A pseudorange extraction and pairing method in wireless two-way time alignment

Abstract

This invention relates to a pseudorange extraction and pairing method in wireless two-way time comparison, belonging to the field of wireless two-way time comparison technology. It includes: extracting the counter values ​​at each level of the transmitting or receiving module; calculating the counter times at each level of the transmitting or receiving module based on the counter values; calculating the local time and recovery time based on the counter times at each level; calculating the pseudorange based on the local time and recovery time; buffering the pseudorange and frame count; arranging the transmission messages according to the frame structure of the two stations; retrieving the frame count and pseudorange from the received messages of the other station; extracting the pseudorange and frame count buffered in the station's register based on the recovered frame count demodulated from the messages; and calculating the clock difference between the two stations based on the pseudorange parsed from the messages and the locally buffered pseudorange. This invention completely eliminates the geometric path asymmetry error caused by motion, achieving nanosecond-level time comparison accuracy in dynamic environments.

Description

Technical Field

[0001] This invention belongs to the field of wireless two-way time comparison technology and relates to a pseudorange extraction and pairing method. This method can be applied to wireless two-way time comparison technology, and its application advantages are more prominent in dynamic environments. Background Technology

[0002] With the development of joint observation and multi-system fusion technologies such as wireless sensor networks and joint formations, many observation platforms or networks require sensor nodes to be in motion to more accurately observe and track targets. This necessitates nanosecond-level dynamic time comparison technology to support dynamic observation and tracking tasks. For example, UAV technology has developed rapidly in recent years, and UAV networking has become a research hotspot. In special applications, nanosecond-level time synchronization is required between UAVs and between UAVs and the ground for better collaborative work.

[0003] Currently, most high-precision time alignment methods face practical problems in dynamic environments due to limitations in their technical characteristics. For example, time alignment using ordinary cables or high-performance optical fibers cannot support dynamic applications; satellite two-way time alignment technology requires leasing satellites and establishing a two-way data transmission link in advance, resulting in high costs and poor convenience for dynamic applications; navigation satellite common-view technology can also achieve time alignment accuracy of a few nanoseconds, but it requires establishing a data exchange link, and the common-view terminal also needs precise calibration of its position coordinates, limiting its application in dynamic systems.

[0004] A wireless time comparison terminal based on the principle of bidirectional time comparison can be developed to conduct time comparisons between ground stations. However, for moving stations, the wireless time comparison signal transmission and reception paths are not the same. The influence of geometric distance delay, the extraction of pseudoranges observed by the two stations, and the pairing mechanism all affect the performance of dynamic time comparison. If not handled by appropriate methods, the time comparison performance in dynamic environments is significantly worse than in static environments. Summary of the Invention

[0005] The technical problem to be solved by this invention is:

[0006] To overcome the shortcomings of existing wireless two-way time comparison technology in dynamic applications, this invention provides a pseudorange extraction and pairing method for wireless two-way time comparison. This pseudorange extraction and pairing method can almost completely eliminate the geometric path asymmetry error caused by motion, and achieve nanosecond-level time comparison accuracy in dynamic environments.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0008] A pseudorange extraction and pairing method for wireless two-way time comparison, characterized in that it includes two stations, A and B, where station A is the time comparison master station and station B is the time comparison slave station. Both stations A and B include a transmitting module and a receiving module, and both the transmitting module and the receiving module are equipped with a wireless time comparison terminal capable of transmitting and receiving wireless signals; including:

[0009] Extract the counter values ​​at each level of the transmitting or receiving module; calculate the counter time at each level of the transmitting or receiving module based on the counter values ​​at each level of the transmitting or receiving module;

[0010] Calculate local time and recovery time based on the time of each counter level; calculate pseudorange based on local time and recovery time;

[0011] The pseudorange and frame count are buffered. The two stations arrange the transmitted messages according to the frame structure composition. The station receives the messages from the other station and recovers the frame count and pseudorange from the message information.

[0012] The pseudorange and frame count in the local station register cache are extracted based on the recovered frame count demodulated from the message, and the clock difference between the two stations is calculated based on the message parsing pseudorange and the local cache pseudorange.

