first and a second antenna element, determining a carrier to noise ratio for each received signal; comparing the Garner to noise ratios for each received signal to determine a magnitude calibration factor for said second antenna element compared to said lo first antenna element; combining said first and second received signals with equal magnitude; adjusting a phase offset of one of said signals; and determining a value of phase offset which results in a peak in carrier to noise ratio of said combined signal, wherein said offset is a phase calibration factor for is said second antenna element compared to said first antenna element.

2. A method of calibrating an antenna arrangement as claimed in claim 1, wherein the positioning satellite is a GPS satellite

3. A method of calibrating an antenna arrangement as claimed In any preceding claim, wherein said step of combining comprises using said magnitude calibration factor such that said first and second signals are combined with equal magnitude

4. A method of calibrating an antenna arrangement as claimed In any preceding claim, wherein said step of adjusting comprises adjusting a phase offset of said second signal.

5. A method of calibrating an antenna arrangement as claimed In any preceding claim, further comprising the step of repeating said method steps to compare received signals from each of said plurality of antenna elements to a received signal from said first antenna element to obtain a set of magnitude and phase calibration factors for said positioning satellite - 11 6 A method of calibrating an antenna arrangement as claimed in any preceding claim, further comprising the step of repeating said method steps for all visible positioning satellites 7. Apparatus for calibrating an antenna arrangement, the antenna arrangement s composing a plurality of antenna elements, and the apparatus comprising.

a beamformer having a plurality of inputs, each said input connected to one of said plurality of antenna elements, said antenna elements receiving signals from a positioning satellite at a known position, and a receiver, connected to an output from said beamformer and arranged to lo determine a carrier to noise ratio for each signal output from said beamformer, and a processor, the apparatus having a first mode of operation in which the beamformer is arranged to sequentially pass the signal from each antenna element to said Is receiver, wherein the processor is arranged to compare the carrier to noise ratios for each received signal to determine a magnitude calibration factor for said second antenna element compared to said first antenna element; and the apparatus having a second mode of operation In which the beamformer combines first and second received signals with equal magnitude and adjusts a phase offset of one of said signals, wherein the processor is arranged to determine a value of phase offset which results In a peak in carrier to noise ratio of said combined signal, wherein said offset is a phase calibration factor for said second antenna element compared to said first antenna element 2s