WO2008103170A1 - Horloge assistée - Google Patents

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Publication number
WO2008103170A1
WO2008103170A1 PCT/US2007/062452 US2007062452W WO2008103170A1 WO 2008103170 A1 WO2008103170 A1 WO 2008103170A1 US 2007062452 W US2007062452 W US 2007062452W WO 2008103170 A1 WO2008103170 A1 WO 2008103170A1
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WO
WIPO (PCT)
Prior art keywords
time
client
clock
server
packet
Prior art date
Application number
PCT/US2007/062452
Other languages
English (en)
Inventor
Kishan Shenoi
George P. Zampetti
Original Assignee
Symmetricom, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Symmetricom, Inc. filed Critical Symmetricom, Inc.
Priority to PCT/US2007/062452 priority Critical patent/WO2008103170A1/fr
Publication of WO2008103170A1 publication Critical patent/WO2008103170A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L12/00—Data switching networks
    • H04L12/28—Data switching networks characterised by path configuration, e.g. LAN [Local Area Networks] or WAN [Wide Area Networks]
    • H04L12/40—Bus networks
    • H04L12/407—Bus networks with decentralised control
    • H04L12/413—Bus networks with decentralised control with random access, e.g. carrier-sense multiple-access with collision detection [CSMA-CD]
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
    • G06F1/04—Generating or distributing clock signals or signals derived directly therefrom
    • G06F1/14—Time supervision arrangements, e.g. real time clock
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
    • H04J3/0658—Clock or time synchronisation among packet nodes
    • H04J3/0661—Clock or time synchronisation among packet nodes using timestamps
    • H04J3/0667—Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/02—Details
    • H04J3/06—Synchronising arrangements
    • H04J3/0635—Clock or time synchronisation in a network
    • H04J3/0638—Clock or time synchronisation among nodes; Internode synchronisation
    • H04J3/0658—Clock or time synchronisation among packet nodes
    • H04J3/0661—Clock or time synchronisation among packet nodes using timestamps
    • H04J3/0664—Clock or time synchronisation among packet nodes using timestamps unidirectional timestamps

Definitions

  • Embodiments of the present invention generally relate to methods of controlling the clock of an element deployed within a communications network and, more specifically, to an assisted wall-clock.
  • each element in the network has its own clock (referred to herein as a “client wall-clock”) providing frequency as well as absolute time and date information (the latter referred to herein as "time-of-day” information).
  • Quartz oscillators typically serve as local oscillators within client wall- clocks, providing frequency to support local timescale generation. While quartz oscillators offer good frequency stability over short term measurement intervals, their intermediate and long term frequency stability does not meet the telecommunications standards. Therefore, client wall-clocks must be checked and corrected against external sources. Furthermore, given the immense number of elements in the network, the client wall-clock of each element is required to not only provide good frequency stability over all measurement intervals, but also be reproducible throughout the entire network with the lowest cost per element.
  • NGNs Next generation networks
  • PONs Passive Optical Networks
  • DSL Digital Subscriber Line
  • cable modems or wireless networks.
  • packet-switched networks are optimum for delivering non-real-time services (e.g., file transfer, email, etc.), obtaining and maintaining accurate time and frequency information may be challenging since each packet in a flow may experience different delay with a significant random component, a phenomenon referred to as "packet delay variation" (PDV).
  • PDV packet delay variation
  • FIG. 1 illustrates the use of the NTP/SNTP to generate time-of-day information at the client wall-clock 100, according to prior art.
  • the client wall-clock 100 includes, without limitation, a local oscillator 110, a system time clock (STC) 120, and an NTP/SNTP client module 130.
  • the local oscillator 110 e.g., quartz oscillator
  • the local oscillator 110 continuously generates an periodic timing signal, referred to herein as a "local time-base" 115.
  • the local time-base 115 is supplied as one of the two inputs to the STC 120.
  • the other input to the STC 120 is an STC adjustment signal 143 from the NTP/SNTP client module 130 to support adjusting the local time in the STC 120 to the current value determined by the NTP/SNTP methods.
