METHOD AND APPARATUS FOR SYNCHRONIZING THE TRANSMISSION OF DIGITAL SIGNALS BETWEEN GROUND STATIONS VIA A COMMUNICATIONS SATELLITE

• US 3,646,444 A
• Filed: 10/08/1969
• Issued: 02/29/1972
• Est. Priority Date: 10/16/1968
• Status: Expired due to Term

• Current Assignee: Wolfgang Bitzer
• Original Assignee: Wolfgang Bitzer
• Inventors: Bitzer, Wolfgang
• Primary Examiner(s): Griffin, Robert L.
• Assistant Examiner(s): Weinstein, Kenneth W.
• Application Number:
  US04/864,818
• Time in Patent Office: 874 Days
• Field of Search: 325/58,419,421,4,5 179/15BS 343/225,7.5
• US Class Current: 375/356
• CPC Class Codes: H04B 7/2126 using a reference station

Abstract

A method and apparatus for synchronizing the transmission of data between a plurality of ground stations via a satellite by transmitting a fixed frequency reference signal from a reference ground station, receiving this signal at every other ground station, producing at each other ground station a first local signal identical in frequency and phase with the received reference signal, producing at each other ground station a second local signal, transmitting this signal to the satellite and receiving this signal back at the ground station from which it originated, and varying the second local signal until it has a value such that upon its reception back at the originating station it is identical in frequency and phase with the first local signal produced at that station.

Claims

1. A method for synchronizing the time-division multiplex transmission of digital data signals via a satellite between a plurality of ground stations, one of which is a reference station, comprising the steps of:

a. generating at the reference station a signal having a fixed frequency constituting a basic reference frequency for such transmission and transmitting reference bursts of this fixed frequency from the reference station to the satellite;

b. receiving these reference bursts from the satellite at a second ground station;

c. generating at such second ground station a local reception control signal whose frequency is substantially equal to such fixed frequency;

d. electronically comparing, at such second station, the phase of the received reference bursts with the phase of the local reception control signal;

e. varying, on the basis of such comparison, the local reception control signal in a direction to eliminate the phase difference between the received reference bursts and the local reception control signal, whereby the local reception control signal provides a reference for processing the data signals received by the second station;

f. generating at the second station a local transmission control signal and transmitting local reference bursts of this signal to the satellite;

g. receiving these local reference bursts back from the satellite at the second station;

h. electronically comparing, at the second station, the phase of the local reference bursts received back from the satellite with the phase of the local reception control signal; and

i. varying the local transmission control signal in a direction to eliminate the phase difference, at the second station, between the received local reference bursts and the local reception control signal, whereby the local transmission control signal provides a reference for transmitting, from the second station, data which will arrive at the satellite coherently with data transmitted from the reference station.

2. Apparatus for synchronizing the time-division multiplex transmission of digital data signals via a satellite between a plurality of ground stations, one of which is a reference station, comprising the steps of:

a. first generating means located at the reference station for generating a signal having a fixed frequency constituting a basic reference frequency for such transmission and for transmitting reference bursts of this fixed frequency from the reference station to the satellite;

b. receiving means at a second ground station for receiving these reference bursts from the satellite;

c. second generating means at the second ground station for generating a local reception control signal whose frequency is substantially equal to such fixed frequency;

d. comparator means at such second station connected to said receiving means and said seconD generating means for comparing the phase of the received reference bursts with the phase of the local reception control signal;

e. generator control means connected between said comparator means and said second control means for varying, on the basis of such comparison, the local reception control signal in a direction to eliminate the phase difference between the received reference bursts and the local reception control signal, whereby the local reception control signal provides a reference for processing the data signals received by the second station;

f. third generating means at the second station for generating a local transmission control signal and for transmitting local reference bursts of this signal to the satellite, said receiving means acting to receive these local reference bursts back from the satellite at the second station;

g. second comparator means at the second station connected to said receiving means and said second generating means for comparing the phase of the local reference bursts received back from the satellite with the phase of the local reception control signal; and

h. second generator control means connected between said second comparator means and said third generating means for varying the local transmission control signal in a direction to eliminate the phase difference, at the second station, between the received local reference bursts and the local reception control signal, whereby the local transmission control signal provides a reference for transmitting, from the second station, data which will arrive at the satellite coherently with data transmitted from the reference station.

3. An arrangement as defined in claim 2 wherein said comparator means and said second comparator means are each constituted by a low-pass filter and said low-pass filter of said comparator means has a longer time constant than said low-pass filter of said first defined comparator means.

4. An arrangement as defined in claim 2 wherein at least one of said generator control means is constituted by a digital control device.

5. An arrangement as defined in claim 2 wherein said apparatus is employed for synchronizing the carrier frequencies of the plurality of stations with respect to the satellite, and said first generating means generates a signal whose fixed frequency bears a predetermined relation to the carrier frequency transmitted by said reference station.

6. An arrangement as defined in claim 5 further comprising a second set of all of said means provided for synchronizing the clock frequencies of said ground stations, wherein the fixed frequency produced by said first generating means of said second set of means is equal to the clock frequency at which signals are transmitted by said reference station.

7. An arrangement as defined in claim 2 wherein said third generating means comprise a frequency-controllable local oscillator having its input connected to the output of said second generator control means, and a burst generator connected to the output of said local oscillator, said arrangement further comprising fourth generating means for generating a signal having a fixed frequency and switch means connected to said burst generator, said local oscillator and said fourth generating means for selectively connecting the input of said burst generator to one of said local oscillator and said fourth generating means.

8. An arrangement as defined in claim 2 wherein there are more than two ground stations each provided with a respective set of said means located at said second ground station.

9. An arrangement as defined in claim 2 further comprising a source at the second ground station of bursts synchronized with the received reference bursts and means for selectively connecting a respective one of said source and said receiving means to said comparator means.

10. An arrangement as defined in claim 2 wherein at least one of said comparator means and second comparator means is constituted by a frequency comparison cirCuit for providing an output signal whose amplitude is proportional to the frequency difference between the two signals applied to said comparator means and whose polarity represents the sense of such frequency difference.

11. An arrangement as defined in claim 2, wherein said second generator control means are constituted by a keyed control device periodically connected to said second comparator means.

12. An arrangement as defined in claim 2, wherein said second generator control means comprise:

a switch connected in series with said second comparator means and arranged to be periodically closed at a rate such that the periods during which said switch is open are longer than the travel time of the local reference bursts from the second ground station to the satellite and back to the second ground station, and the intervals during which the switch is closed are short with respect to this travel time;

multiplying means connected to said switch for receiving the output from said second comparator means and for multiplying such output by a first multiplication factor;

integrator means connected to said switch for integrating the output from said comparator means and for multiplying the resulting integral by a second multiplication factor; and

a summing circuit connected to said multiplying means and said integrating means for producing an output proportional to the algebraic sum of the outputs of said multiplying means and said integrating means, the output of said summing means being connected to said third generating means.

13. An arrangement as defined in claim 12 wherein at least one of said multiplying means and said summing circuit is provided with a further input for receiving a voltage whose amplitude is proportional to the difference between the frequencies of the signal supplied to said second phase comparison circuit and whose polarity is representative of the sense of such difference.

14. An arrangement as defined in claim 12 wherein said integrating circuit is constituted by an operational amplifier and a capacitor connected to said amplifier to form a feedback loop therefor.

15. An arrangement as defined in claim 13 wherein said summing circuit is constituted by an operational amplifier and a resistor connected to said amplifier to form a feedback loop therefor.