[0013] A further technical solution of the present invention: the extraction of the calculator values ​​at each level of the transmitting module and the receiving module specifically includes:

[0014] The master station uses the frame pulse signal of the transmitting module as a counter to extract the start signal, and the slave station uses the frame pulse signal of the receiving module as a counter to extract the start signal. When the rising edge of the counter extraction start signal of the master station and the slave station arrives, the code phase count value code_phase, the chip count value code_chip, the bit count value bit_count and the frame count value frame_count of the transmitting module and the receiving module are extracted.

[0015] A further technical solution of the present invention: the calculation of the counter time at each stage based on the calculator values ​​at each stage of the transmitting module and the receiving module specifically includes:

[0016]

[0017] Where code_freq is the pseudocode frequency, which is f1; code_phase is the code phase counter value in the code NCO module; n is the NCO counter bit width; code_chip is the chip counter value; bit_count is the bit counter value; bit_len is the duration of 1 bit of data, in nanoseconds; frame_len is the length of a frame of data, in nanoseconds; and frame_count is the frame counter value.

[0018] A further technical solution of the present invention: the calculation of local time and recovery time based on the time of each level of counters specifically includes:

[0019] The local time is obtained by summing the times of each counter in the transmitting module; the recovery time is obtained by summing the times of each counter in the receiving module.

[0020] A further technical solution of the present invention: the calculation of pseudorange based on local time and recovery time specifically includes:

[0021] The difference between local time and recovery time is used as the pseudorange.

[0022] A further technical solution of the present invention: the calculation of the clock difference between two stations based on message parsing pseudorange and local cache pseudorange specifically includes:

[0023] Half of the difference between the local cache pseudorange and the message parsing pseudorange is taken as the clock difference between the two stations.

[0024] A computer system is characterized by comprising: one or more processors, and a computer-readable storage medium for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors cause the one or more processors to implement the method described above.

[0025] A computer-readable storage medium is characterized by storing computer-executable instructions, which, when executed, are used to implement the above-described method.

[0026] A computer program product is characterized by including computer-executable instructions, which, when executed, are used to implement the above-described method.

[0027] The beneficial effects of this invention are as follows:

[0028] This invention utilizes the recovery frame pulse from the receiving module as the start signal for pseudorange extraction at the slave station, and the local frame pulse from the transmitting module as the start signal for pseudorange extraction at the master station. The lag in pseudorange generation between the slave and master stations is only the sum of geometric distance delay and transmit / receive hardware delay. For wireless time comparison within line-of-sight, it can be almost assumed that the master and slave stations generate pseudoranges simultaneously, thus solving the time comparison error problem caused by the asynchronous generation of pseudoranges between the master and slave stations, and almost eliminating the time comparison error caused by geometric distance asymmetry due to motion. Furthermore, by combining pseudorange, frame counting buffering, and message transmission functions, pseudoranges from the two stations at the same frame counting time are paired to prevent accuracy reduction caused by pseudorange misalignment during bidirectional wireless time comparison. Attached Figure Description

[0029] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0030] Figure 1 This is a diagram illustrating the working mechanism of the counters at each stage of the launch module;

[0031] Figure 2 This is a diagram illustrating the working mechanism of the counters at each stage of the receiving module;

[0032] Figure 3 It is a table of message frame structure arrangement;

[0033] Figure 4 This is a flowchart of the wireless two-way time comparison clock difference calculation process.

[0034] Figure 5 It is a dynamic two-way time comparison error map. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0036] Taking time comparison between two stations, A and B, as an example, station A is the master station for time comparison, and station B is the slave station. Both stations A and B are equipped with wireless time comparison terminal equipment, which can transmit and receive wireless signals. The technical solution adopted by this invention to solve its technical problem includes the following steps:

[0037] 1) Connect the 1PPS and 10MHz signals to be tested from stations A and B to the wireless time comparison terminals of the two stations, and power on the terminal equipment.