  • the STC 120 in turn, produces an STC output 125, which is an accumulation of elapsed time from some arbitrary epoch, where the accuracy of the elapsed time is governed by the accuracy of the local time- base 115.
  • the accuracy of the local time-base 115 is on the order of 100 parts per million (ppm), which means that the time drift of the STC 120 is 100 ⁇ s every second, resulting in the time drift of about 8.6 s every day.
  • the NTP/SNTP client module 130 uses the algorithms set forth in the NTP and/or SNTP standards to determine a clock correction value, CLK_CORR 145.
  • the client wall-clock 100 adds the CLK_CORR 145 to the STC output 125 and produces a time-of-day 155, which is the best estimate of absolute date and time, typically represented in seconds relative to Oh UTC on 1 January 1900.
  • One embodiment of the present invention sets forth a method for operating a client wall-clock to obtain time-of-day information.
  • the method includes the steps of receiving an incoming signal from at least one server, where the incoming signal contains at least one packet and the packet includes one or more server time stamps, extracting the server time stamps from the received packet, associating a terminating client time stamp with the received packet, extracting a frequency reference carried in a physical layer of the incoming signal, and stabilizing a local oscillator by synchronizing the local oscillator with the frequency reference.
  • the method also includes the steps of determining a clock correction value using an enhanced Jam- Sync method and adding the clock correction value to the STC output to obtain the time-of-day information, where the enhanced Jam-Sync method includes the steps of waiting for a reception of a valid server-to-client packet, initializing a first value of the progression of the client wall-clock over time, initializing a second value of at least one estimate of the constant offset between the true time-of-day and the STC output, calculating the clock correction value, and adjusting the STC.
  • control of the client wall- clock may be achieved by extracting and utilizing the frequency reference contained in the physical layer of the incoming signal, such as Network Timing Reference (NTR) or synchronous Ethernet.
  • NTR Network Timing Reference
  • local time-base with an accuracy of 1x10 '11 may be achieved.
  • An accurate local time-base in turn, enables a method for obtaining the clock correction value that is simpler and more reliable than the prior art methods.
  • the disclosed enhanced Jam-Sync method includes the steps of performing iterations to derive the estimates of the time offset between the true time-of-day and the STC in forward and reverse directions of data transmission and calculating an overall estimate of the clock correction.
  • the time-of-day information at the client wall-clocks can be generated with lower cost, lower power oscillators without adding the burden to the servers, thereby reducing costs for both the servers and the clients.
  • Figure 1 illustrates the use of the NTP/SNTP to generate time-of-day information at the client wall-clock, according to prior art
  • Figure 2 illustrates the use of the enhanced Jam-Sync method to generate time-of-day information at the client wall-clock, according to one embodiment of the present invention
  • Figure 3 illustrates the notions of time stamps used by the client wall-clock of Figure 2, according to one embodiment of the present invention
  • Figure 4 sets forth a flow diagram of enhanced Jam-Sync method steps for operating the enhanced Jam-Sync module of Figure 2, provided the time stamps of Figure 3, according to one embodiment of the present invention.
  • Figure 5 illustrates a computing device configured to implement one or more aspects of the present invention.
  • Figure 2 illustrates the use of the enhanced Jam-Sync method to generate time-of-day information at the client wall-clock 200, according to one embodiment of the present invention.
  • the client wall-clock 200 includes, without limitation, a local oscillator 210, an STC 220, an enhanced Jam-Sync module 230, and a physical layer synchronization module 250. Similar to the system of Figure 1 , the local oscillator 210 continuously generates an AC signal, referred to herein as a "local time-base" 215. The local time-base 215 is supplied as one of the two inputs to the STC 220.
  • the other input to the STC 220 is an STC adjustment signal 243 from the enhanced Jam-Sync module 230 to support adjusting the local time in the STC 220 to the current value determined by the enhanced Jam-Sync method.
  • the STC 220 in turn, produces an STC output 225, which is an accumulation of elapsed time from some arbitrary epoch, where the accuracy of the elapsed time is governed by the accuracy of the local time-base 215.
