in the following description , numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention . it will be apparent to one skilled in the art , however , that not all these details are necessarily always needed for practicing the present invention . in this instance , well - known circuits , control logic , and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily . turning now to the drawings , reference is initially made to fig1 , which is a pictorial illustration of a system 10 for detecting areas of abnormal electrical activity and performing ablative procedures on a heart 12 of a living subject 40 in accordance with a disclosed embodiment of the invention . a probe or catheter 14 having a tip 18 is a component of the system 10 , and is percutaneously inserted by an operator 16 through the patient &# 39 ; s vascular system into a chamber or vascular structure of the heart . the operator 16 brings a distal portion of the catheter 14 into contact with the heart wall at a target site that is to be evaluated . electrical activation maps are then prepared , according to the methods disclosed in the above - noted u . s . pat . nos . 6 , 226 , 542 , and 6 , 301 , 496 , and in commonly assigned u . s . pat . no . 6 , 892 , 091 , whose disclosures are herein incorporated by reference . electrical signals can be conveyed from the heart 12 through one or more electrodes 32 located at or near the distal tip 18 of the catheter 14 and through wires 34 to a console 24 . pacing signals and other control signals may be conveyed from the console 24 through the wires 34 and the electrodes 32 to the heart 12 . the electrodes 32 also function as components of an impedance - based positioning system for locating the catheter , which is described below . wire connections 28 link the console 24 with body surface electrodes 30 . additionally , areas determined to be abnormal by evaluation of the electrical activation maps can be ablated by application of thermal energy , e . g ., by passage of radiofrequency electrical current through wires 34 in the catheter to the electrodes 32 , which apply the radiofrequency energy to the myocardium . the energy is absorbed in the tissue , heating it to a point ( typically about 50 ° c .) at which it permanently loses its electrical excitability . when successful , this procedure creates non - conducting lesions in the cardiac tissue , which disrupt the abnormal electrical pathway causing the arrhythmia . the principles of the invention can be applied to different heart chambers , to mapping in sinus rhythm , and when many different cardiac arrhythmias are present . the catheter 14 typically comprises a handle 20 , having suitable controls to enable the operator 16 to steer , position and orient the distal end of the catheter as desired for the ablation . a positioning processor 22 calculates location and orientation coordinates of the catheter 14 . the console 24 contains a generator 25 , the output of which is connected to one or more electrodes 32 on the outer surface of the catheter 14 by wires 34 . the electrodes 32 are at least dual - purpose , being employed to transmit first electrical signals to the heart 12 through the body of the subject 40 to body surface electrodes 30 , to be ultimately evaluated by the positioning processor 22 . in some embodiments , the operator 16 may cause second electrical signals , containing ablative radiofrequency energy to be conducted to the electrodes 32 from an ablation power generator 36 , which can be incorporated in the console 24 . such techniques are disclosed in commonly assigned u . s . pat . no . 6 , 814 , 733 , which is herein incorporated by reference . as noted above , the catheter 14 is coupled to the console 24 , which enables the operator 16 to observe and regulate the functions of the catheter 14 . the positioning processor 22 is preferably a computer with appropriate signal processing circuitry . the processor is coupled to drive a display monitor 29 . the signal processing circuits , typically including an electrocardiographic device 38 , receive , amplify , filter and digitize signals from the catheter 14 , including signals conveyed via the electrodes 32 . the digitized signals are received and analyzed in the console 24 to derive electrical information of medical interest . the information derived from this analysis is used to generate an electrophysiological map of at least a portion of the heart 12 or related structures such as the pulmonary venous ostia . the map may be employed for diagnostic purposes , such as locating an arrhythmogenic area in the heart , or to facilitate therapeutic ablation . other signals used by the positioning processor 22 are transmitted from the console 24 through the wires 34 and the electrodes 32 in order to compute the position and orientation of the catheter 14 . the electrocardiographic device 38 may provide an ecg synchronization signal to the console 24 , which may be displayed on the display monitor 29 or on a separate display ( not shown ). the system 10 typically also includes a reference position sensor , either on an externally - applied reference electrode attached to the exterior of the subject &# 39 ; s body , or on another internally - placed reference catheter ( not shown ), which is inserted into the heart 12 and maintained in a fixed position relative to the heart 12 . by comparing the position of the catheter 14 to that of the reference catheter , the coordinates of catheter 14 are accurately determined relative to the heart 12 , irrespective of heart motion . alternatively , any other suitable method may be used to compensate for heart motion . reference is now made to fig2 , which is a schematic illustration of an impedance - based positioning system 26 , which is a