Adaptive microphonics noise cancellation

Abstract

To provide systems and methods for compensating for mechanical acceleration at a reference oscillator.SOLUTION: In a communication system 100, a reference oscillator 102 provides an oscillator output signal 103, and an accelerometer 104 on the same platform 105 as the reference oscillator detects mechanical acceleration at the reference oscillator to produce a measured acceleration. An adaptive filter assembly 108, having an associated set of filter weights, receives the measured acceleration 106 from the accelerometer and provides a tuning control signal 110 responsive to the measured acceleration to a frequency reference, which is another system component that utilizes the output of the reference oscillator, associated with the communication system. An adaptive weighting component 114 receives the oscillator output signal of the reference oscillator and an external signal 116 that is provided from an external source to the platform and adjusts the set of filter weights 112 for the adaptive filter assembly based on a comparison of the external signal and the oscillator output signal.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] (Related Application) This application claims priority from U.S. Patent Application No. 16 / 223,777, filed on December 18, 2018, the entire contents of which are incorporated herein by reference.

[0002] (Field of the Invention) The present disclosure generally relates to the field of communications, and more specifically to adaptive microphone noise cancellation.

Background Art

[0003] Microphonics or microphony means the phenomenon in which certain components within an electronic device convert mechanical vibrations into unwanted electrical signals. Mechanical accelerations such as vibrations or shocks can cause frequency modulation in an oscillator, resulting in microphonic phase noise sidebands in the signal. Piezoelectric crystals can be particularly vulnerable to this effect, where mechanical vibrations transiently change the resonance frequency of the crystal, and extremely large phase noise sidebands can be captured by an unexpected frequency modulation. This error can propagate and multiply throughout the system because any oscillator phase-locked to a reference oscillator, such as the sampling clock for analog-to-digital converters and digital-to-analog converters, will be affected.

Summary of the Invention

[0004] According to one embodiment, the system includes a reference oscillator that provides an oscillator output signal and an accelerometer on the same platform as the reference oscillator. Mechanical acceleration at the reference oscillator is detected by the accelerometer, and a measured acceleration is generated. A filter assembly having an associated set of filter weights receives the measured acceleration from the accelerometer and supplies a tuning control signal corresponding to the measured acceleration to a frequency reference of the system. An adaptive weighting component receives the oscillator output signal of the reference oscillator and an external signal provided from a source external to the platform, and adjusts the set of filter weights for the filter assembly based on a comparison between the external signal and the oscillator output signal.

[0005] According to another embodiment, a method for compensating for mechanical acceleration at a reference oscillator is provided. Mechanical acceleration is detected by an accelerometer on the same platform as the reference oscillator, and a measured acceleration is generated. A tuning control signal corresponding to the measured acceleration is supplied to a filter assembly having a set of filter weights. The set of filter weights for the filter assembly is adjusted based on a comparison between an external signal provided from a source external to the platform and the oscillator output signal of the reference oscillator. The tuning control signal is supplied to a frequency reference associated with the system.

Brief Description of the Drawings

[0006] The foregoing and other features of the present invention will become apparent to those skilled in the art related to the present invention by reading the following description with reference to the accompanying drawings.

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Best Mode for Carrying Out the Invention

[0014] Various examples of the systems and methods described herein provide a noise cancellation system. This noise cancellation system can be used to generate a tuning control signal that modulates a reference oscillator to cancel or minimize noise caused by mechanical acceleration in the reference oscillator. For this purpose, the acceleration is measured in place and supplied to an adaptive filter that includes an associated set of weights, and a tuning control signal is generated. The weights can be adapted at regular intervals in response to a measured phase error (or frequency error) of the oscillator output signal of the reference oscillator using an external signal provided to the system. This is to account for the fact that the response of the reference oscillator changes in response to acceleration. Thus, a lower-cost, less rugged reference oscillator can be used without significantly increasing microphonic noise or cost and without increasing the weight of the mechanical isolation structure. Further, small variations between reference oscillators incorporated during manufacturing can be compensated without the time-consuming testing of individual units.

[0015] FIG. 1 shows a communication system 100 that uses a reference oscillator 102 to generate an oscillator output signal 103. The reference oscillator 102 can comprise, for example, an electronic oscillator such as a Hartley oscillator or a Colpitts oscillator, or a crystal oscillator that includes a piezoelectric crystal. The communication system 100 includes an accelerometer 104 on the same platform 105 as the reference oscillator 102 and is configured such that any mechanical acceleration in the reference oscillator is detected by the accelerometer. Thus, the accelerometer 104 can continuously and periodically generate a measured acceleration 106 that represents the acceleration received at the reference oscillator 102. In some implementations of the reference oscillator 102, it will be understood that the oscillator will have various sensitivities to accelerations from different directions and that the accelerometer 104 can be implemented as a three-axis accelerometer that measures accelerations along three mutually perpendicular axes.

