The field of the invention is that of determining the geographical position of a mobile station in a digital cellular communications system of the type operating in time division multiple access (TDMA) mode. 
The digital cellular communications system to which the mobile station discussed herein belongs may be of the following types: Global System for Mobile communication (GSM); or personal Communications Network (PCN); or digital European cordless telecommunication (DECT). 
It is recalled that a mobile station is the physical equipment used by a user of a digital cellular communications system to access the telecommunications services on offer.  In general, there are three types of mobile station, namely: vehicle-mounted stations, portable stations, and hand-held terminals. 
More precisely, the invention relates to a mobile station combining telecommunications and positioning functions. 
The invention has numerous applications, such as locating stolen mobile stations (and simultaneously locating stolen vehicles in the event that the mobile stations are fixed to the vehicles), for example. 
More generally, the invention can be applied in all cases where it is desired to determine the geographical position of a mobile station at a given instant. 
In the description below, the state of the art is described essentially in terms of known hand-held terminals.  It is clear that this discussion can be generalized to any type of mobile station.  The present invention is applicable to any type of mobile station, and not only to a hand-held terminal. 
In the state of the art, several techniques are known that enable telecommunications and positioning functions to be combined in a hand-held terminal. 
A first known technique consists in combining two independent terminals in a single housing, specifically a radio telecommunications terminal (e.g. a GSM terminal) and a positioning terminal (e.g. a GPS terminal), and in interconnecting them by means of a data line.  Under such circumstances, it has also been proposed to use the telecommunications terminal to transmit the positioning information to an operating center.  In other words, positioning information is transmitted over the radio telecommunications network (e.g. via control channels or using a short message service). 
Conventionally, a positioning terminal receives positioning signals transmitted from one or more reference points.  In particular, the Global Positioning System (GPS) is known which implements a constellation of 24 satellites distributed in such a manner that at any given point it is always possible to receive signals transmitted from three separate satellites.  A GPS type positioning terminal can determine its own position from the positioning signals it receives from three separate satellites. 
The major drawback of that first known technique is to be significantly more expensive than an ordinary telecommunications terminal.  The positioning terminal has receiver means and processor means (generally operating in baseband) and so does a telecommunications terminal. 
A second known technique sets out to remedy that drawback, but does not fully achieve its objective.  That technique consists in using a common baseband processing unit either with a system for receiving positioning signals (operation in positioning mode) or else with a transceiver system for telecommunications signals (operation in telecommunications mode).  The terminal includes switching means enabling one or other of its two operating modes to be selected (positioning or telecommunications). 
Thus, compared with the first known technique, there is a saving of one baseband processor unit.  Unfortunately, that second known technique suffers from the major drawback of not being able to operate simultaneously both for positioning and for telecommunications. 
A particular object of the invention is to mitigate those various drawbacks of the prior art. 
More precisely, one of the objects of the present invention is to provide a mobile station combining telecommunications and positioning functions, and which is of small extra cost compared with a mobile station that performs the telecommunications function only. 
The invention also seeks to provide such a mobile station that is capable of operating simultaneously for positioning and for telecommunications purposes. 
Another object of the invention is to provide such a mobile station that enables positioning information to be transmitted to an operating center, while simultaneously performing voice and/or data communication. 
These objects, and others that appear below, are achieved according to the invention by means of a mobile station of a digital cellular communications system, the station being of the type comprising:
means for transmitting and receiving telecommunications signals by radio in time division multiple access mode;
means for receiving positioning signals transmitted by a positioning system; and
processor means for processing said telecommunications signals and said positioning signals;
wherein said processor means are continuously connected to said means for transmitting and receiving telecommunications signals and to said means for receiving positioning signals, said processor means making use of received positioning signals during moments in which said telecommunications signals are not being transmitted and are not being received. 
The principle of the invention thus consists in taking advantage of the moments of inactivity of the means for transmitting and receiving telecommunications signals in order to perform positioning measurements.  Thus, the mobile station of the invention can operate simultaneously both for positioning and for telecommunications purposes.  It should be emphasized that such moments of radio inactivity in a telecommunications system are always present in a digital cellular communications system of the type operating in time division multiple access (TDMA) mode. 
It will be observed that the invention requires only one processor unit, without that requiring switching between two operating modes (positioning and telecommunications).  The processor unit receives positioning signals continuously, but only some of them are used, i.e. at least some of those which are received during moments when radio telecommunications signals are not being transmitted and are not being received.  It is recalled that in TDMA mode, each mobile station has such moments assigned to it, corresponding to the spectrum being segmented in time, both in the up direction (mobile station to base station) and in the down direction (base station to mobile station). 
In a preferred embodiment of the invention, said telecommunications signals are structured for transmission and for reception in successive frames, each transmission frame and each reception frame including at least one time slot for transmission or for reception of telecommunication signals by said mobile station, and the positioning signals used by the processor means are received during unused time slots, i.e. time slots that correspond neither to a transmission time slot nor to a reception time slot for the telecommunications signals. 
Advantageously, said mobile station includes single receiver means common both to said telecommunications signals and to said positioning signals. 
This saves one receiver.  However the single receiver used must generally be a multiband receiver so as to be capable of receiving both telecommunications signals and positioning signals. 
Advantageously, said processor means include means for extracting positioning data contained in said positioning signals that are used, and for inserting said positioning data, optionally after transforming it, into telecommunications signals transmitted by said transmission means to an operating center. 
In this way, the telecommunications network is used for relaying positioning data to an operating center. 
Preferably, said positioning data is transmitted to said operating center via at least one control channel and/or via a short message service. 
Preferably, said positioning system is of the type comprising a constellation of satellites transmitting said positioning signals. 
Advantageously, said digital cellular communications system belongs to the group comprising GSM type systems and PCN and DECT type systems. 
The invention also provides a method of receiving positioning signals by a mobile station of a digital cellular communications system, said mobile station being of the type that transmits and receives telecommunications signals in a time division multiple access mode, the method including a step of making use of positioning signals received during moments when said telecommunications signals are not being transmitted and are not being received.