Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Patent
1998-05-15
1999-10-19
Tarcza, Thomas H.
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
34235701, 701213, G01S 502, H04B 7185
Patent
active
059696710
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
The purpose of this invention is a process for determining the time difference between stations in a radio-positioning-navigation system, in real time.
2. State of Prior Art
A radio-positioning-navigation system usually includes reference stations located on the ground and connected to a processing center. One or more satellites form a bi-directional link between the stations. Vehicles (land, air, sea) equipped with appropriate radioelectric means may be positioned and/or guided with reference to the stations.
For example, known systems include the Global Positioning System (GPS) and its Russian variant GLONASS, and the preliminary plan for the European complement to the GPS system, called the CE-GPS.
In this type of system, ground stations usually include a clock acting as a time reference and it is often essential to know the difference between the references on the different stations.
One known means is to use the "Two-Way Time Transfer" technique. This technique consists of transmitting a "One Pulse Per Second" (1PPS) type signal from a station 1 to a station 2, through a repeater satellite and symmetrically transmitting from station 2 to station 1. In this case, the instant at which the pulse specific to the station is transmitted, and the instant at which the pulse from the other station is received, are measured. Each station then informs the other of the result of its measurements. This communication takes place through any type of link (telephone, electronic mail, disk, etc.). Each station is then in a position to calculate the time difference between station references, a difference which is a simple function of the measurement results carried out by each station (usually a half-sum).
Although this means is satisfactory in some respects, it is clear that it has one disadvantage, which is its delayed nature. Each station must wait to receive information from the other station before starting calculations and determining its difference with respect to the other station. The purpose of this invention is to correct this disadvantage, by proposing a process which is capable of determining the time difference between stations, in real time.
DISCLOSURE OF THE INVENTION
A known "spectrum spreading" technique is used in radio-positioning-navigation systems, that makes use of pseudo-noise codes (abbreviated as "PN" through the rest of the document). It is known that a pseudo-noise code is a periodic sequence of an odd number of bits or "bribes".
Spectrum spreading consists of modulating the useful signal by this type of pseudo-noise code, this code having a high binary flow compared with the data flow, and then modulating a carrier by the composite signal obtained. The distance of moving objects can be measured by using this type of PN codes by dating the instants at which a specific event in the code is transmitted and received with respect to the clock in a spread spectrum signal receiver. This special event is the start or end of a specific bribe in the code.
Further information about this spectrum spreading technique using pseudo-noise codes in liaison with radio-positioning-navigation may be found in the collective book entitled "Techniques et technologies des vehicules spatiaux" (Space vehicle techniques and technologies), Volume 1, Cepadues-Editions, (1994), and specifically in Module 6 "Localisation spatiale" (Space positioning) by Jean-Luc ISSLER, pages 561-634.
In this type of technique, it is known how to synchronize the instant at which a code is retransmitted by the geostationary satellite with a time reference (GPS or other reference). This is done by offsetting the time at which this code is transmitted in a station (which correspondingly offsets the time te at which the same code is received in the said station), until the half-sum of the transmission and reception times (te+tr)/2 is equal to one reference instant tref. This is equivalent to synchronizing the satellite retransmission time to this reference.
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REFERENCES:
Alta Frequenza, vol. 51, No. 82, Sep. 1982--Oct. 1982, Milan, pp. 263-274, XP002011485 E. Detoma & S. Leschiutta: "The SIRIO-1 timing experiment."
Measurement Techniques, vol. 37, No. 1, Jan. 1, 1994, pp. 29-34, XP000471137, Gurevich E. L. et al., "Comparison of Remote Time Scales by Satellite Communications Channels Method, Apparatus, and Error".
Centre National d'Etudes Spatiales
Phan Dao L.
Tarcza Thomas H.
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