[0038] 2) Two-station pseudocode clock frequency control word generation mechanism: The frequency control word of the code clock counter in both transmitting modules remains constant and is calculated using the pseudocode frequency f1. The pseudocode clock of the two receiving modules is a recovered clock, which adjusts the code frequency control word according to the received pseudocode signal to achieve phase synchronization with the received signal's pseudocode clock. Therefore, the code ring NCO frequency control word of the receiving module fluctuates based on the frequency control word of the transmitting module;

[0039] 3) Two-station counter counting: When the rising edge of the 1PPS test edge arrives at the master / slave station, the chip NCO counters in the two-station transmit and receive modules are triggered to start working. The chip NCO counter of the receive module accumulates based on the recovered pseudo-code clock, and the chip NCO counter of the transmit module accumulates based on the local pseudo-code clock. The chip counters, bit counters, and frame counters of the transmit and receive modules accumulate based on the overflow signal of the previous stage counter. The pseudo-code clock output of the transmit module is a square wave signal with a frequency of f1, and the period of the chip NCO counter is 2. n (n is the bit width of the NCO counter), the chip counter period is the chip length S, the bit counter period is N, and the frame counter period is M. After all counters have completed their cycles, an overflow indication signal is generated and the count is reset to zero, starting a new round of counting. The frequency of the square wave signal output by the receiver module's pseudocode clock is related to the received tracking signal and fluctuates around f1. The design of the remaining counters in the receiver module is the same as that in the transmitter module. When the NCO counter reaches 2... n An overflow indication signal is then generated, and the chip counter automatically increments by 1 upon receiving the overflow indication signal. When the chip counter has counted for one pseudo-code cycle S, an overflow indication signal is generated, and the bit counter automatically increments by 1 upon receiving the overflow indication signal. When the bit counter has counted for one cycle N, an overflow indication signal is also generated, and the frame counter automatically increments by 1 upon receiving the overflow indication signal, and generates a frame pulse high-level signal for one clock cycle. When the frame counter has counted for M, it is reset to zero and starts counting again from 1.

[0040] 4) Extraction of counter values ​​at each level: The master station uses the frame pulse signal of the transmitting module as the counter extraction start signal, and the slave station uses the frame pulse signal of the receiving module as the counter extraction start signal. When the rising edge of the counter extraction start signal of the master station and the slave station arrives, the code phase count value (code_phase), chip count value (code_chip), bit count value (bit_count), and frame count value (frame_count) of the transmitting module and the receiving module are extracted.

[0041] 5) Calculation of time for each level of counter: The time represented by each level of counter is obtained according to formulas (1) to (4), and the unit is nanosecond.

[0042] phase_ns = (code_phase / 2) n ) / (code_freq *10 9 (1)

[0043] chip_ns=code_chip / (code_freq *10 9 (2)

[0044] bit_ns=bit_count * bit_len (3)

[0045] frame_ns=frame_count *frame_len (4)

[0046] code_freq is the pseudocode frequency, which is f1; code_phase is the code phase counter value in the code NCO module; n is the NCO counter bit width; code_chip is the chip counter value; bit_count is the bit counter value; bit_len is the duration of 1 bit of data, in nanoseconds; frame_len is the length of a frame of data, in nanoseconds; and frame_count is the frame counter value.

[0047] 6) Calculate the local time and recovery time of the two stations: The local time is calculated by the counters of each level of the transmitting module, and the recovery time is calculated by the counters of each level of the receiving module. The calculation methods for local time and recovery time are the same, as shown in Equation (5), with the unit being nanoseconds.

[0048] Local time = phase_ns + chip_ns + bit_ns + frame_ns (5)

[0049] 7) Calculation of pseudorange: The pseudorange of the master station and the slave station is calculated in the same way. The pseudorange is obtained by formula (6) in nanoseconds.

[0050] Pseudorange = Local time - Recovery time (6)

[0051] 8) Pseudorange and frame count buffer: Both stations store their calculated pseudorange and transmission module frame counts into registers. The registers can store more than 30 sets of pseudorange and frame counts.

[0052] 9) Data framing: The two stations follow the... Figure 3 The frame structure shown is used to arrange and transmit messages. Figure 3 The message arrangement is based on two stations. If a station can be compared with multiple stations in terms of time, redundant bits are enabled. Figure 3 The fifth piece of information is the recovery frame count, which is the second piece of data that this station demodulates from the message information received from the other station, namely the "frame count of this station's transmission module" of the other station.

