A space-based detection radar channel consistency calibration method based on an active calibrator

Abstract

The application belongs to the field of space-based microwave radar, and discloses a space-based detection radar channel consistency calibration method based on an active calibrator. The active calibrator is arranged on the ground to actively emit a point frequency continuous wave signal, and the space-based detection radar beam direction is adjusted to make the beam dwell and continuously scan the active calibrator, so that a high signal-to-noise ratio, stable and controllable external radiation echo signal is obtained. The received point frequency continuous wave signal is multiplied by a carrier frequency signal and then subjected to quadrature demodulation to obtain demodulation signals of each channel; distance direction FFT is performed on the demodulation signals to obtain peak amplitude and peak phase; the phase difference and amplitude difference of each channel relative to a reference channel are calculated to generate a channel consistency calibration parameter file for radar calling. The precision and stability of the space-based detection radar channel consistency calibration are improved, the method is suitable for a multi-frequency band and multi-channel system, and has operability and popularization value in engineering application.

Description

Technical Field

[0001] This invention relates to the field of space-based microwave radar, and specifically to a channel consistency calibration method for space-based detection radar based on an active calibrator. Background Technology

[0002] Multi-channel design is one of the key technologies for ensuring the performance of space-based radar. With the increasing quantitative requirements of space-based radar applications, the consistency calibration between radar channels has become a crucial aspect of ensuring system performance. Channel consistency mainly involves amplitude consistency and phase consistency; if these are not effectively calibrated, it will affect the fidelity of the detected information, the accuracy of interferometry, and the quality of multi-channel imaging.

[0003] Currently, the most common channel consistency calibration methods for space-based radars include internal calibration, distributed natural scatterer calibration, and artificial corner reflector or active calibrator methods.

[0004] Internal calibration can perform calibration without relying on external targets. However, since the internal calibration signal does not pass through the antenna and external radiation path, it cannot fully reflect the overall channel characteristics of the device. Furthermore, in environments with strong electromagnetic interference or during long-term operation, the stability of the internal calibration loop may be insufficient, affecting calibration accuracy.

[0005] Distributed natural scatterer calibration is suitable for observation over large areas, but naturally distributed targets are highly random and have low echo signal-to-noise ratios. Therefore, calibration requires time averaging of results from multiple repeated trials, making rapid calibration difficult. Furthermore, the accuracy of the calibration results may vary significantly for different distributed targets.

[0006] Artificial corner reflectors or active calibrators are used to provide targets with high radar cross section (RCS) by deploying corner reflectors or active calibrators on the ground, enabling radiation link calibration. While trihedral corner reflectors are easy to deploy for absolute amplitude calibration of atmospheric cloud radar, their strong scattering characteristics are highly dependent on the incident angle, which may limit the signal-to-noise ratio in multi-channel consistency testing due to angle limitations. Active calibrators, which actively transmit radar signals, can achieve stronger scattered echoes and more flexible applications; however, these techniques are mostly used for overall radar amplitude and phase calibration or polarization calibration, and in space-based detection environments, dynamic changes in radar beam pointing can introduce additional errors into the calibration results.

[0007] Therefore, there is an urgent need for a consistency calibration scheme for space-based radar channels to achieve high-precision and rapid calibration of channel consistency. Summary of the Invention

[0008] To address the existing technical problems and achieve high-precision and rapid calibration of channel consistency, this application provides a channel consistency calibration method for space-based radar based on an active calibrator.

[0009] High-precision and rapid calibration refers to a phase accuracy better than 2° and an amplitude accuracy better than 0.5dB in a single experimental compensation.

[0010] Firstly, this application provides a channel consistency calibration method for space-based detection radar based on an active calibrator, adopting the following technical solution:

[0011] The radar switches to external calibration mode, with the beam center aligned with the active calibrator;

[0012] An active calibrator transmits a point-frequency continuous wave signal, which is consistent with the radar's operating frequency.

[0013] The radar receives point-frequency continuous wave signals and samples and records the received raw data to generate raw calibration data;

[0014] The raw calibration data is downloaded to the ground station;

[0015] The amplitude and phase differences of each channel are extracted as follows: The point-frequency continuous wave signal in the received original calibration data is multiplied with the carrier frequency signal and then orthogonally demodulated to obtain the demodulated signal of each channel; the range FFT is performed on the demodulated signal to obtain the peak amplitude and peak phase; the phase difference and amplitude difference of each channel relative to the reference channel are calculated.