[0016] The communication system 100 further includes an adaptive filter assembly 108 that receives a measured acceleration 106 from an accelerometer and generates a synchronization control signal 110 corresponding to the measured acceleration 106 according to a set of filter weights. The synchronization control signal 110 is supplied to a frequency reference of the system, which is the reference oscillator 102 in this implementation. However, it will be understood that the frequency reference can be another system component that utilizes the output of the reference oscillator 102. It will be understood that the filter weights correspond to the response of the reference oscillator 102 to acceleration. Thereby, the filter assembly 108 can correct the oscillator against disturbances caused by the measured acceleration.

[0017] In some implementations, the response of the reference oscillator 102 to acceleration will change over time, for example, due to aging of components and changes in the operating environment. Thus, the adaptive filter assembly 108 can use adaptive weights that are adjusted over time to account for changes in the response of the reference oscillator 102. Since the response of the reference oscillator 102 to acceleration generally changes slowly, the adaptation can be slow for the system, for example, in the range of 3 Hz to 2 kHz. However, it will be understood that the optimization used to generate the weights takes a certain amount of time to converge, and the adaptation must be performed at a high enough frequency such that the weights converge more quickly than the changes in the response at the reference oscillator 102. The initial values of the filter weights can be set according to known characteristics of the reference oscillator 102 to facilitate convergence of the filter 108.

[0018] The weight 112 of the filter assembly can be provided by an adaptive weighting component 114 that receives the oscillator output signal 103 and the external signal 116. As used herein, the term "external signal" refers to a signal provided from a source external to the platform that includes the reference oscillator 102. Thus, the external signal 116 is generated so as not to be affected by any acceleration received at the reference oscillator. The adaptive weighting component 114 adjusts a set of filter weights 112 for the filter assembly based on a comparison of the external signal and the oscillator output signal. The adaptive weighting component 114 can be implemented, for example, in digital logic as a field programmable gate array or an application specific circuit, in software on a non-transitory computer-readable medium executed by an associated processor, or in some combination of hardware and software. It will be appreciated that the adaptive filter assembly 108 can be provided with an initial set of weights at the time of manufacture or installation and can be periodically provided with adaptive filter weights 112 to adjust for changes in the response of the reference oscillator 102.

[0019] FIG. 2 shows an example of an adaptive weighting process 200 incorporating an adaptive weighting component 210 that can be used in the system of FIG. 1. The adaptive weighting component 210 includes a demodulator 202 that determines a phase error 203, Θ e (n) in the oscillator output signal 204 of the reference oscillator 205 from the oscillator output signal 204 and the external signal 206. A frequency estimation filter 216 within the adaptive weighting component 210 calculates an instantaneous frequency f(n) from the phase error 203 determined in the oscillator output signal. In one implementation, the frequency estimation filter 216 is a differential filter having a frequency response; H(f) = 2jπf, a phase difference filter; f(n) = Θ e (n) - Θ e (n - 1), or any other suitable implementation.

[0020] Values 222 - 224 corresponding to the accelerations along their respective axes measured by the accelerometer 104 are filtered by corresponding adaptive filters 226 - 228, added by the adder 230, and the compensation frequency 232; f c (n) corresponding synchronization control signal 232 is generated, and this synchronization control signal 232 is supplied to the reference oscillator 205. Each of the corresponding adaptive filters 222 - 224 and the adder 230 can be realized, for example, as digital logic in a digital signal processor, an application - specific integrated circuit, or a field - programmable gate array. The adaptive filters 226 - 228 can represent a part of the filter assembly 108 shown in FIG. 1, and it will be understood that by combining the corresponding outputs 236 - 238 of the adaptive filters 226 - 228, the synchronization signal 110 can be provided. The synchronization control signal 232 is supplied to the frequency estimation filter 216, and the compensation frequency corresponding to the synchronization control signal is compared with the instantaneous frequency to generate a frequency error 242; f e (n). This frequency error 242 is utilized in the weight calculation component 244 together with the values 222 - 224 of the accelerations along their respective axes, and new weights of the adaptive filters 226 - 228 that minimize the frequency error 242 are generated. Further, the frequency error signal 242 is provided to the reference oscillator 205 (not shown in FIG. 2) as indicated by line 110 in FIG. 1, and its frequency is adjusted. In another embodiment, the frequency error signal 242 can be used for digital correction of the frequency as described in FIG. 6. The weight calculation component 244 can use algorithms such as the Least Mean Square (LMS) algorithm, the recursive least mean square algorithm, and the gradient descent algorithm to minimize the mean square of the frequency error.

