Abstract:
A method for determining a yaw direction of a wind turbine includes the following steps, receiving at a component of the wind turbine a signal broadcasted from a source, determining a direction from the component towards the source based on the received signal, determining the yaw direction of the wind turbine in relation to the determined direction towards the source is provided. Further, a wind turbine and a device as well as a computer program product and a computer readable medium are disclosed for performing the method.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to European Application No. EP 14179302.6, having a filing date of Jul. 31, 2014, the entire contents of which are hereby incorporated by reference. 
       FIELD OF TECHNOLOGY 
       [0002]    The following relates to a method, a wind turbine and to a device for determining a yaw direction of a wind turbine. In addition, an according computer program product and a computer readable medium are suggested. 
       BACKGROUND 
       [0003]    A wind turbine in operation will not always experience wind perpendicular to a rotor plane. When the rotor plane (which is also referred to as heading) of a wind turbine is not perpendicular to the wind, the efficiency will decrease. Therefore, actual wind turbines comprise a yaw system designed to automatically adjust their heading, like, e.g., rotating the rotor plane perpendicular to the incoming wind or to maintain an angle relative to the wind to maximize the surface area of the turbine rotor. 
         [0004]    Usually, the yaw system is part of a nacelle, which may be involved in a yawing movement, i.e. being rotatable mounted on top of a tower via at least one yaw bearing. A rotor is attached to an upwind side of the nacelle. The rotor is coupled via a drive train to a generator housed inside the nacelle. The rotor includes a central rotor hub and a plurality of blades mounted to and extending radially from the rotor hub defining the rotor plane. 
         [0005]    It is important for wind power plant operators to know an actual position or direction of the rotor plane or heading of the respective wind turbine, the plane or heading being correlated with an actual position or direction of the nacelle. The actual direction of the nacelle is also referred to as a yaw direction or a yaw position or, in relation to a predefined direction (e.g. a cardinal direction), as a yaw angle. Alternatively the yaw angle may be defined as the direction of the nacelle in relation of the direction of the incoming wind. 
         [0006]      FIG. 1  shows in a schematically top view an exemplary scenario of a wind turbine  100  in relation to the well known cardinal points or compass points which are indicated as a compass rose in the background of  FIG. 1 . A rotor hub  120  including a plurality of blades  130  defining a rotor plane  140  is mounted at the upwind side of a nacelle  110 . According to the scenario of  FIG. 1 , an actual yaw direction  150  (which is also referred to as “compass heading”) of the wind turbine  100 , i.e. the actual direction of the nacelle  110  points towards the cardinal direction “North East” or “NE”. As exemplarily shown in  FIG. 1 , an absolute yaw angle “θ YawAngle ” is referencing the actual yaw direction  150  of the wind turbine in relation towards the cardinal direction “North” or “N”. The absolute yaw angle θ YawAngle  is indicated by an arrow  160 , wherein θ YawAngle =45°. 
         [0007]    Information concerning the yaw direction is a common used basis for analyzing data concerning a wind turbine or performing sector management control like, e.g.,
       site wind mapping and historical data collection on wind patterns,   limiting wind turbine noise by avoiding operation in wind directions where noise generation is excessive,   automatic curtailment and regulation of a wind turbine at yaw angles where significant wind turbulence might be present,   prevention of shadow flicker/light pollution for neighboring residents or businesses at certain times of day and yaw angles,   remote manual control of a wind turbine yaw position,   efficiency testing and wind turbine power curve validation, or   safe positioning of the rotor during ice conditions when service teams are approaching.       
 