  • the physical layer synchronization module 250 synchronizes the local time-base 215 to a traceable physical layer provided in a downstream transmission 251 , such as network timing reference (NTR) or synchronous Ethernet, thereby assuring that in normal operation the local oscillator 210 produces the local time-base 215 accurate to 1 part in 10 11 .
  • NTR network timing reference
  • the process of synchronizing the signal produced by the local oscillator 210 to a traceable physical layer is referred to herein as "stabilizing of the local oscillator” because it results in improved frequency stability of the local oscillator 210.
  • Synchronization of the local time-base 215 to the traceable physical layer is enabled by the fact that almost all proposed variations of arrangements for delivering packet-based services are capable of carrying in its downstream transmission 251 a frequency reference with long-term frequency stability better than 1x10 "11 .
  • variations of DSL namely, SHDSL, ADSL, and VDSL (and variants thereof) have the downstream transmission 251 capable of transporting a NTR from the DSL server (not shown) to the client wall-clock 200.
  • This NTR is based on a clock available to the DSL server from the Building Integrated Timing Supply (BITS) that, in normal operation, is traceable to a primary reference source (PRS).
  • BTS Building Integrated Timing Supply
  • the physical layer synchronization module 250 can extract from the downstream transmission 251 (in this case, the incoming electrical signal) a frequency reference with long-term frequency stability better than 1x10 '11 and synchronize the local time- base 215 produced by the local oscillator 210 with this stable frequency reference.
  • the accuracy is determined by the holdover mechanism in the DSL central office as constrained by established clocks specifications such as G.811 , G.812, G.813 and ANSI T1.101.
  • variations of PONs namely, APON, BPON, and GPON have the downstream transmission 251 governed by a clock that, in normal operation, is traceable to a PRS. That is, in normal operation, the physical layer synchronization module 250 can extract from the downstream transmission 251 (in this case, the incoming optical signal) a frequency reference with long-term frequency stability better than 1x10 "11 and synchronize the local time-base 215 produced by the local oscillator 210 with this stable frequency reference. In holdover operation, the accuracy is determined by the holdover mechanism in the PON central office as constrained by established clocks specifications such as G.811 , G.812, G.813 and ANSI T1.101.
  • a local time-base accuracy of 1x10 11 introduces a very small error to the STC 220 that, for all practical purposes, can be ignored. Therefore, the enhanced Jam-Sync module 230 does not have to consider any time drift in the local time-base 215.
  • the enhanced Jam-Sync module 230 may operate in a one-way time transfer (OWTT) mode or a two-way time transfer (TWTT) mode. While the enhanced Jam- Sync module 230 may be based on NTP/SNTP principles, it does not have to implement any frequency correction algorithms.
  • the enhanced Jam-Sync module 230 executes the steps of the enhanced Jam-Sync method, as described in more details in Figure 4, and produces the STC adjustment 243 and a clock correction value, a CLK_CORR 245.
  • the client wall-clock 200 adds the CLK_CORR 245 to the STC output 225 and produces a time-of-day 255, which is the best estimate of absolute date and time, typically represented in seconds relative to Oh UTC on 1 January 1900.
  • Figure 3 illustrates the notions of time stamps used by the client wall-clock 200 of Figure 2, according to one embodiment of the present invention.
  • the two-way time transfer (TWTT) methods and the one-way time transfer (OWTT) subset methods are all based on the notion of time stamping the ingress and egress of designated packets that constitute the flows between the client wall-clock 200 and the server clock (not shown).
  • the enhanced Jam-Sync method executed by the enhanced Jam-Sync module 230 to extract the CLK_CORR 245 also utilizes variants of a time stamping process illustrated in Figure 3 to support a one-way time transfer mode.
  • a time measurement process may involve four time stamps along a timeline 300, which are defined as follows: Ti is an originating client time stamp representing the best estimate of the transmit originating epoch of a packet originating from the client wall-clock 200 of a client 310,
  • T 2 is a server time stamp representing the best estimate of the receive termination epoch of a packet terminating at the clock of a server 320
  • T 3 is a server time stamp representing the best estimate of the transmit origination epoch of a packet originating from the clock of the server 320.