[0021] In one embodiment, the least mean square algorithm is used with a vector w of k filter coefficients for each of the adaptive filters 226 - 228. Here, k is a positive integer greater than 1. The measured acceleration values 222 - 224 along each axis at time n can be represented as a vector α containing the most recent k measurements. For time n + 1, the filter weights can be calculated as follows. w x (n + 1) = w x (n) + μα x (n)f e (n - d) w y (n + 1) = w y (n) + μα y (n)f e (n - d) Equation 1 w z (n + 1) = w z (n) + μα z (n)f e (n - d)

[0022] Where μ is a convergence coefficient selected according to the implementation form, d is a delay, and this delay is calculated to temporally align the frequency estimation value from the received phase and the measured acceleration, and compensate for the filter delay along the two signal paths. When the acceleration measured by the accelerometer 104 is small, for example, when the magnitude of the measured acceleration is below a predetermined threshold, it will be understood that the adaptive weighting component 210 may stop adjusting the weights in the filters 226 - 228 for a while and incorporate significant values into the acceleration vector for optimization calculations.

[0023] Figure 3 shows an example of a communication system 300 that uses a reference oscillator 302 to generate an oscillator output signal 303. The oscillator output signal 303 is supplied to at least each of a receiver front end 304 and a transmitter 305. An accelerometer 306 on the same platform 307 as the reference oscillator 302 detects mechanical acceleration at the platform 307. In the illustrated example, the accelerometer 306 can be implemented as a three-axis accelerometer. An adaptive filter assembly 308 receives a measured acceleration 309 from the accelerometer 306 and supplies a tuning control signal 310 corresponding to the measured acceleration to the reference oscillator 302.

[0024] A weight set 312 for the adaptive filter assembly 308 is determined in an adaptive weighting component 314. An external clean signal 316 is received at the receiver front end 304 and supplied to the adaptive weighting component 314 together with the measured acceleration 309. The oscillator output signal 303 supplied to the transmitter 305 is adjusted in the adaptive filter assembly 308, and it will be understood that the influence of local acceleration on the reference oscillator 302 is removed. Thus, the signal 318 transmitted by the transmitter 305 is a "clean" signal like the external signal 316. The adaptive weighting component 314 can determine the degree of phase error in the oscillator output signal 303 based on the received external signal 316. From this phase error, the adaptive weighting component 314 determines appropriate weights for the adaptive filter assembly 308 by minimizing the square of the frequency error derived from the phase error. This can be performed periodically in consideration of changes in the response of the reference oscillator 302 to acceleration due to aging or changes in the operating environment.

[0025] Figure 4 shows another example of a communication system 400 that uses a reference oscillator 402 to generate an oscillator output signal 403. The oscillator output signal 403 is supplied to at least each of a receiver front end 404 and a transmitter 405. An accelerometer 406 on the same platform 407 as the reference oscillator 402 detects mechanical acceleration at the platform 407. In the illustrated example, the accelerometer 406 can be implemented as a three-axis accelerometer. An adaptive filter assembly 408 receives a measured acceleration 409 from the accelerometer and provides an adjustment control signal 410 in response to the measured acceleration 409. A set of weights 412 for the adaptive filter assembly 408 is determined in an adaptive weighting component 414.

[0026] An external clean signal 416 is received at the receiver front end 404 and supplied to the adaptive weighting component 414 along with the oscillator output signal 403 and the measured acceleration 409. The adaptive weighting component 414 can estimate a phase error 415 at the reference oscillator output, shown as 203 in the example of FIG. 2, based on the received external signal 416, and determine appropriate weights for the adaptive filter assembly 408 by minimizing the square of the frequency error derived from the phase error 415. This can be performed periodically in consideration of changes in the response of the reference oscillator 402 to acceleration due to aging or changes in the operating environment. The adaptive weighting component 414 provides the set of weights 412 to the adaptive filter assembly 408.