         [0015]    In order to determine, e.g., an absolute yaw angle, a wind turbine may be equipped with a yaw encoder, measuring the relative yaw direction in relation to a stationary object like, e.g., a tower being secured to a foundation at ground level. The yaw encoder is typically calibrated by determining a reference yaw direction or reference yaw angle after finalization of the wind turbine installation. 
         [0016]    In some scenarios the initial calibration of the yaw angle is incorrect or less accurate due to applying a rough estimate or rule of thumb to determine a cardinal direction as a basis or reference for the yaw angle calibration. 
         [0017]    A further possible reason for an inaccurate yaw angle calibration is a wind turbine installation based on a design including powerful permanent magnets, eliminating the possibility of applying magnetic compasses to determine the yaw direction or yaw angle. A magnetic compass, as a further general disadvantage, comprises inaccurateness per se, in particular at installations located at high geographic latitudes. 
         [0018]    Alternatively, compasses based on GPS (Global Positioning System) or other satellite-based positioning systems have been applied to determine the reference yaw direction of the wind turbine. 
         [0019]    [EP 2 599 993 A1] refers to a method to determine the yaw angle of a component of a wind turbine wherein at least one receiver of an automated and autonomous positioning system is used to generate position-data of the receiver. The receiver is arranged at a wind turbine location being subjected to a yawing movement. 
         [0020]    However, applying such kind of automated and autonomous positioning systems for calibration issues is restricted due to high costs and limited accuracy. 
       SUMMARY 
       [0021]    An aspect relates to improving the approach for determining an accurate yaw direction and/or yaw angle of a wind turbine. 
         [0022]    A further aspect relates to a method is provided for determining a yaw direction of a wind turbine comprising the following steps,
       receiving at a component of the wind turbine a signal broadcasted from a source,   determining a direction from the component towards the source based on the received signal, and   determining the yaw direction of the wind turbine in relation to the determined direction towards the source.       
 
         [0026]    Determining the yaw direction based on a received signal broadcasted from a source can be implemented into a wind turbine in a cost effective way. As a further advantage, no active yawing movement of the wind turbine is necessary to enable the determination of the yaw direction with sufficient accuracy, i.e., the determination of the yaw direction is possible even when the wind turbine is stationary. 
         [0027]    In an embodiment, the yaw direction is determined based on a Radio Direction Finding (RDF) method. 
         [0028]    In another embodiment, the Radio Direction Finding method is based on a Pseudo-Doppler method. Implementing RDF based on a Pseudo-Doppler method can be implemented at a very low cost wherein the results of the RDF are based on a high quality. 
         [0029]    In a further embodiment,
       the signal is received via an antenna and/or receiver being attached to the component, the antenna and/or receiver having a calibrated 0°-direction in relation to a direction of the component,   an offset angle is determined based on the calibrated 0°-direction in relation to the determined direction, and   the yaw direction is determined based on the offset angle and the determined direction.       
 
         [0033]    In a next embodiment,
       the signal is broadcasted from the source located at a source-specific geographic position,   the broadcasted signal is received at a component-specific geographic position,   a relative compass heading is derived by processing the component-specific geographic position and the source-specific geographic position, and   a yaw angle of the wind turbine is derived based
           on the offset angle, and   on the relative compass heading.   
               
 
         [0040]    The relative compass heading or the relative cardinal direction between the receiver and transmitter of a broadcasted signal may be determined by comparing, i.e., processing respective coordinates of the geographic positions according to, e.g., triangular calculations. Such processing based on standardized geographic coordinate systems is well known and will be shortly summarized at the end of the description. 
         [0041]    It is also an embodiment that the yaw angle is determined in relation towards a defined cardinal direction. By determining the yaw angle in relation towards a defined cardinal direction the resulting yaw direction and/or yaw angle (which is also referred to as “absolute yaw direction and/or angle”) can be determined with sufficient accuracy for each wind turbine of a wind park installation individually. As an example, the individual yaw angle/direction may be determined for each wind turbine in relation to the cardinal direction “North”. 
         [0042]    Pursuant to another embodiment, the broadcasted signal is received at a nacelle or rotor of the wind turbine. Basically, the broadcasted signal may be received via an antenna or receiver located at any part of the wind turbine being involved in yawing or rotating movement causing a change in the direction between the antenna/receiver and the source of the signal. 
         [0043]    According to an embodiment, the yaw direction is determined
       continuously, or   periodically, or   within at least one defined time interval, or   one-time.       
 