  • T 4 is a terminating client time stamp representing the best estimate of the receive termination epoch of a packet terminating at the client wall-clock 200 of the client 310.
  • Each time stamp represents a critical epoch in a protocol transaction. It is assumed that the server 320 has the correct time-of-day and therefore the server time stamps T 2 and T 3 are indicative of the absolute time-of-day associated with the time-of-arrival, ⁇ 2l and the time-of-departure, 0: 3 , respectively.
  • the originating time stamp Ti and the terminating time stamp T 4 are meant to represent the time-of-departure, ⁇ -i, and the time-of-arrival, ⁇ 4 , respectively but these are estimates based on the client wall-clock 200 that may not be perfectly accurate.
  • T 4 (n) T 3 (n) + A m
  • T 4 (n) - T 4 (n -l) T 3 (n) - T 3 (n -l)
  • the entity AM S is the estimated network delay from the server 320 (commonly referred to in the telecommunications industry as a "master”) to the client 310 (commonly referred to in the telecommunications industry as a "slave") that may be erroneous because T 4 is based on the client wall-clock 200.
  • the network delay is constant, and the local time-base 215 at the client 310 is accurate, then the time interval between the transmit epochs of two successive packets is identical to the measured time interval between the reception of the two successive packets. This can clearly be extended to the time interval of transmission/reception of any pair of identifiable packets.
  • T 2 (H) - T 2 (H - V) T 1 (Ti) - T 1 (Ti - Y)
  • entity ⁇ S M is the estimated network delay from the client 310 to the server 320 that may be somewhat erroneous because Ti is based on the client wall-clock 200.
  • the minimum delay will be constant unless there are changes in the network due to rearrangements, failures, etc. However, the actual delays will be greater than the minimum by a random non-negative amount that quantifies the packet delay variation across the network and is denoted by ⁇ MS ( ⁇ ) and ⁇ SM ( ⁇ ) . Thus, the following relations hold:
  • T 2 (n) [T 1 (n) + ⁇ (n)] + A SM + ⁇ SM (n)
  • the non-constant delay of the network as well as the client wall-clock error has been included.
  • the client 310 has available T 1 , T 2 , T 3 , and T 4 , which are supplied to the enhanced Jam- Sync module 230 to execute the enhanced Jam-Sync method and generate the CLK_CORR 245 and the STC adjustment 243.
  • FIG. 4 sets forth a flow diagram of enhanced Jam-Sync method steps for operating the enhanced Jam-Sync module 230 of Figure 2, provided the time stamps of Figure 3, according to one embodiment of the present invention.
  • the index n means the number of nominally equally spaced time stamp transaction since the start at time index zero
  • Ts(n) represents the progression of the client wall-clock 200 over time
  • TM( ⁇ ) represents the progression of the server wall- clock over time (which is assumed to be accurate)
  • v is a CLK_CORR 245 value, which is an estimate of the constant offset between the true time-of-day and the STC output 225
  • YF and YR are the two estimates based on transmission between the client 310 and the server 320 (referred to herein as "client-to-server” transmission), and the server 320 and the client 310 (referred to herein as "server-to-client” transmission), respectively.
  • the method begins in step 412, where the enhanced Jam-Sync module 230 determines whether the client 310 is operating in a one-way time transfer mode (i.e., server-to-client transmission). If so, then the method proceeds to step 413, where the enhanced Jam-Sync module 230 sets the CLK_CORR 245 to an static compensation value that may be set to zero if there is no external knowledge of the one-way delay between the server 320 and the client 310 or, alternatively, be set to a one-way compensation value supplied externally from this process (e.g. from one-way QOS metrics).
  • a one-way time transfer mode i.e., server-to-client transmission.
