CN104320176B - A kind of satellite communication system and its forward direction Calibration Method - Google Patents

Abstract

The invention discloses a satellite communication system and a forward calibration method thereof. The ground station of the satellite communication system generates a plurality of mutually orthogonal spread spectrum correction signals, which are used as each forward channel data for frequency division multiplexing and then transmitted from the ground to the star. After demultiplexing on the star, each The correction signal in the channel is connected to the coupling network on the back of the satellite’s forward receiving antenna, and the superimposed signal of the correction signal is output and transmitted to the ground as a return channel data. To the channel delay, amplitude and phase information, and then complete the forward calibration. The invention does not need to set up multiple calibration stations, which reduces the complexity of system maintenance; the spread spectrum correction signal is generated on the ground instead of directly on the satellite, which reduces the amount of equipment on the satellite.

Description

A satellite communication system and its forward calibration method

technical field

The invention relates to a satellite communication system and calibration means in the satellite communication of the satellite communication system, in particular to a satellite communication system and a forward calibration method thereof.

Background technique

In the satellite communication system based on ground beamforming, since the forward transmission beam of the satellite is formed on the ground, the data of each channel needs to be transmitted from the ground to the star through the forward feeder link after being weighted, and the data of multiple forward channels usually passes through The wideband signal synthesized by frequency division multiplexing is transmitted to the satellite through the forward satellite-ground feeder link. After the satellite receives the broadband signal of the forward feed link, it separates the data of multiple forward channels, and sends them to the forward transmit channel of the satellite user segment respectively, and then forms a beam. However, due to the influence of the wireless channel and microwave components of the forward feed link, different frequency bands of frequency division multiplexed broadband signals will experience different channel fading, that is, different channels received by satellites will experience different amplitude attenuation and phase Variations and delays affect the forward beamforming performance of the user segment. Therefore, it is necessary to adopt certain technical means to improve the situation that different channels are affected by the environment differently during the signal satellite-to-ground transmission process. Forward calibration refers to the use of certain calibration methods for the signal transmission in the forward link from the ground station to the satellite to correct the time delay, amplitude and phase difference between different forward channel signals.

The commonly used forward calibration method is to set up a calibration station on the ground, and use the calibration station to perform calibration. The calibration signal of the forward link is sent by the ground station, passes through the forward processing channel on the star, and reaches different calibration stations. The correction station transmits the obtained amplitude, phase and delay difference to the ground station to complete the forward calibration process. The traditional method of setting up a calibration station to correct channel signals requires a large amount of equipment, which makes the system complex, high maintenance costs, and a waste of manpower and material resources.

Contents of the invention

Based on the traditional forward calibration method of setting calibration stations outside the array, the amplitude, phase and time delay between channels caused by antenna deformation, changes in the distance between the feed source and the antenna, and the deformation of the feed source itself due to temperature changes, etc. The result is relatively accurate, but the cost is high and the system is complicated. In order to solve this problem, the present invention proposes a satellite communication system and its forward calibration method in satellite communication.

The present invention is realized like this, a kind of satellite communication system, it comprises at least one ground station and the satellite that adopts satellite communication with this ground station: This ground station comprises first frequency division multiplexer, ground transmitting antenna, ground receiving antenna, The first demultiplexer, channel corrector, spread spectrum signal generator, and despread processor; the satellite includes a receiving antenna on the star, a forward processor on the star, a second demultiplexer, a forward Receiving antenna, second frequency division multiplexer, on-board return processor, on-board transmitting antenna, coupling network, filter converter;

The spread spectrum signal generator is used to make the correction signal generate N mutually orthogonal spread spectrum correction signals in the form of spread spectrum signals; the first frequency division multiplexer is used to modulate the N spread spectrum correction signals to N front Perform frequency division multiplexing into the data channel; the ground transmitting antenna is used to send the signal after frequency division multiplexing by the first frequency division multiplexer to the satellite as the transmitting signal of the ground station;