         [0048]    As an advantage, the power consumption of the transmitter can be optimized, i.e. the waste of energy minimized. As an example, for power consumption purposes, the transmitter could be timed to broadcast the signal at regular intervals (i.e. every 24 hours) in conjunction with receivers mounted on the wind turbine. 
         [0049]    According to another embodiment, the geographic position is defined according to
       a Geographic Latitude and Longitude coordinate system, or   an Universal Transverse Mercator (UTM) coordinate system, or   an Universal Polar Stereographic (UPS) coordinate system.       
 
         [0053]    The problem stated above is also solved by a wind turbine comprising
       a receiver for receiving a signal broadcasted from a source, and   a processing unit that is arranged for
           determining a direction from the receiver towards the source based on the received signal,   determining the yaw direction of the wind turbine in relation to the determined direction towards the source.   
               
 
         [0058]    The problem stated above is also solved by a device comprising and/or being associated with a processor unit and/or hard-wired circuit and/or a logic device that is arranged such that the method as described herein is executable thereon. 
         [0059]    In a further embodiment, the device is a yaw encoder. 
         [0060]    The solution provided herein further comprises a computer program product directly loadable into a memory of a digital computer, comprising software code portions for performing the steps of the method as described herein. 
         [0061]    In addition, the problem stated above, is solved by a computer readable medium, having computer-executable instructions adapted to cause a computer system to perform the steps of the method as described herein. 
     
    
     
       BRIEF DESCRIPTION  
         [0062]    Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
           [0063]      FIG. 1  shows in a schematically top view an exemplary scenario of a wind turbine in relation to the well-known cardinal points or compass points which are indicated as a compass rose in the background; 
           [0064]      FIG. 2  shows an exemplary scenario of an off-shore wind park installation; 
           [0065]      FIG. 3  exemplarily illustrates in a schematic view a basic principle of the original Doppler-RDF; 
           [0066]      FIG. 4  illustrates in a graph a more detailed view of a sinusoidal curve representing the wavelength/frequency of a received signal according to Doppler-RDF; and 
           [0067]      FIG. 5  shows in a block diagram a possible embodiment of a Pseudo-Doppler RDF receiver. 
       
    
    
     DETAILED DESCRIPTION 
       [0068]      FIG. 2  shows an exemplary scenario of an off-shore wind park installation  200  thereby illustrating a determination of a yaw direction of a wind turbine according to the proposed solution. 
         [0069]    According to the example of  FIG. 2  an off-shore wind turbine  210  is located at a specific geographic position  211 . The geographic position  211  may be exemplarily defined according the UTM (Universal Transverse Mercator) coordinate system comprising a first datum or coordinate X 1  (also called “eastings”) and a second datum or coordinate Y 1  (also called “northings”). 
         [0070]    The wind turbine  210  comprises a nacelle  216  being rotatable mounted on top of a tower  217  via a yawing system  219 . A rotor is attached to an upwind side of the nacelle  216 . The rotor includes a central rotor hub  213  and a plurality of blades  212  mounted to and extending radially from the rotor hub  213  defining a rotor plane  220 . 
         [0071]    The nacelle  216  may be involved in a yawing movement, e.g., rotating the rotor plane  220  perpendicular to an incoming wind. 
         [0072]    As a further exemplary member of the off-shore wind park installation  200  an electrical substation  230  is located at a specific geographic position  231  which is different from the geographic position  211  of the wind turbine  210 . The geographic position  231  may be also defined according the UTM (Universal Transverse Mercator) coordinate system comprising a first datum or coordinate X 2  and a second datum or coordinate Y 2 . 
         [0073]    The substation  230  includes a transmitter  232  representing a source of a radio signal  233  being broadcasted to be processed with the help of a Radio Direction Finding (RDF) method. 
         [0074]    Radio Direction Finding (RDF) refers to the determination of a direction from which a received signal is transmitted thereby using a specialized antenna or antenna system in combination with triangulation to identify the precise location or direction of a transmitter, i.e. the source of the broadcasted signal. This may exemplarily refer to radio or to other forms of wireless communication. 
         [0075]    As shown in  FIG. 2 , the signal  233  broadcasted from the transmitter  232  is received by a receiver  215  attached on top of the nacelle  216 . According to the proposed solution, the receiver  215  comprises an antenna  218 , both configured as a Radio Direction Finder or RDF receiver for finding or determining a direction towards the source  232  of the signal  233 . In the scenario  200 , the antenna  218  is configured according to a single-channel RDF system which is based on the use of a multi-antenna array in combination with the receiver  215  as a single channel radio receiver. 
         [0076]    Thereby, the antenna array  218  may be installed or calibrated such on the top of the nacelle  216 , that a 0°-position or 0°-direction of the RDF receiver is equal to a forward facing direction of the wind turbine  210 , i.e., is in line with an actual yaw direction  214  of the nacelle  216 . 
         [0077]    Two main categories are applicable for single-channel direction finding:
       direction finding based on amplitude comparison   direction finding based on phase comparison       
 