  • step 414 the enhanced Jam-Sync module 230 waits for a valid time stamp T 3 from a properly formed, error-free packet from the server 320. After such a time stamp is received, the method proceeds to step 416, where the enhanced Jam-Sync module 230 determines if this is the first valid time stamp packet. If so, the method proceeds to step 418, where, for the purpose of initialization, the enhanced Jam-Sync module 230 sets T 5 (O) to be equal to T 3 . This is implement by the STC adjustment 243, which synchronously adjusts the value of the STC 220 to T 8 (O). As a result, the error is then bounded by (A MS + ⁇ MS (0)) . If, however, the enhanced Jam-Sync module 230 determines that this is not the first valid time stamp packet, the method proceeds to step 420, where the enhanced Jam-Sync module 230 sets T s (n) as follows:
  • T s (n) min ⁇ T s (n-1 ); T 3 (n) ⁇
  • T 3 (n) is the new valid server time stamp just received. This is implement by the STC adjustment 243, which synchronously adjusts the value of the STC 220 to T.(n).
  • step 418 or 420 the method returns to step 414 for iteration.
  • the client wall-clock 200 Given the known behavior of the PDV, the client wall-clock 200 will asymptotically approach the value corresponding to (T M (n) -A MS ) . The convergence will be rapid when the network congestion is low and slow if the network congestion is high.
  • the transit delay is bound by the accuracy of one-way time transfer methods. Nevertheless, the error will always be bound from above by (A MS + ⁇ MS (0)) .
  • step 412 if the enhanced Jam-Sync module 230 determines that the client 310 is operating in a two-way time transfer mode, the method proceeds to step 422, where the enhanced Jam-Sync module 230 waits for the next periodic transaction start time. At the start time, the method proceeds to step 424 where the client 310 launches a packet to the server 320 with the originating time stamp Ti (n) equal to T s (n), where T s (n) is the current value of the STC output 225 without correction. In step 426 the enhanced Jam-Sync module 230 waits for the reception of valid response packet from the server 320 to the client 310.
  • step 428 the enhanced Jam-Sync module 230 determines whether this is the first valid response packet. If so, the method proceeds to step 430, where the enhanced Jam-Sync module 230 sets T 3 (O), YF(O), and ⁇ R (0) as follows:
  • YR(O) - YF(O) where Max_Delay is largest one-way delay expected.
  • This initialization includes a one time adjustment by the STC adjustment 243, which synchronously adjusts the value of the STC 220 to T s (0).
  • the method proceeds to step 432, where the enhanced Jam-Sync module 230 sets ⁇ F (n), YR( ⁇ ), and ⁇ (n) as follows:
  • ⁇ F (n) min ⁇ ⁇ F (n-1 ); [T 2 (n) - T s (n-1 ) ] ⁇
  • ⁇ R (n) max ⁇ ⁇ R (n-1 ); [T 3 (n) - T s (n) ] ⁇
  • step 432 the method returns to step 422 for iteration.
  • the two estimates for the constant offset between the server wall-clock, TM, and the STC output 225 are derived iteratively and improved with additional knowledge gained with each packet exchange.
  • the clock correction, ⁇ (n) is developed as the average of the "forward" and "reverse” estimates, i.e., the estimates obtained from client-to-server and server-to-client transmissions, respectively.
  • the limiting accuracy of the asymptotical estimate is limited only by the asymmetry in transit time between the two directions of transmission. Persons skilled in the art will understand that this asymmetry bounds the accuracy of the two-way time transfer methods.
  • the client 310 may be able to query multiple servers in order to ascertain its client wall-clock 200. Denoting the K possible servers with an index k, the clock correction estimates can be made in both forward and reverse directions utilizing all K servers as follows:
  • FIG. 5 illustrates a computing device 500 configured to implement one or more aspects of the present invention.
  • the computing device 500 includes a processor 510, a memory 515, and an application circuitry 520.
  • the memory 515 and the application circuitry 520 are coupled to the processor 510.
  • the processor 510 includes the client wall-clock 200.
  • the client wall-clock 200 executes the steps described in the present invention and produces the time-of-day synchronized with the precise server wall-clock on the network.