[0053] 10) Calculation of clock difference between two stations: Extract the pseudorange and frame count from the local station register buffer based on the recovery frame count read from the message. The two frame count values ​​must be the same. Calculate the clock difference between the two stations according to formula (7) to obtain the time comparison result in nanoseconds.

[0054] Clock difference between two stations = (local cache pseudorange - message parsing pseudorange) / 2 (7)

[0055] To enable those skilled in the art to better understand the present invention, the present invention will be described in detail below with reference to specific embodiments.

[0056] Example 1:

[0057] Taking time comparison and clock difference calculation at two stations, A and B, as an example, station A is the time comparison master station and is a fixed station, while station B is the time comparison slave station and is a mobile station with a maximum speed of 100 km / h. The pseudocode clock frequency of the transmission modules at both stations is 10.23 MHz, the pseudocode length is 10230 chips, the duration of 1 bit of data is 1 ms, 1 frame of data contains 300 bits, and the duration of 1 frame of data is 0.3 s.

[0058] This embodiment includes the following 1 to 10 steps.

[0059] Step 1: Connect the 1PPS and 10MHz signals to be tested from stations A and B to the wireless time comparison terminals of the two stations, set up the transceiver antennas of the two stations, and ensure that there are no obstructions between the transceiver antennas of the two stations. To improve the quality of the received signal, the antenna height can be adjusted appropriately. Turn on the terminal equipment of the two stations.

[0060] Step 2: Implement the pseudo-code clock in the FPGA chip of the two-station wireless time comparison terminal. The NCO counter has a bit width of 32 bits. The frequency control word of the code clock counter in the two-station transmitting modules remains constant, calculated using the pseudo-code clock frequency of 10.23MHz. The pseudo-code clock of the two-station receiving modules is a recovery clock, which adjusts the code frequency control word according to the received pseudo-code signal to achieve phase synchronization with the received signal's pseudo-code clock. Therefore, the code ring NCO frequency control word of the receiving module fluctuates based on the frequency control word of the transmitting module.

[0061] Step 3: Implement the various stages of counters for the transmitting and receiving modules in the FPGA chip of the two-station wireless time comparison terminal. When the rising edge of the 1PPS test edge arrives at the master / slave station, the chip NCO counters in the transmitting and receiving modules of both stations are triggered to start working. The chip NCO counter of the receiving module accumulates based on the recovered pseudo-code clock, and the chip NCO counter of the transmitting module accumulates based on the local pseudo-code clock. The chip counters, bit counters, and frame counters of the transmitting and receiving modules accumulate based on the overflow signal of the previous stage counter. The pseudo-code clock module of the transmitting module outputs a square wave signal with a frequency of 10.23MHz, and the counting period of the chip NCO counter is 2. 32 The chip counter has a period of 10230, and the bit counter has a period of 300. After all counters have completed their full cycles, an overflow indication signal is generated and the count is reset to zero, starting a new round of counting. When the NCO counter reaches 2... 32An overflow indication signal is then generated, and the chip counter automatically increments by 1 upon receiving the overflow indication signal. When the chip counter reaches a count of 10230 chips, an overflow indication signal is generated, and the bit counter automatically increments by 1 upon receiving the overflow indication signal. When the bit counter reaches a count of 300, an overflow indication signal is also generated, and the frame counter automatically increments by 1 upon receiving the overflow indication signal, and generates a frame pulse high-level signal for one clock cycle. The frame counter has a longer counting period, and after reaching a full count, it is also reset to zero and starts counting again from 1. The working mechanism of each stage counter in the receiving module is the same as that in the transmitting module;