         [0080]    According to an exemplary embodiment of the scenario  200  illustrated in  FIG. 2 , the applied RDF method is based on a Pseudo-Doppler method (“Doppler-RDF”). Doppler-DRF is a phase-based direction finding method producing a direction estimate based on the received signal  233  by measuring a Doppler-shift induced on the signal at the antenna  218  of the RDF receiver by sampling around the elements of a circular antenna array. 
         [0081]      FIG. 3  exemplarily illustrates in a schematic view the principle of the original Doppler-RDF using a single antenna  310  that physically moves along a circle or rotating platform  320 . In short, when the antenna  310  moves in a direction  330  towards a transmitter  350  representing a source of a signal, the antenna  310  detects a signal with a shorter wavelength, i.e. a signal with a higher frequency. On the contrary, when the antenna  310  is moving in a direction  340  away from the transmitter  350 , the antenna  310  detects a signal with a longer wavelength, i.e. a signal with a lower frequency. 
         [0082]    Using this principle, an antenna mounted on a rotating platform as shown in  FIG. 3  would detect a wavelength of the received signal which increases and decreases sinusoidal in relation to the frequency of the signal as originally emitted from the transmitter. 
         [0083]      FIG. 4  illustrates in a graph  400  a more detailed view of a sinusoidal curve  410  representing the wavelength/frequency of a signal received via an antenna  310  as shown in  FIG. 3 . Thereby, an abscissa  420  of the graph  400  is representing the angular position of the antenna  310  and an ordinate  430  is representing a Doppler-shift frequency of the received signal indicating a level of increase or decrease of the frequency of the received signal in relation to the frequency of the signal as originally emitted from the transmitter  350 . 
         [0084]    When the antenna  310  is moving towards (i.e. towards direction  330 ) the source  350  (i.e. position “D” in  FIG. 3 ), the wavelength of the received signal is at a local minimum, i.e. the Doppler-shift frequency is at a maximum (i.e. position “D” in  FIG. 4 ). 
         [0085]    When the antenna  310  is at a position nearest to the source of the signal (i.e. at position “A” in  FIG. 3 ) the wavelength of the received signal is unchanged, i.e. the Doppler-shift frequency is zero (i.e. at position “A” in  FIG. 4 ). 
         [0086]    When the antenna  310  is moving away (i.e. towards direction  340 ) from the source  350  (i.e. at position “B” in  FIG. 3 ) the wavelength of the received signal is at a local maximum, i.e. the Doppler-shift frequency is at a minimum (i.e. at position “B” in  FIG. 4 ). 
         [0087]    When the antenna  310  is at a position with a maximum distance to the source  350  of the signal (i.e. at position “C” in  FIG. 3 ) the wavelength of the received signal is unchanged, i.e. the Doppler-shift frequency is zero (i.e. at position “C” in  FIG. 4 ). 
         [0088]    Consequently, those sections in the graph  400  without any Doppler-shift, and in particular such areas in curve  410  marking an angular position with a decreasing “zero crossing” towards the abscissa  420  (i.e. position “A” in the curve  410 ) are representing those positions of the antenna  310  closest to the source of the signal (i.e. at position “A” in  FIG. 3 ). Thus, applying a decreasing zero crossing detection in graph  400  results in an accurate indication of the direction towards the source of the received signal. 