The receiving antenna on the star is used to receive the transmission signal of the ground station; the forward processor on the star is used to perform forward processing on the transmission signal of the ground station; The signal is demultiplexed to separate N correction signals, and the N correction signals are sent to the coupling network on the back of the radiation surface of the forward receiving antenna of the satellite; the coupling network and each of the forward receiving antennas The forward transmitting channel is connected, and the coupling network outputs superimposed signals of N correction signals; the filtering frequency converter filters the superimposed signals and converts the frequency to the signal center frequency point of the return receiving channel of the satellite; the second frequency division multiplexer is used The frequency-converted superimposed correction signal is used as a separate channel for frequency division multiplexing with the return channel data of the satellite; the on-board return processor is used to filter, amplify and convert the frequency-division multiplexed signal As the transmitting signal of the satellite, the transmitting signal of the satellite is transmitted to the ground station through the transmitting antenna on the satellite;

The ground receiving antenna is used to receive the transmitted signal from the satellite; the first frequency division multiplexer is used to demultiplex the transmitted signal of the satellite to obtain a superposition correction signal; the despreading processor uses the signal generated by the transmitting end N mutually orthogonal spread spectrum correction signals are despread, and then the amplitude attenuation, phase change and delay information of each forward channel are obtained; the channel corrector is used to obtain the amplitude attenuation, phase Using the change and delay information as a reference, the amplitude, phase and delay of other forward channel data are corrected.

As a further improvement of the above scheme, the second frequency division multiplexer is used to use the frequency-converted superimposed correction signal as a separate channel and perform frequency division multiplexing with the return receiving channel data of the satellite, and the center of the returning receiving channel signal The maximum frequency is modulated to f _h , and the superimposed correction signal is modulated to the center frequency point f _h +Δ, where Δ is the frequency interval between the data channel and the correction signal.

The present invention also provides a forward calibration method in satellite communication, which is applied to the above-mentioned satellite communication system, and is used for compensating the process of transmitting channel signals in the forward feed section between the satellite and the ground station by means of frequency division multiplexing Different channels are affected by different distortions caused by the environment, wherein: the forward calibration method in the satellite communication includes the following steps:

The first step, the ground station: generate N mutually orthogonal spread spectrum correction signals, and modulate the N correction signals into N forward data channels for frequency division multiplexing, and the ground station converts the frequency division multiplexed The signal is processed by radio frequency and transmitted through the antenna;

The second step, the satellite: receive the signal of the ground station, filter, amplify, down-convert, demultiplex, separate out N correction signals, and send the N correction signals to the radiation surface of the forward receiving antenna The coupling network on the back; the coupling network is connected to each forward transmission channel, and the coupling network outputs the superimposed signal of N correction signals; the superimposed signal is filtered and converted to the signal center frequency point of the satellite return receiving channel; The frequency-converted superimposed correction signal is used as a separate channel to perform frequency division multiplexing with the satellite's return receiving channel data; the frequency-division multiplexed signal is filtered, amplified, and frequency-converted, and transmitted through the satellite-ground return feeder link to the ground station;

The third step is to return to the ground: demultiplex the received signal to obtain the superposition correction signal, and use the N mutually orthogonal spread spectrum correction signals generated by the transmitting end to perform despreading processing, and then obtain each forward For the amplitude attenuation, phase change and delay information of the channel, the ground station uses the amplitude attenuation, phase change and delay information of the first forward channel as a reference to correct the amplitude, phase and delay of other forward channel data.

As a further improvement of the above scheme, the frequency-converted superimposed correction signal is used as a separate channel for frequency division multiplexing with the return receiving channel data of the satellite: the center frequency of the returning receiving channel signal is modulated to f _h at the maximum, and the superposition The correction signal is modulated to the center frequency point f _h +Δ, where Δ is the frequency interval between the data channel and the correction signal.

Compared with prior art, the present invention has following advantage:

1. The forward calibration method adopted in the present invention does not need to set up multiple calibration stations on the ground, which can reduce a series of problems caused by the construction and maintenance of calibration stations;

2. The design of the forward calibration in the array of the present invention makes the coupling network itself a part of the on-board system, which greatly reduces the space and equipment of the entire system, and also reduces the complexity.

Description of drawings

FIG. 1 is a schematic structural diagram of a satellite communication system provided by Embodiment 1 of the present invention, that is, a schematic diagram of a forward calibration method of the present invention.

Fig. 2 is a frequency division multiplexing diagram of the forward channels of 10 calibration signals of the ground station in Fig. 1 .