[0016] Iterate through all operating frequencies and extract the amplitude difference and phase difference of each channel at each frequency.

[0017] Generate a channel consistency calibration parameter file for radar to use.

[0018] Furthermore, the point-frequency continuous wave signal for:

[0019]

[0020] in, Indicates signal amplitude; Indicates the carrier frequency; Indicates the intrinsic phase of the active scaler; Indicates time; It is a complex exponential signal; It is the imaginary unit.

[0021] Furthermore, when the radar receives point-frequency continuous wave signals and samples and records the received raw data to generate raw calibration data, the steps are as follows:

[0022] 3-1) All radar receiving channels synchronously acquire point-frequency continuous wave signals, and the sampling clock is distributed from the same local oscillator;

[0023] 3-2) The acquired data is stored in binary format in complex form, which includes two channels, I and Q, where I is the in-phase component and Q is the quadrature component;

[0024] 3-3) Synchronously record the platform attitude data and beam pointing, and store them in an auxiliary data file. The auxiliary data file is associated with the original calibration data index.

[0025] Furthermore, the platform attitude data includes pitch angle, roll angle, and yaw angle; the beam pointing includes azimuth angle and pitch angle.

[0026] Furthermore, the received point-frequency continuous wave signal is multiplied by the carrier frequency signal and then orthogonally demodulated to obtain the demodulated signal for each channel. for:

[0027]

[0028] in, Channel number, ; This represents the total number of channels. It is a positive integer; For time; For the first The amplitude of each channel; It is the common part composed of the phases of each channel; For the first The phase caused by the delay and temperature characteristics of each channel; It is a complex exponential signal; The imaginary unit; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path.

[0029] Furthermore, when performing a range-direction FFT on the demodulated signal to obtain the peak amplitude and peak phase, the specific steps are as follows:

[0030] Perform a range-direction FFT on the demodulated signals of all channels, and extract the peak amplitude and peak phase from the FFT results:

[0031]

[0032]

[0033] in, Channel number, ; This represents the total number of channels. It is a positive integer; For the first Each channel peak amplitude; For the first Each channel peak phase; For the first Demodulated signals of each channel; Indicates the amplitude of the signal; This indicates the phase of the signal.

[0034] Furthermore, the phase difference and amplitude difference of each channel relative to the reference channel are calculated as follows:

[0035] The phase difference is calculated as follows:

[0036]

[0037] in, Channel number, ; This represents the total number of channels. A positive integer; channel number For reference only; Indicates the first The phase difference between each channel and the reference channel; For the first Each channel peak phase; For the first Each channel peak phase; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path;

[0038] The amplitude difference is calculated as follows:

[0039]

[0040] in, Channel number, ; This represents the total number of channels. A positive integer; channel number For reference only; Indicates the first The amplitude difference between each channel and the reference channel; For the first Each channel peak amplitude; For the first Each channel peak amplitude.

[0041] Secondly, this application provides a computer-readable storage medium having a computer program stored thereon, wherein when the computer program is executed by a processor, it implements the steps of the space-based detection radar channel consistency calibration method based on an active calibrator as described in the first aspect.

[0042] The beneficial effects of this invention are as follows:

[0043] This invention proposes a channel consistency calibration method for space-based radar based on an active calibrator. This method enables high-precision extraction and compensation of channel amplitude and phase differences under external radiation link conditions, solving the problems of insufficient signal-to-noise ratio (SNR) of calibrated targets, difficulty in reflecting full-link characteristics, and susceptibility to beam dynamics in existing technologies. By deploying an active calibrator on the ground and transmitting a point-frequency continuous wave, a high SNR, stable, and controllable calibration signal can be provided to the satellite radar, effectively ensuring the reliability of inter-channel difference measurements. By continuously pointing the radar beam at the active calibrator in external calibration mode, the consistency of received data from each channel is ensured, thus avoiding the problems of limited SNR of distributed targets and strong dependence on the incident angle of corner reflectors. By introducing beam pointing difference compensation in the ground processing stage, the additional phase error caused by beam deflection can be eliminated, ensuring that the extraction results accurately reflect the characteristics of the channel itself. Through multi-frequency point repetitive processing, a complete set of channel consistency parameters can be formed across the entire frequency band, facilitating long-term on-orbit application. In summary, the method of this invention can significantly improve the accuracy and stability of channel consistency calibration for space-based radar, and is applicable to multi-band, multi-channel systems. It has operability and promotional value in engineering applications. Attached Figure Description

