IL95976A - Time dissemination - Google Patents

Description

TIME DISSEMINATION FIELD OF THE INVENTION : The invention relates to a system and method for high accuracy tim.e dissemination. Time dissemination is carried out between a main station, and one or more substations, the ultimate result being a time signal submitted to the substation with a high degree of accuracy, of the order of better than 30 psec. accuracy. The time dissemination is effected via a two-way link between the two stations, generally by means of a telephone line.

BACKGROUND OF THE INVENTION : Presently, many methods for time dissemination exist. Most of these methods and some of their realizations are described in PTTI conferences. These methods may be divided into three categories : 1) One-way transmission of timing pulses accompanied by time codes defining the time of the timing pulses. These methods include one-way radio transmission from ground stations or satellites such as OMEGA, LORAN-C and one-way GPS. The method of system of vocal telephone time announcement belong to this category as well. 2) Common mode time dissemination in which two timing stations may be compared by measuring a signal received from a third station. This method is commonly utilized in the Common View GPS system where the third station is a satellite. Assuming similar radio transmission paths to both stations, a time comparison between the two ground stations may be relatively easily accomplished with a high precision. 3) Two-way time transmission. This may be further divided into two methods : 3A) Where both stations are transmitting their time signals to each other.

Assuming that the transmission delay through the common medium does not change during both transmissions, it is possible in principle to eliminate the effect of this delay and its noise and which will result in a very high precision. 3B) A timing station is transmitting a signal to a station under calibration which echoes a signal backwards to the first station.

This echo is used to calculate the transmission delay between the two stations. This delay is taken into account in the process of synchronization of the first station.

The present invention fits category (3B) and has the advantage of being suitable for precise time dissemination via telephone lines and the advantage of a relatively cheap realization.

SUMMARY OF THE INVENTION : The invention relates to a system and to a method of time dissemination with a high degree of accuracy, between two stations, the communication between the main and the substation being via a telephone link.

The novel system is suitable for precise time dissemination and is comparatively simple and inexpensive.

The invention further relates to a system of such time dissemination which provides a high accuracy of time indication, better than at least 30psec. Another feature of the invention is a novel method of time dissemination from a main station A to a substation B, which essentially comprises generating at station^a pulse, transmitting same to Station B and back via a telephone line, correcting the time according to the duration taken by the pulse in both directions, calculating the required time correction, and based on this correction, transmitting from Station A to Station B a pulse indicating the correct time at an accuracy of better than 30 sec. The correction at Station A is effected using a suitable computer (such as a PC), modem, programmed I/O, serial and counter interface means, and a suitable atomic clock, such as a caesium or rubidium clock. Station B comprises computer means, modem, programmed I/0+. serial interface means, heat-rstabil ized high frequency oscillator and digital clock.

The invention is described by way of illustration with reference to the enclosed schematic drawings, which; . are not according to scale and in which Figure 1 - is a block diagram of the main station and of the substation; Figure 2 - is a diagram of some of the time relationships of the relevant signals; Figurs 3 and 4 illustrate the systems for the first phase of the signal procession; Figure 6 - is a time flow diagram of a system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT : Tfte invention relates to a system for time dissemination from a transmitting sub-system to a receiving sub-system via a two way communication link. Figure 1 is a block diagram of the system in which 11 , is the time transmitting sub-system and 12, the time receiving sub-system and 8, is the two-way communication link. In Figure 1 , is the system's precise clock, whose time is to be disseminated, such as a caesium atomic clock.

Two relevant signals are derived from 1, a high frequency wave function 2,. such as a 100 KHz sine wave and, a low frequency wave function 3 (which will be designated 1/pps derived from 2.

Assuming that counter 4 is reset, then pulse 3 will trigger it to count pulse 2. Pulse 3 will trigger counter 5 to count .down pulses I. 1 he initial value from which counter 5 starts to count down is D. D is a number generated by the processor 6 and is related to the transmission delay in medium 8 as will be explained below.

Counter4 and counter 5 may be the same or two separate physical counters. Figure 2, shows the time relationships of some of the relevant signals.

Pulse 21, which is a specific pulse in the train of pulses 3, triggers the sending of signal 41 from 7 to 9. At the time 24, 9 sends back signal 42, which arrives at 7 delayed by t with respect to 21. At time 21, counter 3 will start its counts till signal 42 is detected and counter 5 will start counting down, after time 21 D at instant 31 counter 5 will output zero and a signal 43 will be sent to 9. This signal will arrive at time 34, Signal 43 is used to synchronize the clock in the sub-system. Assuming that the transmission delay of signal 43 is the same as that of 41, and that the relation between the measured time t and the relation between transmission delays of 41 and 42 is known, it is possible to calculate D so that 34 will coincide with any time which may be for example pulse 22, the next pulse in the train of pulses 3. For example, if the transmission delay in 41 and 42 is equal and 9 sends back 42 with negligible delay then the delay of 43 will be t/2.

At start-up the value of D is unknown so it may take several sequences of the kind described to obtain D. If the transmission medium 8 is noise, this sequence has to be repeated many times to gain enough confidence in the calculated D before first signal of the kind 43, is sent.