[0062] Step 4: When the rising edge of the frame pulse signal of the master station's transmitting module arrives, the code phase counter value (code_phase), chip count value (code_chip), bit count value (bit_count), and frame count value (frame_count) of the transmitting and receiving modules in the master station FPGA are extracted by the program; when the rising edge of the recovery frame pulse signal of the slave station's receiving module arrives, the code phase counter value (code_phase), chip count value (code_chip), bit count value (bit_count), and frame count value (frame_count) of the transmitting and receiving modules in the slave station FPGA are extracted by the program. Assuming the distance between the master and slave stations is 1 km, and the slave station's speed is 100 km / h, the slave station's receiving module recovers the frame pulse after the master station's transmitting module's frame pulse by approximately (3333 + hardware delay) ns. Since the hardware delay is relatively small, the lag between the slave station's receiving module and the master station's transmitting module is less than 4 μs. It can almost be assumed that the pseudorange of both stations occurs simultaneously. Within this 4 μs timeframe, the change in clock difference between the two stations is far less than the picosecond level and can be ignored. On the other hand, the geometric distance asymmetry caused by the slave station's movement is approximately 2*4*10... -6 *100000 / 3600≈0.0002 meters, which is less than 1 ps in time. Therefore, the influence of geometric distance asymmetry caused by motion can be ignored.

[0063] Step 5: Calculate the time represented by each level of the transmitter and receiver modules of the master station and slave station according to formulas (8) to (11), in nanoseconds.

[0064] phase_ns = (code_phase / 2) 32 ) / (10.23*10 6 *10 9 (8)

[0065] chip_ns = code_chip / (10.23 * 10) 6 *10 9 (9)

[0066] bit_ns = bit_count * 10 6 (10)

[0067] frame_ns=frame_count *0.3*10 9 (11)

[0068] code_phase is the code phase counter value in the code NCO module, code_chip is the chip counter value; bit_count is the bit counter value, and frame_count is the frame counter value;

[0069] Step 6: Calculate the local time and recovery time of the two stations using Equation (12). The local time is calculated using the counters of each stage of the transmitting module, and the recovery time is calculated using the counters of each stage of the receiving module. The unit is nanoseconds.

[0070] Local time = phase_ns + chip_ns + bit_ns + frame_ns (12)

[0071] Step 7: Calculate the pseudorange using equation (13). The pseudorange calculation method is the same for both the master station and the slave station. The unit is nanosecond.

[0072] Pseudorange = Local time - Recovery time (13)

[0073] Step 8: Both stations store their calculated pseudorange and transmit module frame count into registers. The registers can store more than 30 sets of pseudorange and frame count. For example, the master station stores the pseudorange as Pr_1 and the transmit module frame count as frame_count_1, while the slave station stores the pseudorange as Pr_2 and the transmit module frame count as frame_count_2.

[0074] Step 9: The two stations follow Figure 3 The frame structure shown is used to arrange the transmitted messages. For the master station, frame_count_1 is placed in... Figure 3 The location of the second piece of information, Pr_1, is... Figure 3 The location of the 5th item; for slave stations, frame_count_2 is placed in... Figure 3 The second piece of information, Pr_2, is located in... Figure 3 The location of item 5. If both the master and slave stations are transmitting a message to each other for the first time... Figure 3 If the fourth information item is 0, the frame count is 0; otherwise, it is the second data item that this station demodulates from the message information received by the other station, which is the "frame count of this station's transmission module" sent by the other station.

[0075] Step 10: Pseudorange pairing and clock difference calculation. Taking the master station as an example, the receiving module parses the message data. If the recovery frame count value read from the fourth data item of the message is frame_count_1, then the pseudorange of the fifth data item of the message is extracted and set as Pr_3. The master station's frame count and pseudorange storage unit is searched to find the pseudorange value Pr_1 when the frame count is frame_count_1. The clock difference between the two stations is calculated according to formula (14) to obtain the time comparison result in nanoseconds.

[0076] Clock difference between the two stations = (Pr_1 - Pr_3) / 2 (14)

[0077] As can be seen from the above implementation steps, the pseudorange extraction and pairing method in wireless two-way time comparison proposed in this invention mainly includes pseudocode clock generation, counting of each level of counters, frame pulse signal detection, counter value extraction, pseudorange calculation, frame counting and pseudorange buffering, pseudorange pairing and clock difference calculation, etc. The pseudocode clock is the reference for the operation of each level of counters, and the counter values ​​at each level are components of the pseudorange. The extraction of counter values ​​and pseudorange calculation at each level are restarted by detecting the rising edge of the frame pulse signal. Frame counting and pseudorange buffering prepare for pseudorange pairing. Pseudorange pairing is achieved by finding two sets of pseudoranges with the same frame count, thereby performing two-station clock difference calculation.