component of the system 10 ( fig1 ), shown connected to a patient body 42 , in accordance with a disclosed embodiment of the invention . this arrangement is similar to that described in the above - mentioned publications by osadchy and govari , modified to operate in accordance with the principles of the present invention . a brief description follows for convenience of presentation : a plurality of body surface electrodes 30 , which can be adhesive skin patches , are coupled to a body surface 44 ( e . g ., the skin ) of the subject . the body surface electrodes 30 are sometimes referred to herein as “ patches ”. in cardiac applications the body surface electrodes 30 are usually distributed so as to surround the heart , three on the chest of the subject and three on the back . however , the number of the body surface electrodes 30 is not critical , and they may be placed at convenient locations on the body surface 44 in the general vicinity of the site of the medical procedure . a control unit 46 , normally disposed in the console 24 ( fig1 , includes current measurement circuitry 48 and one or more catheter electrode transmitters 50 for driving a current through one or more of the electrodes 32 to one or more of the body surface electrodes 30 at respective working frequencies . the control unit 46 is linked to the positioning processor 22 ( fig1 ). the control unit 46 is linked to an ablator 54 , which comprises at least one ablation generator 52 . currents through the body surface electrodes 30 and an ablator body surface electrode 56 flow in a circuit with the ablation generator 52 and are measured by respective current measurement circuits that are disposed within body electrode receivers 58 , sometimes referred to herein as “ patch measurement circuits ”. the body electrode receivers 58 are typically incorporated in the control unit 46 . alternatively , they may be affixed to the body surface electrodes 30 . catheter electrodes are represented in fig2 as measurement electrodes 60 ( circles ) and a dual - purpose electrode 62 ( ellipse ). the dual - purpose electrode 62 functions as an ablation electrode and also serves as one of the measurement electrodes . the body surface electrodes 30 are connected to the body electrode receivers 58 via a patch box 64 , which protects the system from ablation and defibrillation currents . typically the system is configured with six body electrode receivers 58 . the patch box parasitic impedances 66 ( z ), are measured during production and thus known a priori . these impedances are discussed below . typically , although only two measurement electrodes 60 are shown for convenience , about 80 measurement electrodes are used for impedance measurements . typically there are one or two ablation electrodes . the coordinates of a catheter inside the body are determined in the positioning system 26 by passing currents between electrodes on the catheter and the body surface electrodes 30 . the control unit 46 may also control an ablation circuit , comprising ablator 54 , and the dual - purpose electrode 62 . the ablator 54 is typically disposed externally to the control unit 46 and incorporates the ablation generator 52 . it connects with the ablator body surface electrode 56 and to an ablator filter 68 , which in this example is shown within the control unit 46 . however this location is not essential . a switch 70 configures the ablator circuit for different modes of operation as described below . voltage measurement circuitry 72 is provided for determining the output of the catheter electrode transmitters 50 . it will be noted from inspection of fig2 that the ablation circuit is connected to one of the catheter electrode transmitters 50 . the significance of this connection is described below in the section entitled “ ablation leakage training phase ”. reference is now made to fig3 , which is a schematic of an exemplary body electrode receiver 58 ( fig2 ), which is constructed and operative in accordance with a disclosed embodiment of the invention . ideally , the impedance between the body surface electrodes 30 and ground should be zero . in practice it is not zero and thus it affects the current distribution among the body surface electrodes 30 . the effect is frequency dependent and as such , it affects each electrode differently . as will be apparent from the discussion below , during calibration currents of respective frequencies flow through the body surface electrodes 30 . this makes it impossible to predict one electrode location based on a mapping performed by another electrode . another effect that preferably requires compensation is the leakage of positioning current generated by the catheter electrode transmitters 50 ( fig2 ) through the ablation generator 52 and the ablator body surface electrode 56 to the body surface electrodes 30 . the objective of the calibration and compensation procedures is to estimate the current that would have flowed if the input impedance of the body surface electrodes 30 were zero and there were no ablator 54 ( fig2 ). it will be noted that the body electrode receiver 58 includes a body surface electrode generator 74 , a current measurement device 76 , and a voltage measurement device 78 . the body surface electrode generator 74 in different instances of the body electrode receiver 58 may be assigned respective frequencies . alternatively , the body surface electrode generator 74 may be assigned the same frequency in all instances of the body electrode receiver 58 and they may be time - division multiplexed . the quantities described with reference to fig3 are as follows : i — body surface electrode index . j — frequency index . this denotes the frequency f j , which is transmitted through body surface electrode j . z ij — the a priori known impedance of the patch box 64 ( fig2 ). this quantity may be fixed during manufacture , or determined in a post - production procedure . in any case it is treated as a known stable quantity . q ij — the a priori known impedance of a component of the transmission path through the body surface electrode that is not included in voltage measurement . r ij — the a priori known impedance of a component of the transmission path through the body surface electrode that is included in voltage measurement . e i — voltage source ( unknown ) that drives the body surface electrode i with frequency f i . i ij — current measured at body surface electrode i at frequency f j . v ij — voltage measured at body surface electrode i at frequency f j . cv ij — the a priori known voltage calibration constants ; and ci ij — the a priori known current calibration constants . the quantities q ij and r ij are also referred to as “ body surface receiver parasitic impedances ”. reference is now made to fig4 , which is an electrical schematic of the ablator filter 68 ( fig2 ), showing a notch filter 80 and a current measurement element 82 . in normal operation , the notch filter 80 stops most of the current transmitted through the measurement electrodes 60 ( fig2 ), from leaking through the ablator 54 and the ablator body surface electrode 56 . the current measurement element 82 measures the residual leakage current through the ablator 54 . this measurement is used for ablator leakage compensation during normal operation . reference is now made to fig5 , which is a schematic circuit diagram of the positioning system 26 ( fig2 ), which is configured for crosstalk calibration , in accordance with a disclosed embodiment of the invention . the body electrode receivers 58 are connected to a network of resistors 84 that approximately simulate body impedance . each of the resistors 84 has a value of about 10 ohms . the resistive network replaces the connections to the body surface electrodes 30 of normal operation as shown in fig2 . crosstalk calibration is normally done once , either after completion of manufacture , or during initial field installation . during the crosstalk calibration process , the ablator 54 ( fig2 ) is omitted the switch 70 is closed . the catheter electrode transmitters 50 are all turned on with their outputs grounded . the voltages of the catheter electrode transmitters 50 are measured simultaneously . the ratios between measured crosstalk current and the transmitter voltages are computed : i ij — current measurement at patch i , from electrode j ( frequency f j ). v j — electrode j transmitter measured voltage ( frequency f j ). the following calibration data is saved in order to perform crosstalk compensation : x ij ≡ abs ( i ij )— absolute crosstalk value at patch i , from electrode j ( frequency f j ). φ x / v ij ≡ arg ( k ij )— phase between current at patch i , from electrode j , and voltage at electrode j . reference is again made to fig2 . the calculations which follow may be performed by software programs incorporated in the positioning processor 22 ( fig1 ). additionally or alternatively the calculations may be performed using hardware implementations in the positioning processor 22 . during normal operation , the transmitter voltages and the phase relations between the voltages and total current output are stable . thus it is possible to perform training infrequently . alternatively , by training the system as a preliminary to patient procedures , the operator may achieve a higher degree of confidence in the accuracy of the crosstalk compensation . referring again to fig1 , during the training phase of calibration , at least one of the electrodes 32 should be in the mapping volume , i . e ., within a chamber of the heart 12 . the positioning system 26 transmits current through this electrode , and the system operates in a first mode , wherein transmitter voltage is measured along with the resulting patch currents . a second ( normal ) mode of operation , which involves measurements of ablator leakage currents , is described below . during the training phase the switch 70 ( fig2 ) is closed . the training phase is nearly frequency - independent . thus it is necessary to perform training with respect to only one of the measurement electrodes 60 . the ratio between the transmitter voltage and the sum of the patch currents is averaged over 5 seconds . we then calculate the phase between the transmitter voltage and total current ( sum of patch currents ): e — is the transmitting electrode . v e — is the measured transmitter voltage . i i e — current measurement at patch i , from the transmitting electrode . during normal system operation , the crosstalk current is calculated for every transmitting electrode as follows : x ij , φ x / v ij — crosstalk calibration constants ( as defined above ). φ v / i — phase between electrode transmitters and currents ( see below ). i ij — current measured at patch i at frequency f j . the values q ij are used in the discussion below . estimation of the body impedance matrix is essential for ablator leakage compensation and frequency compensation , as described below . the measurements are represented as dft ( discrete fourier transform ) results : q ij ( after crosstalk compensation ) for i ij measurement and p j for { tilde over ( v )} j measurement . denote voltages on the patch as x ij ( for patch i and frequency f j ). also represent s respective body surface electrode generator 74 , which is incorporated in the body electrode receiver 58 ( fig2 ) as multi - frequency e ij ≡ δ ij e j ( which actually means that patch i transmits only frequency f j ). then : the value of i ij can be calculated from the measuremenet q ij by : the patch currents and