         [0089]    In practical applications of Doppler-RDF a physically rotating disc would have to be moving at a very high rotating velocity to make the Doppler-shift “visible”. Because of this limitation, Pseudo-Doppler RDF was developed simulating the rotation of the antenna disc electronically. 
         [0090]      FIG. 5  shows in a block diagram a possible embodiment of a Pseudo-Doppler RDF receiver  500 . Pseudo-Doppler RDF is based on an antenna array  510  including multiple antennas  511  . . .  514 . Each antenna  511  . . .  514  is connected to an antenna controller  520 . The antenna controller  520  is connected to a FM (Frequency Modulation) receiver  530  which is communicating with a demodulator  521 . The demodulator  521  is coupled to a band pass filter  532  which is connected to a zero-crossing detector  533 . 
         [0091]    The antenna controller  520  is further connected to an antenna position selector/multiplexer  540  driven by a clocking signal unit  541 . The antenna position selector/multiplexer  540  is further coupled to a direction comparator  542  which is also communicating with the zero-crossing detector  533 . The direction comparator  542  is further communicating with an orientation output  543  indicating the resulting direction of the source of the signal received at the antenna array  510 . 
         [0092]    According to  FIG. 5 , signal reception at the antenna array  510  is rapidly shifted (indicated by a sequence “1-2-3-4” in  FIG. 5 ) from antenna to antenna  511  . . .  514  driven by the antenna position selector/multiplexer  540  in combination with the controller  520  thereby simulating a single antenna rotating rapidly on a disc. As an example, for UHF (Ultra High Frequency) signals the rotation speed may be about 500 Hz. 
         [0093]    After receiving the frequency modulated signal via the antenna array  510  and further processing via the FM receiver  530 , the received signal will be demodulated by the demodulator  531 . After demodulation, the frequency of the processed signal is equal to the frequency of the pseudo antenna rotation. After a band pass filtering via the filter  532  the positions with decreasing zero-crossings of the Doppler-shift frequency can be identified by the zero-crossing-detector  533  in combination with the direction comparator  542 . Based on the identified zero-crossings, the resulting direction from the antenna  510  towards or in relation to the source of the received signal will be indicated via the orientation output  543 . 
         [0094]    Further, dependent from the calibration of the 0°-position or 0°-direction of the Pseudo-Doppler RDF receiver  500 , a relative offset between the 0°-position/direction, e.g. the actual yaw direction of the nacelle and the identified direction towards the source of the received signal may be also presented as a further result at the orientation output  543 . 
         [0095]    The Pseudo-Doppler RDF receiver  500  as presented in  FIG. 5  may be part of a yaw encoder of the wind turbine. 
         [0096]    It should be noted, that each kind of Radio Direction Finding (RDF) method may be used for implementing the proposed solution. 
         [0097]    Applying Pseudo-Doppler RDF may be the preferred solution for the following reasons:
       antenna array and processor can be sourced at very low cost,   antenna array can be small for UHF frequency band (15 cm×15 cm or smaller),   small individual antenna length (whip style length around 19 cm for 400 MHz),   high degree of accuracy (&lt;1 degree to 5 degrees depending on design),   possibility to identify beacon direction at all angles, and   no direction aliasing       
 