[0044] Figure 1 This is a flowchart of a channel consistency calibration method for space-based radar based on an active calibrator. Detailed Implementation

[0045] The present invention will now be described in further detail.

[0046] This invention provides a channel consistency calibration method for space-based detection radar based on an active calibrator. This method utilizes an active calibrator to perform channel consistency testing. A point-frequency continuous wave signal is emitted through the active calibrator, while the radar operates in receiving mode with the beam center continuously pointing towards the active calibrator. After compensating for the phase difference caused by the radar beam pointing, the amplitude and phase differences between channels are obtained by comparing the signal with a reference channel.

[0047] In this embodiment, the operating parameters of the active calibrator are set as follows:

[0048] Transmission frequency: Consistent with the radar's operating frequency;

[0049] Signal type: Point-frequency continuous wave;

[0050] Transmit power: 20dBm (signal strength must meet radar receiver sensitivity requirements);

[0051] Use a stable frequency source (such as a temperature-controlled crystal oscillator OCXO, with a frequency stability ≤10). -8 Ensure signal frequency stability.

[0052] The radar external calibration mode parameters are set as follows:

[0053] Number of channels: 64;

[0054] Beam pointing control: The beam pointing angle is calculated in real time based on the geographic coordinates of the active calibrator;

[0055] Data sampling rate: 10MHz;

[0056] Single data collection duration: 10 seconds.

[0057] A channel consistency calibration method for space-based radar based on active calibrators, such as Figure 1 As shown, it includes the following steps:

[0058] 1. Switch the radar to external calibration mode and align the beam center with the active calibrator.

[0059] The satellite radar system switches to external calibration mode. In this mode, the radar stops transmitting and only maintains reception. The receiving beam is pointed at the location of the active calibrator deployed on the ground to ensure observation stability during the calibration process. This includes the following steps:

[0060] 1-1) The ground control center sends a preset command to the radar, and the radar switches to external calibration mode.

[0061] 1-2) The radar transmitter is powered off, only the receiving link is powered on.

[0062] 1-3) The beam control system adjusts the phased array antenna according to the coordinates of the active calibrator (calculated by orbit ephemeris and active calibrator position) so that the beam center is aligned with the active calibrator.

[0063] 2. The active calibrator transmits a point-frequency continuous wave signal, which is consistent with the radar operating frequency.

[0064] The ground-based active calibrator starts operating according to preset parameters, transmitting a point-frequency continuous wave signal consistent with the radar's operating frequency. A carrier signal transmitted by the active calibrator is also provided. for:

[0065]

[0066] in, Indicates signal amplitude; Indicates the carrier frequency; Indicates the intrinsic phase of the active scaler; Indicates time; It is a complex exponential signal; The unit is imaginary. The signal maintains stable frequency and amplitude, and continuously radiates with a signal-to-noise ratio greater than 25 dB at the radar receiver (e.g., the equivalent isotropic radiated power of the transmitter is not less than 30 dBW).

[0067] Third, the radar receives point-frequency continuous wave signals and samples and records the received raw data to generate raw calibration data.

[0068] In receiving mode, the radar simultaneously receives continuous wave signals from the active calibrator using all receiving channels, and samples and records the raw data received by each channel in chronological order to form the corresponding raw calibration data. Specifically, the steps include:

[0069] 3-1) All radar receiving channels synchronously acquire signals (64 in this embodiment), and the sampling clock is distributed from the same local oscillator to ensure synchronization.

[0070] 3-2) The acquired data is stored in binary file in complex form (I / Q channels, where I is the in-phase component and Q is the quadrature component), which is the original calibration data.