[0027] It will be appreciated that the adaptive filter assembly 408 compensates only for frequency errors due to microphonics. Other phase and frequency errors, such as Doppler, crystal drift, and scintillation, are not compensated for in the adaptive filter assembly 408. To address the sources of these errors, the phase error 415 can be further supplied to a phase locked loop (PLL) 420. The phase locked loop 420 includes a phase locked loop filter 422. In one implementation, the phase locked loop filter 422 is implemented as a low pass filter that removes any undesired high frequency components present in the estimated phase error. The resulting filtered signal is combined at an adder 424 with the output of the adaptive filter assembly 408 to provide a tuning control signal for the reference oscillator 402.

[0028] FIG. 5 shows yet another example of a communication system 500 that uses a reference oscillator 502 to generate an oscillator output signal 503. The oscillator output signal 503 is supplied to at least each of a receiver 504 and a transmitter 506 that operate via a diplexer 507. An accelerometer 508 on a first platform 510 having the reference oscillator 502 detects mechanical acceleration on this platform. In the illustrated example, the accelerometer 506 can be implemented as a three-axis accelerometer. An adaptive filter assembly 512 receives a measured acceleration 513 from the accelerometer and provides a tuning control signal 514 responsive to the measured acceleration. The measured acceleration is also supplied to the transmitter 506 for transmission to a second platform 520. In one embodiment, the first platform 510 is a user terminal within the communication system and the second platform 520 is a satellite access node, and communication between the first platform and the second platform is via a satellite connection. Alternatively or in addition, the first platform 510 can be a mobile platform implemented, for example, in an automobile, a ship, an aircraft, a train, or other vehicle. However, it will be understood that other configurations of the system are possible where one or more user terminals are used, for example, to correct for vibrations at a satellite access node or to correct between two user terminals.

[0029] The implementation of FIG. 5 takes advantage of the fact that components located on the second platform 520 will not be affected by any mechanical acceleration in the first platform 510, whereas the signal 522 transmitted from the transmitter 506 on the first platform 510 will include any microphonic errors induced by mechanical vibrations on the platform that have not been corrected by other means. Thus, the transmitted signal 522 can be received at the second platform 520 and demodulated by an adaptive weighting component 524 associated with a local receiver (not shown). The phase error in the signal can be determined while demodulating via a frequency reference (not shown) local to the second platform, and this phase error can be used in the adaptive weighting component 516 to determine the appropriate weights for the adaptive filter assembly 512 by minimizing the square of the frequency error derived from the phase error. Determining the appropriate weights by the second platform also requires acceleration information (not shown) transmitted from the first platform to the second platform. The calculated weights 526 can then be transmitted to the first platform 510 via the receiver 504 for use in the adaptive filter assembly 512. In an alternative implementation, the adaptive weighting component 516 can be distributed between the first platform 510 and the second platform 520. In this implementation, a value indicative of the frequency error, such as the frequency error, phase error, or any other metric that can be used to determine the frequency error, is determined at the second platform 520 and can be transmitted to the first platform 510 for use in the calculation of the filter weights 526.

[0030] Exchanging the accelerometer data and the filter weights is understood to correspond to the overhead in a communication system. To reduce this overhead, the update frequency for the weights can be limited by updating the weights either periodically or at any given time schedule. When the weights are not updated, the most recently updated value is maintained and used in the adaptive filter assembly 512 to correct the mechanical acceleration at the first platform 510. Since the change in the response of the reference oscillator 502 to acceleration changes slowly, gating the update function in this way can reduce the overhead in the system while minimizing the loss of accuracy in the oscillator output signal.

[0031] FIG. 6 shows yet another example of a communication system 600 that uses a reference oscillator 602 to generate an oscillator output signal 603. The oscillator output signal is used to drive a numerically controlled oscillator, at least one application specific integrated circuit (ASIC) 605, and an associated receiver front end 608. This implementation is provided for illustrative purposes only, and it will be understood that other implementations of numerically controlled oscillators, such as field programmable gate arrays, may be used. An accelerometer 610 on the same platform 611 as the reference oscillator 602 detects the mechanical acceleration at the platform. In the illustrated example, the accelerometer 610 can be implemented as a three-axis accelerometer. The output 612 of the accelerometer 610 is supplied to each ASIC 605 via a first analog-to-digital converter (ADC) 613. Similarly, an external clean signal 614 is received at the receiver front end 608 and supplied to the ASIC 605 via a second ADC 615.