  • the physical layer synchronization module 250 and the local oscillator 210 may be included in the application circuitry 520.
  • the STC 220 and the enhanced Jam-Sync module 230 may operate within the processor 510 or alternatively be supported in the application circuitry 520.
  • the computing device 500 may be any type of computing device that includes application circuitry requiring a client wall-clock to produce a timing signal.
  • Some examples of computing device include, without limitation, a personal computer, a data center server, a router, an IP telephony device, a cellular phone, and a personal digital assistant. The more likely instantiation is in embedded computing devices with built-in timestamp and clock support. These computing devices find general application is telecommunication and industrial applications required client synchronization including DSL aggregators, passive optical network units (ONUs), wireless base stations, and access points.
  • the present invention enables the computing device 500 to
  • control of the client wall- clock may be achieved by extracting and utilizing the frequency reference contained in the physical layer of the incoming signal, such as Network Timing Reference (NTR) and synchronous Ethernet.
  • NTR Network Timing Reference
  • By stabilizing the local oscillator with the extracted frequency reference local time-base with an accuracy of 1x10 "11 may be achieved.
  • An accurate local time-base in turn, enables a method for obtaining the clock correction value that is simpler and more reliable than the prior art methods.
  • the disclosed enhanced Jam-Sync method includes the steps of performing iterations to derive the estimates of the time offset between the true time-of-day and the STC in forward and reverse directions of data transmission and calculating an overall estimate of the clock correction.
  • time-of-day information at the client wall- clocks can be generated with lower cost, lower power oscillators without adding the burden to the servers, thereby reducing costs for both the servers and the clients.
  • the present invention can be implemented in hardware or software, with the software being delivered as a program product for use with a computer system.
  • the program(s) of the program product defines functions of the embodiments (including the methods described herein) and can be contained on a variety of computer-readable storage media.
  • Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive) on which information is permanently stored; (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive) on which alterable information is stored.
  • Such computer-readable storage media when carrying computer-readable instructions that direct the functions of the present invention, are embodiments of the present invention.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Theoretical Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • General Engineering & Computer Science (AREA)
  • General Physics & Mathematics (AREA)
  • Electric Clocks (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)

Abstract

Un mode de réalisation de la présente invention se rapporte à un procédé pour extraire une référence de fréquence véhiculée par la couche physique du signal entrant, permettant de stabiliser l'oscillateur local, et pour générer une estimation adéquate de la correction d'horloge afin d'obtenir une information précise sur l'heure du jour d'après l'horloge client d'un dispositif informatique. Le procédé Jam-Sync amélioré comprend les étapes de réalisation d'itérations afin de déduire les estimations du décalage de temps entre l'heure véritable du jour et le temps d'horloge système (STC ou System Time Clock) en sens direct et inverse de la transmission des données, et de calcul d'une estimation de la valeur globale de correction d'horloge. Grâce à ces étapes, les effets négatifs d'une variation de retard de paquet peuvent être atténués, et une valeur nécessaire pour corriger le temps d'horloge système dans l'horloge client est déterminée. Par conséquent, le contrôle de qualité de l'horloge client est réalisé à un coût significativement réduit et à un niveau de complexité inférieur en comparaison avec les approches de l'art antérieur.
PCT/US2007/062452 2007-02-20 2007-02-20 Horloge assistée WO2008103170A1 (fr)

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EP2991261A1 (fr) * 2011-05-31 2016-03-02 NEC Corporation Procédé et système de synchronisation
CN107147462A (zh) * 2017-04-18 2017-09-08 福建天泉教育科技有限公司 一种时钟校准方法及系统
CN112968748A (zh) * 2021-04-14 2021-06-15 中国人民解放军海军航空大学岸防兵学院 软件同步误差补偿方法、系统、介质及设备
CN115981130A (zh) * 2023-01-09 2023-04-18 哈尔滨工程大学 一种基于多普勒补偿的水下目标授时方法

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Cited By (13)

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Publication number Priority date Publication date Assignee Title
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