Figure 2 shows two such sequences where 22 is equivalent to 21, 32 to 31, 46 to 43, 44 to 41, 35 to 34, 25 to 24, 23 to 22, and 45 to 42.

As a practice, signal 43 contains a word from which the time at one specific bit in the train of 43, is defined.

The invention is further illustrated with reference to the enclosed Figures 3 to 6.

The main Station A, designated in Fig. 1 as 11 , and the sub-station B, designated in Fig. 1 as 12, are illustrated in detail.

The main Station A is composed of a computer (PC computer), modem, programmed I/O + serial + counter interface means, and an accurate atomic clock.

The sub-system B (Station B) comprises a computer, modem, programmed I/O + serial interface means, high frequency ,temperature-stabi 1 ized oscillator (such as 5 MHzK and digital clock. As shown, the two counters 4 and 5 are united to one, in Counter 54.

The system operates in two phases, in the first the system measures transmission delays and in the second, the system uses these delays to send corrected time code to arrive at the sub B just at the right time.

During the first phase the system operates as. follows : In order to transmit a time code (which includes hour, minute, second and sync.signal) so that the sync, signal arrives at 12 on time, the master sub system (11) uses a binary counter (54) to measure the time elapsed between sending (41) and receiving (42), a check character.

The counter starts to count from the sending of the check character (41) until this character is returned back from sub-system B.

This is further illustrated in Figure 5.

This time delay t is averaged several times, then divided by two to get the one-way delay, assuming equal times for both transmissions.

We assume in our realization that this value t/2 ,is a fraction of a second, otherwise we take the fractional part of H. This concludes the first phase of the operation.

D«iH ngthe second phase, the system operates as follows : Using a clock rate derived from the atomic clock (1) of 50 KHz, the counter (54) is preset with (1-t) 50000.

Z Upon arrival of the l/pps pulse from clock 1 the counter starts to count down; the communication link sends the time code hour, minutes, seconds (hh:mm:ss) to sub-system B and waits for the counter to reach zero.

When this occurs, the communication link sends the sync.character to sub B. The time the sub system B receives this character is the time given in the time code.

DESCRIPTION OF SIGNALS : Description of the first phase of processing : (see Figures 3-4).

The incoming 100 KHz signal arriving from the caesium clock is shaped by circuit 51 and divided by 2 by binary divider 52.

This signal, which is gated by the count enable signal 55 coming from the computer 53, goes into 16 bit of binary counter 54.

The counter is now reset to zero and the count enable signal sets the counter to count the incoming pulses within the sending of a check character via the RS232C interface.

When the check character is returned back from sub B (60) the count enable signal (55) stops counting.

This counter's value is now divided by two and subtracted from 50000 counts. (50000 counts = 1 sec.) The new value is loaded into the counter (54).

The second phase of the process is as follows : The counter now waits, for 1/pps pulse (56) from the caesium clock (50) upon the arrival of this signal (56), the counter (54) starts to count down.

The RS232C interface (56) sends the time code hh:mm:ss over the communication line (56-66) to sub system B (60).

The RS232C interface (56) waits for zero counter state, and when this occurs, a sync character is sent to sub system B (60). When the RS232 interface (64) of system B (60) senses the arrival of the sync. character (a special character) a reset pulse (65) is sent to load the time code into the digital clock (62) and a reset pulse (actually the same pulse) (65) is sent to the binary divider (61) to synchronize the next 1/pps pulse (67). This completes the second phase operation (see Figure 6 for time flow diagram) .

In our realization the baud rate of the communication link was 4800; the jitter in the time transmission was G"*= 30/us.

The length of the line was about 9-10 km in point to point line.

We have seen that the £ was baud rate dependent and in 9600 baud a better sigma can be obtained.

Claims (7)

CLAIMS :

1. A method of high accuracy time dissemination from a station A to a station B, via a two-way telephone link^ which comprises generating at Station A a pulse, transmitting same to Station B and back via the telephone link, correcting the time according to the duration of time taken by the time to travel in both directions, calculating the time correction required, and based on such correction, transmitting from A to B a pulse indicative of the correct time, the accuracy being better than 30usec.

2. A method according to claim 1, where the correction is effected by means of computing means, a modem, programmed I/O serial, and counter interface.

3. A method according to claim 1 or 2, where the time delay is integrated a plurality of times, divided by two to give the one-way delay, and using this to establish the correct time signal which is transmitted to Station B.

4. A system for high accuracy time dissemination via a telephone link between Station A to substation B which comprises generating at Station A a pulse and transmitting same to Substation B, returning the signal to station A, measuring the time taken by this two-way trans= mission, dividing this by two and re-transmitting a corrected accurate time signal from Station A to Station B, which has an accuracy better than 30 gsec.

5. A system according to claim 4, where Station A comprises computing means, modem, programmed I/O, serial and counter interface means and an atomic clock.

6. A system according to claim 4 or 5, where substation B com= prises computer means, modem, programmed I/O, serial and counter interface means, thermally stabilized pscillator and digital clock.

7. A system according to any of claims 4 to 6, where station A comprises an atomic clock, two counters, a processor and transmitting means for sending the signal to station B, where the atomic clock comprises means for generating a high frequency wave function such as 100 KHz sine wave and 1 pps signal. For Applicant? COHEl ZEDEK & RAPAPORT