[0032] An exemplary ASIC 605 is illustrated in detail that includes a numerically controlled oscillator 622 that provides a reference signal to an associated transmitter 623. In ASIC 605, an adaptive filter assembly 624 receives a measured acceleration 612 from an accelerometer 610 and provides a tuning control signal 625 responsive to the measured acceleration 612. The tuning control signal 625 from the adaptive filter assembly 624 can be complemented by an additional tuning signal from a loop filter 630 in an associated adder 626 to track other sources of frequency error such as Doppler shift, oscillator drift, etc., as discussed in connection with FIG. 4. The numerically controlled oscillator 622 can receive a digital tuning signal and thus it will be understood that there is no need to convert the digital output of the adaptive filter assembly 624 to an analog signal.

[0033] Each of an external signal 614, an output 628 of the numerically controlled oscillator 622, and an output 612 of the accelerometer 610 is provided to an adaptive weighting component 632. The adaptive weighting component 632 includes a demodulator (not shown) that estimates a phase error 634 (shown as 203 in the example of FIG. 2) in the numerically controlled oscillator output 634 based on the external signal 614. This phase error 634 is provided to the loop filter 630 to track other sources of frequency error as described above. The adaptive weighting component utilizes the estimated phase error together with the accelerometer output 612 to estimate a frequency error and determine appropriate weights for the adaptive filter assembly 624 that minimizes the square of the frequency error.

[0034] Considering the foregoing structure and the functional features described above, an exemplary method will be better understood with reference to FIG. 7. For purposes of brevity, the exemplary method of FIG. 7 is shown and described as being performed sequentially, but it should be understood and recognized that the present embodiment is not limited to the order shown and that in other embodiments, processing can occur multiple times and / or simultaneously in an order different from that shown and described herein. Further, not all of the described processes need to be executed to implement the method.

[0035] FIG. 7 shows an example of a method 700 for compensating mechanical acceleration in a reference oscillator. At 702, mechanical acceleration is detected by an accelerometer on the same platform as the reference oscillator, generating a measured acceleration. At 704, a tuning control signal corresponding to the measured acceleration is supplied to a filter assembly having a filter weight set. At 706, the filter weight set for the filter assembly is adjusted based on a comparison of an external signal provided from a source external to the platform and the oscillator output signal. For example, a phase error in the oscillator output signal can be determined from the external signal and the oscillator output signal, a frequency error can be estimated, and the filter weight set can be adjusted according to the determined frequency error.

[0036] It will be appreciated that determining the adjustment to the filter weight set can be performed locally, remotely, or in combination of local and remote components. In one embodiment, a signal is generated using the oscillator output signal, transmitted from the platform to a remote platform, and a phase error in the oscillator output signal is calculated from the external signal generated at the remote platform and the signal generated from the oscillator output signal. The calculated phase error is then transmitted to the platform, and the filter weight set for the filter assembly is adjusted by the phase error calculated at the remote platform. In one implementation, the filter weight set for the filter assembly is determined only periodically, and the accelerometer and filter are active when the filter weight set is not being determined.

[0037] At 708, a synchronization control signal is supplied to a frequency reference associated with the system to correct for errors caused by the detected acceleration. In one implementation, the frequency reference is a reference oscillator. In another implementation, the frequency reference is at least one numerically controlled oscillator driven by an oscillator output signal. It will be understood that the synchronization control signal can correct for errors other than those caused by mechanical acceleration. In one implementation, correction values can be calculated in a phase-locked loop and added to the synchronization control signal to account for additional sources of phase error and frequency error.

[0038] What has been described above are examples. Of course, it is not possible to describe every conceivable combination of components or methodologies, but those skilled in the art will recognize that many additional combinations and permutations are possible. Accordingly, this disclosure is intended to embrace all such alternatives, modifications, and variations that fall within the scope of this application, including the appended claims. As used herein, the term "includes" means including but not limited to, and the term "including" means including but not limited to. The term "based on" means based at least in part on. Additionally, if this disclosure or the claims enumerate an element or its equivalent as "a", "an", "a first", or "another", the "first" or "another" element (or its equivalent) should be construed to include one or more than one such element and not to require or exclude two or more such elements.

Claims

1. A system comprising: a reference oscillator that provides an oscillator output signal; an accelerometer on the same platform as the reference oscillator, wherein mechanical acceleration at the reference oscillator is detected by the accelerometer to generate a measured acceleration; a filter assembly having an associated filter weight set, the filter assembly receiving the measured acceleration from the accelerometer and supplying a tuning control signal corresponding to the measured acceleration to a frequency reference associated with the system; an adaptive weighting component that receives the oscillator output signal and an external signal provided from a source external to the platform, and adjusts the filter weight set for the filter assembly based on a comparison between the external signal and the oscillator output signal.