[0078] As can be seen from the above embodiments, the main feature of this invention is that the master station uses the frame pulse signal of the transmitting module as the start signal for pseudorange extraction, and the slave station uses the recovered frame pulse signal of the receiving module as the start signal for pseudorange extraction. It can be considered that the master and slave stations generate pseudorange almost simultaneously, thus solving the time comparison error problem caused by the asynchronous generation of pseudorange between the master and slave stations. Furthermore, the pseudorange of the master and slave stations is extracted almost at the same time, greatly reducing the asymmetry of bidirectional geometric distance caused by motion. For time comparison between moving platforms traveling at speeds of several hundred kilometers per hour, the influence of motion can be completely ignored at the nanosecond-level time comparison accuracy. The pseudorange, frame counting buffering method, and message transmission function of this invention are all guarantee measures for successful pseudorange pairing, preventing the reduction in time comparison accuracy caused by pseudorange misalignment between the two stations.

[0079] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the scope of the technology disclosed in the present invention, and such modifications or substitutions should all be covered within the scope of protection of the present invention.

Claims

1. A method for pseudorange extraction and pairing in wireless two-way time comparison, characterized in that, It includes two stations A and B, A is the master station of time comparison, B is the slave station of time comparison, A and B both include transmitting module and receiving module, and both of them are placed with wireless time comparison terminal which can realize the transmission and reception of wireless signal; it includes: Extracting the counter value of each level of transmitting module or receiving module; calculating the counter time of each level of transmitting module or receiving module according to the counter value of each level of transmitting module or receiving module; Calculating local time and recovery time according to the counter time of each level; calculating pseudo-range according to local time and recovery time; Caching pseudo-range and frame count, two stations arrange transmitting text according to frame structure composition mode, and the station recovers frame count and pseudo-range from text information by receiving the text of the opposite station; Extracting the pseudo-range and frame count cached in the register of the station according to the recovered frame count demodulated from the text, and calculating the clock difference between two stations based on the pseudo-range parsed from the text and the locally cached pseudo-range; The specific steps of extracting the counter value of each level of transmitting module and receiving module are as follows: The master station takes the frame pulse signal of transmitting module as the counter extraction start signal, and the slave station takes the frame pulse signal of receiving module as the counter extraction start signal; when the rising edge of the counter extraction start signal of the master station and the slave station comes, the code phase count value code_phase, the chip count value code_chip, the bit count value bit_count and the frame count value frame_count of transmitting module and receiving module are extracted; The specific steps of calculating the counter time of each level according to the counter value of each level of transmitting module and receiving module are as follows: phase_ns=(code_phase / 2 n ) / (code_freq *10 9 ) chip ns = code chip / (code freq * 10 9 ) bit_ns = bit_count * bit_len frame_ns = frame_count * frame_len Wherein, code_freq is the pseudo code frequency, which is f1; code_phase is the code phase counter value in the code NCO module; n is the NCO counter bit width; code_chip is the chip counter value; bit_count is the bit counter value; bit_len is the time length of 1bit data, unit nanosecond; frame_len is the length of one frame data, unit nanosecond, and frame_count is the frame counter value.

2. The method of claim 1, wherein, The specific steps of calculating local time and recovery time according to the counter time of each level are as follows: Adding the counter time of each level of transmitting module to obtain local time; adding the counter time of each level of receiving module to obtain recovery time.

3. The method of claim 2, wherein the pseudoranges are extracted and paired in a wireless two-way time comparison. The specific steps of calculating pseudo-range according to local time and recovery time are as follows: Taking the difference between local time and recovery time as pseudo-range.

4. The method of claim 3, wherein: The specific steps of calculating the clock difference between two stations based on the pseudo-range parsed from the text and the locally cached pseudo-range are as follows: Taking half of the difference between the locally cached pseudo-range and the pseudo-range parsed from the text as the clock difference between two stations.

5. A computer system, characterized by It includes: One or more processors, computer readable storage medium, for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors realize the method of claim 1.

6. A computer-readable storage medium, characterized in that Computer executable instructions are stored, which when executed, implement the method of claim 1.

7. A computer program product, characterised in that Computer executable instructions are included, which when executed, implement the method of claim 1.

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