[0071] 3-3) Synchronously record platform attitude data (including pitch, roll, and yaw angles) and beam pointing angles (azimuth and pitch angles), and store them in an auxiliary data file. This auxiliary data file is indexed by timestamps and the original calibration data in step 3-2, and is used to compensate for phase differences caused by beam pointing deviations in the future.

[0072] 4. Download the original calibration data to the ground station.

[0073] After receiving and recording, the satellite will transmit the original calibration data of each channel collected by the radar to the ground station via the data link.

[0074] 5. Extract the amplitude difference and phase difference of each channel.

[0075] The point-frequency continuous wave signal in the received raw calibration data is multiplied by the carrier frequency signal and then orthogonally demodulated to obtain the demodulated signal for each channel. The carrier frequency signal is a local reference signal independently generated by the ground station, space-based radar, and active calibrator, all three having the same frequency. The carrier frequency signal generated by the ground station is used to demodulate the downlinked raw calibration data; the carrier frequency signal generated by the space-based radar is used for radar signal transmission and reception; and the carrier frequency signal generated by the active calibrator is used for signal transmission. Range-direction FFT is performed on the demodulated signals of all channels, and the peak amplitude is extracted from the FFT results. and peak phase ; Calculate the phase difference and amplitude difference of each channel relative to the reference channel; the specific steps are as follows:

[0076] 5-1) After multiplying the point-frequency continuous wave signal in the received raw calibration data with the carrier frequency signal, orthogonal demodulation is performed to obtain the demodulated signal of each channel. :

[0077]

[0078] in, Channel number, ; This represents the total number of channels. It is a positive integer; For time; For the first The amplitude of each channel; It is the common part composed of the phases of each channel; For the first The phase caused by the delay and temperature characteristics of each channel; It is a complex exponential signal; The imaginary unit;

[0079] For the first The phase of the antenna pattern for each channel is determined by the beam pointing angle contained in the original calibration data. The information was calculated. Among them, It is the azimuth angle. It is the pitch angle.

[0080] For the first The phase of each channel transmission path is calculated as follows: Based on the coordinate sequence of the antenna phase center and the coordinates of the active calibrator, the actual pointing of the active calibrator relative to the antenna is calculated. , and then calculate the first Phase of each channel transmission path .in This is the actual azimuth angle. This is the actual pitch angle.

[0081] 5-2) Perform range-direction FFT on the demodulated signals of all channels and extract the peak amplitude from the FFT results. and peak phase :

[0082]

[0083]

[0084] in, Channel number, ; This represents the total number of channels. It is a positive integer; For the first Each channel peak amplitude; For the first Each channel peak phase; For the first Demodulated signals of each channel; Indicates the amplitude of the signal; This indicates the phase of the signal.

[0085] 5-3) Calculate the phase difference and amplitude difference of each channel relative to the reference channel.

[0086] In this embodiment, let the channel number This is for reference only.

[0087] The phase difference is calculated as follows:

[0088]

[0089] in, Channel number, ; This represents the total number of channels. It is a positive integer (in this embodiment, ); Channel number For reference only; Indicates the first The phase difference between each channel and the reference channel; For the first Each channel peak phase; For the first Each channel peak phase; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path.

[0090] The amplitude difference is calculated as follows:

[0091]

[0092] in, Channel number, ; This represents the total number of channels. A positive integer; channel number For reference only; Indicates the first The amplitude difference between each channel and the reference channel; For the first Each channel peak amplitude; For the first Each channel peak amplitude.

[0093] 6. Traverse all operating frequencies and extract the amplitude difference and phase difference of each channel at each frequency.

[0094] After extracting the channel differences for a single frequency point, steps two through five are repeated according to a preset frequency sequence to process all operating frequencies of the space-based radar one by one, and the amplitude and phase differences of each channel at each frequency point are obtained respectively. In this embodiment, a 64-channel space-based radar is used as an example, and data is collected within the observation time (5 minutes) of a single experiment using the above method. The processing results show that the original phase inconsistency standard deviation between channels is 12.5°. After calibration and compensation using this method, the residual phase consistency error is reduced to 1.1°, and the mean amplitude error is less than 0.3dB.