Fig. 3 is a schematic diagram of 10 superimposed calibration signals output by the on-board coupling network in Fig. 1 .

FIG. 4 is a schematic diagram of the frequency domain of the signal after frequency division multiplexing of the on-board channel in FIG. 1 .

Fig. 5 is a schematic structural diagram of the satellite communication system provided by Embodiment 2 of the present invention, that is, a system framework diagram of the ground multi-station joint forward calibration method described in the present invention.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

Example 1:

Referring to Fig. 1, Fig. 2, Fig. 3 and Fig. 4 together, describe the first embodiment of the satellite communication system of the present invention, the star communication system includes at least one ground station 1 and the satellite 2 that adopts satellite communication with this ground station 1. In this embodiment, there is a ground station 1 on the ground, and the data of 10 forward channels of the ground station 1 needs to be transmitted to the satellite, that is, the satellite 2. The data bandwidth of each channel is 7MHz, and the data of the 10 channels adopts frequency division multiplexing Transmission is carried out in the way, considering the isolation of each channel, the interval between each adjacent frequency point is taken as 10MHz when performing frequency division multiplexing.

The ground station 1 includes a spread spectrum signal generator 12, a first frequency division multiplexer 13, a ground transmitting antenna 14, a ground receiving antenna 15, a first frequency division multiplexer 16, a channel corrector 17, and a despreading processor 18 . The satellite includes a receiving antenna 22 on the star, a forward processor 23 on the star, a second frequency division multiplexer 24, a receiving antenna 25 forward, a coupling network 26, a second frequency division multiplexer 27, and a return signal on the star. Processor 28 , on-board transmitting antenna 29 , filter frequency converter 30 .

The spread spectrum signal generator 12 is used to make the correction signal generate N mutually orthogonal spread spectrum correction signals in the form of spread spectrum signals; the first frequency division multiplexer 13 is used to modulate the N spread spectrum correction signals respectively Frequency division multiplexing is carried out in the N forward data channels; the ground transmitting antenna 14 is used to send the signal frequency division multiplexed by the first frequency division multiplexer 13 as the transmission signal of the ground station 1 to the Satellite 2.

The receiving antenna 22 on the star is used to receive the transmission signal of the ground station 1; the forward processor 23 on the star is used to perform forward processing on the transmission signal of the ground station 1; the second frequency division multiplexer 24 It is used to demultiplex the forwardly processed signal to separate N correction signals, and send N correction signals to the coupling network 26 on the back side of the radiation surface of the forward receiving antenna 25 of the satellite 2; the coupling The network 26 is connected to each forward transmitting channel of the forward receiving antenna 25, and the coupling network 26 outputs superimposed signals of N correction signals; The signal center frequency point of the channel; the second frequency division multiplexer 27 is used to carry out frequency division multiplexing with the return receiving channel data of the satellite 2 as a separate channel through the superimposed correction signal after frequency conversion; the return on the star The processor 28 is used for filtering, amplifying and converting the frequency division multiplexed signal as the transmitting signal of the satellite 2 , and the transmitting signal of the satellite 2 is transmitted to the ground station 1 through the transmitting antenna 29 on the satellite.

The ground receiving antenna 15 is used to receive the transmission signal from the satellite 2; the first frequency division multiplexer 16 is used to demultiplex the transmission signal of the satellite 2 to obtain a superposition correction signal; the despreading processor 18. Use the N mutually orthogonal spread spectrum correction signals generated by the transmitting end to perform despreading processing, and then obtain the amplitude attenuation, phase change and delay information of each forward channel; the channel corrector 17 is used to use the first Based on the amplitude attenuation, phase change and delay information of the forward channel, the amplitude, phase and delay of other forward channel data are corrected.