         [0104]    Regarding the signal being broadcasted, a transmitter representing the source of the signal may broadcast a steady signal at a constant reference frequency. As an example, the UHF frequency band (300 MHz to 1 GHz) may be the preferred frequency range for the broadcast due to the following reasons:
       multiple UHF frequencies are available for public use,   UHF allows the use of compact antenna systems (&lt;1 m), and   UHF is best for medium range line of site applications such as a large wind farms       
 
         [0108]    In the following, the determination of the actual yaw direction of wind turbine according to the proposed solution will be explained in more detail. 
         [0109]    For that, a further diagram  250  is embedded in  FIG. 2  visualizing in top-view a geographical situation of the off-shore scenario  200 . At the bottom left side of the diagram  250  the nacelle  216  is indicated in top-view together with the antenna  218  located at the origin of the diagram  250  representing the geographic position  211 . Accordingly, the geographic location of the substation  230 , in particular the geographic position  231  of the transmitter  232  is indicated at the upper right side of the diagram  250 . 
         [0110]    It should be noted, that the geographic positions  211 ,  231  maybe defined according to any geographic coordinate system enabling every location on earth to be specified by a set of numbers or letters which are also referred to as coordinates. Such coordinates are often chosen such that one of the numbers represents a vertical position and two or three of the numbers represent a horizontal position. Examples for geographic coordinate systems are “Geographic latitude and longitude” or “UTM” (Universal Transverse Mercator) and “UPS” (Universal Polar Stereographic). 
         [0111]    In the example shown in  FIG. 2 , the diagram  250  is configured according to UTM wherein an abscissa  251  is exemplarily representing a cardinal direction “East” and an ordinate  252  is representing a cardinal direction “North”. 
         [0112]    Alternatively, the abscissa  251  may represent a “Longitude” information and the ordinate  252  may represent a “Latitude” information according to the Geographic Latitude and Longitude system. 
         [0113]    According to a first step of the proposed solution, a relative cardinal direction or a relative compass heading between the antenna or antenna array  218  of the wind turbine  210  and the transmitter  232  will be determined by comparing, i.e., processing the respective coordinates (X 1 , Y 1 , X 2 , Y 2 ) of the geographic positions  211 ,  231  according to, e.g., triangular calculations. Such calculation of the relative compass heading based on a standardized geographic coordinate systems is well known and will be shortly summarized at the end of the description. 
         [0114]    The resulting relative compass heading is indicated by an arrow  253  in the geographic diagram  250 . According to  FIG. 2 , the relative compass heading  253  comprises a first coordinate (indicated by an arrow  260 ) representing the UTM-specific “eastings” and a second coordinate (indicated by an arrow  261 ) representing the UTM-specific “northings”. 
         [0115]    The relative compass heading  253  is permanent and will never change over time as long as the wind turbine  210 , i.e. the antenna  218  and the substation  230 , i.e. the transmitter  232  will remain at the same geographic position. Therefore, the relative compass heading  253  can be calculated individually for each wind turbine one-time and be stored into a configuration file as a reference information. 
         [0116]    In a next step, by applying the Pseudo-Doppler RDF based on the signal  233  received at the receiver  215  via the antenna  218 , the direction from the antenna  218  towards the transmitter  232  is determined. 
         [0117]    It should be noted, that the direction from the antenna  218  toward the transmitter  232  is the same or almost the same as the direction from the nacelle  216  toward the transmitter  232  and the same or almost the same as the direction from the wind turbine  210  towards the transmitter  232 . 
         [0118]    Further, the determined direction which is presented at the orientation output  543  of the Pseudo-Doppler RDF receiver  500  is equal or almost equal to the calculated relative compass heading  253 . Thus, the determined direction and the relative compass heading are labeled with the same index  253  in the description hereinafter. 
         [0119]    As already mentioned above, the receiver  215  and the antenna  218  are calibrated such, that the 0°-direction is equal to the actual yaw direction  214  of the nacelle  216 . 
         [0120]    Consequently, as a further output of the Pseudo-Doppler RDF, a nacelle offset angle θ NacelleOffset  (indicated by an arrow  254  in the diagram  250 ) between the 0°-direction of the antenna  218  and the determined direction (which is equal to the calculated relative compass heading  253 ), can be derived. Based on the determined direction and the offset angle  254  the actual yaw direction (indicated by an arrow  214  in the diagram  250 ) can be determined. 
         [0121]    Based on the offset angle  254  and/or the actual yaw direction  214  and based on the calculated relative compass heading  253  further geographic information may be derived dependent on the orientation or calibration of the geographic diagram  250 . 
         [0122]    As an example, a reference angle θ UTM  may be derived based on the relative compass heading  253  in relation to the cardinal direction “North” (indicated by the ordinate  252 ). The reference angle θ UTM  is indicated by an arrow  255  in the diagram  250 . 
         [0123]    Further, by subtracting the offset angle  254  from the reference angle  255  an absolute turbine yaw angle θ YawAngle  may be derived which is specific for each wind turbine  210  being part of the wind park installation  200 . The absolute turbine yaw angle θ YawAngle  is indicated by an arrow  256  in the diagram  250 . 
         [0124]    The absolute turbine yaw angle  256  or the actual yaw direction  214  may be either updated continuously or sporadically to determine the actual yaw direction  214  or any further information concerning the actual position or direction of the rotor plane  220  or heading of the wind turbine or to calibrate the existing yaw encoder. 
         [0125]    The proposed solution may be applicable to any wind turbines according to any of the following configurations:
       front mounted rotor (Forward facing) with active yaw,   rear mounted rotor (Rear facing) with active yaw,   any non-traditional direction dependent rotor configurations, and   any passive yaw wind turbine with a direction dependent rotor configuration       
 