[0095] 7. Generate a channel consistency calibration parameter file for radar to use.

[0096] The amplitude and phase differences extracted from each channel at all frequency points are organized and stored according to a predefined data structure to generate a channel consistency calibration parameter file, which is stored in the radar system's non-volatile memory. For multiple receiving channels of the radar system, the amplitude and phase differences of each receiving channel relative to the reference channel at different operating frequencies are obtained. According to a predefined data structure (such as channel index, frequency point, difference value mapping, etc.), the amplitude and phase differences are associated and stored with the corresponding channel identifier and frequency point identifier; a channel consistency calibration parameter file containing the above associated stored data is generated. When the radar executes subsequent data processing and application algorithms, it calls the channel consistency calibration parameter file to compensate for the inconsistencies in amplitude and phase between channels, performing real-time correction on the original echo data, thereby ensuring the signal consistency of each radar channel.

[0097] The proposed method for channel consistency calibration of space-based radar based on active calibrators enables high-precision extraction and compensation of channel amplitude and phase differences under external radiation link conditions. This solves the problems of insufficient signal-to-noise ratio (SNR) of calibrated targets, difficulty in reflecting full-link characteristics, and susceptibility to beam dynamics in existing technologies. By deploying active calibrators on the ground and transmitting point-frequency continuous waves, a high SNR, stable, and controllable calibration signal can be provided to the satellite radar, effectively ensuring the reliability of inter-channel difference measurements. By continuously pointing the radar beam at the active calibrator in external calibration mode, the consistency of received data from each channel is guaranteed, thus avoiding the problems of limited SNR of distributed targets and strong dependence on the incident angle of corner reflectors. By introducing beam pointing difference compensation in the ground processing stage, the additional phase error caused by beam deflection can be eliminated, ensuring that the extraction results accurately reflect the characteristics of the channel itself. Compared to natural scatterer calibration in the prior art, this method can complete the calibration task in a single experiment, with residual phase consistency error accuracy better than 2° and amplitude accuracy better than 0.5dB, significantly improving the accuracy and efficiency of on-orbit real-time calibration (existing technologies typically require phase accuracy better than 5° and amplitude accuracy better than 1dB). Through repeated processing at multiple frequency points, a complete set of channel consistency parameters can be formed across the entire frequency band, facilitating long-term on-orbit applications. In summary, the method of this invention can significantly improve the accuracy and stability of channel consistency calibration for space-based radar, is suitable for multi-band, multi-channel systems, and has operability and promotional value in engineering applications.

[0098] In some embodiments, certain aspects of the above-described techniques may be implemented by one or more processors of a processing system executing software. The software includes one or more sets of executable instructions stored or otherwise tangibly implemented on a non-transitory computer-readable storage medium. The software may include instructions and certain data that, when executed by one or more processors, manipulate one or more processors to perform one or more aspects of the above-described techniques. The non-transitory computer-readable storage medium may include, for example, magnetic or optical disk storage devices, solid-state storage devices such as flash memory, cache, random access memory (RAM), or other non-volatile memory devices. The executable instructions stored on the non-transitory computer-readable storage medium may be source code, assembly language code, object code, or other instruction formats interpreted or otherwise executed by one or more processors.

[0099] Computer-readable storage media can include any storage medium or combination of storage media that can be accessed by a computer system during use to provide instructions and / or data to the computer system. Such storage media can include, but are not limited to, optical media (e.g., optical discs (CDs), digital versatile optical discs (DVDs), Blu-ray discs), magnetic media (e.g., floppy disks, magnetic tapes, or magnetic hard disks), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or flash memory), or microelectromechanical systems (MEMS) based storage media. Computer-readable storage media can be embedded in a computing system (e.g., system RAM or ROM), fixedly attached to a computing system (e.g., a magnetic hard disk drive), removably attached to a computing system (e.g., an optical disc or universal serial bus-based (USB) flash memory), or coupled to a computer system via a wired or wireless network (e.g., network-accessible storage (NAS)).

[0100] While the present invention has been disclosed above with reference to preferred embodiments, these embodiments are not intended to limit the invention. Any equivalent changes or modifications made without departing from the spirit and scope of the invention are also within the scope of protection of the invention. Therefore, the scope of protection of the present invention should be determined by the claims of this application.