voltages are related via the patient body impedance matrix ( which does not depend on frequency ): the minus sign is due to a convention that positive current flow into the body — but measured as current flowing out of the body . in matrix notation : − i = σ · x . the patient body impedance matrix is estimated by σ =− i · x − 1 , here , i represents the current matrix , and not the identity matrix : some additional corrections to σ follow , in which there is a transposition of σ at the end : in this section all the currents are “ true measured currents ”, which means that the patch current dft values are multiplied by the corresponding calibration constant ci p , f j , and the ablator leakage current dft values are multiplied by the corresponding calibration constant ci abl f j . referring again to fig2 , the ablator 54 connects to the patient and the positioning system 26 . the switch 70 is closed . an ablator electrode is typically located at the tip 18 ( fig1 ) of the catheter 14 and corresponds to dual - purpose electrode 62 . not all of the current that is driven into the dual - purpose electrode 62 flows through the patient body 42 into body surface electrodes 30 . part of the current produced by the catheter electrode transmitters 50 also goes into the ablator 54 , entering the patient body 42 through the ablator body surface electrode 56 , and finally flows into the body surface electrodes 30 . the measurement electrodes 60 are affected , too . components of their currents follow a path leading through the dual - purpose electrode 62 , through the ablator input resistance into the ablator 54 , the ablator body surface electrode 56 and finally through the body surface electrodes 30 . continuing to refer to fig2 , the ablation leakage training phase of the positioning system 26 begins once the body surface electrodes 30 and the ablator body surface electrode 56 are in place . during ablation leakage training , the switch 70 is open , so that all the current that would otherwise be driven through the dual - purpose electrode 62 by the corresponding catheter electrode transmitter 50 is forced to flow through the ablator 54 via the ablator body surface electrode 56 to the body surface electrodes 30 . the currents through the body surface electrodes 30 i abl p , f m1 ≡{ right arrow over ( i )} abl f m1 ( p is patch index ; f m1 is m1 ( ablation electrode ) frequency ) are measured together with the total current through the ablator body surface electrode 56 , i 8 f m1 . the sum of these currents should be equal to the output of the ablation generator 52 within 20 %: { right arrow over ( i )} cal f m1 =( i + σ · r f m1 ) { right arrow over ( i )} abl f m1 now we can calculate the currents { right arrow over ( i )} abl f k for every working frequency by using the estimation of the patch - to - patch conductance matrix σ as described above : { right arrow over ( i )} abl f k =( i + σ · r f k ) − 1 ·{ right arrow over ( i )} cal f m1 r f k — diagonal matrix with ( r ik + q ik + z ik ) as the i th diagonal element ( the catheter transmits frequency f k ). the ablation current ratios at every frequency are then calculated as : where i abl p , f k is the p component of the vector { right arrow over ( i )} abl f k . the parameters α p , f k should be averaged over a predefined time ( 30 seconds ). continuing to refer to fig2 , during normal system operation , the switch 70 is closed , and it is assumed that the ablator 54 may be operating at any time thereafter . the currents through the body surface electrodes 30 i p , f k are measured together with the current through the ablator body surface electrode 56 i 8 f k . using the parameters α p , f k , estimated at the initialization phase , the compensation is performed as follows : the resulting compensated currents i a p , f k ≡{ right arrow over ( i )} a f k are transferred forward to a frequency compensation module , which may be implemented as a software routine or a hardware module in the positioning processor 22 ( fig1 ). where we separate σ body into a catheter component and patch component as follows : e — total current emitted from the catheter electrode ( if excited by a 1v source ) let the matrix { tilde over ( r )} f k represent the electrode and patch resistances at frequency f k ({ tilde over ( r )} f k is a diagonal matrix with the electrode and patch resistances at the diagonal ). we will separate { tilde over ( r )} f k into catheter ( no resistance ) and patch parts , where r f k is a diagonal matrix , with ( z ik + q ik + r ik ) as the diagonal element number i . { tilde over ( σ )} body =( i + σ body { tilde over ( r )} f k ) − 1 σ body in the final step only relevant quantities were computed . this means that the ideal measurement ( where patch resistances are zero ) s can be estimated from the real measurement { tilde over ( s )} by : writing the ablation compensated current in place of { tilde over ( s )} we get the frequency - compensated currents as : { right arrow over ( i )} c k = abs (( i + σr f k ) ĩ a f k ). here we convert complex values to real by taking their absolute values . r f k — diagonal matrix with ( r ik + q ik + z ik ) as the i th diagonal element ( the catheter transmits frequency f k ). { right arrow over ( i )} a f k — current after ablation leakage compensation . the resulting vector { right arrow over ( i )} c k is a compensated , frequency - independent measure that depends only on the electrode position . it will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove . rather , the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove , as well as variations and modifications thereof that are not in the prior art , which would occur to persons skilled in the art upon reading the foregoing description .