         [0130]    The proposed solution is independent from the design of the rotor or the nacelle, e.g., independent from the number of blades or from the shape of the nacelle. 
         [0131]    Further, the proposed solution may be applicable to any Radio Direction Finding (RDF) method or technology capable for measuring or detecting the relative direction of a signal source. 
         [0132]    The proposed solution may be further applicable to any embodiment of a radio transmitter as a source for broadcasting a signal at any transmission frequency. The possible range of possible frequencies to be used for the proposed solution maybe within or outside the UHF frequency band. 
         [0133]    The proposed solution may be used for a constant or permanent monitoring of the yaw direction or yaw angle of a wind turbine or for a one-time only calibration of an existing yaw encoder. 
         [0134]    According to a further embodiment of the proposed solution, the transmitter  232  may be configured such, that the signal  233  is broadcasted only within defined time intervals like, e.g., every 24 hours. Accordingly, the receiver  215  mounted at the wind turbine has be activated, i.e. synchronized, within the same time intervals. Beneficially, power consumption can be reduced at transmitter side as well as on receiver side. 
         [0135]    Calculating the relative compass heading between two defined geographic positions: 
         [0136]    Using a geographic coordinate system according to UTM: 
         [0137]    The UTM (Universal Transverse Mercator) system of coordinates is a common system used in industry. This system breaks the globe into 60 zones each of which is then measured using meters north and east. These measurements are called “eastings” and “northings” and are designated as mE (meters east) and mN (meters north), respectively. 
         [0138]    In nearly all cases a wind farm will exist entirely within one of the 60 zones. In the event that it falls on the border between two zones, it will be important that both the turbine and the reference point are in the same zone. 
         [0139]    Calculating an angle from one point to another using UTM coordinates is straightforward. To determine a bearing θ, (which is corresponding with the reference angle  255  of  FIG. 2 ) from the turbine coordinates (Easting 1  (i.e. X 1  in  FIG. 2 ), Northing 1  (i.e. Y 1  in  FIG. 2 )) to a reference coordinate (Easting 2  (i.e. X 2  in  FIG. 2 ), Northing 2  (i.e. Y 2  in  FIG. 2 )) the following equation can be used: 
         [0000]    
       
         
           
             θ 
             = 
             
               
                 tan 
                 
                   - 
                   1 
                 
               
                
               
                 ( 
                 
                   
                     
                       Easting 
                       2 
                     
                     - 
                     
                       Easting 
                       1 
                     
                   
                   
                     
                       Northing 
                       2 
                     
                     - 
                     
                       Northing 
                       1 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0140]    The expression tan −1 (x) will only calculate the correct bearing when the reference coordinate is to the northeast of the turbine coordinate. 
         [0141]    This is because 
         [0000]    
       
         
           
             
               tan 
               
                 - 
                 1 
               
             
              
             
               ( 
               
                 y 
                 x 
               
               ) 
             
           
         
       
     
         [0000]    produces the same result as 
         [0000]    
       
         
           
             
               
                 tan 
                 
                   - 
                   1 
                 
               
                
               
                 ( 
                 
                   
                     - 
                     y 
                   
                   
                     - 
                     x 
                   
                 
                 ) 
               
             
             . 
           