Claims

1. A method for calibrating the channel consistency of a space-based radar based on an active calibrator, characterized in that, Includes the following steps: The radar switches to external calibration mode, with the beam center aligned with the active calibrator; An active calibrator transmits a point-frequency continuous wave signal, which is consistent with the radar's operating frequency. The radar receives point-frequency continuous wave signals and samples and records the received raw data to generate raw calibration data; The raw calibration data is downloaded to the ground station; The amplitude and phase differences of each channel are extracted as follows: The point-frequency continuous wave signal in the received original calibration data is multiplied with the carrier frequency signal and then orthogonally demodulated to obtain the demodulated signal of each channel; the range FFT is performed on the demodulated signal to obtain the peak amplitude and peak phase; the phase difference and amplitude difference of each channel relative to the reference channel are calculated. Iterate through all operating frequencies and extract the amplitude difference and phase difference of each channel at each frequency. Generate a channel consistency calibration parameter file for radar to use.

2. The channel consistency calibration method for space-based detection radar based on an active calibrator according to claim 1, characterized in that, The point frequency continuous wave signal for: in, Indicates signal amplitude; Indicates the carrier frequency; Indicates the intrinsic phase of the active scaler; Indicates time; It is a complex exponential signal; It is the imaginary unit.

3. The channel consistency calibration method for space-based detection radar based on an active calibrator according to claim 1, characterized in that, When a radar receives a point-frequency continuous wave signal and samples and records the received raw data to generate raw calibration data, the steps are as follows: 3-1) All radar receiving channels synchronously acquire point-frequency continuous wave signals, and the sampling clock is distributed from the same local oscillator; 3-2) The acquired data is stored in binary format in complex form, which includes two channels, I and Q, where I is the in-phase component and Q is the quadrature component; 3-3) Synchronously record the platform attitude data and beam pointing, and store them in an auxiliary data file. The auxiliary data file is associated with the original calibration data index.

4. The channel consistency calibration method for space-based detection radar based on an active calibrator according to claim 3, characterized in that, The platform attitude data includes pitch angle, roll angle, and yaw angle; the beam pointing includes azimuth angle and pitch angle.

5. The channel consistency calibration method for space-based detection radar based on an active calibrator according to claim 1, characterized in that, The received point-frequency continuous wave signal is multiplied by the carrier frequency signal and then orthogonally demodulated to obtain the demodulated signal for each channel. for: in, Channel number, ; This represents the total number of channels. It is a positive integer; For time; For the first The amplitude of each channel; It is the common part composed of the phases of each channel; For the first The phase caused by the delay and temperature characteristics of each channel; It is a complex exponential signal; The imaginary unit; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path.

6. The channel consistency calibration method for space-based detection radar based on an active calibrator according to claim 5, characterized in that, When performing a range-direction FFT on the demodulated signal to obtain the peak amplitude and peak phase, the specific steps are as follows: Perform a range-direction FFT on the demodulated signals of all channels, and extract the peak amplitude and peak phase from the FFT results: in, Channel number, ; This represents the total number of channels. It is a positive integer; For the first Each channel peak amplitude; For the first Each channel peak phase; For the first Demodulated signals of each channel; Indicates the amplitude of the signal; This indicates the phase of the signal.

7. The channel consistency calibration method for space-based detection radar based on an active calibrator according to claim 6, characterized in that, The phase difference and amplitude difference of each channel relative to the reference channel are calculated as follows: The phase difference is calculated as follows: in, Channel number, ; This represents the total number of channels. A positive integer; channel number For reference only; Indicates the first The phase difference between each channel and the reference channel; For the first Each channel peak phase; For the first Each channel peak phase; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path; For the first Phase of the antenna pattern for each channel; For the first Phase of each channel transmission path; The amplitude difference is calculated as follows: in, Channel number, ; This represents the total number of channels. A positive integer; channel number For reference only; Indicates the first The amplitude difference between each channel and the reference channel; For the first Each channel peak amplitude; For the first Each channel peak amplitude.

8. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by the processor, it implements the steps of the space-based radar channel consistency calibration method based on an active calibrator as described in any one of claims 1-7.

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