2. The system according to claim 1, wherein the frequency reference is the reference oscillator.

3. The system according to claim 1, wherein the frequency reference is at least one numerically controlled oscillator driven by the oscillator output signal.

4. The system according to claim 1, further comprising a phase-locked loop that calculates a correction value from a comparison between the external signal and the oscillator output signal and adds the correction value to the tuning control signal.

5. The system according to claim 1, wherein the adaptive weighting component does not adjust the filter weight set when the magnitude of the measured acceleration is below a threshold.

6. The system according to claim 1, wherein the reference oscillator, the accelerometer, the filter assembly, and the adaptive weighting component are all implemented on the platform, and the external signal is provided from a remote location via a receiver on the platform.

7. The system according to claim 6, wherein the oscillator output signal is provided as a reference for a transmitter, whereby a signal transmitted by the transmitter is not affected by mechanical acceleration at the reference oscillator.

8. The platform is a first platform, the reference oscillator, the accelerometer, and the filter assembly are mounted on the first platform, the adaptive weighting component is mounted on a second platform remote from the first platform, the first platform transmits the oscillator output signal to the second platform via an associated transmitter, and the second platform transmits the filter weight set to the first platform via an associated second transmitter. The system according to claim 1.

9. The platform is a first platform, the reference oscillator, the accelerometer, and the filter assembly are mounted on the first platform, the adaptive weighting component is distributed between the first platform and a second platform remote from the first platform, the first platform transmits the oscillator output signal to the second platform in an associated first transceiver, and the second platform transmits an indication of a frequency error in the oscillator output signal to the first platform in an associated second transceiver. The system according to claim 1.

10. The adaptive weighting component includes a demodulator that determines a phase error in the oscillator output signal from the oscillator output signal and an external signal, and a weight calculation component that adjusts the filter weight set based on the phase error determined in the oscillator output signal. The system according to claim 1.

11. The adaptive weighting component includes a frequency estimation filter that calculates an instantaneous frequency from the phase error determined in the oscillator output signal, a compensation frequency represented by the synchronization control signal, and a measured acceleration corresponding to the phase error determined in the oscillator output signal, and the weight calculation component determines a value that minimizes a difference between the instantaneous frequency and the compensation frequency for a set of filter coefficients. The system according to claim 10.

12. The system according to claim 10 further includes a phase-locked loop that calculates a correction value from the phase error in the oscillator output signal and adds the correction value to the synchronization control signal.

13. The system according to claim 1, wherein the external signal is provided via a satellite associated with the system.

14. The system according to claim 1, wherein the accelerometer is a three-axis accelerometer that provides measured accelerations along each of a first axis, a second axis, and a third axis, and the filter weight set includes filter weight subsets for each of the first axis, the second axis, and the third axis.

15. A method for compensating for mechanical acceleration in a reference oscillator, comprising: detecting mechanical acceleration in an accelerometer on the same platform as the reference oscillator to generate a measured acceleration; supplying a tuning control signal corresponding to the measured acceleration to a filter assembly having a filter weight set; adjusting the filter weight set for the filter assembly based on a comparison between an external signal provided from a source external to the platform and an oscillator output signal of the reference oscillator; supplying the tuning control signal to a frequency reference of the system.

16. The method according to claim 15, wherein the frequency reference is the reference oscillator.

17. The method according to claim 15, wherein the frequency reference is at least one numerically controlled oscillator driven by the oscillator output signal.

18. Determining the filter weight set of the filter assembly includes: transmitting a signal generated from the oscillator output signal from the platform to a remote platform; calculating a frequency error in the oscillator output signal from an external signal generated at the remote platform and the signal generated from the oscillator output signal; transmitting an indicator of the calculated frequency error to the platform; adjusting the filter weight set for the filter assembly from a phase error calculated at the remote platform. The method according to claim 15.

19. further comprising calculating a correction value in a position synchronization loop; adding the correction value to the tuning control signal. The method according to claim 15.

20. The method according to claim 15, wherein the filter weight set for the filter assembly is determined only periodically, and the accelerometer and the filter are active when the filter weight set is not being determined.

21. Adjusting the filter weight set for the filter assembly comprises determining a phase error in the oscillator output signal from the external signal and the oscillator output signal, and adjusting the filter weight set from the determined phase error, the method according to claim 15.

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