         
       
     
         [0142]    To correct this, the common function atan2(y,x) can be used to identify which quadrant the angle is in. 
         [0143]    The results of atan2(y,x) will show angles greater than 180° as negative numbers. To convert this result to a range from 0° to 360° the following expression can be used: 
         [0000]      0 360 =mod(0 deg +360, 360) 
         [0144]    Here mod(a,b) is the modulo function that returns the remainder of a divided by b. 
         [0145]    The only thing remaining is to make sure that the result of atan2(x,y) is converted back to degrees by using the relation below. 
         [0000]      180° =π radians  
 
         [0146]    By combining this all it is possible to calculate the bearing θ from one UTM coordinate to the other. As an example, a line of computer code could be written as the following: 
         [0000]      θ=mod(ATAN2(Easting2−Easting1,Northing2−Northing1)*(180/π)+360,360)
 
         [0147]    Using a geographic coordinate system according to Latitude and Longitude: 
         [0148]    In place of using UTM coordinates, it is also possible to use the more traditional latitude and longitude coordinates. Calculating a bearing using this coordinate system is a bit more complicated; although it is still possible using simple trigonometric functions. 
         [0149]    Approximating the earth as a sphere, the initial bearing θ from the turbine coordinate (long 1  (i.e. X 1  in  FIG. 2 ), lat 1  (i.e. Y 1  in  FIG. 2 )) to the reference coordinate (long 2  (i.e. X 2  in  FIG. 2 ), lat 2  (i.e. Y 2  in  FIG. 2 )) can be calculated using the following equation: 
         [0000]    
       
         
           
             θ 
             = 
             
               
                 tan 
                 
                   - 
                   1 
                 
               
                
               
                 ( 
                 
                   
                     
                       
                         cos 
                          
                         
                           ( 
                           
                             lat 
                             1 
                           
                           ) 
                         
                       
                        
                       
                         sin 
                          
                         
                           ( 
                           
                             lat 
                             2 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       
                         sin 
                          
                         
                           ( 
                           
                             lat 
                             1 
                           
                           ) 
                         
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           
                             lat 
                             2 
                           
                           ) 
                         
                       
                        
                       
                         cos 
                          
                         
                           ( 
                           
                             
                               long 
                               2 
                             
                             - 
                             
                               long 
                               1 
                             
                           
                           ) 
                         
                       
                     
                   
                   
                     
                       sin 
                        
                       
                         ( 
                         
                           
                             long 
                             2 
                           
                           - 
                           
                             long 
                             1 
                           
                         
                         ) 
                       
                     
                      
                     
                       cos 
                        
                       
                         ( 
                         
                           lat 
                           2 
                         
                         ) 
                       
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0150]    However, for short distances, such as those on a wind farm, the lines of longitude around the earth can be considered to be parallel. Using this simplification the complex equation above can be simplified to the following: 
         [0000]    
       
         
           
             θ 
             = 
             
               
                 tan 
                 
                   - 
                   1 
                 
               
                
               
                 ( 
                 
                   
                     
                       lat 
                       2 
                     
                     - 
                     
                       lat 
                       1 
                     
                   
                   
                     
                       cos 
                        
                       
                         ( 
                         
                           lat 
                           1 
                         
                         ) 
                       
                     
                      
                     
                       ( 
                       
                         
                           long 
                           2 
                         
                         - 
                         
                           long 
                           1 
                         
                       
                       ) 
                     
                   
                 
                 ) 
               
             
           
         
       
     
         [0151]    The expression tan −1 (x) only gives correct answers for coordinates located in the Eastern Hemisphere of the globe when using the Decimal Degree format to represent latitude and longitude. 
         [0152]    Therefore, this function will also use atan2(y,x). Similarly, mod(a,b) is also used as before. 
         [0153]    It is also necessary to make sure that the angle within the cos(x) function is expressed as radians and that the result of atan2(x,y) is converted back to degrees by using the relationship between degrees and radians above. 
         [0154]    By combining this all it is possible to calculate the bearing 0 from the turbine coordinate to the reference coordinate. As an example a line of computer code could be written as the following: 
         [0000]      θ=mod(atan2(lat 2 −lat 1 ,COS(lat 1 *π/180)*(long 2 −long 1 ))*(180/π)+360,360)
 
         [0155]    Although the invention is described in detail by the embodiments above, it is noted that the invention is not at all limited to such embodiments. In particular, alternatives can be derived by a person skilled in the art from the exemplary embodiments and the illustrations without exceeding the scope of this invention.