Ground station 1 generates 10 mutually orthogonal spread spectrum correction signals of 7MHz bandwidth, and performs frequency division multiplexing as signals of 10 forward channels, and ground station 1 uses the first frequency division multiplexer 13 to perform frequency division multiplexing on the forward correction signals. The frequency domain signal of signal A after division and multiplexing is shown in Fig. 2 ;

The satellite 2 uses the receiving antenna 22 on the star to receive the signal of the ground transmitting antenna 14, uses the forward processor 23 on the star to filter, amplify, and convert the signal, and then uses the second demultiplexer 24 to decompose the signal. Frequency division multiplexing, obtain 10 spread spectrum correction signals, and send 10 spread spectrum correction signals to the coupling network 26 on the back side of the radiation surface of the forward receiving antenna 25 of the satellite 2; the coupling network 26 is connected with each forward transmitting channel , the coupling network 26 outputs superposition signals of 10 spread spectrum correction signals, and the frequency domain schematic diagram of the superposition correction signals is shown in Figure 3; the satellite 2 filters the superposition correction signals and converts the frequency to the signal center frequency point of the return receiving channel of the satellite 2 , Satellite 2 uses the superimposed correction signal after frequency conversion as a separate channel to perform frequency division multiplexing with the data of 10 return receiving channels of the satellite. The frequency domain schematic diagram of signal B after frequency division multiplexing is shown in Figure 4, and then Signal B is filtered, amplified, and frequency-converted by the return processor 28 on the satellite, and transmitted by the transmitting antenna 29 on the satellite.

The ground station 1 uses the ground receiving antenna 15 to receive the signal of the satellite transmitting antenna 29, and uses the first frequency division multiplexer 16 to perform frequency division multiplexing to obtain 10 return channel signals and superposition correction signals. The ground station 1 pairs The superimposed correction signal uses 10 spreading codes for despreading processing, and compares the amplitude, phase and time delay with the 10 spreading correction signals sent, and then obtains the amplitude fading, phase change and time delay of the 10 forward channels Information, ground station 1 uses the amplitude attenuation, phase change and delay information of the first forward channel as a reference, and corrects the amplitude, phase and delay of other channel data to complete the forward calibration work.

In the present invention, the forward correction signal adopts a plurality of mutually orthogonal spread-spectrum signals, which are superimposed together after coupling on the satellite and transmitted back to the ground through the return link. Since the spread-spectrum signals are orthogonal, the superposition will not Affects the despreading process of Ground Station 1. In addition, the forward calibration signal in the present invention is generated by the ground station 1 and transmitted to the star, and then coupled on the star and then transmitted back to the ground station 1 via the return link, which avoids the problem that the traditional calibration method needs a calibration station , thus simplifying the system design and later maintenance.

Example 2:

Another embodiment of the present invention is a scenario where distributed ground stations are used. Please refer to Figure 5. Two ground stations are arranged on the ground, namely ground station one 3 and ground station two 4. There are 80 channel data in the forward direction of the system that need to be transmitted by the ground To the star is satellite 5. There are 40 channels of data in the return direction that need to be transmitted from the star to the ground, and the bandwidth of each forward channel data and return channel data is 7MHz. Two directional receiving antennas are configured on the star, that is, the first receiving antenna 51 on the satellite and the second receiving antenna 52 on the satellite, pointing to the first ground station 3 and the second ground station 4 respectively.

The ground station-3 to the on-star receiving antenna-51 transmit 40 forward channel data, and the data of 40 channels is transmitted by frequency division multiplexing. Considering the isolation of each channel, when performing frequency division multiplexing, each adjacent The interval of frequency points is taken as 10MHz. The ground station 2 4 transmits another 40 forward channel data to the receiving antenna 2 52 on the star, and the data of the 40 channels is transmitted by means of frequency division multiplexing, considering the isolation of each channel, when performing frequency division multiplexing, each phase The interval between adjacent frequency points is 10MHz.

Ground station 2 4 generates 80 mutually orthogonal 7MHz bandwidth spread spectrum correction signals, 40 of which are sent to ground station 1 3 for frequency division multiplexing as 40 forward channel signals. Ground station one 3 uses the first frequency division multiplexer one 31 to carry out frequency division multiplexing on the forward correction signal and then uses the ground transmitting antenna one 33 to transmit; ground station two 4 uses the second frequency for the other 40 spread spectrum correction signals The second multiplexer 32 performs frequency division multiplexing and transmits it using the second ground transmitting antenna 34.

The satellite 2 uses the receiving antenna-51 on the star to receive the signal of the ground transmitting antenna-33, and uses the forward processor-53 on the star to filter, amplify, and convert the signal, and then uses the second frequency division multiplexer-55 The signal is demultiplexed to obtain 40 spread spectrum correction signals. The satellite 2 uses the receiving antenna 2 52 on the star to receive the signal of the ground transmitting antenna 2 34, uses the forward processor 2 54 on the star to filter, amplify, and convert the signal, and then uses the second frequency division multiplexer 2 56 The signal is demultiplexed to obtain another 40 spread spectrum correction signals, and a total of 80 spread spectrum correction signals are obtained.

The satellite 2 sends the obtained 80 spread spectrum correction signals to the coupling network 26 on the back side of the radiation surface of the forward receiving antenna 25 of the satellite 2. The coupling network 26 is connected to each forward transmission channel, and the coupling network 26 outputs 80 spread spectrum correction signals. Superposition of signals. Satellite 2 uses filter frequency converter 30 to filter the superposition correction signal and converts it to the signal center frequency point of the return receiving channel of satellite 2. Satellite 2 uses the superposition correction signal after frequency conversion as a separate channel and 40 return channels of the satellite. The receiving channel data is frequency-division-multiplexed by the second frequency-division multiplexer 27 , then filtered, amplified, and frequency-converted by the on-board return processor 28 , and transmitted by the on-board transmitting antenna 29 .

The ground station 2 4 uses the ground receiving antenna 15 to receive the signal from the satellite transmitting antenna 29, and uses the first frequency division multiplexer 16 to perform frequency division multiplexing to obtain 40 return channel signals and superposition correction signals. The ground station 24 uses 80 spread spectrum codes to despread the superimposed correction signal, and compares the amplitude, phase and delay with the 80 spread spectrum correction signals sent, and then obtains the amplitude fading, For phase change and delay information, the ground station uses the amplitude attenuation, phase change and delay information of the first forward channel as a reference to correct the amplitude, phase and delay of other channel data to complete the forward calibration work.

In this embodiment, two ground stations are arranged on the ground, which are respectively responsible for the transmission of part of the forward channel signals, which is a simple extension of the present invention.

In the present invention, the forward correction signal adopts a plurality of mutually orthogonal spread-spectrum signals, which are superimposed together after coupling on the satellite and transmitted back to the ground through the return link. Since the spread-spectrum signals are orthogonal, the superposition will not Affects despreading processing at ground stations. In addition, the forward calibration signal in the present invention is generated by the ground station and transmitted to the star, and then coupled on the star and then transmitted back to the ground through the return link, which avoids the problem that the traditional calibration method needs a calibration station, thereby Simplifies system design and facilitates post-maintenance.

In summary, the ground station of the present invention generates a plurality of mutually orthogonal spread spectrum correction signals, and transmits them from the ground to the star after frequency division multiplexing as each forward channel data, and after the frequency division multiplexing is demultiplexed on the star Connect the correction signals in each channel to the coupling network on the back of the satellite’s forward receiving antenna, output the superimposed signal of the correction signal, and transmit it to the ground as a return channel data, and the ground station despreads the superimposed spread spectrum correction signal Obtain the delay, amplitude and phase information of the forward channel, and then complete the forward calibration. The invention does not need to set up multiple calibration stations, which reduces the complexity of system maintenance; the spread spectrum correction signal is generated on the ground instead of directly on the satellite, which reduces the amount of equipment on the satellite.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. within range.

Claims (4)

1. a kind of satellite communication system, it includes at least one earth station and the satellite of satellite communication is used with the earth station： Wherein, the earth station solves frequency division multiplexer including the first frequency division multiplexer, ground launch antenna, terrestrial receiving antenna, first, led to Channel correction device；The satellite includes reception antenna on star, on star before to processor, the second solution frequency division multiplexer, Forward receiving antenna, Back processor on second frequency division multiplexer, star, transmitting antenna on star；It is characterized in that：The earth station also includes spread-spectrum signal Generator, despreading processor；The satellite also includes coupling network, filtering frequency converter；

The spread-spectrum signal generator is used for the N number of mutually orthogonal spread spectrum correction of Form generation for making correction signal using spread-spectrum signal Signal；First frequency division multiplexer, which is used to N number of spread spectrum correction signal be modulated to respectively in N number of forward data passage, carries out frequency division Multiplexing；The ground launch antenna be used for using the signal after the first frequency division multiplexer frequency division multiplexing as the earth station transmitting Signal is sent to the satellite；

Reception antenna is used for the transmission signal for receiving the earth station on the star；It is used for before on the star to processor to the earth station Transmission signal carries out preceding to processing；The second solution frequency division multiplexer is used for carrying out solution frequency division multiplexing point through the preceding signal to processing Separate out and isolate N number of correction signal, and N number of correction signal send into satellite the Forward receiving antenna the radiating surface back side coupling Close network；The coupling network is connected with each forward emitted passage of the Forward receiving antenna, and the coupling network exports N number of correction The superposed signal of signal；The filtering frequency converter is filtered and is converted to the signal of the back receiving channel of satellite to superposed signal Center frequency point；Second frequency division multiplexer is used to assign the superposition correction signal after frequency conversion as an individual passage and satellite Back receiving channel data carry out frequency division multiplexing；Back processor is used to the signal after frequency division multiplexing be filtered on the star Ripple, amplification, frequency-conversion processing as the satellite transmission signal, the transmission signal of the satellite by transmitting antenna on the star transmit to The earth station；

The terrestrial receiving antenna is used to receive the transmission signal from the satellite；The first solution frequency division multiplexer is used for the satellite Transmission signal solution frequency division multiplexing obtain superposition correction signal；The despreading processor is N number of mutually orthogonal using transmitting terminal generation Spread spectrum correction signal carry out despreading processing, and then obtain amplitude fading, phase place change and the time delay of each forward path Information；The channel correcting device is used on the basis of the amplitude fading of first forward path, phase place change and Delay, to it Amplitude, phase and the time delay of his forward path data are corrected.

2. satellite communication system as claimed in claim 1, it is characterised in that：Second frequency division multiplexer is used for handle and passes through frequency conversion Superposition correction signal afterwards carries out frequency division multiplexing as an individual passage and the back receiving channel data of satellite, and back is connect The centre frequency maximum modulation of channel signal is received to f_h, superposition correction signal is modulated to center frequency point f_h+ Δ, wherein Δ are number According to passage and the frequency interval of correction signal.

3. the forward direction Calibration Method in a kind of satellite communication, it is applied in satellite communication system as claimed in claim 1, is used In compensation satellite and earth station between forward direction feed section use frequency division multiplexing mode channel signal during different passages by The different distortions that ambient influnence is caused, it is characterised in that：Forward direction Calibration Method in the satellite communication comprises the following steps：

The first step, earth station：N number of mutually orthogonal spread spectrum correction signal is produced, and N number of correction signal is modulated to respectively N number of Frequency division multiplexing is carried out in forward data passage, the signal after frequency division multiplexing is carried out radio frequency processing and launched using antenna by earth station Go；

Second step, satellite：Receive the signal of the earth station, be filtered, amplify, down coversion, solution frequency division multiplexing, isolate N Individual correction signal, and N number of correction signal sent into the coupling network at the Forward receiving aerial radiation face back side；The coupling network with Each forward emitted passage is connected, and the coupling network exports the superposed signal of N number of correction signal；Superposed signal is filtered simultaneously It is converted to signal center's frequency of satellite back receiving channel；It is individually logical using the superposition correction signal after frequency conversion as one The back receiving channel data of road and satellite carry out frequency division multiplexing；Signal after frequency division multiplexing is filtered, amplified, at frequency conversion Reason, by star back feeding link transmit to earth station；

3rd step, returns to ground：Solution frequency division multiplexing acquisition superposition correction signal is carried out to the signal of reception, and produced using transmitting terminal Raw N number of mutually orthogonal spread spectrum correction signal carries out despreading processing, so obtain each forward path amplitude fading, Phase place change and Delay, earth station on the basis of the amplitude fading of first forward path, phase place change and Delay, Amplitude, phase and time delay to other forward path data are corrected.

4. the forward direction Calibration Method in satellite communication as claimed in claim 3, it is characterised in that：The superposition after frequency conversion Correction signal carries out frequency division multiplexing as an individual passage and the back receiving channel data of satellite：Back receiving channel letter Number centre frequency maximum modulation to f_h, superposition correction signal is modulated to center frequency point f_h+ Δ, wherein Δ be data channel with The frequency interval of correction signal.